S2's Build Thread

This car has so much tech. Execution is phenomenal. On paper the car could be so fast assuming all the complicated systems work. What ambition Scott has to try and program a pneumatic shifting albins and work out an active wing. I wish you were my neighbor Scott.

As far as the weight issue goes, i tend to agree. I look for ways to make my car as light as possible because thats the easy way out. yet look how heavy all modern cars have gotten, especially the fast ones. yet they still get faster, safer and cleaner.
 

Scott

Lifetime Supporter
as they say in aviation, "With enough horsepower, even a barn door can fly."
Neil, I take your point, but I haven’t seen a SL-C that approaches the barn door comparison.

The nice thing about this hobby is that one can pursue whatever path he chooses toward whatever end he seeks
The better thing is that, to my wife’s dismay, I’m not limited to one home-built car. You’re advocating a lightweight, no-frills build which is a good objective, but I’ve been there and done that 32 years ago (wow I’m old) I built an ERA cobra – their cobras are as nice as their GT40s. It has a 1966 427 side-oiler with Webber 48 IDAs. The engine had a 428 stroker crank and, when I had it rebuilt, I simply requested the best internals. I never ran it on the dyno because I wasn’t chasing a specific number, so the official HP spec is “more than enough.”

The only modern nods are a 5-speed transmission, electric radiator fan, MSD ignition, a hidden USB charging port and ceramic coating on the headers and side pipes. Smile per mile is high and it doesn’t have a once of non-required weight.

My SL-C is a completely different animal. It would have been impossible to build a car like my SL-C when I built the cobra. I didn’t have the time, money, knowledge or tools. More importantly, the ecosystem didn’t exist – forums like this, deeply knowledgeable automotive friends, internet-based research/tutorials/suppliers, affordable CAD/CAM, 3D scanning/printing, configurable ECUs, active aero, electromechanical accessories that aren’t tied to a belt or vacuum, manufacturing services like SendCutSend and 3D Hubs, etc.

This is my lightweight, analog build (I need to take some better pictures).

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Neil

Supporter
Neil, I take your point, but I haven’t seen a SL-C that approaches the barn door comparison.



The better thing is that, to my wife’s dismay, I’m not limited to one home-built car. You’re advocating a lightweight, no-frills build which is a good objective, but I’ve been there and done that 32 years ago (wow I’m old) I built an ERA cobra – their cobras are as nice as their GT40s. It has a 1966 427 side-oiler with Webber 48 IDAs. The engine had a 428 stroker crank and, when I had it rebuilt, I simply requested the best internals. I never ran it on the dyno because I wasn’t chasing a specific number, so the official HP spec is “more than enough.”

The only modern nods are a 5-speed transmission, electric radiator fan, MSD ignition, a hidden USB charging port and ceramic coating on the headers and side pipes. Smile per mile is high and it doesn’t have a once of non-required weight.

My SL-C is a completely different animal. It would have been impossible to build a car like my SL-C when I built the cobra. I didn’t have the time, money, knowledge or tools. More importantly, the ecosystem didn’t exist – forums like this, deeply knowledgeable automotive friends, internet-based research/tutorials/suppliers, affordable CAD/CAM, 3D scanning/printing, configurable ECUs, active aero, electromechanical accessories that aren’t tied to a belt or vacuum, manufacturing services like SendCutSend and 3D Hubs, etc.

This is my lightweight, analog build (I need to take some better pictures).

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Nice car! I put a deposit on a new medium-riser 427 Cobra at Bill Watkins Ford in Phoenix when I was in Tucson on business in 1968. $8200 was the price then! When I returned to Charlottesville, I found a Ferrari 250 GTE and bought it instead. No regrets.
 

Scott

Lifetime Supporter

I finally got around to mounting the EC for the active wing. It has four feet with mounting holes so that should be simple, right? Nope…

There are no instructions regarding mounting and when I went to install it I realized that it had a cooling fan. I called Aeromotions and they confirmed that the ECU will not tolerate any moisture. Front-engine cars mount the ECU in the trunk and the Ferraris mount it under the passenger seat, neither of which is feasible in the SL-C.

The ECU must be in one of two orientations for the accelerometers to work and you need access to the control panel. The only location was under the dash where a normal car would have a glove box. I already hate doing anything under the dash and that’s only going to get worst when the interior is finished which made it important that the control panel was easily accessible. I considered hinging the ECU, but there wasn’t enough space for it to pivot in place. The solution was a dual motion bracket; (1) slide down to clear the bottom of the dashboard and (2) pivot to provide access to the control panel.

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Laser-cut 0.060” stainless steel and stainless-steel piano hinge. I subsequentially added slots to the glide plate to provide access to the guide rail mounting screws.

As mentioned above I really don’t like doing anything under the dash, so I looked for a quarter turn fastener that didn’t require a tool and could be easily released without seeing the fastener. I found these AeroLoc fasteners with bail handles at Pegasus Auto Racing. Unlike winged Dzus fasteners, the bail handle can be flipped flat (and they stay that way even if the fastener is inverted). I can reach under the dash, flip the handle down and twist it a quarter turn without needing to see anything.

