GT4 Spring Rates and Handling
Discussion
There have been numerous discussions on roll bars and camber settings, generally focusing on improving GT4 handling, but nothing so far as I can see on spring rates or dampers.
I have been looking at spring rates on my car (not a GT4) and the GT4 rates front to rear seem to be quite different to other Caymans, whether cooking, Clubsport, or one-off versions.
The GT4 main spring rates are 45 and 80 Nm/mm with a front:rear ratio of 1.00:1.78. The rears are fitted with tender springs (not sure of their rate) but from what I have seen none at the front.
Mathey fit 140 and 140 Nm/mm main springs to their MR Clubsport so a ratio of 1.00:1.00 with front and rear tender springs of 80 and 80 Nm/mm - ratio is typical for Cayman race cars
An example of a sporty cooking model is a 981 X71 manual which has 30 and 47 Nm/mm main springs so ratio of 1.00:1.57
For what it is worth I am running 60 and 80 Nm/mm main so 1.00:1.33 and tenders of 40 and 50
What puzzles me is why the sportiest road Cayman has a front/rear spring rate ratio (1.78) greater than cooking models (typically c1.55), and further away from race cars’ 1:00.
Any thoughts on what characteristic Porsche were trying to overcome/achieve with this part of the GT4 chassis specification?
I have been looking at spring rates on my car (not a GT4) and the GT4 rates front to rear seem to be quite different to other Caymans, whether cooking, Clubsport, or one-off versions.
The GT4 main spring rates are 45 and 80 Nm/mm with a front:rear ratio of 1.00:1.78. The rears are fitted with tender springs (not sure of their rate) but from what I have seen none at the front.
Mathey fit 140 and 140 Nm/mm main springs to their MR Clubsport so a ratio of 1.00:1.00 with front and rear tender springs of 80 and 80 Nm/mm - ratio is typical for Cayman race cars
An example of a sporty cooking model is a 981 X71 manual which has 30 and 47 Nm/mm main springs so ratio of 1.00:1.57
For what it is worth I am running 60 and 80 Nm/mm main so 1.00:1.33 and tenders of 40 and 50
What puzzles me is why the sportiest road Cayman has a front/rear spring rate ratio (1.78) greater than cooking models (typically c1.55), and further away from race cars’ 1:00.
Any thoughts on what characteristic Porsche were trying to overcome/achieve with this part of the GT4 chassis specification?
I'm guessing the front tyre to rear tyre sizes have something to do with it. The clubsport runs slicks with a relatively much bigger front vs rear. The boggo cayman also has bigger fronts to rears as the gt4 has a rather large rear tyre.
Are your tender springs fully blocked under static load btw?
Are your tender springs fully blocked under static load btw?
Tarett do a 50 front and 63 NM spring kit for the GT4
The Ohlins txx comes in two rates
100/120
Or
120/140
It seems well known that the GT4 dives and rolls to much oem at the front end.
The DSC can help firming up damping under braking and bends.
The Ohlins txx comes in two rates
100/120
Or
120/140
It seems well known that the GT4 dives and rolls to much oem at the front end.
The DSC can help firming up damping under braking and bends.
Edited by Porsche911R on Sunday 19th November 13:24
Porsche911R said:
It seems well known that the GT4 dives and rolls to much oem at the front end.
Has your car got a universal joint in the middle? Most cars (probably Morgans excepted) roll exactly the same amount at the front as they do at the rear!Edited by Porsche911R on Sunday 19th November 13:24
Edited by bcr5784 on Sunday 19th November 16:46
gtsralph said:
The tender springs are not fully compressed at rest and under load they first compress fully then the load goes to the main spring, if that answers your question.
It does indeed cheers. Just one final question if I could, is the transition perfectly seamless between the springs and you don't feel a 'step' so to speak under load when the tender gets fully loaded and the main spring takes over? Thanks.bcr5784 said:
Has your car got a universal joint in the middle? Most cars (probably Morgans excepted) roll exactly the same amount at the front as they do at the rear!
