schaibaa

Hi Guys, just wondering if anyone has some sage advice.

I switched to Hoosier R7's last year and can't get the pressures right. They seem to be great between 31-33psi, maybe as high as 34. After they hit 35 I lose a ton of grip and it gets a bit dangerous! I'm starting them out around 25-26psi, and bleeding air before every session, starting them back down at 25-26. It seems like after ~15 minutes on the track, I'll have them up to 37 (mostly the fronts). The problem is that it takes me ~7-8 minutes to get them up to 33, which is where I am more comfortable pushing them.

Right before the next session, they are usually 28-29 and I will bring them down to 26 and repeat the cycle. It's much less of an issue for a 20 minute session, but I have to pull off and air down for a 30 minute session.

Can I start lower than that? Can I start at ~23? Am I being too cautious not pushing the car when under 30psi?

SilveradoSS500

30 hot, whatever you need to do to get to 30 hot.

schaibaa

Thanks man -- so how low do you start yours? I take the outlap easy, and put 2psi of heat in them each lap. I just read a lot of warnings about running them too low - at 20psi do I need to be concerned about rolling them off of the rim or rotating them under braking?

Rookieracer

Are you using nitrogen?

schaibaa

I'm not, probably a good idea though. Not sure if it is easy to get a tank filled, but that's probably my best bet.

Bill Dearborn

Hoosier recommendations say to divide your car weight (including driver and fuel) by 100 to get a reasonable hot pressure. That probably puts your hot tire pressure around 37 not 30. I have been running A6s and A7s and usually start with a cold pressure of 28 front and 26 rear with hot temps coming in right around 37 at all 4 corners. The camber on my car is close to -2.0 front and rear. Since I am not running at least -2.5 deg camber as Hoosier recommends I am wearing the outside edge more than the inside edge.

https://docs.google.com/viewer?url=h...y%2FHO0914.pdf

Bill

schaibaa

I have read their recommendations but the tires seem terrible at 37psi. They seem to lose a good amount of grip and the slip angle gets weird - meaning around 33psi I can slip them around smoothly but at 37 they grab and let go with much less predictability.

I did feel like 30psi felt a bit better than 33 but it was more subtle and I was a bit worried about the tires that far from the Hoosier recommendation of 37.

lawdogg149

buy some pirelli's. Scrubs from GT racing tire. You want ever look back.

Starting at 24 psi should not be a issue. I usually keep the tire pressure menu up on the screen and pull in once they reach 36 PSI. Usually mid session. Keep my pressure gauge in console I bleed down on pit road to 30 psi and back at it.

Poor-sha

Usually you can "lease" the tank where you pay so much up front and then they do exchanges. It costs me ~$35 to refill one of the smaller tanks and they last a long time.

In order to do it right though you need to pull a vacuum on the tire, then refill with Nitrogen. Ideally repeat that process a couple of times. I wonder if the shop that mounted your tires had an air dryer and it might just be a lot of moisture in the tire.

I usually start at 26 cold and they usually peak around 35 PSI which is about the max you want them. As you know though I'm tired of cording the outside edges so I'm going to stick with Michelin's for a while.

Anybody want a set of 295/18F 345/18R R7s with just two sessions on them? $1200 plus shipping.

SilveradoSS500

30 HOT guys, I'm a tire trainer and train with many pro race car drivers.

My R8 with R7s at COTA, I start at 23 cold. There are so many factors at play.... Temps, Type of asphalt, track, driving style blah blah blah.

For example, my front right tire at my local track is always 2psi higher, so I start that tire 2psi lower than the others. Whatever you gotta do to get to 30 hot for a semi slick. 32 hot for a street tire. 30 or below for a real slick.

Straight line braking will heat up your tire the quickest and don't force the heat, heat up
the brakes and tires equally.

Bill Dearborn

I know what you are saying but they do address it in the literature. They say the tire will feel like it isn't sticking as much but not to lower the pressure. Typically a tire that loses grip at a certain radius/G Force just moves out to a wider radius where the available grip matches the G Force being generated. It may not be as progressive in warning you as a tire with a lower pressure but it still isn't a huge increase in radius.

