rear droop/camber set up

[Don]

sometime a little knowledge is.................

For an intial rear camber set up (6 deg neg on the ground with driver, and max 1 deg neg off ground at full droop per SR Racing book). My car has zero roll with outside coil over shocks and droop cables on each axle, and an adjustable cross rod between the pivot uprights (old style zero roll before mono shock set ups). I have 3 ways to set the camber; springs, cross bar and cables. I have the car set up for 6 deg neg static with driver and correct ride height and now need to set the droop. I can adjust the cross bar for 1 deg neg at droop with shocks fully extended and then readjust springs on the ground to get my 6 back, resulting in a harder spring setting, but then the droop cable does nothing, or shorten the droop cables and then the springs bottom out on the cable and not the shock at droop. My question is is it better to start out with a softer spring setting or harder spring setting at the rear for an initial set up?

thanks, don

[FV80]

Don,
Regardless of HOW you limit the droop - you really need to have a S O F T droop. Reaching the end of a cable or shock is about the same if it's a HARD stop - bad!. I'd suggest you use the cable for a limit since you can add a rubber bushing or something in the cable 'path' to soften it. It's usually quite tough to control what happens inside a shock.

FWIW - I'd put the static (driver in car) and max droop closer together. IMHO, 6 degrees of static is too high - I'd suggest starting with about 1/2 that, but your suspension seems to be quite different from mine (new style zero role with single shock) so YMMV. (I am talking COMBINED camber here ... ~ 3 degrees per side = 6 total -> recommend about 1.5 - 2 degrees per side = 3 - 4 total [but you might get additional suggestions from some with more similar rear setups])

Good luck!
Steve, FV80

[FV80]

Hmmmm - follow up. On reread - if you have TWO INDEPENDENT droop cables and not a single cable between both sides, I withdraw my suggestions and have NO IDEA how that should be set up . Maybe someone else can help.

Steve, FV80

[brian]

Post a picture, the rod between the shocks may be position sensitive. Id set the camber with the two springs, then the droop with the cables after you add some rubber dampening.

[hardingfv32-1]

1) If we assume that the shocks can fully extend and the droop rod adjusted to control the droop setting, it would seem the droop cables on each shock are for pre-loading the springs. Pre loading the rear spring(s) is not normal practice and would seem to limit rear suspension movement or compliance. I think!

2) I would say current tires are using much lower total camber numbers 4.5 - 5 deg.

Brian

[hardingfv32-1]

Steve

What is the purpose of the soft droop limiter stop? Why can't the shocks rebound valving do the job of controlling the stop's "landing"? You only have the the rear spring in rebound and the jacking force to control, neither has much velocity approaching the droop stop.

Brian

[Mystique Racing]

Pre loading the rear spring(s) is not normal practice and would seem to limit rear suspension movement or compliance.

I would say that preloading the rear spring on a zero roll FV is quite normal, In fact It takes about 3" of preload on a 200LB spring in my car to maintain normal ride height/camber settings. I know the motion ratio will affect this and is different for different cars, but I don't believe I have ever seen a FV without preload in the rear spring. I would have to run a 600lb rear spring to avoid any preload.

[hardingfv32-1]

You could be correct. A low spring rate and/or long spring could require you to pre-load the spring. I guess the droop limiting system could also cause this pre-load. I can say my car has a 400 lb. spring and very little pre-load.

My we assume that the pre-loaded springs are not detrimental to tire compliance?

Brian

[fvracer27]

Pre loading the rear spring(s) is not normal practice and would seem to limit rear suspension movement or compliance.

I would say that preloading the rear spring on a zero roll FV is quite normal, In fact It takes about 3" of preload on a 200LB spring in my car to maintain normal ride height/camber settings. I know the motion ratio will affect this and is different for different cars, but I don't believe I have ever seen a FV without preload in the rear spring. I would have to run a 600lb rear spring to avoid any preload.

If you have a straight rate spring and preload it 3" all you are doing is making the spring compressed you are not increasing the rate with preloading it. 200lbs spring takes 200lbs to compress 1" once it's compressed the 3" it only takes another 200lbs to make it work. Only problem will be if it's compressed and your getting coil bind which is bad.

Mark

[FV80]

Steve

What is the purpose of the soft droop limiter stop? Why can't the shocks rebound valving do the job of controlling the stop's "landing"? You only have the the rear spring in rebound and the jacking force to control, neither has much velocity approaching the droop stop.

Brian

I guess, if the rebound velocity was PERFECTLY IDEAL for all conditions, then it would not be needed -- however, I doubt that is possible. I could certainly be WRONG (I hear that's POSSIBLE for some people <G>), but I remain concerned about the 'snatch' aspect of hitting a hard stop in EITHER direction under ANY conditions. I just don't think you should allow it to happen.

