SL-C ABS Feasibility Question/Discussion

[James]

So as I am researching an SL-C build, I wanted to look at the feasibility of adding ABS and wanted some input from more experienced builders, specifically SL-C builders. What this discussion is NOT is a philosophical one about whether ABS "should" be added to the SLC, that has already been discussed here before. Since my build would be a street build, I am interested in ABS for use on the street in a surprise "panic" stop, which I am sure all of us have encountered at least once or twice (or more) in our driving careers. So now on to my idea......

After researching ABS retrofits for the SL-C a bit, I noticed two common themes: 1) The system you use should be from a car with similar weight and weight distribution characteristics and, 2) You should use a complete system from a donor car, and not try to piece something together. So I took a look at the 2nd generation Toyota MR2 (1989-1999). The pros of this ABS donor are:

1. The MR2 is a mid engine car with a curb weight of around 2500+ pounds and similar weight distribution (which is comparable to a street build with all of the "bells and whistles")

2. The system is relatively simple. It does not interface nor is it dependent on other systems such as traction control or even the main engine ECU. The system basically consists of a pump, ECU, wheel speed sensors and reluctor rings.

Mechanically, the ABS pump unit has two separate brake lines going to the front, one for each wheel. It has one brake line that supplies both rear brakes. If you take a look at the attached wire diagram for this system, you will notice how relatively simple the system would be to incorporate electrically. The other thing I like about this system after looking at the repair manual for it is that diagnostic codes for the system can be read using the ABS light, and can be cleared using a sequence of pressing the brake pedal. This makes testing of the system during and after the retrofit possible without the need for an external reader.

Gen 2 MR2 ABS Wire Diagram

The biggest challenge that I see is properly attaching the wheel speed sensors and the reluctor rings to each of the hubs such that a clean wheel speed signal can be produced and the rings and sensors will stay put. After that it is a matter of mounting the two units, plumbing the hard brake lines, and wiring everything up.

Now as for the cost, the pump/actuator can be had for $60-$100; ECU for $50-80, the reluctor rings for $30 each, and the wheel speed sensors for $50-$150. The reluctor rings could only be found new. Wheel speed sensors were harder to find used, but doable. The ECU and actuator/pump were relatively easy to find used. Even when taking into account the extras such as fittings, connectors, wiring, etc, this modification is not that expensive, especially when compared to the only aftermarket/race ABS system at over $8k.

As a side note, another potential donor is a 3rd gen MR2 Spyder (2000-2007). It is also a mid-engine car but has a curb weight of 2100+ pounds. The parts would be a bit more expensive and the front wheel speed sensors are a different design than the gen 2 MR2 (the rear sensors are the same). One advantage of this system is that the pump and ECU are a combined unit, vice separate. However, I could not find a shop manual with wiring schematic for that model so I could not verify how adaptable it might be compared to the gen 2.

So what do you think?

 

[Fran Hall RCR]

This is great info for the more experienced builder....nice work..

I am sure that if you were able to piece together and prove with experience and real world testing thats its truly fool proof and fully capable , you would have a nice aftermarket option to retail to many guys...

 

[David]

I quite seriously looked into ABS and came to the conclusion that unless you can find an identical car (same wheelbase, track, brakes and wheels) its not going to work properly.

You need to specify a number of parameters and I couldn't find anything that would allow you to do that outside of either the full on motorsport kit or the very limited Audi (Bosch) which would allow you to specify different models.

If you want ABS for your SL-C it's easy but it'll cost. Google Bosch ABS M4

 

[Fran Hall RCR]

I have had very long and serious talks with Bosch and their system seems reasonably simple until you realise how tunable the software really is....part of the integration service required by Bosch is the hiring of Matt , one of their highly qualified techs for at least one, possibly two days for track set up ...this runs around $1000 per day plus expenses . This is the only way they will support their systems for any future issues . To be honest it seems like a great solution and I have looked at adding it to the race car , so we can then offer it to our road customers....but the full system with harness, integration and support is close to $20k...
reardless of what the Bosch website may say , these are the numbers on my last quote..

 

[James]

Wow, $20k? In any event, i know this is not a perfect solution since the MR2 is not a perfect match. But builders (of other project cars) have apparently had some success retrofitting ABS systems that were "close" to the specs of the project car.

I think one of the keys is to try to keep things as simple as possible. This is why I like this system as a potential viable option. But of course, it still needs more "development", which is why I think it is worthy of some discussion since I know there may be some other SL-C implementation issues that I am not aware of, like the design of each wheel hub such that the wheel speed sensors can be installed, just to name one potential installation issue that I can think of.

