PHEOX

I just learned a lot from this page I think, or maybe learned things that I already knew but wouldn't have been able to identify and explain. Yay internet arguments!

FrankL

How'd you do yesterday?

Leafy

iTrader: (1)

2nd. Lost to the Elise on a hero run. I beat 50% of the drivers of the panda though. It was cold and I had no tire warmer. They felt great but I was trying a new set of tire pressures because this set of tires has been showing some weird *** wear its whole life, I was able to put the car closer to where I wanted it with a lot less effort. Who would have thought that actually following the hoosier tire engineer's recommendations for pressure would seeming be good. I just looked at the data quickly and at some spots I'm still able to find the same level of sustained g's but not in others where I normally would have. The whole event I kind of felt like I was under driving and I just couldnt really get into it mentally to push it in the spots I needed too.

TNTUBA

iTrader: (1)

So you went all the way down to 22-23 PSI? I've not ran that low....but close.

Leafy

iTrader: (1)

No I got up into the range of what hoosier tire recommends for my corner weights. So like 34 front 33 rear or there abouts. I think this was the first time ever I was able to sustain a full effort turn while full on the gas without having the front wash out. I wish I had better data analysis software so I could plot some friction circles to compare last week to this week, I feel like the gg plot moved from football shaped to more like rugby ball shaped.

I did just get some free Davis takeoffs that set top pax at our event before nationals, ~100 runs, that I'm going to swap on since there's almost some cords peaking through on the passenger's side tires.

TNTUBA

iTrader: (1)

I thought Hoosier's formula for a Autocross car with IRS was total vehicle weight including the driver / 100.

Leafy

iTrader: (1)

1800-2200 lbs hot pressure 30-33+, I took front and rear axle weights and multiplied by 2 to get the vehicle weight on that end to get an estimate of a starting pressure for each end. And it comes with the note "+Higher pressures will improve the performance capability but will require a more sensitive feel to take advantage of the
increase." The formula you're thinking of is the minimum safe pressure that probably wont cause tire damage.

TNTUBA

iTrader: (1)

Reading is fundamental:

The above chart is a general recommendation which is intended for a standard configuration vehicle (i.e. front engine, rear wheel drive).
Factors which can radically affect your pressure set up would include front wheel drive, independent rear suspension, rear engine, McPherson
vs. control arm front suspension

Then two paragraphs lower you will find this

Independent rear suspension
With IRS and proper geometry up front, tire pressures can be reduced from the recommendations listed above. When there is adequate
camber gain and good roll control, the Hoosier radial tire will perform very well at the reduced air pressure. This results in a bigger
“sweet spot” and easier control at the limit.

But if 33 is working for you...that's what you need to run. I was just looking for clarification.

Leafy

iTrader: (1)

But you missed "One characteristic of the tires is the tendency to “skate” initially (when inflation pressures are correct). It is important to resist lowering the
pressure to attempt to eliminate this feeling. Dropping the pressure too far may improve the “feel” of the tire however it will also lower the
performance and increase the wear rate." I didnt get this skate feel unless I was really jerking at the wheel sunday.

We are referencing a pretty old write up though, I just noticed it when I finished reading the end and they talk about using dirt stockers as rain tires. I know there's been at least 1 maybe 2 compound changes since that dirt stocker info would have been accurate.

TNTUBA

iTrader: (1)

No. I didn't miss it at all. I read the IRS portion of the page as superseding the earlier mentioned setup advice for "live axle" cars.

Leafy

iTrader: (1)

Thats quite an assumption to make, since it doesnt say that at all. I might be implied but I would find it weird if they considered the standard configuration for a road race car to be solid rear axle. There's actually no mention of live axle of solid axle in the whole document.

On another note. My fuel pump wiring is melting. But only between the 15amp fuse and the relay. Its perfectly fine everywhere else. The relay is also real hot. The relay coil power wire actually is burnt and crispy and as an 18ga control wire I find it hard to believe it got that hot without popping the 15amp fuse. But even the 12ga power wires got hot enough to get melty. I might have finally gotten to the bottom of my hot car breakup problems. What weird is that I've been fighting this problem pretty much since it was an STR car and at this point literally zero of the original fuel pump wiring exists, net even the in tank wiring and the tank bulk head connector are oem. Like WTF?

