Could lack of droop cause coilovers to knock?

[derek182]

Recently had a new set of Spax RSX coilovers fitted to my 182 Cup but can't get rid of a knocking noise from the front whenever I hit any kind of bump on our local country roads.
The fitting was done by a very experienced race and rally mechanic who has been right through the front suspension and can't find anything loose or worn which could be causing this. We are actually running the suspension quite high for coilovers, not much lower than standard as I wanted it to ride reasonably well on roads and be useable for some club motorsport on a variety of surfaces.
As it is quite high the mechanic has measured that we only have about 15mm of droop on the front shocks and he wonders if them reaching the bottom of their travel is causing the knocking, could this be the case?
If so presumably the only solution is to run it a bit lower to give the shocks more chance to extend down over bumpy roads?
Any help much appreciated!

 

[_WILL_]

Do you hear it, feel it or both.

I would be surprised if it was the above but surely the easiest thing to do would be to drop it low and try it, you can always put it back.

Really you should A-B-A it. So you have a knock now. Drop the suspension, not changing anything else, if you get no knock then great. But put it back up to make it knock again to prove that was the fault.

What top mounts are you running?

 

[derek182]

Hear it and feel it, certainly something wrong somewhere.
Got the K-Tec solid top mounts, just changed the bearings as there seemed to be a small amount of play in them but that wasn't the answer.
Did try to talk to someone technical at Spax today but they didn't have anyone available with any technical knowledge! Will try again tomorrow.
Trying a lower ride height is the next plan but as I'm paying for the mechanic's time thought I would see if anyone had any other ideas first.

 

[derek182]

Just looked at the specs of the RSX, they are advertised as for a 15-50mm ride height drop, think we are less than 15mm so maybe that is the problem.

 

[Steven103]

Lack of rebound travel can definitely cause a knock (afterall there is something mechanical inside every damper that will limit travel, more expensive dampers have internal springs and flexible plastic washers to stop metal on metal contact causes knocking noises etc).

15mm of droop isn't correct. 300 KG corner weight * gravity / 15mm / motion ratio of about 0.8 = a front spring rate of nearly 250N/mm which is absurd for a Clio road car.

(bear in mind depending on weight, tyres, damping you could have 45mm of droop and it still might run out of travel and knock on severe bumps)

 

[derek182]

Thanks for the replies, I'm not sure if I'm using the right terminology, by 15mm droop I meant that when we jack the car up the wheels only drop 15mm before reaching the bottom of the suspension travel which I think is the problem, having spoken to someone helpful at Spax they agree and we are lowering the car so it runs about 25mm lower than standard, well within the operating range according to Spax and hopefully avoiding what they call topping out.
I think Steven103 is talking about what I would call sag, ie how much the suspension compresses just with the weight of the car on it.
Anyway, with a bit of luck I'll have a lower, better handling and better riding Clio next week!

 

[Steven103]

No no broadly speaking you've got the correct definition of maximum droop travel. You're just having a funny moment regarding "sag" as you call it.

- If you move a door 1 meter to open it, then you have to move it 1 meter to close it.
- If "just the weight of the car" makes the suspension "sag" by 15mm, then removing "just the weight of the car" will make it "un-sag" (droop) by the exact same amount (15mm). Hence, with some reasonable assumption, working out one can tell you about the other.


But back to your problem, just raise the car and see if it gets better! If it's within Spax's safe limits then perhaps consider the dampers are faulty. Maybe the knocking is just the (lowered) damper hitting the (standard length) Renault bump stops.

Frankly it probably just knocks because something else is, by complete chance, now in the way of your suspension since you've moved it all 25mm. Your mechanic can really easily spot if things clash in droop with the wheels on the car. But it's difficult to visualise whether things will clash when the suspension has moved 100mm into full bump, the engine is vibrating +- 10mm on it's mounts, the airflow & general vibrations are making a heat shield move 5mm etc etc.

 

[dann2707]

Are you sure the top mounts haven't worked their way loose? Mine did that and knocked going over bumps after fitting coilovers.

Easiest way to tell is by jacking the front up and try lifting the wheel up, if you have movement and it's making a similar noise to what you're experiencing that will be it. Ended up sticking another nut on the top mount acting as a locknut.

Another problem I had when fitting solid top mounts to coilovers was that the locating spigot platform was too bit for the coilover top platform so required an m22 washer to keep it nice and solid on the spring.

 

[derek182]

Have now lowered the 182, we think about an inch lower than standard, maybe a bit more, and it's driving nicely, with no bangs going over the same bumps as before.
Now need to find a trackday to do!