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Old 05-24-2008, 03:37 PM   #11 (permalink)
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You are right about the size, I haven't seen much data either. But I was talking about the pressure relation to RR.

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Old 05-24-2008, 05:41 PM   #12 (permalink)
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I actually watched a show on the History channel about rubber or automobiles, but they had tires and showed what happens to a tire at high speed, but in slow motion. It's basically what they have on tirerack http://www.tirerack.com/tires/tirete....jsp?techid=72.

They have a lot of stuff there.
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Old 05-24-2008, 08:36 PM   #13 (permalink)
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Quote:
Originally Posted by ihatejoefitz View Post
http://www.officer.com/article/artic...on=19&id=27281

This shows the relationship between temperature and tire pressure. Their test indicates that for a 20 minute drive you can expect ~+4Psi.
http://www.tirerack.com/tires/tirete...e.jsp?techid=1

http://www.tirerack.com/tires/tirete...e.jsp?techid=1

-Notice, no mention of higher than normal wear, or blowouts.
Right in the text you quoted it mentioned irregular tread wear.
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Old 05-24-2008, 08:58 PM   #14 (permalink)
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Yeah, irregular tread wear could be an issue, but in the people who have done it for 50k+ they haven't reported it.
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Old 05-25-2008, 01:07 AM   #15 (permalink)
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Quote:
Originally Posted by CapriRacer View Post
I've been reading about folks inflating tires over the maximum printed on the sidewall and this is one site where this has been reported. I've joined just to add technical background as to why that is a bad idea!
Welcome to EM! Looking forward to what you have to say.

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Old 05-25-2008, 01:45 AM   #16 (permalink)
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I may be able to provide some useful info here as I know a ton about tires because of my racing school and driving on the track.

on the track my tires have gone up as much= as +12 psi over what i had them inflated . as an example i run my tires at 38psi front and 36 rear on the c230 for handling reasons to help curb my understeer problems and at the end of a particular 20 minute heat, on a 100 degree day my fronts were 50psi and the rear were 47psi the tires themselves were very very hot. the brakes were near 500 degrees and the tires were around 170, the fenders were over 200 degrees from the heat rolling off the wheels.

obviously noone drives like that on as street but there is a big relationship between temperature and pressure. at the track i use a tire pyrometer its like a needle you poke in the tread to measure tread tempurature. use the lazer one for the rest. also you do need a very acurate tire pressure guage being off by 1 psi on any particular tire can make handling weird, they sell em at racing supply stores. I would order the things they sell not the stuff at at pep boys or auto zone for tire monitoring duties.

at the track you measure the tempurature of the outside, middle and inside of each tire this gives you a map of how your tire pressure is working and will indicate how the tire will wear.

key: inside/middle/outside in degrees

as an example ideally on a 160 degre tire youd see 160/160/160

if a tire is overinflated you might see 145/180/145

if underinflated 155/135/155

numbers here are exaggerated for examples sake

the over inflated tire has a hump in the middle and the middle will go bald first and you will have lots of tread left on the outside.

the under inflated tire has a trough in the middle and the edges will go bald and have lots of tread left on the middle.

I am no tire engineer I do imagine that there is some over engineering, ie a tire says its max is 50 maybe you could put in 65 before it would be really dangerous but I dont know how you could tell what would be an ok amount. but thats cold and when you drive 80-100mph on the highway on a nice hot day for an hour I'd worry about explosive tire pressure loss with a blowout.

CapriRacer can most likely answer the (how much they overengineer tires) question

everyone here should get good tire pressure guages and tire pyrometers too you can better monitor your tires health that way, also keep those pieces of equipment in your car always use the tire pressure guage you own and always the same one every time, we have a gauge in each of our cars it stays with that car permanently so that tire pressures are accurate

obviously under inflated tires cost you gas mileage because they are so soft, but over inflated tires i think works because if you pump them up enough then they bulge in the middle the more the buldge the less of the tire touches the road and the less rolling resistance you have, but the bulge area will wear faster. but this is also dangerous because you really do not know how small your tires contact patch is thats all that connects your car to the road is your tires. and if you need to stop in an emergency or it rains you might be in big trouble on tires that are too over inflated think bicycle wheel instead of car tire and thats what you get out of a grossly overinflated tire.

