I another post, I decided on a 205 width tire for the Fiesta ST. But I went to a 40-series to lower it a bit. I have coil-overs, so I can bring it down a bit on the suspension too. Lower, with plenty of travel and clearance, is better right? It lowers the CG which benefits handling on the autocross, which is where the tires will be used. Is that sound logic, or is that a concern? Should I stick with 215/45 or 215/50?
For that particular car, I think anything that lowers the center of gravity might be a good idea.
While lowering the CG seems like the right thing to do, you have to understand what changing the geometry did to the roll center. If the roll center went down further than the CG did, then you have increased your roll moment, creating more roll when you intended to get less. I have no idea what lowering a Fiesta does to it's roll center.
As long as you can keep tires on the ground, gearing is the most important reason to change tire size. Going at least 60 mph in 2nd gear is really nice for autocross if you have the midrange to support it. If you run on a small lot, that number could be a little lower. How often do you bounce off the limiter at events now?
If you lower it a lot, look at driveshaft angles too.
In reply to DeadSkunk (Warren) :
Also, if it's low enough that the ball joint/inboard control arm pivot point axis vs. upper strut mount/ball joint axis goes past 90* he could end up with a falling-rate suspension.
That may not be a concern on a FiST, but it was on l MkIV (and older) VWs.
In reply to Pete Gossett (Forum Supporter) :
I did find an English company that sells "roll center correction' devices. Could be offset bushings to raise the pivot point of the control arm.
In reply to DeadSkunk (Warren) :
On VWs there used to be(maybe still is) a metal bushing you bolted to the stud on the ball joint, then bolted the knuckle to it, to level out the control arm. I seem to recall at one point they were illegal for autox due to safety concerns though.
What people here are saying about roll center could definitely be a thing. At its most basic, try not to have the ball joint higher than the inner control arm pivot points.
You say you're on coilovers. What is the spring rate? Spring rate and dampers are going to be your most effective way of controlling roll. Far more so than ride height. So, what are you working with?
Very interesting stuff, I see why the people who design suspension systems are engineers!
Currently, I am on 8000# front and #5000 pound rear coils. The dampers have 24-steps of damping adjustment, I run the rears at 50% stiff and the fronts at about 80% stiff. I have a 2-point front subframe brace, and slotted towers and camber bolts that bring me to -2.6 degrees up front. (-2 in the rear)
On my street tires, the ball joints are about level with my pivot point, maybe a smidge higher. My autocross wheels have a 1/2" smaller radius, which means that the ball joints will move up 1/2" elative to the pivots...or down 1/2" (I can't visualize it. I will have to measure the next time they are mounted.)
I ran the smaller diameter wheels yesterday at the autocross and was pretty happy with the setup. The shift to second was much faster, so I have to adjust to that. I hit the rev limiter once, on longer courses I could see that being an issue, but most of mine are small and tight. If there is a really long section, I go to third I guess. I thought that turn-in was improved and the car was much more responsive to steering inputs.
If there was an issue, like a falling-rate suspension, how would that manifest itself? I tend the follow the "if it seems OK, send it" mantra, but that may not be ideal. : )
Your control arm angle (ball joint relative to pivot) doesn't change with different size wheels/tires. If they are level, you probably have just a little bit of travel before your camber curve starts to go bad.
I'm assuming you are on 8Kg/mm springs which = 450lb. (and 5Kg/mm rear)
Yes, 8K and 5K. I'm learning more every day
When going to a lower profile also remember that this may require suspension changes. Tires are part of the suspension and the likely stiffer sidewall may change an optimal set up. Smaller changes tend to require testing.
Besides the smaller circumference, smaller tires tend to be lighter tires so there is less unsprung weight to go along with the shorter gearing.
In reply to pinchvalve (Forum Supporter) :
For falling-rate it would feel like the springs get softer as you approach mid-corner(which is effectively what's happening). So I'd say you're ok there as long as you don't lower the front much more.
In reply to pinchvalve (Forum Supporter) :
That looks higher than my stock ST.
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