rear droop/camber set up

[hardingfv32-1]

Say we assume a rear weight of 600 lb. and then pre-load the spring with an additional 200 lb. beyond what it would take to maintain the proper ride height. When the car hits a bump will it not take more than 200 lb. of force from the bump to cause any movement in the suspension? Would this not be detrimental to tire compliance/grip?

It would seem that the droop limiter should always be loose when at operating ride height.

For Don's original question: With the cross bar at an appropriate length (some trial an error), he would set the droop first and then adjust the coil-overs to get the desired height. He does not want the droop cables taut when the car is at ride height. How does that sound?

Brian

[hardingfv32-1]

Mark

Could you be using springs with the wrong free length? I would say you should do all of your ride height adjustment with the spring collar.

Brian

[problemchild]

It would seem that the droop limiter should always be loose when at operating ride height. Brian

Some FVs like D-13s and Adams Aeros (and Shirley MacLanes and Problemchilds) have non-solid droop limiters that are under load at static ride height.

[Mystique Racing]

It would seem that the droop limiter should always be loose when at operating ride height.

Most modern Formula cars, and possibly other race cars, have preload designed into the suspension. My FA had preload designed into the suspension.

So, I guess the answer to your statement would be, not necessarily.

[hardingfv32-1]

True or false?

Say we assume a rear weight of 600 lb. and then pre-load the spring with an additional 200 lb. beyond what it would take to maintain the proper ride height. When the car hits a bump will it not take more than 200 lb. of force from the bump to cause any movement in the suspension? Would this not be detrimental to tire compliance/grip?

A soft rubber droop stop does cloud the issue a little. I would estimate that the soft rubber stop is only adding a few lb. of pre-load. What is the benefit of this pre-load IF compliance is affected?

Brian

[Mystique Racing]

The benefit of preload would be that you can run a softer spring rate. The preload just keeps the car from dragging on the ground.

How else could you run a single 200lb spring to support the rear of a car that weighs 600lb.

[hardingfv32-1]

ALL cars pre-load their springs when they are at static ride height. A 600 lb. rear car weight provides 600 lb. of pre-load. Your particular coil-over/rocker system may require you to compress your spring to fit on the shock. Most people would not consider this the definition of pre-load.

"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? "

You need expand on your statement.

1) What do you mean by "stiffer". The next 100 lb. of force applied to the rear suspension beyond static ride height:

A) Does the suspension move with greater resistance or
B) Does the suspension not move at all?

2)How are you increasing pre-load beyond what is required for the static ride height? Is it correct to assume that you adjust your ride height with the coil-over collars?

Brian

[Matt King]

For any given vehicle weight and spring rate, changing the installed length of the spring (preload or whatever you want to call it) will only change the static ride height. If you want to change the stiffness of the rear suspension, you need to change the rate of the spring.

[hardingfv32-1]

I think in this case we might be talking about using some kind of shock rebound limiter. We need an explanation of how pre-load is achieve beyond what is normally used to achieve the normal ride height.

Brian

[Matt King]

In compression when the rebound limiter is not functional, stiffness is determined by the spring and shock. Wouldn't the rate in rebound be determined by the bumpstop on the droop control, usually a rubber bumper ? I assume this is why several people have told me that a metal on metal stop isn't a good idea, as the rate become infinite in a hurry when things bind up solid!

[fvracer27]

Mark

Could you be using springs with the wrong free length? I would say you should do all of your ride height adjustment with the spring collar.

Brian

Brian

Yes I believe they are to long and that is what leads me to be thinking about this. But this is the way I purchased the car. I also believe ride height should be done on my D13 with spring collars.

With the current springs I have to compress them a little just to get them on the shock.

I have some testing do

Mark

[hardingfv32-1]

"Wouldn't the rate in rebound be determined by the bumpstop on the droop control, usually a rubber bumper ?"

I think it is more accurate to that the state spring rate for that the portion of rebound in which the bump rubber is engaged is a combination of the rear spring rate and bump rubber rate.

I think the "soft landing" theory for the droop limiter is a myth. Your are in rebound, stopping at the ideal camber angle for the rear suspension. Do people think the rear wheels will be pulled off the ground by the chassis' upward momentum?

The bump rubber is probably just a spring rate modifier/variable rate device that functions when the rear suspension approaches the droop limit. This was cover in an earlier thread. It is not clear what advantage this approach has.

Brian

[Mystique Racing]

You need expand on your statement.