I personally would love to try when it comes to my build, even with the potential unknowns. I just like to have as much data/research as possible.

 

[David]

As an example BMW had different programs for the 'base' e46 M3 and the competition pack M3 so it could run slicks, Ford have different ABS units for their track versions. Audi have different setups for the S4 to the A4 etc etc. That's in the *same* car! An SL-C is totally different to almost any road car in terms of performance that you're likely to *reduce* performance not increase it as the system won't 'believe' you can slow down that quickly and will engage ABS far too early. More rather than less likely to lead to an accident.

Older 'dumber' ABS computers may work as they are far simpler but they're also far less effective and very heavy.

Now theoretically the M4 is 'just' a consumer ABS computer with the ability to reprogram it, if you can manage to reprogram a standard ABS5.7 for example you've got 95% of the setup (the M4 also has the ability to swap maps which I don't believe any 'consumer' ABS computers do). I'm just not sure if you can do that without the special Bosch/dealer/manufacturer tools (which even if available cost a fortune).

On my build I'm leaving the option for an ABS computer in and I've got two to play with but I'm not putting one in. I do however plan to try and re-program one in the future but restricting it to a test rig until I'm comfortable it's safe to go in a car. Of course first I need to build the test rig, then I need to work out how to re-program it and frankly I don't have time while I'm building the car!

As Fran says, the basic cost for the M4 is very high and then you need to set it up, and if you don't fancy hiring Matt just the lead to program it is ~700EUR!

 

[David Nunn]

Very interesting discussion! As far as tuning or calibrating an ABS system is concerned, what are the actual variables involved?

If I'm not mistaken, ABS works by detecting a locked wheel and releasing pressure at that particular brake caliper. When versions of the same car have different ABS computers, I would think that would be due to different brake calipers and/or wheels and tires. Is the weight of the vehicle and/or the weight distribution really a significant variable? After all, if you put a 500lb load in the back of your pick-up truck, you've changed the weight and weight distribution significantly but the ABS still works fine. The same logic applies to commercial aircraft, where the weight and weight distribution of a particular aircraft can vary significantly due to cargo and fuel.

 

[Alex]

If it were me i'd just wait until you physically have the car there before deciding on what you want to do .... biggest mistake (in terms of lost time and money) of all my builds has been pre-planning and not realizing how different it is when the car is actually there. E.g., in my cobra I wanted ABS so it would be like a modern car, spent my time researching and plotting it out, then when I got my kit I said 'f-it, if the car is able to stop ... eventually .... under its own power, good enough'

 

[Fran Hall RCR]

Pantera...slip angle, yaw, pitch, roll...many many variables

The Bosch system is so good that you can actually make the car under or oversteer by the release timing of the ABS system...its actually very very cool....

But I can tell you we have NOT got it on the race car and it has not prevented us from being able to stop pretty well....

 

[Ruth]

If it were me i'd just wait until you physically have the car there before deciding on what you want to do .... biggest mistake (in terms of lost time and money) of all my builds has been pre-planning and not realizing how different it is when the car is actually there. E.g., in my cobra I wanted ABS so it would be like a modern car, spent my time researching and plotting it out, then when I got my kit I said 'f-it, if the car is able to stop ... eventually .... under its own power, good enough'

Echo what Alex said for 3 reasons. 1) Some better product/solution comes along 2) What you thought would work, obviously won't once you look at some specific part of the car 3) You change your mind.

All 3 have happened at least twice for us and half the time its cost us money.

 

[James]

Good discussion guys....

Dave, good points and I agree that the the "older, dumber" ABS units are more viable candidates. This unit in particular falls into that category (I thought it ironic that you used a BMW E46 M3 in your example, I actually have one and have tracked it with r-compound tires and no ABS issues, but I digress).

The only input to this system, according to the manual, is wheel speed and then the brakes to turn the system "on" in a panic stop situation. No slip angle, yaw, pitch or roll sensors, which is common on the ABS systems on more modern cars. I don't know how much added weight the system would be, but for a street car build, I would think it would be negligible.

As far as performance, well there are definitely no guarantees since the MR2 and SL-C are still different cars, even if they are very close in the weight and weight distribution. My thoughts are that the MR2 ABS system, like many other OEM ABS systems on the road today, is not too precisely tuned to account for the varied driving conditions and loads that a street car will encounter, compared to a very precisely tuned track/race car. That built in system "slop" if you will, may help bridge the gap between the SL-C and MR2, thus increasing the likelyhood that it would work. I would suspect that since the brakes on the SL-C are better than the MR2 brakes, they could possibly cause the wheels to lock up before the ABS system could respond as opposed to the ABS not engaging at all.

