27 replies to this topic

[jed]

Stu,

If it helps you out at all, there are many papers on the subject available from the SAE

Suggest Cyberman looks at Understeer

[bw64870]

Its quite possible for an LSD to induce understeer, a logical way of looking at it is the rear now has more grip (traction) so this extra drive from the rear can push the front wide. I don't see it as a major problem though, a change of driving style or adjustment to geo/setup should easily solve the problem and the benefits from an LSD in most cases would be much appreciated, but like all things it isn't something that you just drop in and get a magic result everytime, it might need some other work or like i say a change in driving style to get the most out of it.

[cyberman]

Ian,

Discussing understeer at a theoretical level......erm...back in a mo

Stu

Stu

For the avoidance of confusion I do know what understeer is - its where you go in the hedge front first.

What I wanted to know was why you seem to suggest that a LSD will always induce understeer. It clearly isn't an unmanageable problem as I've driven loads of of road and track vehicles with different types of LSD and none of them was intrinsically understeering.

I once drove the 5 litre car with the differential totally locked. If anything it gave a very nice predictable bias towards oversteer under power. So, I am confused and really want to understand the engineering issues which cause you to say that a LSD induces understeer.

Kind regards - Ian Douglass

[jneill]

Google found > this <:

...it will also increase understeer in tight corners under dry conditions. This is simply due to the fact that with a limited slip, the drive wheels tend to want to turn at the same speed, making the car tend to want to go in a straight line. When it is slippery, however, both drive tires will tend to lose traction at the same time, increasing oversteer.

HTH, James

[cyberman]

Its quite possible for an LSD to induce understeer, a logical way of looking at it is the rear now has more grip (traction) so this extra drive from the rear can push the front wide.

It seems a good suggestion but surely most of the time the differential rotation rate between one rear and another is modest and the LSD will not greatly affect characteristics in this state.

The LSD allows a percentage of difference in rotation speed between the wheels. As this difference increases some work progressively to limit the difference and some just lock up with a bang. When restrained one or both wheels are going to have increased surface slip in other than a straight line (same rolling rate on different radius bends). This seems to me to preferentially induce oversteer and not understeer. That aligns with my experience with DG400s and so on.

Where they come into their own is when one wheel loses adhesion and the other doesn't. Here without LSD you can get fairly extreme differences in wheel speed which can lead to all sorts of unpleasantness - a need to back off for example or subsequent asymmetric thrust causing extreme rear steer. It doesn't have to be variable mu, bouncing or lifting a wheel can cause it.

If you want to describe eliminating rear steer as induced understeer I don't mind, its all relative, but I'd never have thought to do so.

Kind regards - Ian Douglass

[cyberman]

Google found > this <:

Oh that is interesting.

However, I think all these discussions of handling characteristics are a bit incomplete because they are not identifying the conditions they are commenting on clearly. The google snip deals with a car being deviated from a straight line without rear wheel slip and a car with loads of rear wheel slip.

I am not sure the first is very important for if going slow enough for it to be true the handling characteristic is barely interesting. Hrrm, need to think about this in order not to introduce more confusion to the discussion. I'll come back later: basically its the situation where a lift on the way in disengages the LSD puts some rear wheel lateral slip on and the application of power then nicely maintains the attitude.

Kind regards - Ian Douglass

[cyberman]

Dear all Please forgive an ignorant newbie but is the VXT gearbox what is known as an F23? Cause if so I now know where to buy two types of LSD. If not please tell all. More generally is there a list anywhere of all the parts cannibalised from something else (e.g. Lotus, Vauxahall etc) which would help making substitutions? For a small production volume vehicle a list of generic parts and substitutions has got to be a big help at difficult moments. Kind regards - Ian Douglass

[cyberman]

Question concerning F23 now less urgent. Quaife have just sold last unit and next come in April I had the dealer it was going to tracked down and had found another website advertising them. Sadly dealer one had sold it to dealer two and dealer two didn't have the stock his website reported and this one is sold mein herr. Bother. Back to drawing board. Now I wonder if Stu would part with the one under his desk? Kind regards - Ian Douglass