Understanding Roll Centers/axis RF95

[bassracer]

Hello
In 2022 I started racing a RF95 Honda fresh from frame-up. It was converted previously from FC to FF, but only by engine and reduction of aero and wheels. This generation of car had exclusive rear suspension width and uprights, so I widened the rear to 1600 spec during the frame up but did not change the uprights. We measured XYZ of everything while it was torn down but didn't let super sensitive setup details get in the way of getting on track. Generally, we had a good first season getting the car racing for a shake down season.

I am now digging into the geometry and looking for opportunities to improve the car. With radial tires and low ride height, the roll centers appear to be quite low - but other than understanding the large moment to the CG and "roll=grip" I do not have the tribal knowledge or experience to analysis this further.

Looking at some immediate options, I've mapped out some things that can be done and the resulting impact. Based on what I know about the 1600 uprights, the lower arm mount moves further down and changes angle of lower arm. They all revolve around RC, Raxis, and ride height. I welcome any advice anyone has whether it's these options or a complete "start over and think this way..."!

One thing I did find odd while racing is a very noticeable tire wear that would build up through the entry of the corner and migrate towards the outside and fling off. I tried getting extra sensitive with the corner weights, but it never went away and I was curious if this could be symptom of high roll? Pictures attached

Thanks! Brandon

[tlracer]

If it was excessive front roll, I would have expected to see the outside tire peeling as it becomes over-loaded. In your pictures it looks like the inside one and I'm guessing you haven't had any obvious symptoms of too much load on the outer tire?

As the inside wheel is less loaded but that tire is peeling and - if I read correctly - the rubber is moving towards its outer shoulder, I would be looking at how the wheel attitude to the road changes with steering lock, causing the excessive scrub.

[Rick Kirchner]

Steve Lathrop started an extensive analysis of the suspension on my RF94 FC. I laid the chassis on the garage floor on setup blocks corresponding the the RH I was using at the time and made measurements of all the mounting locations (as best I could without a faro arm).

I was chasing some instability issues around reducing caster. What we found was that the left side arms were not jigged the same as the rights, and you couldn't see it with the naked eye but it showed up in suspension measurements when you used a trammel bar to jig the parts before assembly.

This led to me building a precision trammel bar as well as a 3D printed jig to eliminate the differences in the left to right arms to arrive at equal caster.

An additional part of this study was to determine if the setup data you can get from Primus that was used as a starting point "back in the day" was good. it was. This makes sense, because the cars didn't change much in the suspension department from 90-95 so VD had a lot of time to get it right. I've never been able to use the measurements to set up arms and such, I don't know what the issue is there - different year car?

Call Steve and tell him that you can use my data/results and you can take it from there. Please pass me any analysis you do and the results you come up with.

[DaveW]

This may be useful. Link:

https://www.apexspeed.com/forums/att...3&d=1539115694

[bassracer]

Rick
Thanks. I certainly have unique left and front lower A arms. I was not successful using the Primus data or having perfectly symmetrical A-arms using the trammel bars. When doing the XYZ on the frame pickup points as well as the assembled corners (on chassis), we found things that were not perfect left to right. So we adjusted and setup all hubs to have the same symmetrical XYZ Left to right, and modified arm lengths as needed from there to achieve caster/camber.

Dave, appreciate the notes and I think I'll be keeping a copy around for just about everything in there. How does one understand "excessive RCH?" I suppose I'm bringing this all up on the assumption that I have excessively LOW front RCH.

Thanks

[DaveW]

...Dave, appreciate the notes and I think I'll be keeping a copy around for just about everything in there. How does one understand "excessive RCH?" I suppose I'm bringing this all up on the assumption that I have excessively LOW front RCH.

Thanks

What I'd look for on too high roll centers is lack of grip and "skating" instead of hooking up to the track. OTOH, sloppy, slow response to direction-change inputs might indicate roll centers too low.