TNTUBA

iTrader: (1)

If Hoosier didn't think IRS ............You know what....nevermind. id have better luck talking to a mentally deficient brick wall.

Like i said....if 30+ psi works for you...that's what you should run.

FrankL

30 psi on 100+ run tires. Looks like the Davis car outran you by 2.3s. I wonder if they had 30something psi.

TNTUBA

iTrader: (1)

Just to be clear. Im not trying to be a dick. Im trying to help you. Ive already dealt with a lot of what you are struggling with. Im not going to spoon feed you......but I dont know how much more obvious my hints could be.

Leafy

iTrader: (1)

2.3 is closer than I normally am even when there isnt a massive tire disparity. And to preemptively counter the inevitable strawman, I never said that high run hoosiers were as fast as sub 40ish run hoosier, just that they were sufficient.

mcfandango

iTrader: (1)

I thought the right way to set tire pressure was with a pryometer. That is how I have to do it with the bias ply slicks. Start high and go down until the temps across the thread come back mostly even (with an exception on the rear inside due to some toe in). And then check the sidewalls aren't getting raped and call it good. Oddly enough, it does tend to end up ~weight of vehicle (car+driver) / 100. Weight bias also factors in. Usually requires +1psi on the heavy end.
It's been a while since I ran the A6s but I was pretty sure the answer there ended up in the 25-26psi range.

codrus

My experience was that my car got faster the lower the pressures I ran on my A6s, all the way down to the 1-psi-per-100-pounds lower limit (26.5 psi on my car -- it's heavy). I was running 225 Hoosiers, though, not the 275s.

I did get odd wear patterns on them, though. It wore the most just inboard of the outermost groove, which I've seen referred to as the "hoosier groove 'o doom". I still got over 100 runs on them before they corded, which is plenty.

--Ian

WAM

Actually, the right way to set tire pressures is with a stopwatch. A pyrometer might get you close, but so can the Hoosier spec sheet. Unfortunately stop watch development kind of requires nice test events. Hard to do at a 3-4 run autocross.

One question for the two pugilists here: Have you both found that optimum rear pressures tend to be about 2psi below the fronts? I'm running 30/28 on A6's @ 2500lbs. Sounds like I can go as low as 25/23 looking for the sweetspot? If I do it, I'll probably do it in 2psi steps.

And TNTUBA -- I kinda read the spec sheet the way you do but not sure it makes sense like that. The IRS advantage cited was the ability to run negative camber, which clearly invites lower REAR pressures. Not obvious to me why the front end of an IRS car is now happier with lower pressures than it would have been with a live axle back there.

Leafy

iTrader: (1)

With how auto-x loads tires I dont really trust any pyrometer data correlating with grip. On a skid pad? **** yes, lets pull out the pyrometer and measure the **** out of the tire to work on pressures and alignment. But at an auto-x its going to lie to you. For example, at just under 3° of front camber the tires are in their happy place based on the pyrometer after an auto-x run, but as long as there's not a lot of heavy braking the car keeps getting faster as you add more front camber. I personally haven't gone to silly numbers but I know Guy Ankeny went to some completely absurd bonkers numbers (like 5° iirc) on an S2k while doing testing and the damn thing kept on getting faster and was already well beyond the amount of negative camber the pyrometer was saying to run. Of course its not a miata, but no one has ever made a suspension setup for these cars capable of pushing the front camber that far. Even my new arms are going for like a max possible number of 4°.

When I was run 27 psi, I run it square, but its somewhat pushy, I tried dropping the front a bit more and it just made the push worse. Now trying the higher pressure with a 1psi less in the rear feels like a good balance on similar tires that would give the slight push at 27psi. Now that goes out the window with sub 40 run tires which just give an undriveable push at 27 square and lower in the front makes it worse, so does higher in the rear. I never even though to try higher pressure back when I had the tires that new.

TNTUBA

iTrader: (1)

Yes. I typically run 2psi lower in the rear unless its a pro solo