Alignment

A tire is NOT a circle. It's a toroid. Its rolling circumference can change based on how domed or pushed inwards the outer edge becomes.

As the pressure decreases and the sidewall of the tire relaxes, it moves outward... this draws the tread inward... the area at the contact patch moves inward based on the weight of the car as the tire SPREADS OUT against the ground. The rest of the tire is "doming" at this point to offset the volume of air being displaced at the contact patch.

I wish I knew some 3D modeling... I'd throw this together as an animation.


Anyway, yes that's the point we were trying to make... the tire does not have to turn the same number of times per distance traveled, it travels more times because the rolling radius is shorter.


That's the magic of the wheel, and why mathematicians have been so fascinated by the concept of Pi since the constant was first discovered.

Thanks for the great posts guys. So now we understand that different sized tires travel at different speeds. Tires that rotate at different speeds tax the differentials. Making turns means that tires are all rotating at different speeds. So is making turns bad for the car? I don't think so, I think that is what the differentials are for, to allow varied tire rotation in the awd system. So then it should not matter if all of your tires are excatly the same size or proper tire pressure. Otherwise everyone would be bucking when driving an awd car around or it would be bad to drive the car because turning taxes the differential.

Actually, turning can tax the differentials in an LSD equipped car if you continue it long enough.

The reason that turning doesn't cause issues is that at some point you STOP turning... and the differential in speeds between the sides of the car ceases.

The reason that the center diff runs into problems is that in a straight line on a car with differently sized tires the differential in speeds never stops... it continues to heat up the viscous fluid in the center diff and cause partial lockup.


If you do donuts continually for several miles, your rear differential will probably be VERY unhappy.





In other words, that is the difference between the three types of differentials...

A) "Locking" differentials that are totally engaged, which would cause strange bucking sensations as you describe.

B) Open differentials, which are in the majority of cars.. these have no lockup whatsoever and will transfer all power to the path of least resistance (the spinning wheel).

C) Limited Slip Differentials - i.e. Subaru Viscous Coupling units such as the rear diff on the SVX, or electronically locking viscous coupler units such as the center diff on the SVX. These differentials allow for partial lockup depending on the difference in speeds between the output shafts.... at zero differential in speeds, there is no lockup and the torque characteristics are "open". As the differential in speeds increases, the fluid heats up between the plates in the diff and causes rapidly increasing friction that eventually results in almost full lockup.

There are a number of different types of LSDs, including Viscous Couplers, Torque Sensing Planetary Gearsets (TorSen, Quaife, SensaTrac), and clutch based hard locking LSDs used for racing such as the KAZZ differentials.

Hope this helps!

[immortal_suby]

Originally posted by Porter
You guys crack me up... It was good seeing you last night, Matt! And your lovely wife too. Mike was convinced that she thought we were nuts or something. I was like, no way man.. she has one too!

We both had a great time. She does not think either you or Mike are any more nuts than I am. Remember - she lived through the days of the little red wagon with ram-air and dual exhaust sidepipes. If she can put up with me, most of the other subafreaks seem mild mannered in comparison.

Did you see that honda almost sideswipe you when you pulled off 29? I creased my seat when I saw him veer towards you. He was heading straight for your rear quarter panel Jimmy Spencer style.

Glad you made it home safe, we'll have to do that again soon.

Oh yeah- you better get your ass to lafayette - you have no excuse not to go! We may not be as crazy as the rex guys, but it's a fun group to hang out with.

see ya

[oab_au]

Gee I'm glad that you all got that worked out.

But I was thinking.

Does a flat tyre deliver more torque to the ground than an inflated one?

The tyre circumference, over the working radius point is a good one, but it really does lower the gearing as the tyre deforms.

Look at a fuel drag car, The slicks at blast off deform to ripple at the bottom, this increases the contact patch, but more importantly reduces the radius leverage of the wheel. It gears the car down, till the speed rises and the centrifugal force expands the diameter of the tyre, to raise the gearing, to give the top speed.

Question, How can the gearing change if the rotational speed does not???

Harvey.

[Trevor]

Originally posted by lee


I think Trevor came closest to explaining the situation. Or at least it may have turned on a bulb in my head. Trevor, if you're still reading this drivel, let me know. Here goes my explanation....

Porter/Trevor, either I'm getting closer, or one of you is gonna have to come shoot me and put me out of this misery

Yes Lee you are spot on. No not drivel but the thread has been inflated with a lot of very hot air which increases the rolling resistance !

The EFFECTIVE radius of the wheel between the wheel centre and the road governs the speed at which the wheel rotates.

Good show, fellas.

