Dual Rate Springs

[VT-R]

I’ve been thinking about dual rate springs for a Peugeot 205 Gti hillclimb/sprint car that is also used on the road. My idea was to have a main spring of around 200 lbs/in and a tender spring of 1000 lbs/in
This would give an initial rate of 167 lbs/in, so fairly compliant, then after maybe an inch of travel the tender would go coil bound and the rate would stiffen up to 200 lbs/in.
This would hopefully keep the car compliant for rough airfield surfaces and on the road, but would help reducing roll when cornering without the downsides associated with stiffer ARB’s.
Question is what would the transition between the spring rates be like and how bigger percentage jump could you tolerate between the initial and main spring rates?

Cheers
Ed

[tristancliffe]

Coil springs are not perfectly linear, especially near coil bound, so the change in rate would not be instant. But it wouldn't be far off. Yet cars still use it, often with progressive 'rated' bump stops as well, so it can't be that bad (once you get used to it, or have the confidence to make use of the change?)

As for percentages I don't know what could be tolerated. I doubt there is a single answer, or even a single range.

You may get rid of the downsides of stiff ARBs, but you'll also be getting rid of the good things too. If you think the overall change is beneficial then give it a try. But I'd be wary of pioneering too much in your class without fully understanding what you're doing.

Of course, if other people in your class or with the same car are doing similar things then you might be more tempted to follow the same, pre-trodden path.

If you can afford to try it, and possibly waste your money, go for it. If you can't afford that, stick with what you've got and refine that.

[phoenix]

Originally Posted by VT-R

I’ve been thinking about dual rate springs for a Peugeot 205 Gti hillclimb/sprint car that is also used on the road. My idea was to have a main spring of around 200 lbs/in and a tender spring of 1000 lbs/in
This would give an initial rate of 167 lbs/in, so fairly compliant, then after maybe an inch of travel the tender would go coil bound and the rate would stiffen up to 200 lbs/in.
This would hopefully keep the car compliant for rough airfield surfaces and on the road, but would help reducing roll when cornering without the downsides associated with stiffer ARB’s.
Question is what would the transition between the spring rates be like and how bigger percentage jump could you tolerate between the initial and main spring rates?

Cheers
Ed

There are a couple of things about your proposed setup that would worry me.

For the helper spring to become coilbound after a total of 1" compression, the total clearance between the working coils of the 1000lb spring at static ride height would need to be 4.23mm. This is a very small gap spread amongst however many working coils you would have, and I would be concerned that
getting such precision in the spring manufacture might be difficult if not impossible. The left and right hand springs would need to be identical, and not just millimetre perfect but maybe 0.1mm perfect, because if the left and right hand springs were different you might get odd handling under braking, for example.

The increase in stiffness in your setup would be like a switch - one moment 167lbs and an instant later 200lb. I wouldn't think the 20% increase would be a problem in itself, but if the setup between the front and the rear of the car is different - as it will almost certainly be - then the end that suddenly gets stiffer first will immediately carry more load. If this happens at the front, then you may get sudden understeer; if it happens at the rear, then you could get instant oversteer.

I would imagine it would be very difficult to get the transition points for the front and rear springs perfectly matched for all significant circumstances - i.e. turning into a corner, mid corner and accelerating out of a corner. One end is almost certain to get stiffer before the other.

Anti-roll bars do at least offer a more progressive solution than dual rate springs.

[ubrben]

Originally Posted by phoenix

There are a couple of things about your proposed setup that would worry me.

For the helper spring to become coilbound after a total of 1" compression, the total clearance between the working coils of the 1000lb spring at static ride height would need to be 4.23mm. This is a very small gap spread amongst however many working coils you would have, and I would be concerned that
getting such precision in the spring manufacture might be difficult if not impossible. The left and right hand springs would need to be identical, and not just millimetre perfect but maybe 0.1mm perfect, because if the left and right hand springs were different you might get odd handling under braking, for example.

The increase in stiffness in your setup would be like a switch - one moment 167lbs and an instant later 200lb. I wouldn't think the 20% increase would be a problem in itself, but if the setup between the front and the rear of the car is different - as it will almost certainly be - then the end that suddenly gets stiffer first will immediately carry more load. If this happens at the front, then you may get sudden understeer; if it happens at the rear, then you could get instant oversteer.

I would imagine it would be very difficult to get the transition points for the front and rear springs perfectly matched for all significant circumstances - i.e. turning into a corner, mid corner and accelerating out of a corner. One end is almost certain to get stiffer before the other.

Anti-roll bars do at least offer a more progressive solution than dual rate springs.

Why not just use bump rubbers and packers?

Ben

[CNHSS1]

Eibach used to do progressive rate springs for cars (have a front set for a chevette somewhere) which have varying coil spacing, the theory being the closer coils go coil bound and effectively increase the rate. to look at (dead technical i know ) the upper 3 or 4 coils are equidistant, the next few increase slightly in their respective coil 'gaps' and then the rest ( the majority) are reasonably well spaced as you expect for a saloon car spring. this is supposed to give a more progressive effect than the dual rate achieved with spring and tender method. the springs i have are 'tarmac' spec so am guesing in region of +/- 700lb/in dependant on whether they were designed to be softer initially, increasing to the rated rate, or vice versa (never fitted them).
not hugely technical i know, but might help in your quest, they may even do them for Pugs still
CNH

[Zico]

Not the cheapest option but would 3 way dampers not be a better way to give the best of both worlds?

You have adjustments for slow bump and rebound ie body roll and weight transfer but also a seperate adjustable valve for fast bump that deals with the faster frequencies of keeping the tyre in contact with the road ie rough airfield surfaces.

AVO have a very good range of 2 and 3 way adjustables for most applications, very competatively priced in comparison to the likes of Proflex or AST.