:. Racing .:. 2005/05/26 - IMI Test Day
You test for a reason!
I had a great idea for what was going to fix the Radical's understeering tendencies going into this test day at IMI. Based on my experience with the Corvette, this *had* to be the solution!
From talking to folks who believe that part of the Radical's issue is the roll-understeer due to not a huge amount of camber gain in the suspension design, I thought that perhaps stiffening the front of the car would help keep the front tires where they ought to be, rather than flopping over into positive camber like they do in production cars.
My theory was to try a 400# spring on the front of the car, coupled with the 350# rears on there now. In theory, it worked OK, and fit with my previous experience. In fact, it was truly awful!
We started with just a few warmups to get the car "woken up" from having slept for a week or so. Part of this test was to ensure that the chain was clearing the frame OK with the new 50 tooth sprocket I installed on the rear. All seemed well on that front. Additionally, I wanted to see if the 2 (more!) turns towards the rear on the brake bias had affected anything, and it seemed mostly OK at that point. Most braking is straight-line braking at IMI - as it turns out, I think there's a touch too much on the rear, so I'll move it to the front by a 1/2 turn and see if the car is a bit more stable again.
After the initial checks and shakedown, I went out with the existing 300/350 configuration from the Mead trials outing to get some basline laps. As before, the car felt pretty balanced front-to-rear. I ran a 56.30, 56.20, 56.08 all right back-to-back. Things felt quite good. With the 300/350 (.86 ratio) balance, the car feels like it reacts very similarly front-and-rear - i.e. they move "in sync" with eachother. Hard to describe. I do recall that the 250/300 combo had a similar "balanced stiffness" feel to it - shame it's too soft, since the 250# front spring seems to work so well.
After this, we tried what I had hoped would be much better with the front, and put the 400# springs on the front. Total disaster. Lots of understeer, and a definite feeling of the tires having lost a lot of grip, like they were taking a long time to come up to temperature? I did several laps to see if they were in a "temperature dead spot" or something like that. Things improved somewhat, or perhaps I just adjusted my driving a bit? At any rate, most of my times were in the 57's, with a best of around 56.7 or so.
Since this was clearly a move in the wrong direction with the existing rear spring (or, at least, for low-speed driving), we switched back to the 250/350 combo I had on the car before Trials. (recall I'd changed it due to worries about bumps & downforce handling...). With this combo, I was able to run consistently faster than with either of the other two combinations, running a new best of 55.73, backed up by a 55.93, 56.00, and a 56.15.
With the 250# front spring, you can definitely feel the car move around a lot on its front suspension. It really rolls and takes a set in corners - definitely none of the skatey feeling the car first had when on old/cold tires and the 350/450 combo. It will be interesting to run this setup at higher speeds and see how it behaves.
Because of that "extra" movement from the front, it does rather seem like there's a mismatch between the front and rear of the car, since the front moves so much more than the rear seems to. In tight corners, it feels like the car really leans onto its outside front tire. This was the same effect I felt when running 250/400 when testing last time - it just felt a bit strange. (Note that 250/400 was the fast-lap combo last time at 55.90...) The same feeling of having to "muscle" the car around a bit more is still there, as is the generally slightly less responsive nature to all inputs - braking (easier to avoid lockup), lift-throttle (takes bigger lift to encourage sliding).
One driving epiphany that I have never so directly felt was the effect of feeding in throttle to transfer weight to the rear and plant the back of the car when it is sliding. Exiting onto the straight, I started getting fairly good at turning the car in, getting a bit of rotation, and then stopping that rotation by feeding in the throttle. The weight on the rear immediately would stop the slide and really catapult the car down the straight. Very cool!
The car is definitely better than it has ever been at this moment, and I'm certainly learning some things that *don't* work! The current balance may even pretty good for higher speed road racing work, even if it's still a bit push-y for autocross use. Perhaps the larger front Hoosiers will take care of part of that, the replacement shocks may also help, and perhaps even the front splitter/diffuser. Getting closer!