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To implement the sliding motion, I considered machining slots on the glide plate and fabricating flanged bushings on the lathe. However, I didn’t want things rattling so I used low-profile sleeve bearing carriages and guide rails at McMaster. The carriages apply a small amount of pretension to the rails which provides smooth motion and prevents rattling.

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Bracket installed on chassis and locked in place with AeroLoc fasteners. I need to trim the tops of the guide rails a bit.

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ECU installed

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ECU slid down and rotated to provide access to the control panel. The guide rails are clearly visible.

Well, that was several orders of magnitude more work than what I expected, but the results were worth it.
 

Randy V

Moderator-Admin
Staff member
Admin
Lifetime Supporter
I’ve loved the active wing concept ever since Jim Hall used it.
I have never implemented one, but have tuned wings on racecars for a number of years.
Admittedly - I know nothing about this particular system.
With regard to wings and weight distribution / balancing - my only concern is how you’ll address front downforce to be commensurate with rear For given speeds.. From my experience with static wing tuning, too much rear downforce can easily overcome the downforce from the splitter / airdam combination in the front of the car. Not being able to turn in at high speed can lead to a religious experience….
 

Neil

Supporter
"Not being able to turn in at high speed can lead to a religious experience…. "

Or soiled underwear!
 

Howard Jones

Supporter
My limited experience has indicated that the fastest corner on the course is the one you try to aero balance. Since I have run my car mostly at COTA that would be the 16-17-18-19 complex and secondarily t-10. Terminal speed on the front straight is about 130mph and on the back straight about 140mph. That's with 440ish HP and a 5-speed G50 geared at 3.44 final and a .85 5th.

So I would assume that any type of active wing would be in full downforce mode in the corners and full low drag on the straights going to full downforce as the car enters the brake zones. Given those parameters, it is pretty clear that the aero balance tune is unaffected by straight line speeds on most road courses generally speaking. Notable exceptions being, Road America, Daytona, and Monza.......

Other than that I can say that spring rates will need to account for the highest download. With my car that is at the ends of the straights whereas with an active wing it would be in the highest speed corners AND in the first few meters of the high-speed brake zones. This would create an interesting tuning question. In my racecar case, it might result in higher spring/ higher download rates because the terminal speeds will be higher due to reduced drag. Using softer springs thinking lower download rates will result from the low drag open wing would create aero ride height compression right as the brakes are applied and that is also the problem with soft rear springs at high speeds.

In any case, the front end will need to be tuned for max downforce to match the rear on a racecar. Whereas the streetcar rear will need to be tuned for open wing low drag. and low download on the front. The positive on a streetcar is reduced drag/fuel consumption and on a racecar higher reduced drag/ higher straight-line speeds

Interestingly on a street car, it might even allow for even slightly softer spring rates, even if driven at high speeds in straight lines. The front aero downforce should be tuned for low drag/ downforce so that the car will aero balance with the rear when trimmed out but closed. More than likely the rear wing setting will be near zero angle of attack anyway on a street car. Therefore when the wing is opened on the highway the resulting downforce reduction will be minimised and not upset aeroballance as much as if the rearwing is set for max usable downforce.

As you can see racecar technology doesn't really translate to a streetcar or if it does then minimally. Especally racing chassis setups that increase corner performance. As always you can't have your cake and eat it too or streetcars can't be racecars and racecars wont work on the street.

Like I said an interesting tuning problem. Active ride height anyone?
 

Attachments

I was looking into a custom active wing for my GTM build. Would be maximum downforce in resting state for cornering, then essentially a DRS low downforce/low drag for the straightaway via a momentary button on the steering wheel button plate.

My 2 cents is that I was going to have a mechanical sensor that told me via a dash indicator (green light) when the wing was back in the resting/maximum downforce position. So I would know if there was a malfunction and I wasn't going to have the expected downforce in the next corner.

I chickened out and went with a fixed spoiler, a proven 200mph design.
 

Scott

Lifetime Supporter
Before I respond to the questions and comments, I’d like to make a couple of points regarding the active wing.

Given all the discussion above regarding weight it’s important to understand that the wing, end plates, wing support pods, actuators (located inside of the pods), ECU and wire harness only weigh about ten pounds. My guess is that it’s comparable to the Superlight carbon fiber wing, end plates and average support implementation. The reason for this is that the wing is much smaller than Superlite’s wing. This is achieved via a high-performance “concave pressure recovery” airfoil. Why don’t other companies use the same approach? Consider a 6K RPM engine vs. a high-performance 18K RPM engine. The former will easily tolerate imbalances that would turn the later into a grenade. Aero works the same way and high-performance airfoils are very sensitive to small manufacturing variances. Apparently, a lot of the company’s wind tunnel efforts were spent understanding what tolerances are acceptable and a significant amount of their intellectual property is the tooling, manufacturing and surface finishing processes to maintain those tolerances.

One of the large advantages of an active wing is that it can be adjusted from the cockpit while driving. I can do this via my steering wheel or a passenger can do it via the wired, handheld remote. With a fixed wing you need to pit the car which takes valuable time, particularly if you have short run sessions. Even if you have someone in the pit to make the adjustment for you, it’s better to compare back-to-back laps rather than disjointed laps. If you add some AoA and it’s better, will you keep adding a little more until you get to your optimal setting? Getting to the best solution in any design or tuning endeavor (e.g., software development, part design, performance tuning, etc.) is about reducing the change, test and analyze cycle time. With an active wing that cycle time is one lap.

