2WD

[CharBaby]

Regardless which vehicle is 2WD(FWD/RWD), which wheel has the drive?
Or can both have the drive or some drive(Not being Posi-Traction)?

For example, the everyday family FWD car. Can either wheel have the drive depending on traction(snow/ice)?

Same for RWD? ^^^ AGAIN, w/o being Posi!

 

[NHGUY]

Open differentials send the power to the wheel with the least traction.Generally going forward its the right wheel that will spin/slip.

 

[CT8]

Originally Posted By: NHGUY
Open differentials send the power to the wheel with the least traction.Generally going forward its the right wheel that will spin/slip.
this !!!

 

[dishdude]

 

[Traction]

Sorry, that link didn't work, Anyway this is a video made in 1937 by Chevrolet, explaining how differentials work. It totally explains why and how they work. Same today. You will never understand without watching it. I never knew after driving 45 years.

 

[440Magnum]

Strangely worded question, not sure I really understand it.

An "open" differential (no limited slip) always puts exactly the same TORQUE to each wheel under all conditions. That means that the wheel with the best traction still gets no more torque than the wheel that has the worst traction. Think of the extreme case- jack one wheel in the air. That wheel has zero torque, so the wheel on the ground also gets zero torque and the wheel in the air just spins.

A limited slip differential always tries to turn both wheels at the same SPEED, and therefore puts at least the holding torque of the clutch packs to the wheel with the best traction. A locker can put 100% torque to the wheel with the best traction. In the example above, the wheel on the ground will get either the full torque (locker) or the holding torque of the clutch pack/cone (regular limited slip) and will drive the car off the jackstand.

 

[Jetronic]

Originally Posted By: 440Magnum
Strangely worded question, not sure I really understand it.

An "open" differential (no limited slip) always puts exactly the same TORQUE to each wheel under all conditions. That means that the wheel with the best traction still gets no more torque than the wheel that has the worst traction. Think of the extreme case- jack one wheel in the air. That wheel has zero torque, so the wheel on the ground also gets zero torque and the wheel in the air just spins.

A limited slip differential always tries to turn both wheels at the same SPEED, and therefore puts at least the holding torque of the clutch packs to the wheel with the best traction. A locker can put 100% torque to the wheel with the best traction. In the example above, the wheel on the ground will get either the full torque (locker) or the holding torque of the clutch pack/cone (regular limited slip) and will drive the car off the jackstand.




Just to add: a limited slip diff will turn the same amount of torque it's directing to the slower wheel into heat: 100 Nm transferred also means 100Nm that stays in te gearbox and has to be cooled away. A limited slip diff therefor can not redirect more than half of the output torque aswell.

Makes a compelling case for usin the brakes to mimic a limited slip differential (combined with a real one maybe?

 

[sicko]

Check out the above videos (both of them) because they're both good at explaining it. Then consider this, with a FWD vehicle the power is going to both front wheels and with a RWD vehicle the power is going to drive both rear wheels. However, as mentioned in different ways in the videos, an open differential allows power to follow the path of least resistance. This means if one wheel has total grip of the ground beneath it, and the other one has almost no grip, then the wheel with no grip will spin and the vehicle wont move. That's because the power is following the path of least resistance, and it's easier to make a tire spin without grip than it is to spin a tire that has to overcome the force of moving the vehicle as it spins.

There are different types of differentials (locking, limited slip, etc) that work in different ways to prevent the tires from completely spilling. Ultimately what they're doing is limiting the amount one tire can can spin. Some will force power to be split 50/50 between the wheels, some will always ensure that some level of power (but not necessarily 50%) is always going to each wheel.

It goes beyond the question, but some vehicle systems don't even use the differential itself. The vehicle senses that one tire is spinning, and will automatically apply the brakes to just that tire. This reduces the path of least resistance for the energy to flow to, forcing some energy to go to the other wheel. This will result in the other wheel either getting enough energy to move the vehicle, or getting more energy than the friction/grip can handle and causing that tire to spin in place, or some combination of the two.

 

[CharBaby]

I guess what I am asking is...is there one wheel that is more dominant over the other wheel?

 

[dishdude]

Originally Posted By: Char Baby
I guess what I am asking is...is there one wheel that is more dominant over the other wheel?


The one with less traction unless you have a locking differential.

 

[Traction]

Originally Posted By: Char Baby
I guess what I am asking is...is there one wheel that is more dominant over the other wheel?

