gkmccready

Okay, there're only four things that I can think of that will stop a shock in compression:

1. Piston bottoms inside shock body (ie. smash the piston internally)
2. Shock body bottoms on bump rubber (before #1 occurs)
3. Wheel/tire hits body work
4. Chassis hits ground

Add one for coilovers:

5. Bottom the spring.

For a street car nothing but option #2 or #5 seem reasonable. LG (& DRM?) run without bump stops and I'm not sure which method they choose. Pfadt and HardbarUSA appear to use bump stops.

Pfadt lowers the car by physically shortening the overall shock length. But if you do this which of the above stop the shock in compression?
You've effectively changed where the wheel starts relative to the body work, left the shock travel alone, but now you can compress the wheel further upwards, effectively... does this mean you sacrifice shock travel you could have had if you ran higher?

With the others where you just remove preload on the spring, removing preload means at static ride height you have less bump travel. You're more likely to exhaust bump travel and hit the bump stops (or whatever is the bump travel limiter).

Other than Pfadt, you increase ride height by adding preload to the spring. That's all fine and dandy -- you sacrifice some droop travel, gain some bump travel, and hopefully everything is happy.

Short version is, if you and the coilover vendor like different ride heights, and you like to run higher it's pretty easy to get into #5 above and not use the bump rubbers... or if you like to run lower then you can pretty easily smash the bump rubbers (or body work!?) to bits.

Randy mentioned that running a lower ride height to keep from smashing the bump stops all the time you would want a shorter shock. Randy, if you're reading this, my impression is that if you shorten the shock body you had better also shorten the stroke of the shock or you'll start stuffing the wheel higher up and either experiencing #1, #3 or #4...

I'd love to hear some vendors say what ride height they built their packages for, or if you deviate from those ride heights how far you can go before you put yourself in trouble with either droop travel, or bump travel.

Obviously I focused on coilovers above. Running leafs has all the same troubles when you change ride height. Outside of all the talk above, if your minimum overall shock length stays the same (ie. at full bump travel) no matter what ride height you run you'll stick the wheel just that far into the bodywork at max compression. You still lose bump or rebound travel if you lower or raise the car...

Again, from my POV shocks/coilovers a designed for a given ride height to work best. There's probably an acceptable range around that ride height where they work, but they're not optimal... after that you have to be careful about minimum overall length of the package if you want to keep the wheel/tire from hitting body work.

Reality? Crazy talk? Why don't vendors tell us what ride height they built their package for? What's the acceptable ride height range?

Aaron Pfadt

Glen,

Well written post. The point of travel is often lost in the discussion of shocks and coilovers.

Obviously (or maybe not), you would like the wheel to be able to travel as far as possible without having a radical change in the force level (i.e. bottoming out in one of the methods you mention above).

The reality of the Corvette is that there is very little packaging room for a shock relative to the travel required. I would guess that all of the shocks mentioned above will bottom out the mount on the outside of the body (or bump stop) before the piston crashes into the bottom. That is because we all protect for a certain gas volume and that gives some spare room in the body.

When you try to make a shorter shock to allow the wheel to travel to the limit of the bodywork you have to shorten the shaft also so that it does not cause item 1 above. The effect of this is that you limit the droop travel. If the car is being run low, this can be OK. If the droop travel is sufficient to allow the car to roll/dive/squat without topping out the drooping shock, the suspension will work. If you are trying to run a car on short shocks at stock ride height for example, you may be at nearly full droop at static ride height. As the car rolls in a corner, the inside shock can top out causing a non-linearity in wheel loading. That is bad for vehicle dynamics.

How we have handled this whole situation at Pfadt is to make our body lengths adjustable. If you run the car at stock ride height, you are in the middle of the travel range of the damper. If you lower the car, you shorten the damper body while still keeping the shaft working in the middle of it's stroke. That allows the shock to not bottom out if run low and not top out if run at stock ride height.

We think that this is an important feature and we are designing our new Featherlights to have this also.

At Pfadt an acceptable ride height range for our coilover is stock height to about 26" at the fender arches (that holds pretty close for C5 and C6). Anything below that and you start to lose bump travel when you hit our bump stops. They can be cut shorter if you have clearanced your inner fenders and wish to allow the wheels to travel further.

Our shocks are similar but they will hit the bump stops about the time your factory wheels with start to touch the inner fenders. I have had a couple of people want to stretch the travel and cut the bump stops on them. This will not damage the internals of the shocks, but it may allow your tire to hit the inner fender.

