The V8 Owners Forum -

How can i make 'em better?

Currently muy capri is relatively standard, poly bushes thruout, bilstein dampers, biggest available a/r/b's, lowered springs, decambered leafs, adjustable shocks etc, the usual.

It handles great. . . . . . unless its bumpy!

Or infact anything that requires suspension movement is when it goes a bit wrong eg the brakes skip and lock up coming off bumps and various other situations like that.

Now the back end will get sorted by way of 4 linking and maybe a watts linkage if i can get it under there (petrol tank may hinder the watss) if not a panhard rod will do then i'll finsh with some nice coil overs but its the front thats the problem.

It just binds up not allowing hte springs and dampers to do their job properly so i'm thinkin nice free moving rose joints thus allowing the dmapers to do all the damping not the bushes by way of friction and also allow the springs to react correctly to the bumps and also any weight transfer caused by driving like a nutter.

I was thinking may be ditching the standard a/r/b and putting some compression struts in then getting another a/r/b and connecting it up by way of drop links and using some lower arms with the rose joint in place of the inner bushes but that still leaves me with the big ass polybush still in place on the lower arm (where the a/r/b would originally have connected to and where the comp strut would now go in place of) and the ball joint which connects to the bottom of the strut.

What do you reckon?

Stay with wot u got and see if u can fine tune it, have u got adjustable shocks? wot setting do u have them on? wot poundage are the front springs? with a rover motor being lighter than 2.8 u can probably leave stock poundage springs in with lower ride height? john

You could go the rose joint route but I don't think it will help you any in this case and the road noise and maintenance hassles on a road car just aren't worth it. I don't think its the polly bushes that are causing you grief either as there just isn't enough stiction in there to stop your suspension extending properly. IMHO your whole problem is centred around your shocks. You either have far too much compression, rebound damping or both. Either your wheels aren't moving to go over the bump making the car jump or they are moving but aren't extending quickly enough to keep contact with the road. It could also be your springs are far too hard as well. You might get some results by loosing some poundage but increasing the preload so your car sits at the same hight but it takes less force to move the suspension. No idea what your ride height is or what your spring rates are but something in there is too stiff and it isn't the bushes.

Admittedly the car does sit LOW but it has alot of suspension travel left. Regards to springs irrc they're 170lb -2". I should consider if maybe the front is bottoming out hence the problems.

The dampers are 2.8 bilstein units, i can't iamgine their damping action is far from ideal, they made a huge difference to the cars handling and not because the old ones were knackered.

What are your thoughts on coil overs? I've heard from reliable sources that because the springs seat isn't offset that it can binding issues for the damper?

GT40 Kits 300lb fronts and 400 rears

Engine in the middle bot about 40 / 60 weight with more on the rear.
Total weight about 1100kg

So I would think your springs are way too soft.

Now I happen to know where I can get abatch of these in a box if you want to do trial and error tests. They would need a donation to our club for the ones you use.

Ian

Hi gav
What wheels and tyres are you using? Large wheels and low profile tyres by any chance?
If so then I would look at lowering the spring and damper rates all round and the damper rates in rebound especially, with lower profile tyres you are relying on the suspension to do the work normally done by the tyres (absorbing the small amplitude surface bumps, the tyres need to be given a chance to stay in contact with the road. (look at an F1 car, the side walls are actually quite high and watch the way deform as they go over an apex rumble strip, that is why the comentators droan on about tyres so much) in that case the suspension is keeping the car flat so the airodynamics can work and the tyres are left to do the shock absorbing).
Unless you can get a rising rate spring for the car try using softer springs and big rubber bump stops to stop the suspension bottoming out on the chassis, rubber makes a good rising rate spring just look at the old mini set up.
Another result of alloys and lower profile tyres is that the unsprung mass will be much lower and again the spring rate will be to high as a serult of this and also the damper rate will be too great, the set up does not have so much energy to absorb for a given bump in the road (especially on rebound).
Hope this helps.
Mike

The wheels are 15x7 with 195 tyres the same setup as the brooklands capri which has the exact same struts as mine so i wouldn't thought is was that.

