Having now read numerous books on the subject, made "string computers" and virtual geometry, I was getting fairly confident that I was
really making some progress on suspension, to the point that I'm happy the wishbones and associated parts I've designed for my scratch build
aren't going to throw the car through a hedge on it's first corner taken above 20mph.
One thing I'm still not truly "getting" though, is ride comfort. Judging cars against each other for handling performance versus ride
comfort, there really seems little correlation between the standard accepted variables - so what am I missing? I've been in cars with rubber band
tyres and 6 inch long springs with a beautifully smooth ride but little roll and excellent grip, then others with high profile tyres and nice long
springs that is harsh as hell with any potholes but feels like a blancmange through the twisties?! How do you get a harsh ride with such soft
springing, and how do you get such a smooth ride with such hard springing?!
I have made provision in the design for Citroen suspension to be interchangeable with coilovers so I can get it dialed in on a fairly standard setup
before going on to fiddle with hydraulics - but obviously I'm going to need to get the coilovers basically correct first!
Elastic band tyres will make your ride harder as the tyres act as part of the springs but handling will improve in corners. Bigger profile tyres
actual bend sideways in corners decreasing grip.
Your spring rates effect body roll and should be strong enough to carry the car loaded.
Shocks affect the amount of time that a tyre is in contact with the road after a bump. Finding the optimun shock setting is made easier by adjustable
shocks.
Build your car first with an approx spring rate and soft shock settings.
You will find that after driving your car (no matter how many calcs you make) I guarantee you will be adjusting your shocks and changing your
springs.
I am fitting air suspension from arnott industries. They are inflated with air to change my ride height.
[Edited on 14/3/2008 by nitram38]
quote:
Originally posted by nitram38
Elastic band tyres will make your ride harder as the tyres act as part of the springs but handling will improve in corners. Bigger profile tyres actual bend sideways in corners decreasing grip.
Your spring rates effect body roll and should be strong enough to carry the car loaded.
Shocks affect the amount of time that a tyre is in contact with the road after a bump. Finding the optimun shock setting is made easier by adjustable shocks.
Build your car first with an approx spring rate and soft shock settings.
You will find that after driving your car (no matter how many calcs you make) I guarantee you will be adjusting your shocks and changing your springs.
I am fitting air suspension from arnott industries. They are inflated with air to change my ride height.
[Edited on 14/3/2008 by nitram38]
It's all about the shocks on the car and their hydraulic characteristics. Most car shocks are v poor as far as this is concerned as the shock
designer will try and create a damping curve to cope with a worst case scenario.
In everyday use the shock is simply much too hard on low speed compression damping. Equally there are loads of shocks around where the damping
characteristics are just way out. High pressure gas monotube shocks also have a tendency towards jarring as the force necessary to start the shock
closing is too high due to the excessive force needed to open the shim stack which controls the damping.
Choosing the right type of shock is important to start with, then the damping rates and right at the end, the spring rates.
Edited to add: The length of a spring has got absolutely nothing to do with it's rate or performance. If anything, the shorter the better. Weighs
and costs less!
[Edited on 14/3/08 by MikeCapon]
quote:
Originally posted by MikeCapon
It's all about the shocks on the car and their hydraulic characteristics. Most car shocks are v poor as far as this is concerned as the shock designer will try and create a damping curve to cope with a worst case scenario.
In everyday use the shock is simply much too hard on low speed compression damping. Equally there are loads of shocks around where the damping characteristics are just way out. High pressure gas monotube shocks also have a tendency towards jarring as the force necessary to start the shock closing is too high due to the excessive force needed to open the shim stack which controls the damping.
Choosing the right type of shock is important to start with, then the damping rates and right at the end, the spring rates.
Edited to add: The length of a spring has got absolutely nothing to do with it's rate or performance. If anything, the shorter the better. Weighs and costs less!
[Edited on 14/3/08 by MikeCapon]
Over the last 14 years I've made or been responsible for making around 50000 shocks, mainly for bikes but I've done a few cars too. Mostly
competition cars but a lot of road bikes and ATVs.
Comfort, or the lack of it through jarring is one of the biggest complaints on OE suspension. Mainly on recent BMWs believe it or not! All these
jarring type problems where the discomfort is at it's most obvious over small road irregularities like surface changes and manhole covers can be
eliminated by a change of shock. The BMs all use Showa which is fundamentally good kit but suffers a little from the "worst case scenario"
problem and a lot from the high load necessary to start the compression stroke of the shock. A change to another type of shock will generally
eliminate this problem completely.
Lets not forget that in normal cars they have complicated bushes to for your ride and NVH performance.
quote:
Originally posted by chriscook
Lets not forget that in normal cars they have complicated bushes to for your ride and NVH performance.
Many thanks for your comments Mike, they have been noted! Strange you should mention BMW - my brother has just bought a brand new 5 series Estate and
I have to say it's got one of the worst rides I've ever experienced in a luxury car! It is the epitome of everything I DONT want to
replicate!
What would you recommend as a good make of shock in this respect? I'm presuming I'll have to have something special knocked up anyway as
it's a scratch build.
The BMs I was talking about were the bikes but I imagine that the same bean counters choose the shocks for both. For what is perceived and priced to
be a luxury machine they really are appalling.
For your build it depends on a number of factors. The primary one is the type of surface, on or off road? What type of shock installation? If there is
a linkage is it progressive or linear? What is the wheel movement and the equivalent shock movement? Weights per wheel would also be useful.
If I don't get back straight away don't worry. SWMBO's just got back from work and I've got some stuff to sort out.
Cheers,
Mike
It's for the car in my Avetar - so road going, coil over inboard mount with rocker 100mm total travel on the wheel, but rocker and spring length were left variable pending the outcome of this thread! not sure on wheel weight - will see if I can find out.
