Maybe prompted by the passing away of Paul Frere I have just re-read his great book "the racing Porsches" which is a study in methodical development of racing cars. However sometimes Porsche did something a bit radical and in 1968 they were obsessively reducing the weight of the hillclimb 910 down to 382kg ( for a 3 litre car!). As part of this , and to address low speed understeer, they deleted the front coil springs and went to a Z bar set-up.
This has a conventional anti-roll bar but also a z shaped link between the front wheels which has zero roll stiffness but full bump stiffness. Thus the rates for roll and bump are completely seperated (except in one wheel bump where there is some cross feed).
The idea was to provide good bump control on the rough hillclimbs but to go very low on front roll stiffness to get less turn in understeer.
My question is , has this z bar approach been tried elsewhere on race cars, and was it successful?
It is somewhat similar to the third spring system but potentially lighter and you could, in principle, get seperate damping characteristics for roll and bump by suitably locating the dampers.
Z bar springing
Started by
mariner
, Apr 08 2008 13:45
5 replies to this topic
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#2
phantom II
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Posted 08 April 2008 - 19:36
A Z bar is just a bent Tie Rod to prevent bump in high travel suspensions. You see Z bars on 4 wheel hunting vehicles and off road vehicle to reduce bump steer. No matter how you cut it, a steel spring or roll bar doing the same work, be it a coil or a torsion bar or a Z Bar or a blade will weigh the same so I can't see how Porsche saved weight if indeed they used a Z bar as a spring. Not only that, if you loaded the steering arm in that manor, you will encounter some pretty strange feed back on the steering wheel. Have you got a picture?
Bump steer has been the hallmark of Porsche suspension design. You can feel it on a brand new 997 and even on the Boxster.
They have been illegal for road vehicles since the 70s. It has a ball joint or Heim on one end and a rotational bearing at the drag link or wrack end. When the wheel drops, the Z rod just rotates and because of the Z shape, prevents bump steer because the distance between the two ends doesn't change on the X axis. It can bend on high cornering loads though(not in torsion) and there are even Z bar elimination kits for 4 wheelers. They are used in high travel suspensions where the height of the steering arm and wrack are far apart. If it were designed to handle the stresses of normal springs loads, it would be heavy and cumbersome. Maybe there was another spring used somewhere else.
Never liked Paul Frere because he has never said a nice thing about American cars in his life and his technical explanations of race cars proved he never knew what he was talking about. This could be another example of it.
I just read his last column in the latest Road & Track where he gives a technical analysis on the Nissan Skyline GT-R comparing it to the Corvette Z06 and Porsche Turbo. Not one mention of the Vette and not a mention of the tire compounds used on the Nissan which gave it a distinct advantage over the other two cars.
Bump steer has been the hallmark of Porsche suspension design. You can feel it on a brand new 997 and even on the Boxster.
They have been illegal for road vehicles since the 70s. It has a ball joint or Heim on one end and a rotational bearing at the drag link or wrack end. When the wheel drops, the Z rod just rotates and because of the Z shape, prevents bump steer because the distance between the two ends doesn't change on the X axis. It can bend on high cornering loads though(not in torsion) and there are even Z bar elimination kits for 4 wheelers. They are used in high travel suspensions where the height of the steering arm and wrack are far apart. If it were designed to handle the stresses of normal springs loads, it would be heavy and cumbersome. Maybe there was another spring used somewhere else.
Never liked Paul Frere because he has never said a nice thing about American cars in his life and his technical explanations of race cars proved he never knew what he was talking about. This could be another example of it.
I just read his last column in the latest Road & Track where he gives a technical analysis on the Nissan Skyline GT-R comparing it to the Corvette Z06 and Porsche Turbo. Not one mention of the Vette and not a mention of the tire compounds used on the Nissan which gave it a distinct advantage over the other two cars.
Originally posted by mariner
Maybe prompted by the passing away of Paul Frere I have just re-read his great book "the racing Porsches" which is a study in methodical development of racing cars. However sometimes Porsche did something a bit radical and in 1968 they were obsessively reducing the weight of the hillclimb 910 down to 382kg ( for a 3 litre car!). As part of this , and to address low speed understeer, they deleted the front coil springs and went to a Z bar set-up.
This has a conventional anti-roll bar but also a z shaped link between the front wheels which has zero roll stiffness but full bump stiffness. Thus the rates for roll and bump are completely seperated (except in one wheel bump where there is some cross feed).
The idea was to provide good bump control on the rough hillclimbs but to go very low on front roll stiffness to get less turn in understeer.
My question is , has this z bar approach been tried elsewhere on race cars, and was it successful?
It is somewhat similar to the third spring system but potentially lighter and you could, in principle, get seperate damping characteristics for roll and bump by suitably locating the dampers.
