:. Racing .:. 2005/05/05 - IMI Test Day
Once again, I've made a zillion changes to the car, so I felt a test-day was warranted to see what more I can learn about my little race car.
As before, Ro came out with me as 'crew', and we worked from a test plan I came up with as I made changes the week before, to make sure everything got a double-check.
the test plan had about 3 major parts: The most important was to fix the jetting issues which have bedeviled me since I've gotten the car running.
Prior to the test day, I'd gone down 3 sizes on the main jets, and 2 sizes on the idle jets. I knew that the idle jet was a bit too small, since the screw was adjusted 3 turns out to be "about right", so I'll be installing one size larger (48, from 45) to get that about right. The 178 main jet turned out to be maybe a hair too small, so we put in 180's, and all seems to be happy. According to the correction factor math, going from 185 and 50 at sea level to 5000' feet and 70* calls for 176 and 47.6 jets, respectively. Going to the 180 main gives me a margin of safety - especially as we get some fairly wild temperature variation here in Colorado - and the 48 idle jet will cover me over a fairly wide range as well, with luck. I may have to change the main jets more often than I think for "optimum" performance, but with luck, it will always be "good enough" that I can ignore it - maybe I'll have money to put in a fuel injected Hayabusa some day, so I can stop #**! with carburetors.
Anyway, long story short, I had absolutely zero issues with the engine performance all day. I'll be leaving the 180's in for the time being - and checking plug stuff from time-to-time, and putting the 48 idle jets back in, and with luck, I should be able to quit screwing with them for a little while and just drive the car.
Second goal was to scrub in the new R35 front tires, so I have a nice good set for use at trials next week - it looks like all is well there. No major issues on scrubbing.
Third goal was to play with springrates a bit. Based on the wide variance of spring setups I've seen folks run on Radicals (front ranges from 250 to 450, rear from 350 to 700), it seems there is a wide window in which the car works "decently". From talking to Joe Stimola, he was of the opinion that for solo speeds, I should go softer than my existing 350/400, and suggested I try 250/300 or 300/350. He believed that 400 rear was going to be too stiff for solo speeds, especially.
Ro and I started with the 350/400 combo to get some baseline times. The car's ride heights were set based on the cornerweighting that Lawrence and I did - so the front was: LF: 2 3/8 RF: 2.25, and the rear was around 2.75 or so. From last time, I'd done an alignment on the car, with very little front toe (maybe 1/16 total), and added a bit of front camber, to 1.5*, and taken a bit out at the rear, to about 0.5*. The additional camber in front coupled with less in the rear made the car far more neutral, which was great. With this baseline, my best time was a 56.30. Behavorially, the car was quite sensitive to throttle position changes - lifting made the car tuck very quickly, and the car understeers a bit under neutral throttle. Lastly, it put power down *decently*, but not great. We took tire temps to check the new camber settings, and got the following:
Following this, we changed the car to 250/400 - dropping the front springrate 100lbs, keeping front ride heights the same. Initial feel was that the car definitely did not transition as well as previously - it felt a bit slower to respond, and I felt like I had to do more to "muscle" it around the tighter transitions. The mismatch between the springs and the shock bump valving became all the more apparent too, with the softer front springrates. We'll see how well the car slaloms on the softer front springs, but I bet it might not be as good as I like. My other initial impression was that it was a much smaller difference than I'd thought it would be - certainly smaller than going from 600# to 960# in the Corvette.
Quick time for this session was a 55.90, the fastest time I was able to see *for sure* on the day. (more on that later). The car seemed to me harder to drive, and the additional front roll made it feel like the car did not put power down as well, since more weight was off the inside rear. Besides the one quick 55.90, most of the other times were in the high 56's and low 57's. However, we discovered on coming back into the pits that the lugs were slightly loose on two wheels, which may have contributed to some of the "odd" feel of the car. One thing I did notice with the softer front spring is that braking was slightly easier to be consistent with. Again, we took tire temperatures to confirm our first set of numbers, and got:
Our next change was to drop the rear spring down to 300#, to see if the softer setup worked. Based on how the car did not transition as well with the 250# front, we did not think this setup would be good. However, it turned out to feel very nicely balanced. Unfortunately, it appears that it simply lets the chassis move around too much at even the relatively high ride height the car was set at (~2.75" rear), as the tires were rubbing in the corners. Further, Ro reported that down the straight, at ~80-90mph, the downforce was sufficient to push the bodywork down on the rear tires, and create quite a smokescreen.
It was here we noticed that the left side of the body is about a finger's width lower than the RHS in the rear, so that is one thing I have to investigate. However, there was noteworthy rubbing on the RHS tire as well, so I think that 300# in the rear of a Radical just isn't enough for any kind of high speed driving. From looking at the position of the o-rings on the shocks, it's clear we were using more suspension travel with these springs than with any other combo. On just the 3 laps I did with this setup, I believe I saw a 55.50 on the Stack, but I accidentally marked a 10-second best lap, so I can't confirm that. a 55.5 on 5/5/2005. Interesting, eh?
