Proposed braking test: comparing tire pressure @ 50 psi vs 35

[brucepick]

Hi MetroMPG,

Good to hear that you'd do it if not done first by someone outside the snow belt.
Thanks!
Of course if you run the test you can definitely adjust the procedure if find you need to.

I've seen skid marks that are a line of evenly spaced "dash" marks and assumed they were from ABS, but what do I know??

[SVOboy]

Just to note, these two tire rack articles discuss (not in detail) some issues with traction and tire inflation:

- http://www.tirerack.com/tires/tirete...e.jsp?techid=2
- http://www.tirerack.com/tires/tirete...e.jsp?techid=1

[vtec-e]

Just had a service on my kia today and the serviceman mentioned that my tires were seriously overinflated. Sidewall of 51psi and mine were up to it! He put them back down to 32psi saying that my insurance company would "walk away from me" if i was involved in an accident with tires at that pressure. He wouldn't listen to anything i had to say on the subject and got very snotty. So, where can i get my hands on DEFINITIVE proof that going to sidewall isn't dangerous. Everywhere i look says overinflation will wear the centre of the tread. I doubt my insurance company would look at my tires with 105,000km on them at sidewall pressure and even wear to boot!
Has anyone done the braking experiment yet? I think it really needs to be done, and most definitely done in the wet because thats where the grip is really needed. By wet i mean a damp surface where the water doesn't go above the stones in the ashphalt. You know, where there's just been a shower for a few minutes but has gone away.
Anyway, sorry for going on, i'm just a little freaked out now and need this info. It's impossible to find info that goes over this magical figure of 32psi.

ollie

ps. I have 2 cars, a civic and a kia cee'd. The kia weighs way more than the civic and is a diesel too so how is it that 32psi is ok for the two?

[ebacherville]

Dang i would havbe let that service guy have it.. what a tard.. you were at the max side wall pressure.. the tire company is saying that safe.. and as for the insurance company.. i have never heard of anything like this.. ridiculous.. if my insurance company tried pulling that off id sue them for every penny i have every paid them.. because if you arn't protected by there service at there whim then your never protected.. realize they take a risk when they insure you.. thats there business.

[Duffman]

vtec-e,
Just air your tires back up to where you want them. This is just another example of somebody working at a dealership that somewhere along the line got an idea in their head and also happen to have the "I know everything" personality to go with it. Trust me it is very common and is not worth the stress to get worked up over it.

Look at all the vehicles on the road with aftermarket tire sizes and lifts, are they denied insurance? It is absolutely ridiculous, I use it as a measure of the quality of the service that your dealer provides.

[vtec-e]

Thanks lads. I'll not give up my quest though!

ollie

[Peter7307]

The "wearing the centre of the tread" logic was made obsolete when steel belted radials became available but it still persists from the days of "balloon tyres".

A classic case of people not keeping up with progress in their own industry.

Personally I would just go and spend my money in some other place.
The value you get couldn't be worse.

Pete.

[diesel_john]

Originally Posted by Frank Lee

No, it wasn't. That belt does flex, and it does bulge too.

Frank Lee, how much over rated pressure can you go before center wear becomes an issue.

At 5 psi over rated i'm measuring 1/32 inch more wear in the center than at the edges, after 110,000 miles.

When autocrossing i always ran the tire pressures over rated for the best times. If there is a more abusive tire test than autocrossing, i would like to know what it is.

[vtec-e]

Hi frank, how did you do that test? Considering you said the contact patch was rounder, i'm thinking you jacked up the car and put paint on the tire then put it down on a sheet of paper or something, so when you lifted the car again, you could see the result. Am i close??
What did you find in relation to the contact patch up to sidewall pressure? Was there much of a decrease in size?

ollie

[SuperTrooper]

First of all, you'd be better off with a car WITHOUT ABS. The common misconception is that ABS improves braking. Wrong. On dry surfaces you get quicker stops without ABS since it is easier to feel the lockup point of the tires and make minor pedal pressure adjustments to maintain maximum braking without skidding. The point of ABS is to maintain DIRECTIONAL control in low traction situations, allowing you to steer around obstacles rather than running into them.

There are many variables involved in this type of test that make repetition difficult:

Tire temp
road temp
road traction
brake fluid temp

You'd wind up testing the braking system on the car as much as the tires. For testing cars, magazines do several braking tests in succession and use the statistical mean in generating the number they use. This would be a difficult test to generate consistent results that I would want to use to make intelligent choices with.

I think we could all agree on the concept that, for a given tire, traction is inversely proportional to rolling resistance.

How do you decide HOW MUCH traction to give up to justify lower rolling resistance? I'd hate to be on the wrong end of that equation even once.

After reading most of threads on the inflation subject I don't see the point of inflating tires over the maximum recommended pressure listed on the sidewall. MetroMPGs rollout tests (though a teeny tiny sample) show limited results beyond max recommended. I see no reason to "push the envelope" in everyday driving.