pre-load & anti droop

[test run]

Hi, can anyone help exspain what pre-load is on a suspention set up. I am running a 1980's FF2000. it has ant droop fixtures added but I am told i need to run pre-load as well. What is pre-load, how do i put it on and what effect will it have???

Thanks

[mattray]

As I understand it Preload is "with your weight in the car the springs should be shortened (equally) until the damper is at maximum extension. As this happens you are at 0 preload.

On cars without push or pullrods (i.e. the rideheight is only adjustable via the spring lengths) a droop stop is needed to effectivly create a maximum length of the damper. So with you in the car and the droop stop fitted, wind the springs up evenly until the end of the damper touches the droop stop. Every turn you add after that is the preload.

final check is to make sure the front springs are an equal length. Some people see preload as turns from when the spring platform touches the spring but thats more on Renaults and F3 cars so don't worry bout that.

Hope that makes sence!

Matt

[Mackmot]

Only thing to add to Matt is that when you set it up put a piece of paper between the damper and the droop stop. When you tighten the spring up at the point that you can only just get the piece of paper out then that is zero droop

[Triple J Motorsport]

And you asked the effect it has.....

Stops the front diving under braking and weight transfer whilst cornering.

The more preload you add the less the suspension will move add too much and you could have no suspension at all (all ways on the stops).

Generally you want it set at zero on modern cars (no droop and no preload with the driver in the car).

[THR]

but u want droop at the rear, else u will tend to pick the rear off the floor under braking.
but none at front.

[mattray]

nice tip from the top Wayne!

THR not to mention wheel spin like a dragster, and have about 80mm rear ride height! Rear droop will probably be fixed by the damper length and ride height.

the effects will basically be that you will have a load of understeer with droop at the front, as you get rid of it the understeer will get better. Also as you brake into corners it will stop it trying to climb over the outside front and swapping ends.

[test run]

Thank you guys, it makes sense to me now, and I understand what to do with the anti droop devise that is fitted to the front end.

Cheers Test Run

[Triple J Motorsport]

Obviously I was just talking about the front....

Originally Posted by RayFF

And you asked the effect it has.....

Stops the front diving under braking and weight transfer whilst cornering.

The more preload you add the less the suspension will move add too much and you could have no suspension at all (all ways on the stops).

Generally you want it set at zero on modern cars (no droop and no preload with the driver in the car).

[tristancliffe]

This doesn't make sense to me. Please correct me if I'm wrong, as there is so much useless information about preload that doesn't actually make sense, and I'm trying to figure it out.

Preload is when the spring is compressed on the damper, with the damper off the car. If the spring perch just touches the spring, then that is zero preload, and any more is "pre" "loading" the spring on the damper.

Put the spring and damper on the car with zero preload, and it will take a ride height. That is likely to be between full droop and bump. Raising the spring platform will add preload, and raise the ride height, reducing droop travel, and increasing bump travel (from static). It will NOT change the spring rate (despite what people have written elsewhere!). More load will be needed to overcome the spring preload (which is normally opposed by the damper's internal bump stops).

With rising rate suspension, adding preload (which will move the rocker position, even if the car is subsequently lowered via the push/pull rods) will move the rocker so that it SOFTENS the wheel rate (again, at static, but the change will be throughout the suspension travel in the same direction), which is one area to tune the car.

So, if you ignore running zero droop, there appears to be no real need for preload, unless you want to change ride height (without push/pull rods) or if you want to slightly soften the suspension for, say, the wet or to cure slight understeer.

Then you get to droop limiting. I don't get this - partly because I haven't thought about it much, and partly because I don't use it. I thought load transfer was a function of CoG height/position, wheelbase/track and the acceleration, but NOT suspension movement, which is a reaction to that. If the suspension can't react, don't you ultimately just unload the front tyres. To take an extreme, think wheelies. Surely that is acceleration with insufficient front droop travel (and possibly excessive rear bump), and that hardly reduces understeer!

Feel free to point out my errors, because I'm quite new to visualising chassis behaviour, and I am trying to learn. But websites that claim the actual spring rate increases with preload are more abundant than I would have thought.

Oh, and sorry for digging up an old thread, but I felt it more appropriate than starting a new one.

[Matt22]

Well this is the way I think about it -

If you had a damper/ spring with loads of travel then you wouldn't need any pre-load, just wind up the spring base until the spring is 'tight', so it doesn't rattle around when unloaded (with the car off the ground!).

If your damper has, say, 3inches travel then you may find that 2inches are used up just by putting the car on the floor. This leaves only 1inch to account for bumps and braking (and aero if you've got any).

If you add 1inch of preload (meaning wind the spring base up 1inch from the zero position) then the car will only use 1inch travel when put on the floor (the droop), leaving 2 inches for bumps and stuff.