[mbtoloczko]

I got myself some white-out, found a straight piece of empty road, marked all four tires, and tested the idea. I set all four of my 225/45-17 tires to 32 psi. It took almost exactly 0.5 miles for the front tires to roll one less revolution than the rear tires. I did it twice and got the same result both times. Assuming an average tire diameter of 25", then the loss of a revolution equates to a 0.25% difference in rolling radius between the front and rear tires when the tires are at 32 psi. Now I just need to find out what tire pressures are required to get a 0% difference.

Typically a 3 to 5 psi difference front to rear at usual operating pressures (35-45psi).

I have 51psi max tires on mine, so I run 49f/45r.


Remember... if you are running 44psi tires at 35psi, you are underinflated. Bumping the tires up to their optimal range will significantly improve handling and road behavior! I always run 44psi tires at 44f/40r.

[CigarJohnny]

Originally posted by mbtoloczko
I got myself some white-out, found a straight piece of empty road, marked all four tires, and tested the idea. I set all four of my 225/45-17 tires to 32 psi. It took almost exactly 0.5 miles for the front tires to roll one less revolution than the rear tires. I did it twice and got the same result both times. Assuming an average tire circumfrence of 25", then the loss of a revolution equates to a 0.25% difference in rolling radius between the front and rear tires when the tires are at 32 psi. Now I just need to find out what tire pressures are required to get a 0% difference.

Thanks for the test, Mychailo! That is proof enough for me. As I roll this whole thing around in my brain now, it is starting to understand how the sidewall deformation takes up the slack in the original circumference of the tire. I guess it does a better job than I originally thought. I suppose I am more of a doubting Thomas than I thought I was. The more I think about it (now that I know it to be true) the more it makes sense. Sorry to have dragged out this debate as long as I did () but I really enjoy topics like this. They teach us what we did not know, explain why we thought some things and confirm what we already knew to be true. Now I have to dig through this thread to find the proper difference in pressure to set the "rolling circumference" equal front and rear. Something like 3 or 4 psi, no?

Thanks again, guys! You can teach a middle aged dog new tricks.

Haha, cool man! I think it's fun to have these kinds of discussions, even when I'm wrong (which is a lot)! This particular one is really counterintuitive due to our concept of the rubber being a solid item.

I crossposted with you above about the pressure difference. 3-5psi depending on the range that you're running in... higher pressures will require a slightly higher difference in pressure, because you're adjusting a percentage and not an exact number. The difference is small between the ranges though... 4-5psi at high pressures (40 and above), 3-4psi at lower pressures (35 and below).

[CigarJohnny]

Originally posted by Porter
Typically a 3 to 5 psi difference front to rear at usual operating pressures (35-45psi).

I have 51psi max tires on mine, so I run 49f/45r.


Remember... if you are running 44psi tires at 35psi, you are underinflated. Bumping the tires up to their optimal range will significantly improve handling and road behavior! I always run 44psi tires at 44f/40r.

Cool! I got my answer before even asking! One question though...doesn't running that high a pressure cause the tires, especially wide ones like ours, to wear more in the center of the tread? I know autocrossers run max pressures to increase teh rigidity of the tire and increase responsiveness, but I was always led to believe that the car manufacturers recommended tire pressures should be maintained since they are calculated to put even pressure along the footprint of the tire so that the tread wears evenly.

[CigarJohnny]

And so starts another debate!

Gosh, I hope not!

At one time tires were more round in shape, back in the days of wire spokes and whitewalls.... Tire pressure "common wisdom" is largely a result of those days, and applies little to modern tires with steel belts and rigid tread patterns.

The only part of the tire thin enough to actually deform with tire pressure is the sidewall itself. Your tread will stay flat and wear evenly all the way across at any pressure under about 55 on most tires. Obviously this changes depending on the tire and the construction of the tire, but I can tell you I've had no problems with wear whatsoever, even running 35psi tires at 42 or 45psi for mountain runs where we scrub a lot of rubber.

Most "sporty" all season tires wear excessively at the outer edges at 35psi... bumping them up actually makes them wear evenly.

Obviously these rules apply to modern, well constructed tires. The same experience may not apply to that set of $25 Sears Weathermaster balloons.

[mbtoloczko]

I'm going to have to disagree with Porter somewhat on tire pressure. If you are using the stock size tire, I think that running pressures near the Subaru-recommended pressure is fine as long as you don't rail every corner at 2x the posted speed limit. I run my 225/45-17 tires at 36f/33r. I don't use the Subaru-recommended difference in pressure of 4 psi because the lower profile tires don't sag as much at the stock 50 series tires.