Also if the front is too low compared to the rear, it would likely cause oversteer, and if the rear is too low compared to the front, it would likely cause understeer. Higher RC is similar to a stiffer swaybar as far as F-R balance is concerned.

[pooch776]

I don't claim to be an expert and I will admit to knowing very little about the set-up of a modern bell crank car but 2.5 inches of roll axis rake seems crazy to me. I general am happy when I get within a window of .375-.75 of roll axis rake. I also try to maintain my roll centers above ground under all conditions. To me the rear roll centers look reasonable with a nice linear change in ride hight adjustment. A .25 change in ride hight is equal to a .25 change in roll center. Can you make changes to the front of the car to get the roll center above ground? I will be very interested to see if Steve Lathrop responds to this thread. I always learn something when he shares his knowledge.

Good Luck, Joe

[S Lathrop]

There is nothing wrong with the geometry of the VD suspension. Look for the fixes else where.

The front suspension of the VD that I have looked at are almost identical to what I have on the Citations.

[Rick Kirchner]

After looking over your first post I'd start with removing the front anti-dive. On my car there's a spacer above the front lower a-arm attachment and the trailing leg goes into the lower pocket.

My car came with anti-dive and anti-squat and it gave me fits at first - push in, loose out.

[bassracer]

Thanks all. I fear I've tried to ask too many questions at once.

If we forget about the tire issue for a moment - I have a partially converted 1600 rear suspension expect uprights which would raise the roll center by 3/4". I am trying to determine if continuing with converting the uprights is necessary - it would be "OE van diemen" this way, and also a pricey update. I was hoping to run the numbers on paper and see if I could learn about how it would change the car before plunging in. When comparing to the front it seemed like something was already not optimized.

But essentially, is there a theoretical way to evaluate RC without testing?

[Rick Kirchner]

isn't the difference in uprights the vertical position of the bottom pin? I didn't think it was as much as 3/4".

I bought a upright form VD when they went out of business and it the lower pin was a slightly different height from the originals - I didn't even notice it until I put AP calipers on and there was interference with the outside joint on the track bar.

[bassracer]

isn't the difference in uprights the vertical position of the bottom pin? I didn't think it was as much as 3/4".

I bought a upright form VD when they went out of business and it the lower pin was a slightly different height from the originals - I didn't even notice it until I put AP calipers on and there was interference with the outside joint on the track bar.

Correct, the lower arm/track rod pin moves down and it's not 3/4" at the upright - Without having one in front of me I had to assume a change based on the casting geometry and calculated the resulting impact to RC as approx. 3/4". If you or anyone reading this knows the exact difference please do share!

[Rick Kirchner]

by the way, you won't necessarily need to buy new uprights. I buggered one on a royale and had a machinist drill it oversize, shrink in a bronze bushing, and re-bore.

[DaveW]

After looking over your first post I'd start with removing the front anti-dive. On my car there's a spacer above the front lower a-arm attachment and the trailing leg goes into the lower pocket.

My car came with anti-dive and anti-squat and it gave me fits at first - push in, loose out.

Anti-dive is a major cause of understeer. That geometry and steering angle results in cornering force adding to the outside tire's vertical load and subtracting from the inside - just like a stiff front swaybar.

[2BWise]

But essentially, is there a theoretical way to evaluate RC without testing?

Yes. I would maybe suggest taking a step back. Roll centers for some reason get the reputation of being tricky. Probably since they are hard to measure/locate and they move, but their contribution is pretty straight forward. They are just anti geometry but in the other plane from anti dive or squat. As such they just apply an anti roll moment that is taken up by the chassis instead of the springs/bars. Due to this it is actually pretty easy to calculate how they effect your roll stiffness, moment, and the distribution. If you do the calculations you can play around how you have to change springs/bars to compensate for RCH changes. It also will give you insight on the sensitivity to changes at one end of the car vs the other or how the vehicle's motion can influence the suspension stiffness. Similar to rise/falling rate bellcranks you could have falling/rising rate based on RCH migration.