Dual-rate rear suspension springs, your experiences?

[John Orchard]

In my quest to know everything, what is the go with dual or triple rate  (or progressive) rear suspension springs?  I tend to not be in favour of them, my doubts are following ....

* Surely the suspension sags through the soft part of the spring, leaving too little travel to suck-up a decent hit?

* Aaaand, surely if you have enough rebound damping to conrtol the stiff main spring, then it will be overdamped for the soft spring, and vica-a-versa.

I feel that if you have the majority of your total travel available, then it will appropriately suck-up the small bumps as the light spring is 'said' to do.

I have just swapped springs to do some comparison testing at The Farm hopefully this Friday.  I'm interested in your thoughts on the subject?  I know many riders run dual-rate springs BUT have you compared to a straight-rate spring?

[evo550]

Assuming your talking about non linkage rear end, A progressive or dual rate spring is designed to give you spring rate for all conditions small spring for the small choppy bits, main spring for the big hits.
A straight rate spring doesn't give you that option, it's either stiff for the big hits, giving you a rough ride through the small stuff, or good for the small stuff but bottoms on the big hits....it's was a conundrum...until linkages came along at least.

[Tahitian_Red]

Kinda of strange, because I buy into the dual rate spring setup, but they lose me when they explain the triple rate springs.

 

CAPT. KIRK: "He lied. Everything Harry tells you is a lie. Remember that. Everything Harry tells you is a lie."
HARRY MUDD: "Listen to this carefully, Norman. I am lying."
NORMAN: "You say you are lying, but if everything you say is a lie then you are telling the truth, but you cannot tell the truth because everything you say is a lie. You lie. You tell the truth. But you cannot for. Illogical! Illogical! Please explain."
(Smoke comes out of Norman's head.)

[evo550]

...I think I have the perfect solution, why not run a soft spring on the left shock and a stiff spring on the right shock. Gotta work right?

[Ktm181]

Lots of pros and cons John.

Just gives a greater range of tuning, i have used both setups on both twin shocks bikes and single shock bikes with no linkage.

If setup correctly or as well as can be for the amount of time and dollars invested it can be better, if guessed at with no prior knowledge then the result can be worse or better depending on your luck.

Twin shock...seems to give it more of a "rising rate" so can offer better bottoming resistance (slowing it down before bottoming), it means your larger main spring can be softer offering better tracktion and better small bump absorption and still give a decent bottoming resistance, thats how i found it anyway, your looking for "progression" and "tuneability" to eliminate a problem or improve it anyway.

Trying a different setup is good, but trying a different known working setup is always better is it not?  What bike, what shock, what spring rate do you have on it now and what are you switching it too?  How much preload do you have on it now? How much preload should you have on a particular bike (twin shock i guess?)?

Kt.

[John Orchard]

Sorry Guy's, yes twin-shock.

I understand the simple thinking of a stiff spring for the big hits & soft spring the little bumps but it doesn't change my questions of the amount of damping for the changing spring-rates and the reduction in available travel with a sagged-out soft spring.

Plus another concern is the more radical changing steering geometry & weight transfer/distribution?

I think an important consideration might be the amount of travel of a soft spring until it coil-binds and the main spring can take over, I think you are right Ktm181; there are lots of varying settings to alter the end result.

With mono-shock linkage suspension, the rising rate is in the damping by the linkage speeding up the shocks piston speed for the same given axle travel.  This will keep the suspension supple at part travel on small bumps and give the increased damping/cushioning on the big hits.

I think the KTM PDS linkless rear suspension increases the damping through the shock travel internal of the shock, sounds like a good idea to me if the technicians get it right.

Getting back to my above query, on my last test of my KDX/KX I found that with enough rebound damping to control the main spring (or the stiffer part of the progressive spring), I found the rear suspension dead on the small bumps and deceleration on the braking bumps, the soft portion of the spring is soft enough for the small bumps but the damping prevented the suspension from being supple enough.

I truly feel that an 'anti bottoming' action (for the big hits & jump landings) should be controlled over the shortest distance possible, like an internal shock spring or rubber bump-stop.

I will test again and come back to you, I am never one to just follow trends, I have to determine it for myself.

[JohnnyO]

Sorry Guy's, yes twin-shock.

I understand the simple thinking of a stiff spring for the big hits & soft spring the little bumps but it doesn't change my questions of the amount of damping for the changing spring-rates and the reduction in available travel with a sagged-out soft spring.

A good pair of shocks has a progressive shim stack with more damping for the big hits and less for the smaller ones, both compression and rebound. So when the shock is rebounding on the main spring the high speed damping is in control and the low speed damping controls the slower rebound speed of the softer spring.
A properly sprung pair of twin shocks with dual springs should only sag the correct amount with rider seated then the harder main spring takes over on big hits and jumps. Similar to the rising rate linkage of modern bikes. If you've got too much sag then your spring rate is too soft.
Maico and Husky used dual springs on their Twinshock bikes in the late 70's early 80's and had the best suspension of the time.

