'05 FJR1300 OEM spring testing

We finally did some testing here at the shop today. The test subject was my own '05 FJR1300 rear shock. The rear OEM shock is an interesting design. It uses a very elegant and simple mechanism to alter rear spring rates. Many people probably think the hard/soft lever is a preload adjuster. It's not. It's actually a device that allows selection of two entirely different spring rates. Below is a more detailed description.

The shock is constructed so it uses a twin cable activated spring lock out mechanism to provide a soft or a hard setting. The mechanism itself is a cam and ramp type of unit similar to the normal cam and ramp type of preload adjuster seen on many motorcycle shocks. When a ramp is turned, it either activates or deactivates one of two springs used in this particular setup. There is a short spring and a longer spring. The short spring tested separately at close to 1400 lb/inch. The long spring tested separately at 670 lb/inch. When both springs are stacked on top of each other and tested, the result was a 440 lb/inch total spring rate. So, when the soft setting is activated with the lever, you are riding on 440 lb/inch spring. When the hard setting is activated, the short spring is "locked out" and only the long spring is used providing a 670 lb/inch rating. The amount of total spring compression (free length vs. installed length) measured during disassembly/reassembly was slightly less than 32mm. This last bit of information is probably invalid or not useful due to the excessive coil end gaps on the longer spring. Initial installed minimum spring preload is only useful information when the spring coil end gaps are zero.

We wanted to mount the shock in our SPA shock dyno but a special holder was necessary for the top shock mount since the bushings were so wide. We decided not to spend more time on this project since the shock is a simple unit without full adjustability. It has no facility/fitting for repressurization if it is gas pressurized. So, in our opinion, it's not a worthy unit to consider for rebuilding, revalving or servicing.

it's not a worthy unit to consider for rebuilding, revalving or servicing.

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Um, that sez it all.

So, can either spring be replaced?

1. I know Hyperpro do one progressive spring to replace both, but can the two spring set up be retained whilst changing either or both springs for better/stronger items?

2. Being a bit dim on the technical side, I don't understand how both springs together give a lower spring rate:

"The short spring tested separately at close to 1400 lb/inch. The long spring tested separately at 670 lb/inch. When both springs are stacked on top of each other and tested, the result was a 440 lb/inch total "

When force is applied to two springs in parallel, both give a little sharing the load.

Parallel spring calculations: Spring 1 is hence forth named S1; Spring 2 is now S2

Spring Rate = (S1 * S2) / (S1 + S2) -or- (1400 * 670) / (1400 + 670) = 453 Pretty close to the measured 440 in/lb

If you had two springs in series and both had the same spring rate you would get twice the compression distance but the Spring Rate would remain the same. If you had two springs of different Spring Rates first the weaker spring would compress until coil bind then the second spring would compress. The weak spring would compress easliy then the stiffer spring would compress upon larger force. This crudely describes how Progressive Wound springs work.

There are a lot of formulas that are common between electronics and mechanics, this parallel spring formula works for parallel resistors also.

So, can either spring be replaced?

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Because of the design of the lockout mechanism and the high rate of the short spring, I wouldn't recommend changing out the short spring. The mechanism has a nice advantage offering a soft and cushy soft setting for droning down a rough frost heave infested interstate.

If you wish to continue to use the OEM shock, you might be able to source a stiffer spring to replace the lower longer OEM item. The difficult thing will be finding one that has two ends matching ID to the OEM item. The upper large end is a little over 2.5" ID. The lower end on the OEM long spring is close to 2" ID. Or, a spring could be purchased with the same ID on each end that matches the upper larger ID. One would then have to fabricate/machine a new lower spring seat or collar that would be used to replace the OEM small ID collar/retainer.

1. I know Hyperpro do one progressive spring to replace both, but can the two spring set up be retained whilst changing either or both springs for better/stronger items?

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IF you can find a solution to the oddball end IDs, the fact remains the OEM shock valving is best suited for spring rates between 440 lb/inch and somewhere close to 700 and maybe even 800 lbs. With a spring in the 950-1000 lb/inch range, it would surely need to be revalved. You might be able to find a suspension specialist that would want to tackle the revalving project on the shock. It might be expensive and probably not worth all the trouble, downtime and expense especially for a shock that only has rebound damping adjustability with a soft/hard setting for spring rates. You might as well spend the bucks and get a quality shock instead. To give you an idea, Penske shocks normally sell for 60-75 percent of new cost when they are 2-3 years old or older. You get ready to sell the FJR, put the OEM shock back on and sell the Penske separately to recover a substantial portion of your money. You've had a great shock to use for 2-3 years or longer and the overall cost to you was far less than revalving/respringing a shock with little to no adjustment capabilities. And, that's with a shock that is truely rebuildable. So far, I'm not sure if any suspension expert would attempt to revalve the OEM FJR shock. Maybe there is, but it wouldn't be Traxxion Dynamics. I know we could do it, but how much R&D time do we need to spend on a shock to make it a worthwhile project? We know most owners would be far happier with a Penske.

2. Being a bit dim on the technical side, I don't understand how both springs together give a lower spring rate:

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If two springs with the same wire size were compared to each other but one spring had 8 coils in it and the other spring had 4 coils in it the one with the lesser number of coils would be stiffer. Or, if you had a spring with a certain rating at it's original length but cut part of it's length off, it would be stiffer than what the original length was.

With the FJR setup, the shorter spring has less than 2 coils in it and is approximately 3 inches long. The longer spring has 6 or so coils in it and is around 6-7 inches in length. Separately, the springs have different rates. Combined, they press against each other and have an overlapping effect and also simulate a longer spring. In other words, the short spring gives a little bit for each amount that the longer spring compresses. This softens the over all combined rate in the same way as having a longer spring with more coils in it.

I'm not a spring or mechanical engineer and I'm sure someone else out here can better explain this to you. Hopefully, I've not confused you even more.

Well, how do I reply to this?

Fantastic!

Thanks guys.

This is what "Technical Discussions" is all about.

Thanks again.