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

[vtec-e]

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

[equation112]

Originally Posted by SuperTrooper

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.

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.

[getnpsi]

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.

[MazdaMatt]

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.

[getnpsi]

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.

[MazdaMatt]

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.

[tasdrouille]

Originally Posted by getnpsi

[...]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.[...]

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

[MazdaMatt]

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.

[IndyIan]

Originally Posted by MazdaMatt

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.

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

[dcb]

I found this link to be extremely useful in these situations