[John Orchard]

I guess shock valving type is another aspect to add to the aquation.

But surely a "progressive stacked shim" comes into play with shock piston speed, the 'stacked shim' will blow-off at pre-selected sudden and/or high piston load/speeds.  A stacked-shim cannot relieve damping at low speed/loads and then be heavier damping at high speed/loads.  Although I guess if the a low-speed valving circuit could have limited passage relief to relieve low-speed restriction but it be negligable at high speed/loads?

But then to have variable valving based on high & low shock speeds, why would you need multiple springs?

If a different shim-stack could be utilized on the first half of the shock stroke when the light spring is in effect, and then another shim-stack in the second half of the shock stroke to cater for the heavy spring, then that would work.

I agree that certain older Euro bikes had the best suspension back in the 70's and they had dual-rate springs BUT with technology back then you only had to build a better valving mechanism and the suspension would be better no matter what springs you were using.

I think the variables that matter when it comes to using dual springs are .....

* Valving stack.
* Travel of the primary spring.

But in an old-school shock with just plain orrifice control and not stacked-shim things become even worse.

Now, high & low speed compression damping is just that 'compression damping', the rebound damping (for a given temperature) has only to be set to deal with the strength of the spring ..... but what spring?  A stacked-shim on rebound, is there really such a need on a non rising-rate (non linkage) suspension? Unlike compression that has to deal with various size & shape bumps, the rebound is always mid-speed (spring strength).  To put in a stacked-shim is to relieve damping on sudden loads (reduce hydraulic-lock), the opposite to what you need with dual springs (it would relieve damping for the strong spring and keeping it for the light spring!).  Which again gives me my problem, too much rebound damping for a light spring, causing the suspension to pack-down against the stiffer spring.

I think my brain is hurting.  Hopefully the penny-will-drop for me soon and all will be clear?  ;-)

[John Orchard]

Walter could you post-up a stacked-shim valving diagram?

[JohnnyO]

 The shim stack on a pair of 1980 ohlins is the same as the shim stack on a new pair of ohlins which is basically the same as the shim stack on a modern bike but smaller shims in the twin shocks, so basically the same technology.
Rebound needs a shim stack the same as compression because the piston speed varies, higher speed returning from a fully compressed shock spring compared to lower piston speed returning from a partially compressed shock. The shim stack has a range of shims from the small clamp shim to the biggest shim against the piston. Big shims open at low shock speed( small rolling bumps), smaller shims open at higher shock speeds(big square edged bumps and flat landings off big jumps).
Modern bikes have rising rate linkages to increase stiffnes as the suspension goes through its stroke and the old twin shockers use a dual rate or progressive spring set up to try and get a similar result.

[John Orchard]

But just as you say  "Rebound needs a shim stack the same as compression because the piston speed varies, higher speed returning from a fully compressed shock spring compared to lower piston speed returning from a partially compressed shock"  but a stacked-shim blows-off (flexes back from the orifice) with increased load/speed, so, again, my/the problem is, even with stacked-shim on rebound, too much rebound damping when controlled by the lighter spring ..... 

Sure the stacked-shim on rebound will relieve the hydraulic-lock effect when the main spring fires the shock back from bottomed but does nothing to relieve the damping effect when being pushed back by a primary spring.

Does what I am saying make sense?

[JohnnyO]

Not exactly sure what you mean but when rebounding on the smaller softer spring only the larger softer shims will open compared to rebounding on the big hard spring where the whole shim stack (stiffer shims) will open. It is possible to revalve the low speed rebound (lighter spring) to make that part of the rebound faster without affecting the action of the high speed rebound.
Does that help any?

[Nathan S]

I think you're either over-thinking it, or missing the bit about valving being shaft-speed dependant.

A properly set up shim stack will have more dampening at higher shaft speeds, which largely negates the problem you're worried about.

Back on the original question:
Dual rate springs work. They're not a magic bullet, but they help.
Many "progressive" springs are not actually progressive - the ones that actually work will have the softer/thinner coils of the spring go to coil bind part way through the shock's travel.

[Nathan S]

What does" progressive mean" and how does a progressive spring have to be constructed , Nathan ?

Non-linear rate = progressive.

A spring that has useful/noticable progression will put some of the coils into coil bind, mid stroke. For a single, progressive spring, this generally means a tapered wire that has the coils made from the thinner section closer together.

[John Orchard]

A wire coil that tightens-up is progressive, more coils means more wire length, which in-turn means, more wire to flex = softer.

Look at the stock coil springs on KX250A5 (progressive), same as the progressive springs I got from Walter, on the springs I have  they are 13 N/mm on the soft end and 18 N/mm on the firm end.