Thursday 23rd February 2012

A question of diffs

Why do limited-slip differentials matter? Your chance to ask the expert

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Every budding hoon knows rear-wheel drive and a limited-slip differential are two essential ingredients for driving fun. But, as ever, life is a bit more complicated than that.

Relevant to the M Differential how exactly?

And in a world of active differentials, torque vectoring, ESP-based 'electronic' limited-slip diffs and more besides there's a lot to take in. So in an opportunist 'all you wanted to know about differentials but were afraid to ask' kind of forum we're going to talk later on today to Jorg Trommer, product development manager at TransAxle Solutions at GKN Driveline.

So if there's anything you'd like to ask him let us know!

As for Jorg's expertise he's worked at GKN for 24 years and includes the BMW Active M Differential among his back catalogue. Intended or not, the sideways ability of BMW M cars and the M Differential's contribution to this is legendary, as demonstrated recently by our own Mr Harris. So we're thinking Jorg is probably the kind of guy we could get along with.

Jag shows BMW how to do it

A pity then that even after an extensive browse through BMW's press photo selection there seemed to be precisely no shots of M cars sideways in clouds of tyre smoke. We did find one of a (stationary) M5 scaring a horse though. No? Anyway, loosen up fellas and take your lead from the guys at Jaguar, who never waste an opportunity for a good sideways press shot.

We digress.

If you've any questions for Jorg let us know below and we'll get them across to him before we meet up with him later on today.

 

 

[lgomgf]

You don't have a pic of a sideways M car with a cloud of smoke???? Call BBC, they might lend you one.... they have a fat dude there that loves to drive like that...

[goron59]

Ooo, looking forward to this.

One area of confusion I have is how rear LS diffs and centre diffs work in combination... Though that's not exactly an area of BMW expertise.

[jmjo88]

Will brake-steer systems replace mechanical diffs in the coming future?

[Mermaid]

Is BMW's M diff (M3's M3 CSL's) reliable? More costly t maintain?
How does is M3 diff, for the want of a better word different, to that in the 1M?

[velocgee]

1) do LSDs induce understeer? - as what was stated by a Lotus Development engineer when discussing why the Evora, on launch, didn't come with one.

2) with advances in tyre and damper technology[and along with electrical advances] are LSDs really necessary for road cars? (I understand for track use....)

Edited by velocgee on Thursday 23 February 09:33

[varsas]

velocgee said:

1) do LSDs induce understeer? - as what was stated by a Lotus Development engineer when discussing why the Evora, on launch, didn't come with one.

Jaguar say that too. How does the BMW engineer respond to that, does the M diff also promote understeer on turn-in? And if not, why not?

[Steff]

2) How does a geared LSD like the quaife work compares to a conventional plate LSD?

[Dagnut]

3) Do sheep get really heavy when it rains?

[FWDRacer]

Can LSD Diff technology help very powerful front wheel drive vehicles in the future? Application so far has been a miss and hit (head on-oncoming traffic).

[SimbaWC]

Where in the realm of LSDs, torsen diffs etc did the Ferrari e-Diff lie and what desirable and/or undesirable handling characteristics did it give to the models that were fitted with it.

[clarkey]

velocgee said:

1) do LSDs induce understeer? - as what was stated by a Lotus Development engineer when discussing why the Evora, on launch, didn't come with one.

2) with advances in tyre and damper technology[and along with electrical advances] are LSDs really necessary for road cars? (I understand for track use....)

Edited by velocgee on Thursday 23 February 09:33

Yes, an LSD can induce understeer. If you are going round a bend, the inside wheel needs to turn more slowly than the outside wheel. An LSD will limit the difference in turning speed between the two wheels, which could cause the car to push on in the bend rather than turn (ie understeer)
Think of it like a kart, which has no differential. To avoid understeer in a kart you have to get the inside rear wheel to lift.

[Big E 118]

What's next technology wise for mechanical diffs? The E92 M3 uses the same diff (slightly modified) as the E46 M3's which is "old technology". Is there anything new on the cards or are electronic diffs the future?

Dagnut said:

3) Do sheep get really heavy when it rains?

[rhinochopig]

Motor-In-Wheel technology, which is being driven by the demand to lower CO2 emissions will, in all likelihood, kill of the diff as 4 independently controlled motors can provide the ultimate in torque vectoring. Given that, where do you see the future of GKN? Are you developing electric drive-trains?

[V12 Migaloo]

In the past Cars were set up without traction control etc and generally the higher end models had LSD's, now days I think cars are set up already with the traction control in place and hence when you turn them off the rear wheels tend to loose traction much more easily. Why dont manufactuers set up their cars without traction control and then add this afterwards... I also believe that the same applies for ABS equipped cars..?? The reason I say this is that I have driven every single Jag saloon since the 90's up to the X351 (all equipped with traction control and ABS) and in my opinion the have a lower grip threshold then my 85 LSD equipped V12, the same applies to the brakes, the ABS cuts in far sooner then when the brakes lock on my V12.... Just a thought..

[Strawman]

Dagnut said:

3) Do sheep get really heavy when it rains?

They get heavier, but not as heavy as you'd expect due to water repelling oils in the wool.

[Crusoe]

From the other thread where m diff is being used on a Z4m coupe.

What is the effect of not replacing both rear tyres in the event of a puncture. Is having one new and one part worn tyre going to be an issue?


http://www.pistonheads.com/gassing/topic.asp?h=0&a...

Edited by Crusoe on Thursday 23 February 10:18

[Some Gump]

If you put a 747 on a conveyorbelt, and it has an LSD fittted - will it understeer off?

[rhinochopig]

Second question:

Having been the recipient of a big repair bill from trying to fault find and subsequently rebuild the micro hydraulic system as used in the Mitsi ACD/AYC system, do you think these clever diffs are a ticking bomb with respect to second, third, fourth, etc. owners?

[_g_]

I'd be interested in a description of how each type of LSD works.
Only finally worked out how a normal differential works when I saw a friend's lego model tractor (which I'm sure should have had a Locking differential option )!

rhinochopig said:

Motor-In-Wheel technology, which is being driven by the demand to lower CO2 emissions will, in all likelihood, kill of the diff as 4 independently controlled motors can provide the ultimate in torque vectoring. Given that, where do you see the future of GKN? Are you developing electric drive-trains?

I've got a hub-motor'd mountain bike. Coming from motorbikes, there's no way I'd want extra weight in a wheel of a performance orientated machine - people spend a lot of money reducing unsprung weight as you get the most benefit from that generally.
However, of course even for performance application, this doesn't preclude separate motors in-board, of course.

[Kawasicki]

Is it always the case that a lsd will induce corner entry understeer?

Surely partially locking the rear tyres together can also induce (non power) corner entry oversteer, as the total grip of the rear axle will be reduced because each tyre is not independently free rolling, and therefore not capable of producing maximum lateral grip.

Edited by Kawasicki on Thursday 23 February 10:42