Gilson/Sebac Shocks

Regarding the Sebac rear shocks for the high-line Gilsons, why are the springs installed upside down on every single photo I've ever seen? Progressive coils start out resilient and eventually become "near-solid" or flat in response. With that said, coils at the bottom are spaced far apart in comparison to those at the top of travel. Look at any photo of a 525 or a 988 with sebacs...


Active Member
I wonder if a bunch got put together wrong and Gilson got a good deal on them as 2nds? Will it make any difference on how well they work?
..Will it make any difference on how well they work?
Certainly will. Simply stated, the spring rate is variable as you can see. The "active coils" or high spring index turns can store more energy/give-up more rebound and are more flexible than the low index, inactive coils (fewer turns) which are stiff.

In practice, on a properly installed set of dampers, riding over small bumps work the active coils as the wheel rises and falls gradually (low force). Larger bumps compress the spring into the stiffer coils and prevent bottoming-out the suspension (high force).

Mounting shocks with the low-index spring end closest to the wheel typically provides for a rough ride on average.
Seems to me less coils means more springy because they can compress more with led resistance hence a softer ride. Tighter coils at the top work to progressively reduce larger impacts. Leaf springs have the smaller leaves on the bottom.

Please send me $.02 for my opinion.
^I'm referencing number of turns so folks know which end of the spring to visualize. That's all. A few factors go into the choice of windings and spacing to arrive at target frequency. Y'all can find this exciting info on the internet...
I remember a big stink a few years ago in the pro touring and autocross about which way a progressive coil over should be. Some company came out with a product and the old school thinking said it was upside down. The company posted testing where it frankly didn't matter which way the spring was turned as the weaker coils are still gonna compress first. I can't remember if it was the s10 or nova racing forum. I know the one group still would not accept it a spring will compress the same doesn't matter if its up, down, left, or right.
^Yes and all of that talk relates to a-arm or multiple-link suspension technology. Swingarm tech is a different set of dynamics.

Lets skip the theory and have some "rural" individual, north of 200# try it out and get back with the butt-o-meter results. Sadly, I live in a concrete jungle and my mass is wasting away...

And to capitalize on this near-meaningless thread, I'm on the hunt for 12" Sebacs (upside down or right side up versions)
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Think about riding over a small bump. What is the first few parts of the bike that react? The tire absorbs energy, the force exerts itself on the wheel which is attached to the swingarm which pivots and transfers which way, up or down? That motion vector is upwards bound through the more compressible section(s) of the spring until the energy "breaks through" to the next series of less-compressible windings. Since that motion is "sequential", hitting more compressible spring windings before reaching tightly-wound coils and eventually the frame is preferable in my mind...
I wouldn't think it would matter, the weaker part will compress before the stiffer part no matter what end they are on right?
Until steel develops a brain and decides it refuses to flex unless it's pointed in a certain direction. It won't matter if it's a 2 link rear suspension, a unequal a-arm front suspension, or a swing arm the weakest part will always give first.

Now on some coil over shocks some of them will not work upside down due to the valve design inside and whatnot but that has nothing to do with the spring on it.

Now with valve springs in an engine if you have progressive springs you generally install them with the tighter wound part towards the head. The reasoning behind this is the tighter wound part is more stable base and looser wound weighs the tiniest amount less so you have that tiny less moving mass. Some also have a slight smaller diameter on the looser side meaning thats the top (not talking about behive springs). Trying to get that .000009 hp it matters but the ones that are the same diameter will work "upside down".
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Until steel develops a brain and decides it refuses to flex unless it's pointed in a certain direction...
The spring doesn't react until it receives energy. The energy could come from the swingarm end or the seat end. Again, think of a SMALL bump, one that has such low energy that can be absorbed by the soft, springy coils of the shock ( in a properly positioned unit) before ever reaching the tighter non-compliant windings. Now think of an impact with so much energy as to overcome those upside-down tighter coils and then transfer that same energy up through the loose coils (effortlessly I might add) and then on to the frame.

I'm not about to do "bench testing" on my 40+ year old Sebacs but you can do a search here and find a few folks that have experimented and found they work better positioned properly. BTW, how's it going on that 424 of yours?

Contrary to all of this, perhaps the rationale for placing the spring the way they did back then was also to maintain tire-to-trail contact over a relatively even plane. Small rocks sending the wheel airborne repeatedly doesn't make for an efficient trip.
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red baron

Active Member
With all due respect and I'm stressing respect. I think you might be over thinking this a bit at least from a minibike stand point, I just don't see much thought going into a mini bike...well anything except maybe the motor design. Mini bikes are nothing but a toy competing with homemade mini bikes. I'm sure it was all cost driven and they surely sourced the springs from the motorcycle industry and progressive springs where what they had. If they were that concerned about performance wouldn't they have used hydraulic shocks? This is a great thread! This is thought provoking and I'm learning lots from you guys.
Well, you could be discussing and then doing the calculations for centerlines for a two speed clutch/jackshaft assembly, calculating sprocket teeth (and grinding your own), and then counting necessary inks. Now that would be a thread worth ignoring...this poster says tongue-in-cheek. Somebody's going to look for answers to this question someday, so it's a good brain buster.:laugh: :thumbsup:
Yeah, that would be lunatic-fringe. I just cut new chains for a Gilson/comet dual clutch last night. Call me nuts but I just lined-up new with old and made the punches. Neanderthal methodology...
^Horse before cart. While F=MA, Newton's 2nd law may not be specific as to which vector the F takes. However, Newton's 3rd law specifically states F in the opposite direction of contact. Ask yourself, which direction does the A term take on a mini bike impacting a bump. Think of the grade school science experiment where two kids hold each end of a rope. Kid on the right starts moving the rope end up and down. The oscillation moves away from him and travels towards the kid on the left. This is how motion behaves.