So, to keep the car off of itself, we installed a 350# rear spring, to get us to 250#/350# on setup. I went out for about 8 more laps, and predictably enough, the car felt right in between the 250/400 and 250/350 setup. Put power down a bit better than the 400# rear springs, but was a slight-bit harder to drive than with the softer rear. I was very consistently in the low 56's with this setup - 56.24, 56.28, and believe I saw another high 55 with this combo.
All in all, I was surprised with the relatively small change the spring changes made in the car, at least to my sensitivity levels. We ran out of time to try the super-stiff 400/500 setup I'd planned on trying out, but I imagine it'd be just too stiff for the hardly-any-downforce levels that we have at Solo speeds. My basic feelings confirm Joe Stimola's feelings that 250/300 or 300/350 might be the softer rates required for solo. The car sits now with a 250/350 setup, with the rear bar at full hard.
Having gotten closer to "good" to a good compromise setup between solo'ing and road racing, next time out we're going to try a 300/350 combo with the rear bar still at full hard, and then try disconnecting it completely to see how the balance of the car changes. We'll see if it sticks the power a bit better in the lower gears (1st, mostly, is the only time it's an issue). Further, we'll try the bar disconnecting game at the current 250/350 settings, and then I also want to try a stiffer front swaybar when I get the chance, to see if that has desireable effects or not.
One important change for solo is going to be the addition of the 20x7.5 front tire. If I can run that and no rear rollbar, the car should handle very nicely, and then for road racing, reconnect the rear bar when running the 20x7, and hopefully have a bit 'safer' balance for high speed as a result. We'll see what kind of balance I can find between the two setups.
So what did I learn about springs today?
Unfortunately, I wasn't able to take a lot of "learning" home about the steady-state balance changes due to the springs, as I just wasn't really able to detect large changes in the understeer/oversteer balance of the car as a result. The softer front was easier to drive when properly matched at the rear, so I'll have to balance that versus the need for fast transitional ability in solo2 slaloms. It will also be interesting to see what affects the swaybars have on the car when I play with those a bit.
- Softer front springs made the car not transition as easily
- Softer front springs make it a bit easier to brake?
- Softer rear springs make the car less sensitive to lift-throttle weight transfer
- Softer rear springs make it possible to put more power down sooner
- Softer rear springs make braking a bit easier too?
- Due to downforce, if you go *too* soft, you'll ground out the body on the tires
- Softer springs everywhere make the car less "sudden", and a bit easier to drive, for now
- I can effectively eliminate the *very* stiff setups from my list of possibilities
So, for next time, having found out some good basic info, I've got a new plan to try and arrive at some rates, basically what Caroll Smith says:
At least I have a few answers now - and they were fun to get.
- Determine Your Ride Heights - This, for me, is going to be based primarily on the emperical experience of others. Tom Robinson is running Stimola's zero-droop setup, and says he runs 2.0"/2.5". Several others have told me the same numbers - so it appears that the POS stock Radical splitter is basically the limitation here. Since Tom is running soft 250# front springs, since they are running at high speeds, that should be just fine for what my car will see - maybe 120 at the most - in terms of download generation. So, the ride heights I will try tentatively for now will be 2 1/8" and 2.5", and we'll see how things work.
- Find the softest rear rates that keep the car off of the ground at the ride height you want to run - having chosen a rear ride-height ot 2.5", how I can see what springs I'll need to keep the car off of the bodywork and the end of bump travel. It appears that 300 is insufficient for anything above solo speeds, which means next I will try 350, or perhaps even back to 400#. Could be the rear rates will be one of the things that changes between the solo'ing and road racing setups. Also, I'll have to balance the desire for soft springs against the need for decent rear camber control. Some tire temps with softer rear springs will be educational.
- Balance the car with front springs - next, I'll have to choose front springs that give the car decent balance given the bars I have to work with. Smith says to do this in medium speed corners, so as to get a nice steady state, and to remove aero from the equation (60mph corners or so). Perhaps I can try disconnecting both swaybars for this?
- Play with rollbars to get to "optimum" roll stiffness. Here, I'll try the stiffer front nik bar, and also see how changes to the rear bar affect balance versus the ability to get power to the ground well.
So what else? one important note: I got much more comfortable with the car today. By the end of the day I was pushing much harder in all the corners, getting much more comfortable with the car sliding a bit, and really learning the characteristics of my new vehicle as they differ from the Corvette. Next time should be even more fun, as the setup will get better, and the driver is improving a lot too! It bears in mind that I might even try the 'baseline' 350/400 setup again at some point in the future, and see how it compares to the various work I've done so far - a good A-B-A test to make sure I'm not going backwards...
- Additional front camber, less rear
- Jet changes to 45/178
- cornerweights set.
- played with springs a bunch