If you add 2inches of pre-load, then this will be zero-droop. What then happens is that in a steady corner, the inside wheel will have NO grip (or will be in the air), putting all the load on the outside wheel. Whether you want this or not depends on the car/ tyres, etc....I don't like this, but then I'm not at the front so what do I know?......

Matt

[tristancliffe]

I can see why you'd want to increase bump travel, but limiting droop is surely not a good thing, but perhaps the only thing. Two wrongs don't make a right, although I suppose it could make you quicker. Does it ultimately mean that zero-droop suspension is a friendly way of saying the car was poorly designed in the first place by now allowing sufficient bump and droop travel for it's intended purpose.

Do people voluntarily use lots of preload (zero droop) even when bump travel isn't an issue?

[Matt22]

___________________________________________
Does it ultimately mean that zero-droop suspension is a friendly way of saying the car was poorly designed in the first place by now allowing sufficient bump and droop travel for it's intended purpose.
____________________________________________________

More likely that most classes have tyres imposed upon them which are less than ideal, so F3, for example uses tricks to overcome understeer such as narrow rear track and mounting the front pushrods on the uprights

[Bob Pearson]

We find front spring preload a powerful tool for controlling turn in understeer, but that is on cars with mono shocks and disc springs controlling roll.
We did however convert a classic FF2000 to no droop front suspension with enormous improvements

[tristancliffe]

If I assume the CoG doesn't move (which of course it does) then zero droop surely just takes load OFF the front tyres? Which means less grip, right?

But if the CoG moves with the car, and the car doesn't wheelie [not exactly likely!], then zero-droop acts to limit the CoG movement, and hence load transfer?

It's all well and good telling me it works - I'm sure it does, as stopwatches don't lie and drivers don't generally get quicker with crapper cars. I want to know why it works. Reducing load on the fronts is a bad thing. And zero-droop will [ignoring CoG movement] reduce the front loads more, surely? So it must be something connected obscurly with preload, rather than the preload itself.

Sorry for being a pain about it. I just like to know WHY rather than just know.

[Bob Pearson]

We find it increases the load on the tyre in the corner and during breaking, that is an observation by looking at the condition of the tyres. I find it difficult to explain fully, so perhaps I shouldn't have got involved, but I think the extra load which passes through you tyre with no droop, or just generally stiffer springs results from load not being soaked up by compression of the springs. There is of course a limit to this or else you find yourself driving something like a large kart.
What I have just said applies to non monoshock cars, the effects of front spring preload are only apparent during breaking and tirn in on a monoshock car as the cars roll is dealt with by the disc spring pack.
In theory non droop front suspension does encourage a wheelie, but if you stand beside your car and look at the points of pivot which would be necessary to achieve this you can see instantly that downside of the suspension change is barely measurable. Understeer exiting the corners, which is the first step towards this mythical wheelie is best controlled by working on the back suspension.

[Bob Pearson]

I've just read through what I posted and realised that I missed out the vital link. Stiffer suspension tends to create more tyre temperature, and providing you aren't taking the tyres outside their design envelope, there is grip to come from that. ( and extra cost of course)

[Matt22]

In reality I think it works like this:-

On corner entry, under braking, the front has 'droop' because of the extra load on the front, so both tyres work properly

As the throttle is applied, the front goes back into zero-droop and will lift the inside front wheel on corner exit. This will create understeer, but this is kind-of what you want as it counteracts the oversteer caused by power going to the rears.

SO...It's possible to end up with a setup which has oversteer on entry (i.e turns in well) and understeer on exit (so can get on power early)...= FAST

[tristancliffe]

Hmmm, could be... Seems a bit weird though, as some people are running zero droop, so any non-braking cornering will tend to lift the inside wheel, not just accelerating out of the corner. How horrid would a fast corner be (e.g. Coram at Snetterton) with the right front half an inch in the air!?

[mattray]

You could think of it this way, the stiffer you have the front the harder the wheels are being pushed into the ground. Inside wheels being lifted are usually a result of the rear rolling and outside rear tyre compressing and the front right (in a RH corner) being at maximum droop while the front left is being compressed.

You dont get a front wheel lifting on the exit, it will only lift at the apex or the time of most load which should be the apex as you are releasing from apex to exit.

think that makes sense.

[mattray]

I guess if you have a really stiff front roll bar as well it could be pulling the inside wheel up?

[mattray]

http://www.facebook.com/photo.php?pi...e&id=511922954

check out the rear tyre!
this has 2 turns pre-load on the front, full stiff front roll bar and a fairly soft rear bar.

[oily mitts]

You've got a left rear puncture mate! You want to sort that out PDQ, can ruin your handling.

And your front suspension is looking a bit stuck. Have you run over something?



Get your car to an expert to sort it out.










LOL

[SAMD]

BIG LOL that was very funny.

It is a class photo though, and does demonstrate what was being described.

[mattray]

Trouble is the only guy I know that can sort that is busy making composites

[kartingdad]

Oh I know that blokes company.

Is is Wrecktech?