SLC 24 Howard Jones

[flatchat(Chris)]

Your first para sounds like it dives under braking which means you need to dial in some anti dive - achieved by raking the lower control arm -i think?
"just my thoughts anyhow"

 

[Roger Reid]

There is also the possibility of porpoising. Here is a link. What is porpoising and why are 2022 F1 cars doing it? - The Race (the-race.com)

 

[Howard Jones]

Chris, I had considered "anti-dive" geometry as a source of investigation but this is a very momentary, transitionary, and very high-speed specific thing I am feeling. I still believe my setup values are the source and it's not a deeper foundational chassis design issue.

Once I have eliminated the toe change idea I will go to what I think is the real problem. I still am running 900-pound springs on the rear, 750F, and the original QA-1 shocks. The shocks are at the functional limit of their adjustable damping range with both compression and rebound settings almost at the full hard limits. I think more R rebound and F compression would be my first thing to do if I could.

AND I really can't add any rear wing (I'm at 8-9 degrees of AOA ) because of high-speed download suspension compression. Again too light springs. The reason I have not done this is the car is really good on slower tracks with 110-120mph straights and slower speed corners.

So in the end I will need to more than likely, re-valve or replace shocks, go to about 1050R/ 850F springs, and add rear wing AOA. Maybe I need two sets of shocks and springs. Then just swap them out for the track I will run at. Lots of work and money but, it is what it is.

Rodger, I don't think my car makes anything like the downforce that it would take to cause F1-style porpoising. It's not an up and down thing, it's more of a rear-steer side-to-side feeling as the suspension aero unloads. And it is a feeling and it only lasts a second or two. Not a real controllability problem that requires a lot of steering correction. But it's there and it does bug me a bit.

I'll get it.............I like this stuff and it's why I bought my SLC in the first place, so I could enjoy the journey of the development of the car. Kind of a poor man's version of what it must be like to learn how to race a real prototype.

 

[Neil]

Chris, I had considered "anti-dive" geometry as a source of investigation but this is a very momentary, transitionary, and very high-speed specific thing I am feeling. I still believe my setup values are the source and it's not a deeper foundational chassis design issue.

Once I have eliminated the toe change idea I will go to what I think is the real problem. I still am running 900-pound springs on the rear, 750F, and the original QA-1 shocks. The shocks are at the functional limit of their adjustable damping range with both compression and rebound settings almost at the full hard limits. I think more R rebound and F compression would be my first thing to do if I could.

AND I really can't add any rear wing (I'm at 8-9 degrees of AOA ) because of high-speed download suspension compression. Again too light springs. The reason I have not done this is the car is really good on slower tracks with 110-120mph straights and slower speed corners.

So in the end I will need to more than likely, re-valve or replace shocks, go to about 1050R/ 850F springs, and add rear wing AOA. Maybe I need two sets of shocks and springs. Then just swap them out for the track I will run at. Lots of work and money but, it is what it is.

Rodger, I don't think my car makes anything like the downforce that it would take to cause F1-style porpoising. It's not an up and down thing, it's more of a rear-steer side-to-side feeling as the suspension aero unloads. And it is a feeling and it only lasts a second or two. Not a real controllability problem that requires a lot of steering correction. But it's there and it does bug me a bit.

I'll get it.............I like this stuff and it's why I bought my SLC in the first place, so I could enjoy the journey of the development of the car. Kind of a poor man's version of what it must be like to learn how to race a real prototype.

How much static rear caster & toe-in are you running?

 

[Mesa]

How much static rear caster & toe-in are you running?

Oh now.... that question strikes a cord.
Neil, I have been searching for intelligent discourse on the effect of rear caster. Please enlighten us (me) on the benefits (or not) of rear caster.
On a properly set-up rear suspension is it even possible to generate active rear caster (camber alteration)?

Thanks

 

[Neil]

Oh now.... that question strikes a cord.
Neil, I have been searching for intelligent discourse on the effect of rear caster. Please enlighten us (me) on the benefits (or not) of rear caster.
On a properly set-up rear suspension is it even possible to generate active rear caster (camber alteration)?

Thanks

Whether it's possible or not depends on your type of suspension. I used Porsche 996 rear uprights and a slightly modified 996 5-link system.

My recommendation from John Horsman was to design for 7 degrees. You do not want the wheels to go into a "grocery cart" mode under any condition.

 

[Roger Reid]

This is a good video to watch. Suspension Geometry - Part 1 (Camber, Toe, Caster, KPI, Scrub Radius) - YouTube
There is more than just camber/caster involved. SAI, steering axis inclination (kingpin) and scrub radius is rarely mentioned but very important. Most of these discussions only talk about the front suspension. The rise and fall of the front suspension during turning will tend to return the steering wheel to center if you release the wheel.
This video does talk about SAI and scrub radius at the front, but it applies to the rear as well. At the rear the difference between the intersection of the SAI and ground level and the centerline of the tire contact patch will introduce massive twisting forces under acceleration and braking. Any compliance in the rear toe link, bearings or bushings, or mounting points will introduce rear wheel steering. At the rear, bump steer is minimized with proper toe link mounting using washers to raise/lower the pivot points. There is no or minimal (bump) steer so there is no self-centering so why do you need rear caster? Even with beefy toe links and mounting, there is still compliance. The rear tire contact patch is rubber and has its own compliance. The contact patch will try to rotate around the SAI intersection stressing every piece of steel up to and including the frame. It's possible that introducing rear caster will change what happens to the rear contact patch under power or braking but I haven't seen anything published.