]
roll stiffness balance front to rear is pointless then, screw spring rates and let’s throw our adjustable ARB'S in the bin. Has your son taught you how to hold a steering wheel yet ? ]
Porsche911R said:
bcr5784 said:
Has your car got a universal joint in the middle? Most cars (probably Morgans excepted) roll exactly the same amount at the front as they do at the rear!
]
roll stiffness balance front to rear is pointless then, screw spring rates and let’s throw our adjustable ARB'S in the bin. Has your son taught you how to hold a steering wheel yet ? ]
Relative front to rear roll STIFFNESS is a completely different matter - which, as your comment about rear roll stiffness not affecting front grip shows, you don't understand either. It's about different weight transfer across the car front and rear, not about different ends of the car rolling a different amount - which they don't (can't unless the chassis flexes).
Simplistically to maximise the cornering grip at one end of the car you need to minimise the difference in load between the inside and outside tyres. That is because the load/grip curve of a tyre is non-linear. So if you double the load on a tyre you don't get double the grip. Hence on something like a race 911 at the limit you will find cars cornering have stiff front roll stiffness, with the inside wheel off the ground to reduce front grip and TRANSFER it to the rear, by reducing rear roll weight transfer. On front wheel drive cars you do the reverse and they generally aviate a rear wheel by having a lot of rear roll stiffness, TRANSFERING grip to the front to reduce understeer by reducing lateral weight transfer at the front.
I think before you offer to show people how to hold a steering wheel perhaps a read of (pretty much) any vaguely serious book on car set-up theory might be in order. It might give your dogmatic statements about car set-up more credibility.
gtsralph said:
isaldiri said:
It does indeed cheers. Just one final question if I could, is the transition perfectly seamless between the springs and you don't feel a 'step' so to speak under load when the tender gets fully loaded and the main spring takes over? Thanks.
No stepbcr5784 said:
Simple matter of common sense (or geometry if you want to get "technical") - if the front rolls 2 degrees the back rolls 2 degrees unless the body twists. It really can't get any more simple than that.
Relative front to rear roll STIFFNESS is a completely different matter - which, as your comment about rear roll stiffness not affecting front grip shows, you don't understand either. It's about different weight transfer across the car front and rear, not about different ends of the car rolling a different amount - which they don't (can't unless the chassis flexes).
Simplistically to maximise the cornering grip at one end of the car you need to minimise the difference in load between the inside and outside tyres. That is because the load/grip curve of a tyre is non-linear. So if you double the load on a tyre you don't get double the grip. Hence on something like a race 911 at the limit you will find cars cornering have stiff front roll stiffness, with the inside wheel off the ground to reduce front grip and TRANSFER it to the rear, by reducing rear roll weight transfer. On front wheel drive cars you do the reverse and they generally aviate a rear wheel by having a lot of rear roll stiffness, TRANSFERING grip to the front to reduce understeer by reducing lateral weight transfer at the front.
I think before you offer to show people how to hold a steering wheel perhaps a read of (pretty much) any vaguely serious book on car set-up theory might be in order. It might give your dogmatic statements about car set-up more credibility.
All my statements are valid, you are just trying to be a dick., you change the way the thread goes to cause a debate over a subject you made up. You did it in the gear change thread and you have done it here. why do it ?Relative front to rear roll STIFFNESS is a completely different matter - which, as your comment about rear roll stiffness not affecting front grip shows, you don't understand either. It's about different weight transfer across the car front and rear, not about different ends of the car rolling a different amount - which they don't (can't unless the chassis flexes).
