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Posted by: Joseph Mills Jun 15 2003, 08:49 PM

At the last AX, after several runs, I went from 36# to 30# for the front tires (Hoosiers). At 32# and above, the inside was about 5 deg. hotter than the outside. Below 30# the outside started heating. At 30# they were about 117 deg. all the way across the tire. Good. And the car does not push.

The rear tires were about the same temps. However, even when I went as low as 26#, they are STILL about 6+ deg. hotter on the inside. And at 26# the car was starting to get a little loose, but easily controllable. Below are some things I've considered, but really, I don't know where to go from here.

My car does have 1/2 deg more neg camber in the back than the front....

It was a short AX - around 38 sec's with some straight ahead driving - which would tend to heat the inside tread....?

Maybe I need a faster/longer AX? Or can you only do proper readings on a skid pad or figure 8 only....?

Could the engine heat up the inside of the tires? Usually spend 5 -10 minutes with engine idling and slowly creeping up the line.....???

My "pyrometer" is very cheap, but seems to be very reliable with repeated readings.

Any ideas out there??? HELP!

Posted by: Racer Chris Jun 16 2003, 07:23 AM

Lowering the pressure to get the outside edge of the tire to heat up doesn't seem like a good idea to me. The tires work best within a certain pressure range. You will have to alter your car's camber settings to even out the tire temps, then fine tune the pressure so the middle is not more or less hot than the sides. A 5 degreee temp variation is not something you should worry about. At an auto X the temps won't get high enough to be real accurate anyway.

Posted by: john rogers Jun 16 2003, 10:19 AM

Tire temps seem to be a black art, almost. Here's what I have noticed over the years: many PCA 914 auto-x'ers run pressure too high, such as 34# or even more but get even temps across the tire. Many have gone much faster when they lowered pressures to the 26# range but get uneven temp readings, usually the outside is getting too hot. This temp increase can be from some tire roll-over and the fact it is gripping much more since the foot print is larger. To compensate more negative camber (varies as to which tire) must be used but since alignments are expensive they do not get done. You need a person to check the tire temps for you immediately after your run and if possible use a probe type pyrometer but any type is better than none. 914s can get some heat build up on the rears if you are sitting in linefor extended periods and this can raise the rear tire pressures a pound or two which can affect the way the car handles from one run to the next. A very accurate tire pressure gauge is a must so you will know exactly what the tires are doing. As Chris said, a 5 degree difference is fine but if you get over that then tunning is needed.

Posted by: Joseph Mills Jun 16 2003, 12:15 PM

Thanks Chris.

Several emails I got off-list suggested that I try a skidpad for more accurate readings. And from reading your comments, maybe this is what I need to try.

I know I need to use a "tall" gear to minimize the effect of throttle on a skidpad, but that's about it. Do you have any suggestions or tips?

What size diameter should a skidpad be anyway? Does it matter if I take readings across the tire, make adjustments and go on, or should the tires cool back down some after adjusting before another run?

Posted by: Don Wohlfarth Jun 16 2003, 02:29 PM

For ax you should be within 1-2 pounds of the recommended HOT pressure for your first run. You will not generate enough heat to get the tires up to the ideal temperature. Hoosier's usually like 40 hot +/-. When you use lower pressure trying to generate heat you are forcing the tire to do something it was not designed to do such as letting either the sidewall or the tred have excessive flexing that is generating the heat. Using the recommended hot pressure will help to eliminate this strange heat source.
Hot pressure is built up by the shearing action of the pavement shearing the rubber from the tire. The recommended hot pressure will not allow the sidewall to flex and will help to keep the tred flat across the pavement.
The "black art" comes into play when we have lightweight cars with oversized tires and to a certain extent suspension settings. With a larger tire we may not have enough weight to flatten the tire forcing us into all kinds of strange setups trying to generate tire temperatures.
Ax just makes it that much more difficult because the run times are so short.

Posted by: kdfoust Jun 16 2003, 02:29 PM

I don't see a problem with measuring tires temp at an AX. I've got a Longacre memory pyrometer and while I haven't done a gauge R&R study on the test my temp readings pass the test of reason. They are remarkably consistent with all conditions being equal and reflect changes in tire pressure and speed in a logical way. Like John indicates I too have noticed that people seem to run really high tire pressures compared to what I've found on my car by tuning the pressure with a pyrometer. While my temps haven't been even across the tread, the temp progression across the tread has always made sense to me. Now that I've got better camber adjustability and a camber gauge I plan to really work/optimize the pressures and camber settings on my car.

