500 vs. Mini - Multi-link and Torsion-beam characteristics

[Wilburo]

I was reading the 500 Abarth vs Mini Cooper S article in the latest issue of Evo when I came across the following:

evo said:

The 500 has tenacious front-end grip. It's just as well it does, too, because you get precious little help around bends from the torsion-beam back axle, the rear tyres doggedly following the tracks marked out by the fronts and refusing to join in the cornering process.
As a result the 500's cross-country prowess is entirely governed by that front suspension and the slight adjustment of line allowed by the throttle.
Around the same corner the Mini is far more three-dimensional. [T]he helm is very well weighted and responsive, and cornering angles are more easily adjustable, the rear suspension playing a noticable role. Barrel in hard and it moves over the road, working the stiff sidewalls of the runflat tyres, making you feel part of the car, hunkered down behind the wheel.

Having moved from a Ford KA (torsion-beam) to a mk.1 Mini Cooper S (multi-link), this has been the biggest impact between the handling of the two cars - the rear-end on the KA always felt really planted, while the Mini seems to move about - as they say, barrel in hard and it moves over the road.

So, I understand that multi-link should give a better ride-quality / handling compromise and better grip on bumpy surfaces, as the angle of each wheel can be indepentaly controlled, while the torsion-beam is simply cheaper and better on space.

But, why does the multi-link car "moves over the road, working the stiff sidewalls of the runflat tyres" when it should provide more grip than the torsion-beam, on which: "the rear tyres doggedly following the tracks marked out by the fronts".

What's going on!? Why does the torsion-beam follow those tracks so much more precisely!? Why does the multi-link hang it's arse out!?

Edited by Wilburo on Sunday 22 March 19:13

[Wilburo]

Hooray - I think I've found an answer from a GT5 forum, where they're trying to get hot hatches to oversteer (I have the same problem in Forza 2, actually).

GT5 Forum said:

...but here's why hot haches lift-off oversteer - the rear is usually light, and when you let go the throttle the weight transfers to the front, taking away grip from the rear. On cars like Honda Civic Type-R (not the latest one) lift-off oversteer was induced because of the car's tendency to raise its inner rear tyre when turning without accelerating due to suspension setup.

GT5 Forum said:

Yes, the Civic Type-R, hot Peugeots, older GTIs tend to raise their inner rear wheel when turning, thus inducing oversteer, and now I know why. I did some research, and it seems that the cause of this are very stiff anti-roll bars (stabilisers).

GT5 Forum said:

Lift-off is not because of the inside rear cocking up... (that would just add extra grip to the oustide rear)... it's all because weight transfer forward unloads the rear tires and allows them to "skip" across the pavement as the rear anti-rollbar prevents weight transfer to the outside rear tire... keeping it flat against the ground and allowing it to lose grip.

So, it sounds like it's to do with the rear anti-roll bar, which the 500 and KA don't have, as the torsion-beam doesn't require them. I gather that independent suspension allows the manufacturer to finely tune the rear handling, so they firm it up to reduce understeer, to the cost of an occasionally frisky backend.

Edited by Wilburo on Monday 23 March 17:31

[Mr2Mike]

Wilburo said:

So, it sounds like it's to do with the rear anti-roll bar, which the 500 and KA don't have, as the torsion-beam doesn't require them. I gather that independent suspension allows the manufacturer to finely tune the rear handling, so they firm it up to reduce understeer, to the cost of an occasionally frisky backend.

Increasing rear roll resistance reducing front tyre loading and increased rear tyre loading during cornering, which is a sure way to increase the cars tendencies towards oversteer.

Torsion beams can be used in conjunction with anti-roll bars, my old Astra GTE 16v had one for instance. However, the torsion beam set-ups are effectively engineered as a budget solution (you can't really design anything cheaper to make than a few bits of pressed steel welded together), rather than a performance one. They have absolutely no camber control for starters, the wheel adopts the same angle as the body during cornering.

[HellDiver]

I had an aftermarket rear anti roll bar added on to the torsion beam on my Astra. Brought the car from a seriously understeery deathtrap toward a very neutral handling car. I could invoke understeer or liftoff oversteer at will, or just get on with the driving. It cocked a leg once the ARB was fitted, which annoyed the ESP a little.