David Jenkins
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 posted on 4/9/04 at 03:48 PM |
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Bumping on the stops
When I took the car out for its first run today it became clear that my back suspension was hitting the stops on some of the bigger bumps - this is
not kind to the spine!
Mine is a book chassis, live Escort back axle, Avo shocks (mounted vertically) and 180lb springs. The ride height is currently set to 6" and
the damping control is set to 'minimum damping'.
Should I fit another set of springs (I have a pair of 200lb jobbies), raise the ride height, change the damping control, or what?
rgds,
David
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Surrey Dave
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| posted on 4/9/04 at 04:05 PM |
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Design........
When I 1st built mine I fitted 140 lb springs on the rear ,and they would bottom out unless the dampers where wound up.
although the ride was OK , the problem I believe is that the standard design gives shorter rear dampers with less travel ,they should actually be
longer with more travel than the front because they are mounted vertically.
The only way I could cure this was to fit harder springs 180lb which limits the suspension movement but gives a harder ride than neccesary,
If I wasn't lazy I would change the rear damper mounts and fit longer dampers around( 12-13 inches) , I think you will find that a Westfield has
dampers this kind of length on the rear.
Then go back to softer springs........
[Edited on 4/9/04 by Surrey Dave]
[Edited on 4/9/04 by Surrey Dave]
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stephen_gusterson
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| posted on 4/9/04 at 06:24 PM |
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surely damping only controls the spring rebound - need bigger springs.
Are you saying you drove your car illegally on the road, david
atb
steve
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pbura
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| posted on 4/9/04 at 06:57 PM |
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180# is a good-sized spring for a live axle. I'd try taking the dampers up a couple of clicks (and the front as well).
Try pushing down each end of the car and releasing. You should have a quick return to equilibrium with no extra jiggling. If you get the dampers too
hard, every little bump will give you a jolt.
How are your tire pressures?
Pete
Pete
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David Jenkins
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| posted on 4/9/04 at 07:11 PM |
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quote:
Originally posted by stephen_gusterson
Are you saying you drove your car illegally on the road, david
Steve,
No, totally legal - it went for its MOT (and passed)
Pete,
My pressures are 18psi all round.
Surrey Dave,
I think you're right - but I'm not going to redesign the back end just at the moment!
David
[Edited on 4/9/04 by David Jenkins]
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GeoffT
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| posted on 4/9/04 at 09:39 PM |
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My car is book, live axle, 150 lbs, GAZ shocks and like yourself my spine complains, mainly on the downside of speedbumps. Sorry mate, haven't
got the answer, the real problem as you state is the lack of rear end travel on the book design.
My plan of action on this was to wind up the platforms to give a bit more more bump travel (albeit with less droop) and increase the spring
rate a bit. Worryingly this sounds like exactly what you've got right now.
Keep us posted on your results, I'm sure there's a few ageing spines like mine looking for a solution on this
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stephen_gusterson
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| posted on 4/9/04 at 09:50 PM |
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im the first to admit that suspension isnt my bag, but here is my theory when I set mine up.
Spring preload is just that. If you preload 200lb springs to the effect of 200lb, for the first 200lb pressure, you get no movement. Sounds fairly
obvious to me.
Ideally, you set the preload such that the springs are perhaps 1/3 compressed so that you get the std 1/3 bump and 2/3 compression.
Ok so far?
Now, if you increase the preload to give more bump, then you will compromise on droop?
Wont that tend to cause the suspension to bounce back and hit the bottom stop, which also isnt gonna feel comfortable or help handling.
atb
steve
[Edited on 4/9/04 by stephen_gusterson]
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pbura
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| posted on 4/9/04 at 11:34 PM |
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Don't know why I asked about the tires, except a vague thought that they provide spring action. Wholly inapplicable here
When you have a normal load in the car, are you seeing about 2/3 of the damper shaft? If not, then you could add preload, per Steve G's
comments, which will raise your ride height. If you want a lower ride height, the correct way would be to relocate the shock brackets. If you get
into this, a bracket with alternate mounting holes might be worthwhile.
2" isn't a lot of shock travel. Assuming your preload is correct, how about chopping half an inch off your bump rubbers?
Pete
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Surrey Dave
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| posted on 5/9/04 at 11:21 AM |
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My ride hieght was set so that the damper is 1/3 compressed allowing another 2/3 of travel , the last inch being on the bumpstop.
although tyre pressure does not tackle the problem the ride can be improved by keeping them low , 15 - 18 psi.
The only real fix is longer travel on the rear dampers with a softer spring ,then using the dampers correctly to control them.
Yes Steve- stiffening the damper doesnt help the spring rate or travel but it seems to buy a bit of time being the suspension hits the bumpstop,
[Edited on 5/9/04 by Surrey Dave]
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Rob Lane
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| posted on 5/9/04 at 12:06 PM |
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It's not possible on a standard live axle chassis to fit long shocks.
This is because the lower trailing arms hit the chassis, limiting the droop.
I have the same problem two up. On my own it's fine but the extra weight of a passenger makes it bottom out occasionally.
I have put shocks click up but it makes for a hard bumpy ride. Better to slightly uprate the springs and keep rebound lower for a more compliant ride.
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Surrey Dave
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| posted on 5/9/04 at 12:11 PM |
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Another way to get more travel is to fit the shocks at an angle, I believe I saw this approach on the Isonblade where the rear shocks where fitted on
the centre of the axle tube and went up an angle to the top mounts.
Or you could even angle them back or forward slightly, which will give a little more travel for the same damper length.
Why cant I post without having to edit!
[Edited on 5/9/04 by Surrey Dave]
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