LSD's - Not the trippy stuff....

S

spinningfish

Member
Surely there has got to be an easier way of getting a LSD on the back of a P6.

I see lots of talk of Jag conversions, but these look very, very expensive. We're P6 owners (nearly!) after all....

Does anyone have any more details on Alan Butterworth's LSD conversions, or can the internals of a BMW or Capri LSD, for example, be fitted?

Thanks

Fishyyyyyy
 
I very much doubt if there is a cheap way of doing this!

Odly enough the key problem is the inboard rear brakes. Virtually all rear diff's that were (or are) available with LSD's have outboard brakes; the only one I can think of that has inboard brakes is the Jag.... Converting a P6 to outboard brakes is a complete non starter using the current hub carriers. I spent a bit of time looking at how possible it would be to fit a Sierra diff, as being the set up closest in spirit to the Rover that I knew, and eventually concluded it couldn't be done without such major work that you might as well go down the Jag or Alan Ramsbottom route.

The only solution that does still intrigue me would be to do a complete transplant from one of the 80's rear drive Alfa's. I have yet to see an Alfa rear end in sufficient detail to assess how feasible this is though. The potential advantage is that the Alfa has a de dion rear end so it could be possible to do a complete transplant onto the existing Rover suspension pick ups. It's still going to be a pretty major operation though, cos the Alfa has the gearbox in unit with the diff, so you need to find some space in the Rover floor pan for that, as well as devising a dummy bellhousing for the engine to fit the clutch and prop take off into. The other disadvantage is that the Alfa has sliding half shafts and fixed de dion rather than the Rover's sliding de dion and fixed half shafts. Sliding half shafts lock up under power giving rise to the Alfa's well known rear end wriggle in fast corners...

If you were to try to do a complete transplant from say a Sierra or BMW, the lower trailing arms would be OK but you then have to find a pick up point on the body for the upper trailing arms. I would want to put a huge amount of reinforcing in to accept suspension loads where none were before.

That pushes you back on complete Jag rear end or the Alan Ramsbottom mod.

The Jag rear end has the advantage of being self contained on its own subframe - so strengthening to accept suspension loads is relatively starightforward; but has the disadvantage that it is too wide in standard form, so that you have to shorten the drive shafts and lower links. The suspension geometry is inferior to the Rover's de dion as well.

The Alan Ramsbottom mod is brilliant in using all the existing casings etc. Not so good in that, if you actually need an LSD then you are probably going to come up against the Rover diff's principle weaknesses - the long input shaft isn't strong enough (although Alan will sell you a strengthened one) and you are likely to start eating output shafts. The latter is potentially very serious as there are not many new ones left! I have a stock and Alan has a stock, but it's a long time since I've seen any anywhere else. Look on the bright side, we'll soon have to start making reproductions, which gives us the opportunity to improve the design!

Of course if you really have money to burn the Aston Martin V8 rear suspension is astonishingly similar to the P6....

The issue with where you put suspension loads into the shell is that the Rover is even more unusual than appears at first sight. The front trailing arm pickups are pretty obvious but how many people realise where the lateral cornering loads enter the shell? Yep, down the half shafts and into the diff! That makes the half shafts and diff internals pretty well bomb proof because the cornering loads far exceeed the driving loads. But it makes the diff pick ups onto the shell the only real stong points at the rear of the car.

I'm pretty sure the problems with the diff input shaft and the output shafts failing in torsion is to do with what happens when a road wheel that is spinning finally grips. This scenario is a major torsional shock. It is possible that an LSD might substantially reduce the occurence of this.

The other way to solve this conundrum is to fit wider tyres. The standard 185 (165 on the 4 cyl cars) is very narrow by modern standards for the power output. If you get the correct wheel offset it is possible to go up to 205 in the rear mudguards without bodywork (D post) modification. 205 section tyres ought to reduce the propensity to spin wheels significantly. (see other threads of mine on the intricacies of getting wide wheels into the rear arches!)

Hope that gives a bit of food for thought!

