tire pressure changing

[joyriide1113]

last night i tried running a different pressure on my tires.

i put 38 in the rear and 35 in the front as recommended from a sales person on tirerack.com

i noticed that before i inflated the tires i was driving a consistant 75mph on my way home on the turnpike so the tires were pretty hot.

when i got home after having dinner and giving myself a shower i got the same digital read i used to measure the pressure and noticed that they were no longer 38R 35F. they were 36.4R and 34F.

whats the best time to inflate tire pressure as i feel the reason the reading was wrong was because the tires were driven hard prior to inflation.

[Bigs]

I find that funny he would recommend more air in the rear than front. I would reverse that as the front is heavier. They should be filled when cold, not driven on for a while.

[joyriide1113]

i drive a 330i if that makes a difference. the car is rear wheel drive. the manufactiers recommended pressure also stated the rear as more pressure too.

i didnt go with the manufacturers recommendation since they are for bridgestone turanza`a and i upgraded to potenza s-03`s.

if i were to drive 2 miles at moderate speed, would that be cool enough for me to inflate my tires or would that heat them up too much.

[DetailCad]

First, I suggest you stick with the car manufacturers pressure recomendations. The guy from tire rack doesn`t have deep enough pockets should his recommendations lead to a handling incident. And, proving he said it might be difficult also. Check your car for TSBs to see if there have been any changes in recommended tire pressures from what was on the placard.

However, if you have changed wheel and tire sizes, you may well need to use different pressures but I would consult with the wheel manufacturer in that case.

The best time to check pressure is when the tires are COLD. I check my pressures at least once a month. I do it in the mornings after my car has been parked all night, and at the time I normally leave for work.

I believe the rule of thumb is there is a 1 pound increase or decrease for every 10 degree increase or decrease in the temperature.

Edited to add: Changing to a different design or brand of tire should not lead you using a pressure different than stated on the car`s placard. Specification of the car as it was built determine what the proper pressure should be, not the tire unless perhaps you are changing to something radically different.

[joyriide1113]

the problem here is that the car manufactuers recommendation is for all-season runflat tires. run flat tires have a harder more denser rubber, therefor requiring a bit mroe pressure.

the tires i upgraded to are max performance summer tires that are not run flat.

as for tireracks recommendation, there are one of the elading tire distributers and actually use e46 and e90 bmws (i have a e90) for their testing. he said based on my cars weight distribution that 38r and 35f should give me optimal tire performance.

the max psi for this tire is actually 51psi. he said the only difference i should have is a bit more stiffer ride but the handling should improve.

when i got the tires installed there were filled to 32 front and back but thats just the general psi they inflate tires with.

what pressure are u guys running?

and would driving 2 miles at 40mph heat up the tires enough to alter the pressure reading?>

[DetailCad]

I don`t think the change you made in tires is radical enough to warrant deviating from the car manufacturer`s recommendations. However, if you have doubts, I suggest you ask BMW. It`s a good car manufacturer and I bet if you email them, they will respond.

I use to have a `65 Cadillac that originally was equiped with bias ply tires. I switched to radial tires because I didn`t want to spend $150 per tire for bias tires anymore. Now, that was a radical change and the car placard was no longer relevant.

NHTSA has a very good website related to car safety with a section on tires that you might like to look at.

http://www.safercars.gov/Tires/Index.htm

Look at their FAQs - especially the last one.

[joyriide1113]

thanks for that link

my car gives me a range to use. all depending on the number of people normally riding in the car, ect. it goes from 42r-32r/ 38f-28f. but this is for runflat tires. arent runflat tires required more pressure than regular tires.

i question not to disagree, but to learn more.
i say this as tone of speech cant be read and i dont want to come off as stubborn.

obviously i shouldnt be driving with 20psi or 60psi but im pretty sure chnging the pressure and altering it (tweeking) gives different results in traction and performance.

my bimmer mag had an article regarding this.

[joyriide1113]

I’ve heard that the best time to get an accurate reading of tire pressure is when tires are “cold.� What is a “cold� tire?

While some people think that the term “cold� tire relates to the outside temperature, it really has nothing to do with the weather. It can be 90 degrees outside and your tires can still be considered cold. Rather, a cold tire is one that hasn’t been driven on for at least 3 hours, which is the optimal time to get an accurate pressure reading. A tire that has been driven on for short distances — even just 1 mile — is no longer considered “cold.�


---quote

[CharlesW]

I’ve heard that the best time to get an accurate reading of tire pressure is when tires are “cold.� What is a “cold� tire?

While some people think that the term “cold� tire relates to the outside temperature, it really has nothing to do with the weather. It can be 90 degrees outside and your tires can still be considered cold. Rather, a cold tire is one that hasn’t been driven on for at least 3 hours, which is the optimal time to get an accurate pressure reading. A tire that has been driven on for short distances — even just 1 mile — is no longer considered “cold.�
---quote

You got it!
My usual starting place for tire pressures has always been the vehicle mfgs. recommendation.
Right or wrong, I have always added 2# to that figure if I was going to be doing any sustained high speed travel. 50 to 100 miles at speeds above 80 for instance. My AWD Turbo Talons even had a pressure recommendation for high speed travel of 7# higher.
I never intentionally run pressures lower than the mfgs. recommendation.

FWIW, you might want to ask your question on a BMW forum where you might find someone with the same car and quite possibly the same tires. It seems that different vehicles have different reactions to tire pressure changes. Heck, the NASCAR guys even make 1# adjustments.

Charles

[DetailCad]

Something that could be interesting in the not too distant future relates to (1) tire pressure monitors and (2) on board systems with communication abilities or black boxes and (3) insurance companies requiring access to this data as a requrement of them insuring you.

Remember the Ford Explorer roll-overs where Firestone blamed Ford and Ford blamed Firestone. Much of it had to do with tire pressures.

Tire Pressure monitors will soon be required. The on board black boxes will probably have stored information as to the tire pressures at the time of the accident. Insurance companies are already seeking volunteers to let them tap into their black boxes.

Somebody not running the manufacturer`s recommended pressures could be opening themselves to a lot of trouble. But they could also if they speed, do rolling stops, drive late at night, etc.

Big brother is watching.

[pogo123]

I go slightly over the mfg spec pressure on my Mustang because I have noticed that increasing pressure by six or more lbs really effects the handling. The front end tends to wander and the rears break loose way to easy (especially on wet roads). My wifes Nissan Quest has tire pressure monitors and they are really annoying because they are inaccurate and they`re always saying the tires are low when they really aren`t (according to 3 different digital pressure gauges). I don`t know if that is something that the dealer can set or not, but I plan to ask next time it`s in for service.

[joyriide1113]

cars already come with these things. mine does atleast. giving u;re car 3 psi more will not cause a crash. giving u`re car 20 more psi will...

adjustments liek these are to our prefererence as small adjustments in pressure allow for different grip and different ride quality.

[joyriide1113]

got a response from tirerack.com that states that the pressure they ran on all their e90 325i`s was 39r 36F.

Just thought i would share.

my vehicles recommendation ranges way past that when driving with 5 adults and over 100mph.

[joe.p]

I find that funny he would recommend more air in the rear than front. I would reverse that as the front is heavier. They should be filled when cold, not driven on for a while.

if the car is front wheel drive then you might add a little more to the front then rears. i always check my tire pressure when thier cold,but won`t the tire heat up if you have to drive to a location to add air? therefor heating up the tire enroute, just asking.

[TrueDetailer]

What does the tire say on it? Go by that. And yes inflate when your tires are cold.

Everyone has their own opinion, but the company that made the tire knows best.