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Permantently lock centre diff?

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Hi all,

The drivetrain in a 1990 Celica GT-Four consists of an open front differential, a viscous coupling centre and a torsen rear differential.

I intend on changing my rear diff to a clutch type LSD - reason being at for example Hungry corner at Lakeside, the inside rear wheel lifts off the ground and when this is happening it's spinning wildly trying to get traction. I figure with a clutch type LSD in there, I'd at least still get drive from the outside rear to push me out of the corner.

Now, one product that is on the market now is a clutch type front LSD which fits inside the transaxle. This effectifely replaces the open front, but in doing so, it deletes the viscous coupling - effectively locking it permanently.

The rally guys love it, I'm not so sure this would be such a good thing in a circuit racing car and it might make the car more prone to understeer?

It's just food for thought at the moment - I hadn't intended on changing the front or centre diffs at this stage. But I'd be interested in hearing any feedback from people who might have experience in this area.

Thanks.

I would expect you to suffer badly from transmission 'wind up' as the front and rear axles will be be forced to rotate at the same speed which will cause massive drag as the tyres are forced to slip in anything other than a straight line. The result will be a considerable reduction in net power to the track, increased understeer, increased steering loads, and kick-back.

Way back in the day, AUDI used a locked centre diff' on tarmac, relatively successfully, but they had less competition and I understand it needed a rather violent technique to upset the car and lift a front, or rear, wheel and/or provoke sliding of the car.

The car will probably be more susceptible to transmission breakage, too.

Hi Nick, in my experience with 4wd circuit cars a locked centre is too aggressive. It can be effective (but still considered a brute force approach) tool for rally cars, but obviously they do so much slipping and sliding anyway it's not as much of a problem.

A plate type rear diff is a no-brainer, will be a good thing! I've never had much luck with a Torsen type in any car, but maybe I never used them right!

The front I think is a little more nuanced. I would be inclined to run the car as is and wait to see how much inside front wheel spin I ended up with (maybe you already know this is an issue?) which in my experience is the limiting factor, a lot of this come down to the circuit layouts you will use. If you have inside front spinning, then you may as well not carry around all that stuff in your car that makes it 4wd 😅. If you do, then I would suggest a plated diff is likely a good option. There's usually a lot of tuning options for these too - ramp angles, preload and plate stacking options.

The most effective option for a proper centre diff is an externally adjustable/controllable unit. May or may not be an option for you but will likely be extremely pricey if it is. I have no time for viscous diffs, these belong in RAV4's, CRV's and the bin. But if I had to bet I would guess staying with the viscous centre is probably a better option than locking it for what you're doing.

In my car I have a 1 way (acceleration) locking front differential, a ECU controlled DCCD and a 2 way locking rear differential. I did have the front as a 1.5 way but this caused understeer on turn in, I do suffer from some understeer coming out of a corner if I am too aggressive on the throttle with the 1 way front, but a lot less than I had with the open or 1.5 way. The open front diff used to just want to push under cornering, especially if I was braking into the corner as well.

I do run the rear with a stiffer rear bar and minimal droop so it does tend to unload the inside rear wheel on cornering, but from looking at the wheel speed traces this is not causing the rear wheel to spin.

Tim, might have been partially due tot he way the chassis was loading the tyres? With the "Torsen", or gear type, torque biasing diff' there needs to be a resistance for the transmission to work against and if the inside tyre is too lightly loaded it won't work correctly. IIRC, the maximum ratio is around 6:1, so if the inner is unloaded enough there may not be enough for the total torque capacity across the axle - if it's actually lifting, the diff is effectively working like a conventional 'open'type. AFAIK, this is the main reason they're not generally used for track and rally use where at least one drive wheel is normally very lightly loaded or even lifting. I believe QUAIFE, for a short while, had a variation with a small plate-type LSD incorporated into the centre of the ATB to give the required resistance even if one wheel was lifted, but don't recall that being available for years - probably down to cost, demand, and maybe reliability issues.

For a FWD road car, the ATB seems to be very popular because it does work well, is progressive in that it's action is smooth without undesireable feedback through the steering or tyre slippage or 'chirping', and as there is usually rather limited load transfer there's not the drive loss from unloading the tyre excessively in corners.

Thanks for the replies everyone, yes unfortunately the nicer electronically controlled centre diffs as seen on Evos and GTRs just isn't an option on the GT-Four, even aftermarket, unless you go the the extent of fitting a whole new drivetrain in there (not an option for me).

Plan is definitely to go with the clutch type LSD on the rear and keep the rest as is, your comments have confirmed my thoughts on the matter.