A question of diffs


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?
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
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.

 

 

Comments (98) Join the discussion on the forum

  • Marf 28 Feb 2012

    Kozy said:
    Marf said:
    Kozy said:
    For a FWD racing car, is a helical or plate diff a better option?

    Do Quaifes/Torsens/helicals work under braking?
    Helicals require both wheels to be loaded in order to work, i.e. if one wheel is in the air, then no power is being transmitted to either.

    Plate type diffs do not have this little foible.
    That's all very well. But since plates effectively lock rather than bias the torque, surely they will tend towards under steer and also lose the torque vectoring effect that the helicals have, felt as the car steering itself into the corner?
    I can only speak from experience of driving the same car with both a torsen and plate type diff and say that both felt pretty similar during cornering (entry, apex and exit), though the plate type could be felt locking and unlocking markedly when adjusting the throttle, whereas the torsen was smooth as butter.

  • Kozy 28 Feb 2012

    Marf said:
    Kozy said:
    For a FWD racing car, is a helical or plate diff a better option?

    Do Quaifes/Torsens/helicals work under braking?
    Helicals require both wheels to be loaded in order to work, i.e. if one wheel is in the air, then no power is being transmitted to either.

    Plate type diffs do not have this little foible.
    That's all very well. But since plates effectively lock rather than bias the torque, surely they will tend towards under steer and also lose the torque vectoring effect that the helicals have, felt as the car steering itself into the corner?

    This is outweighed by the pros on a RWD car, but on a FWD, increased understeer and steering effort must surely be detrimental. With a heavy front bias on weight distribution, is unloading the wheel an issue like it is on a RWD?

  • Max_Torque 25 Feb 2012

    Kawasicki said:
    Yes, I meant apex speed. Thanks for the discussion. Besides the tech, the future looks a little boring.
    Just as long as there is one of these:



    we'll be fine ;-)


  • Kawasicki 25 Feb 2012

    Max_Torque said:
    Kawasicki said:
    Max_Torque said:
    Kawasicki said:
    What I am talking about is different. Take a corner, where the maximum speed a vehicle could carry through is 60mph. Which would be better if you entered that corner at 61mph, DSC or torque vectoring?
    neither, you'd be unable to make the turn in either case!
    If calibrated correctly a stability control system would recognize in the corner entry phase (before the required peak lat acc was reached)that the vehicle was nearing the absolute limit. If the system brakes, then it is conceivable that the vehicle could slow to 60mph before it slides off the corner. Adding to the speed however is a disaster, so using torque vectoring alone in my opinion is not logical.

    Now, give me a few minutes to read the rest of your post!
    But you said the maximum speed the car could carry through is 60mph, so 61 = crash ! No DSC system can break the laws of physics. If you mean't that 60mph was the maximum apex speed, and the road was wide enough to allow the car to scrub off that extra 1mph without hitting anything then yes, DSC is the one to have ;-)

    Currently of course the systems are only using Handwheel angle as the driver directional demand, so it is still up to the driver to do the correct thing (and why so many people just drive straight into the thing they hit as in the "heat of the moment" they forget to steer.........)

    The next-big-thing in active chassis dynamics in undoubtedly "holostic trajectory planning" where a large number of sensors, some operating outside of human capacity (think IR sensors to see at night, or even roads embedded with corner ID/info tags that relay the severity and surface conditions of the next bend to the oncomming car etc) all add information to the dynamics controller so it can preempt the future !!


    In conbination with peer-to-peer inter-car networking it's an interesting time to be in vehicle development ;-)
    Yes, I meant apex speed. Thanks for the discussion. Besides the tech, the future looks a little boring.

  • Kawasicki 25 Feb 2012

    Max_Torque said:
    Kawasicki said:
    What I am talking about is different. Take a corner, where the maximum speed a vehicle could carry through is 60mph. Which would be better if you entered that corner at 61mph, DSC or torque vectoring?
    For example, in a fully 3 diff active WRC car, you must not use any significant countersteering lock ! (in effect the system now thinks you want to turn left in the middle of a big drift in a rhd corner, which is bad ;-) Except, when you are beyond the yaw authority of the system you have to use countersteering to augment the systems capability to return the front of the car to, er, infront, of the back (if you see what i mean). Hence you have a car where initally you deliberately don't react as a driver to yaw instablility, then, at some point depending on a million factors such as road surface, camber, friction, speed/momentum, etc etc, you must not react.

    Learning that point is difficult, and why a lot of privateer drivers who got into full fat WRC machinery went a lot slower initally untill they had adapted to those changing responses
    Thanks for the info. There are obviously many parallels with DSC systems, as both systems are comparing driver requests to estimated path. The opposite lock comment is also very familiar as that is also used by DSC to fire up oversteer control which really kills speed. That's sort of my point, I can only imagine how much I would brick it if I got a shot of torque propelling me to the right in a long fast left hand bend.

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