Alignment

[Beav]

You know I can't be trusted, don't you?

Another consideration for keeping the tires the same approximate size is even more important (well, to some people anyway. A lot of you know I don't give a flip about this.) The ABS depends on wheel speed detection at each wheel. Now that I've tossed this out there y'all can discuss the merits (or lack of) this new entry. Does the ABS controller have the sophistication to accomodate for tire size differences? No. Not vast differences, anyway. What is a vast difference? How about "I'm driving 30mph on a snow covered road. I mash the brake pedal so hard that all four wheels instantly lock. Why doesn't the ABS cycle?" Or does it? Does the controller have the fuzzy logic to know that the car can't normally stop that fast? And if it does, how does it know when it is actually stopped?

You know I'm messing with y'all, don't you?

[lee]

maybe, but it's fun anyway....so I'll bite...I'm bound to learn something new from the great Obi-Wan (sorry I meant Obi-Beav ). I vote it cyles because the system is monitoring wheel rotations before the brakes are applied and therefore recognizes the deceleration no matter how fast the wheels are locked up. Do I win??????

But what would happen if you brought out a car on a frozen lake bed (or like surface) and were in a long sideways drift. If the front wheels stopped rotating, and the back had the parking brake on (stopping them), then the brakes were applied via the pedal...what happens to the ABS system?

[Beav]

My '92 would slide all the way down my driveway. I doubt they updated the program in later years. Newer model cars (but probably not all) do have the fuzzy logic necessary to recognize the situation.

As for your icy lake scenario, it doesn't matter. Once the vehicle is in a skid there's nothing that can be done but to ride the skid out. Once traction is lost inertia takes over. I have yet to see an inertia guidance system on a car. Side thrusters? Nuttin' like barbecuing some onlookers at turn four... (then again, if it's a frozen lake they might appreciate the heat)

[Subafreak]

Gard rails, trees, telepoles, other cars, all forms of inertia guidence systems.

[Ron Mummert]

Since alignment has become brakes, I have a question for the creme-de-la-creme (excuse my French), that have assembled here. Forget ABS for the moment. Driver "A" is cruising at 50 mph, comes upon a red light, & steadily, evenly, brakes to a stop. Driver "B", same situation, applies brakes in more of a "press hard" then let up, "press hard", let up cycle until the vehicle has stopped. Which vehicle's pads & rotors will last the longest? I'm saying driver "B" because I think each "let up" on the brake pedal provides some cooling, whereas slow steady braking doesn't dissipate the heat as well. Also, since I'm a type "B", I'd hate to think I've been a total idiot for 40 years. As a rule, however, I seem to change brake pads a lot less often than other folks. Any comments?

Ron(Go ahead, say I'm an idiot.I can handle it).

[Earthworm]

Is that just like a blonde trying to drive through a flashing read light? vroom...screech...vroom...screech...

(I'm blonde so I can say that)

actually, you do save your rotors stopping as stated in "B"
brakes change kinetic energy to heat, dragging the brakes for a long time will offer the best escape for heat, pounding the brakes at the last second will offer the least.

[ensteele]

I'll vote for driver "C" The one who has dimple drilled / slotted rotors and performance brake pads!

Earl (using my fuzzy logic)

[CigarJohnny]

Originally posted by Chicane



uh, yes it does. Think about it. A tire with a circumfrence of 100 inches will travel (duh) 100inches if rotated once. A tire with less PSI will have a smaller circumfrence than the one with 35psi and thus not travel as far.

Think about it.

- Rob

A tire with less air pressure does not have a smaller circumference, Rob. It merely has a larger contact patch. While the lower pressure tire has a smaller radius at the center point of contact with the pavement, the remainder of the tire does not. A tire with a "circumfrence of 100 inches will travel (duh) 100inches if rotated once". Do you honestly think all the extra rubber and steel belts vanishes when air pressure is lost? Do yourself a favor and educate yourself with this little experiment, it will explain things far easier than words can. Let the air out of one of your tires (make sure you have a compressor handy to refill it later. Bicycle pumps suck for filling car tires). Then, using a white grease pencil or piece of chalk, make a mark on the front and rear tires (same side where you let the air out of one of them) at the exact center point of contact with the pavement. Next drift your car forward (or backward) exactly one tire's revolution. You will find that both wheels rotated the exact amount.

Your reasoning is a common misconception that people make by basing their thought on the concept that a circle with a smaller radius will always turn faster than a circle with a larger radius. This only holds true when the smaller radius is maintained along the entire 360 degrees of the circle. In the case of a flat tire, however, the fixed circumference ( or "rolling diameter") of the tire will never change due to lower air pressure. The contact patch of the tire will simply increase to make up for the shorter radius distance to center of the wheel at the point of contact with the pavement. The flat tire will still rotate at the same speed as a properly inflated tire of equal circumference.

While an improperly inflated tire will cause premature wear on that tire and in the long run will have a small effect on the circumference of it if left uncorrected, improper inflation in and of itself will have no effect on "rolling diameter" of a circle.