Ref the gt40 that has a different suspension setup to mine so the spring rates will be alot different to mine.

tyre section?
are you running fairly modern alloys?
assuming the capri originally ran 13" wheels and a 185 tyre then the tyre side wall is about 3/4" t0 1" less than originally designed for? sorry I don't know capri specs by heart (a major sin around here ). assuming you are running standard rolling diamiter.
not as much as it could be but I still think the lighter wheel and the symptoms you describe you need to soften the damping alot (both bump and rebound ) and soften the spring rate , add a bit of preload on the springs if the ride hight starts to drop.
Mike

It was the same before i had these wheels on it so i really don't think its that. Even if this wheel/tyre combo does wheigh less i'm sure its offset by the heavier brakes.

I think Marki has hit the nail on the head really, i need to try a stiffer or longer spring as it is likely that its bottoming out the more i think about it.

Ian Anderson wrote:GT40 Kits 300lb fronts and 400 rears

Engine in the middle bot about 40 / 60 weight with more on the rear.
Total weight about 1100kg

So I would think your springs are way too soft.

Now I happen to know where I can get abatch of these in a box if you want to do trial and error tests. They would need a donation to our club for the ones you use.

Ian

You cant go by weight of car and say what spring should be on. For example my car is lighter than yours and has 50-50 weight with 200 pound springs in each corner which is standard. At the front this is currently too hard and the car wont come off the bump stops. Its all to do with the geometry of the suspension and how much preload is on the springs. At a guess I would say your GT has virtually no preload on the springs or they are very inboard.
There is two ways you can set up springs.
You can use light poundage with a lot of preload. This gets the car at the proper ride height but leaves a very supple ride. French cars used this a lot, Renault 18 being one of them where it had 5 or 6 inches of preload. You never felt a bump in the car but it felt soft and bouncy.
Other way is high spring rates but virtually no preload. Again car sits at the right height but you end up with very firm, what you would call sporty suspension. Great for smooth race tracks but not much use on the street. A typically set up F1 car would break your back driving on most of our roads for example.

As kiwicar says it all has to match as well. AN F1 car has large profile tyres so they are part of the suspension as a result the springs are stiffer. Likewise the new trend is low profile tyres on road cars mean the tyres virtually don't move so you need softer suspension. On top of that you need dampers that match your springs or its just not going to work. Typically harder springs softer damping and vice versa to stop it acting like a pogo stick on soft springs. Sadly it isn't as simple as that though.

What you have done Gav is completely changed the geometry of your suspension. Again coil overs work but at the end of the day all you have to do is work out what values you now need for what you have and it will work fine so long as like you said you aren't hitting the bump stops.

Work out what's happening with the suspension and take it from there. Is it too hard, too soft, hitting the bump stops? After that you will get a better idea of where to go with what. Too big a role bar isn't good either by the way as a bump on one wheel can lift the other when it shouldn't be causing all sorts of problems.

an over simplistic view of it is
pre load controls ride height
spring rate contols how far the wheel moves up when it hits a bump
compression damping controls how fast the wheel comes to a stop when disturbed by the bump and
Rebound damping controls how fast the wheel returns to ride height
The reality is that all four interact with each other.
An alternative university maths version of it looks at it as a "critically damped harmonic oscilating system which does not help much more but is does recognise you are trying to make the wheel return to its ride height position in half a cycle from being disturbed so there is a "correct" damping rate for a given mass of spring rate/preload/ wheel mass.
The other thing that the tyres are a critical part of the suspension system, a stiffer side wall on the tyre (or a shorter one) will change completly the best settings for the spring rate/ damper set up (probably not ride height but may tune to a different preload so need a different length spring.
Mike

Gav, Where do your problems seem to come from? front or rear,, are you using a A frame, what front poundage are you using,

I should also say it cant be too hard getting the info you need with it being a Capri. Try giving Tony's M8 a phone Capri sport??

my set up is:
175lb less 2 " front coil spring with 2.8i bilstien struts,
2.8i anti roll bars frotn and rear kitted out with poly bushes,
rear end heavy duty multi leafs with 2" lowering kit 2.8i bilstien shoxs and Axle locator kit (A frame)

your set up is same as mien Gav and i have no probs at all.

Smple harmonic movement here :- http://www.boreme.com/boreme/funny-2006 ... ing-p1.php

It does sound like the suspension is being overdampened causing the twitchyness in the handling, have u tried lowering tyre pressures by 2 psi at a time? it's critical on a good handling car, i found major issues with my set up in a "hot rod" rears were too hard and shocks were set up to high, it just bounced everywhere, a no cost adjustments put it right, john