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Helper/Tender Spring and Roll Stiffness?

Suspension Tuning & Optimization

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Hello at all,

how does a Helper/Tender Spring affect load and roll distribution/stiffness in cornering with stiff Springs on Coilover Suspension?

We use actually 140N/mm Main Springs with 10N/mm Helpers and i can see from Damper Potentiometers that during cornering the inner Spring

expand so much that the Helper get open. So at this moment the effective Spring rate drop to 9,3N/mm because of 2 Springs in series.

Does this have an effect on roll stiffness? Because the outer Spring remain 140N/mm and the inner change to 9,3N/mm, so the combined Spring rate at that Axle drop from 240 to 149,3N/mm total?

If i change the Helper Spring to a stiffer one, say 30N/mm, what will be the result? Did i get more roll stiffness because of the higher total Spring rate (in this case 24,7N/mm) or will i get more roll angle because of more travel from lower Spring rate?

With the stiffer Helper the travel to the Point where it get open and so reduce the total Rate of the Spring get less because of the higher "Blockload" from the stiffer Helper Spring.

I think also that at the moment the Stiff Main Spring get out of travel and the Helper get open, there must be a change in the vertical loading of the Tires overall on the Car, thinking in Corner weightning?

I hope i can get some answers and get light on this.

Kind regards, Michael

I'm not an expert, by any means, but some discussion points/comments.

I think you've got the main points. When the 'helper' spring is fully compressed the spring rate is just the main spring, but as soon as the load on the spring assembly drops to the point where the 'helper'starts to extend the spring rate will be the product of the two springs. By adjusting the spring rates. and lengths, you could place the transition where you needed it. The 'helper' could even be a higher spring rate - possible applications may be if a main spring works perfectly for most of the travel, but there's a bump, or yump, that would normally bottom out the suspension and the added spring rate of the helper may be useful if the main is fully compressed, to give a more cushioned transition to possibly bump stops.

In your case, and I may be mis-understanding your questions, I would expect more roll after the transition as the inner suspension will lift more for the same amount of load transfer, with the softer 'helper' springs - but less gain in roll between what would be the transition point with the stiff spring to the soft spring. By that I mean the roll would be expected to be the same until the stiff helper starts to open up, and them to have the same roll at full inner extension.

Any experts able to help?

Thank you Gord for thinking about it.

The Question is if i can cange roll stiffness(angle) and vertical load distribution by optimising helper springs.

The Car is for Hillclimb and has some aero, so for this my goal is to reduce body movement to bring the aero working better and stable.

The other goal is to maybe get better loading the corner inside tires if that is possible.

You're not providing much information, and as the cars used in the Euro' hill climb series cover a huge range of configuations, the more info' you can give, the better the discussion may aide you (or not?).

I think you've got the main points already. First and foremost, the overall load transfer is dependent on three main points - the height of the CoG, the width of the track, and the lateral (side) load. If the only variable is the 'helper' spring, the softest possible would be the best option, as that would keep the CoG the lowest at (almost) all times, fitted to the top to reduce unsprung mass. Remember, even though it is weak, the helper spring is still applying a small force that contributes to lifting the vehicle will both increase roll and also lift the CoG slightly. Minimising that will reduce lift - and roll - slightly.

Reducing roll can be done with stiffer springs and/or stiffer ARBs, but the actual, overall, load transfer will be about the same if the track width and CoG height is the same. Depending on the specific application, and all else being equal, you may find a net gain from lowering the car at the expense of compromising the roll centre(s) - perhaps by using stiffer ARBs to balance the vehicle in roll resistance?

I don't know if you have limitations imposed by the rules, or finances, but you may find most gains (if you haven't already done so) from reducing the mass (weight) of the vehicle by gutting the interior, fitting plastic windows, lowering the seating position, etc - as much of this should be above the CoG height, it will be lowered in turn. Wider wheels, or same width, using an increased offset, will increase the track and also help reduce the load transfer.

A few things to bear in mind, especially if FWD. If stiffening the rear to increase it's load transfer, once the inner tyre lifts it's at 100% and further stiffening does nothing but potentially compromise other aspects. Once the inner rear lifts, stiffening the front end may bring improvements from reducing camber gain in roll. A second point is many will run rear springs that are far too stiff for the application - especially if the vehicle is lightened - and this can promote skipping, or bouncing, over bumps and surface irregularities. On a similar vein, many don't have the damper settings correct on the rear, which can also promote skipping or bouncing. Further, especially with lightened vehicles, many run the rear pressures too high, having failed to take the reduced mass into account - again, it will promote skipping/bouncing and lower pressures 'should' bring gains from contact patch increase and inclreased slip angles.

NOTE - remember, the tyres are effectively auxiliary air springs - the higher the internal pressure the higher the 'spring rate' and the lower the internal damping of the spring. You can see this by bouncing two wheel assemblies, one with 20PSI and one with 30 PSI - the 30 PSI tyre will bounce higher for longer.

Sure i can provide you moore info from the car. It`s an AWD with about 640whp turbo engine. Weight is about 1300kg at the start line, distribution 60/40 front/rear. McPherson front and rear, paddleshifted 6-speed sequential dogbox, track and tyre-width nearly maxed out (270-65/18 slick). Actual spring rate is 160N/mm front and 120N/mm rear with helpers 10N/mm, uncompressed 80mm long, compressed 20mm.

We are not very restricted by rules and the car is modyfied nearly at every part of it. It is already fast, but i think there is to find a little more in the suspension setup, getting more out of the tyres from what i think they are sometimes overloaded on front outside.

In this link you could see the car at last race from 6:47 to 7:47 https://youtu.be/Ov_H-Gi7zn4

My thoughts on helper springs are mainly to keep the main (high spring rate) spring positively located and maintain some amount droop travel. The maths of adding two springs in series says the spring rate will decrease:

Combined spring rate = Keqs = (Ka*Kb)/(Ka +Kb) = 140*10/(140+10) = 9.33kg/mm

This is only true until the helper spring becomes coil bound (max compression), at which point it's no longer a spring, so your spring rate is back to 140kg/mm. So. i'd say it does nothing for your roll stiffness.

On a car running serious aero i'd be tuning roll stiffness with Anti-Roll Bars or, depending what your rules say, with some progress bump stops. This is the equivalent of running springs in parallel. This is where you simply add the spring rates. A bit trickier with a progress spring but, there a still a number of ways to get there.

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