Helper Springs vs. Tender Springs- a Yumping Question

Our rally car, pictured here doing what this thread is about:  

currently uses flat, zero rate helper springs like these:  

They do an acceptable job of keeping the main springs from flopping around when the car is in the air for service, but are not strong enough to push the suspension to full droop during a typical yump.  What I'm wondering is, if I replace them with 25-50lb/in tender springs like these:  

would the added suspension travel help?  Assuming they can push everything to full droop shortly after leaving the ground, the dampers would get another inch or so of travel to soak up the landing.  A reasonable thing to test?  Or would I be better off spending my money on higher quality bumpstops instead?  I think the answer is "both" but I'm interested in the forum's thoughts.

Can you change the rebound settings on your shocks?

Less rebound could allow the suspension to fully extend more quickly.

To compensate you may need to increase your compression damping.

In reply to Stefan :

The shocks are not adjustable and the valving seems perfect for basically everything else the car encounters on a rally stage, so I'd rather not change it.  It's not that it takes a lot of force to fully extend the suspension, just that the current helper springs have no rate to speak of.  The suspension does pretty well on landings, but it seems like a waste not to let it use that extra bit of travel.

It's difficult talking specific spring rates on different platforms, but I'd go for a secondary spring that's fully compressed at your static ride height - but only just. That'll keep them out of action whenever there's a normal load on the suspension but give you maximum help when it's unloaded. On a Miata, we're talking about 100 lb.

Alternately, if you can package longer main springs, you can get them to a point where they're not fully unloaded at full droop. This is going to mean dropping the lower perch down a fair bit, so it may not be possible.

In reply to ¯\_(ツ)_/¯ :

I have no idea if it may be irrelevant, but what happens to camber/caster/toe at full-droop? You may find out that while the car is docile upon touchdown now, it could become a handful when landing at full-droop? 

In reply to Pete Gossett :

The front doesn't do anything really weird, but the rear suspension does some wonky E36 M3 at both ends of its' travel- that said, it's pretty stable despite a lot of toe and camber change at full bump, which it hits on landing, so I'm not super worried about full droop where it's just beginning to be loaded.

In reply to Keith Tanner:

We're in agreement about the secondary spring rate.  I do have room to package longer main springs, but I like the rates where they are now- just like the damping, everything works very well when the wheels are on the ground.  I could go higher on the static ride height (getting me more bump travel with the same amount of effective droop I have now) to see how it works, maybe keep raising the car until the handling is negatively impacted.  This is the current ride height, the car could reasonably go up an inch (and a half, maybe?) from here before the rear suspension geometry starts to get weird:  

In reply to ¯\_(ツ)_/¯ :

No need to change rates if you just want longer springs.

In reply to Stefan :

There is if I want to keep the ride height where it currently is without having the same issue of droop travel being taken up by the helper spring.

Longer springs will only help if you have a coil over design that allows for ride height to be set independently from spring perch height, or if you're willing to increase ride height equivalently.  Just dropping the lower perch on longer springs of the same rate to the same static ride height would leave the exact same amount of full-droop spring-gap needing to be taken up by a secondary spring, but would reduce the amount of coil bind on the main spring under full compression.

When considering secondary springs with a 'non-zero' rate, make sure that what you gain in available droop travel isn't less than what you could lose in maximum bump travel, depending on how close you are to coil bind at full compression right now.  Those 'non-zero' rate springs also have a much more 'non-zero' bind height than the 'zero' rate springs.

I was assuming adjustable perches.

Extend the existing spring by 1", drop the perch by 1". Ride height is the same, rate is the same. Keep doing that, you'll get to a point where the shock is fully extended so the spring can't fully unload. Voila. I think I've got the geometry right in my head. Longer springs are also less likely to coil bind.

When you pull the current lightweight springs, move the perch up by the same amount as the thickness of the compressed stack.

In reply to Keith Tanner :

They are adjustable perches, and the springs are nice and long already- coil bind is not an issue.  The spring rate is the spring rate, and the amount of load at static ride height is consistent- this means that, keeping the same spring rate, and the same preload (or lack thereof in this case) produces the same height, no matter whether the spring is 10" or 20" long.  100 lbs/in compresses one inch for every 100lbs, regardless of how long the spring is.  If I adjust the perches up until the spring touches at full droop with no tender, the ride height will move up by that much (give or take motion ratio).