 

[Clayton]

Roger
Caster plays a huge effect on roll steer in the reverse A arm, trailing link suspension like the original GT40’s
From memory, they ran 7 degs and on my RF40 I ended up with 3.5 degs to minimise the roll steer the best.
Not sure if caster would have much of an effect on an A arm with toe link rear suspension, but I’ve been wrong many times before…

Clayton

 

[flatchat(Chris)]

Chassis torsional rigidity ??
check simple things first -motto
I have seen cracked aluminum chassis welds before

 

[Don V.]

Howard I think you are spot on.

1) There was a reason Fran changed the rear toe link from the street version to the race version. The street version under high loads flex.

Street version:

Race version:

2) The suspension has anti-dive built in. So that is not the issue.

 

[Mesa]

Thanks Roger (will watch as time allows)

My question specifically pertains to double wishbone on the rear of an SLC and similar chassis.
Without purposeful 'steering' input how can caster be actively altered? Or beneficially actively altered?

 

[DaveC]

Thanks Roger (will watch as time allows)

My question specifically pertains to double wishbone on the rear of an SLC and similar chassis.
Without purposeful 'steering' input how can caster be actively altered? Or beneficially actively altered?

I think it comes down to what the hub is doing in compression and rebound. This is independent of toe link effects on bump steer with compression and rebound. Depending on the geometry of the upper and lower control arms (ie rear caster), different toe curves can be resultant, I'm assuming much less pronounced than bump steer but nevertheless detectable and noticeable at performance limits.
Here's a good writeup regarding the C7.
https://www.corvetteforum.com/forums/c7-tech-performance/4324718-what-is-the-function-of-caster-alignment-on-the-rear-suspension.html

Then you are getting into complex geometry like ride and roll rates...
https://archit-rstg.medium.com/sprung-back-ride-rates-in-a-vehicle-cc969443cdee

 

[Fran Hall RCR]

The reason I changed the toe link on the race car is that the top plate has shim-style camber adjustment and the toe link needed to be on the same plane as the upper control arm.

 

[Neil]

Howard I think you are spot on.

1) There was a reason Fran changed the rear toe link from the street version to the race version. The street version under high loads flex.

Street version:

View attachment 126232

Race version:

View attachment 126233

2) The suspension has anti-dive built in. So that is not the issue.

View attachment 126234

The street version has a rather long, slender vertical member so it is not surprising that it flexes under load.

 

[bill douglas]

Flexes far less than you may think Neil, much less than oem bushings used in oem sports car (Porsche/Ferrari/Maclaren etc )suspension components

 

[Howard Jones]

Heil. I set the car up with 1/16" toe in per side at the rear. The rear caster is set by placing washers alongside the upper or lower or both Inboard rod ends that locate the A-arms. Since this is a double a-arms design the caster is then fixed. Fran told me to center both A-arms in their washers and that was how it was designed. I did and have never changed that setting.

After I get to the setup exercise I am going to go for no toe at all and see I if can get that throughout the rear vertical travel for this setup. If not then absolute minimum neg. toe (in). Note: is very hard to work with less than 1/32" adjustments with the string method I use. So in practice I will try to achieve 1/32" toe in per side.

Robert. I have not found any reference to "caster setting" in any of the racecar books I have when considering a double a-arm rear suspension. My guess is it isn't a consideration and they design them with a straight up and down upright (both upper and lower pivot points in a vertical line). Our cars look like that is what is going on, although I haven't figured out a way to accurately measure that. I think that as long as both sides are the same, then changing the angle will only alter the wheelbase and as long as they are the same angle then both sides of the car will be the same wheelbase with the axle centers in the same place in relation to each other. We are talking fractions of an inch with the washer method we have anyway.

I should say again. This a very small " unpredictable twitch" that's last for little more than a second or two and then only at one or two points of the lap. I don't really consider it dangerous but I do tend to leave a little more on the table because it's there and I would like to understand what is going on, more to increase my comfort that anything else. Like I said.............I'll get it.

 

[Neil]

Flexes far less than you may think Neil, much less than oem bushings used in oem sports car (Porsche/Ferrari/Maclaren etc )suspension components

Bushings are unacceptable in my application. Porsche used rubber bushings in their 996 rear suspension and I found that using spherical rod end bearings required a slight modification of the inner pick-up points to cure bump steer. I ascribe this as to Porsche taking into account the expected rubber bushing deflection in their original design. Maybe...?

 

[Howard Jones]

Robert: Here it is! Dave nailed it! read this.

https://www.corvetteforum.com/forums/c7-tech-performance/4324718-what-is-the-function-of-caster-alignment-on-the-rear-suspension.html

My takeaway is you want 0 caster at the rear. At least on a C8 Corvette. Now how can we accurately measure caster? Question for everyone. If the top of the rear upright is completely horizontal, does that get us 0 Caster? I'll have to really look at the relative position of the top and bottom "ball joint" centerlines in relation to the top plate. If they are and set at 90 degrees to the top plate plane then it will be simple to at least estimate caster angle.

 

[DaveC]

If the top of the rear upright is completely horizontal, does that get us 0 Caster?

yes, as long as both ball joints are in a plane 90 degrees to this. Fran could answer this based on CAD data. To clarify, you are only looking for it to be 0 degrees in the fore/aft direction, you can ignore camber.

 

[Howard Jones]

Dave, I have been looking at open-wheel car suspension pictures all day and what I think I see is the rear at 0 caster and the fronts are angled reward at the top for the positive front caster. Many look like they are the same uprights with steering arms, and caliper brackets, etc. bolted on as required.