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Old 12-15-2007, 11:41 PM   #1 (permalink)
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Proposed braking test: comparing tire pressure @ 50 psi vs 35

OK, this isn't an extensive mod at first glance but it IS a case of setting up your car as the maker never intended. So I say running tires at anything like 50 psi is a mod if your car was spec'd somewhere around 27-35 psi.

I've been running my own tires way over the car maker's original recommend, since this past summer. I wouldn't have it any other way. Currently running 44 psi tires at 50-52 psi; original spec is 27 and 32 (owner's choice).

I've mentioned 50 psi tires as a good way to help FE on some other brand-dedicated message boards and received responses saying this would be unsafe. One of the biggest objections is that a reduced contact patch will result in reduced traction. Hard to argue against, and possibly true.

It would be good to know if running tires at about 20 psi over the car's spec actually does help or hurt traction.

Is someone here willing to test traction with 28, 30 or 32 psi in a vehicle that is so rated by the maker vs 50 psi? Doing an A-B-A comparison with tires at 50 psi or higher?

One tough part is that this would take a toll on your existing tread life. Maybe someone driving a rented car or one they are about to sell??
Or someone whose buddy owns a tire or service shop and has access to a used tire collection?

Anyway, I think this method would give meaningful results:

Car with ABS braking and cruise control (mine has neither)
A really big empty parking lot (maybe a beach or campground lot in off-season)
Or an empty parking lot with a straight-ahead entry from road so you don't have to brake when entering

First test at 50 or 55 psi, or whatever is your standard high pressure
Determine a location where to hit brakes
Accelerate and stabilize speed at 30 or 40 mph with cruise control
Lock up brakes at predetermined point + come to full stop
Mark tracks with chalk stating psi and run #
Measure length of skid marks

Let out air to the lower pressure level, about 30 psi
Keep car idling when adjusting pressure to maintain the same cruise control setting

Repeat the locked-brake test, tracking about a foot to the left or right of previous run so you can see the different set of tracks
Mark tracks with chalk stating psi and run #
Measure length of skid marks

Pump up tires back to the original pressure
Back at the lot, check pressure for proper level
Repeat the high pressure test

Compare the lengths of the three sets of skid marks
Any difference in traction at high vs. low pressure should show up as a change in stopping distance.
ABS braking should give standardized equal braking performance for all runs.
It's important to hit the brakes hard, immediately, to achieve matched braking input from driver.

For the record, note the tires' max sidewall rating.

A last comment - since pressure increases maybe 4 psi after a good driving warmup, you'd probably give your car that warmup before doing the tests and noting the actual pressures at that time. So if your usual pressure is 50 and they read 53 or 54 just before the test, that's OK, just record it. If the door sticker says 30 psi then release air to achieve 33 or 34 psi, note the psi, and run the 2nd test at that pressure. Then back to 53/54 for the final run.

Probably cars from early to mid '90's will have oem recommendations for psi around 30 or less. Newer cars seem to be spec'd for higher pressures. Before '90 you likely won't find ABS on most cars.

Anyone ready to do this?

Coast long and prosper.
Driving '00 Honda Insight, acquired Feb 2016.

Last edited by brucepick; 12-15-2007 at 11:56 PM..
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