Tender springs?

[Theo]

At present I'm running 600 lbs/in front springs and by in large really like how the car performs with them. That said, when driven on the infamously bumpy Nordschleife I found that it some areas the car was exceptionally harsh and wondered whether some tender springs might be of benefit.

Is there any 'rule of thumb' I should follow when choosing the spring rate of the tender?

[SteMarsden]

Im running 550lb fronts/275lb rears and GAZ Golds on my PBMW, but when the suspension is unloaded the springs become 'free'.....
My opinion is that this dangerous as the retaining cap on the rears can be removed by hand when this happens.... and I only hope that this never happens to anyone else running this set-up
I have fitted 4" 75lb tenders springs at the rear so that it all remains in compression and hope that this also helps to 'push' the unloaded corner towards the tarmac (and hopefully some grip!)
I have also bought some 4" 90lb tenders for the front which I have not yet fitted.....

Uwe can usually explain this better than I......

[RPM]

This should help:

ERS-CALCULATING FORMULA / TUNING INSTRUCTIONS
To determine the initial rate of a double spring system (dual-rate), the following formula applies:



Ci = initial spring rate
CM= spring rate main spring
CT = spring rate tender spring

The final spring rate (effective after the tender springis closed) corresponds to the active spring.It is often asked which tender spring / main springcombination improves handling and traction.There is no clearly defined answer due to a multitude of variables in the racing suspension.
However, there are several rules of thumb to follow which are listed below:

a) In road racing (ie Sedans or Formula cars with no downforce), the car needs a very stiff set up in order to reduce body roll and maintain proper suspension geometry. Such a stiff spring produces only a slight amount of pre-load. This condition usually results in little or no preload when the suspension is unloaded. This causes a momentary loss of traction at the tire because no slip angle is generated. Therefore, in an unloaded condition, the tyre will have minimal contact with the racing surface and the end result is lost traction.
The tender spring will either increase or provide the necessary pre-load on the unladen tyre. The result is improved tire contact with the racing surface anda proportionate increase in traction. Therefore, it isbest to have a tender spring set up at coil bind when the vehicle is at static ride. The tender spring should open immediately when the suspension becomes unladen. Review the type list for the corresponding solid load (Fc theor.) for a given tender spring application.

b) In Rallye or Off-Road racing, traction is most critical due to the various racing surfaces which are encountered. This usually requires a soft suspension setting so the tire maintains contact with the racing surface at all times. Large deflections in the suspension are usually required because of the soft spring rates. However, a soft spring rate may cause the suspension to bottom on the chassis. This condition generally results in a loss of traction. To absorb these large deflections and suspension loads, a progressive spring system can produce the desired improvement in traction.

Therefore, a tender spring combination, which provides a soft initial rate and stiff final rate in the loaded condition, will increase the suspension load capacity. Therefore, one should select a tender spring which becomes inactive at a higher load than static ride. There are general assumptions based on previous data. However, other combinations are possible which are not outlined here. As an example, on wet road courses, a tender spring which provides a soft initial rate just before going to solid, has proven effective (especially on the drive axle). Closed tender springs have also proven effec-tive on Rallye cars where asphalt roads are raced on during the various stages.

Drop us an email if you need anymore help or advice, we can supply the full range of Eibach springs

[randomspeedfreak]

This should help:

ERS-CALCULATING FORMULA / TUNING INSTRUCTIONS
To determine the initial rate of a double spring system (dual-rate), the following formula applies:



Ci = initial spring rate
CM= spring rate main spring
CT = spring rate tender spring

The final spring rate (effective after the tender springis closed) corresponds to the active spring.It is often asked which tender spring / main springcombination improves handling and traction.There is no clearly defined answer due to a multitude of variables in the racing suspension.
However, there are several rules of thumb to follow which are listed below:

a) In road racing (ie Sedans or Formula cars with no downforce), the car needs a very stiff set up in order to reduce body roll and maintain proper suspension geometry. Such a stiff spring produces only a slight amount of pre-load. This condition usually results in little or no preload when the suspension is unloaded. This causes a momentary loss of traction at the tire because no slip angle is generated. Therefore, in an unloaded condition, the tyre will have minimal contact with the racing surface and the end result is lost traction.
The tender spring will either increase or provide the necessary pre-load on the unladen tyre. The result is improved tire contact with the racing surface anda proportionate increase in traction. Therefore, it isbest to have a tender spring set up at coil bind when the vehicle is at static ride. The tender spring should open immediately when the suspension becomes unladen. Review the type list for the corresponding solid load (Fc theor.) for a given tender spring application.

b) In Rallye or Off-Road racing, traction is most critical due to the various racing surfaces which are encountered. This usually requires a soft suspension setting so the tire maintains contact with the racing surface at all times. Large deflections in the suspension are usually required because of the soft spring rates. However, a soft spring rate may cause the suspension to bottom on the chassis. This condition generally results in a loss of traction. To absorb these large deflections and suspension loads, a progressive spring system can produce the desired improvement in traction.

Therefore, a tender spring combination, which provides a soft initial rate and stiff final rate in the loaded condition, will increase the suspension load capacity. Therefore, one should select a tender spring which becomes inactive at a higher load than static ride. There are general assumptions based on previous data. However, other combinations are possible which are not outlined here. As an example, on wet road courses, a tender spring which provides a soft initial rate just before going to solid, has proven effective (especially on the drive axle). Closed tender springs have also proven effec-tive on Rallye cars where asphalt roads are raced on during the various stages.

Drop us an email if you need anymore help or advice, we can supply the full range of Eibach springs

a totally excellent post, thankyou very much
i was thinking of buying cheap coilovers for my road e30..im beginning to think about going with rpm now!

[ross_jsy]

RPM, you can do a Gaz Gold package with optional tender springs can't you?

