Road Vehicle Dynamics - spring rates and dampening

Changing the ride height gives different geometry.. normally inferior.

the cut springs will be stiffer.

If you don't use stiffer springs then you will be at risk of running out of ground clearance. Even if you don't ground out you will have poor camber angles with independent suspension systems.

Higher rate springs require stronger damping to work properly.

I regard the lowering of the C of G as a helpful extra to reworking a suspension system with stiffer ( and thus shorter ) springs to reduce body roll when cornering. The stiffer suspension works better on smooth tarmac, but may be inferior on rougher roads. ( when did you last see a drainhole cover on a race track ? :-) )

If you are making a large change to the ride height, the suspension geometry needs attention.

"Steve" wrote in news:YXUOd.4988$ snipped-for-privacy@news20.bellglobal.com:

Lowering the CG height is beneficial. Cutting the springs will increase their rate. This will reduce your grip on rough surfaces, but will also restrict the travel of the suspension, so if you have wide tires you may be better off.

If it was it would but it isn't so it isn't.

and the dampening would be the same

as above

yes, usually.

Not really. You'll get faster load transfer into corners, so you'll get more understeer going in. The reduced compliance will give you more problems with surface irregularities. To offset that, with less suspension travel your tires might be held square to the road for longer.

Shrug. Anyone can make predictions. I predict that if you cut the springs George Bush will not be the next President of the USA.

Grin

Thanks ...

"-If the 'cut' spring rate is the same as the original spring - ride

There is a set of lowering springs available with a rate that is almost the same as stock? the way to go? considering I run a bumpy track and solo in the parking lot on street tires (215/45/17) and a camber plate kit to take care of the alignment change.

"You'll get faster load transfer into corners, so you'll get

If the rate was the same do I still get faster load transfer? If so run a little more spring in the back to neutral it out?

thks

"Steve" wrote in news:lHcPd.33756$ snipped-for-privacy@news20.bellglobal.com:

Yes, the ride on smoothish roads would then be the same. This makes the (fairly safe in the USA) assumption that you are not running constant contact jounce bumpers.

No, that won't change, in a simple model. You might even get slightly slower load transfer for two reasons - (a) less roll due to CG height and (b) your roll centre heights will have dropped.

Nonetheless your handling should improve, dropping the CG height is (I think always) a good thing.

Cheers

Greg

Thanks Greg...

any suggested reading to gain knowledge for basic understanding Vehicle Dynamics?

"Steve" wrote in news:24wPd.35513$ snipped-for-privacy@news20.bellglobal.com:

Milliken and Milliken Race Car Vehicle Dynamics has the basics covered and lots of other interesting bits and pieces as well. It is a bit expensive (~$100) but you'll easily spend that on your first modification, and you'll end up buying it sooner or later.

If that is too much (or too big) then the Carroll Smith or Herb Adams books usually get a good rap. I haven't read them.

On the net the following is a classic

and the following is quite a pretty website but I seem to remember it has errors and a strange set of priorities

Cheers

Greg

Cheers

Greg

Greg -- thanks for the "plug" for our book, always appreciated.

I came in late, nice to see that the original poster has been getting sensible advice for a low budget project. One small point is that quite a few cars over here (USA) now have long bumpstops that are engaged at (or very near) static ride height. It's often all about the stylists not leaving enough room for real bump travel...worth checking all the clearances (including with full lock steering) when planning to lower any car.

Note that ordering our books is normally quickest direct from the publisher . Many other sources (amazon.com, etc) do not carry stock and must then place the order with SAE anyway.

-- Doug Milliken

"Steve" wrote in

Doug Milliken wrote in news: snipped-for-privacy@bfn.org:

Yes, constant contact jounce bumpers. The nice thing with them is they are very non linear and easy to tune - just stick them in the lathe and turn a bit more off them. The big trick is the significant non linearity as they just come into contact - usually dealt with by forming a very soft lip that buckles as it engages.

The nasty thing, as you allude to, is that being non linear they really need a height sensitive shock absorber to go with them.

But overall, used sensibly they are more of a blessing than a curse, for road cars.

Cheers

Greg

Ok this is what I was afraid of - I knew going in that it wouldn't be as simple as just putting in a camber kit and swapping springs.. how would one recognize the bumpers and how to understand the effect.

And armed with what Greg has mentioned (re: the stock rate spring in a shorter height) doing more research I find that most but one company offers either variable rate or linear rate springs with a higher rate. Wondering out loud if a variable rate would be a good alternative if I am unable to find the 1st choice, but my concern is as the higher rate comes in the dampening wouldn't be there. I also am considering the fact that the factory has already done a fair bit of work (SVT Focus) on the handling and any steps that I would take would be a step backwards... comments

Thanks

S

"Steve" wrote in news:cF0Qd.1761$ snipped-for-privacy@news20.bellglobal.com:

You are messing with a Focus? Is there a more extreme suspension tune available from the factory? If so why not use that?

Failing that, you need to decide what it is you are trying to achieve. Dropping the ride height is a sensible move if you are prepared to accept a different compromise between ground clearance, suspension travel, and handling.

I'm not going to tell you if Focus has cc jounce bumpers, look at the shocks, if there is a foam cylinder around the shock rod that is almost in contact with the top of the shock tube, that's a cc jounce bumper. Don't mess with it. They are more likely to be on the rear, but there is no reason not to use them at the front.

They already give you the effect of a variable rate spring, so you might get away with constant rate springs.

AFAIK there is no magic formula to decide whether you'll need stiffer shocks, but you probably will, and your supplier will tell you that you do. I'd fit the shorter springs and look around for advice on shocks as a separate exercise. Your big problem is that you don't know where you are starting from, in terms of effective spring rates and shock characteristics, so deciding where to go is a bit tricky.

Cheers

Greg

Already has the factory extreme tune on it ...

thanks for the words..

S