Some wings use predrilled holes for adjustment. This works, but the increments are larger than ideal. The active wing provides very granular adjustments. This level of granularity can also be achieved via a link with rod ends, but you either need to count threads or use a level to determine the wing’s angle.

If any of the active features make you nervous, you can lock them out electronically.

my only concern is how you’ll address front downforce to be commensurate with rear for given speeds
Randy, excellent question. Front downforce is fixed and once you understand how the ECU operates it becomes apparent that this works well. Because I’m focused on mechanicals I haven’t dug deep into the ECU, but here’s my understanding.

There are four wing modes, each of which has a configurable angle-of-attack (AoA). The ECU determines the mode based on mode-specific threshold settings, two high-quality accelerometers and the vehicle speed sensor (VSS).
  • Low-Speed Cornering: The wing’s default mode. The AoA is set to balance the car such that the correct level of downforce is applied at the limit of traction in a turn.
  • High-Speed Cornering: The AoA is set to value that’s lower than the Low-Speed Cornering value to keep the car from pushing in high-speed corners. This means that you don’t need to leave downforce on the table in the low-speed corners. It’s my understanding that this mode has a speed threshold which makes sense. It seems to me that transitioning into/out of this mode is more subtle than the other modes to not upset the balance of the car, something that I haven’t had time to investigate.
  • Braking: Increases AoA to provide maximum downforce and high drag to increase stopping power. A deceleration threshold configurable in 1/10 G increments combined with lack of lateral acceleration triggers this mode. .
  • Straightaway: Reduces AoA for low drag during high-speed straights. This mode is activated when a configurable speed is exceeded and there are no lateral forces.
So, the wing isn’t constantly varying downforce during cornering based on speed which would require the front end to apply a commensurate amount of downforce to prevent the car from becoming unbalanced.

However, as can be seen in the video below, the wing is split in the middle and the AoA of the wing on the inside of the turn is increased and as the outside is decreased as the car turns which I assume only occurs in the Slow-Speed Cornering mode.

Why is this done? If the car is balanced at an AoA that’s below the max downforce angle (it usually is), the wing has unused potential downforce. In this case, the ECU adds AoA/downforce to the inside wheel and reduces it a commensurate amount from the outside wheel. This maintains the front-to-back balance while transferring weight to the inside wheel.

Howard, in response to some of your points……………..

I would assume that any type of active wing would be in full downforce mode in the corners and full low drag on the straights going to full downforce as the car enters the brake zones... Notable exceptions being, Road America, Daytona, and Monza.
The ECU stores tunes for up to ten tracks to account for these types of exceptions.

My limited experience has indicated that the fastest corner on the course is the one you try to aero balance
That sounds right because aero push in a high-speed corner is somewhere between uncomfortable and dangerous. With a fixed wing, if the car is pushing too much on the fastest corner, you’ll decrease AoA which results in lost (i.e., usable) downforce in the slow corners and increased lap times. This would be most noticeable on a track with a long high-speed sweeper and lots of slower turns.

With an active wing, you would configure the High-Speed Cornering mode (AoA and speed threshold) to optimize the downforce for the corner(s) that you deemed high speed. You would then configure a larger AoA for the Slow-Speed Cornering mode. This improves lap times vs. a static wing.
While I’m not advocating it, it would be easy to integrate GPS and have an AoA setting for each turn.
 
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Scott

Lifetime Supporter
CONTINUED FROM ABOVE

the front end will need to be tuned for max downforce to match the rear on a racecar. Whereas the streetcar rear will need to be tuned for open wing low drag. and low download on the front.
I agree that you maximize downforce on the front end, but I disagree that you match the front to the rear. Instead, you match the rear to the front.
Like most non-formula and prototype cars, the SL-C’s rear wing will easily overpower the front end. All of the race SL-Cs that I’ve seen have gone to great length to increase downforce on the front end; race splitter with end plates (according to Superlite’s site it can generate up to 900 lbs. of downforce at 120 MPH), large canards, large front fender vents (i.e., much larger than the ones sold on Superlite’s website), additional cutouts in the wheel arch, completely redesign and duct the radiator outlet, etc. All of that effort on the front end and I’ve noticed very little change to the tail. The national-championship car did have a cool swan-neck wing and a few cars have added rear wheel well vents, but I think that they did so out of good aero hygiene as opposed to keeping pace with frontend downforce. So, do everything possible to maximize downforce on the front end and then simply put the wing in clean air and set the AoA to achieve the desired balance. This is the same for both a fixed and an active wing.

IMO, the approach for a street SL-C is the same as a race SL-C within the limitations of what’s practical and legal on the street. The race splitter is too low, but pnut used the track splitter on his street SL-C. That said, I know that he replaced it at least once.

My car has the street splitter (I live in Boston), a CFD-designed radiator duct and outlet, fender vents and small canards. During the design process I asked my aero guy about adding a LaFerrari-style foil in the radiator outlet and he told me that it would reduce drag and the cost of downforce. Since this is on the front end, the foil was a no-go.

