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?

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!

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.

:turtle: for the idea.

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

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.

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.

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.

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?

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.

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.

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??

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

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?

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.

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.

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

ollie

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.

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.

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

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.

Hi Frank,

Thats a good test alright. I suppse it's a given that the contact patch gets smaller at higher pressures but its surprising that you encountered a rounder patch. This happened below sidewall pressure right? Anyway, i was searching for info on contact patch size vs traction and found this: www.stevemunden.com/friction.html
He is mostly talking about bike tires but also refers to car tires. I think it's conclusive enough evidence for me that traction doesn't decrease with higher pressures/ smaller contact patch. So, getting back to Brucepick's experiment, i'm looking forward to seeing how the real world results correlate to physics theory.

ollie

thing is... ABS is really a band-aid used to cover up poor driver training. in a low traction situation, especially in a curve or other direction change scenario, braking is the absolute worst thing you can do - braking upsets the weight distribution and inertial vectors of the vehicle, and will cause the vehicle to break completely free from the road surface in very short order.

Well its definately not snowing anywhere...did anyone attempt any testing biased or not? I think we should have 2 members rent the same car model and do all kinds of tests for fun back to back. Abs can be disabled on most cars by yanking a fuse or pulling a switch.

Testing not required. Find an article about tuning your tire pressures for autocross and you will see that there is definately a "peak grip" pressure and a fall-off on both sides of that pressure. Every car/tire combo is different.

As for higher pressure makes for better grip... nah... higher pressure gives you better road feel and more responsive handling. Ultimate grip is not the same as responsive handling. You will again find that for a car/tire combo, there is a "peak cornering grip pressure". The peak cornering grip and peak braking grip are typically not the same. Usually braking is lower from my experiences in autocross and on race tracks in both my street vehicle and my race car.

Although the methodology may be sound, the test is only good for that pavement with those tires, with that tread life remaining, at that PSI while attached to that vehicle for those road and air temperatures while carrying a driver of that weight... etc etc etc. The effects of everything form your cargo to the age of your shocks is being tested here.

I know that braking may suffer if there is less rubber on the road, but i dont think I'm going out on a limb to say that if you are already ecodriving the feet you may lose in a panic situation is moot if you were not speeding, were watching your traffic, and timing your lights. I dont think we are testing the under/oversteer limits of our car AND trying to break last week's mpg record at the same time. If going outside factory rating nets an improvement most people are going to try it so being careful is necessary. I will say my tires are very dry and they slide no matter what psi they are inflated. I know to behave.

Yup, and that's exactly how you should approach something like this. Yes, it will remove a slight margin of safety from your car. So, you need to compensate with your other equipment (the nut behind the wheel). But always remember: kids, dogs, rabid monkeys may at any time jump in front of your car and it is my opinion that my equipment should be in the best condition to avoid hitting them. If you hit me and I found out you had your tires inflated not only above your vehicle spec, but also above the sidewall max, i would sue you for every dollar you've ever made, and twice for every dollar you hope to make in the future...

but I don't live in america, so that won't work.

Hypermiling is also about momentum conservation. Some people take corners pretty fast when coasting.

Also true... so setting yourself up just barely above the peak cornering pressure might be the best optimization. You stay safe with good braking ability, and you sacrifice just a little rolling resistance for harder cornering ability, and you will have good feedback and response as well.

I believe there are minimum braking performance safety standards in the US and Canada that vehicles have to meet. So I think this covers your butt in cases like these, even with tires "over" inflated any car would still meet these standards (I couldn't find actual numbers in a quick google search). No one gets sued for driving a 3 ton SUV instead of a better stopping car when they crunch someone.

I put pizza cutter 155/80R13 snow tires on my Neon in the winter. My dry pavement handling is now much worse than stock tires. If I get in an accident on dry pavement will anyone blame me for having snow tires on? Not likely.

Hopefully people won't be so paranoid about frivilous lawsuits to stop getting some free efficiency gains from their car.
Ian

I found this link to be extremely useful in these situations :)

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

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.

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!

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.

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

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:eek:

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

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.

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.

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.

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.