Cutting bump stops

[AMMO]

Got to start by saying that I know practically nothing about car suspension. I have enough knowledge to turn a motorcyle from a bad handling piece of crap into something that can win races.

 

I am not happy how my Seven handles at the moment. It is useable but could be improved so much. The spring rates and damping need work. The Nitrons probably need a re-gas at lower pressure and the damping works best with adjusters backed off to maximum 2 clicks. Going to Nitron next week to get it sorted. Probably need a re-valve and shock dyno session.

 

I am applying what knowledge of motorcycle suspension to my car. I was not happy that the front suspension showed only around 25 mm of exposed damper rod. This means that as the supension compresses it doesn't take much travel before you are on the bump stops which then up the spring rate at the end of the travel. In motorcycle racing everybody uses linear springs (or at least used to). Linear springs do have higher spring rates as they compress towards the end f their travel anyway. To add another spring in the form of a bump stop makes things even more confusing.

 

The solution at the front was to cut the bump stop to expose more rod leaving enough to stop the body going metal to metal on full bump. Pleased to say that this has worked very well. The oil witness on the damper rod shows that the suspension on full bump is not quite going to the cut down bump stop but would have impinged quite a way into the old bump stop had it still been there. In my view this is how it should be. I like the spring and damper to do the work. This is how we set up bikes at front and rear (although bikes have more parameters to play with) and it works.

 

Cutting the rear stops today as the rear is very bumpy and uncomfortable on bumpy roads. My bum leaves the seat as the rear goes airborne. Over a series of bumps you can hear the engine rev up as the wheels leave the deck. On a fast bend a bump that should have been absorbed made the back leave the ground and step out. Not nice. Got a bad back from a couple of longish trips. This didn't happen before on the old setup.

 

The front feels great in comparison. The drive back from the Cornwallis Seven Club meet was brilliant. The front end is fluid, less bumpy / twitchy. Still needs more work.

 

Hopefully will be just what I want when I have finished with it.

 

Anybody else tried cutting their bump stops or am I on my own adopting this way of thinking?

 

AMMO

[MorganMan]

How and by how much did you cut your bump stops? Does the same apply to standard Bilsteins?

[Mike Bees]

Similar effect by a different route - widetrack cars have a spacer fitted to the end of the damper (just makes the whole damper unit longer) to account for the wider track. However it's quite common to remove the spacer to reduce the likelihood of running on the bump stop.

 

It sounds as though your dampers are too long. Or your ride height is a lot lower than expected by whoever specced the damper length.

 

Mike

[Casbar]

Narrow track, Mo Bilsteins on the front. Can't hardly see any piston rod, with the springs wound up and 65mm under sump.

 

Spoke to CC, they told me that the bump stop on Bilsteins is designed to act as part of the damping hence why they are so big

 

Suspension at the front seems ok, been like it for a while, but as Ammo mentioned, this goes against everything I have seen before on supsension

[tom7]

On a wide track car there is so little of the damper-rod showing, the damping seems as if it is almost all done by the bumpstops if you set the ride height to 65mm.

I cut mine in half. I can't believe that the bumpstops could be 'designed' as part of the damping

[Iain A]

Hi

 

Cautionary bump stop tale - I have Ledas on my widetrack car. Going through a dip in the road the front suspension fully compressed grounding the sump... which then met a raised cats eye coming the other way at high speed. Result - one cracked Raceline sump on my Duratech. ☹️

I have had the sump welded up and will be trying it back on the car at the weekend. At the same time my spring damper units have been back to Leda to have longer bumpstops fitted.

When setting the bumpstop heights, try assembling the front suspension without the springs so it settles fully on the bumpstops... then sit on the front of the car to compress them further to simulate the compression in a dip in the road. Now measure the clearance under the sump. If you can't fit a cats eye (about 15mm) under there then it is too low for me!

For a track car, the above doesn't really apply I suppose.

 

One of the Duratectives

[Mike Bees]

Also (whilst the springs are off) check that the wishbones don't make contact with the cutouts in the ali skin at full bump.

 

I would have thought that the bumpstop would be considered a (very variable rate) spring rather than having much in the way of damping properties.

 

Mike

[AMMO]

Thanks for your views guys

 

Have ordered lighter rear springs to try and will be going to Nitron next week for additional work. Will let you know the outcome.

 

AMMO

[AMMO]

Have changed the 140 lb rear springs for 110 lb ones. The ride is still very harsh. I can now bounce the car and feel it needs a couple of clicks of damping. The problem is that even with the lighter springs the suspension travel is inadequate. The witness on the damper rod shows that the damper body is not travelling anywhere near the bump stops. Will send shocks to Nitron for re-gassing at a lower pressure.

 

 

AMMO