I am designing a track-day package in which a splitter with endplates will bolt under the street splitter (I might need to redesign it to make that work). In addition, large canards will utilize the mounting holes for the smaller canards. The smaller canards add negligible downforce and look cool but, their real purpose is to hide the mounting holes for the track-day canards. The wing’s ECU will have a tune for the street and the track-day package.

spring rates will need to account for the highest download. With my car that is at the ends of the straights whereas with an active wing it would be in the highest speed corners AND in the first few meters of the high-speed brake zones. This would create an interesting tuning question. In my racecar case, it might result in higher spring/ higher download rates because the terminal speeds will be higher due to reduced drag. Using softer springs thinking lower download rates will result from the low drag open wing would create aero ride height compression right as the brakes are applied and that is also the problem with soft rear springs at high speeds.
.
I don’t have any practical experience tuning the wing or springs yet, but IMO, an active wing doesn’t complicate and may potentially simplify spring choice because it mitigates the peak aero-driven spring-rate requirements.
  • Terminal speed: As you point out, terminal speed will increase because AoA is decreased which results in less drag. This will also reduce downforce which will reduce the required rear spring rate for this scenario.
  • High-Speed Corner: The High-Speed Cornering mode decreases AoA and downforce and therefore the required rear spring rate.
  • Braking: An active wing will increase AoA to maximum downforce which reduces rear-to-front weight transfer and therefore front spring compression. In addition to keeping more of the downforce on the rear, this will increase drag and thereby reduce the amount of energy that the brakes need to dissipate which will further mitigate front spring compression. Given that there is more downforce on the larger rear tires, the driver will want to increase the rear brake bias.
It’s possible that the wing doesn’t pop up fast enough during hard braking to mitigate the initial compression of the front springs. I won’t know for sure until I get the car on the track. Fortunately, I have a bunch of sensors and MoTeC logging so I’ll have access to track position, deceleration, front & rear brake pressure, front & rear spring compression, AoA, speed, etc.
You missed the biggest benefit for a street car… it gives you something cool to talk about at a cars and coffee event! But my car is a street car that will do track days and as such it will gain the same benefits, with less effect, as a race car.
Benefits to a race car go way beyond increased straightaway speed:
  • Improved braking; probably the most important thing that any car needs to do well.
  • Improved low-speed cornering; no need to reduce slow-corner AoA to prevent the car from pushing on the high-speed corners and the split wing will apply more downforce to the inside wheel.
  • Ability to tune AoA for each of the four modes while driving the car. No wasted run sessions, rapid iteration leads to better tunes.
  • Fine-grained AoA adjustment
  • Lower peak aero-related spring requirements
 
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Scott

Lifetime Supporter
CONTINUED FROM ABOVE

As you can see racecar technology doesn't really translate to a streetcar or if it does then minimally.[\QUOTE]
That’s like saying aerospace/military tech doesn’t’ transfer to race cars… Howard, you do have AN fittings all over your engine compartment, right? The Nissian GT-R in the video above claimed a 1.29 second apples-to-apples lap improvement at Buttonwillow. Specifically, it compares the wing locked at the High-Speed Cornering AoA (the aero balance point that you mentioned above) vs. allowing the ECU to vary the AoA while maintaining the same High-Speed Cornering setting. No doubt that required some tuning, but it’s an excellent translation of race car tech to street car for a bolt on aero mod which is tuned via a control panel.

I would put race tech applied to a street car into the following categories:
  • Enhanced Street: Works as well on the street as it does on the track.
  • Enhanced Track: Doesn’t hurt anything to have on the street, but
  • No-Go: Don’t do for a car that driven on the street because it’s Illegal, unsafe or unusable.
Enhanced Street Tech in My Car
I feel a lot safer with a FIA-certified fuel cell, seat belts, solid-state battery isolator and fire suppression. The MoTeC ECU, PDMs, DHBs, and display make wiring and programming a street car easier and more reliable. The steering wheel quick release makes it easy to get into and out of my car. Wiggins fittings, dry-break disconnects and air jacks make it easier to service the car. Extensive logging makes it easier to tune or resolve issues. The Electric Power Assisted Steering (EPAS) system I have is a racing product, but I’d venture it’s more useful for a car with big sticky tires navigating a parking lot than a race car doing 20-30 minute run sessions (an endurance race driver would obviously see the largest benefit).

Enhanced Track Tech in My Car
Penske shocks, cockpit-adjustable sway bars, active wing and dry sump.

No-Go Track Tech
  • Racing Slicks: aren’t DOT certified and illegal, but can be swapped in minutes
  • FIA-Certified Roll Cage: Dangerous unless you’re going to wear a helmet everywhere you go
  • Low Ride Height or Race Splitter: Completely impractical for anything driven on the street.
  • Superlite Race Suspension Option: Superlite indicates that it’s race only
  • Polycarbonate Windshield: Not DOT certified
Especally racing chassis setups that increase corner performance.
All SL-Cs are low and don’t have any bushings which makes them lean towards a racing chassis setup. While I don’t have a breakdown of where the aforementioned lap time gains were made, I know that a fair amount of it was obtained in the slow-speed corners. Are you running Superlite’s Race Suspension upgrade? What other chassis setups are you running other than lower ride height and higher spring rates? I’m curious, because I like to push the envelope and I might have missed some Enhanced-Track approaches or tech.