Simplistically to maximise the cornering grip at one end of the car you need to minimise the difference in load between the inside and outside tyres. That is because the load/grip curve of a tyre is non-linear. So if you double the load on a tyre you don't get double the grip. Hence on something like a race 911 at the limit you will find cars cornering have stiff front roll stiffness, with the inside wheel off the ground to reduce front grip and TRANSFER it to the rear, by reducing rear roll weight transfer. On front wheel drive cars you do the reverse and they generally aviate a rear wheel by having a lot of rear roll stiffness, TRANSFERING grip to the front to reduce understeer by reducing lateral weight transfer at the front.
I think before you offer to show people how to hold a steering wheel perhaps a read of (pretty much) any vaguely serious book on car set-up theory might be in order. It might give your dogmatic statements about car set-up more credibility.
Edited by Porsche911R on Monday 20th November 08:35
Porsche911R said:
All my statements are valid, you are just trying to be a dick., you change the way the thread goes to cause a debate over a subject you made up. You did it in the gear change thread and you have done it here. why do it ?
I have done nothing of the sort. You said "It seems well known that the GT4 dives and rolls to much oem at the front end". That is a silly statement since the car rolls the same amount at both ends. If you can't recognise when you have said something silly I can't help you. You might have responded "yes that is a loose/misleading/unhelpful way of putting it, what I meant was .... but instead you went on to talk about ARBs etc as if that had anything to do with "It seems well known that the GT4 dives and rolls to much oem at the front end". It's not me that went off topic!Edited by Porsche911R on Monday 20th November 08:35
Edited by bcr5784 on Monday 20th November 09:28
bcr5784 said:
I have done nothing of the sort. You said "It seems well known that the GT4 dives and rolls to much oem at the front end". That is a silly statement since the car rolls the same amount at both ends. If you can't recognise when you have said something silly I can't help you. You might have responded "yes that is a loose/misleading/unhelpful way of putting it, what I meant was .... but instead you went on to talk about ARBs etc as if that had anything to do with "It seems well known that the GT4 dives and rolls to much oem at the front end". It's not me that went off topic!
why do you think the OP is posting, the whole thing is about balance and roll stiffness, FFS.I did not talk about the back end as under brakes and in bends it's the front end having issues ! hence the spring rates questions.
To answer the original question. You can increase roll stiffness either by increasing the spring rate or by fitting a stiffer anti roll bar. However fitting a stiffer (or softer for that matter) ARB has no effect on squat at the rear or dive at the front. If the the race cars go for relatively stiffer front springs (rather than ARB) it is presumably because the effect of dive at the front under braking - and, in particular, the loss of braking that results from the increased camber (when the car is already running significant static negative camber) is a serious issue. Given that slicks are used ground clearance under braking may be an issue too, which might also lead to the need for less dive at the front.
In the case of the GT4 to counter the understeer which would likely result from the larger rear tyres, Porsche could have gone for either a stiffer rear ARB or stiffer springs to increase rear roll stiffness. However on a bumpy road (as opposed to a smooth track) stiff antiroll bars can produce undesirable effects (such as arb "waddle"), so there is a limit to stiff you would like the ARB to be. So I suspect that is the reason they have gone for realtively stiff rear springs. Since the road cars run much less negative camber than race cars the inclination to run stiff front springs is less.
All that said there are lots of ways to skin a cat, and any setup will have its pluses and minuses. On track the only arbiter is the stop watch, on the road other issues come into play.
In the case of the GT4 to counter the understeer which would likely result from the larger rear tyres, Porsche could have gone for either a stiffer rear ARB or stiffer springs to increase rear roll stiffness. However on a bumpy road (as opposed to a smooth track) stiff antiroll bars can produce undesirable effects (such as arb "waddle"), so there is a limit to stiff you would like the ARB to be. So I suspect that is the reason they have gone for realtively stiff rear springs. Since the road cars run much less negative camber than race cars the inclination to run stiff front springs is less.
All that said there are lots of ways to skin a cat, and any setup will have its pluses and minuses. On track the only arbiter is the stop watch, on the road other issues come into play.
Edited by bcr5784 on Monday 20th November 19:21
Edited by bcr5784 on Tuesday 21st November 08:57
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