Keith Tanner said:I know that shortening the shock shaft will add preload without changing static ride height.
Now think about how to add preload without changing the shock shaft length nor static ride height.
Keith Tanner said:I know that shortening the shock shaft will add preload without changing static ride height.
Now think about how to add preload without changing the shock shaft length nor static ride height.
Keith Tanner said: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.
I just went on Flyin' Miata and looked at this: V-MAXX XXTREME COILOVERS FOR NA - SPORT PACKAGE. Using just the rear for simplicity, you've got a 258lb/in main spring and a 112lb/in tender spring. Let's put 500lbs on that combo and assume minimal preload (the perch is touching)- the main spring compresses 500/258=1.94", and the tender 500/112=4.46 so it probably bottoms out. I don't know how much travel it has, let's say 2" since that seems typical for a tender spring. This means your suspension moved about 4", so an equivalent spring for the same ride height with minimal preload again would be 500/4=125lbs/in so the case you're talking about probably doesn't have equivalent preload in both scenarios. If the long spring is minimal preload, and your setup with the tender has 1" of preload (112lbs) the equivalent becomes 500/3=167lbs/in. 1.5" of preload on the tender gets us 500/2.5=200lbs/in.
What I'm saying there is that a preloaded tender spring could change the result significantly in your case, and (as would make sense) the more preloaded it is the more things converge until eventually the tender spring does nothing and you get very similar ride heights out of similar main spring rates, with and without the tender.
In my car's case, the tender has minimal preload, so it's more similar to the initial case above, meaning equivalent ride height would mean a softer spring if I'm going without a tender and putting things at minimum preload.
In reply to Driven5 :
Are you and I saying the same thing? I feel like I'm speaking gibberish, and it looks like Keith had to go do his job before he could explain whether or not I'm an idiot and why.
In reply to ¯\_(ツ)_/¯ :
LOL...Yes, just in different ways.
What you essentially have now is a progressive spring that is really soft during the initial stroke. You want to make that initial stroke a bit stiffer. Doing this will probably make the overall pack stiffer, so you may have to readjust the perch seat, but assuming the damper can take it, I think your plan will work reasonably well. Especially for the minimal cost of the stiffer springs.
Of course, it'd be easy to buy custom shocks that cost more than you've spent on the car...
Since I already posted a bunch of numbers, here's a shock dyno plot (not mine, but an example):
The point here is that, if the suspension doesn't get to full droop while airborne, I'm missing out on compression damping (the top lines) helping to smooth things out with hundreds of pounds of force as the car comes crashing out of the sky at 10in/s.
Do you know how far the shock is from reaching full extension on a 'typical' jump?
On another thought, what is your current spring rate and do you know how far your current springs are compressed at static ride height? Are we talking 2.25" ID springs? And what's the spring-to-perch gap at full droop?
Driven5 said:Do you know how far the shock is from reaching full extension on a 'typical' jump?
As far as I can tell, it's about an inch and a half front and rear. I really need to put a camera in the actual wheel well at some point.
2.25" ID springs, something in the realm of 275 lb/in front and 225 lb/in rear*** I think, but gravel blast has removed most of the numbering. I think the first thing I might try is to copy the ride height from the old Group A setup sheets and see where that puts things- it looks like 220-245mm was typical, which I think is higher than I was running the car.
EDIT: Here are some setup sheets, one of the few advantages to the Merkur is that its' European cousin the Sierra was rallied extensively and everything was published. It also looks like they used longer bump stops than I do.
***EDIT EDIT: I had a brain fart, those were the spring specs for our old rx7. 2.5" ID, 350lb/in front 275 (I think, numbers are REALLY gone on those) rear.
I wonder if you're looking too soft for your secondary springs, such that they still wouldn't provide enough force to overcome the shocks quickly enough. Have you looked at tender springs that would have substantially more effect? Eibach makes tender springs having 1.75" travel with up to 300 lb/in rates, and 2.25" travel with up to 250 lb/in rates. Maybe something in that range could potentially work, depending on how your actual numbers math out.
For example, lets say you have a 750 pound sprung (with driver) corner weight on each side in front. Your 275 springs with a 1:1 motion ratio would have 2.73" static compression, plus lets say 1.5" of 'free' travel beyond the max extension of the spring...Which is what you're currently trying to account for. So you actually have 4.23 of total droop travel to take up. 750/4.23 = 177 lb/in effective spring rate. Back calculating spring rates in series gets you 1/(1/177-1/275)) = 496. So in this example the desired secondary spring would have a ~500 lb rate with 750/500 = 1.5" of travel before bind, and assuming there is enough room for both springs between the perches, it would be able to maximize available droop travel usage without changing ride height.