As always you can't have your cake and eat it too or streetcars can't be racecars and racecars wont work on the street.
A street-legal SL-C will always be compromised on the track vs. an equally well-built race SL-C. That said, the cake idiom demands a zero-sum scenario whereas a positive sum outcome is possible. In other words, technology can magically create more cake, so you need to be less concerned about eating some of it.

[QUOTE]Active ride height anyone?
Bob lives 15 minutes away and I visited his shop yesterday to check out his electric-powered SL-C. It has active ride height and spring rate… I can’t wait to see it up and running. The Boys from Boston are doing some wild stuff.

I was going to have a mechanical sensor that told me via a dash indicator (green light) when the wing was back in the resting/maximum downforce position. So I would know if there was a malfunction and I wasn't going to have the expected downforce in the next corner.
Dave, that’s a good idea. Ideally the ECU would put a message on the CAN bus, but I don’t think that it does. However, it does provide an analog output to log the AoA which I was planning to map to show in degrees on the MoTeC display. I’ll think about how to implement a warning light.

The positive on a streetcar is reduced drag/fuel consumption and on a racecar higher reduced drag/ higher straight-line speeds
You missed the biggest benefit for a street car… it gives you something cool to talk about at a cars and coffee event! But my car is a street car that will do track days and as such it will gain the same benefits, with less effect, as a race car.
Benefits to a race car go way beyond increased straightaway speed:
  • Improved braking; probably the most important thing that any car needs to do well.
  • Improved low-speed cornering; no need to reduce slow-corner AoA to prevent the car from pushing on the high-speed corners and the split wing will apply more downforce to the inside wheel.
  • Ability to tune AoA for each of the four modes while driving the car. No wasted run sessions, rapid iteration leads to better tunes.
  • Fine-grained AoA adjustment
  • Lower peak aero-related spring requirements
As you can see racecar technology doesn't really translate to a streetcar or if it does then minimally.[\QUOTE]
That’s like saying aerospace/military tech doesn’t’ transfer to race cars… Howard, you do have AN fittings all over your engine compartment, right? The Nissian GT-R in the video above claimed a 1.29 second apples-to-apples lap improvement at Buttonwillow. Specifically, it compares the wing locked at the High-Speed Cornering AoA (the aero balance point that you mentioned above) vs. allowing the ECU to vary the AoA while maintaining the same High-Speed Cornering setting. No doubt that required some tuning, but it’s an excellent translation of race car tech to street car for a bolt on aero mod which is tuned via a control panel.

I would put race tech applied to a street car into the following categories:
  • Enhanced Street: Works as well on the street as it does on the track.
  • Enhanced Track: Doesn’t hurt anything to have on the street, but
  • No-Go: Don’t do for a car that driven on the street because it’s Illegal, unsafe or unusable.
Enhanced Street Tech in My Car
I feel a lot safer with a FIA-certified fuel cell, seat belts, solid-state battery isolator and fire suppression. The MoTeC ECU, PDMs, DHBs, and display make wiring and programming a street car easier and more reliable. The steering wheel quick release makes it easy to get into and out of my car. Wiggins fittings, dry-break disconnects and air jacks make it easier to service the car. Extensive logging makes it easier to tune or resolve issues. The Electric Power Assisted Steering (EPAS) system I have is a racing product, but I’d venture it’s more useful for a car with big sticky tires navigating a parking lot than a race car doing 20-30 minute run sessions (an endurance race driver would obviously see the largest benefit).

Enhanced Track Tech in My Car
Penske shocks, cockpit-adjustable sway bars, active wing and dry sump.

No-Go Track Tech
  • Racing Slicks: aren’t DOT certified and illegal, but can be swapped in minutes
  • FIA-Certified Roll Cage: Dangerous unless you’re going to wear a helmet everywhere you go
  • Low Ride Height or Race Splitter: Completely impractical for anything driven on the street.
  • Superlite Race Suspension Option: Superlite indicates that it’s race only
  • Polycarbonate Windshield: Not DOT certified
Especally racing chassis setups that increase corner performance.
All SL-Cs are low and don’t have any bushings which makes them lean towards a racing chassis setup. While I don’t have a breakdown of where the aforementioned lap time gains were made, I know that a fair amount of it was obtained in the slow-speed corners. Are you running Superlite’s Race Suspension upgrade? What other chassis setups are you running other than lower ride height and higher spring rates? I’m curious, because I like to push the envelope and I might have missed some Enhanced-Track approaches or tech.

As always you can't have your cake and eat it too or streetcars can't be racecars and racecars wont work on the street.
A street-legal SL-C will always be compromised on the track vs. an equally well-built race SL-C. That said, the cake idiom demands a zero-sum scenario whereas a positive sum outcome is possible. In other words, technology can magically create more cake, so you need to be less concerned about eating some of it.

Active ride height anyone?
Bob lives 15 minutes away and I visited his shop yesterday to check out his electric-powered SL-C. It has active ride height and spring rate… I can’t wait to see it up and running. The Boys from Boston are doing some wild stuff.