1) What do you mean by "stiffer". The next 100 lb. of force applied to the rear suspension beyond static ride height:

A) Does the suspension move with greater resistance or
B) Does the suspension not move at all?

2)How are you increasing pre-load beyond what is required for the static ride height? Is it correct to assume that you adjust your ride height with the coil-over collars?

1) Sure, that will work
A) All suspensions have greater resistance as they compresses right?
B) Well, even F1 suspension moves a bit. I am just trying to keep my car from bottoming out under compression loads in the rear.
2) By compressing the spring more and using the shock droop limiter, and no to your second question. I adjust the ride height with the drive rods.

Again, my real question, that may not have an answer, is...... is it better to increase the stiffness of my rear suspension by adding more pre-load to my existing 200lb spring or should I use a 300lb spring with less pre-load? anyone have an opinion on this? Bruce, where are you when I need you?

I think the "soft landing" theory for the droop limiter is a myth. Your are in rebound, stopping at the ideal camber angle for the rear suspension. Do people think the rear wheels will be pulled off the ground by the chassis' upward momentum?

Yes, I 100% Agree with you, I think that is a myth also.

[hardingfv32-1]

OK, you are pre-loading a force into your spring using the coil-over collar. Say we need 600 lb. of spring force for the static ride height, but we also screw in an additional 200 lb. of pre-load. That means that the rear spring could actually be maintaining the ride height for a 800 lb. car. Is this correct?

We take our 600 lb. rear suspension (+ 200 lb. pre-load) setup to the track and encounter a bump that generates 100 lb., of vertical thrust. The rear suspension of the car will not move because it needs at least 200 lb. of vertical thrust plus the cars 600 lb. weight to over come the rear spring system that is at a total of 800 lb.

Sounds like tire compliance will be hampered a little. That is why the guys at the PRI show did not have an answer for you, it is not good practice.

This was correctly covered pretty well in an earlier post.

Brian

[jpetillo]

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?

Not having read the rest of this thread yet, I'll make a comment.

Assuming that you're not against the droop limiter, changing preload only changes ride height, and has nothing to do with stiffness. To change the stiffness you have to change the spring rate.

You can change the effective spring rate by running up against the droop limiter - the more you squish the limiter rubber, the stiffer. If you're running up against the droop limiter and then add spring preload, you will squish the droop limiter less and have a lower spring rate and less stiff suspension. John

[jpetillo]

For any given vehicle weight and spring rate, changing the installed length of the spring (preload or whatever you want to call it) will only change the static ride height. If you want to change the stiffness of the rear suspension, you need to change the rate of the spring.

Exactly!

[Mystique Racing]

OK Brian, a couple of thoughts on your statement.

1) I would not necessarily add another whole inch of preload. I may add something less than that amount.

2) Assuming this 600lb mass, or weight, is rolling down the track I would think that a bump that causes a 100lb vertical load, which is only 16% of the total weight of this mass, would be pretty minor and may not need suspension compliance?

You are going beyond my closet engineering abilities, so I am not exactly sure how to interpret or answer your question.

[hardingfv32-1]

Keep this simple at first! Do not concern yourself with the variable rates that the bump rubber brings to the system.

Use a simple straight forward 200 lb. pre-load with a steel to steel bump stop.

John, you should know better.

Brian

[jpetillo]

"Wouldn't the rate in rebound be determined by the bumpstop on the droop control, usually a rubber bumper ?"

I think it is more accurate to that the state spring rate for that the portion of rebound in which the bump rubber is engaged is a combination of the rear spring rate and bump rubber rate.

Yes, I agree.

I think the "soft landing" theory for the droop limiter is a myth. Your are in rebound, stopping at the ideal camber angle for the rear suspension. Do people think the rear wheels will be pulled off the ground by the chassis' upward momentum?

Not sure about this.

The bump rubber is probably just a spring rate modifier/variable rate device that functions when the rear suspension approaches the droop limit. This was cover in an earlier thread. It is not clear what advantage this approach has.

Exactly!

[jpetillo]

Keep this simple at first! Do not concern yourself with the variable rates that the bump rubber brings to the system.

Use a simple straight forward 200 lb. pre-load with a steel to steel bump stop.

John, you should know better.

Brian

Brian, what did I say that was wrong? John

[jpetillo]

Again, my real question, that may not have an answer, is...... is it better to increase the stiffness of my rear suspension by adding more pre-load to my existing 200lb spring or should I use a 300lb spring with less pre-load? anyone have an opinion on this?