#3
mariner
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Posted 08 April 2008 - 19:51
This Z bar is not connected to the steering, it is connected to the wishbones/uprights in the usual way and provides pure bump rate as it has no effect in roll as one end rises ( relative to the chassis) as the other falls.
As Porsche did it to save weight I presume it did, to get down to 382kg was quite an acheivement. I think it would work out lighter than a third spring as the loads are being fed into the chassis at its edges and are relatively low. A third spring ends up on big linkages near the chassis centre line with high loads. Not an issue on a single setaer as that is where the chassis is but on a sports car you can put loads in better places.
Of course GM went one better many years ago on a UK Vauxhall Viva which just had a single transverse leaf spring with two clamps set far enough apart to give a combined roll and bump stiffnes from one spring. It handled god awfully but I bet the cost target was neatly met!
As for Paul Frere I dont know about whether he understood cars but he was a racing driver of some considerable skill so maybe he did know something, if criticising US cars is a criterion then 99% of European journalists over the last 50 years are off -limits!! I personally think their criticisms are somewhat myopic but they all do it.
As Porsche did it to save weight I presume it did, to get down to 382kg was quite an acheivement. I think it would work out lighter than a third spring as the loads are being fed into the chassis at its edges and are relatively low. A third spring ends up on big linkages near the chassis centre line with high loads. Not an issue on a single setaer as that is where the chassis is but on a sports car you can put loads in better places.
Of course GM went one better many years ago on a UK Vauxhall Viva which just had a single transverse leaf spring with two clamps set far enough apart to give a combined roll and bump stiffnes from one spring. It handled god awfully but I bet the cost target was neatly met!
As for Paul Frere I dont know about whether he understood cars but he was a racing driver of some considerable skill so maybe he did know something, if criticising US cars is a criterion then 99% of European journalists over the last 50 years are off -limits!! I personally think their criticisms are somewhat myopic but they all do it.
#4
Greg Locock
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Posted 08 April 2008 - 20:17
Packaging a traditional U bar is bad enough, adding another bar would be tricky. It's not obvious to me that it is necessarily a lighter solution than a coil spring - the same metal is being used in the same fashion (torsion). That being said, if you design around it then yes, I can se some advantages. Of course you don't get the main advantage of a 3 spring setup.
The trouble with criticising people for criticising the handling of US cars is that for many years it was a valid target - at least up until 1990. We are now informed that the Corvette is now a masterpiece etc equivalent to Europe's best, trouble is they've been saying that since oo 1983 at least. And it wasn't. Nice engine, looked pretty mean. Ride and handling compromise, hohoho. Refinement likewise. Any suspension will work if you don't let it move.
The trouble with criticising people for criticising the handling of US cars is that for many years it was a valid target - at least up until 1990. We are now informed that the Corvette is now a masterpiece etc equivalent to Europe's best, trouble is they've been saying that since oo 1983 at least. And it wasn't. Nice engine, looked pretty mean. Ride and handling compromise, hohoho. Refinement likewise. Any suspension will work if you don't let it move.
#5
cheapracer
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Posted 10 April 2008 - 06:30
Z bars (well the style I know of) were used on competition VW Beetles to stop the car pitching in an effort to stop tuck in at the rear. It may of had some value to help stop the Porshe rear suspension which (then) is also notorious for lift off oversteer. Its purpose can be to stop pitching while still using softer suspension as they might for a rough hillclimb.
Think of it as an 'anti roll bar' but front to rear instead of side to side.
Think of it as an 'anti roll bar' but front to rear instead of side to side.
#6
britishtrident
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Posted 19 April 2008 - 19:14
The whole purpose of this type of device is to transfer all of the roll stiffness to the other end of the car in a car with a swing axle. Z bars were more or less normall fitting in Formula Vee cars that had to retain the early beetle suspension.
Z bar is just one form od "compensator spring" a leaf spring can also be used and was typically fitted when modifying the suspension on Triumph Herald/Spitfire based cars. http://members.cox.n...ompensator.html
Mercedes Benz were the only firm I know that used a compensator spring on production models, the 1960s fin tail Mercedes range and some Mercedes SL sports car were fitted with swing axles, the compensator spring was fitted above the diff and took theform of a coil spring on lower spec models and self-leveling unit on the more expensive saloons and estate cars.
Z bar is just one form od "compensator spring" a leaf spring can also be used and was typically fitted when modifying the suspension on Triumph Herald/Spitfire based cars. http://members.cox.n...ompensator.html
Mercedes Benz were the only firm I know that used a compensator spring on production models, the 1960s fin tail Mercedes range and some Mercedes SL sports car were fitted with swing axles, the compensator spring was fitted above the diff and took theform of a coil spring on lower spec models and self-leveling unit on the more expensive saloons and estate cars.
