SLC 24 Howard Jones

Roger Reid

Supporter
The way I now see it is there are 2 ways to change bump/roll steer at the rear. Caster and toe link geometry. I get the feeling that the corvette 0 degree caster spec is set as not to confuse how bump/roll steer is achieved. Keep the caster consistent and adjust the toe link as needed. I feel you nailed the solution in your first post (1,100) Quote ........"So I convinced myself that it was related to toe change as the rear goes full droop. I tried to measure this statically in the shop and did see some toe change from the target setting of 1/16" in, to a little above zero or even just a hair positive at full droop...... I feel you are on the right trac by setting a negative 1/16 per side toe in at full droop and staying the same or increasing negative toe at full compression. If you have 0 or positive rear toe, the rear will be "loose".
 
Positive caster on the front suspension helps the steering return to neutral after a turn. This results in much more stable straight-line tracking of the vehicle at speed. As your front caster approaches zero, the car will feel much more squirrley at speed, gives the car and the driver a more nervous feeling. The other benefit of increased positive caster is camber gain. As the steering wheel is turned to enter a corner, positive caster will drive the hub/wheel into increased camber, right when you need it. So you can run a relatively lower static camber for a better tire contact patch and more even tire wear.
The downside to increasing caster is increased steering effort at low speed with an unpowered rack.
I like +7.5.
 

Howard Jones

Supporter
Rodger, I was reading all my books again and trying to understand the rear "caster". Among them are, Alan Staniforth-Competition car suspension, Carroll Smith's books, and Len Terry and Alan Baker-Racing car design and development. Nobody uses the term "rear caster" But somebody used the term "rear upright incident angle" or something like that. So I will now not use "rear caster" anymore because I don't think it exists except as a description of misadjustment.

So I think the Corvette "0" spec is meant to eliminate misadjustment errors and establish a preliminary basis for further adjustment of toe. The more I think of it, the more I believe you hit the nail on the head. If the "ball joints" can only move in a perfectly vertical plane then the only thing that affects the rear toe is the toe link, its adjustment and it's geometry. Given that, it seems to me that "caster" on a double a-arm rear suspension really has no effect. The term really does not have any meaning in this application.

Another way to think of this is the two A-Arms on the rear are fixed at all three pivot points. Two inboard and one on the upright each. None of them swing on an arc and only move vertically. Therefore they can't CHANGE the toe from its at-rest setting. ONLY the toe link geometry can do that. There is no DESIRED "rear caster".

Mounting the toe link on the A-arm at one end and the upright at the other prevents the toe link from moving in an arc as well. Thus resulting in a stable toe setting throughout the suspension travel. Every race car pic I found does it this way including the green RCR SLC. Pretty much done and dusted on this question for a race car.

I think I will try and set the rear toe as close as possible to 0. The above texts do warn that too much neg rear toe will cause oversteer so it should be kept to a minimum with a preference on preventing ANY positive toe. That will also produce oversteer but much more dramatically.

Dave, since my SLC is a track-only car and I am running R7 Hoosiers I need to run a lot more camber. The Hoosier guy I talked to wanted me to run at least -2 and as much as -3.5 depending on tread section temps and tread wear. It is currently set to -2.5 F and -2.0R. I ran -.75 on my GT40 when it was a dual use car and then went to -.5 as I returned it to street only use. Runing a lot of camber on the street will really effect tire life. I view about -.5 as a street car maximum.

Here's a C8 specs.


Claimed stock C8 alignment specs
10-19-2019, 12:38 AM

So they claim :

The stock curb alignment specs (a full tank of fuel with no passengers) alignment specs :

FRONT

Caster: 7.4 +/-0.6 deg
Camber: -0.5 +/-0.6 deg
Sum toe (left + right): 0.10 +/- 0.20 deg

REAR

Caster: 0 +/-0.8 deg
Camber: -0.5 +/-0.6 deg
Sum toe (left + right): 0 +/- 0.20 deg


Track Alignment Specs

FRONT

Caster: 8.0 deg
Camber: -3.0 deg
Sum toe (left + right): 0.1 deg

REAR

Caster: 0
Camber: -2.5 deg
Sum toe (left + right): 0.1 deg"


Of particular note is that the rear caster specification is tightened from 0-(-.8) to absolute 0. This would confirm the information Dave posted above. Also, you can see that GM thinks -.5 degrees of camber is correct for a street car. I suspect this is to preserve tire life.

