Valkyrie

I'm wondering what's the best way to set tire pressure that gives you the best handling and fastest lap times.

Normally I will run a few laps at stock recommended pressure (which for my MR-2 is 29 in front and 32 in the rear) and then drop the hot tires back down to those numbers, and that seems to work pretty well, but there's gotta be a way that's best.

I know if you have a pyrometer you can tell measure the heat and see if you've got a maximum contact patch, but is there anything you can do to find out what your exact optimal pressure for max grip is without one?

Like putting something on the tires to leave a tread mark?


Also, I'm curious if changing to a different tire and wheel size from the stock sizes will change your optimum tire pressure. I've got 235 / 45 / 17 in the rear and the stock size is more like 225 / 60 / 15 or so (not sure about the sidewalls though).

Does the size of the wheel and tire effect this, or does it not matter since the car's weight is the same?

Or does both tire brand, model, and size all effect the optimum pressure?


Also, should I run my hot temperatures higher or lower the same as recommended pressure? I know there's the issue of sidewall stiffness (although in an ideal world you should just use tires with stiffer sidewalls) versus max grip, but I wonder if it's really worth doing anything besides that gives you maximum grip.

jgrewe

iTrader: (1)

EVERYTHING you listed matters when it comes to the correct tire pressures.

A pyrometer is really the only way to tell what your car needs. If you are dealing with DOT racing tires, the manufacturer will have some good baseline numbers to start with.

You are looking for hot pressures. Example, EP RX7 on Hoosier bias slicks, we start out about 20 psi cold because we want 26-28 hot per Hoosier's tech help. We end up pretty close to 26.5 and the pyrometer agrees. DOT tires will run higher pressures, on the Miata we are looking for pressures in the mid to high 30's

Brekyrself

iTrader: (30)

There is no set magic number. Aim for mid 30's hot and see how the car feels along with taking temps of the inner/middle/outside tread. Tread temp however is just one piece of the puzzle as camber will skew the inside temp.

Valkyrie

I guess the easiest thing to do is just go to a skidpad and see which setting gives me the most grip.

I wonder if the iPhone G-meter apps have decent accuracy...


By mid 30's hot do you mean slightly over the recommended pressure?

I just read an article saying that plus-size wheels and tires need to be increased a few PSI.


FWIW, I'm using regular street tires (240 treadwear), not DOT R-comps (or what they call 'S tires' in Japan) or slicks.


At the very least, is hot pressure number for max grip going to be slightly higher than the recommended stock cold pressure?

jgrewe

iTrader: (1)

Yea, the cold pressure will be slightly different everytime unless you are filling your tires with nitrogen. The amount of moisture in the air you put in the tire affects how much the pressure increases.

The hot pressure is what is important and it will feel best well above the stock cold settings. And as Brekyrself said even the pyrometer will lie to you if you don't watch out. At Sebring it is useless to use a pyrometer on the right side of the car when you pull into the pits. The last turn is a looooong right hander after the long back straight. In fact three of the last four turns are right handers so it has been about 1/4 lap since you've leaned on the right side tires hard!

Skid pad is handy for those last 1/2 to 1 psi adjustments for car balance.

Valkyrie

Thanks for the info.

I just got a tire wear gauge yesterday because I thought it was wearing unevenly, and the front's weren't too bad, maybe a 1 mm or 1.5 mms thinner on the sides.

The rear's were probably about the same, maybe a little, although it's more pronounced because the tires were worn twice as much as the fronts (2.5 mm left versus 5 mm left).

I had thought the wear was much more uneven, but I think it just looks that way because Federal 595's have tread with very rounded edges and because driving on the track as much as I do causes the top edges of the sidewalls to get scrubbed off...

I wish I had my racing air gauge with me. While millimeters is clearly a better way to measure tire wear, BAR is a shitty way to measure tire air pressure. That's way too big of a unit, and my gauge feels like it's not particularly accurate or consistent.

Valkyrie

I went to the track yesterday and I tried all sorts of pressures, and I was oversteering like mad with anything slightly over the recommended pressure while hot, so I dropped it WAY below the recommended pressure (when cold) and it was like I was glued the road until it started raining where I started to get a bit of oversteer again. It was great!

I ended up starting at 23 and 26 PSI! I forgot to write down what it was aftwards but I think it was something like 28 and 32, maybe a little less.

This is with Federal RS-R high-grip radials. Ironically it seems like the more grip my tires have the better I am at drifting...

thewird

iTrader: (8)

HOT pressures are the only pressures that matter. It really depends what tire your running. On my Hoosier R6's, I run 32-33 PSi HOT. On say my street tires, I'd probably do 36-38 HOT. Hot meaning you just finished a hot lap and come into the pits with zero cooldown and steaming brakes. Once you get the pressures stable, you usually dont have to mess with them for the rest of the day. Usually takes about 2 sessions for me. Cold pressures are different from day to day and track to track so its useless trying to figure them out. Just set 30 front and 27 rear cold and do the hot adjustments.

thewird

Valkyrie

I was asking about hot pressures. I just mentioned cold pressure because of how much lower I dropped it than usual, and because I forgot to write down the hot pressure after the race because it started raining during the race and I needed to keep my stuff from getting wet.

j9fd3s

iTrader: (3)

the full procedure for us to come up with hot pressures actually is to use the pyrometer, we'll start with something reasonable cold, and then adjust the final pressure when the tire is hot.

something reasonable cold is around 24-27psi, because most tracks are asymmetrical, so our final setup will also be asymmetrical. the tire that sees the most load (ouside front) will run the hottest, and the inside rear is the coldest. we actually do keep notes, so we know the starting pressures at each track.

we will then go out for a session and use the pyrometer to dial each tire in for camber and pressure. i think this is backwards, i think most people try to pick a PSI number and use the pyrometer to tune the alignment around that.

so once we have a good pressure/alignment, the temperature of the track goes up. at thunderhill we can start with a track temp of like 50F and it can be 150F at the end of the day!

so we usually end up taking a PSI out of the hot tires at the end of the day.

Valkyrie

What do you do before an actual race, though? Just get as close as possible to ideal with your cold pressures based on your practice/qualifying sessions and just hope for the best?

I don't have camber adjustment (I do need a few more degrees because my old tires got worn pretty badly on the outside edge, and the new ones are showing outside wear after one day at the track), and I was thinking of getting a pyrometer... it's just a lot of data to take down, and I figure it would be wasted on an amateur like me. Getting a combination laser and probe pyrometer would let me check track temps, brake temps, and tire temps all with the same tool, though... That would be nice.

jgrewe

iTrader: (1)

Its all about notes. I have a weather station in the trailer that is probably older than me. Temp, humidity and barometric pressure. I use a IR laser for track temps. All this info goes on the track notes sheet along with time of day, cloud cover, date any anything else that jumps out at me when I'm writing. Alignment/suspension settings and cold tire temps also. When the car comes in the hot temps are written down and any thing that the driver has for feedback.

All that and then we start to look at the dataq on the laptop.

If I'm serious every session gets a sheet. After a while you have a great note book that will tell you what you need for conditions for your car.

j9fd3s

iTrader: (3)

it seems like we either leave it, or let about .5 psi out of the outside front. for us the race session is always at the hottest temp.

jgrewe is right, keep a binder/notepad it really helps. its especially nice when you can have a month or more between races/sessions, so with the note pad you don't need to remember what you did last time.

no brains no headaches!