Chris
 
Only point I'd like to bring up with respect to increasing tyre grip, is that this effectively increase the max torque loading in the drive train, as in standard form the lack of grip limits the peak torque.

Although I used to run 195/60's on the back of my 2.2TC and span the wheels on a daily basis (a tight roundabout near work meant it was always tail out) along with constant thrashing, and I never managed to break any of it. But it was only a standard 2.2TC

What about the terribly retrograde step of fitting a LSD live rear axle instead ?
 
chrisyork said:
The Jag rear end has the advantage of being self contained on its own subframe - so strengthening to accept suspension loads is relatively starightforward; but has the disadvantage that it is too wide in standard form, so that you have to shorten the drive shafts and lower links. The suspension geometry is inferior to the Rover's de dion as well.

Chris
Click to expand...
Are you serious? The jaguar rear end has outstanding geometry, camber changes to keep teh tyre upright during pretty much the entire suspension travel.
It is for all intents and puropses as close as you're going to get to a perfect setup in a production sedan.
The Rover is not, it makes the rear wheels travel in one plane only, the plane the hubs swivel in the DeDion tube. They are far from being at zero camber at all body roll angles.
Yes the Rovers are lovely, but the rear does roll steer and is a litle tail happy. My Series 3 XJ6 did not roll steer at all........I've never driven a more surefooted and confidence inspiring car, and in all conditions.
The Jaguar also has much less unsprung weight than a DeDion setup.

Regards, Andrew.
 
Hi Andrew

The de dion in the Rover actually guarantees that the wheels are vertical to the tarmac irrespective of body roll - it's equivalent to a live axle without the unsprung weight of the diff. That is not to say that it counteracts the effects of tyre-roll on the rim. Some of the modern multi link systems have a stab at this - but remember that each time you put wider tyres on the back of your BMW (be it down the local wheel shack or care of BMW's M sport department) you would actually need to change the complete geometry of the suspension to retain the same characteristic, and then it would only really work at a particular cornering G force! So ALL systems are a compromise to one degree or another!

The Jag, by contrast, sets out to mirror a double wishbone system. That's fine as long as you get the centre point of the wishbone coincident with the roll centre of the car. No problem in a race car with a very low roll centre but not feasible in a saloon. The Jag then further compromises the position by using the drive shafts as the upper wishbone in the interests of cheapness. They ought to be mounted way above the diff if you were to get something like the right result. Go look at the back of any formula one car since at leasst the mid 60's to understand the proper layout for this system. For a saloon, the pivot points of both lower and upper arms would need to be much higher relative to the diff because of the higher roll centre.

Sorry, the Jag is a VERY poor man's set up. You might get lucky and strike a good result in (say) an E type, but by the time it's in a tall saloon and you've shortened the lower wishbone and driveshaft to get the width right the chances of coming any where near a de dion in performance are extremely thin indeed.

Chris
 
I was checking my Ebay tonight, and for those committed to the Jag route look at item no 130124285348 .

In my last reply above I didn't address the comment about roll steer on the P6 properly. This is a rather different issue to the rear suspension geometry. While it is influenced to some degree by the angle of the wheels to the road, this is primarily a handling issue. What I mean by that is that we are really talking about the balance of the car between front and rear. This is a discussion about the relative roll stiffness front to back rather than it's absolute value. The French used to demonstrate superb handling cars that rolled onto their door handles (the 2CV springs to mind) and the Rover follows in that tradition with soft long travel springing and relatively low roll stiffness. Like all coil sprung cars (multi-leaf springs seem less affected by settling and have their own inbuilt damping through friction between the leaves) the Rover seems to be especially badly affected by spring settlement and weak dampers at the rear. If you are suffering from "roll steer" - I'd interpret that as relatively unprovoked oversteer? - then first port of call would be to check the rear ride height (the workshop manual has simple instructions and dimensions for this). After the likely change of springs and dampers, if still not satisfied, then its time to play with front and rear roll stiffnesses. This is done BOTH by increasing spring stiffness AND by increasing roll bar stiffness (or fitting one in the case of the rear!). And don't forget its the balance front to back that is the key issue!

