Dumb A$$ Question on Tire Pressures

[RSBob]

I have 205/60/15s on my early car. I need recommendations for high mileage freeway driving - about 1000 miles in a week, into the most expensive gas prices in the U.S. - Calif. And I need another recommendation for spirited backroad driving on the twisties back home. I have currently have 24 F and 26 R. I am not looking to losen my fillings or need a kidney belt for the drive to and from Caly.

TIA,
Bob

[Victor]

Up it by 10psi.

What are you doing in an early 911 if you are concerned about fillings and kidneys?

[Emission]

I agree with Victor.

Keep them about 35 psi - 24 is way too low.

[moazam]

I have 38psi in the front and 46psi in the rear. This is the standard Modderman (of Mtn. View) setup. Works great for me. The ride is definitely not cushy, but it handles amazingly. I also have an upgraded Weltmeister suspension and my car sits lower than stock though.

[911-32]

Originally posted by moazam
I have 38psi in the front and 46psi in the rear. This is the standard Modderman (of Mtn. View) setup. Works great for me. The ride is definitely not cushy, but it handles amazingly. I also have an upgraded Weltmeister suspension and my car sits lower than stock though.

Curious? Why would you run such high pressures? Have you checked the hot pressures after a spirited drive or DE - they would be over 50psi!!! This is way too high and you are sacrificing huge amounts of grip for what, low rolling resistance and a rock hard ride? Remember, all other things being equal, a lower pressure will result in a larger contact patch.

IIRC the handbook (for 3.2s) recommends something like 29/36psi - and many people think the 36psi (rear) is too high. For 60 profile tyres on 15 inch rims you can certainly run pressures in the high 20s without a problem for everyday driving and whether you go alittle higher or lower is probably down to personal preference.

Richard

[Wil Ferch]

32 front / 34 rear (cold) works very well if you're about to do a spirited run that brings everything up by 5-6 psi hot.

---Wil Ferch

[moazam]

Originally posted by 911-32
Curious? Why would you run such high pressures? Have you checked the hot pressures after a spirited drive or DE - they would be over 50psi!!! This is way too high and you are sacrificing huge amounts of grip for what, low rolling resistance and a rock hard ride? Remember, all other things being equal, a lower pressure will result in a larger contact patch.

Richard

Honestly, the only reason I am running such a high PSI is because the previous owners mechanic always filled it to that level. All the records for the last 3-4 years show the mechanic setting the PSI to 38/46.

I run the following tires:

205/50WR-16 Bridgestone Potenza RE750
225/50WR-16 Bridgestone Potenza RE750

Maybe I should lower the PSI eh?

[Rot 911]

38/46 is crazy high for the reasons outlined by Richard. In my '86 I am running 32/36.

[Emission]

For everyday driving: 32F / 29R

For spirited driving: 32F / 35R

[Superman]

I do not agree with those who conclude that lower tire pressures increase grip. I believe this is false. I believe that lower tires pressures increase tire FLEX and rolling resistance. Higher pressures decrease rolling resistance (providing more power, essentially) and they also help the tire resist flexing. I tend to run 34-38 cold tire pressures. At autocross I bleed pressure off but keep them in the 36-38 psi area when warmed up. On the street I just let them build to 40 or 42 psi. Our asphalt is cold.

Another thing that low-tire-pressure officianados believe is that higher pressures cause excess wear in the tire's center. I believe the opposite. Actually I verified the opposite when I rode a street bike. Lower pressures cause the tread to flex and heat up. The center of the tire gets the least air/cooling, so it wears faster when pressures are low. Higher pressures avoid this heating of the tread, and higher pressures DO NOT cause your tires to balloon out in the center as some might suspect. The inside of the tire carcass is round, so the more pressure you put in it, the more firmly the tire retains its designed square shape. It is when you put too little pressure in the tire that centrifugal forces can pooch the center out.

So, it's fair to say I prefer higher tire pressures.

[Emission]

Originally posted by Superman
"...Another thing that low-tire-pressure officianados believe is that higher pressures cause excess wear in the tire's center. I believe the opposite. Actually I verified the opposite when I rode a street bike. Lower pressures cause the tread to flex and heat up. The center of the tire gets the least air/cooling, so it wears faster when pressures are low. Higher pressures avoid this heating of the tread, and higher pressures DO NOT cause your tires to balloon out in the center as some might suspect. The inside of the tire carcass is round, so the more pressure you put in it, the more firmly the tire retains its designed square shape. It is when you put too little pressure in the tire that centrifugal forces can pooch the center out."

I run my rears lower pressure to prevent the middle of the tread from experiencing accelerated wear.

Can you prove your comments above? I may change my habits.

[geof33]

If you want to be very specific in terms of tire wear and contact patch... do a chalk test. Take a piece of chalk and mark from the sidewall and over the shoulder of the tread in four spots around the tire. Go out and hit some corners after your tires have warmed up. If there is chalk on the tread, your too high, if the wear mark is right on the shoulder, your just right. If the wear pattern goes into the side wall your too soft.

