rear droop/camber set up

[Mystique Racing]

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.

John,
I agree with your first statement, however, I am not sure that the second part of your statement is correct. I think I can increase the opposing force, or maybe we should call it wheel rate, of the rear suspension by preloading the spring more. I don't have to increase the rate of the spring to do this. Of course if I do this there will have to be a droop limiter involved.

You can change the effective spring rate by running up against the droop limiter

I don't think the spring rate changes based on preload, only the opposing force is effected by adding preload.

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

My droop limiter does not have any rubber dampener. I guess it would be considered a hard stop. I don't understand how the suspension can become less stiff by adding more preload?

[SR Racing]

??? NOw I don't get it... Ignoring the droop limiter stop for the moment..

As I said, to move the 200 lb spring 1 inch requires 200 lbs. To move it another inch takes another 200 lbs. Correspondingly lhe same with a 300lb spring.

If you use the droop limiter (a solid one) to preload the spring 1 inch you have introduced a stored force of 200 lbs. So now to move it the first inch will require 400 lbs. Yes, you have a stiffer spring for the first inch.

You DO have a "stiffer" suspension, but you have affected other dynamics that may or not be good.

The only way to make a "stiffer" suspension is with a higher rate spring (or bellcrank and geometry ratios). The spring rate is the only way to do it linear (at least at the spring pick-ups).

[jpetillo]

Scott, I was addressing a more typical FV configuration with a soft stop, not yours in particular with the hard stop. We're also all still suffering from slightly different definitions of the same thing to some extent. But let me rephrase where needed and respond to your posting. Also I think Jim's comments were right in line with what I was trying to explain.

We all seem to be pretty close in defining stiffness as an effective spring rate. That effective spring rate is a combination of what is affecting the suspension from moving (spring, droop limiters - hard & soft, shock top-out springs, tires, etc.), but let's not include tires for the moment and also not include any damping, yet.

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.

John, I agree with your first statement, however, I am not sure that the second part of your statement is correct. I think I can increase the opposing force, or maybe we should call it wheel rate, of the rear suspension by preloading the spring more. I don't have to increase the rate of the spring to do this. Of course if I do this there will have to be a droop limiter involved.

Scott, I see what you mean. If by increasing the preload you are topped out against a hard stop while driving, then you will have an infinite spring rate at that time, and the tires will be doing all your spring work until you overcome the preload, as Jim mentioned. When you hit a sufficiently hard bump that it overcomes the preload and the spring now moves, at that time you will have your old spring rate back. I guess on average you will have a higher spring rate - stiffer suspension - as you move into and out of that regime. But do you ever really want to be running up against the hard stop aside from just pressing against it momentarily now and then?

You can change the effective spring rate by running up against the droop limiter

I don't think the spring rate changes based on preload, only the opposing force is effected by adding preload.

I agree. You can't change the rate of the coil spring, but that's why I said "effective" spring rate. If you had a squishy limiter, then the limiter in parallel with the coil spring has a higher combined (effective) spring rate. Two springs in parallel have a higher rate. In your case, if you top out against a hard limiter, the combined/effective rate would be an infinite rate at that time.

The bottom line is that, in times when you're running and not sitting up against your hard stop limiter, no matter how much preload you have in the system the rate does not change, like Jim said. If you spend more time into the limiter during normal operation because of your increased preload, then on average your suspension will be stiffer. Perhaps we're in violent agreement.

Merry Christmas, all. John

[cendiv37]

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

I'm going to comment on this post since it seems the closest to what I think happens. I'm not familiar enough with dual spring zero roll set-ups so I'm going to stick with the common single spring zero roll arrangement. I think we have to ignore the shock forces or this will get even messier that it already is.

I've come to think that the only practical state to define and measure preload is at static ride height. So I've come to agree with Scott and Greg on calling this (and only this) condition "preload" of the rear spring. Any other definition of rear spring preload seems to have little or no usefulness for us. The only functional way to "preload" any suspension (compression) spring would be to control or limit its extension with something. A droop limiter can do exactly this.