[GeoffBob]

The tender spring will either increase or provide the necessary pre-load on the unladen tyre. The result is improved tire contact with the racing surface and a proportionate increase in traction.

I disagree. The amount of traction provided by any given wheel (or to be more precise, a measure of the horizontal force that need only be applied to the tyre in order to cause the tyre to slip) is a function of the vertical force pushing down upon the wheel. Under static load this vertical force is, quite simply, the static mass of the car distributed accordingly over each of the four wheels.

Now, a car will only pick up a wheel (or alternately the spring fall slack) when the dynamic redistribution of the cars weight (while cornering, for example) is sufficient to reduce the vertical force over that wheel to zero. This is simply an illustration of Newton’s second law: Since the vertical force on that wheel is reduced to zero, the reaction force of the spring will likewise reduce to zero (the spring falls slack). Of course, with very stiff springs it will take minimal vertical displacement (suspension travel) before the vertical load on the wheel reduces to zero (and the spring accordingly falls slack). Conversely, with soft springs, the suspension travel will be greater before the spring likewise falls slack.

The point I wish to make, however, and I cannot emphasise this enough, is that the level of traction at any given wheel is a function of the vertical force pushing down on that wheel, and has nought to do with the vertical displacement of that wheel (ignoring the effects of camber changes for the moment). Even while still in contact with the ground, a wheel that has little or no force pushing down upon it is capable of generating little or no traction!

And it is for this reason that tender springs offer little or no added value since the total spring-constant (Ci in the above equation) is dominated by the tender spring-constant (Ct above) when both springs are in operation. And since the tender spring is markedly softer than the main spring, the force that is communicated to the wheel is markedly less. The fact is, while a tender spring results in increased suspension travel while in extension (thus keeping the tyre in contact with the ground) it does little or nothing to improve traction at that wheel due to the fact that there is little or no vertical force communicated to that wheel.

On a car that demands a stiffer suspension, the only mechanism to inhibit the consequences of reduced suspension travel is to ensure that the limits of the suspension travel are seldom (preferably never) exceeded. This, unfortunately, can only be achieved on a vehicle of fixed wheel-base by lowering the cars COG, which thus ensures that the dynamic weight distribution differs only fractionally from the static weight distribution, thus minimising suspension travel.

Finally, if a cars wheels vertical displacement regularly exceed the limits of the suspension travel (causing the springs to fall slack) and the car has already been lightened to its limits (all glass removed, sun-roof removed) in order to lower the COG, then the car needs softer springs - since stiffer springs clearly aren’t doing it any good!

[Theo]

RPM - thanks for the info, unfortunately I have no idea what sort of initial rate would work well on the car so I can't really put the equation to use.

Geoff, thanks for your input. My problem isn't that I'm suffering from spring dislocation, I have 4" helper springs fitted to the rear to prevent this and the fronts are just long enough to not require helpers.

What I was hoping to acheive by using tender springs is a slightly more compliant setup over bumps. Perhaps I'm being ambitious seeking a setup which works as well on a smooth circuit such as Silverstone as it does on a bumpy one like the 'Ring.

[Mikey_Boy]

So - do RPM do their coilovers with tender springs? I have just bought a set from them and the only (minor) disapointment is the dislocation of the springs on full rebound - I am sure my MOT tester is going be a bit narky about this! Otherwise, a fab bit of kit...!

Any advice gratefully received!

Cheers,
Mikey_Boy

[GeoffBob]

[GeoffBob]

So - do RPM do their coilovers with tender springs? I have just bought a set from them and the only (minor) disapointment is the dislocation of the springs on full rebound - I am sure my MOT tester is going be a bit narky about this! Otherwise, a fab bit of kit...!

Any advice gratefully received!

Cheers,
Mikey_Boy

Mikey, to prevent spring dislocation simply requires a set of helper springs. Helper springs are not the same as tender springs. They look very similar, but differ in stiffness, helper springs being markedly softer than tender springs.

Nice article here explaining the differences.

Geoff

[Mikey_Boy]

Goeff - thanks for clearing that up! Always appreciated... By the way - the 262 gearbox for my M20 engine is being rebuilt as I write this - I shall do an installation thread soon...

Cheers,
Mikey_Boy

[GeoffBob]

Goeff - thanks for clearing that up! Always appreciated... By the way - the 262 gearbox for my M20 engine is being rebuilt as I write this - I shall do an installation thread soon...

Cheers,
Mikey_Boy

Pleased to hear it Mike! I look forward to reading about it. Any chance you could post up a few pics of the 262CR internals before it gets put back together? It is a 262CR that you bought, right? The plain 262 is a 4-speed box.

Did you buy a bell housing from the US after all, or find one elsewhere?

Also, keep in mind (as I think I must have described previously) that 5th gear on the Getrag 262 has 1:1 ratio. This means that you may need to look at lowering the ratio of your final-drive if you require a higher top speed than attainable with your current diff.

If you would like, PM me or post up here a) your current final drive (diff) ratio, b) your rear wheel tyre size, and c) your rev limit and I’ll post up a plot of your vehicle speed as a function engine speed (rpm) and selected gear. Then you can decide if you want to lower your diff ratio (at the cost of reduced acceleration).

Regards
Geoff

[Mikey_Boy]

Thanks Geoff - PM sent!

[RPM]

So - do RPM do their coilovers with tender springs? I have just bought a set from them and the only (minor) disapointment is the dislocation of the springs on full rebound - I am sure my MOT tester is going be a bit narky about this! Otherwise, a fab bit of kit...!

Any advice gratefully received!

Cheers,
Mikey_Boy

We can supply helper or tender springs with any kit at an additional cost. Either our standard springs or Eibach springs.

Drop me an email if you are interested in a set

[Theo]