LSD What's it all about
Discussion
hi ben
when you go into a corner in a rear wheel drive car the power is apoplied evenly to the rear wheels
if too much power is applied the inside wheel spins but the outside wheels grips but less forward force is exerted. basically the same as the inside wheel
with an LSD the outside wheel experences greater forward force untill that looses grip then you start to spin
so it comes into its own in the region between 1 wheel slipping and two wheels slipping
really depends on driving style and whether your style is suited to drifting or driving sensibly on the limit
i am a ham fisted animal so require an LSD the other option is to learn throttle control
the LSD on a seven clone should not be needed as they are suited to carring speed through a corner with finess.
however an mr2 turbo needs one :-)
when you go into a corner in a rear wheel drive car the power is apoplied evenly to the rear wheels
if too much power is applied the inside wheel spins but the outside wheels grips but less forward force is exerted. basically the same as the inside wheel
with an LSD the outside wheel experences greater forward force untill that looses grip then you start to spin
so it comes into its own in the region between 1 wheel slipping and two wheels slipping
really depends on driving style and whether your style is suited to drifting or driving sensibly on the limit
i am a ham fisted animal so require an LSD the other option is to learn throttle control
the LSD on a seven clone should not be needed as they are suited to carring speed through a corner with finess.
however an mr2 turbo needs one :-)
If you can afford a limited slip diff, get one.
They greatly increase driveability on throttle.
:: Long Technical Explanation Alert ::
Without a limited slip diff, it becomes very easy to spin the unloaded (inside) wheel on corner exit. The more lateral cornering force you generate, the more unloaded the inner wheel will be and that means you can apply less power before the wheel starts to spin.
A non-slip (open or standard) diff directs all torque to the side that has least resistance. Nothing you can do to stop that. In practice, it means once the wheel's started to slip all the torque will be directed that way and wheel speed will inevitably increase. Inside wheel spins up, all lateral load transfers to the loaded wheel, which if you're on the limit or thereabouts will exceed its grip threshold and the back will step out, fairly quickly and uncontrollably. Armfuls of lock and off the gas waiting for it all to come back to you.
Even if not that severe, you have a reduced amount of grip remaining on that loaded wheel and you can only back off the throttle and reapply, either more gently or once the rear is settled and level again, in order to make progress.
With a limited slip diff, to start with the unloaded wheel does receive more of the torque, but as the relative speeds of the inner and outer wheels get larger (inside wheel spins up further) the diff tightens and permits more and more torque to be directed to the loaded (non-spinning) wheel.
This means you retain traction under power for far longer, and even when traction is broken at both wheels, the wheels don't both spin up in the same way as the unloaded one with an open diff did, so you don't have to lift and reapply the throttle. Nor do you get a sudden sideways moment because there is no dramatic weight transfer once one wheel starts to spin.
The rear of the car may step sideways but not aggressively or unpredictably - it's far more catchable and allows you to maintain forward progress.
Then there's adjustability on throttle.
If you're running out of track on exit and both wheels have just broken traction, more throttle will push the tail wider. You don't have to lift to tuck the nose back in.
If you have loads of track remaining on exit and both wheels have just broken traction, a slight lift and reapply will bring the rear back under control and mean you are back on the throttle sooner with full traction.
They're very intuitive to drive with, they won't make the rear of the car lairy, quite the reverse. You won't end up drifting round every corner unless the suspension and the diff are intended to provoke that.
Also, you get more control over the driven wheels on corner entry. Again, engine braking on the rear wheels through an open diff means one wheel does the majority of the slowing, and it's not necessarily predictable which wheel that will be. A LSD gives a more predictable and equal engine braking effect into the corner.
The only downside to a limited slip diff is the cost, both to buy and maintain. LSD diff oil isn't cheap, and it will need checking and adjusting once every few years by someone who knows their stuff (rally prep specialist, most likely). But for the extra smiles they give you, well worth it.
Like I say, if you can justify the cash (£350 for a Cosworth diff, isn't it?) I'd leap at the chance.
They greatly increase driveability on throttle.
:: Long Technical Explanation Alert ::
Without a limited slip diff, it becomes very easy to spin the unloaded (inside) wheel on corner exit. The more lateral cornering force you generate, the more unloaded the inner wheel will be and that means you can apply less power before the wheel starts to spin.