Now all that said, I'd love to find a skid pad or someplace that I could treat like a skid pad for an afternoon. Given a skid pad I'd run the whole thing as a DOE using pressure, camber, and toe for factors and grip (time around the pad) and tire temp as results. That would be cool. Anybody know of a skid pad in SoCal (that I can play on)?

Have fun,
Kevin

Posted by: Joseph Mills Jun 16 2003, 06:23 PM

Whew! This is a wealth of info and observations! It's definitely made me think and reconsider options.

John Rogers, you are right on with the black art comment. Your overview is somewhat similar to what I am finding at my local level. Lighter cars using lower pressures going faster. Usually - but not always

Don Wohlfarth, after reading you post, I perused the recommended tire temps for Hoosiers. They lump the track and AX tires together when giving information - that seems odd. Anyway, for my weight of car (2k#), the recommended pressure for cold is 31-36# and 39-42# for hot. Is the cold setting just the point to start so you end up with hot pressures near their recommendation for hot pressures? Or is the cold settings the pressure they work well at with lesser temps?

If so, since my tires never get near track temps of 180-200 on an AX course, it would make sense to run them at the recommended lower pressure of a cooler tire. I'm not arguing this point, I'm just trying to fully understand the basis of the recommended pressures.

They do warn about running tires at too low a pressure.

Posted by: Don Wohlfarth Jun 17 2003, 06:49 AM

"Don Wohlfarth, after reading you post, I perused the recommended tire temps for Hoosiers. They lump the track and AX tires together when giving information - that seems odd. Anyway, for my weight of car (2k#), the recommended pressure for cold is 31-36# and 39-42# for hot. Is the cold setting just the point to start so you end up with hot pressures near their recommendation for hot pressures? Or is the cold settings the pressure they work well at with lesser temps?"
All comp type tires need a couple laps to come up to "recommended" temperature. The F1 guys use heated blackets on the tires so when they hit the track they're ready to go. When drivers are on a 1 or 2 lap qualifying run they hit the track with the recommended hot pressures and that's one of the reasons the 2nd lap is usually slightly slower as the tire has exceeded the recommended hot pressure.
When you see racers zigzag across the track for a restart it is not to warm the tires but to try and scrape loose rubber from the tred that they may have picked up. The cars are going too slow with very shallow turns to have any effect on tire temps.
With ax your runs are too short to properly warm the tires. Start out with recommended hot pressures for 1st run, then check pressure after each run to keep the ideal pressure. As every ax course is slightly different you may have to add or subtract pressure to help with understeer/oversteer and balance the car.
I don't think it's odd that Hoosier has the same press/temp for track/ax.
They design a tire for the track and figure out what the "best" pressure should be to get the tire temps within a range, usually 160-200 degrees. The ax tire has a softer compound for the tred but the carcass of the tire is still the same. You can use Hoosier track tires for ax but you should not use ax tires on the track as you will overheat them and they are not speed rated.
A tire with a cold pressure of 31 will not handle as well or have as much grip as the same tire with a recommended hot pressure of 40 which will allow the tire to get close to the proper temperature. When you end up running a hot pressure of 31 you are putting a band aid on a setup problem and forcing the tire to flex to generate the heat necessary for good traction.

Posted by: john rogers Jun 17 2003, 03:00 PM

I noticed something I thought was unusual at the last NASCAR race I was at at the CA Speedway. When the tires came off of Jeff Gordon's car after racing laps, usually only 75% of the tread was really used and that was the side on the left side of the car. I know the left-turners use and unusual camber setup to help flatten the tires in the turns, but not using full width of the tires seemed strange. I asked a couple of Goodyear reps and they said that's the way they set'm up?

Posted by: Joseph Mills Jun 17 2003, 04:01 PM

Tell me the truth now.

Would this be over-kill for an autocross???

Posted by: Don Wohlfarth Jun 17 2003, 04:23 PM

It's enough of a problem that it's specifically prohibited. SCCA National SOLO 6.11
Still think it's overkill?