I think it would be safer to just downsize your tire width and run near max psi you still get a smaller contact patch but you can operate the tire at a safer pressure so the contact patch stays flat which is what you want for optimal wear, and maximum traction.

ie if you have tires that are 205 width get a set of 195's or 185's that will drop your contact patch a few square inches per tire and you can then operate them at correct pressures while still achieving less rolling resistance. I always stay at least 10psi under the posted max on my tires. I do drive spirtidly quite a bit and would hate to have a tire go over max due to temperature.

tire rack is actually very helpful they will be able to tell you if say a 195mm tire will fit on your factory wheels if it wont you will need to buy a narrower set of wheels ive done that in reverse for track use gotten wider wheels than stock so that i can run 255mm tires instead of stock 225mm this also increases my contact patch and therefore increases traction.

I don't know what the safe trade off is on tire width I am a big believer in having sticky enough wheels for emergency braking and rainy weather.

i think im going to go down to 195mm on the c230 first and see how that is if its too skittish and easy to have the car do a random 360 on me i will maybe go up to a 205, as I start my ecomodding process I can give my feedback on tires.

I have not even tapped the tire performance vs temperature issues yet and this is already way to long, but let me briefly say that a tire has to warm up before maximum stickyness/traction is achieved its why cars wreck while racing so much on the first lap of races because noones tires or brakes are up to tempurature so they do not have peak traction it takes a lap or 2 to get the tires good and warm. so a tire used by everday folks on the road is really not hot enough to get its best traction. that is also why they use different compounds for street tires. ones that perform better at lower temperatures.

ok I am done for now sorry that was so wordy

Last edited by kompressor; 05-25-2008 at 01:51 AM..
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Old 05-25-2008, 02:36 AM   #17 (permalink)
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I told you guys this was bad! Sidewall max is just that. It's not the car manufacturer recommending a comfortable number, it's max you should run them at. You guys who are overinflating are counting on margins of error and safety, thinking they don't apply to you. Tire mfg's aren't responsible for errors in crappy pressure gauges, so that's not one of the margins. One day might be significantly hotter than the next, or you might hit a pothole, or you might need to slam on your brakes. When you need your tire the most, it might blow up.

Saying I know _______ has been running his tires ___% over, doesn't say too much. If you use the same tire, the lot it came from might not be as resilient and you might end up blowing yourself up or rolling your car because of it. You'll never know how much pressure is too much until something really, really bad happens.

Overinflating has its limits, and for each psi, you get diminishing returns. How much safety do you want to forfeit for that fraction of an MPG?
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Old 05-25-2008, 11:28 AM   #18 (permalink)
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If the underlying argument here is contact patch why not test it. Take YOUR tire down to 36 psi or whatever, lift the car, roll some water based paint over the tread and drop the car on some heavy duty paper. Then re-inflate to YOUR max psi and repeat, then measure. If the patches are the same, case closed right? This should even show weight distribution by how much of the treads are printed on the paper, to see which treads are making contact and how much.

Once that is laid to rest the next question would be side wall flexing. Now you guys keep talking about air and it varying pressure with heat. What about Nitrofill? Which has a higher balance of nitrogen than air and claims to stabilize the heat expansion in tires. Which is why aircraft and NASCAR use it.
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Old 05-25-2008, 11:40 AM   #19 (permalink)
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Quote:
Originally Posted by adam728 View Post
Right in the text you quoted it mentioned irregular tread wear.
Irregular =/= higher than normal, they could just as easily mean lower than normal. Furthermore, there is a huge difference in somebody saying irregular tire wear could happen, and that it will happen. I've seen this statement posted a few time on this site with no data to support it.
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Old 05-25-2008, 11:47 AM   #20 (permalink)
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Quote:
If the underlying argument here is contact patch why not test it. Take YOUR tire down to 36 psi or whatever, lift the car, roll some water based paint over the tread and drop the car on some heavy duty paper. Then re-inflate to YOUR max psi and repeat, then measure. If the patches are the same, case closed right? This should even show weight distribution by how much of the treads are printed on the paper, to see which treads are making contact and how much.
I seriously doubt that the correlation between contact patch size and rolling resistance is linear. The ultimate goal is not contact patch size, but MPG. Forget intermediate details like patch size and just produce an MPG to PSI plot. You'll see the diminishing returns I'm talking about.

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