Keith Tanner said:

Extend the existing spring by 1", drop the perch by 1". Ride height is the same, rate is the same. Keep doing that, you'll get to a point where the shock is fully extended so the spring can't fully unload. Voila.

When only adjusting via spring perches, the shock would not be extending any further when the static ride height isn't changing.  No change in static shock position and no change in static spring compression, means no change in full droop spring gap or preload.

I'm thinking about the edge case, when you run out of shock extension and the shock tops out. That's where you start to get preload. Think of it the opposite way - if you kept your current setup as it is right now but shortened the shock shaft considerably, you could end up with preload at full droop but no change in the static position or behavior until you get to full droop.

In reply to Keith Tanner :

Right, and if I have 1.5in until I start to get preload, and adjust that out using the spring perch, the static ride height would move up 1.5in accordingly.  Because the spring begins to act 1.5in sooner when moving from droop into bump.

Is your shock of infinite length? I'm not talking about moving the perches up. I'm talking about a longer spring and moving the perches down to compensate. You'll get to a point where the shock is topped out and you get preload on the spring because the shock isn't any longer.

We do this exact thing with V-Maxx shocks - they come either with a preloaded long spring or a shorter spring with secondaries. Same rates, same ride height, same shock body.

Keith Tanner said:

Think of it the opposite way - if you kept your current setup as it is right now but shortened the shock shaft considerably, you could end up with preload at full droop but no change in the static position or behavior until you get to full droop.

What you are describing, physically adjusting the max/min shock length, is what I referenced when I mentioned the ability to change ride height independently of the spring perch height.  This effect cannot be achieved only via adjusting the spring perches.

It's a thought experiment, intended to separate concepts of preload from static ride height.

You guys are forgetting that shocks are finite.

Keith Tanner said:

I'm talking about a longer spring and moving the perches down to compensate. You'll get to a point where the shock is topped out and you get preload on the spring because the shock isn't any longer.

Why even bother going through the motions of lowering the perch all the way while increasing coil length if coil bind isn't an issue. Once the perch is bottomed out, all you're talking about is adding spring length while not being able to lower the spring perch any further...Just the same as simply adding longer springs to the current perch position.  This necessarily increases the ride height.

Until you hit the limits of the shock. This happens. I've done it. I can go out to our warehouse and physically lay my hands upon a setup that works exactly how I describe.

Or you can just forget all this and just bung some high rate secondary springs on there.

I understand that, Keith, but what I'm saying is that the only way nothing else changes in that scenario is if the spring is also a softer rate.  Let's look at one corner of the car, and say that it has a 1:1 motion ratio, 600lb corner weight, and a 150lb/in spring.  We can agree that, no matter what, the 600lb weight will compress the 150lb/in spring 4 inches, right?  So let's say you have an 8" spring and a tender with 1" of travel.  Ride height will be 5" lower than full droop, 1" of tender travel and 4" of spring travel.  Now make that a 10" spring, same rate, remove the tender, and adjust the perch to suit.  That longer 150lb/in spring still only moves 4", and since the perch is adjusted to meet it, the car is now 1" higher than with the tender.

I'm not trying to argue with you, I'm just trying to figure out where in this line of reasoning we're interpreting things differently.

Keith Tanner said:

Until you hit the limits of the shock. 

In this situation, you can't physically hit the extension limit of the shock without changing ride height.  Period.

Since we're at Period, the discussion is over.

I'll go take that suspension off the shelves because it's not possible. That's going to disappoint a lot of people.

In reply to Keith Tanner :

Go ahead and try.  Grab a pair of coil-overs and drop the perch until there is a 1" gap between the spring and perch at full extension.  Place it on a car and check the ride height.  Now grab a 1 inch longer spring with the same rate that will extend the shock fully when unloaded, and see what the ride height ends up at.

I'm trying to get my head around this. I know that shortening the shock shaft will add preload without changing static ride height. And I understand that the distance between the perches AT STATIC RIDE HEIGHT will determine the ride height, and I do remember enough high school science to remember how spring rates work. And I know from practical experience that we have a suspension in the building that has either a preloaded main spring or a main/tender setup that has the same rates, same shock dimensions and same ride height in both situations.

But I think that particular suspension may be bumping up against a different edge case, maybe because the secondary springs are fairly hefty. I'll think about it on the drive home, but I'm out of time during the workday.

So carry on.