Losing tire pressure during high speed autocross
 

This past weekend was our big local event by the Canada border at an old air force base.

Car is a 2004 g35 with 275/35 re71rs on 18x10.5s. Rsr springs on stock "track pack" shocks.

Before my first run, tires were 36/33. When I got back to grid, they were 26/28ish. Aired up, codriver went out, same thing. I ran out of air in my tank on the 2nd runs and spent the rest of the 2 day event with a borrowed air compressor, putting them at 41/37ish and it dropped to 33/33. Was a pita. And car was never set up right all weekend.

I've been running the tires and wheels for 5 events already and about 3k of street driving, no issues. Our local lots are small and hardly even get to the top of 2nd. This event was mostly 3rd gear courses, sweepers around 4.5k in 3rd.

The tires were a pita to mount. I took them to a buddy's shop and we mounted them. We had to use seam sealer to get them to seal. One guy said that might be causing the issue.. I just flipped 2 of them. Other two are the same as first event.

Tires hold air all night and on the street.

What do you guys think? Seam sealer? Too stretched? Are the springs and shocks causing too much load on the outside tires on speed sweepers? Have you ever even heard of this?

Something difficult about matching tires and wheels sizes appropriately?

Thanks, I guess. Tire rack lists up to an 11" wheel. They aren't visibly too stretched in my opinion. Am I missing something?

What kind of tires and wheels?

I've never run into this problem on road course (nor have any racers I supported back in the day). I've also never had to use (nor seen in my or anyone's tire shop) a "bead sealer" before. Have heard of it but it's not something a typical shop will have on hand.

I'm suspecting something to do with the wheel bead. If you had problems seating them, something's amiss as the tire/wheel sizing is OK.

Autocross does put a much more severe lateral loading on the tire/wheel assembly but again, never seen an air down like that.

Is this too stretched? Seriously looking for an opinion.
https://cimg0.ibsrv.net/gimg/my350z....4a10222bfc.jpg

Without even looking at the pic, you are technically fine.

There's two tire "laws" one should always adhere to:

1.2 Factor: Tire width should NOT be more than 1.2x wheel width for maximum performance and....

...the converse, the "90% rule" where tire width should NOT be less than .9 for max perf.

You can physically run a tire under or over the above but optimal performance is difficult to achieve outside of these parameters - in a performance car/tire application. In normal commuter car situations, you rarely see these rules adhered to.

You're fine...

275 = 10.8" or easily within this range on a 10.5" (266mm) wheel.

You shouldn't have to use "seam sealer" on a proper tire install.

I'm just curious ... is there a low-speed autocross?

Sadly, yes. We never ever get out of second gear at my local region autox :icon22:

if they are like here in TX they are usually just in big parking lots. most of the time second gear, maybe third on a long stretch

Average speed was around 60mph for both days. 1.4 mile course at mid 80 second runs in the fast guys from NER. They had to tone the course down this year because some of the power cars have been hitting 100+ in the fast sections the last couple years. Our local lots are tiny and I rarely get to the top of 2nd. We used to have a nice really local air base, but idiot spectators got us in trouble.

same place a couple years ago in my old Evo. back sweepers were well into 3rd gear (5spd). Car made 400whp, 285 ZIIs and 12/14k rates.