As I read the explanation of a LSD in How stuff works, it doesn't absolutely limit the difference in speed between the two wheels, it means that the wheel that wants to spin faster has first to overcome the resistance of the differential clutches before it can do so. So if you used too much power you could still override the clutches?
It's action sounds very like that old trick of half-applying the handbrake to get grip. The brake doesn't stop a wheel from turning, but it has to overcome the braking effect before it will spin.
|
There are two extremes
1) The standard differential, which divides the torque equally between the driven wheels, and doesn't care what speed they do. i.e., one wheel can be stationary, and the other doing many rpm, but the torque in the driveshafts is still equal.
2) The solid final drive, which drives each wheel at equal speed, and doesn't care what torque is in each drive shaft. (fine until you want to go round a corner!)
A limited slip diff is a device to enable operation at some point between the extremes of 1) and 2)
As the extremes work using an equal split of torque in one extreme and angular velocity in the other, then perhaps the best diff would split power equally, i.e. the product of torque and angular velocity would be equal in each driveshaft. Such a device would, doubtlessly, be even more inelegant in realisation than most limited slip diffs!
Number_Cruncher
|
Wouldn't do on a modern car of course but a solid axle is the truly elegant solution, as on a chain-drive Frazer Nash. Turned the car into a happy and cheerful low-speed oversteerer. The back axle was quite short (i.e. the car crab-tracked as they say) to minimise undesirable scrubbing.
Actually though the only time I went in a chain drive Nash - alas, I didn't drive it - the thing that really impressed me was the brakes, four-wheel mechanical, operated by cables, but with tiny little drums in the middle of those very big, narrow spoked wheels. They worked very well indeed, although would perhaps have suffered fade if worked very hard. Doubtless so effective because the car is so light.
Wouldn't mind one of those as a pet if I had the (considerable) money needed to acquire a nice one.
|
|
|
As the extremes work using an equal split of torque in one extreme and angular velocity in the other, then perhaps the best diff would split power equally, i.e. the product of torque and angular velocity would be equal in each driveshaft. Such a device would, doubtlessly, be even more inelegant in realisation than most limited slip diffs!
Such is the objective of viscous coupled diffs.
|
|
|
|
|
|
BMW fit it to their M cars.
|
Thanks guys.
That reminds me, I'm just off for a Frazer...
|
This reminds me of the typos you still see on Ebay, but used to be in AutoTrader etc.
Cars with this feature would often say in the ad "limited slipped disc"
|
"But when a wheel is spinning it means the other one isn't being driven at all. You have to brake the spinning wheel to a stop to get the other wheel to turn. As I said, this is fine at low speeds in the mud but not what you need at high speeds in a corner."
Not true, when one wheel spins, and the other stationary, the spinning wheel will turn twice as fast as they both would be if both were turning at equal speeds. If the spinning wheel is braked to 50% of it's speed, the other 50% of speed will be given to the stationary wheel. Of course it isn't 100% efficient, but the actual brake force required is minimal, if you work on a car on ice or snow where one wheel is spinning and one stationary, you can often use leather-gloved hands to slow the spinning wheel and make the other turn. Once the other wheel starts spinning too, You can completely stop the whell you are holding with your hands, and the other wheel will turn twice as fast as the speed shown on the speedometer.
|
Ashok
An interesting technique..... But who drives you to A+E with eight mangled fingers after you slip on the, presumably inclined, ice and it all goes a little bit wrong.....?
|
|
You haven't lived until you've found yourself stuck in wheel arch staring at the snow below with a damper squashing your cheek.
|
Traction control slows you down when a wheel starts spinning.
Limited slip diff keeps you going when one wheel starts spinning.
|
|
|
You haven't lived until you've found yourself stuck in wheel arch staring at the snow below with a damper squashing your cheek.
and dont try this at home children ;-)
|
|
As I said, slow it with the handbrake.
|
Would an LSD (on a rear wheel drive car) tend to push the car wide (understeer) on bends?
I remember driving one of those 4 wheel off-road 'bikes' once, that apparently had no differential, and it really didn't want to turn.
|
An LSD shouldn't push the car wide on corners as the effect of the clutches is only enough to prevent the wheel that is unrestrained from spinning. The difference in speed between the wheels in even tight corners is much less.
Mitsubishi have a system called AYC which is Active Yaw Control. They use two pendulum sensors, one at each end of the car. These detect under and oversteer, and a control box sends signals to a hydraulically braked differential. When you are tending to oversteer, the diff puts more torque to the outside wheel (I think) and pushes it back in line. The opposite happens when understeer is detected.
I have this on my Galant Estate VR-4, but it first appeared on the Lancer EVO series from about the 5th generation on. It makes the car corner like, well, nothing else. It's especially good on roundabouts.
|
Good tip, Cliff Pope, I'll remember that.
The old pre diff Dafs had a crude sort of non slip diff arrangement designed into the belt drive system. They were non slip anyway as each belt drove one rear wheel, but the diff effect was achieved by the belts settling into their 'most comfortable' position in the 'Vario' pulleys. This resulted in broken half shafts and rear tyres that lasted at most 20k miles.
|
my MR2 has an LSD - being rwd and 240wbhp it definitely needs it. The versions without were much more tail happy, so it really does help getting the power down and keeping back end in line.
They can be a bit clunky though, if they're plate type diffs.
The car doesn't have TRC though, so can't comment on it's similarities / differences.
Ian
|
This thread reminds me of a conversation I had with my trike mechanic, who builds trikes for a living and has a lot of experience with the old Beetle chassis/gearbox. He said he didn't like the ones with LSD at all, one nearly put him through a hedge once. Didn't get to ask why as we were both in a hurry ...
--
andymc
Vroom, vroom - mmm, doughnuts ...
|
|
|
|
|