-Aaron

gkmccready

I noticed another shock thread this morning that is heading directly for the content of this thread. Shock length vs ride height...

autoxer6

Does anyone know how much wheel travel can be gained with a shortened shock in a C5 Z06 with stock shocks or Koni 3013's?

0Randy@DRM

Glen,
I would prefer to leave somethings to ourselves. The stock will never bottom on it's self. The tire can hit the fender and slow the shock tower impact, rubber bump stops don't offer much for spring rate, coil springs are raising rate.

Here is a short video about our setups.




Randy
www.dougrippie.com

gkmccready

Completely understand, Randy.

BTW, Penske publishes their bump rubber rates on their website:
http://www.penskeshocks.com/index.html select "Manuals & Tech Sheets" and they're there. The heaviest one is >400lbs/in...

ghoffman

Randy, coil springs are rising rate? Please explain. So are leafs if a coil is. Our dual rate system is rising rate, but most single springs are not! And some vendors put bump rubbers on for cosmetics, but the Penske 400 pound/inch bump rubbers are certainly not for cosmetics.

0Randy@DRM

I'm sorry Gary, I used the wrong word. They are pretty linear but increase as the suspension goes into bump.

Randy

froggy47

mousecatcher

You also affect the handling. Well, on my 1000# formula cars you certainly do and that's why (in part) the preload on those cars is separate from ride height. On a 3000# car I don't know what the affect would be (actually this is interesting as I have recently been trying to grok the affect of preload) but I imagine it is noticeable.

0Randy@DRM

The setup on any car is important. Making small changes in ride height don't give major spring rate changes.

Randy

ghoffman

I think you mean wheel rate changes?

gkmccready

I've been working this through in my head, too. The thing to remember is that a 1000#/in spring is always 1000#/in. If you add 500# (aka 1/2") of preload to the spring and then set a 500# weight on it, the spring doesn't compress at all. If you set 1000# weight on it, it will compress another 1/2in. (Obviously, with no preload a 1000#/in spring with 1000# on it compresses an inch.)

I've stared at this for a while... adding preload seems to remove droop travel and add bump travel. But until you start putting in so much preload that you have >= the corner weight I don't think it changes any of the other behaviours. So if you have the droop travel available, using preload to adjust ride height seems completely reasonable.

How do you affect the handling with preload? You shouldn't really be able to notice unless you're using a tonne of droop travel, or have "way too much" preload?

gkmccready

Refering to the motion ratio and spring angle changing to ultimately affect the wheel rate? I was under the impression that the rear MR changed quite a bit through travel, but the front didn't change nearly as much...

0Randy@DRM

The spring doesn't change much, and the wheel rate would be less.

Randy

mousecatcher

I think you may be right. There's an article in a recent racecar engineering about preload and from what i've been able to get out of it so far they are talking about "lots" of preload. I think both to tune turn-in characteristics and to limit droop.

0Randy@DRM

You can put 5 inches of preload into a spring and still have the exact same amount of droop travel.

How does preload affect handling?? Easy it increases spring rate, the more preload you put into the spring, sence the spring is getting shorter.

Randy

mousecatcher

i have seen the little rubber bumpers some folks use to shorten the spring and fine tune the spring rate. i guess preload is the same thing? until you get to the static corner weight then you have a totally different thing going on.

fatbillybob

To the OP,

Coil overs and lowering seem to go hand in had with stiffer springs. All the coilover makers seem to increase the wheel rate over stock. You pros can tell me if I am right on you increasing wheel rates with your systems. As such the increase wheel rate reduces the relative bump and rebound travel so a short shock with less travel is fine when matched to the right springs. Do I have that right or am I totally lost?

gkmccready

Can you explain this a bit, Randy? At full droop the shock piston is at the end of the body. When you put the corner weight of the car down the spring compresses some amount and the shock piston moves inside the body the amount the spring compresses to handle the corner weight.

Let's say you have an 8" 500# spring and a 1000# corner weight. (And let's throw out the MR and spring angle, etc.) With 0 preload in the spring you set the 1000# load on the 8" spring and it compresses to 6" in length. If I put 500# of preload in to the spring so it starts at 7" in length, when I put the 1000# load on it the spring will only compress to be 6". Right?

Okay, so in the first case from full droop you compressed the spring 2", giving you 2" of droop travel. In the second case, with preload, you compressed the spring only 1" from full droop, so now you have 1" of droop travel.... no?

Hold on now. Preload does *nothing* to change the spring rate. Not a thing. It can't. The spring is the spring. Preload changes the amount of force you initially need to place on the spring to start it moving, that's all. A 1000# per inch spring will always need another 1000# of load to compress it an additional inch.

Spring rate does not increase the more you compress a spring...