I was going to have a mechanical sensor that told me via a dash indicator (green light) when the wing was back in the resting/maximum downforce position. So I would know if there was a malfunction and I wasn't going to have the expected downforce in the next corner.
Dave, that’s a good idea. Ideally the ECU would put a message on the CAN bus, but I don’t think that it does. However, it does provide an analog output to log the AoA which I was planning to map to show in degrees on the MoTeC display. I’ll think about how to implement a warning light.
 
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Neil

Supporter
John Horsman told me that JWAE researched many airfoil profiles in the MIRA wind tunnel and in the end found that the NACA 4412 was the best overall.
 
something cool to talk about at a cars and coffee event!
I agree, the cool factor is 10/10.

large canards will utilize the mounting holes for the smaller canards
Don't discount the effect that front canards have on rear wing efficiency! Your front canard size and shape WILL LIKELY affect your rear wing downforce (basically through affecting how much efficient airflow is reaching the rear wing surface). I'd encourage you to pick up this book if you don't already have it, interesting section on wind tunnel testing with canards, and the surprising effect they had on changing airflow to the rear wing:

 
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Howard Jones

Supporter
"I agree that you maximize downforce on the front end, but I disagree that you match the front to the rear. Instead, you match the rear to the front."

That's what I meant to say thanks for the correction.

Once you have maximized the front grip on the car with aero downforce in the fastest corner then add rear downforce to balance the car to nurtural. That would be the basic setup. From there I personally like to take a step back and allow for just a bit of understeer. Personally, I like a car that if it loses grip I want it to understeer first. Then just a bit of reduction in speed will bring the car back, If the rear goes first at 120MPH then, especially in a mid-engined car, it can get beyond saveable very quickly. This is my personal preference.

I of course don't advocate returning to the pre-war automobile construction methods and materials. And of course, there is a balance between race car tech and high-performance street cars. However, even the grip levels my SLC generates would be very dangerous if routinely driven at those levels on the street. The corner at the end of my block can simply be rolled through at 45MPH in my SLC when I drive it around the block to load in on my trailer. My M4 at 45 through that corner is..........well not prudent for adults. 50 might even be doable in the SLC and 50 in the M4 will put me into my neighbor's front room.

How would I build a max-performance street/race car? Make it light as possible. Leave off all the wings and their drag, suck all the air out from under the car with a big ass fan and skirts (in sections to balance front to rear and maybe even left to right and then control ride height to suit with actuators and electronic controls. Then make the motor as big as room allows and add boost to make as much power as desired. The only real difference between street and track would be the tires.

Very little of that potential performance would ever be useable on the street, well until you hit something at least it would be a hoot!

See I said it badly.

So I guess what I am saying, badly, is that at some level of performance the resulting car is so fast that a majority of the performance envelope can't even be used on the street regardless of legalities. That doesn't apply to a one-off home-built personal hobby showcase car. I love those creative out of the box builds. But that is just what they are in the end.
 
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Randy V

Moderator-Admin
Staff member
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Lifetime Supporter
Thanks for allowing the drift in your thread, Scott…
One last thing to be mindful of…. At least within the SCCA, they had outlawed active suspension controls that required driver input. This was done to actually reduce the load carried by the driver - who needs to remain focused on his environment being his/her car, the other vehicles on the track, safety and corner workers and flaggers. Since your car is track-day (non competitive), you’re at a somewhat lighter loading in terms of the driver being very busy. Still, it’s something to be mindful of. Possibly use radios to communicate changes made / cause-affect to a helper… Pretty difficult to remember or even jot down events from corner to corner when on the track for 10 laps or more…
Noting front aero changes - We would adjust the deflection angle of the splitter infrequently, but we did adjust those turn-buckles as necessary..
Cars & Coffee - yes, cool factor will be off the charts….
 

Scott

Lifetime Supporter
Don't discount the effect that front canards have on rear wing efficiency!
Dave, thanks for the book recommendation. I've learned that aero is fickle and often opposite to what I thought. I have retained an xF1 aerodynamist to do the CFD modeling on the radiator outlet. I'll use him for the splitter/canards and diffuser. It's not critical to my use case, but I've enjoyed learning from him.

One last thing to be mindful of…. At least within the SCCA, they had outlawed active suspension controls that required driver input. This was done to actually reduce the load carried by the driver - who needs to remain focused on his environment being his/her car, the other vehicles on the track, safety and corner workers and flaggers.
Randy, that's a sensible regulation. Drivers, especially an amateur like me, doesn't need any distractions. I wouldn't consider using any of the controls on a hot lap. Once things are configured, the ECUs automate everything. I think F1 drivers need to press a DRS button whereas the active wing does it automatically. As mentioned above, the wing's ECU has a wired, handheld remote which allows a passenger to tune the wing. The passenger will also be able to adjust the sway bar settings through a MoTeC rotary controller and console display.

some level of performance the resulting car is so fast that a majority of the performance envelope can't even be used on the street regardless of legalities.
Howard, I agree, but that applies to family sedans these days as well. My M5 is driven on a daily basis rain, snow or shine to go grocery shopping, pick up plants at the garden center and haul construction materials from Home Depot. It has 627 HP, does 0-60 in 2.6 seconds and is restricted to 190 MPH. You can't use much of that on the street and even though the competition sport is allegedly 230 pounds lighter than the standard model and has large carbon ceramic disks, I don't think I would enjoy it on the track. So, given that the car will never be tracked, that's a lot of unutilized performance.