I love this forum. Always a good thoughtful discussion. I couldn't get by without it.
 
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Neil

Supporter
Rodger, I was reading all my books again and trying to understand the rear "caster". Among them are, Alan Staniforth-Competition car suspension, Carroll Smith's books, and Len Terry and Alan Baker-Racing car design and development. Nobody uses the term "rear caster" But somebody used the term "rear upright incident angle" or something like that. So I will now not use "rear caster" anymore because I don't think it exists except as a description of misadjustment.

So I think the Corvette "0" spec is meant to eliminate misadjustment errors and establish a preliminary basis for further adjustment of toe. The more I think of it, the more I believe you hit the nail on the head. If the "ball joints" can only move in a perfectly vertical plane then the only thing that affects the rear toe is the toe link, its adjustment and it's geometry. Given that, it seems to me that "caster" on a double a-arm rear suspension really has no effect. The term really does not have any meaning in this application.

Another way to think of this is the two A-Arms on the rear are fixed at all three pivot points. Two inboard and one on the upright each. None of them swing on an arc and only move vertically. Therefore they can't CHANGE the toe from its at-rest setting. ONLY the toe link geometry can do that. There is no DESIRED "rear caster".

Mounting the toe link on the A-arm at one end and the upright at the other prevents the toe link from moving in an arc as well. Thus resulting in a stable toe setting throughout the suspension travel. Every race car pic I found does it this way including the green RCR SLC. Pretty much done and dusted on this question for a race car.

I think I will try and set the rear toe as close as possible to 0. The above texts do warn that too much neg rear toe will cause oversteer so it should be kept to a minimum with a preference on preventing ANY positive toe. That will also produce oversteer but much more dramatically.

Dave, since my SLC is a track-only car and I am running R7 Hoosiers I need to run a lot more camber. The Hoosier guy I talked to wanted me to run at least -2 and as much as -3.5 depending on tread section temps and tread wear. It is currently set to -2.5 F and -2.0R. I ran -.75 on my GT40 when it was a dual use car and then went to -.5 as I returned it to street only use. Runing a lot of camber on the street will really effect tire life. I view about -.5 as a street car maximum.

Here's a C8 specs.


Claimed stock C8 alignment specs
10-19-2019, 12:38 AM

So they claim :

The stock curb alignment specs (a full tank of fuel with no passengers) alignment specs :

FRONT

Caster: 7.4 +/-0.6 deg
Camber: -0.5 +/-0.6 deg
Sum toe (left + right): 0.10 +/- 0.20 deg

REAR

Caster: 0 +/-0.8 deg
Camber: -0.5 +/-0.6 deg
Sum toe (left + right): 0 +/- 0.20 deg


Track Alignment Specs

FRONT

Caster: 8.0 deg
Camber: -3.0 deg
Sum toe (left + right): 0.1 deg

REAR

Caster: 0
Camber: -2.5 deg
Sum toe (left + right): 0.1 deg"


Of particular note is that the rear caster specification is tightened from 0-(-.8) to absolute 0. This would confirm the information Dave posted above. Also, you can see that GM thinks -.5 degrees of camber is correct for a street car. I suspect this is to preserve tire life.

I love this forum. Always a good thoughtful discussion. I couldn't get by without it.
If the rear suspension has any "anti-squat", then the movement is not just vertical...
 

Ian Anderson

Lifetime Supporter
Hey Howard I remember a few years back Jac Mac said on the TVR? Race car they found that the rear toe measurement would change under power as the whole axle would flex, and if that can happen on a live axle just wondering if you could be getting some flex in your system meaning power on toe in and braking toe out, which at high speed would make for an unstable ride which would be correctable at lower speeds.
 

