Springs and myths

[Larry J-Croft]

Right - I may be setting myself up for the dumb*ss question of the year, but talking to people in the paddock and trying to remember my engineering training is scrambling my brain cells. I race a monoposto single seater and there is lot of talk of 'precompressing/preloading' shock absorber springs. This has come about whilst I am trying to suss out a rather nasty bout of oversteer which I think is due to the back of the car being too stiff.

A standard combined spring/shocker unit has an adjustable platform which can be used to compress the spring - it does not effect the unit lenght as it is already fully extended. It has virtualy no effect on car ride height as the springs are relatively very stiff compared say to a rally car (which is why we use the pushrods to adjust height). However by winding the platform in you can obviously increase the compression of the spring. To my mind this is will have no effect if using a linear rate spring - a '100lb' spring takes 100lbs to compress it one inch and 200 lbs to compress it two inches etc (Hookes law). So if you have pre-compressed it by one inch then it will still only take a further 100lbs to compress it the next inch. Yet people in the paddock have been quite adamant that preloading the spring has a significant effect on suspension performance.

So - have I got it completely wrong and should I stand in the corner with a pointy hat on and if so what is the point I am missing?

If I am right - why the adjustable spring platform???

Confused of Someret

[greenamex2]

'Only' an extra 100lbs is quite a lot, and I suspect you have much harder springs than that. I also suspect you wouldn't be adding an inch of preload.

Even if you had 100lb of preload then that 100lb of weight transferred to another corner that would otherwise be 'absorbed' by that corner. On a heavy saloon car that has a fair amount of effect on controlling weight transfer and hence grip. On a lightweight car like yours that is a MASSIVE amount of weight to be playing with (I'd guess 10% ish).

Obviously you have to factor in the actual wheel rate and tyre spring rate which reduces the spring wheel rate significantly but that still leaves significant weight figures that can be controlled.

I reckon that preloading the front springs will transfer weight to the back and reduce oversteer. That is of course providing that the front still sticks and doesn't just slide.

I use spring preloading on my kit car to ensure that, even at the very end of suspension travel, I still have a decent spring rate 'pushing' the tyres on the ground. However my car has serious suspension travel issues at the front.

Hope that helps but I have no doubt more knowledgable people will contribute.

[Larry J-Croft]

Denis,

Thanks for that, I was only using 100lbs nominally to illustrate the point ie that preloading does little more than put a static tension in the shocker which then does not effect the subsequent spring/deflection. Ie preload a 100lb nominal spring to 100 or 400 lbs - it will still only deflect another inch per extra 100lbs added so I beleive the amount of preload is irrelevant.

As the preload is put on the shocker with the the piston already at full extension I don't see how it could be transferred as weight transfer to any other corner?

(by the way I use about 1100lb springs and my corner weights are about 300lbs so you can see how little the suspension actually moves - hurts the teeth!!!)

[greenamex2]

That's the point, the 'weight' has to go somewhere so if one corner doesn't/can't compress then another will.

Providing the chassis doesn't just bend (back to my kit car!)!

[BrettFrancis]

True - the spring still requires 100lb to compress it an inch. But now to compress it that first inch it needs 100lb just to get to the "breaking point" (where just starts to compress) plus another 100lb to move it the inch. So you've doubled the force it takes to move it that first inch.

Therefore any force from the 0lb to 100lb range is not compressing the spring, but pushing the tyre into the track harder.

If you find that by increasing the preload you are also increasing the grip on that tyre, you probably need to have a stiffer spring (or you could try to stiffen the low speed bump on the damper).

More preload/stiffer springs might also help a car that has poor suspension geometry. If you have poor bump steer characteristics, poor camber control in roll, or a number of other things, putting stiffer springs in will effectively "lock" the suspension, not allowing all of these nasties to happen.

If you are stiffening the front of the car and find that your oversteer magically goes away, more than likely you have managed to maintain more load on the inside rear tyre (less weight transfer across the back end). Stiffening the front bar will quite often have the same effect.

PS. I hope that when you say winding the spring platform has no effect on the ride height of the car you are referring to the front end and not the rear. I don't think most monoposto cars (or most racecars for that matter) are designed to have zero rear droop.

[marcush.]

You forgot that one little detail:the compressed spring exerts a force already to the extracted damper ,so you add your linear rate to the preloaded forces as the cars suspensions comresses.In other words ;the damper will start to move as soon as you OVERCOME the preload ,if there´s no preload it will move instantly(bleed and shimstack permitting).
Of course ,stiffening the front may bring back your balance and help your laptimes but the true objective is to maximise grip not getting rid of it.
so my word is:work on the end that is not working!so if you feel the rearend has overstear(when?corner entry,under braking,midcorner neutral throttle or corner exit under acceleration ,or over bumps or rough surface)
Only when you won´t be able to improve the bad axle you have to resort to sheding grip on the other end to survive.

[ubrben]

This is a thread on fsae.com on zero-droop, i.e. preloading.

http://fsae.com/groupee/forums/a/tpc...8910968321/p/1

Ben

[Cameron Winton]

Also remember that at extremes of compression/extension, springs go non linear. ie the rate X travel equation fails.
Have you tried your springs in a spring tester to make sure the rate you have is correct?

