Is the left or right front tire the dominant driven tire? Or does it change?

[idosubaru]

Only one tire up front is driven in an open diff - is it usually the right or left?
Or is it constantly changing?

 

If you installed one mismatched tire up front - would the other side then become the driven tire most of the time  because of propensity to bind?

[mikaleda]

In an open diff the tire that is spinning fastest is the one that gets the power. If you put a slightly more worn tire on one side, the side with the slightly smaller circumference will get the power while going straight. When turning the outside tire is traveling farther and slightly faster than the inside tire so it gets the power transfered to it.

On a left hand turn your right hand tire is the dominant tire and on a right hand turn the left tire gets the power.

 

As a rule of thumb the tire that is smaller will be the dominant tire.

[ocei77]

^??????

Not sure I get this answer. ie What if all four tires are the same?

 

Can't say this is definitive, ( I only remember 3 occasions, an 07,a 96 and a 99), but with all four wheels off the ground and the car in gear, the left front and the right rear turns until and unless you start the others to turn, then they will remain doing so.

 

O.

[mikaleda]

^??????

Not sure I get this answer. ie What if all four tires are the same?

O.

Front to rear power is controlled by the center diff and does not control lateral power unless you have symetrical all wheel drive or traction control.

[WoodsWagon]

No, tires are equally driven off of an open diff. The same amount of torque is being applied to each tire when driving normally. The difference comes when one looses traction before the other, then most of the power heads out the spinning wheel. However, an equal amount of torque to the resistance of the spinning wheel's remaining traction is still being applied to the other tire. It's always a 50/50 split in an open differential, but the total power put to the tires is equal to double the one with the least traction. With a wheel in the air and the other on the ground, it takes no force to spin the airborn wheel so 0x2=0 torque applied to the wheel on the ground. If you applied the brake to the airborn wheel (like some people with dual handbrakes do offroad), then the torque it takes to overcome the brake is also equal to what's applied to the wheel on the ground.

 

With all wheels off the ground (like running it up on a lift in a shop) the wheels with the least brake and wheel bearing drag are the ones that are going to spin. There isn't a natural bias to one wheel or another.

 

If you have one tire bigger than the other, the differential will average the speed difference between the two of them while still applying equal torque to both. The tires will wear normally, but since the spider gears in the diff will constantly be turning doing the averaging work, they will wear a bit more.

 

Now the deal of a "dominant" tire comes from one tire consistantly wearing faster than another, usually in a RWD solid axle it's the right rear. That's due to a torque reaction in the suspension from the rotation of the driveshaft. As the driving force goes up, the axle gets torque applied in the direction the driveshaft is turning. That torque compresses the right rear spring toward the frame, lowering the effective weight of the vehicle pushing down on that tire. So the "dominant" tire is the one that ends up with the least weight on it under load, and therefore the least traction, so it's the one that breaks loose first. It's the loss of traction and slippage on the road that causes tire wear, so that's why it wears out first. There's also more likely to be sand ect on the shoulder side of the lane, so tires on the right side of the car would tend to have reduced traction and more wear from that.

 

So, going back to the two different tires scenario; the bigger diameter tire will have a longer lever arm between the axle and the road, so it will be harder to break loose and loose traction, assuming the same tread width and road surface as the smaller tire. So under hard driving, the smaller tire will tend to wear faster because it will break loose first. It's also making more rotations for the same tread depth, so it will wear faster on that front too. But those are both very long term concerns, and with an open differential the car will drive normally.

 

The concern on tire size differences on an AWD car comes into play when there are limited slip differentials in play. They will fight one wheel or axle turning consistantly faster than the other, and that fight creates heat which eventually destroys the limited slip componant of the differential. If you have open diffs in all positions, front, center, and rear, the drivetrain couldn't care what size tire was on each wheel. They all get averaged out. But if you have a limited slip center, then the average speed of the front and rear axles must match. You could have a 25" tire on one side and a 28" on the other as long as you did the same on the front axle as you did on the rear. The axle differentials do the averaging work and the center differential sees equal speeds. If you also have a limited slip in the rear, then both rear tires must match and be equal to the average circumfurance of the two front tires.

 

With subaru's, the AWD system differs based on the trim model, so the safest advise is all tires must match. With a Brighton or L with open diffs front and rear, then really only two pairs front to back must match. You could have two new tires on one side and two worn on the other and the AWD wouldn't care. But a GT or Outback would have a rear LSD that would be hurt by that same configuration.

[ocei77]

^ Thanks for this detailed explanation

 

O.

[Loyale 2.7 Turbo]

This video is Very Old (1937)

 

it was made to explain how a Differential works

 

 

 

 

Kind Regards.

 

 

 

[idosubaru]

thanks miles.

 

hmmm, a differential averages the speed of two tires?