Let's go with a 200 lb/in spring. With a 600 lb rear axle load requiring a 600 lb force to hold it up, we are assuming the wheel rate equals the spring rate (without droop limiter engaged). So, if our car needs 600 lbs of spring force to hold it (up) at ride height (without engaging the limiter), we can add preload to the spring if we set the droop limiter to be engaged at static ride height (droop limit camber = static camber). If it's a solid stop limiter, we can simply set the droop limit where we want it (total rear camber) and this will by definition become our static rear camber. We are going to crank up the spring platform to make this so. After setting the droop limit we set the car on the ground. Assuming the spring platform starts backed off so that the limiter is not yet engaged, rotation of the spring platform (in the right direction will at first raise the car (without compressing the spring). Once the limiter engages, further rotation of the platform will can longer lift the car (and change camber) but will instead, compress and thus preload the spring. If we want 200 lb of preload, with our 200 lb/in spring, we will have to compress the spring another inch from where it first engaged the limiter. This will give us a force of 800 lb against the spring platforms even though it only takes 600 lbs to hold up the car. But the force against the rockers from the spring/shock ends will still be only 600 lb. since that's what it takes to hold the car up.

So what does this do? I believe it will give a very non-linear wheel rate. The wheel rate will be infinite until the preload force (200 lb) is overcome. So over small bumps, the suspension will not comply and all springing/compliance will be by the tires. Only when the tire loads reach 800 lbs (as when the car hits a larger bump) will there be enough force at the spring/shock to rocker connection ( >800 lb), to compress the spring. At that point the wheel rate will drop to 200 lb/in. The force will be 800 lb (or more) but the rate will still be 200 lb/in when operating off of the limiter.

So for the same spring rate spring is the suspension "stiffer" with the preload than without? Yes. But it is not a linear system. It doesn't have an easily defined "wheel rate". It is infinitely stiff for small load changes but drops to 200 lb/in for larger ones. The average wheel rate is higher, but once off the limiter the wheel rate is the same as without the preload (assuming the same spring). However, you won't be off the limiter as soon or as often so the car will certainly be and feel feel stiffer overall. I would call this is a kind of "regressive" rate spring system.

Using a soft limiter changes things in ways I'm still trying to understand. I'm thinking that it does something quite different since it's opposing the spring force yet still allowing compliance. It will also be quite non-linear for wheel rate.

my $.02 for now.

[jpetillo]

Bruce, Yes, the way I see it, that was exactly right. Yes, it's a regressive (decreasing) rate spring system. With regard to your last comment about what the soft limiter is doing, I see that it's doing the exact same thing you described with the hard limiter, but gives the system a smooth transition between 200 lb rate and the infinite rate. I agree that it's still a nonlinearly varying rate changing in the same direction from soft to stiff as we go into droop, but not nearly as aggressive as with the hard stop. What I think the soft limiters give you is the ability to choose your stiffness at full droop by changing the combination of coil springs and the droop limiter rubber - as you said for tire compliance. John

[hardingfv32-1]

"What I think the soft limiters give you is the ability to choose your stiffness at full droop by changing the combination of coil springs and the droop limiter rubber - as you said for tire compliance. John"

Expand on this.

What stiffness are we talking about: rebound or compression?
At full droop are we not at full rebound?
How is compliance improved?

Brian

[smsazzy]

As the car jacks in the rear, you come up against the droop limiter and begin to loose a little rear grip. (as you are starting to unload the inside tire) A soft droop stop will cause this to happen slowly. A cement brick will cause it to happen instantly. I don't like to go from grip to no grip in the middle of a turn. So a droop stop with some compliance is preferred.

[Mystique Racing]

Bruce, nicely said. I think we are in agreement.

"What I think the soft limiters give you is the ability to choose your stiffness at full droop by changing the combination of coil springs and the droop limiter rubber - as you said for tire compliance. John"

In my mind a soft, or compliant, rubber on the droop limiter changes nothing at full droop, however, It may affect the transition entering full droop. Although I don't think this "soft" transition matters. We are talking about milliseconds between loading and unloading the droop limiter.

Additionally, without any spring preload on the droop limiter, the limiter is really only controlling the un-sprung weight of the suspension, and possibly some jacking force, so we may be only talking about an 100lbs or so.

[Mystique Racing]

As the car jacks in the rear, you come up against the droop limiter and begin to loose a little rear grip. (as you are starting to unload the inside tire) A soft droop stop will cause this to happen slowly. A cement brick will cause it to happen instantly. I don't like to go from grip to no grip in the middle of a turn. So a droop stop with some compliance is preferred.

