Cars overturning due to too much tyre grip

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FrenchLeave
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Cars overturning due to too much tyre grip

Post by FrenchLeave »

A previous posting referred to the possibility of vehicles overturning if they have too much grip.
As the maximum frictional coefficient between tyre and road cannot exceed 1 (unless you utilise aerodynamic downforce), the problem of overturning comes down to a simple matter of moments. The moment trying to overturn the vehicle is formed by the weight of the vehicle x the height of the C of G above road level. Opposing that is the moment formed by the vehicle weight (because C of F = 1) x half the track (assuming the C of G is laterally central).
Most car designers try to keep the C of G height at less than half the track so as to avoid any possibility of overturning. There are of course exceptions like Chelsea tractors and the early Merc "A" class. Dnsey's cars of long ago had high C of Gs and narrow tracks.
Tripping up over kerbs doesn't count!

Derek
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Post by dnsey »

I was once told (but I haven't done the calculations to verify it) that it's theoretically possible to grab the door handle of a car as it passes at just the right moment and turn the car over.
As you say, the C of G of most cars is lower these days, so if it was ever true, it probably no longer applies.
Wasn't there a Suzuki 4X4 a few years ago which did have a habit of falling over when cornering hard?
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Post by Homer »

dnsey wrote: Wasn't there a Suzuki 4X4 a few years ago which did have a habit of falling over when cornering hard?
Yes, the SJ wasn't it, the little Jeep, before the Vitara. I drove one in Greece and it was an ... umm... experience.

Almost turned my Reliant Regal over a couple of times but they have their own problems.
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Post by dnsey »

Almost turned my Reliant Regal over a couple of times but they have their own problems.
They're actually quite hard to turn over - if you don't ground the font corners on bends, you're not trying hard enough :twisted:
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Post by deian »

i remember jeremy clarkson turning over a merc a-class when it first came out, he managed to turn it over quite easily, and mercedes panicked and VERY quickly went back to the drawing boards, fixed the problem and gave him the fixed version to try, merc saved their skins by giving him another one to try, it showed the public they fixed it!
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Post by jeremy »

Its generally reconed to be impossible to overturn a proper car unless there is some aggravating factor such as it hitting a bump.

Having said that I do recall a Morris Minor overturning in front of my family on a quiet country road when the driver braked hard and the thing slewed sideways. We pushed it up the right way for him and he didn't really seem to understand when we told him to drive it home! (or for that matter why we were all laughing)
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Post by PowerLee »

deian wrote:i remember jeremy clarkson turning over a merc a-class when it first came out, he managed to turn it over quite easily, and mercedes panicked and VERY quickly went back to the drawing boards, fixed the problem and gave him the fixed version to try, merc saved their skins by giving him another one to try, it showed the public they fixed it!
They revised the suspension & fitted ESP as standard.

Jean Alesi & Gerhard Berger turned over a Punto mk 1 on to its roof at the launch, It had something to do with Berger pulling the handbrake up at speed as Alesi was driving it round the track :lol:
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Post by ACTIVE8 »

deian wrote:i remember jeremy clarkson turning over a merc a-class when it first came out, he managed to turn it over quite easily, and mercedes panicked and VERY quickly went back to the drawing boards, fixed the problem and gave him the fixed version to try, merc saved their skins by giving him another one to try, it showed the public they fixed it!
Well, they really should have tried harder (back to school report time for Mercedes), and designed it properly in the first place ! :roll:
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Post by jeremy »

Somehow I think there are probably other modern unstable cars - as it seems to me to be the inevietable consequence of the 'People Mover' layout with its higher centre of gravity.

It seems that good design can overcome most of the problems - and if you can't design it properly - you can always use a computer system like Mercedes.

Mind you I believe a journalist mananged to get a 3 series BMW to overturn in a straight line shortly after its anouncement in the 70's. Lethal rear suspension was considered acceptable in those days - I recall an experienced mechanic and amateur rally driver getting out of a 3 series and saying '****** thing - when it goes round a corner it tucks its outside back wheel underneath itself and shouts 'Yipee - I've got independant rear suspension!' - At the time he drove a Princess!
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Post by FrenchLeave »

Yes Jeremy, the Triumph Herald (among others) suffered from the same foible. This beast was fitted with swing axle rear suspension which, of course, caused camber changes when operating. There was also an interaction between the roll centre height and the C og G which caused the back end to jack up when cornering hard, this caused the rear wheels to change to positive camber and the outer one to start to tuck under which jacked up the back end even more and so on until the thing flipped over.

I'm not familiar with the early Series 3 but it sounds as if it had a similar setup.

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Post by citronut »

it used to be said the AUTIN A35 was pron to falling over,but i belive its when people fitted radial tyres gave it to much grip,where as with cross plies they just slide,also the honda acty van used to fall over with out any notice
regards malcolm
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Post by jeremy »

Early 3 series had the dreaded semi trailing arms as fitted to Triumph 2000's, TR5/6 and Stags. I my Stag being lethal in the wet and I remember when I was working in the law, driving some distance to a court rather fast as I was late. It was raining and I had no problem until I turned into the court driveway when the thing did a 90 degree turn in front of a Police Inspector!

Not quite the same problem I know at that speed but I was never happy about the rear suspension and it was one of the reasons I replaced it with a Jaguar.
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Post by reblack68 »

The trouble with good roadholding is that there is much more energy built up when the car does let go.
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