 

some tire shops will install two new tires in a AWD Subaru - one front and the opposite side rear (let's assume non-LSD for the rest of this discussion, that's easy/obvious.)

 

why do they install them opposite sides?  if the front is on the left they install the rear on the right.  

is that just for traction then and not related to the differential activity?

they could presumably install them both on the same side?

 

what about installing one new tire up front instead of two?  what happens then?

i guess based on your description it will "average" the two and it'll not be the same as the rear....

[nipper]

A differential is basically a mechanical computer. It divides the speed of the drive shaft equally between the two wheels. 

 

Subarus have awd. Subaru AWD has a center diff that is controlled either by a clutch pack, or a viscous coupling. 

 

When you have a front driven axle and a rear driven axle on a car all four wheels (and input shafts of the diffs) turn at the same speed if all tires are of equal size. If you put two new tires on one axle, you will have one diff input shaft spinning faster the the other (like it does when the car turns). Subarus with AWD can tolerate this for short times, like turning a corner, parking etc. If they have this condition constantly the limited slip center diff will over heat from the friction.

 

The link will explain the AWD/4wd systems.

 

I hope that is clear as Moms been in the hospital making for a very tired nipper.

 

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

[mikaleda]

We are going from talking about latteral distribution to front to rear distribution which are controlled by seperate systems.

 

As the op origianly asked about the open diff in the front, my explination was about how the open diff in the front works. An open diff will split power 50/50 only if both tires are spinning at exactly the same speed. As the crown of the road and slight directional correction comes to play one tire is going to become the dominant tire and will wear that tire faster thus causing the need for rotations

The smaller tire will always have power transfered to it doninantly, something you can do to try at home to help you understand this is take a 2wd car put one significantly larger or smaller tire on one side and spin out. The smaller tire will always be the domient tire.

The whole reason open diff's were invented were because a solid 50/50 power transfer to your axle ment that the car was wanting to keep going straight while going around corners and made handling a nightmare, but with an open diff the power could be transfered to one tire allowing for tighter turns.

 

The reason for the open diff to transfer power to one side or the other was based on one scientific principle. When turning the inside tire travles a much shorter distance than the outside tire so if both tires were trying to travle at the same rate and travel the same distance the inner tire binds and has to spin faster than neccisary for the amount of travle.

 

On an open diff it is rare to see power split equally unless you are driving perfectly straight on a perfectly level driving surface with a perfectly level load with perfectly matching tires

Edited January 18, 2015 by mikaleda

[mikaleda]

I have driven a 4wd with one non matching tire before. It was okay as long as I was going perfectly straight, but as soon as I started to turn twords the opposite direction of were the tire was mounted it would bind. The reason for this was as I turned the power was being to equal out as both tires began turning at the same speed and as I got turned far enough the larger tire got power transfered to it which ended up changing the gear ratio causing the binding. Since AWD Subaru's are so sensitive to this the slightly worn tire will get power and will match the other 2 in the back until you go into even a slight turn (say for correction purposes of the crown of the road) and since the overall circimfrence differance isn't that great the power will end up being transfered to the larger diamiter tire (depending on circumstances) even when going straight causing binding.

 

I am on my phone ane autocorrect is starting to irratate me so I hope I was able to explqin this properly

[Loyale 2.7 Turbo]

...On an open diff it is rare to see power split equally unless you are driving perfectly straight on a perfectly level driving surface with a perfectly level load with perfectly matching tires

 

Somehow, that is what the above posted Video, explains in a Graphic mode.

 

General Motors did that Movie back then... interesting.

 

Kind Regards.

[Fairtax4me]

Only one tire up front is driven in an open diff - is it usually the right or left?

Or is it constantly changing?

 

If you installed one mismatched tire up front - would the other side then become the driven tire most of the time because of propensity to bind?

The "driven" wheel or what we think of as the driven wheel because of its tendancy to spin, is the wheel with the Least weight on it.

 

Torque twist of the engine and drivetrain, as well as the weight of the driver on the left side of the vehicle will generally cause the right side to spin more easily.

 

When you start talking mismatched tires, now you get into traction differences between the tires, and the amount of torque applied to the traction surface of the tire, which will change depending on the size of the tire. A smaller tire will have greater torque applied at the point of contact with the road. If the smaller tire is on the left side, whether that tire spins first will depend on its friction coefficient and the weight applied to it, vs the same two factors on the other side.

If the smaller tire is on the right, its pretty much guaranteed to spin first.

 

Mismatching tire sizes isn't a good idea if you can avoid it. Forcing the differential to have to spin faster on one side all the time causes accelerated wear of the spider and side gears in the differential, as well as the center post that the spider gears ride on. Just because it can work that way doesn't mean it's designed to do that ALL the time.