davidl

iTrader: (26)

I have run it that high on a medium/high traction carpet. Had great results as it let the car get up out of the carpet, as Mike H. stated, but didn't loose rear stability or forward bite. One of the fastest runs I have "ever" made with a 1/12 car. I couldn't believe it and wondered after looking at the score sheet if I really drove that car to those numbers. Now this is all relative and doesn't take into consideration the other deviations from most setups I had in the car. But for 8 minutes, I felt like someone more skilled and consistent than in my past.

Casey

iTrader: (5)

BIG GT 12 race this weekend at 180 Raceway,, come on out and see what my roll center is

MC Hamilton

iTrader: (3)

A more technical description - lowering the rear pivot lowers roll center and adds stability to the rear - raising the rear roll center will allow the chassis to roll more and should reduce rear grip and reduce traction roll.

glennhl

iTrader: (16)

Your first statement is true, however, raising the roll center does not increase chassis roll. When you increase the roll center it gets closer to the car's CG and thus lowers the chassis roll.

davidl

iTrader: (26)

So, here is a challenging question for the masses. Is there really a roll center for the rear of a pan car. This type chassis has a pivot point between the front chassis and the rear pod. But, how can that be analyzed as a roll center. The rear side springs have a direction that is vertical, but are located half the width of the car away from the pivot. The dampers are the tubes that have a direction that is transverse to the chassis, but high above the pivot. The center shock and spring have a direction that is angled towards the front suspension and crosses the position of the pivot very high up. How does all of this play together? Is the pivot a roll center for all of this? What say you?

vafactor

iTrader: (6)

I say 🥴

wingracer

iTrader: (24)

Short answer is yes, the pivot is the roll center. Just like the pivot on a Watts Link style suspension is the roll center.

Spring and damper configurations have zero impact on roll center.

I should note that more recent actual testing of actual cars has shown that geometric roll centers are actually not the point a car rolls around. But for setup purposes, geometric is good enough as it's close and moving it up or down does move the actual roll center up or down in a similar fashion. Also most of that testing involves multi-link suspensions where the geometric roll center is a virtual point in space, not a physical pivot so that might be why there is a discrepancy.

glennhl

iTrader: (16)

I know where you are coming from. You are right, it's more of a chassis roll center than a rear axle roll center, but in reality, that's all we have with a pan car. And as far as roll center calculations, a pivot point is the simplest. You always know exactly where the roll center is, it doesn't move around. And to me the only thing that reacts against the roll are the side springs. So for me the main tuning parameters other than tire compounds are the side springs and the front springs. Second tuning parameter is adjusting rear roll center up and down, down to glue the rear and up to unglue it.

And front roll center? I have an X12 that just slides up and down on the king pin. I've always assumed that this puts the roll center at ground level since it's the same as perfectly parallel A arms and since the lines from the a arms never intersect (other than at infinity), then the line back to the outside tire is on the ground (since it came from infinity). And even though I'm not certain, I don't think the front roll center is adjustable on this type of front end.

davidl

iTrader: (26)

🍦 I wanted to give you two scoops but this is all I could find.

old_dude

iTrader: (1)

That is a interesting point about the X12 and I believe the A12 also. Old pan car front ends were basically rigid kingpins like those. Then Associated came out with their upper "A" arm and everyone jumped on that. Those definitely had a roll center. The oval guys jumped all over that front end and made upper arm mounts that allowed the roll center and upper arm length to be adjusted. As far as I know only CRC added that capability to their current on road front end. Now we have the manufacturers putting the adjustable rear roll center in the kit design. Just comments.

AZ Wolfie

In the process of transitioning to the CK25AR from old Associated cars and have a couple questions.
1) Planning to run the Sanwa Servo direct mounted up front. Will the Xray servo saver fit without modification, or should I instead consider a Tamiya or Kimbrough per the manual?
2) When making rear droop adjustments, how do most hold/rotate the shock shaft to change the length? Smooth Needle Nose, Angled Needle Nose, Special Shock Pliers?

Thanks!

BullFrog

iTrader: (4)

I did try the upfront servo but have switched to the standard servo set-up. I use Kimbrough (always have for years). I either use pliers with a cloth or the shock pliers.

sweems

The Xray servo saver fits just fine. I use the same setup on my car.

AZ Wolfie

Thanks for the quick replies, and happy to hear I can stick with the Xray servo saver!

For holding the shock shaft I saw these round tip pliers, and tempted to give them a try since I haven't discovered a "shock shaft" plier yet that seems ideal.

BullFrog

iTrader: (4)

guess I'm lucky I found mine shock shaft tool at my hobbyshop?