How much is it going to weigh? More or less, a front / rear split would be useful. Sorry, that's really what I meant when I said wheel weight, ie
weight carried by each wheel. I've spent so long over here that there are times when I struggle to speak English
LOL doh, sorry should have spotted that - lack of sleep is doing my brain no favours!
Just changed a few bits around so weight is currently estimated at just less than 1000kg with a 45/55 split.
These are just my own thoughts on spring and shock choice. You need to make your own choices at the end. I take no responsibility for any of this. I
don’t like saying that but then these days it’s all “actual product may differ” “may contain nuts” etc…..
There you go an error. Edited to pull out unsprung weight. Assume 25kg per corner.
OK so we've got 1000kgs split 450 front and 550 rear. Is that including the driver? Ideally the base weights should include the driver. If not
just add his weight and redo the maths.
Front
Weight on wheel 205kgs
Assume ride height of 30% of wheel movement – This leaves 70% bump movement
Assume for the moment a rocker ratio of 2:1
Assume for the moment a spring preload of zero. The spring is just held between the spring seats with the shock fully open.
So, shock stroke = 50mm
We place the car on its wheels. The wheel moves upwards by 30mm with a load of 205kg. This gives us a wheelrate of 6.8kg/mm.
At the same time the opposite end of the rocker moves 15mm with a force of 410kg. This gives us a shockrate of 27.3 kg/mm.
So to obtain equilibrium, with zero preload we would need a spring rate of 27.3 kg/mm or 1525lbs/in!!!!!
Fortunately we are going to preload the spring. Assume a preload of 20mm. This is a reasonable figure that will enable the preload to be altered later
in both directions.
The force stays the same at 410kg but the total spring compression (preload plus shock movement) is now 35mm. So the front spring rate needs to be
11.7kg/mm or 654lbs/in.
This is a pretty high spring rate. For good performance and reliability I would prefer to see less. The higher the spring rate, the more damping is
needed to control the spring. Higher damping equals higher internal pressures and less reliability. At the same time, the shock will be moving less
oil which tends to give poorer shock response and greater heat build up.
Let’s try a 1.5:1 rocker.
Shock stroke = 67mm
Wheelrate stays the same at 6.8kg/mm
Shock movement is now 20mm and the force is 307.5kg. To obtain equilibrium with this rocker at zero preload would need a spring rated at 15.4kg/mm.
Now add the preload of 20mm. 307.5kgf divided by 40mm = 7.7kg/mm or 430lbs/in.
This is a bit more sensible. At the rear using the same ratio:
Weight on wheel 250kg
Wheel rate 8.3kg/mm
Shock stroke = 67mm
Shock movement 20mm. Force seen by shock is 375kg. Equilibrium at zero preload we would need a 18.8kg/mm spring
Add in the 20mm preload and the spring we need is 9.4kg/mm or 525lbs/in.
These spring rates are just starting figures. They should however be a good basis to start testing for real. Even though we do not have telemetry we
can find a little information on what is actually happening very simply. Locost telemetry is no more than a tie wrap fixed tightly around the damper
rod. Stopping and checking if the tie wrap is buried in the bumpstop or maybe is miles away from it will guide us toward a good spring rate.
The bumpstop should be around 30% of the total shock travel. Ideally we should only hit the bumpstop in an extreme situation, kerbs, sleeping
policemen etc. The best bumpstop material is cellular polyurethane, usually a beige colour.
Back to the question of shock choice. With these spring rates, wheel movements etc. a adjustable twin tube damper should be ideal. Given my absence
from the UK I’m not completely up to speed on who’s making what these days. By chance I had my hands on a Gaz all steel unit of this type the other
day and it did perform very well on the shock dyno. The only weak point for me was the fragility of the adjuster mechanism. This seems to be very
similar to the Spax adjuster which was pretty much the original twin tube car unit. Looking on their website the other day they don’t seem to make
these any more. For those old enough to remember these were the yellow shocks with a Coke bottle shaped body. Looking at the protech dampers these
have all the outward signs of being twin tubes but I don’t know for sure. Have a word with someone who sells shocks for a living. If he can’t answer
your question(s), shop elsewhere.
As you are free in terms of your design, you don’t absolutely need to have your shocks custom made. Once you have chosen a make, talk to them and see
if there is a unit that has sufficient stroke and damping values likely to suit your spring rates. From there you can design around the open length of
the shock in question. If the shock has too much travel you can easily reduce this with a large OD spacer around the rod. That way you are using a
“stock” shocker which should be cheaper and more easily available.
Make sure that your two shock mounting bolts remain parallel throughout their movement. This will enable you to specify poly bush mountings which are
available in different hardness to find the best compromise. Rose jointed shocks are great for racing. Very precise but noisy as hell and they wear
out quickly.
You will almost certainly need to buy extra springs to fine tune. The length of the spring has got nothing to do with its performance. What matters is
its rate. The only other thing to be aware of is the danger of going coil bound. The spring supplier should be able to tell you the movement available
in the spring. That should exceed the shock stroke plus the preload. Try and keep another 5mm or so of safety margin. When a spring goes coil bound it
breaks almost everything around it. Not good. If you have a spring you need to know the movement for just measure all the gaps between coils with a
vernier and add them up. That’s pretty accurate.
Hope this helps. Come back to me if I can help.
Cheers,
Mike
[Edited on 16/3/08 by MikeCapon]
Many thanks for that Mike, most informative. One thing it has highlighted in particular is the problem with a 2:1 rocker ratio - which I hadn't
considered. (can you guess what my initial ratio was...!)
I'll redress that and speak to some suppliers 
Thanks again.