Alignment

[lee]

Good graphic illustration.

I may be wrong, but I still disagree. Here's why from 2 perspectives.

1. Using your illustration there would be many different rolling radius lines, i.e., at point of first contact to the point of max sidewall deflection (straight down), then back up again. For this to be true, there would have to be slip between the various parts of the tread in contact with the surface. None where contact starts/stops, reaching a max at the bottom with max sidewall flex.

2. The tire has a very stiff belt. In going around the belt does not shrink or grow (it does some depending on pressure I know, but for this discussion lets assume maybe only 4 psi diff). So unless the belt exhibits an elastic behavior, i.e., bunches up at contact or stretches at the top of rotation, once around is once around.

I don't think this applies to flat or seriously under-inflated tires as a lot of deformation would occur, including buckling of the tread in contact with the road; and therefore, a flat does have an impact.

OK, then explain to me why Subaru feels that it's significant to have a 4 psi pressure delta front/rear, which according to their tech literature is to protect the center diff.

[Beav]

I'm glad I pulled my ripcord way up there ^

[lee]

Originally posted by Porter
OK, then explain to me why Subaru feels that it's significant to have a 4 psi pressure delta front/rear, which according to their tech literature is to protect the center diff.

Frankly I have no idea. My knowledge (as a physicist) is all theoretical, and almost no practical/common sense (that's why I love this forum, so many people with real experience).

I am willing to admit I'm wrong, but I'd like to know why. Let me think on the graphic you did. Maybe there is a way I can believe it all works. There are variations in rolling radius on motorcycles, else they couldn't turn as they do, but that's mostly do to tire profile and it's use in turning, not straight rolling....still thinking.

Sorry, I wasn't trying to start an argument with anybody! These are just things that are accepted as basic facts of AWD systems on the "big" board and it's novel to me to have to work to defend such widely accepted paradigms.

[Subafreak]

Porter is correct. Another example of the fact that a tire with lower air pressure turns faster is this: Ford uses a tire pressure moniter system that uses the ABS sensors to track tire pressure, if one wheel starts turning faster that the others then it's losing pressure and will turn on a warning light on the dash. Personally with an AWD car I think it is a good idea to replace all 4 tires at the same time and rotate them religiously. It may be a bigger hit to the wallet all at once but so is a new tranny.

[lee]

Originally posted by Porter


Sorry, I wasn't trying to start an argument with anybody! These are just things that are accepted as basic facts of AWD systems on the "big" board and it's novel to me to have to work to defend such widely accepted paradigms.

If you're talking about me, I didn't think you were arguing, and I don't think I am. I just want to understand and I thought you were helping me.

Subafreak posted in between with support for your stand. I'm not doubting it to be true in real life, I just don't see how. To me it implies the belt length is not constant when it comes to rolling circumference, and that really confuses me. If anybody has a site reference that describes this I would really appreciate letting me know where.

Oops, sorry... I was responding to Beav!

Belt length is constant... what we're trying to point out is that it's strange to contemplate, but the whole wheel actually turns more when the pressure is lower, as it operates like a smaller tire.

The circumference of the tire is the same.... the rolling diameter is different and thus the whole wheel turns more rapidly. There's no magic happening with the belts... they're just going by faster.

simple way to put it is this:
you're driving a truck lets say with a half deflated tire in the back, say the back right tire. anyway
the car would pull to the right, because in essence you have a smaller tire on the back, smaller tire because it is the same size but has less distance from axle to ground constantly while rolling, because it is deflated, the same as with a smaller tire. so would have to spin faster to even out with the larger diameter tire, which puts stress on the differential.

Exactly.

[Earthworm]

So are we all in agreement that pressure does change rolling circumference?

Now say you shift the load to one side of the car will that change the circumference as well even though the pressures are still the same? (ie - more weight on the wheels)

[lee]

OK, I got that picture, as I've felt that in "real-life" terms.

Keep in mind my training is as a physicist, not an engineer. For example we always tell beginning students it makes no difference between a tall-skinny tire and a short-wide one in terms of contact patch - its all based on the normal component of force - a fat tire has a wide but short patch, a tall tire has a skinny but long patch - great theory, sucks in real life.

Here's my problem (and maybe solution). Assume for a moment that a tire did NOT grow in circumference with increased pressure.

As the front edge of a tire contacts pavement I think you would agree that it has the same rolling radius as if it was rotating in air instead of in contact with the road. As the trailing edge leaves contact with the road it has the same characteristic. Anyway, the section of tire contact patch in the middle of these two edges must then be somehow shrinking in order to have a smaller rolling circumference. That doesn't make sense given a rigid radial belt, or implies a lot of slip which I don't think is real. I don't want to use a flat tire for an example as they have to many issues, like buckled tread.

What may be real is that just a few PSI pressure difference does make a tire grow/shrink substantially (or at least the % needed to cause trouble). Think this is a plausible reason for the difference? Maybe tomorrow I'll jack up a corner and test this - assuming my wife doesn't have too many honey-do's -- I could always tell her it was time to rotate them anyway. OF course this doesn't answer David's question, but one thing at a time for my little brain.

Lest someone think I ignore advice, I don't. I rotate same brand/size tires and especially check pressure frequently - been riding a motorcycle too long (35 yrs) to ignore basics - only having two of those rubber things contact the road tends to make one a believer in maintenance items.

You are correct.

[Beav]

No problems. I bailed early, while there was still some air under the wings. It is a good topic to provoke some thought.

[Trevor]

Porter is correct. The effective diameter of the wheel and therefore its rotating speed is governed by the radius between the centre and the road ( centre of contact patch ) at a given point in time and tyre pressure as set must be an important factor. Any variation in tyre circumference as such has no effect.

In point of fact the effective diameter is constantly changing and is affected by the static load carried by each wheel as well as changes in loading as result of acceleration, braking and turning forces and road conditions. In effect a car wheel fitted with a pneumatic tyre is never a perfect circle either stationery or rolling nor is the shape constant under normal operating conditions. The effective rolling diameter and speed of rotation under typical operating conditions is never constant even in respect of a single rotation when considered in absolute terms.

Yes try the one pumped up and one near flat car or bicycle tyre as a practical experiment for proof. Easier still just consider in the mind the ultimate situation whereby one tyre is dead flat and the other fully inflated.