Experiment Request: Traction at 50 psi vs 30 psi - Fuelly Forums

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Old 12-15-2007, 05:50 PM   #1
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Experiment Request: Traction at 50 psi vs 30 psi

I've mentioned 50 psi tires as a good way to help FE on some other brand-dedicated message boards and received responses indicating this practice 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 would be the way.

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
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
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.

Anyone ready to do this?
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Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 12-15-2007, 07:56 PM   #2
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I've only done AB part from just checking and correcting air pressure after the cold weather set in but 15-20 psi makes a difference on the snow at least. Not a lot, but some. I'm using 205/55R16 bridgestone touranza EL42 which suck in the snow anyway. I went drifting around the subdivisions at about 35 psi then pumped them up to 50 later that night and it was a significant difference. more than 150 lbs of sand in the trunk could make up for (RWD car so it IS drifting and weight in the trunk DOES make a difference)
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Old 12-15-2007, 08:02 PM   #3
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yea my chevette has so skinny of tires and thier M+S tires so even tho its rwd the full gas tank behind the axel is more than enough weight. you accually have to try to fishtail to get it to do so, usually the front wheels start to slide before anything
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Old 12-15-2007, 08:33 PM   #4
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I appreciate your answers but I think you've missed my point.
My question has nothing to do with snow traction though that's an interesting question for another experiment.

I'd like to see if someone here would do some tests to see if a car will stop better or worse on typical pavement with the tires pumped up hard. Regular, typical all season tires. "M+S" is actually a type of all season tire so far as I know. Real snow tires aren't marked "M+S".

However I do think you'd need tires rated at least 44 psi max sidewall for the experiment so that you'd be able to pump them up to 50 or more. If the tires are rated 35 psi readers could claim that tire was never meant to be used anywhere near 50 psi and they'd be right.

Any takers?
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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 12-15-2007, 09:07 PM   #5
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Traction under slower Auto-X type driving...high pressures will stiffen the sidewall and lessen flex. Steering input will be much quicker. Of course I am talking about "correct" higher pressures.

Traction over high speed road scenarios like Road Racing...lower pressures will build temps to the correct psi...where temps are even across the tread...ie- outer-center-inner. The tires will heat up quickest under hard braking and sweeping high g-force turns.

What this all means is that it is give and take. With High Pressures you gain FE and lose a bit of traction in most normal driving speeds except parking lot manuvers. Lowering Pressure to the point that it takes heat to build them to the correct psi will hurt FE but gain you traction.

Since most of us here are trying to get the last mpg out of every drop...you give up a bit of traction to gain mpg...but...there is a safe range that you should stay within. Once you pick a psi you can tune your suspension with sway bars (adding or removing) to gain higher speed stability (removing rear sway bar or getting a bigger front sway bar)...or get better low speed traction (adding or getting a bigger rear sway bar and picking the proper front sway bar). All these things can also be changed with spring rates and alingment specs.

Basically to answer your question...pick your main objective (raising FE) and then tune your suspension to gain back some handling.
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Old 12-15-2007, 10:44 PM   #6
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what if said sway bar is broke....
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Old 12-16-2007, 05:11 AM   #7
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M+S most certainly is the designation for snow tires. It stands for Mud and Snow. They aren't rated the same as regular (all season and most other types of tires) tires, and are missing things like tread life ratings.

While I haven't done an actual ABA test on this, I can say that my braking distances did increase after pumping up my tires. It would be interesting to see someone's results after doing this test, but there really is no reason to debate the issue. Harder tires roll better, and more rolling resistance is what you want to help you stop your car. Thus, tires with low rolling resistance that are pumped up to a high pressure will take a longer distance to stop than higher rolling resistance tires that aren't pumped up that high.

I do want to see some actual data on this though...
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Old 12-16-2007, 06:22 AM   #8
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If the tires have less traction when overinflated, which I think makes sense theoretically, they would take longer to stop with.

I've noticed my tires do have traction issues more now that I have them filled to the max, but I really don't mind. The car already isn't the best at stopping "on a dime", so I doubt it's being hurt too much.
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Old 12-16-2007, 07:37 AM   #9
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Re. M+S tires:
Below is quoted from http://auto.howstuffworks.com/tire3.htm

All-Season Tires with Mud and Snow Designation

... The rough translation of this specification is that the tire must have a row of fairly big grooves that start at the edge of the tread and extend toward the center of the tire. Also, at least 25 percent of the surface area must be grooves... However, as you can see from the specification, there is no testing involved.

To address this shortcoming, the Rubber Manufacturers Association and the tire industry have agreed on a standard that does involve testing. The designation is called Severe Snow Use and has a specific icon (see image...), which goes next to the M/S designation...


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Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.

Now driving '97 Civic HX; tires ~ 50 psi. '89 Volvo 240 = semi-retired.
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Old 12-16-2007, 08:59 AM   #10
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My stock xB tires GoodYear - size I can't read right now due to a rain storm and a half a foot of slippery snow in the driveway - when I run at 40psi will spin in first gear on clean roadway almost by hitting the gas in gear. If I am turning even a little they will spin at half throttle at low rpm. As far as braking there has been no problem because the weight transfer to the front wheels increases the load on the tires and does not seem to hurt the traction problem at all. I have not yet locked up the front tires - abs ya know - but I do brake hard for certain turns and don't even hear a squeel out of the tires. If anything running stock recommended tire pressure and heavy braking probably causes loss of traction as the tire flattens.
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