Kumho Victoracer pressure

[cstreit]

Well I've switched from Hoosiers to Kumho tires because of regulation changes and have no idea what tire pressures to use. (That's tyre pressures for those in the UK)

I know Hoosiers needed to be about 40psi hot... What about Kumho's?

THanks!

5 DAYS UNTILL TRACK TIME!!!!

[zotman72]

On my narrow body 72 with 15X6, 205/50 X15 all around, I use 27 front and 29-31 rears. Do the shoe polish on the diamond marks for your optimum pressure. HTH

[logician]

Hi,

Recommended pressure from Kumho is 35-40 psi. I have an '85 Carrera that I drive in Li -- improved -- class. I use Kumho 225/50-15 all round, with 7x15" front wheels and 8x15" rear wheels. I've settled on 35/40psi front/rear pressures for track. For Autocross, I usually run 34/37psi. I'd be curious to hear what pressures others use.

-Juan

[cstreit]

Are those HOT pressures? (I assume so...)

[dtfastbear]

I've got a set up very similar to Juan's, but I run my pressures slightly higher. I have 15x7 and 8 Fuchs with 225/50's victoracers the whole way around.

For autocross I start the pressures at 34.5 F and 36.5 R cold. They typically come up to about 37 F and 39 R after a couple runs.

This is just a starting point! Remember that the "correct" tire pressure will have a lot to do with the grip that a given surface will provide. For instance, at Marina (near Monterey) I have to run a little more pressure because the concrete there provides an amazing amount of grip, and the tire will roll onto the sidewall a bit more without a little more pressure.

Dean
86 Carrera Cab

[John Brandt]

25 fr
30 rear on my '84

Stone cold +/- 2 degrees depending on the day

Use the chaulk or shoepolish to figure it out

better yet go grab a pyrometre

[cstreit]

I have a pyrometer, but never really got good results using it.

Looks like I need to end up in the upper 30's hot, say 35/37 or 36/38... My tires usually rise about 6-8 degrees on the track...

[jtdig64]

Chris,

See you at the track on saturday!!! I use 34 PSI on the front and 35 on the rear.

Todd

[logician]

Hi Chris,

Regarding tire that I reported presures:

> Are those HOT pressures? (I assume so...)

Yes, hot. Measuring the track pressures (35/40psi) the next morning, stone cold, they were a little under 30psi all round (don't remember the exact number). I've gotten as much as 12psi increase in temperature between hot just off of track, and cold the next morning. On track, the rear tires increase in presure a lot more than the fronts.

Regarding tire temperature, Kumo suggests a temperature between 160 and 200 degrees. With my presures, I've been measuring temps around 140 front and 160 rear on the hottest part of the tire.

One other point is that I think that I have more camber on my car than I use with my rookie driving style. That means that the insides of my tires are usually hotter than the outsides by as much as 20 degrees. I have noticed that as I am getting more aggressive on my driving and getting the outside temps higher, that the pyrometer indicates overinflation for the same presure. My conclusion is that if I reduced the camber in my car, or drove more agressively, that I should reduce the presures from what I am currently using.

Good luck on your track event, and let us know what presures you end up using!

-Juan

[SummerSledSix]

Guys, I think I understand the chalk / shoepolish trick, but could someone briefly describe what to look for? Also, if you're running say, 225's all around, why 7" rims front, 8" rims back? Any performance reason, or is it because that's what the car came with?

[Don Wohlfarth]

Originally posted by logician

One other point is that I think that I have more camber on my car than I use with my rookie driving style. That means that the insides of my tires are usually hotter than the outsides by as much as 20 degrees. I have noticed that as I am getting more aggressive on my driving and getting the outside temps higher, that the pyrometer indicates overinflation for the same presure. My conclusion is that if I reduced the camber in my car, or drove more agressively, that I should reduce the presures from what I am currently using.

Juan, sounds like you should reduce the camber and see what it does to tire temps. On a flat level surface see if you can slide a piece of paper under the outside edge of the tire. You are not using the full tire footprint. In a perfect world you want an even tire temp from side to side but that ain't gonna happen. More likely something like 150-155-160. Most tracks are biased with more rights than lefts or vice versa so you won't see the same temps on each tire.
Your pyrometer with a built in brain is reading say 140-155-160 and is telling you that the tire is overinflated. Because of the excessive camber, the outside of the tire is not working, you are getting a faulty reading of overinflation. In cooler ambients you won't get the tire temps that you get on hotter days.
Kumho's have a little triangle embossed on the sidewall about a 1/4 inch from the edge. You want to adjust pressure to allow the tire to slightly roll to the tip of the triangle. With a true street tire the tred comes down the sidewall about 1/2 inch. You want to adjust tire pressure so the tire rolls down to barely touch the end of the tred. The reason you use shoe polish to mark the tire is so you can check what the tire is doing as you adjust the pressure down. You usually make adjustments 2 # at a time.

[logician]

Hi,

Regarding wheel/tire sizes on my '85 Carrera:

> Also, if you're running say, 225's all around,
> why 7" rims front, 8" rims back? Any performance
> reason, or is it because that's what the car
> came with?

My choice of 225/50-15 all round with 7&8" rims is an artifact of the Porsche club rules for Li (improved) class and the best performance for _Autocross_. This was the Dwight Mitchell setup. The idea is to get the smallest diameter tire in order to get a gearing advantage. The rules limit the width of the wheels for each diameter choice, and the profile. My tire choice gives a smaller 23.5" wheel diameter compared to running 16" wheels, which would result in 24.5" diameter, and this results in a 4% gearing advantage.

Now, the question of whether this choice is optimal for track is unclear, and the Li drivers in my club have been discussing this. No definitive conclusion yet, although the best Li time at Laguna Seca last weekend was running 225/50-16 and 245/24/16 on 7&9" rims. It's tight, but I'm told that the Turbo wheels will fit. Maybe you just have to drive really smoothly to keep those tires out of the wheel wells!

-Juan