Do we know for sure that this assumed loss of grip is due to the droop limiter engagement?

Could it be due to the bump in the track?

Improper shock settings?

Unfavorable camber angle?

Improper tire pressure?

Etc, etc,

[hardingfv32-1]

smsazzy

Remember the rear suspension is in rebound, being raised by the jacking force and rear spring. Neither of these forces are being generated very rapidly and you also have the shock rebound valving controling things. I'm not going to say that coming up against the limiter is without some negative compliance impact, but I will state is has much less effect than you are imaging.

Brian

[hardingfv32-1]

Scott

1) "In my mind a soft, or compliant, rubber on the droop limiter changes nothing at full droop, however, It may affect the transition entering full droop. Although I don't think this "soft" transition matters. We are talking about milliseconds between loading and unloading the droop limiter. "

Again, without a tire model we simply have no idea what the negative impact of the impact of a millisecond loading and unloading the droop limiter is on tire compliance. We should assume it is negative and reduce its impact without involving to many other negative compromises.

That said, it is not clear from past discussions that we are actually hitting the limiter. This might be a mute issue.

2) I think what John is getting at with a soft bump stop is that the spring rate is lower when approaching the droop limit. Say we start with a 200 lb/in rear spring rate and use a 50 lb/in spring as a droop stop (instead of the common rubber stop), I THINK the the total rear spring rate would be 150 lb/in when the the rear suspension is fully jacked up. PLEASE DISREGARD AS THIS IS INCORRECT.

John can confirm or restate.

Brian

[jpetillo]

"What I think the soft limiters give you is the ability to choose your stiffness at full droop by changing the combination of coil springs and the droop limiter rubber - as you said for tire compliance. John"

In my mind a soft, or compliant, rubber on the droop limiter changes nothing at full droop, however, It may affect the transition entering full droop. Although I don't think this "soft" transition matters. We are talking about milliseconds between loading and unloading the droop limiter.

I've been equating stiffness to spring rate. The soft limiters I was talking about are the ones I see most FVs running, where the suspension is either already pressing up against the droop limiter at static ride height, or just beginning to press into it with any suspension movement. They can squish 3/4" to a few inches. In this case, it's not milliseconds between loading and unloading the droop limiter, it's seconds or longer. So, this may not apply to what you want to consider.

With regard to the compliance issue, the difference can be significant. The soft droop limiter is limiting the roughly 200 lbs of jacking force you could have in a corner, and squishes a certain amount to do that, but does not squish completely to the droop limit. If you jack the car up as you do when when you set static droop limit, the droop limiter squishes even more since the springs will push down much harder than that 200 lbs. That difference in the amount the limiter rubber is squished gives you compliance. The limiter rubber can squish that much more before it takes the tire off the ground and uses that to deal with road irregularities. That helps compliance as compared with a hard limiter as Steven pointed out. Whether you need that extra compliance or not is another question that Brian has been posing.

Additionally, without any spring preload on the droop limiter, the limiter is really only controlling the un-sprung weight of the suspension, and possibly some jacking force, so we may be only talking about an 100lbs or so.

The droop liter is in parallel with the regular spring (on the designs I've seen), so it has to control exactly the same masses, doesn't it?

2) I think what John is getting at with a soft bump stop is that the spring rate is lower when approaching the droop limit. Say we start with a 200 lb/in rear spring rate and use a 50 lb/in spring as a droop stop (instead of the common rubber stop), I THINK the the total rear spring rate would be 150 lb/in when the the rear suspension is fully jacked up. So now when the rear suspension hits a bump at full droop, it is more compliant. This sounds very helpfully while in a turn.

John can confirm or restate.

Brian, actually when you're up against the droop stop, the spring rate rises. The reason is that you still have the old 200 lb/in spring rate from the coil spring - you can't reduce that - and the droop limiter is in parallel and just adds to that. The way I look at it, the effective spring rate (coil in combination with the droop) can at any time be determined by measuring the movement of the system in response to the change in force - that's just the definition of spring rate so we can agree on that. We know that if we are not up against a limiter at all, the jacking force will raise the car (with a 1:1 motion ratio) up 1" for the 200 lb jacking force with that coil spring. With the squishy limiter involved, the car jacks up less, say 1/2". So, the suspension system has a higher effective/combined spring rate of 400 lb/in at that time.