Regarding not pulling the trigger on such a mod early in the build process, I agree. This is very much still a research project as I personally don't have enough data yet.

But back to the mechanical feasibility for a bit..... Any idea on how to fix the reluctor ring and wheel speed sensor to the hubs? I don't have any close ups of the hubs on the SL-C.

 

[David]

WRT what options variables are modified, download RaceABS from Bosch Motorsport - Software and you can have a look for yourself!

 

[Troy Truglio]

If you do get the mr2 one get the one from 93-95 cars, that system works better than the 91-92 system.

 

[Ben Whittle]

Not that I want to put a downer on the idea but, Toyota didn't manage to get it right in my Aygo first off and that's a production car.

I'm pretty certain whilst it was in for service they toned the ABS down somewhat, I actually got into a couple of situations where the ABS made the situation worse than if the wheels had just locked up (they wouldn't have done or they would have only done for a fraction of a second but the ABS got overzealous).

After a couple of experiments I worked out in which conditions the ABS just wasn't good, I'm now back in a car that doesn't have ABS ('99 Miata) and I thought I'd miss it but so far I don't, very very rarely do I lock up and even then releasing the pressure marginally is enough to rectify it.

ABS in the snow? Forget it, completely and utterly useless and in fact worse than just locking up.

 

[Doc Kaler]

"ABS in the snow? Forget it, completely and utterly useless and in fact worse than just locking up"......

Not a problem now that you live in San Antonio :shy:

 

[Troy Truglio]

I am thinking the reluctor ring you might be able to make yourself something with the same amount of teeth per wheel that can adapt up. As long as the ecu sees the same variables from each wheel it should work I would think. I do not think it matters if you are going 60 or 80 with the older ecu there.

 

[Les]

Here is a question: does the capability of the brake system change the ability of the add-on ABS system to control the brakes?

The MR2, as sited in a couple of enthusiast websites is thus:

Engine:

2003 MR2 1.8-liter dohc 16V inline-4, 138 bhp

Superlight SLC: a lot more.

Brakes:

2nd generation Toyota MR2 (1989-1999)
Rotor sizes are as follows
91-95 N/A and 91-92 turbo front = 258mm (10.157")
91-95 N/A and 91-92 turbo Rear = 262mm (10.314")
93-95 turbo front = 275mm (10.862")
93-95 turbo rear = 280mm (11.023")

Whereas the SLC has:
Brembo Pair Front 4-piston calipers (with OEM dust seals for Worldwide street compliance)
Brembo Pair Rear 4-piston calipers(with OEM dust seals for Worldwide street compliance)
Brembo Pair Front vented 355mm rotors
Brembo Pair Rear vented 365mm rotors
(from the Superlight website)

Monstrous calipers grabbing rotors too big to be extra-large pizza pans produces enormous braking power.

So, whilst we suspend ourselves in the seats from our cinched-too-tight racing belts, the ABS system is being subjected to enormous braking forces as well. Will the ABS system, designed for the modest braking capability of a 138 horsepower MR2, simply give up an electrical or hydraulic ghost, once it is subjected to SL-C forces?

Les

 

[James]

Good question. No telling if the ABS pump of the MR2 can handle the calipers of the SLC. One way to find out would be to see what type of aftermarket big brake kit options, if any, are available for this car. Then compare the caliper size/design between the big brake kit and the SLC brakes to get a better idea.

Edit: Did a search and found some big brake kits by Rotora and KSport with 4-6 piston calipers and rotors between 280-320mm for front and rear kits. Not as big as the SL-C brakes, but promising......

 

[Roark_John]

I think of all ABS options, the MR2/MR-S setup is the most feasible.

If you look at the simplicity of inputs (no pitch, yaw, roll, sensors, traction control, stability control, etc) and the fact that the load in an on-road vehicle can vary by several hundreds of pounds, plus the upgrade-ability of aftermarket caliper/rotor packages for OEM vehicles means that it should function very well in this particular application.

Fran has stated in a previous thread that weight distribution is 47% front/53% rear, which matches the last generation MR-S. Curb weight estimates for a completed SL-C are in the 2300-2500lbs. range, while MR2/MR-S curb weights are in the 2200-2400lbs. range.

These are just a few of the positives I can put forward in regards to this application.

The guys over on the ProTouring forum have more experience with adapting existing ABS platforms to hot rods than any other online community I've ever come across.

 

[Troy Truglio]

My buddy road races with 12.5" front and 12.7" back with larger wilwood calipers and ABS. Works and his is the 91 abs system. They imporved it in 93.