Limited slip differential vs traction control

[Dman877]

My dad is going to order a 135i asap but he mentioned to me his concern over the fact that these cars won't have an LSD. Anyway, when he went to the BMW dealer to reserve a spot in line, he talked to a salesmen about it and this person told him that an LSD wouldn't be necessary since the 135i was going to use the ABS system to control traction (isn't that what traction control does?)

Now, can some give me a concise overview of the differences between a traction control system and an LSD? I realize the LSD is mechanical and traction control is electronic, but as far as the effects they have, what's the difference?

 

[tweekah]

Read this

 

[Dman877]

That helps. Thank you.

 

[Zenmervolt]

Having had cars with both, I personally like a clutch-type limited slip more than I like traction control. However, the car is much more stable in poor conditions with traction control.

The reason is simple. With a limited slip, if one wheel has no traction, it will simply spin at the same speed as the wheel with traction. However, that also limits its ability to provide lateral traction. With traction control, the brake is applied to the wheel without traction, preventing it from spinning and thereby maintaining some lateral grip. This means that the limited slip will make a car more "nervous" in poor conditions as the rear end will tend to jump around more.

It comes down to what the driver is accustomed to, but overall I think that traction control is superior in most on-road situations.

ZV

 

[shabby]

Lsd and traction control are 2 different things, i dont understand why you're comparing them. You want a car with both, not either one.
Im also surprised the 135i wont come with an lsd, it should be standard in a car with 300hp.

 

[mwmorph]

Originally posted by: shabby
Lsd and traction control are 2 different things, i dont understand why you're comparing them. You want a car with both, not either one.
Im also surprised the 135i wont come with an lsd, it should be standard in a car with 300hp.

335is have open diffs too i think.

 

[sjwaste]

The 335i was looking pretty attractive to me a couple weeks ago, but really after thinking about it, there's too much BMW emblem premium built into the price. For $40k, I expect an LSD. I mean, that's not the only point of hesitance I've found on buying one, but it's the one relevant to this thread.

 

[Zenmervolt]

Originally posted by: shabby
Lsd and traction control are 2 different things, i dont understand why you're comparing them. You want a car with both, not either one.
Im also surprised the 135i wont come with an lsd, it should be standard in a car with 300hp.

They perform similar purposes. Functionally, there's little difference in effect between brake-based traction control and a clutch-type LSD. They will both send the torque back to the one wheel with more grip. One through a mechanical reaction to a differential in wheelspeed, and the other through the electronic intervention of the brake on the slipping wheel.

Now, a TORSEN LSD, which used helical gears instead of friction clutches, will still allow a wheel with no traction to spin freely without transferring torque to the wheel with traction, so that particular flavor of LSD will not have the right characteristics for giving traction in poor weather.

A viscous LSD will still transfer torque like a clutch-type, but it reacts more slowly than a traditional clutch type.

ZV

 

[exdeath]

Limited slip is a mechanical unit related to the differential and it passively balances torque to the drive axles and allows the wheels to turn at different rates when cornering and locks them together when going straight or when neither wheel is spinning.

Traction control is a computer module that actively monitors traction conditions based on the ABS sensors and selectively applies braking to independent wheels and also modulates throttle and fuel/timing to control overall power to the wheels. Most are smart enough to determine if the slipping is desirable (intentional) or not based on the angle of the front wheels.

You still need the LSD to allow the rear wheels to mechanically decouple and spin at different rates around corners regardless if you have traction control or not.

LSD is only concerned with getting the power to the ground in uneven traction conditions.

Traction control keeps you out of the ditch when your foot makes a mistake.

 

[Zenmervolt]

Originally posted by: exdeath
You still need the LSD to allow the rear wheels to mechanically decouple and spin at different rates around corners regardless if you have traction control or not.

Not quite. An open differential will do just fine at allowing the wheels to spin at different rates. Heck, so will a locker as long as you remember to manually de-couple it when you don't need it.

Traction control will not keep anyone out of a ditch either. Stability control might, but traction control isn't concerned with lateral movement.

ZV