[Chicane]

>scratches head<

Oddly enough.... I was thinking that very thing in the back of my mind during my last post, but pushed it back under the 'shut up brain, I think I know what I'm talking about here' catergory. Hrm. Yeah, that makes sense.... ... but.... hrm. I still don't think the tiny di... hrm. Dammit, I'll let someone else respond to this who's got a better grasp on this. I see what you're saying though.

- Rob

I also see what Johnny is saying.... and it initially makes sense from a pure physics perspective.


However, Subaru of America disputes his claim.

They recommend a 4 psi pressure delta between the front and rear tires for this reason...

Since the front of the car is heavier, more pressure will be required there to maintain the same rolling circumference as the rear tires....

Subaru indicates this very clearly and quotes that center diff damage will result if this rule is not followed.


People with WRXs have actually eliminated annoying drivetrain shudder on deceleration by resetting their tire pressures accordingly.

I can bring up the threads on the subject from NASIOC if you are interested.



Think about it this way... sure, the tire itself is not changing circumference... however, the part of the tire that is touching the ground IS closer to the wheel hub and therefore the ROLLING diameter of the wheel/tire combination has changed. All parts of the tire that are not touching the ground at any given moment become irrelevant to the physical problem. As the tire is compressed at the point where it touches the ground, it is in effect the same as putting a smaller tire on with full pressure... the distance from the hub while rotating is the same. Therefore, the overall axle ratio to the ground has changed in the back vs. what is happening in the front, causing a differential in rotation where the shafts intersect in the center diff. The diff is forced to constantly slip ever so slightly in order to allow this to occur. This causes premature wear on the plates and a huge increase in heat produced in the housing. This may be one of the things that contributes to high transmission temperatures in the SVX... incorrect tire pressure delta.

Here's another way of looking at it.

Imagine you have an inflated balloon with a stick through it, though it does not lose its air as a result of this... like I said, imagine.

If the pressure is high enough, it will ride around the outer edge of the balloon's circumference.

If the pressure is less high, even if the balloon stays the same size, it allows the bottom half of the balloon to compress more than it did before... moving the stick (axle) lower in relation to the ground. Therefore, the stick actually has to rotate more quickly as a result of this change. If the stick can go all the way to the bottom of the balloon, it would roll very quickly while the balloon kind of waffled its way around above the intersection of the balloon and stick, and the rotational speeds of the two would remain congruent but not identical.

I hope that was in some way readable, I think I lost my train of thought in there. Oops.... haha.

[CigarJohnny]

Originally posted by Porter
Here's another way of looking at it.

Imagine you have an inflated balloon with a stick through it, though it does not lose its air as a result of this... like I said, imagine.

If the pressure is high enough, it will ride around the outer edge of the balloon's circumference.

If the pressure is less high, even if the balloon stays the same size, it allows the bottom half of the balloon to compress more than it did before... moving the stick (axle) lower in relation to the ground. Therefore, the stick actually has to rotate more quickly as a result of this change. If the stick can go all the way to the bottom of the balloon, it would roll very quickly while the balloon kind of waffled its way around above the intersection of the balloon and stick, and the rotational speeds of the two would remain congruent but not identical.

I hope that was in some way readable, I think I lost my train of thought in there. Oops.... haha.

What you are saying is basically that the tire will be rolling on a point along it's sidewall, rather than the tire tread, there by altering it's rolling diameter? I honestly find it hard to believe it could be anything more than a minor difference that might possibly occur. You still have an equal amount of tire that must rotate at the same rate as the axle. For a front tire with lower air pressure to rotate faster than the rear tire, there must exist serious slippage of the lower pressure tire against the ground it is rolling on. That does not happen. If you extend a radius line to a point along the tires circumference, the wheel must travel the same distance in one revolution regardless of the air pressure. The circumference of the tire never changes. I have been looking at it like a tank tread. If you had two tires of equal circumference - one that remained round and one mounted on a wheel that had a radius that could lengthen as well as shorten as it rolled - and you could squash the the latter tire into a long oval that would maintain that tank-tread like shape as it rolled, The single axle that it revolved around would rotate the exact number of times to cover a given distance as the axle on the perfectly round tire/wheel.

What you are saying defies the basic physical principles unless other factors occur to influence the outcome. I am not saying that those cannot exist and I can think of ways in which they could occur to support what you are saying. It is hard to relate them without a pencil and paper to illustrate. Considering all the factors that could come into play I can create scenarios where what you are saying could occur. We can discuss them in Reading if you are going.

Nearly all manufacturers recommend higher tire pressure in the front to compensate for the extra weight that the front axle usually bears compared to the rear axle does. This is usually done to equalize tire wear and ride quality when they are not AWD.

The part that you're forgetting is that when the tire has less air pressure and thus deforms against the ground slightly, it moves the axis of rotation closer to the ground. An oval "tank tread" arrangement is not what results... what we have here is a constantly deforming sphere that operates as the diameter of the point closest to the axle, that point is the interface with the ground due to tire deformation.

Therefore, the axis of rotation is no longer in the center of the rotating mass... it is below the "center", and thus must turn faster.

The net result is that the rear axle turns more quickly than the front, not the other way around.

Check it out, I made a picture of what I'm trying to describe...