Bottoming down on acceleration
Nitrogeno25 - 23/12/13 at 02:20 PM
I'm trying to solve a problem with my car that became more evident when I replace the engine (more power duratec).
When I accelerate hard in first the rear suspension is bottoming down (more evident in rough surfaces). I was advice to use hard springs, but on the
track the car tends to oversteer a bit, so a firmer rear will be worse.
Is there anybody experiencing bottoming down in hard launches?
Thanks!
Cheers.
[Edited on 23/12/13 by Nitrogeno25]
ReMan - 23/12/13 at 02:46 PM
There are others here more clever than me who I am sure will advise further.
IMHO it's not that you need hard springs, but you need the right springs, which may only be a few lbs heavier than you currently have to dial
this out.
However, have you experimented with the damping at all?
Compresion damping settings will affect squat on the rear and I believe, although to a lesser extent, so do the front rebound settings.
Also as you already know suspension is designed with a purpose. So a drag racer (as you describe) will need different settings to a circuit car. Try
to do both and you have to compromise
Have fun 
[Edited on 23/12/13 by ReMan]
britishtrident - 23/12/13 at 04:18 PM
quote:
Originally posted by Nitrogeno25
I'm trying to solve a problem with my car that became more evident when I replace the engine (more power duratec).
When I accelerate hard in first the rear suspension is bottoming down (more evident in rough surfaces). I was advice to use hard springs, but on the
track the car tends to oversteer a bit, so a firmer rear will be worse.
Is there anybody experiencing bottoming down in hard launches?
Thanks!
Cheers.
[Edited on 23/12/13 by Nitrogeno25]
Unless you want to start moving suspension mounting points on the chassis fit stiffer rear springs and also increase the front roll stiffness by
either stiffer spring or fitting a front anti-roll bar.
If you fit a front anti-roll bar don't make it too stiff the front springs still have to be able to work i roll and single wheel bumps.
bozla - 23/12/13 at 04:45 PM
Have you got adjustable dampers? If you could increase the damping the car may avoid bottoming out, and you wouldn't need harder springs.
Nickp - 23/12/13 at 05:02 PM
Is there anyway to increase the amount of travel you have available? If not then the only way to stop it bottoming out is to stiffen it either with
springs or damping as already suggested.
nick205 - 23/12/13 at 05:22 PM
Do you have scope to raise the rear and lower the front ride height a little?
britishtrident - 23/12/13 at 09:17 PM
Re dampers remember a lot of dampers are only adjustable on rebound which won't exactly help.
Ivan - 24/12/13 at 08:01 AM
An obvious question - are you sure your springs aren't becoming coil bound as they compress - it might not be bottoming but the coils might be
binding before the suspension bottoms out.
phelpsa - 24/12/13 at 11:08 AM
quote:
Originally posted by Nitrogeno25
I'm trying to solve a problem with my car that became more evident when I replace the engine (more power duratec).
When I accelerate hard in first the rear suspension is bottoming down (more evident in rough surfaces). I was advice to use hard springs, but on the
track the car tends to oversteer a bit, so a firmer rear will be worse.
Is there anybody experiencing bottoming down in hard launches?
Thanks!
Cheers.
[Edited on 23/12/13 by Nitrogeno25]
A few more specific bits of information about your suspension setup would help narrow things down. Anything you can give us (spring rates, ride
heights, damper settings etc?).
Nitrogeno25 - 24/12/13 at 02:08 PM
quote:
Originally posted by phelpsa
quote:
Originally posted by Nitrogeno25
I'm trying to solve a problem with my car that became more evident when I replace the engine (more power duratec).
When I accelerate hard in first the rear suspension is bottoming down (more evident in rough surfaces). I was advice to use hard springs, but on the
track the car tends to oversteer a bit, so a firmer rear will be worse.
Is there anybody experiencing bottoming down in hard launches?
Thanks!
Cheers.
[Edited on 23/12/13 by Nitrogeno25]
A few more specific bits of information about your suspension setup would help narrow things down. Anything you can give us (spring rates, ride
heights, damper settings etc?).
Here we go:
Dampers are only adjustable for height not for rebound/compress (changing in the near future though). Spring rate are 220 at the back 350 at the
front. I have 80mm under the sump and 10mm rake. With me in the car the dampers are at half the damper travel.
Maybe the cassis pickup points are not well sorted and the car squats when accelerating hard 
Thanks again!
drt - 24/12/13 at 02:13 PM
What type of car/suspension are you running ?
With bellcranks you could alter the lenghts/angles so the suspension would be progressive through the stroke.
Ofc some anti squat geometry would help... but only marginal, as I suspect that your car is light and with a low CG.
[Edited on 24/12/13 by drt]
drt - 24/12/13 at 03:00 PM
quote:
Originally posted by drt
What type of car/suspension are you running ?
With bellcranks you could alter the lenghts/angles so the suspension would be progressive through the stroke.
Ofc some anti squat geometry would help... but only marginal, as I suspect that your car is light and with a low CG.
[Edited on 24/12/13 by drt]
should have clarified... anti squat for a track car is advisable to keep it around 25% or less.
Only dragsters use 100% (and some FS cars on the acceleration event only!)
Ivan - 25/12/13 at 10:07 AM
quote:
Originally posted by Nitrogeno25
quote:
Originally posted by phelpsa
quote:
Originally posted by Nitrogeno25
With me in the car the dampers are at half the damper travel.
Ma!
I think your problem is that you have insufficient damper travel left at ride height - you should have something like 2/3 of the travel left with you
in the car at normal ride height. You are thus experiencing the rear suspension bottoming out under roll in cornering causing the over-steer and of
course the same problems under acceleration. Hence my original question about spring binding which gives similar symptoms.
I would think re-location of the dampers to give better travel at ride height will go a long way towards solving your problems and you could then
maybe also look at higher spring rates - but first get the damper travel issues sorted..
blakep82 - 25/12/13 at 01:13 PM
What type of rear end is it?
If live axle, you may be able to build in some anti squat geometry? Eg on a live axle you put the top link bar about an inch or so up (so the two bars
arent parallel) and this has the effect of pushing down on the axle under acceleration (or something on those lines)
I'm sure theres a similar set up for irs axles, but a bit of reading up on "anti squat geometry" may hold some inspriration for you.
Check for coil binding, and shock length etc first though as well as all the suggestions above. Might not be as bad as you think