Did I say.... "I could be wrong" ?? <G>.
Steve, FV80

[problemchild]

Pre loading the rear spring(s) is not normal practice and would seem to limit rear suspension movement or compliance.

I would say that preloading the rear spring on a zero roll FV is quite normal, In fact It takes about 3" of preload on a 200LB spring in my car to maintain normal ride height/camber settings. I know the motion ratio will affect this and is different for different cars, but I don't believe I have ever seen a FV without preload in the rear spring. I would have to run a 600lb rear spring to avoid any preload.

Somebody better define preload. I normally agree with Scott but I don't know what he is talking about. Most FVs I see do not have preload. I actually run just a touch but most FVs have play in their droop control when at static ride height ... which to me is "no preload" ...... just the spring at its natural length for the applied load. If you then tighten the droop limiter so the spring compresses .... that would be preload. I know some FF folks who have a different interpretation for the term preload (the change in the distance between static ride height and when actual preload starts) .

[fvracer27]

I agree with Greg
At least that is how I'm looking at things.

Mark

[Don]

Tried to upload pic, but can not form some reason, receive message "Could not upload attachment to./files/595............). Can I email it to someone to load? The pic shows the car is off the floor on a dolly and the rear is at full droop with the cables tight and each spring is compressed about 1/2" to 3/4 ". Total camber at droop is 0 deg. On the ground with driver the car settles out with about 6 deg total camber (3 each side).

don

[problemchild]

Don, It sounds like on a D-13 which has the rear springs preloaded with stillasto rubbers. The spring/stillasto tug-o-war negates much of the shock absorber function to the degree that you can run a variety of/or mismatched shocks with no real change in performance.
Your car may very well be an exception to common FVs as run today. If it works OK, then perhaps tweek what you have. If it works poorly, then you should change to a more conventional setup.

[Mystique Racing]

In my mind when you take a 10" spring that has a rate of 200lbs/inch and compress it 3" to achieve a force of 600lbs, which is about what is needed to hold up the back of a FV I would call that preload.

I also understand that you can tighten down the same spring further using the threaded collar and then hold it with the droop limiter and some would call that preload.

Or you could have a 600lb spring that would not compress at all to equalize the 600 force applied to it by the weight of the car. That in my mind would be an example of no preload.

I understand that the spring rate is different.

[jpetillo]

There are more definitions of preload from different camps than anything else. I think the only folks who define preload as being when we're up against the droop limiter at static ride height is FV. That would suggest that this is not the standard definition, but may be in the FV crowd. In the motorcycle world adjusting preload simply changes ride height, nothing more. I don't think that defining preload will help Don, though. That picture he has would be good to see. Sorry, I added nothing to this thread.

[cendiv37]

Not to pick nits (too much), but a "600lb spring" is actually a 600 lb *per inch* spring so with a 600 pound load applied it will compress 1 inch.

As far as "preload", I can see how the rear of an FV could get confusing. If you are talking about the spring preload, it could be either the spring force/load at full droop (my choice) or the force/load at static ride height (Scott and Greg's choice).

In many mechanisms with a compression spring, the spring is partially compressed before the "working load" is applied. This is the preload. In the case of a FV, I would say that the setting the car on the ground is applying the working load.

If you are setting your droop limiter to be partially engaged at static ride height, I would say you are preloading your droop limiter. To be sure, the spring and droop limiter are working against each other so this also adds more preload to the spring. That makes it a bit more confusing...

[JimN]

I am going to give this a shot. Being fairly new to zero roll, tell me if this makes sense. I am racing a FST.

If I set things up with no preload (spring perch just tight up against the spring) with a 500 lb spring, set the car on the ground, the spring and shock compress to the point where I have very little shock travel left. So, I crank the spring perch in about an inch (500 lb of preload) set the car back down and the shock and spring compress only a little, leaving a reasonable amount of travel to race with. Granted, I have not changed the spring rate but I have changed how the spring reacts to an applied load. The weight of the car resting on the spring has “overcome” the preload force without really compressing it any significant amount. Now I have shock travel and, when I force the rear down, it reacts the way I think it should with a 500lb spring in it.

I don’t think I am making any great point here, just simply trying to confirm my understanding of things. Does this sound right?

Jim (Can’t reply to responses until tonight.)

[Mystique Racing]

Bruce, thanks for that clarification, You are correct that a 600lb spring would need to be compressed 1" before it will reach 600lbs of force. Jim, I think you got it.

I guess my fundamental question would be if I want to make the rear of my car stiffer should I increasing the preload of my 200lb spring, or should I go with a higher rate spring and preload it less?