Bill

CitationZ06@yahoo

Do you have moisture in the tire? I don't use nitrogen but I did add moisture separator on my air compressor. Just deflate with valve down to remove water and then start over with dry air. That should provide a more stable reading.

I wonder if the guys that put your tires on used a bunch of water/lube to mount the tire and a cup of water is in the tire?

Long story but I took a nail in a DR and it was squirting water between the tread and floor and hit my leg. Took off all 4 tires and 2 had a cup of water inside. After that my burnout tire pressure was much more stable. Now it kills me to watch the tire shop lube up the tire with crap before mounting.

dvandentop

good info to know thanks

schaibaa

Regarding water in the tires, good question - I'm not sure. I don't think it would be hard to mount the hoosiers on 18" wheels, lots of side-wall that is pretty soft. I'm debating switching them out for my next track day, I have around 20-25 heat cycles and can definitely feel it. I'm also just interested in running cycled out hoosiers to get a better at car control. That said, I did a few rain days earlier in the season and that helped a lot with car control!

Maybe I'll swap the tires out and have them inflate with nitrogen initially, and then do a tank rental after that.

I was also considering tire warmers - I thought I remembered a post where a company makes a set you can drive over. Not sure how it would impact tire life (reducing heat cycles) but it would definitely help stabilize temps. A set of tires is ~1,800 so if you could get 25% more out of them, doesn't take that long for it to make a lot of sense financially.

X25

TL;DR: We use much wider tires than OEM, which (GM) in turn likely uses wider than dictated for load specs for better performance. As such, Hoosier's recommendations are very likely overkill.

As far as I know, Hoosier's recommendation with that formula is actually COLD, not HOT. Yes, that would mean Hoosier wants you to run 37 COLD, possibly hitting 45+ HOT, and they are OK with it exceeding 44 PSI (the max cold pressure listed on the tire), since that limit is for cold, and accounts for pressure increases that happen as the tire gets hot.
https://trackdaytire.com/wp-content/...re_And_Use.pdf

Anyway, we all know (or at least think) that this tire won't work well at those pressures. The reasoning for Hoosier to go with those recommendations is the fact that these tires have much lower load rating than other tires in the same size. However, we actually do not run tires in stock sizes; we run much wider.
Case of C7 Z06 / GS:

• We don't know what GM needs in terms of minimum load rating of the tires used. GM almost certainly didn't choose wider tires for load rating, but for better performance, so more than likely GM's OEM tire choices come with load ratings much more than we need.
• Anyhow, besides speculation, we only know for a fact that the front tires have the lowest rating of the system. For MPSS ZP, it's 94 (1477 lbs).
• A comparable (but a bit wider) R7 at 295/30/19 is only rated at 89 (1279 lbs). However, most track users use 315/30/18, rated at 91 (1356 lbs), which is much closer to OEM tires' rating.
• I don't think we need to talk much about rears, since 345/30/19 is at 98 (1653 lbs), and 345/35/18 is at 100 (1764 lbs).

Above is all I can say as a fact, but outside Z06/GS, Z51 actually comes with 245/35/19 MPSS ZP tire, a car that is only 150 lbs lighter, which is only rated 89 (1279 lbs), less than 91 (1356 lbs) rated front R7 tire. As such, we're likely safer than we probably think : )

I've been using 25/26 PSI cold with my 315/345 setup, but I also think that it's probably higher than ideal.

Mordeth

On my C6Z race car, which runs 315/345 and sometimes 295/335 R7s, I start with 23-25 psi cold, depending on a few things. The target is 30psi hot. And I never let the tires get or stay above 32psi. I usually have to bleed down once over the course of the day as track temps rise. I have tested this extensively and they are slower above 32psi. And downright greasy above 35psi.

Every racer knows what Hoosier says and why they say it. And still No racer runs a Hoosier R7 above 35psi and for good reason. I have talked directly to Hoosier engineers at the track. Hoosier knows this and makes their statements for liability and warranty reasons. They know what psi and tire temp the racers actually target. They will never put it in writing though. Just like VP Fuels will never put in writing that their fuel jugs are for fuel (they tell you they are made for grain storage, or oil haha).