Mystique Racing wrote:"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?
hardingfv32-1 wrote:
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