At the last autocross event I had three drivers drive my car (myself included) and it really exagerated a problem that I have been finding to be more and more of a problem. As I get more heat into my tires the more the car wants to push however the lap times still come down a little. My guess as to what is happening is that the fronts heat up giving more grip and better times but the rears are getting more heat too and causing more push.

My question is how would you setup the car to keep the car loose like the first run through all 3 runs?

Right now I'm running tire pressures of 40f/37r and I'm thinking of adding pressure to the rear tires after each run (as suggested by GhettoRacer) or even start with 40+ in the rear.

Mike and Matt: What pressures do you guys run?

 

rear toe and camber are how i do it. but i run a different setup.

didn't you just raise your rear spring rate?

nate

 

I run mid-30s all the way around, but that's for Hoosiers.

Do you bleed down your tires after each run? Try bleeding down the fronts and leaving the rears to build up pressure.

With my car I find that the first run is VERY loose and subsequent runs are neutral with oversteer on demand by a variety of means.

Mike

 

yup, 560f/672r

Toe is set to 0 all around
Camber -1.8f/-1.4r

I'm not as interested in changing my setup (well, maybe small changes) but rather keeping the handling consistant through all 3 runs.

 

I agree with RacerMike. Bleed the fronts back to the "desired" pressure, and let the rears keep going up.

Also, what is the frequency of the runs? It's quite possible that you are overheating the tires and simply spraying with water will solve your problems.

Scott

 

why not go with a little more aggresive alignment ?

1/16" out each side is still within factory specs and its night / day compared to 0....

 

Well I've only had one event so far on the rear biased setup (on snow tires!) so I don't think I can help much. I would agree tho to let the rears keep they're gained pressure.

 

I do bleed the fronts only to keep them around 40 and let the rears rise. I might try spraying the rear tires to cool them off if higher rear pressures don't work.

I want to keep as close to 0 toe as I can to maximize straight line acceleration.


[Modified by Geratol, 11:23 AM 3/24/2003]

 

the increased pressure on the rears may actually be adding a grip (moving toward the optimum traction point). Maybe you could try starting the rears above 40 and then letting them rise a little more till you find a good pressure? shoot for maybe 44-45 rear warm?

Just a guess tho...

 

This sounds reasonable. I'll try starting with 42 in the rear next time and see what happens.

 

...and because I like you guys and have way to much free time (don't tell my boss ) I made this graph of how I see what's going on. the first graph is the cold tires. Second is warmer tires. Green line is the point were the rear tires generate the optimal amount of traction to give you the desired amount of oversteer.



EDIT : Forgot the graph!

DISCLAIMER : There is no scientific backing for these numbers!!!!! Exact values may vary!


[Modified by Watkinsm3, 11:50 AM 3/24/2003]

 

i don't get the graph??

fwiw, i don't think small amounts of toe out are going to make any appreciable change to straight line accel. my car feels no different straight line wise going from 1/2" total front toe out to 1/8" total front toe out. and that is with _significantly_ less power then you have, ie, i should feel it easier if there were a change then a higher powered car would.

nate-who runs 1/8" total rear toe out as well...

 

Sorry nate... I didn't really explain it very well...

Vertical Axis is Grip (unquantified)
Horizontal Axis is Pressure (ranging from 30lbs to 50lbs left to right)

The green line represents the level of grip(target grip for the rear) where the car is neutral or has slight oversteer according to taste. Say you are running 40lbs on cold tires. The actual level of grip (indicated by the red line) is higher then prefered so in theory the car will understeer (too much rear grip).

Now what follows is my theory of why some people run higher or lower pressures in the rear to generate oversteer. According to my graph optimal grip would be located at the interection of the green line (optimal grip) and the red line (actual grip). you want (actual grip)optimal for driver preference). right? so there for we can either raise the pressures from 40 to 44lbs or drop them to 36.5lbs to get the car to oversteer the way we want it to. Running higher or lower pressures becomes a matter of personal preference.

you do the same thing with the second graph... except realize this is for warmer tires so the actuall grip of the tires has increased hence the red arc has been shifted up.

 

Just realized a flaw with the second graph....

Because the temperature of the front tires is going to increase they're grip is going to increase.... So inorder to maintain balance in the car we have to increase target grip for the rear so the green line will be shifted up as well... makes sense right .

New graphs...