Well, I think I understand your question. I would have to say that in most cases with a open diff the right tire always want to spin first probably due to torque reaction from the direction of rotation from the engine. If the engine spun the other way it would be opposite. So, yes I would say the left wheel has more traction, and all the more reason to avoid spinning a wheel on slippery surfaces with little traction. Plus when one wheel is spinning it doubles in speed, creating even less traction!

 

[CharBaby]

Thank you all!

 

[Shannow]

An open differential will have the same torque applied to both wheels. If one is on ice, and one is on bitumen, the maximum torque to both is the same, i's your forward motion is restricted by the wheel on ice.

Power is torque x RPM, so the wheel on ice gets "power".

If an FWD, or a rear drive with IRS, there is no torque reaction from the engine, so neither side has an "advantage".

A solid axle has a torque reaction, so one wheel is "planted", and the other "lifted".

LSDs apply a clutch limited torque to the wheel with the most traction, the amount of torque is limited by the design.

A "locker" makes both wheels turn at the same speed, regardless of traction.

 

[CharBaby]

Thank you, good explanation!

 

[440Magnum]

Originally Posted By: Jetronic
Originally Posted By: 440Magnum
Strangely worded question, not sure I really understand it.

An "open" differential (no limited slip) always puts exactly the same TORQUE to each wheel under all conditions. That means that the wheel with the best traction still gets no more torque than the wheel that has the worst traction. Think of the extreme case- jack one wheel in the air. That wheel has zero torque, so the wheel on the ground also gets zero torque and the wheel in the air just spins.

A limited slip differential always tries to turn both wheels at the same SPEED, and therefore puts at least the holding torque of the clutch packs to the wheel with the best traction. A locker can put 100% torque to the wheel with the best traction. In the example above, the wheel on the ground will get either the full torque (locker) or the holding torque of the clutch pack/cone (regular limited slip) and will drive the car off the jackstand.




Just to add: a limited slip diff will turn the same amount of torque it's directing to the slower wheel into heat: 100 Nm transferred also means 100Nm that stays in te gearbox and has to be cooled away. A limited slip diff therefor can not redirect more than half of the output torque aswell.

Makes a compelling case for usin the brakes to mimic a limited slip differential (combined with a real one maybe?


I think if you do the math, dragging the brake on the low-traction wheel burns exactly the same amount of energy as you lose in internal clutch packs in an LS differential. A locker doesn't dissipate any energy internally, but it doesn't activate as "nicely" either. It clanks and pops and sometimes barks a tire just going around a corner at 10 mph in a parking lot for a reason...

 

[Jetronic]

yes, it burns the same amount of energy, but is serviceable (easier than most differentials anyway) and doesn't heat up the transmission.

a limited slip differential like a Torsen needs some resistance to the low-traction wheel to work at all, so that'll benefit from a dragging brake. Otherwise they still burn the same amount of energy as they move to the high-traction side.

 

[rationull]

Originally Posted By: Traction
Originally Posted By: Char Baby
I guess what I am asking is...is there one wheel that is more dominant over the other wheel?

Well, I think I understand your question. I would have to say that in most cases with a open diff the right tire always want to spin first probably due to torque reaction from the direction of rotation from the engine. If the engine spun the other way it would be opposite. So, yes I would say the left wheel has more traction, and all the more reason to avoid spinning a wheel on slippery surfaces with little traction. Plus when one wheel is spinning it doubles in speed, creating even less traction!


Why would the spin direction of the crankshaft affect this? You're positing just from momentum in a FWD transverse application?

In a transverse FWD application the distance from the trans output to the half-shafts would affect this a bit as well due to increased windup in the long axle or the jackshaft in vehicles with equal length front axles. In a front longitudinal engine RWD or Subaru/Audi style AWD (i.e. equal length half-shafts all around) I would not think there'd be much bias side-to-side.

 

[rationull]

Originally Posted By: Char Baby
I guess what I am asking is...is there one wheel that is more dominant over the other wheel?


There are lots of good answers posted but I think it's important to be clear that the specific situation matters. "Normally" when both wheels have the same traction (e.g. driving straight ahead under power on asphalt) both wheels should be getting essentially the same power, as I understand it -- especially in longitudinal engine'd vehicles with equal length drive shafts to each wheel. It's only when a traction difference comes into play that the torque becomes unevenly distributed and one wheel could be called "dominant".

 

[Traction]

For whatever reason, anytime I have done, or seen a burnout on a RWD car with a open diff, it is almost always the RR tire that spins.