The reason it's more compliant is that with the droop limiter when you're cornering you still have a spring rate of around 400 lb/in that can be used for compliance. In contrast, if you're up against a hard stop, you have a infinite spring rate and you're at the mercy of the spring rate of the tires only for compliance.

As you have pointed out, we need to know what tire spring rates are, and I think they're in the 600-800 lb/in range.

Did that make more sense?

John

[hardingfv32-1]

Yes, I see your point about the spring rate climbing toward infinity. The use of a coil spring for the droop stop is very poor when compared to a variable rate rubber stop. We are heading for infinity and the rubber stop adds some curve to the end of the spring rate plot. Although better than a hard stop, it could be a very high spring rate.

I'm starting to see that the spring rate that develops with a standard zero-roll with rubber stop at full droop could be similar to what I find on my high rate spring system.

I guess our goal would be to get to full droop with a falling rate. Obviously not having to come to a stop and hitting infinity would be helpful.

Brian

[cendiv37]

But doesn't "spring rate" have a sign? Remember, the droop limiter will be working in *opposition* to the spring. So does the total spring rate (spring + limiter cushion) go up or down when the cushion is engaged? That's why I'm confused as to what the spring rate is with a compliant droop limiter. My gut tells me one thing, my head the other.

It's interesting to speculate on which is the independent variable and which is the dependent (load or position). For a hard stop, in one case you are talking about a vertical line (infinite slope), in the other a horizontal line (zero slope). In a sense, the hard stop changes the system from a load controlled regime to position controlled regime, allowing 0 position change for load change while the stop in engaged. With a soft stop, this is not the case, so until we reach infinite stiffness of the stop (equivalent to the hard stop), we are staying in the load controlled regime. Obviously, when the cushion compliance gets much smaller than the tire compliance, the tires will be the major component of the total compliance.

I'll see if I can work this out and show it graphically if I can.

[hardingfv32-1]

How do the two types of springs function together? We know that the droop mechanism is compressing the droop rubber while de-compressing the main spring. I assume the main spring will decrease in force at its specified rate moving towards zero. The droop rubber is being compressed into a position where it is in balance with the jacking force + main spring force. I guess it is really not at infinity, but I'm not sure what the rate number might be since it is variable.

It would seem that the spring rate curve of this bump rubber could get very complex and where you are setting on this curve very important. Sure does not sound like there is much respect for the bump rubber's properties in the FV community. Where are all the old timers warning that the bump rubber must be change each year or anytime you get oil on them.

Brian

[hardingfv32-1]

Tire compliance question:

One rear wheel going over a hole in a turn while at full droop or fully jacked:

Why does the the wheel want to fall into the hole (comply)? We are up against the droop limiter, so where is the force going to come from to cause the rebound movement that allows the wheel to fall? If anything the jacking force should be falling and we know the rear spring is not going to provide addition force as it is in rebound by definition.

Brian

[jpetillo]

Bruce, if you think about it, it's not intuitive and it can be confusing. I don't like thinking too hard - I have to make it simple to understand it. Just think of it like a black box and define the spring rate based on what the black box is doing. Then it all works out.

Brian, what you said is right. About your question of how to add springs, since the coil and bump rubber are parallel to each other, their spring rates are simply added. But you need to be careful to properly define their at rest lengths and relative locations to each other correct for it to work out. Also, the spring rate for the bump rubber is not a constant but a more complicated expression.

I've plotted the curves both the forces and the effective spring rates. Each curve has the combined (black box) result, coil, and bump rubber contributions. It all adds up like I was suggesting - the spring rate rises when the bump rubber is pressed as the car jacks.

I can't explain more right now or get the curves posted, family calls! John

[problemchild]

Why don't you make it really simple then?

Black box with 4 adjustments ...... static camber, droop, bump, rebound. Does anything else (beyond which way to turn these 4) matter?

[hardingfv32-1]

Yes, at this point the discussion is indicating the design of the actual droop stop is important.

Since by definition you are near a maximum traction condition in a turn when at/near full droop, improving/maintain maximum tire compliance at this point would be an advantage.

Any ideas on this narrow subject?

