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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?

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Last edited by brucepick; 12-15-2007 at 11:56 PM..
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Old 12-16-2007, 12:03 AM   #2 (permalink)
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Great idea, Bruce. Solid methodology too.

If nobody steps up to the plate, I'll do it. I can borrow a car with cruise & ABS (mine only has cruise).

BUT... I don't know how long I'll have to wait for cooperative weather. Roads have been either wet or snow covered for the past couple of weeks, and the unused parking lots (like where I shot some of the ForkenSwift video) aren't plowed in the winter.

AND we're expecting a big dump of snow tomorrow (something about the biggest winter storm in a decade).

So if someone else in a more hospitable climate wants to offer, it'll probably get done a lot sooner ...

Hey... Rick! Renting any cars in the southern US in the near future? It'll give you something to do after supper one evening instead of sitting in the hotel room!
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Old 12-16-2007, 12:08 AM   #3 (permalink)
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Quote:
Originally Posted by brucepick View Post
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.

Are any tires actually spec'd this low? I usually only see 44, I think. That's what mine are.

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.

Have you ever seen the police article that advocated 50PSI for increased traction and performance? Very interesting. My own anecdotal evidence with overinflated tires shows me little (noticable) difference in traction.

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

Indeed, I hope someone can do it. For my part I have neither ABS nor low-pressure spec'd tires.
I added my thoughts in bold, it would certainly be a very interesting test and something tht a lot of people doing this would be interested in, so I hope someone can pull it off.

I would also wonder how it would go with a non-abs car like mine. Unfortunately I live in a VERY cop/traffic heavy area, and salt on the roads right now makes me think my results wouldn't be consistent.

for the idea.
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Old 12-16-2007, 08:18 AM   #4 (permalink)
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Quote:
Originally Posted by SVOboy View Post
...Are any tires actually spec'd this low? I usually only see 44, I think. That's what mine are.

...Have you ever seen the police article that advocated 50PSI for increased traction and performance? Very interesting. My own anecdotal evidence with overinflated tires shows me little (noticable) difference in traction...
Just to reply to a couple questions you asked:

I'm suggesting using a car where the car itself is originally spec'd for somewhere near 30 psi. Of course its current tires are likely max sidewall 44 psi.

It's the car maker's spec I want to compare against in the testing. My son's 2005 Hyundai Elantra is spec'd for 35 psi all around, whereas my '89 Volvo is spec'd for something like 27. I suspect most cars from late '80s through mid 90's are spec'd somewhere near 30 psi but the tires on them now have max sidewall of either 35 or 44 psi.

So by testing a car from that era we have an OE spec of about 30 psi and we can test that braking performance vs. performance with the same (modern current) tires pumped up to somewhere between 50-60 psi.

Police magazine article
http://www.officer.com/article/artic...on=19&id=27281
I've referenced it in plenty posts on those boards. It doesn't convince all readers. Not that our testing would necessarily convince non-hypermilers, but at least we'd
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Old 12-16-2007, 09:44 AM   #5 (permalink)
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Here's a nice howto on finding the best tire pressure for handling. That being said, increased risk of tire damage from poor road surfaces is a concern with higher pressures. I've read that for best handling, the pressure is usually above the maximum sidewall pressure, but the only way to find out would be to test this.
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Old 12-16-2007, 09:47 AM   #6 (permalink)
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Would be interesting. I been looking for this info for awhile you would think there is some kind of info from the manufacturers on this I'm sure it's been tested. I agree with SVOboy and the police article from my experience but hard data would be nice to have because that one of the reasons we are here. It's a good idea ( except abusing rent cars) but I think for dry performance there would not be enough difference to worry about. I think the real reason that the tires are specs in the 30's is for ride comfort and wet performance. It you were going to test it it should be on wet surfaces. As the pressure increases so does the speed of hydroplaning.
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Old 12-16-2007, 10:21 AM   #7 (permalink)
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I suspect the effect of pressure on straight line braking performance is less significant than the effect of tire pressure on handling. Just guessing, but I wouldn't expect to see much of a difference in straight line braking performance from 35-55 psi.

Unfortunately, it's harder to quantify "handling" effects for the purposes of a test.

Then there's loose surface driving. i'd say that higher pressures are going to reduce traction on gravel.
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Old 12-16-2007, 11:06 AM   #8 (permalink)
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Ah, the car's spec! I missed that point in the late late night,

I agree with others that the handling features might be the most compromised but also hardest to quantify. Perhaps we should contact mythbusters to have them test it?
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Old 12-16-2007, 11:31 AM   #9 (permalink)
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From reading the "police" article I gathered that handling improves with pressure. That would be due to the sidewalls stiffening up with increased pressure.

Steering response definitely seems to improve; I found it got noticeably lighter once I took the pressure past 44 psi to around 50. It let me disable the power steering. My 3070 lb. car handles well and is not too heavy to control with the tires at 50-52 psi.

However, I dove off topic above.
I'm trying to steer back to the brake traction test.

Reason is, I think this is the one most critical safety need. And the one most often quoted as a reason to stay with the "manufacturer spec'd" pressure. It's also probably easier to measure than handling.

Just do a straight line stop from any given standard speed and measure the length of skid marks. ABS will pulse the brakes for a standardized optimum efficiency. It should give a strong indication of basic dry traction.

As to hydroplaning, I think the "police" article shows pretty well that hydroplaning is avoided better at higher pressures. We could test for that but again, it's more of a special purpose condition. As would be a snow traction test.
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Old 12-16-2007, 11:48 AM   #10 (permalink)
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I still think it's a good idea, and unless someone in a friendlier climate does it first, I still plan on doing the experiment.

I don't think measuring the "skid marks" will be an easy measure though, since ABS doesn't leave prominent marks (compared to a locked brake / sliding tire).

I was thinking instead of using something on the pavement as a signal (or lay something down that you drive over). Yes... it introduces the driver's reaction time into the experiment. But I bet a tactile/audible "bump" is a more reliable signal than using a visual cue.

Maybe I'm wrong, and the tire marks will be easy to see.

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