A non-slip (open or standard) diff directs all torque to the side that has least resistance. Nothing you can do to stop that. In practice, it means once the wheel's started to slip all the torque will be directed that way and wheel speed will inevitably increase. Inside wheel spins up, all lateral load transfers to the loaded wheel, which if you're on the limit or thereabouts will exceed its grip threshold and the back will step out, fairly quickly and uncontrollably. Armfuls of lock and off the gas waiting for it all to come back to you.
Even if not that severe, you have a reduced amount of grip remaining on that loaded wheel and you can only back off the throttle and reapply, either more gently or once the rear is settled and level again, in order to make progress.
With a limited slip diff, to start with the unloaded wheel does receive more of the torque, but as the relative speeds of the inner and outer wheels get larger (inside wheel spins up further) the diff tightens and permits more and more torque to be directed to the loaded (non-spinning) wheel.
This means you retain traction under power for far longer, and even when traction is broken at both wheels, the wheels don't both spin up in the same way as the unloaded one with an open diff did, so you don't have to lift and reapply the throttle. Nor do you get a sudden sideways moment because there is no dramatic weight transfer once one wheel starts to spin.
The rear of the car may step sideways but not aggressively or unpredictably - it's far more catchable and allows you to maintain forward progress.
Then there's adjustability on throttle.
If you're running out of track on exit and both wheels have just broken traction, more throttle will push the tail wider. You don't have to lift to tuck the nose back in.
If you have loads of track remaining on exit and both wheels have just broken traction, a slight lift and reapply will bring the rear back under control and mean you are back on the throttle sooner with full traction.
They're very intuitive to drive with, they won't make the rear of the car lairy, quite the reverse. You won't end up drifting round every corner unless the suspension and the diff are intended to provoke that.
Also, you get more control over the driven wheels on corner entry. Again, engine braking on the rear wheels through an open diff means one wheel does the majority of the slowing, and it's not necessarily predictable which wheel that will be. A LSD gives a more predictable and equal engine braking effect into the corner.
The only downside to a limited slip diff is the cost, both to buy and maintain. LSD diff oil isn't cheap, and it will need checking and adjusting once every few years by someone who knows their stuff (rally prep specialist, most likely). But for the extra smiles they give you, well worth it.
Like I say, if you can justify the cash (£350 for a Cosworth diff, isn't it?) I'd leap at the chance.
FNG said:
The only downside to a limited slip diff is the cost, both to buy and maintain. LSD diff oil isn't cheap, and it will need checking and adjusting once every few years by someone who knows their stuff (rally prep specialist, most likely).
Shouldn't be a problem with an XR4x4 diff as Sierras used a viscous coupling LSD rather than the usual clutch type. You can get them from scrappies for under £100.
grahambell said:
Shouldn't be a problem with an XR4x4 diff as Sierras used a viscous coupling LSD rather than the usual clutch type. You can get them from scrappies for under £100.
Ah good man. You have the advantage on me there.
Well done for trawling the whole of my post, by the way!
The XR4x4 diffs use the same 7" casing as the normal Sierra ones and the same CV joints as other Sierras with rear disc brakes. Cossie diffs were bigger, so don't fit easily.
Drum braked cars had a different kind of CV joint. Sometimes you can swap the CV joints on the inside end for those from a disc braked car and sometimes not, but the only way to tell seems to be to remove the joint and try it!
Drum braked cars had a different kind of CV joint. Sometimes you can swap the CV joints on the inside end for those from a disc braked car and sometimes not, but the only way to tell seems to be to remove the joint and try it!
FNG said:
grahambell said:
Shouldn't be a problem with an XR4x4 diff as Sierras used a viscous coupling LSD rather than the usual clutch type. You can get them from scrappies for under £100.
Ah good man. You have the advantage on me there.
Well I wrote an article about diffs for totalkitcar magazine.
Must admit it didn't go into anything like as much detail as to how an LSD can help improve a car's cornering as your post though.
kambites said:
FNG said:
A non-slip (open or standard) diff....
[pedant mode on]
I think you mean something like "infinite-slip". A non-slip diff would force both wheels to spin at the same rate all the time and thus, in fact, wouldn't be a differential.
[pedant mode off]
Should have said "non limited slip diff"
Ta for that.
v8owner said:
FNG.. how about a torque biasing diff? (Quaife?) and what does that do to performance on the track?
Quaife's torque biasing diff is geared rather than viscous (e.g. sierra - thanks Graham) or clutch plates and ramps (e.g. Escort live axle).
Issue with viscous diffs is that as they get worked, their characteristics change as the viscous fluid temperature changes. Probably not something the average non-pro driver will notice where the heat soak on a RWD vehicle will be fairly minor.
