Front Droop and Preload

[FF74]

Hi all

following on from some earlier posts of mine on handling of my 2004 VD, I'm doing a bit more research into front droop and preload.

I have got some good feedback from a supplier in the UK regarding setup and was all set to try it out when one of my fellow competitors lost his brakes (literally a pad separated from the backing plate), spun and hit me at the hairpin and bent 3 chassis rails, smashed up pods and radiators, cracked the bellhousing etc etc. You all know the drill.

Anyway, one point I cant seem to get to the bottom of is the guy I've been talking to in the UK about setup says he runs 300lb springs on the front of the cars with 0 droop, but also says he achieves this with no pre load.

I'm only a few years into this car setup game, but am a mechanical engineer, and I can not see how this is physically possible to achieve given the weight of car and driver.

I run 400lb fronts and need about 300lb (about 3/4") preload to get 0 droop.

The only two things I can think of that he may be doing are:

1: measuring 0 droop without taking off the ARB (I measure mine with it off)
2: droop limiting somehow

Any thoughts?

PS I have previously read the various threads here and on DSR forum on 0 droop

[R. Pare]

Easy. Mechanical stops.

[Brian]

if you are using springs to achieve zero droop, you must have preload equal to the load on the springs. That's kind of the definition of zero droop. If you have zero preload and zero droop, you by definition have zero load.

I think. It's late.

Brian

[R. Pare]

Preload is defined as as the load imposed on the springs by the perches BEFORE any load is imposed by the weight of the car. You can easily have zero preload and zero droop by means of mechanical droop stops (external to the dampers and springs) that are set after the car is scaled.

[Roux]

If I understand correctly could also be done inside the Penskes with a collar. Am I correct?

[R. Pare]

Correct, but it obviously will be much harder to reset the stops if you change the static ride heights.

[S Lathrop]

Anyway, one point I cant seem to get to the bottom of is the guy I've been talking to in the UK about setup says he runs 300lb springs on the front of the cars with 0 droop, but also says he achieves this with no pre load.

Maybe the term zero droop and no preload are the same thing. The shock is fully extended and the spring perch is in contact with the spring.

I have seen setups that call for negative preload which means that the spring perch is set at some distance from the spring. I have even done this to fix a handling problem.

Negative preload could mean droop, the spring drops from the perch when the car is raised and the wheels hang fron the suspension.

The instructions you received don't make sense if zero droop means that the springs are preloaded enough to just support the car at a given ride height.

[Neil_Roberts]

I think the missing element in this discussion is adjustable-length pushrods. With that, it's possible to create any combination of ride height, droop limiting, and preload that you want.

[FF74]

Gents

Am I correct in my earlier assertion that droop is always measured with ARB's disconnected i.e. each side independently?

Also, I note the comment from Neil (thanks) on adjustable pushrod. My car is fitted with such. I don't follow how this allows me to have free adjustment to set 0 droop. Without limiters I achieve 0 droop by adjusting the perch until the through shaft of the shock does not move when the car is lifted from loaded to unloaded. This is iteratively adjusted to ensure 0 droop is only just achieved with minimum preload. I don't follow how adjusting my pushrod would do anything other than moving the required location of the perch up and down the threaded shaft. There is not enough adjustment in the rod to max out the threaded perch shaft in either direction so this is not a factor. Thoughts?

Charles

[Brian]

If you are looking at pre-loading the spring on the shock to achieve zero droop, then you have maximized the shock length - it is held at fully open. So now you just set your ride height with the push-rod length. That's all it is. The adjustable push-rod gives you the ride height and corner weight adjustment while leaving you free to lock out the shock length to achieve zero droop.

Brian