I talked to several people at the PRI show and nobody had a real answer to this question. I guess this is why we have test days.

[SR Racing]

Gee, I understood what all of you meant with "pre-load" <g>

In regards to the OP, the book "Getting Started in FV" was originally written 10+ years ago. We try to update it with every print run (4 so far), but we sometimes miss a few things. Back at the original printing typical rear camber (total R+L) was almost always set at ~6 degrees. Some people ran as much as 8 degrees. Still with driver in it in a warm suspension 6+ is not unusual. However, today most people are closer to 4 deg total. (Depending upon suspension and how well your front end works will vary this.) I have heard of some people running almost straight up.
The book was to get you started ("Getting Started...") and this was certainly a safe set-up, but your car and driving style will dictate.

BTW... I just checked. We have sold over 700 copies of the book to date. When are all of those buyers going to be on track? <g>

Jim

[JimN]

Scott,
I would think you would need a stiffer spring. If you add preload, it will be stiffer initially but once the force overcomes the preload (during which the shock would not move much) you are right back to the spring rate you started with. Maybe this could be described as an abrupt falling rate. It does not sound good to me.

Preload was once explained to me by an engineer (not in racing) by saying that the force created by cranking up the perch on the spring was, in theory, stretching the shock rod. When a force is applied, the shock rod is compressed until the force required to compress the spring is less than the force required to compress the rod. At that point the spring compresses, not the rod. By adding preload, the shock rod becomes one danm stiff spring. Whether that is absolutely correct or not, it didn’t matter a lot to me. I bought it. It helped me apply the idea of preload anyway.

Jim (figured out how to log on from work)

[SR Racing]

I guess my fundamental question would be if I want to make the rear of my car stiffer should I increasing the preload of my 200lb spring, or should I go with a higher rate spring and preload it less?

?? If I understand your question... No matter where you have it set statically, it will take an increase of 200lbs to move the spring 1 inch. With a 300lb spring it will take 300lbs to compress the same amount.
(This assumes a linear spring (which most all are) in it's typical range of operation and about a 1/2" of preload. (There is the preload thing again <g>) On our spring tester you always have to preload about 1/2" and then measure the lbs per inch after that point. For what ever reason the first 1/2 inch is not linear on a typical 6 to 10" spring.

[problemchild]

Bruce, thanks for that clarification, You are correct that a 600lb spring would need to be compressed 1" before it will reach 600lbs of force. Jim, I think you got it.

I guess my fundamental question would be if I want to make the rear of my car stiffer should I increasing the preload of my 200lb spring, or should I go with a higher rate spring and preload it less?

I think your "preload concept" is complicated on a FV because the spring rate is so soft relative to the stiction and shock valving ..... the spring is deforming in your "preload" condition rather than changing the ride height. This is much more noticeable on cars where the bell cranks go over-center and a 2 1/4" spring is used. On my car, which is very rising-rate in configuration, if I back off the shock settings, changing the spring perch location changes the ride-height. That is not the case on many FVs. Even on my car, it is not consistant enough to measure .... and I forget to reset the shocks most often .... which is why I don't even try, but change dimensions of components.

I think your "preload" condition is just the spring deforming and applying forces in directions that are not changing ride height. Anyone who has ever tried to put a long soft spring onto a shock can tell you that there are forces going all different directions.

If you want to change the stiffness .... you must change the spring, assuming all other suspension adjustments remain constant.

[Mystique Racing]

If you want to change the stiffness .... you must change the spring, assuming all other suspension adjustments remain constant.

Greg, I don't think that is true. I guess maybe I need to separate static stiffness and dynamic stiffness.

I can preload my (10" long, 200lb spring) 4" and have 800 lbs of static force, which is 200lbs more force then necessary to hold the back of the car up, but the spring rate will still be 200lbs/inch of spring travel.

Or I can use a 400lb spring and preload it 2" and get the same 800lbs of static force from it.

I guess the real difference between these two situations would be what happens when the rear suspension hits a bump (dynamic) The 400lb spring will only compress half as much as the 200lb spring assuming the same load and motion ratio. Of course all of this assumes a preload condition with a spring limiting device (droop limiter)

OK, I have a headache now.

[fvracer27]

Man I have a headache now too. I think I'm more confussed If funny because I have been working on this with my car for the past few months and when I thought I have it figured out this thread popped up.

On my car D13 zero roll I have Carrera shocks on the rear with 12" free lenght springs. They get compressed 2" with the threaded collar and the the limiters get crank down to compress the springs another 2" for a total of 4" and I have been trying to figure out why? This is how the car was set when I got the car and with other suspension systems I have set up on cars I can not see a need to compress or (preload) the spring with the collar. I can see the limiter being used so the shock will not extend fully. Headache just got worse

Mark