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

[ShadeTreeMech]

That wasn't very nice, dcb. Couln't figure it out at first

[ShadeTreeMech]

I accidentally did this test a while back driving an Explorer without ABS.

I had the front tires around 45-50 psi on the front. These were newer Cooper tires. Anyway, I had to do an emergency stop and went well past the point I would have liked to have stopped and had done before in this truck. Problem was the brakes locked up and i panicked and held the pedal down. Since this time I've been a big fan of ABS. Maybe you can theoretically stop quicker without ABS, but I don't live in the theoretical world. Either way, I lowered the PSI to 40 and the braking was much better.

[marinpa]

Originally Posted by brucepick

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?

My dear friend,

I am an Advanced Tactical Driving instructor and I am running my tires 10PSI over the sidewall max for 8 years now.

8 years ago I was fortunate enough to be in Germany on the same driving training course with 2 German tactical driving instructors that were training, among others, the Berlin Stadt Polizei. From them I've learned that they are using and advocating 5-10 PSI over tire's max pressure rating.

I came back to Canada and I stared to look for information in regards to this topic but there is nothing there. Popped the question in few car forums and almost instantaneously I got my feathers smoking by the flames.

Against all the shouting, screaming and flaming I decided to give it a try and I am using this technique ever since.

I use 10PSI over the sidewall max, on all my training cars (5 Crown Victoria P71) as ell as on my family vehicles; '06 Subaru Tribeca and an '06 BMW 550i as well as my wife's car an '08 Mini Cooper S.

So far I have more than 200,000 km driven using the above methodology and I have never ever had a single incident.

Thru out USA there are a number of organizations, mostly police departments, using the method. Two years ago I advised a taxi company about the 5-10 PSI. I meet some resistance at the beginning but after a series of demonstrations on my own cars they accepted to run on the trial basis two of their 40 vehicles. After two months they decided to go for the whole fleet 10 PSI over the sidewall max.

So far not a single incident reported, on the contrary great feedback.

To all "nay" sayers, that's what I am always keep saying:
Did you have the curiosity to experiment and document 5-10 PSI over the sidewall max. settings? Did you drive around 4-500 miles with each setting and reach the conclusion that the manufactory recommended setting (placard) gives you the best traction, stability, fuel economy and the most optimal tire wear and tear?


Keep in touch!

[marinpa]

For the record, most of the people that I know using the 10 PSI over the sidewall max, me including, do not have as a primary motive fuel economy but driving performance. Fuel economy for us is just a (more than welcome) side effect.. .. so to say.

[some_other_dave]

I have a car that has tire pressures of 26 PSI front and 29 PSI rear recommended. I have done lateral traction testing on it (autoX demands cornering grip!!!) and found that my hard-compound true street tires gripped very well at 36 PSI all around. They were even better at 42 PSI. I chickened out at that point and left them at 42 PSI.

The grip was measured by the RPM that I could drive around a constant-radius circle without losing traction.

I have since changed tires to a type that has fairly well-known pressure settings for optimal traction. (I think they are supposed to be happiest at 38 PSI cold.)

-soD

[trooper Tdiesel]

i run LT tires there rated at 50psi max, and have no ABS or cruse so i cant do it.

many times people get in my ride and say it rids so smooth i then i say its got 50 psi and there like WHAT

i don't even say there 50+ and there scared stiff

[euromodder]

Originally Posted by marinpa

I am an Advanced Tactical Driving instructor and I am running my tires 10PSI over the sidewall max for 8 years now.

Against all the shouting, screaming and flaming I decided to give it a try and I am using this technique ever since.

You won't hear much of that shouting here, as most folks run the sidewall limit or higher

What psi are your cars at then ?

Though I'll agree that tyres need to be run significantly higher than car manufacturer specs - 36psi for my car, when heavily loaded - I don't think you can say to go 5-10 psi above the rated sidewall pressure as a general rule.

My European tires are rated for 51psi, and that's what I run them at.
I've tried running them higher, but the ride gets too bouncy for my liking, especially in the rear. A mere 3psi extra can really be felt.
It's a front wheel drive car, and excess weight has been removed from the trunk (spare tyre, jack, junk, ...) so the rear is rather light.
We'll see if that improves when the shocks get replaced.


Originally, I started to increase the psi to reduce the tyres' tendency to wander about and reduce uneven (shoulder) wear.
At sidewall max, the wear has finally evened out.

[vtec-e]

I used to go to 50psi but experienced savage wheel bounce one day when i braked just before a small step on the road surface. All was well until the tire went over the step, which was about 3/4" to 1". Then my front wheels just bounced like crazy and the ABS was going at it like a minigun! I sharted as i continued to "slide" onwards. Dropped to 40psi and it didn't happen. Maybe my shocks are on the way out but they still pass the NCT which is a biannual test on the car to ensure roadworthiness.
Having said all that, there was great grip on smooth surfaces and even wear, once i didn't go sidewall max from new.

[marinpa]

Originally Posted by trooper Tdiesel

i run LT tires there rated at 50psi max, and have no ABS or cruse so i cant do it.

many times people get in my ride and say it rids so smooth i then i say its got 50 psi and there like WHAT

i don't even say there 50+ and there scared stiff

There are so many anecdotes (scary ones none the less) spread by the oil/car/tire industry about the high pressure that people start to believe and regard them as an absolute true.


I was asking a tie engineer working for one the main stream tire manufacture if he knows or if he had the curiosity to know what is the bursting point of a tire. He said he does not care because this is irrelevant.

I told him that a tire has bursting point between 250 PSI and 280 PSI.
He said this is impossible and very adamant about it, yet he was the one that said he never had the curiosity to test it.

[CapriRacer]

Originally Posted by marinpa

.......

TD,

Remember me? I'm that guy you are talking about - and I'm sorry, you have once again failed to accurately portray things. I mean things got so bad at that other web site, the moderators had to close the threads!

Please note how many posts I have made on this web site, while I note - at the time I am writing this - you have 3. Also notice how many times I have been thanked for my posts.

You must learn to be more careful about what you say. People may ask you to back up what you say.