Scott, if you add more preload to your existing 200lb spring and you're not on the limiter, you will not change the stiffness one bit - it will just raise the car. If you're on the limiter and increase preload, then you will have less stiffness. So, don't do that. You can do one of two things to increase stiffness (maybe more).

If you're not running on the limiter when you want more stiffness, then put in a stiffer spring.

If you are on the limiter when you want the increased stiffness, then put in a lower rate spring. This will cause you to run into the limiter sooner and be deeper into it for the same condition and you'll have a stiffer effective spring rate. John

[hardingfv32-1]

Nobody should worry about their engineering abilities. Raise questions when something does not make sense. There is probably another way of making the point.

" Assuming this 600 lb. mass, or weight, is rolling down the track I would think that a bump that causes a 100 lb. vertical load, which is only 16% of the total weight of this mass, would be pretty minor and may not need suspension compliance?"

First, we really don't have access to a good tire model. For our discussions we should ASSUME every lb. of force applied to the tire matters. Think of it as a trend. If the force is considered a negative to tire compliance, then it is bad. A discussion over how important the actual force level is difficult and not resolvable on this forum.

I should state that the rear of the car weighs 600 lb. Let us just use the 200 lb. pre-load to make the issue very clear and simple. Are you agreeing with me that the suspension will not react to a 100 lb. bump? I could be wrong.

Brian

[hardingfv32-1]

"If you're on the limiter and increase pre-load, then you will have less stiffness"

This statement is not clear. If stiffness is spring rate, then I have no opinion. I am talking about the expansion force of the coil spring which is used to maintain the ride height, etc. Pre-load is being stated as that force greater than is required to keep the car at the proper ride height. In this example we restrict the rebound of the coil-over at a point that provides 200 lb. of pre-load. The spring must be at a prescribed length to provide this force. The droop stop system must achieve a static compression length so the the spring can be set accurately. Could the droop rubber be adding to the the force?

Have I incorrectly stated this?

Brian

[jpetillo]

OK, you are pre-loading a force into your spring using the coil-over collar. Say we need 600 lb. of spring force for the static ride height, but we also screw in an additional 200 lb. of pre-load. That means that the rear spring could actually be maintaining the ride height for a 800 lb. car. Is this correct?

Yes.

We take our 600 lb. rear suspension (+ 200 lb. pre-load) setup to the track and encounter a bump that generates 100 lb., of vertical thrust. The rear suspension of the car will not move because it needs at least 200 lb. of vertical thrust plus the cars 600 lb. weight to over come the rear spring system that is at a total of 800 lb.

I don't think this is correct in general. If you start with the 600 lb car/suspension sitting at ride height, and then you add preload to the point of the shock being topped out that will be some amount of preload. If the amount of preload is then increased by another 100 lbs, then your 100 lb bump will not move the suspension. Any less than that amount of preload will allow some suspension movement. I may be missing some previous constraint you put on the system, so correct me if this is so.

[jpetillo]

"If you're on the limiter and increase pre-load, then you will have less stiffness"

This statement is not clear. If stiffness is spring rate, then I have no opinion. I am talking about the expansion force of the coil spring which is used to maintain the ride height, etc. Pre-load is being stated as that force greater than is required to keep the car at the proper ride height. In this example we restrict the rebound of the coil-over at a point that provides 200 lb. of pre-load. The spring must be at a prescribed length to provide this force. The droop stop system must achieve a static compression length so the the spring can be set accurately. Could the droop rubber be adding to the the force?

Have I incorrectly stated this?

Brian

Brian, I think I understand what you're saying. Our definitions of preload are different - sorry about that. Yes, in the case I was describing the droop rubber is adding the force. That's why, if you increase preload and the car raises (if the car raises), the suspension moves off the droop rubber and it loses some amount of the increased rate it was adding and gets less stiff.

If I understand your definition of preload, then, if you a) have the collar set to attain a certain ride height, and then you b) move that collar to shorten the spring, you are calling step b) adding preload. Is this right? But, as you add any amount of that preload, one of two things will happen. 1) the car will just raise up and not compress the spring any more until it begins to hit the shock limit or 2) it will raise up into the droop limiter until the limiter compresses an amount to counter the force, and the spring will begin to compress only once it begins to interact with the limiter. This is all assuming a constant spring and suspension, etc. Was that clearer or more muddy?
John