Brian

[jpetillo]

I figured it was just as easy to just put the table in the post. The first column is the relative position of one end of the shock, in the direction of the shock lengthening, where zero is chosen to be when only the coil spring is holding up the car, and is just beginning to touch the rubber limiter. The coil spring has a rate of 200 lb/in, and at this point (x=0) the spring is already compressed 3" to hold up 600 lb. The second column is the total force provided by the coil spring and limiter. You can see at x=0, that 600 lb is the total force, so it's negative here. The third column is the contribution of the force from the coil spring only. The fourth column is the contribution of the force from the rubber droop limiter. Like Bruce said, it is a retarding force. The rubber limiter is assumed to be 1" in length (like my car has), and can't squish more than that. This is why the first column doesn't go up to 1".
The two columns for the spring and bump rubber forces add together to give the total/combined force in second column. The table shows the case where at the time the shock lengthens by 0.55", the car is in full droop, and the droop limiter is negating any force the spring is providing. (The tires, axles and anything unsprung is assumed to have zero weight to simplify things.) You can see that the spring rate is the highest (most force difference for the same change in length) when the droop is nearest to the full droop end.
This table is the black box way to look at it (first two columns), but the numbers in the table were all from the basic equations of the separate components inside the box.
John

Code: Select all

x_(in) F_(lb)	F_Coil	   F_BR
 0.00	600.00	600.00	   0.00 <== static ride height (just starting to touch droop limiter)
 0.05	568.95	590.00	 -21.05
 0.10	535.56	580.00	 -44.44
 0.15	499.41	570.00	 -70.59
 0.20	460.00	560.00	-100.00
 0.25	416.67	550.00	-133.33
 0.30	368.57	540.00	-171.43
 0.35	314.62	530.00	-215.38
 0.40	253.33	520.00	-266.67
 0.45	182.73	510.00	-327.27
 0.50	100.00	500.00	-400.00
0.55	  1.11	490.00	-488.89 <== full droop

[Mystique Racing]

John,

What spring rate did you use for the rubber droop limiter?

[hardingfv32-1]

Chances are that he solved for column 4 knowing the values of the other columns. It is not easy to describe a rubber spring.

...IF... you make the droop stop LINEAR at the .40 -.45 compressed range you might be able to get an idea of what the spring rate MIGHT be like. I estimate about 1200 lb/in at that point in the droop stops compression. The tires having a lower spring rate should be coming into play then.

Brian

[smsazzy]

Chances are that he solved for column 4 knowing the values of the other columns. It is not easy to describe a rubber spring.

...IF... you make the droop stop LINEAR at the .40 -.45 compressed range you might be able to get an idea of what the spring rate MIGHT be like. I estimate about 1200 lb/in at that point is the droop stops compression. The tires having a lower spring rate should be coming into play then.

Brian

Why would the tires come into play when you are going in rebound? If you are hitting the droop, you're unloading the tire, not loading it. Unless you are talking about the minute amount of compression in the side wall, I don't think the tire comes into play in rebound.

[hardingfv32-1]

That is exactly what I think could be happening. The component with the lowest spring rate is what becomes active... I think. In this case the tires which we believe are in the 800-1000 lb/in range vs the rear spring system that could be at 1200 lb/in while stopped on the rubber limiter.

We don't have a tire model but it is never good when the tire leaves the ground or reduces it vertical load. If I can REDUCE the variations in the tires vertical loads just a little, I MIGHT be able to operate farther out on the traction circle than you. Of coarse that is assuming all other things are equal....

That is what we are groping for with this discussion.

Brian

[jpetillo]

John,
What spring rate did you use for the rubber droop limiter?

I knew I was going to forget something. Since I mentioned 400 lb/in in an earlier posting, I started with that. So, the rubber has a nonlinear rate that starts at 400 lb/in and goes to infinity at 1" (I chose 1" as the rubber thickness, so it has to be infinite by then). With that, at the 0.55" point the rate has increased to 1,975 lb/in. Brian's in the right ballpark. I could have chosen something else - but this should be good enough for the discussion.

Brian's speculation that the tire spring rate may be lower seems reasonable. I think tire spring rates are in the 600-800 lb/in range, which is much lower than 1,975 lb/in at max droop. But, we should be looking at the combined rate when the car has jacked and not when it has left the ground. When it's jacked the force is reduced by something like 200 lbs, and the combined rate at that point is about 700-800 lb/in (between the 0.25 & 0.30" deflection points in the table). That's squarely in the tire's spring rate territory - interesting.
John