Tires and pressure Options

[lsintampa]

Here goes, maybe I'm not understanding something and there is opportunity for me to learn.

Firstly, what happened to the "tire calculator"?

Now onto the discussion. Porsche 914's for the most part call for tire pressures to be about 23PSI front and 26PSI rear. I may be off some, but for "most" 914's I think those numbers to be "horseshoe" close enough.

BUT, wouldn't one think that the PSI for the car is based on the tire size that came on the car?

IDK, but I would put some big bucks on the fact that most of us are NOT running on stock wheels / tire combination.

So I wonder for my setup - I'm running Yoko Avid envigor's 195/65/15's on 15X6 cookie cutters - what tire pressure should I be using?

It's just hard for me to understand the relationship between the car / tire size / and tire pressure. IE, if I put that same tire on a different car - would that car's recommended pressure be different?

Enlighten me please.

Thanks,

Len

PS - I wish you all a joyous holiday season and may we all enjoy a healthy new year!


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[michael7810]

Sorry for the highjack...but just how does one tell what is best for a given setup? For normal street driving which is what I do on my mostly stock 914/4, I've tried various pressures from OEM (26/29) to 32/32 and the only difference I notice is in the harshness of the ride. I don't drive enough to see abnormal tire wear. I recently did a DE and the instructor recommended staying with the OEM pressure (26/29 cold) to get "mid-30 PSI" when warmed up.

[ConeDodger]

Sorry for the highjack...but just how does one tell what is best for a given setup? For normal street driving which is what I do on my mostly stock 914/4, I've tried various pressures from OEM (26/29) to 32/32 and the only difference I notice is in the harshness of the ride. I don't drive enough to see abnormal tire wear. I recently did a DE and the instructor recommended staying with the OEM pressure (26/29 cold) to get "mid-30 PSI" when warmed up.

Since we've taken the engineer out of the equation by going with non-stock wheels and new tire tech, those stock recommendations don't mean as much anymore except perhaps a place to start.
Your instructor is correct about the psi rising as the tire gets hot, unless you are using nitrogen. Nitrogen is more stable with cold to hot psi.
I suggest using a pyrometer. Drive enough to get the tires warm, then jump out, or have an assistant check your tire temperature in three places across the face of the tread. The goal is equal temps. If your hot on the outsides and cool in the middle, you're under-inflated. Hot in the middle and cooler on the outside is over-inflation. Just keep testing till you get close to even across the tread face. Don't obsess. 1-3 psi difference is ok. Given our 50/50 weight distribution, one tire should give you pretty close to the same as all the others so don't expect huge differences.

[michael7810]

Sorry for the highjack...but just how does one tell what is best for a given setup? For normal street driving which is what I do on my mostly stock 914/4, I've tried various pressures from OEM (26/29) to 32/32 and the only difference I notice is in the harshness of the ride. I don't drive enough to see abnormal tire wear. I recently did a DE and the instructor recommended staying with the OEM pressure (26/29 cold) to get "mid-30 PSI" when warmed up.

Since we've taken the engineer out of the equation by going with non-stock wheels and new tire tech, those stock recommendations don't mean as much anymore except perhaps a place to start.
Your instructor is correct about the psi rising as the tire gets hot, unless you are using nitrogen. Nitrogen is more stable with cold to hot psi.
I suggest using a pyrometer. Drive enough to get the tires warm, then jump out, or have an assistant check your tire temperature in three places across the face of the tread. The goal is equal temps. If your hot on the outsides and cool in the middle, you're under-inflated. Hot in the middle and cooler on the outside is over-inflation. Just keep testing till you get close to even across the tread face. Don't obsess. 1-3 psi difference is ok. Given our 50/50 weight distribution, one tire should give you pretty close to the same as all the others so don't expect huge differences.

Thanks, sounds easy enough to do and makes sense to my old engineer's brain.