Issue with plate diffs is they can be aggressive and grabby, depending how they are set up of course, and setup is crucial to getting the desired handling characteristics. That's number of plates, gaps between them, and the ramp angle that dictates how quickly they engage, so it's not the work of a moment to make changes.
Main advantage with torque biasing diffs as I understand it (having not had experience of them) is that they tighten up very progressively and never fully tighten, so you don't get to the point the diff is fully locked. Nor does it rapidly engage or disengage.
For competition use, torque biasing diffs aren't ideal in RWD applications, but work very well and are very driveable for fast road and trackday work because they're so progressive. A lack of snatching and locking will be a lot more gentle on your drivetrain and tyres, and easier to get good consistent lapping from.
For FWDs, torque biasing diffs are the best solution; the last thing you need in a FWD is a grabby diff causing torque steer at unpredictable moments, or a diff that changes characteristics depending on the heat buildup in the engine bay.
Conclusion:
On a FWD install, get a Quaife. It's probably the only serious option open to you anyway. Escort RS Turbo is about the only realistic FWD donor I can think of with a viscous LSD.
On a RWD, a viscous diff will be fine. If in doubt try and get a diff with a separate oil cooler (my 200SX has one) which will keep it as consistent as possible.
If you have a live axle, make sure you know how the diff's set up before you buy it, or get it set up professionally. Many of them are intended for gravel stages and those boys like going sideways...
Be careful just banging a sierra LSD on though. Remember, the sierra was a lot heavier than your Tiger, and as such it's diff was configured to provide appropriate slip for that weight of car. When that diff is installed in a much lighter car, it needs a lot more "aggression" to persuade it to slip. What this basically means is that there will be more of a tendancy for the 2 rear wheels to rotate at the same speed, which will induce understeer.
There are strong arguments that if you want an LSD in a lightweight car then you should be looking at a Quaife unit. Of course there are also arguments (cost) against the very same
The above said, I have an open diff so it is just what I've gathered rather than experienced.
Regardless, a Sierra LSD, Quaife LSD or any LSD will allow you to do donuts, which we all know is the REAL reason to have one :P
There are strong arguments that if you want an LSD in a lightweight car then you should be looking at a Quaife unit. Of course there are also arguments (cost) against the very same
The above said, I have an open diff so it is just what I've gathered rather than experienced.
Regardless, a Sierra LSD, Quaife LSD or any LSD will allow you to do donuts, which we all know is the REAL reason to have one :P
I put a Quaife ATB in my LSIS when it looked like the previous diff was beginning to go. They're a great company to deal with (I'd bought it from a dealer who had incorrectly told me that it had the right spline fitment for my car when it didn't - I took it to Quaife, and they changed it for me free of charge while I waited).
I only had one sideways moment before it, and I had none after.
I only had one sideways moment before it, and I had none after.
Thanks for all the info so far.
If I went for a sierra lsd would you need the drive shafts that went with it? Would it make it easier if I did?
How easy a job is it to replace the diff. Are we talking simple swap or a major job. i.e could I do it myself or would I need help/garage?
Cheers
If I went for a sierra lsd would you need the drive shafts that went with it? Would it make it easier if I did?
How easy a job is it to replace the diff. Are we talking simple swap or a major job. i.e could I do it myself or would I need help/garage?
Cheers
There's 4 bolts from the driveshaft to the diff to undo and then the mounting bolts. There are 3 long bolts from memory to undo and then it'll drop out.
I have some bolt on sierra driveshafts that I'll be putting up for sale for about 20 quid or so. To be honest any 2nd hand drive shafts like this will have come off an old car and may be pretty worn. However you can then use them to trade in against new ones and you'll get a new set for 100 quid. I don't think that you can simply buy a new pair without a surcharge but if you can then do that. I'd treat any 2nd hand pair are knackered though. You can't detect play in mine but the cvs are pretty jamned up so you wouldn't be able to tell unless you cleaned them out and regreased them.
I have some bolt on sierra driveshafts that I'll be putting up for sale for about 20 quid or so. To be honest any 2nd hand drive shafts like this will have come off an old car and may be pretty worn. However you can then use them to trade in against new ones and you'll get a new set for 100 quid. I don't think that you can simply buy a new pair without a surcharge but if you can then do that. I'd treat any 2nd hand pair are knackered though. You can't detect play in mine but the cvs are pretty jamned up so you wouldn't be able to tell unless you cleaned them out and regreased them.
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