Either way, depending on specs, longer bumpstops, may also be beneficial as well.
I say get the hardest tender springs you can find (to keep the spring rate of both spring acting together as high as possible - it will be softer than your main spring alone no matter what), and consider longer bump stops, or hydraulic bump stops ($$$$) to help handle the landing.
Also if you're done with those helper springs, sell them to me, they're bloody expensive new. I have to keep my total spring heights short for packaging reasons. Edit: Aw never mind, I run 2.5"ID springs.
Don't make the tenders so stiff that they don't fully compress at static ride height. You need to lock them out in that case, otherwise you get some odd behavior. Been there, tried that. Very supple on small bumps, loads of roll until you bind the tender, almost impossible to dial in the damping due to the varying spring rate. AFCO sells lockout rings to prevent this.
Keith Tanner said: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.
That won't change a thing, sadly. Longer main springs just means the lower perch sits lower, and you get more travel before coil bind. If you have a certain spring rate and a certain amount of weight on that corner, the spring will ALWAYS compress only (weight/rate) inches, and if that is less than the amount of droop travel at ride height, the shock will extend more than the springs will.
I use this to my advantage on the RX-7. With the amount of droop travel I have, I have a choice of running slightly positive camber, or a little negative camber but the ball joint will lock before the strut is fully extended. So I just lowered the car until the spring won't push the suspension down far enough to lock the ball joint. I'm already running 14" long springs, because any shorter and they will go into coil bind before the strut bottoms out.
Yeah, I got confused by the fact that our setup uses a non-trivial secondary spring so there's already real preload on the springs. Btw, it's the front and not the rear. Also by the fact that changing the shock shaft length is a different factor. Sorry for the distraction.
Is changable shock shaft length a thing? My suspension science goes to different paths. Is there a benefit to limiting shock travel besides keeping suspension links from going into bind?
(always willing to learn)
Knurled. said:Is changable shock shaft length a thing?
You can shorten them, provided you have something to cut with. I've also extended them before with screw-on adapters, but you have to be very careful about your bump stops in that case. I'm not sure there's any practical reason to do either other than packaging.
Keith Tanner said:Don't make the tenders so stiff that they don't fully compress at static ride height. You need to lock them out in that case, otherwise you get some odd behavior. Been there, tried that. Very supple on small bumps, loads of roll until you bind the tender, almost impossible to dial in the damping due to the varying spring rate. AFCO sells lockout rings to prevent this.
Ah good point. Tenders that don't fully compress at static ride height can actually be a good setup for offroading, but not if you want your suspension to do anything other than be really good at soaking up bumps.
Knurled. said:Is changable shock shaft length a thing? My suspension science goes to different paths. Is there a benefit to limiting shock travel besides keeping suspension links from going into bind?
(always willing to learn)
Many purpose-built race cars use limiting straps that take effect in the "normal" range of suspension travel to affect handling balance:
https://www.f1technical.net/forum/viewtopic.php?t=21565
Also, many modern cars are designed to corner on the bump stops - it's much like running a short dual-rate spring setup, but cheaper. The suspension is soft at first and then suddenly gets much harder when it's compressed past a certain point, so you can have a soft ride without also having door-dragging amounts of body roll. Recent WRX STis even run on the bump stops at static ride height - the springs only work by themselves when the suspension extends!
Posting this link mostly because it's the cheapest I can find them: Flat Wire Springs
I think the 3" tall 126lb/in or 50lb/in ones would work for my application, we'll see if I still feel that way when I'm reassembling the suspension.
¯\_(ツ)_/¯ said:Knurled. said:Is changable shock shaft length a thing?
You can shorten them, provided you have something to cut with. I've also extended them before with screw-on adapters, but you have to be very careful about your bump stops in that case. I'm not sure there's any practical reason to do either other than packaging.
When I'm building suspensions, I can add and remove spacers inside the shock body that will change the amount of shaft travel. The front and rear shocks on an ND Miata, for example, are the same geometry other than a 2.5" shaft travel limiter (!) in the front shock. The front NC application has it as well. You can also change the length of the shaft itself. I know this isn't necessarily the sort of thing you can easily do with parts sourced from a junkyard, but you really don't need anything exotic to do it. My "special tools" are limited to a nitrogen bottle and a clamp to hold the shock body.
Sounds like the new WRX is a classic Mini underneath. They don't even have springs.
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