Once you get the chalk test right on, check your pressures, these pressures are hot pressures and back off 3-4lbs for cold pressures. Guages have inconsistancies so you almost need to "calibrate" your guage, based on this test.

With my guage I run 24 cold front, 33 cold rear... 205's front 245's back

[Emission]

Next time my rears (245's) are off, I am going to drop them to 20 psi and put a straight edge across the tread. Then I am going to pump them to 30, 40, and 50 psi and do the same. It will be interesting to note of the centers "bulge" due to the increased pressure.

[Bill Verburg]

Do your tires have indicators, like these A

[}{arlequin]

I have 205/60-15 and 215/60-15. The plaque on the back says 29F, 34R. I run 30F, 35R for street driving and the tire shoulder seems to wear just right.

[911-32]

Originally posted by Superman
I do not agree with those who conclude that lower tire pressures increase grip. I believe this is false. I believe that lower tires pressures increase tire FLEX and rolling resistance. Higher pressures decrease rolling resistance (providing more power, essentially) and they also help the tire resist flexing. I tend to run 34-38 cold tire pressures. At autocross I bleed pressure off but keep them in the 36-38 psi area when warmed up. On the street I just let them build to 40 or 42 psi. Our asphalt is cold.

Another thing that low-tire-pressure officianados believe is that higher pressures cause excess wear in the tire's center. I believe the opposite. Actually I verified the opposite when I rode a street bike. Lower pressures cause the tread to flex and heat up. The center of the tire gets the least air/cooling, so it wears faster when pressures are low. Higher pressures avoid this heating of the tread, and higher pressures DO NOT cause your tires to balloon out in the center as some might suspect. The inside of the tire carcass is round, so the more pressure you put in it, the more firmly the tire retains its designed square shape. It is when you put too little pressure in the tire that centrifugal forces can pooch the center out.

So, it's fair to say I prefer higher tire pressures.

More interesting than the average tyre pressure thread, certainly, but not all completely accurate. Just wait until the resident board engineers take a look at this thread. Folks who are a whole lot more qualified than me have debated this one backwards, forwards and sideways. The undeniable truth (according to the laws of physics) is that: the lower the pressure, the larger the contact patch. This can be very easily demonstrated and is observable to the naked eye if you drop the pressure in one rear tyre to say 10psi and raise the other to say 50psi.

Now I don't for a moment suggest driving around at 10psi. There is a limit to how low you should go with your pressures and as correctly stated above a lower pressure does increase flex in the sidewall and it can increase rolling resistance. Most (but not all) racers and DEers realise that the lower the tyre pressure the greater the grip BUT for one important caveat: tyres are designed to work with a certain amount of air and below that point they do not work properly or efficiently. With the Pirelli PZeroC track tyres I have been using, the most efficient level is about 31-32 psi (hot). Other track tyres like a little more pressure and some road tyres need a little more pressure to stop tread rollover. Fair points.

I am prepared to go out on a limb though and say that a cold rear pressure of 38psi is (for the vast majority of tyres anyway) too high if you are doing some spirited driving or more importantly a DE session. You should let air out for spirited driving not add more air. Just measure your hot pressures if you don't believe me. For example, I did a freezing wet track day in November a couple of years ago with my road tyres and found that even in these conditions the rear pressures rose from 36psi (cold) to 45psi (hot). This noticably decreases rear grip - I promise you.

I do agree that too low a pressure will wear the centres of wide tyres. I think the reason is centrifugal force that causes the centres to bulge if they are not properly stretched with air pressure. Just a theory, but I believe this is why Porsche recommends quite high rear pressures on later cars with wide rear tyres. The centrifugal effect is more apparent at the potentially high speeds on German autobahns.

HTH
Richard

[geof33]

Richard,

To go along with your post... It would be interesting to see how much centrifugal force was placed on a wide tire and how much rotational velocity caused the deformation, then to see how much the load bearing negates that effect on the actual contact patch.

Anyone doing a video???

[911-32]

Bill,

Due respect to your status as resident Pelican guru, but the indicator in your picture marked "A" just shows where the wear indicator bars are - or at least, it does on every tyre I have owned. The wear indicator bars show minimum safe tread depth. The "A" does not IMHO indicate where you should see scuff marks indicating under/over inflation as per the chalk test. Sure, you could use them as a rough guide but this is not their primary purpose.

HTH
Richard

[Bill Verburg]

They can also be used as a guage for tir pressure. The tire pictured was a bit overinflated. The advantage over chalk is that they are always there.

[Victor]

OK, now let's debate the subject of "heat".

How in God's name are you people getting your tires hot driving around canyons? Warm, - maybe but you have either never had your car on a race track or you must be dound 200MPH around your canyons.