Chris
 
Your use of the 2CV as an example shows up the difference between roadholding and handling, roadholding is exactly what it says, how a vehicle holds the road, handling is how it does it, therefore while the 2CV may have good roadholding, the handling is terrible because of the excessive roll.
Back to the P6, in standard condition and with no faults you'd have to go a long way to find a similar type and size of car that had anywhere near as good roadholding and handling.
 
Having had in, 1979 and 1980, first a p6 2000tc 4spd then an xj6s1 4.2 auto, and later in '87' a rare lwb xj6 s1 daimler auto and now a p6 3500 auto,I can honestly say that when either car is set up properly,the ride quality and handling are second to none, I used to chuck them about when I were a lad,and now enjoy cruising comfort and surefootedness.I have std adjustable rear shocks,but only for when I need to tow.I do on occasions drift the local roundabout when its quiet and a little damp just for fun :p any rwd car will roll and break away if pushed,but with such a weighty car,I dont want to end up pointing the wrong way when a lorrys right up my arse :p
 
I never really tried to understand the physics that govern the P-6 handling, but I was always amazed at its ability to turn down a street after I had almost missed it. As I get older, this ability becomes more valuable

Dick West
 
My amazingly cheap and wide rear tyres, combined with worn rear springs and shocks, capri bump stop rubbers, and a lack of rear end weight (no roof so no rear glass etc), produced a P6 that would oversteer at the slightest provocation, to the point where I was at nearly 90 degrees on a motorway sliproad once and it was still fishtailing at 70 !, at that point I decided not to drive it in the wet anymore :(

Funnily enough, in the dry and in a straight line it was practically impossible to get the rear wheels to light up, it would just grip and go, much to the shock of the local chavs.....
 
I have a mate who raced a P6 in the local classics in Cape Town and he had the Alfa Romeo transaxle installed in his P6.

He later built a parrallel link setup for the back that worked pretty well and I think he is onto version 2 by now with a LSD out of a BMW in a purpose built frame with parrallel links and slip joint halfshafts.

If anyone is interested I can try to get info/ pictures etc from him.
 
I've enjoyed reading chris's theories on "roll steer".

The P6 seems to transfer a lot of weight transfer to the rear of the car when cornering. This is why a P6 with worn out rear springs and shocks handles badly, especially in the wet. When a car enters a corner, the weight transfers to the outside, so if you have a poorly rear spring and shock absorber, a lot of the weight will transfer to the rear wheel,quickly, and dial in the opposite lock! :O The weight has to go somewhere.
The spring controls how far the suspension will move, and the shock absorber is there to control how fast the spring is allowed to react, and also to damp out the natural bouncing effect that a spring has.
Now the tricky part! Matching rear spring and shock rates to suite the car. Generally a car manufacturer will have spent thousands of hours coming up with the best possible trade off against comfortable ride, handling, stability, and traction. And along we come looking for more from our P6! Fitting stiffer rear springs, and uprated rear shocks will help, to a point. But go too stiff, and we're back into oversteer. This time it's not because of too much weight transfer into the rear, but loss of traction due to the rear end being too stiff. Also there is now more weight transferring into the front. This brings us nicely to what most P6's are lacking on the rear.......an anti roll bar. This gives the opportunity to run springs and shocks that aren't excessively stiff, and will give a good balance between having a car with a reasonable ride, and good traction. The anti roll bar helps prevent the excessive roll caused by sudden weight transfer, but again, you need to get an ideally rated anti roll bar, as the same situation can result as having springs that are too soft or too hard.

Sorry to sound like a completely sad anorak!! :D
 
Hi

Unstable Load - I'm very interested indeed in your mates car in SA. Perhaps a few details on the forum and would he be OK if you PM'd me his e mail?

Richard - Spot on about the rear roll bar! They are of course available from Alan at Leighton Buzzard along with heavily uprated front ones!

Chris
 
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