Howard Jones

Supporter
I went to COTA yesterday with the new setup and the upper A-Arms I made. Set camber to F-2.5 and R-2.5. This is the most negative camber I have run to date. Unfortunately, I broke a driveshaft on the third lap of the first session. So what did it feel like at 50-60% pace? Really hard to tell but if anything more on the nose than before. Really I didn't get a chance to even get to full power so who knows?

The driveshafts in my car are the original ones that came with the car. I have a GPS speedometer so I have 2303 total miles on the clock. My guess would be all but maybe a 100 miles driving around the block checkout miles is on track with something like 2/3s full pace. A lot of the early days were at an easy pace so let's call it 1500 miles total full-pace track miles with the rest more or less tuning and check-out laps at less than full power or quick gear changes and downshifts.

A normal track day is about 6 sessions with 8-9 laps per session @ 3,5 miles per lap or 20-25 miles at full pace per session or more or less 150 miles per day at near or full pace adding on maybe 30 miles of warm up and cool down laps per day. 150 miles per day @ 8-10 days per year is about 1200-1500 miles per year. So if I don't change the type of axles I am looking at new ones once a year.

All in all, I think that's not bad for the standard material half shafts that came with the car. So next is to call Fran and find out what part he gave me back then and then call the Drive Shaft Shop and ask them for advice. I'll let you know what I do when I know.

Pictures when I get it apart but I'm pretty sure it failed at the outboard end spline root shearing it in two. The shaft stayed in the boot so there was no other damage. That was good because it was the end of the back straight, 70% power, and about 110MPH when it went to no drive. That's what happens to a Quaife TBD when one side loses any resistance to torque. It goes to no drive on either side.

So there ya go.
 
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The axles being shipped now (and for some time) are made of 300M, which is what DSS will tell you to use, in case others are wondering about their cars.
 

Neil

Supporter
These are Summers Brothers 4140 Chrome-Moly axles that were spares for a Porsche team at Daytona. Offers?
 

Howard Jones

Supporter
I called RCR and they are going to research what was on my car when it shipped for me.

They called me back and believe the best thing to do would be to call the DDS and talk to the guy who they order parts from. He will be able to research the specific order if I want but maybe the best thing to do would be to just have a discussion with DDS taking into account my needs. I think that's right. It really doesn't make sense to use RCR as a middleman.

Thanks, Will, I will mention the 300M spec when I talk to them.

More on this as it develops.
 
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Neil

Supporter
These are Summers Brothers 4140 Chrome-Moly axles that were spares for a Porsche team at Daytona. Offers?
I forgot the photo!
Axles.jpg
 

flatchat(Chris)

Supporter
It’s usually the CV that fails when the ball cage Carks it
Tip :- deburr and polish the ball flights - use quality ( Cro Mo ) ball cages
Assemble CV in its correct orientation and in phase at each end of shaft
 

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It’s usually the CV that fails when the ball cage Carks it
Tip :- deburr and polish the ball flights - use quality ( Cro Mo ) ball cages
Assemble CV in its correct orientation and in phase at each end of shaft

Yep. Here you go. Mike maintains and races the PDG GTM. I bought his CV polish tool for my 930 CVS, what a difference. Removes all the metal from the CV race before the ball does so the metal debris doesn't end up in your grease.
http://www.myraceshop.com/Polishing_Stone.html

 

Neil

Supporter
Yep. Here you go. Mike maintains and races the PDG GTM. I bought his CV polish tool for my 930 CVS, what a difference. Removes all the metal from the CV race before the ball does so the metal debris doesn't end up in your grease.
http://www.myraceshop.com/Polishing_Stone.html

Thanks for posting that video- good information. One wonders why the factory doesn't ship their CV joints already polished? Surely they could use automatic tooling to do it.
 

Ken Roberts

Supporter
I discovered that my original RCR SL-C cv joints from DSS were polished like shown in the video. I disassembled them to install longer axle shafts after converting to SKF X Tracker rear bearings. Last year I bought a couple of #600 grit felt head diamond bobs to go over them before reassembly.

image.jpg
 
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