Trying to decide which lsd to find for my 901/911 street /track 2.2s spec build. I have only run it with an open diff and my perception is this. On transition from trail braking into a corner with slight oversteer it is currently pretty controllable with the throttle ,myassumption isthat this would be even a little more stable with an lsdwith some lockup on decell.would a gt or other lsd give the same effect/ Opinions? Steve
Generally, a limited slip differential should add understeer when you're on the gas. I've never experienced much of anything under hard braking. With a SWB car I tend not to fiddle with "trail braking" so I cannot give you any insight there.
johnt
I tend to prefer Plate type diffs as opposed to ATB's as they are more tuneable to suit different driving styles so it is not an easy question to answer.
There are three major variables that you will need to consider.
Preload - which is the amount of static torque that is needed to allow slip to occur. This will have some influence on understeer. I would tend to try to limit preload to around 25 lbsft
Coast Ramp Angle - by limiting the amount of slip that occurs on a trailing throttle and preventing one wheel from locking up the car will generally beome more stable under braking and you should be able to move the brake bias rearwards which should shorten braking distances. if you trail brake it should give very positive results.
I would tend to consider between 20 and 40 degrees. 20 is more aggressive than 40.
A good coast ramp will also help with reducing Trailing Throttle Oversteer. Lifting in mid corner will tend to promote understeer and limit the cars tendancy to spin.
The problem is that an aggressive coast ramp will tend to increase understeer particulalry if you don't trail brake.
Power Ramp Angle - The LSD will help to transfer power to more heavily loaded outer wheel. It will tend to reduce power oversteer as the inner wheel will drive more effectively. The problem is that if there is too much 'lock up' (such as a spool) it will be failry easy to overcome the outer wheels grip and power oversteer will than take over completely. It is a fairly fine balancing act.
The relative roll stiffness of the front and rear of the car affects the power ramp settings. The stiffer the front the less weigth transfer there is between the rear wheels and more 'lock up' can be applied. It is, however, quite easy to build a car with very rapid break away which would give little warning of spinning.
I would think ramp angle of between 65 and 80 degrees would be a starter.
The number of clutch packs will also have an effect and can affect lock up without having to change the ramp drivers.
The infuence of adding clutch packs is simiar to reducing ramp angle.
(The shallower the angle the gretaer the locking force applied to the clutch pack- 90degrees give zero locking effect)
I think this is a reasonable explanation of what happens, and I believe most of the bad press about plate type LSDs is due to inappropriate selection of the basic characteristics for the intended duty.
thanks for the replies! This is the type of info im looking for re. tuneability ,I have heard mostly specs on "% lockup" how does this compare with the specs you mention?I believe the factory reccomendation at the time (71) was for 40%(I dont have my sports purpose manual in front of me)so thats probably what I would start with. What are the currently available types that offer this tuneability.The ZF lsd with the correct output flanges is difficult to source,so that will factor in my decision. Steve
Trying to compare and figure out differences in LSD behaviour is quite difficult and I am not sure if I understand all of the nuances.
Locking Factor is a common way of describng a plate type LSD with a high number being considered 'better' than a low number. Porsche, for example, used to offer a 40% Locking Unit for road use and an 80% Locking Unit for Motorsport.
In reality conventional 'open' differentials have a zero locking factor; solid drives being 100%. In between these two limiting conditions it is much more difficult to define the precise manner in which torque is distributed without reference to sophisticated test facilities and I think the numbers are misleading and don't tell the full story.
The definition of Locking Factor is:
Percentage Locking = (Torque difference / Torque at the ring gear) x 100
Torque difference = (Torque High Traction Side - Torque Low Traction Side)
This doesn't really explain much in terms of tunability particulalry in terms of coast and power ramps.
I tend to think it is sixties jargon and a bit simplistic.
Torque Bias seems a better way of looking at an LSD.
Torque Bias is simply the ratio of High Traction Side Torque/Low Tracion Side Torque compared to unity.
A 4:1 Bias Ratio will allow a total of 21/2 times more torque delivery to the drive axles than an open diff.
The bias ratio is affected by the ramp driver angle and the number if friction surfaces.
It is possible to make a basic comparison between Locking Factor and Torque Bias and I have agraph but it won't copy (send me a PM with e-mail address and I will forward a copy) but I think it is better to define Plate Type LSD's in terms of preload, Power and Coast ramps.
With regard to manufacture there is quite a bit of choice:
I am not a huge fan of the early ZF for two reasons:
Firstly there seems to be are two basic settings, 40% and 80% but neither define the preload and ramp sets and it is difficult to find alternative ramp drivers if you don't like the way it is configured. Changing prelaod is more staightforward and can be achieved with changing plate thickness.
Seconly the end housings are generally cast iron and they can break in severe conditions.
There are many altenatives depending on budget;
Drexler Motorsport - Very Expensive but has excellent cross pin arrangement and alternative ramp drivers are available.
Drenth - Made in Holland and ramp drivers can be specified.
Carbonetic - Looks a good design but I don't know about ramp driver availability
Guard - I think the Guard diffs are similar in design to the ZH but made in steel
Modena Engineering - Australian well made and very strong ZF Copy
Gripper - UK Good Choice of ramp drivers and plates but plates are only parkerised and tend to be noisy and wear quickly
KAAZ - Excellent diff, well made, well presented but generally suppled as '1.5 Way' which determies the ramp angles and trying to communicate with japan to but alternative ramp drivers has proven almost impossible.
Colotti - Italy Excellent diff but hugely expensive.
Giken Super Diff - Japanese - has some interesting concepts but I am not totally convinced that the adjusments can be made to fine tune the diff.
One problem I have is that most suppliers offer the 'perfect' diff that doesn't need adjusment and this has always seemed an unreasonable claim.
We run a couple of rally cars and we use quite different LSD settings on Gravel and Asphalt. We have spent a great deal of time 'tinkering' and changing ramp drivers, plates and preload to arrive at the best compromise.
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