[phoenix]

I was about to say.....

All the time you still have droop when the suspension is unloaded, pre-load will have no effect on weight transfer - just ride height.

Once you get to a zero droop situation then you are effectively increasing the spring rate. Then as you add more preload you get an ever stiffer spring rate. Bear in mind that this can have all sorts of unpleasant side effects - like losing grip under hard braking, for example, which may harm your lap times. You may also find your dampers are not as effective in rebound as they are now trying to deal with a much stiffer spring.

If you do adjust the front suspension this way then it will, to a greater or lesser extent, tranfer weight onto the front outside wheel in a turn. This will either a) use the tyre more effectively and reduce/remove the oversteer and increase overall grip by using the front tyre more or b) overload the front tyre changing your oversteer to understeer.

Don't worry about the inside tyre too much - it will be offering little or no grip if you are cornering as hard as the grip from the outside tyres will allow. With zero droop you will have to get used to the fact that the inside wheel will spend a lot of it's time completely off the ground.

If it were my car, and if I knew that the existing springs gave the correct wheel frequencies, I wouldn't touch the springs or the spring pans. I'd be looking to reduce the rear anti roll bar stiffness or increase the front anti roll bar stiffness to get the balance right.

[greenamex2]

Inclined to agree with phoenix unless you have no other option (like the rear is already running out of travel somewhere).

I used to have an 'experienced' person tell me that to fix a problem one end you look to the other end. When I stopped listening to him and started fixing the problem at the end that had it I went a lot faster.

[Larry J-Croft]

Guys,

Great advice and a lot more than I originally asked for. As I have only raced the car twice I am going to do some serious measuring over the winter and use Mr Staniforths equations to look at spring rates versus anti roll bar stiffness as I am not convinced this car has been fully assessed since its F3 days 7 years ago. Just out of interest the oversteer is on power application on corner exit, on slow speed corners so I am farily sure it is mechanical rather than aerodynamic.

However I am still having trouble with one concept which is to quote several of you:
'the spring still requires 100lb to compress it an inch. But now to compress it that first inch it needs 100lb just to get to the "breaking point" (where just starts to compress) plus another 100lb to move it the inch. So you've doubled the force it takes to move it that first inch.'

If you compress a spring physically using the platform to a preload condition with the damper at full extension. I don't see why you need to overcome that compresion before it starts to move. A 100lb spring on a table with a 100lb load on it will be compressed an inch - add another 100lbs and it compresses another inch - not 200lbs. I take the point about droop but I only have about .24" droop with my springs and weights (1100lb springs and 300lb corner weight).

larry

[ubrben]

The difference when you push on a spring on a table is that you're reacting the load.

The only way of preloading a normal coilover shock is to wind the spring seat up. This gives you zero drop because you are preloading against the damper. This means that the damper is reacting the preload not the tyre.

Ben

[Larry J-Croft]

Once again many thanks to all of you. I'm off on holiday for a couple of weeks and am going to try and forget about all things motorsport for a while (some chance), but I might just get a spring and do some experiments........

[Chris Y]

Whenever I try and think about these sorts of things, I always think about my radio controlled model cars - most of them have fully adjustable suspension geometry, including pre-load, droop, angles, spring rates, and damper adjustment. If you can get hold of an old radio control race car (not the cheap ones from toy stores), then it might be worth it - just so you can play around with different ideas on your table

[Larry J-Croft]

Just to finish this off, I did a really scientific experiment last night. I wound one rear spring platform tight and loosened the other one off. I then stood on top of the gearbox and jumped up and down. Sure enough you are all correct, the tight one hardly moved and the loose one boinged about a lot!!! More importantly I now understand why the car was probably so oversteery, as the need to raise the ride height for monoposto regs led to the suspension hitting its mechanical droop limit and beyond - putting extra preload on the springs as the only way of relieving the extra pushrod length. Now I understand what has happened it is easily fixed - so thanks to everyone.

[greenamex2]

I can sympathise with you. I spend over a year fighting snap oversteer only to find my rear springs were going coil bound. Easy, and obvious, when you know!

[marcush.]

Originally Posted by Denis Bassom

Inclined to agree with phoenix unless you have no other option (like the rear is already running out of travel somewhere).

I used to have an 'experienced' person tell me that to fix a problem one end you look to the other end. When I stopped listening to him and started fixing the problem at the end that had it I went a lot faster.

this is the point,balance is one thing ,but you don´t know if the bad axle is working under its capabilities or over ,so if you balance the car by shedding grip from the otheraxle.,you will never know.

as for the three wheeling mentioned elsewhere it goes the same ;if a wheel does not have groundcontact it cannot bare any loads,wich is a bad thing as your axle is limited to the load carrying capacity of one wheel during that time.this inevitably will lead to this axle lacking grip compared to the other,plus you have 2 points during cornering when this changes to/from 4 wheel cornering .....and suddenly the gripcapability of this axle changes ...(halves!).