We shifted the read line up because of the increased temperature will generate more actual grip. we also shifted the green line up (now blue) to match the increase in grip in the front tires and maintain our desired balance. this is a somewhat simplistic example. obviously tire type... suspension... etc... will have other effects on this. Just a general theory as I see it.

And FWIW : I don't think we need any toe to get the car to perform properly. again... my $0.02


[Modified by Watkinsm3, 12:18 PM 3/24/2003]

 

didn't you set the cold pressure at 36/35 at first? by the 3rd run i remember it was like 41/36. after ben ran it, it went up to 44/37 then we bled back down.

i would go with ~35/38 cold and bled as necessary. if tires are completely cold 1st run will always be looser than the next 2... i don't think it's possible to get 3 consistant handling runs unless you get a tire warmer.



[Modified by GhettoRacer, 12:19 PM 3/24/2003]

 

cold starting temps are about 38/35 but I bleed the fronts to keep them at ~40 (If I remember to). Next event I'll try the higher rear pressure setup.

 

higher pressure = elongated contact patch
lower pressure = wider contact patch

IMO higher pressure = more gradual breakaway's.

with lower pressure in rear i've experienced turn in oversteer -> power understeer. sucks IMO. see:

http://www.g-speed.com/eg2/tq-midohi...s/IMG_1435.htm


[Modified by GhettoRacer, 12:57 PM 3/24/2003]

 

Spraying down the rear tires worked well on my car when I was playing on the stock tires at a test and tune. I found that I was overheating the rear tires (judging by the iridescent shade of blue/burning hot to the touch/progressively less rear end grip as runs continued) during the event since essentially... You would come off the course, have 1 minute in grid before pulling back up to the line. Spraying down the rear tires gave them back a bunch of stick. In your case, they might take away enough stick to give you the handling balance you need. Plus it's cheap, and if you've got a codriver... the least they can do is water your car for you

 

I'd try spraying the rear tires with ice cold water as well as the fronts if on the 1st run it felt right and on subsequent runs understeer rears its ugly head. This is what happens on my car. I think the rears are heating up a little and getting too sticky (and/or the fronts are overheating and washing out).

On a different note. I think you should go ahead and put some more oversteer into the car if you still have shock adjustment room. I tried my car with street tires (crazy!) in the back this weekend and car turned beautifully.

 

Actually I'll agree with you on some of these points. There always more to it tho. I personally agree that higher pressures result in, not just more gradual, but much more controllable oversteer. In my eyes a soft tire will just be flexing and deforming and doing who-knows-what in a turn.

Just throwing out some ideas of course...

 

I find that over heating the v700 to the point it starts to fell greasy gives the most predictable and forgiving break away. It almost feels like a set of cheap street tires with higher g's but w/o the squeel. Downside is it tend to drift excesively during turn in and corner exit and requires a change in driving to stay on line. At this point, will it be of help to fiddle with tire pressure?

 

are you talking about autox conditions still, or road racing? in RR once a tire feels greasy the grip level is reduced so it seems normal for the breakaway to feel less abrupt. in my experience all R tires breakaway pretty nice and easy once warmed up sufficiently (even Hoosier's).

in RR once a tire feels greasy there isn't much you can do in that session but drive easier and with more finesse to get the most out of it. pressure won't do anything unless it was excessive. i imagine you already set them up hot so, nothing to be gained there...



[Modified by GhettoRacer, 1:47 PM 3/24/2003]

 

i c, thanks.

 

if you have 5 tuning options available to increase oversteer, wouldn't it make sense to use a little of each, instead of a lot of one?? for example, i use a combination of spring rate, tire pressure, toe setting, camber setting, and shock setting to get my car to rotate. i feel strongly that the car wouldn't drive as good as it does if i just tried to do it with springs and tire pressure.

my car drives very consistently throughout the day and even throughout a run. the only time i really have an issue is on my first run on a cold morning. my launch tends to heat the fronts up a lot, then i hit a slalom and the rears don't quite have enough temp yet.

nate-who thinks a consistent handling car is more important than ultimate grip, but just barely

edit: shpelling


[Modified by solo-x, 10:10 PM 3/24/2003]

 

Do some testing.

Do some consecutive runs without adjusting pressure and check your hot pressures at the end of each run. Whatever run works the best is the pressure target you know you need to bleed to at the start of each run. The amount of pressure build-up will naturally depend on the weather and the surface, but you'll figure it out.

Hot pressures are kind of important. Its what I always target.

Scott, who was driving a DSP Integra when it was a CSP Integra .