Posted by: J P Stein Jun 18 2003, 12:32 PM

SCCA AX guys are serious players at the national level. Where else can you see 5-6K worth of Penske
shocks on a "stock" car. Tire warmers are a short step for these guys.

I have seen them keep em' hot pizza delivery containers used to keep in what heat is there hung over tiars. It may be worth .05 sec or sumthin'.

Don:
I sorry you have soured on AX. The non-work, just drive deal ( that you've mentioned a couple of times)
seems to be a PCA problem. SCCA don't put up with that BS.....good way to get banned for a few events here locally. There seems to be a bit of that at the PCA events tho.....the "my shit don't stink cause I have a Porsche" syndrome. 95% of the folks play by the rules, tho.

Posted by: Brad Roberts Jun 19 2003, 04:39 PM

I'm with Don on this one. Hoosiers work best around 40 degrees anything below that and your just screwing up a set of nice tires. I use Hoosier road race tires at AutoX's (after we are done with them at road racing) they work really really well.

B

Posted by: EPK Jun 19 2003, 09:12 PM

Interesting remarks regarding tire pressures but they are all pressures for Hoosier tires. What sort of air pressure experience does anyone have with Kumho Victor Racers V-700.

Posted by: J P Stein Jun 20 2003, 12:03 PM

I run (ran) Vracers between 28 & 32 degrees.

Since Brad says so, I'll try 40 degrees on my Hoosiers....what can it hurt? Maybe the tread won't get all melty like as it does at 30-35 degrees. They are gettin' all ruint.

Whut a great thread. I'm finally begining to understand the black art of tiar temps/pressures.

Posted by: ChrisReale Jun 20 2003, 02:07 PM

Between 28 and 32 degrees or pounds? Is this tiar lingo?

Posted by: J P Stein Jun 20 2003, 02:12 PM

Don't ak me, I'm just trying to communicate/fit in.
Don't even know if it's C or F.

I'm just glad I learned about this before I ordered
the 200 degree springs.

Posted by: Bleyseng Jun 20 2003, 05:00 PM

Come on JP, shake your pacemaker so more blood goes to your brain. Tire air pressure is in lbs not degrees.

Hope you make it up to the race, it will probably rain so bring a effin coat.

Geoff

Posted by: Joseph Mills Jun 20 2003, 06:18 PM

JP - You'l need ice packs (hangover thangs) to cool your tars down ta 40 degrees.

EPK - I have a friend that runs the same tire @45F/50R

The question that begs to be answered is who is faster? The guy with skinny tires @ 40lbs which get hot & sticky, or the guy that's over tired & under inflated, but with a big-ass footprint?

I'm gonna spot check different cars this weekend for tire temps. I wonder if even the skinny tires get hot. It will be a small course this weekend due to some outside restrictions, BUT there will be two continuous laps for each timed run. Should be fun.

Becoming more determined each day to find a skipad.

Posted by: ChrisReale Jun 20 2003, 10:18 PM

Go to a University Parking lot on Sundays

Posted by: Don Wohlfarth Jun 21 2003, 01:11 PM

"Come on JP, shake your pacemaker so more blood goes to your brain."
LOL
Joseph, there's no easy answer. Guys run a lot of goofy shit in ax looking for the easy answer for faster times. Some may work while others won't. One of the reasons some thngs work (or seem to work) is the basic idea of ax, the course is never the same, average of one minute runs, 3 runs and you're done. It's a test of driver skill on that particular day. Even the best drivers have good days and bad days.
If you could make 10-12 runs for 3 or 4 days on the same course you could come up with answers to your question. But then it would be just like running de's.
You never really have the course nailed and a result it is often who makes the least number of mistakes, not necessarily the fastest.
The car that is overtired and underinflated will not be able to take advantage of the potential of the tire. With a underinflated tire only the inner and outer edges of the tire will touch the pavement, there is not enough pressure to force the center of the tire to lay flat. If the tire is overinflated only the center of the tire is touching the pavement.
That's the reason you want to run your first run with a cold tire at the recommended hot pressure and bleed down after each run. Otherwise you will never get from 31-32 cold to 40 hot on a ax. The runs are too short.