"Inflation pressure does not affect grip": Autospeed article.

[thatguitarguy]

Originally Posted by winkosmosis

You're the one who started with the confrontational crap, saying I need to take high school physics.

You are now apparently ignoring that you argued that a smaller contact patch has less traction than a wider one and spewing this nonsense about me "manipulating words". Why can't you just stick to arguing the topic at hand?

If you are going to quote me, then just quote me. Don't interpret what you think I said with your own words and attribute them to me, because you clearly can't understand what I'm saying.

I am quoting your exact words and there is nothing pertaining to the topic at hand.

[Christ]

Originally Posted by winkosmosis

I figured he was talking about a microscopic scale, as in better chance of bumps on the road biting into the rubber

That is quite a possibility, but I can't infer something from it that's not in words already, as I would only be guessing.

Originally Posted by winkosmosis

I don't know what numbers are but I do remember hearing or reading somewhere that the coefficient of friction changes with downforce per unit area.

That's a gimme, basic physics. Force is equal to Pressure*Area, so if you increase either of those variables, force increases, and with more force, you get a higher friction coefficient.

This is why it's harder to slide your hand against any surface the harder you push down, to borrow an analogy earlier used in this thread.

To an extent, a smooth surface does offer more capacity for contact than an abrasive surface does. It compares directly to the ability of the friction material to conform to surface irregularities.

I still have to believe that within the realm of street use tires, which fall under a very narrow range of variables, that inflation pressure will not affect basic tractive properties to any noticeable extent on dry asphalt, that wouldn't be within the realm of "noise" in testing, or some non-arbitrary margin of error.

[Christ]

Originally Posted by winkosmosis

Here's confirmation of coefficient changing. Lower pressure --> higher coefficient of friction
*All Things Motorcycle* - Tire Traction. Is contact patch size important?

This does not jive well with observed events. It might also contradict your previous quote, but I don't have time to read them thoroughly, at the moment.

I'm sure someone else will get to it, though.

[winkosmosis]

Originally Posted by Christ

That is quite a possibility, but I can't infer something from it that's not in words already, as I would only be guessing.



That's a gimme, basic physics. Force is equal to Pressure*Area, so if you increase either of those variables, force increases, and with more force, you get a higher friction coefficient.

This is why it's harder to slide your hand against any surface the harder you push down, to borrow an analogy earlier used in this thread.

No, in basic physics the coefficient stays the same which is what you're talking about with the hand example. Theoretically you press down 2x, and friction increases 2x because the coefficient doesn't change. That's why people are arguing that you can reduce contact patch and end up with the same traction.

With the motorcycle tire example, you press down 2x, and the traction force increases 1.8x. Reduce by your contact patch by 1/2x, and your traction force is 10% less.

He says it's not linear, so it's probably different from 10% for the average car.

[winkosmosis]

Originally Posted by Christ

This does not jive well with observed events. It might also contradict your previous quote, but I don't have time to read them thoroughly, at the moment.

I'm sure someone else will get to it, though.

It does jive with observed events. See the article on wider tires at the rear of the sports car to encourage understeer (aka safe handling).

BTW, I think to get the bigger contact patch with the wider tire you need lower tire pressure. If you ran the same lower pressure with a skinnier tire you'd have more deformation.

Edit:http://www.performancesimulations.co...on-tires-1.htm
According to those numbers, the ground pressure at a given air pressure isn't necessarily the same for two tire widths.
Yes I know it has nothing to do with this discussion between different pressures for the same tire.

[Christ]

Originally Posted by winkosmosis

No, in basic physics the coefficient stays the same which is what you're talking about with the hand example. Theoretically you press down 2x, and friction increases 2x because the coefficient doesn't change. That's why people are arguing that you can reduce contact patch and end up with the same traction.

With the motorcycle tire example, you press down 2x, and the traction force increases 1.8x. Reduce by your contact patch by 1/2x, and your traction force is 10% less.

He says it's not linear, so it's probably different from 10% for the average car.

You're right, I used incorrect terminology there.

What I should have said was basically exactly what you corrected.

I'm still not sure how the Cf changes with a change in pressure, since the Cf should be based on the tractive surfaces, right?

If I understand correctly, the Cf should be equivalent to the force "where the rubber meets the road". Thusly, the Cf would change for different surfaces with the same tire, and likewise for different tires on the same surface. I don't see where force is really an issue that would change Cf, let alone an increase in vertical force changing it negatively.

Of course, I'm not arguing this point, merely trying to understand why that should be the case.

[orange4boy]

SAE paper digested for us by Barry.

Barry's Tire Tech

Another bit of info showing higher inflation pressure (max sidewall) is just dandy, thanks. (to a point)

Peak traction goes up and sliding down a tiny bit. ABS would like this. Also they found cornering response and cornering traction improves. The only downside was bruise resistance and puncture blow out got worse with higher pressure (not surprising.)

[dwtaylorpdx]

If you don't believe tire pressure is a impact, and that lower is sometimes better than higher I think it would be fun to put you in a sprint car with 25 lbs pressure in the left rear... lol We run down in the low teens on hard tracks. Clay is a magical surface, it changes traction every couple minutes, and often when least expected, we carry 2 to 5 different compound tires for each corner of the car.

I've watched some really sharp college folk do all the math set their tire pressure and chase their tales and drag cars off the wall all day long while the old dude at the other end of the pits with his old chev truck and a flat trailer and no computer took their lunch money because he knows how to read the tires and the track...

Of course this is the extreme end of the curve, its about the tire performing inside the boundaries of the need at the moment.

My wifes 318ti gets nasty treacherous as soon as you get the back tires over 35 lbs.

It stops WAY faster with the tires at 26 front 28 rear than it does at 32 front 35 rear.
It gets way higher G numbers on the skid-pad as well.
It will smoke the rear tires with 35 lbs in the rear, it cant break them loose at 25 rear.

Tire & Traction = Pure Voodoo...

Dave

[gone-ot]

...would be nice (better!) if those charts had X- and Y-gridlines to make it easier to "eyeball integrate" the straightness/curviness of those plots.

...and, since those were only "single-point" data collections, those "dips" might actually be from data "round-off" errors, etc. and not really "dips" per se.

[orange4boy]

If you don't believe tire pressure is a impact, and that lower is sometimes better than higher

Of course it makes a difference. I just think it's not as big an issue with traction as some alarmist people think in the case of a passenger car on a typical road.

I like your stories about the clay track but I'm sure it a lot has to do with experience. I bet the old guy could still smoke the college kids if they swapped cars.

Also, it depends on the load of the tire. If your car is very light in the rear then you should probably run lower pressures there. My van is mid engine so running pretty equal front to back works well and for me the higher pressure has meant better handling all round. But that's just my butt dyno. Not a reliable source of data.

It would be nice to have a load and pressure graph to guide us although the mfg placard is still a good guide for the front to back difference.