My SL-C will be used on the track. A large part of the tech is keep things sane on the street and let it do what it can do on the track.
 

Scott

Lifetime Supporter
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The rear is starting to look pretty wicked. Everything after the catalytic converts is 3.5” titanium. The muffler assemblies are fully welded and the rest is tacked. The first step was to connect the catalytic converters to the mufflers via titanium pie cuts. A titanium flex bellow was used to absorb linear growth caused by thermal expansion and to help decouple vibrations.

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The flex bellow is visible on the upper tube

Once the upper tube was completed, the muffler outlets were connected to the X-pipe. The challenge was to determine the exact location of the X-pipe and to keep it there during fabrication. The solution was a piece of 1/4” plywood. The leading edge was clamped to the underside of the billet chassis piece that crosses under the transaxle and two vertical supports were fabricated from right-angle aluminum to affix the trailing edge to bosses on the transaxle. Slots were milled in the supports to enable granular adjustments. This provided a stable platform to locate the X-pipe and enabled the X-pipe to be precisely tilted (the tips will be tilted upwards). This worked much better than a floor stand because everything moved when the car was raised/lowered via the lift and there was no chance of knocking over the stand or dumping everything on the floor.

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Plywood platform to support the X-pipe while fabricating the connecting tubes. Plumb bobs, squares and digital angle finders were used to get everything symmetrical.

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Merge collector (left), bell housing (top middle) and four primaries joined by a flange crossing under the oil pan (bottom middle)

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Merge collector (left) flowing into a transition cone (middle) leading to a catalytic converter after which pie cuts flow into the bellow

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The X-pipe is connected to the muffler outlets. A mock exhaust tip on the left side.

The final step was to connect the mufflers to the X-pipe which required a fair number of pie cuts of different centerline radiuses (CLRs) and slip fittings to enable the muffler assemblies to be removed/installed.

The cat-back system required 82 pie cuts, 6 mandrel bends and 4 straight sections. When V-bands, bellows, slip fittings and transition cones are taken into account there are 113 welds with a 3.5” diameter. That results in a whopping 1,242” or 103.5 feet of welding!

The next steps are to integrate the cutouts, finish weld everything, add hangers with vibration isolators to the X-pipe and fabricate a heat shield between the X-pipe and transaxle.
 

Scott

Lifetime Supporter
Power Brakes: Part 1

Several owners that I know with finished SL-Cs lament not having power brakes. I asked Allan, who’s built 28 SL-Cs and 5 GT-Rs, and he indicated that brake pedal effort is the number one complaint by far. I don’t have power brakes in my cobra, but the SL-C is heavier, more powerful and with active aero, it’s capable of quicker deceleration. I’m also not a spring chicken and my right knee bothers me more than it used to.

Power brakes typically use engine vacuum to operate on a diaphragm. Whomever figured this out back in 1927 was brilliant because vacuum is a side effect of a combustion engine so it’s a “free” power source. Vaccuum-based power brakes have been implemented by at least one SL-C builder (Joel), but the diaphragm takes up a lot of room and it won’t fit inside of the footbox. If I were to place it outside of the footbox, as Joel did, it would collide with my radiator outlet duct.

Electric vehicles (EVs) have power brakes, but no combustion engine and therefore no engine-generated vacuum. So, what do they do? Apparently, early model S Tesla’s used an electric pump to create a vacuum which was a poor solution because the pump ran constantly which was a drain on the battery. Apparently, several years prior the Tesla hack, the Toyota Forerunner employed an electromechanical brake booster, so Elon wasn’t the innovator in this area.


Fortunately, Bosch manufacturers an electro-mechanical brake booster which is used by many OEM EVs. In addition to removing the dependance of vacuum acting on a large diaphragm, the pedal feel can be adjusted through the configuration of braking characteristic curves. This allows the iBooster to be used across models or support different driving modes within a model (see diagram below). While having different braking modes is interesting, just being able to finetune one mode is extremely useful. The only way to change brake pressure on a SL-C is to swap the front and/or rear master cylinders which is both messy and time consuming. The SL-C’s tight footbox makes this a bit of a nightmare and master cylinder increments are at least 1/16,” so changes aren’t granular. While I haven’t found anyone that’s hacked the CAN bus to change the default brake characteristic curves, other builders have figured out the CAN bus messages to brake by wire and determine the master cylinder’s position.

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There are two primary versions of the iBooster; GEN1 and GEN2. I bought a used version of each on eBay to compare. I liked GEN1 vs. GEN2 for the following reasons:
  • It was a bit more compact in the intended orientation.
  • It has a nice cast aluminum body with several machined surfaces whereas the GEN2 is all stamped steel and seems shoddy by comparison. I’m sure this is my perception rather than an issue with the design. GEN1 feels like the engineers were focused on a high-quality innovative solution and GEN2 was engineered to hit a price point.
  • There is more information on how to hack the CAN bus on GEN1 than GEN2 which makes sense because it’s been out longer. While CAN bus integration isn’t required, it’s something that I’d like to do in the future.
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GEN1 (left) and GEN2 (right); the reservoirs vary widely amongst the OEMs

