QuestionQUESTION: Hi Dan -
I am a rather advanced driver of an open wheel car, heavily data acquisitioned. High end autocrossing, so the car is constantly turning, braking, or accelerating. My front suspension geometry is almost infinitely adjustable for all factors. Next year I am going to add a set of three IR pyrometers watching the front tire temps while on course (pyrometers mounted to the upright to stay with the inside/middle/outside of the tire). Rear tire geometry can only be changed for camber (<whisper> "camber cut")
I assume that the key temps will be from an "average" turn, and while the turn feels like it was "well done". Is that correct?
Tuning tire pressure and camber is easy, I have done so on a skid pad with a friend quickly measuring temps (probe). On camber, "common knowledge" is to have the inside of the outer front tire slightly warmer than the outside - is this correct?
More importantly, I have read various places that this setup can help me to adjust my toe (now 3/16" out) and caster (now quite a bit). But no one says how. Do you know how?
Thanks!
Chris
ANSWER: Hi Chris:
Go back and read the part about Ackerman effect. If, with extensive measurement , you find that your two front tires are not following optimum paths through a turn, you must adjust the length of one steering arm or the other to correct this. It's complex because the turning radius of the outer tire is greater than that of the inner one. And if it changes from side to side, your problem becomes even more complex. Even more so if it changes depending on amounts of steering input !
If on the skid pad, you determine that your temps differ from side to side when you reverse directions, you must find the cause and correct it. This will give you more symmetry in corners, and a better feel.
I thought your 3/16" of toe out was excessive, but then I haven't run a solo in 40 years. You might talk to friendly competitors for input, and contact the chassis manufacturer for their advice. If it were me, I'd start with a 1/8" toe out. Then check toe at increasing steering input angles. If you have the wheels turned say 6 or 7 degrees and find you suddenly have a "0" toe (due to Ackerman), you must correct this. With "0" toe most cars are very unstable. When you hear "tight in the middle" during a NASCAR event this is sometimes the cause.
Using temps to arrive at a caster setting would require that you watch for temp changes during the cornering process. If you have notable temp gain on the inside surface of a tire, you have too much camber gain, due to too much caster. If you have a temp gain on the outside, you don't have enough camber gain, and will benefit from added caster. But if you add too much caster, the car will wear the driver out , it adds steering resistance. Remember the rule though "Caster is faster".
For static camber setting I guess I would monitor tire temps immediately on exiting the critical corner. The inside edge of each tire should be about 15 Deg. F warmer than the other two readings, and yes, on a skid pad, the outer tires should be a tad warmer. The center of the tire should read very cloet to the outside edge, this tells you that your pressures are correct.
I usually refer people to the book "How To Make Your Car Handle" by Fred Puhn, it's quite dated, but much of the info is still valid, and it's presented in an easy-to-understand format. There's a chapter on Ackerman effect.
I hope this is more help than hinderance, you've certainly exercised my wrinkled old brain this day !!
Good luck
Dan Liddy
Sarasota, Florida
---------- FOLLOW-UP ----------
QUESTION: Thanks for the good info, Dan! My car is symmetrical left and right, steering rack is in the middle. And the inside tire does turn a fair amount more than the outside, as most autocross turns (that we care about) are rather tight. And in my class, 1/8" to 3/16" is rather standard (measured from a line scribed in the middle of each tire, 55" outer to outer edges of tires, tires about 9" wide).
I've never measured toe while the wheels are turned - it is clear from looking that the inside tire is turning more. I'll do it for fun, though.
Setting caster this way sounds like the ticket. Though my only limiting factor, speed-wise, in a slalom 19 paces or more between cones is my ability to turn the steering wheel. . . And I do weight train for this!
As far as using the temps on corner exit to set camber. Hmmmm. I know that from mid-turn to corner exit the temps might drop 50 or even 100 degrees in this very short time. Why not use the mid-corner temps? Isn't this the time I want the best grip, and want to optimize for?
And toe. . . Toe-out I should expect to see "some" higher temps on the outside of the tires during a straight? Some, but not too much? Any rule of thumb?
Thanks!
Chris
AnswerOK In Re: Pp 1 & 2
The important point is to determine that the two turning radii are tracking the path of the turn. Each wheel separately follows a different track and that track should be ideal for that corner. SO, as we've decided the outer must turn a little less. Or rather have a slightly reduced steering angle.
You might try rolling the car through a few sample turns on smooth sand and evaluating the skid and slip patterns that result.
Pp 3 Id say that once you get to 5 deg. of positive caster, you've probably got enough. Extreme settings tend to create gremlins in strange places. e.g. too much caster cold cause an oversteer due to the fact that the front suddenly has too much grip, more than that for which we can compensate. If the car is difficult to steer in reverse, you probably have enough.
Pp 4 , I don't think you'll see anything quite that dramatic. But certainly grip in the center is most important. So yes. you would want to work toward maximum grip in that tiny time frame. Again, I've been on long tracks for a long time and what tire temps I get I usually get in the paddock after my first bottle of water.
Pp 5 Well, probably not, because the scrub will be minimal, and there will be less weight on that part of the tires, due in part to the negative camber. I don't know of anyone that takes temps to determine
a proper toe setting.
The four things that I recall that made me faster in my early years were:
1) Correcting the camshaft timing through trial and error. and it wasn't anything LIKE the mfgrs. recommendation.
2) Switching to a slightly narrower radial tire to reduce rolling resistance. I use Hoosiers exclusively. I'm carrying out a vendetta against Goodyear.
3) Moving my braking point back (earlier) by about a meter for each of three very sharp corners at Sebring on our short course. I can carry some push through a corner, but it scrubs off speed. Take it easy.
4) Discovering tire temps and camber settings on a very cold day during a Solo I event, same course. One adjustment gained me nearly 3 seconds a lap!
So while dabbling with minutae is important, it's not as vital as getting a good basic setup, and concentrating fiercely on your goals.
Again, Good luck
Dan Liddy
