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Static vs Dynamic, Progressive Springs, which way #561202 09/01/14 10:30 am
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Gordon Gray Offline OP
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I don't seem to have much luck with the site's search function so please forgive me if this has been covered before (and I'm sure it has)

I replaced my B44VR's rear shocks this weekend. I got them from Art Xanders (RIP) and they have been sitting around for a long time unused. They are cheapos...you had to know Art to understand that but if he said they worked well...I believed him. I liked Art a LOT and sure do miss him.

They (the new shocks) have progressive springs. I've always installed the close coils of a progressive spring to the part that doesn't move (hence the static vs dynamic title) but I notice when I look up these shocks online....all of them have the tight coil down (attached to the swingarm)

I figured it was no big deal..and I was in a hurry..so I put them on the way they came (tight coils down) but now when I look at my two bikes sitting side to side I set my B50's Progressive Suspension shocks the way I always have treated progressive rear shock springs, tight coils up and the two together just look like the owner couldn't make up his mind. Which....is the case here.

So what do you do? Can of worms? .....hey, It could be worst. smile

Gordon, who's not going to change either of them right now anyway...too many steps forward to complete to be taking any steps back. blush

Last edited by Gordon Gray; 09/01/14 10:39 am.
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Re: Static vs Dynamic, Progressive Springs, which way [Re: Gordon Gray] #561206 09/01/14 11:12 am
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J. Charles Smith Offline
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The closer coils are the softer coils. Since they are the ones that will compress first, I have always put the tight coils closest to the wheel. I don't really think it matters much, though.

Re: Static vs Dynamic, Progressive Springs, which way [Re: Gordon Gray] #561209 09/01/14 11:31 am
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Magnetoman Online Content
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Originally Posted by Gordon Gray
They (the new shocks) have progressive springs. I've always installed the close coils of a progressive spring to the part that doesn't move (hence the static vs dynamic title) but I notice when I look up these shocks online....all of them have the tight coil down (attached to the swingarm)
I can't remember having seen a serious technical discussion of progressive springs anywhere before (which doesn't mean I haven't seen it, and forgotten). Although, with the internet, something -- right or wrong -- is written about everything, so I suppose it's there if one spent enough time searching (n.b. progressive springs technical gets 13M hits).

The underlying principle being that you would like as much of the weight of your motorcycle supported by the spring, and as little that is not. The latter is termed "unsprung weight." The reasons for this aren't simple, but take it as a rule to try to obey. When you look closer at any given design it's pretty clear whether some components are sprung weight (e.g. the engine and frame) and others are not (e.g. rims and tires), but some things aren't so neatly classified (e.g. the shock absorbers, part of which are in the middle ground between sprung and unsprung).

Anyway, for present purposes the only factor over which you have control when installing a given set of progressive springs over your shocks is the "unsprung weight." Given how the springs are wound more weight is toward one end of the springs than the other so, since you want to minimize the unsprung weight, you should install the spring with the heavier, more tightly wound coils at the top. Having said this, the performance difference between one orientation and the other would be extremely small.

I suspect many people install their springs the other way, with the tight coils at the bottom, because they think the coils at the top will be the first to compress so they should be the ones with the lowest spring constant. It doesn't work that way. When a progressive spring is compressed whatever coils have the lowest spring constant will compress first irrespective of whether they are at the top or bottom. Another reason for installing the tight coils at the bottom is it looks "top heavy" to do it the other way. But, you actually do want it to be "top heavy" in order to maximize the amount of sprung weight.

To digress from this a little., if your shocks provide too little damping ("underdamped") the bike will continue to oscillate up and down for a while after hitting a bump, and if they provide too much ("overdamped") it will take too long to return to equilibrium. So, you also would like your shocks adjusted to "critically damp" the system. Such a condition is no problem to arrange with simple linear springs since the required amount of damping depends on the "spring constant" of the springs.

However, almost by definition progressive springs don't have a single spring constant but a continuously variable range that depends on how much they are compressed. So, for perfect behavior you would need shocks that changed their damping as a function of the amount of compression of the spring. This easily can be arranged to be the case with a computer-controlled system so it isn't of theoretical interest only. Luckily for us, though, progressive springs don't change their spring constant by a large factor, and since the required amount of damping depends only on the square root of the spring constant, the overall system can be tweaked to be close enough to critical damping so as not to be noticed by the rider. Also, with the shocks you purchase, one size (damping) fits all (spring constants), so you're at the mercy of both the shock and the spring manufacturers to give you a satisfactory overall average that works good enough.

Re: Static vs Dynamic, Progressive Springs, which way [Re: Gordon Gray] #561396 09/02/14 3:20 pm
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Gordon, as magneto sez, you're technically right about the spring orientation but in practice you just won't know the difference.

BTW, the way progressive springs work (at least all the ones I've seen) is that, while all the coils have the same spring constant (constant wire diameter and radius throughout), some are closer together and become coilbound when the spring is compressed. When these coils become bound, they no longer contribute to the spring constant, effectively raising the spring rate and giving the effect of a variable spring constant.

Cheers and don't worry, just ride...


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Re: Static vs Dynamic, Progressive Springs, which way [Re: Alex] #561399 09/02/14 3:34 pm
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Gordon Gray Offline OP
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Got it, the good doctor covered what I was looking for.

I'm not worried about it but I would rather "know" something than just guessing.

For the time being....I'll have one set up and one set down so I'm covered any way you want to look at it.

Kinda reminds me of a friend we both know. He was riding an A65 with the stock Girlings turned upside down. When I mentioned that the shocks didn't work like that...I just got a blank stare.

I have a lot of stuff in my head and can't remember where I got it...some where along the line I listened to somebody I trusted about which way progressive springs went and have stuck with it for years.....but now I see new shocks with them turned the other way so I thought I'd ask.

Me ride??? laughing That's what I do... smile

Wheeled wart????? you talking about your head again? laughing

Take care......your friend in NC, Gordon

Last edited by Gordon Gray; 09/02/14 3:35 pm.

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