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GEN1 iBooster donor cars include;
  • Audi A3 e-Tron
  • Chevrolet Bolt
  • Chevrolet Malibu
  • Honda CR-V (MY 2018, 2019)
  • Jaguar i-Pace (MY 2019, 2020)
  • Porsche Panamera (MY 2017+)
  • Tesla Model S (MY 2015+ with autopilot)
  • Tesla Model X (MY 2015+)
  • Volkswagen Passat hybrid
  • Volkswagen e-Golf and Volkswagen e-UP
I didn’t want to pay the Tesla tax, so I purchased a GEN1 from a Honda. The mounting flange, master cylinder and reservoir are easily removed and appear to vary amongst different OEMs. The master cylinder has pressure ports cast on both sides with only one side being tapped. I assume this is accommodate fitment for different cars including left vs right-hand-drive within a model. In addition, I read somewhere that the Honda version was machined differently to accommodate a different master cylinder.

Enterprising builders have utilized the iBooster in homebuilt EVs, restomods, hotrods, etc. They figured out that iBooster can be operated in a fail-safe mode without any CAN bus connections. The ECU only needs four wires (two constant +12v power, ground and ignition) and four wires that connect to the position senser to the ECU. There are multiple wiring kits available (e.g., EVcreate, Tulay’s Wire Works, SGH Innovations) or you could cut an OEM harness.

Given that all of this has already been figured out, installing an iBooster is straightforward. However, the SL-C presents several challenges:
  • The footbox and nose are tight.
  • The pedal box sits on the floor which means that iBooster needs to be properly tilted to align it with the brake pedal.
  • The pedal box uses a balance bar to connect to the front and rear master cylinders and the iBooster has only one master cylinder.
I considered mounting the iBooster in the nose, but the extended footbox projects too far for it to be mounted longitudinally (a standard footbox would probably result in a simple install). I also considered using a bellcrank and mounting it transversally, but that seemed like a hack. In the end, I designed a bracket that locates the iBooster as low and close to the pedal box as possible inside the footbox. I read somewhere that pedal force should be as concentric as possible with the input rod and that it must not diverge more than 3 degrees. My bracket tilts the iBooster’s nose down 15.75 degrees from the vertical to perfectly align, as far as I can discern, the center of the balance bar with the iBooster’s input rod. Any changes to the iBooster’s location would change the angle.

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Laser cut 1/8” 4130. There are 36 tabs and slots. All of the corners are relived and there is only five thousands clearance between the slot edges and the tab edges (i.e., the tabs are 0.125” thick and the slot is 0.135” wide). The only edge that I filed was the bottom of the angled plate. This was by design to keep the weld gap small. If I had CNC machined the plate that wouldn’t have been required, but laser cutting is much cheaper. I assembled the four sides, wiggled them into the bottom plate, added some clamps and welded. As intended, the part pretty much self jigged which ensured that iBooster is at the correct angle.

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Front view, bracket welded and primed with a rattle can

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Rear view. Note the two nuts that are welded to the bottom rear edge. The left vertical plate is larger than the right to mount the GM accelerator which most SL-Cs utilize.

Many builders add a plate under the pedal box to prevent the floor from deflecting. If you look carefully at the pedal box, you realize that it was carefully optimized to reduce weight. There are only five relatively small contact patches with the aluminum floor (see diagram below). The four mounting holes are centered around the brake pedal. Note that the rear bosses are slightly larger than the front bosses and that there is only a small rectangular contact patch in the upper left to counteract force applied by the clutch pedal. In addition, the mounting bolts are only 2.225” apart in the direction of the brake pedal force (i.e., front to back). For these reasons, the bracket extends under the pedal box and provides two additional bolts in the rear of the bracket. When combined with the large truss supporting the iBooster, the bracket significantly stiffens the floor while only raising the pedal box by 1/8.”

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Mounting holes and floor contact patches

The next step was to modify the pedal box to convert the dual cylinder configuration to a single cylinder. I used the mill to remove the top of the front and rear master cylinder brackets. This in no way reduces the strength or integrity of the pedal box base. The cast aluminum is high-quality, there wasn’t even a hint of an air pocket and it felt like I was machining billet.

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The red cap points to the area where the upper portion of the brake master cylinder supports and studs were machined off. Note that the clutch mount in the background is intact. The lower studs are bolted to the bracket.

The bracket mounts to the front and rear brake cylinder lower studs. This allows the entire assembly to be installed and removed as a single assembly. The bracket clears the ECU and clutch master cylinder by about 1/16.”

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The next steps are to:
  • Replace the balance bar with a custom adapter. I’ll know at that point if I left enough space between the pedal box and iBooster to achieve full master cylinder travel.
  • Temporarily wire the ECU and the position sensor.
  • Temporarily plumb the master cylinder to pressure gauages.
  • Determine if the master cylinder has enough volume to support the Brembo GT calipers.
  • Adapt the reservoir inlets to AN fittings.
If anyone has information on iBooster master cylinder volumes or how to set the brake characteristic curves, I’d love to hear about it.
 

Joel K

Supporter
Very nice solution Scott. I saw pics of theses early on in my build design but didn’t have any understanding on how to control it so went a more conventional direction.
Appreciate all the great info you are sharing with us.
 
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