Rear shock preload

[silverback]

My rear shock went Kaput and the dealer paid for a replacement (used). Its sitting in the garage right now waiting on R&R.

So, being that the springs are notoriously weak for my build, is there a chance I could use a spring compressor and put a shim under the spring to up the preloaded before installation? Would this actually help in lieu of a higher spring constant? I guess I could calculate the shim thickness from the spring rate and how much extra "dead weight" the spring has to support...

I.e. if the spring is 708 lbf/in and it needs to support an extra 150 lbf, it would need 150/708 = 0.212". Sound reasonable? Maybe find a stack of appropriately sized fender washers that accumulate to 0.25" and call it good.

Based on the wage for an intern vs the cost of a $1200 ohlins and a $2 stack of washers, you can see my line of thought here...

Yay or nay?

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[El Toro Joe]

Why not just have the dealer put a heavier spring on the replacement shock?

[silverback]

Its a used shock from eBay and they drop shipped it to me. They are from out of town and it took some convincing to get them to buy the used shock to start with.

So, I guess I could have it re-sprung. Which is the soft spring? Long or short?

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[Intech]

I would be hesitant about putting a shim in. The set-able pre-load falls withing the compression range of the spring. I don't think you want to shorten that compression range, because then you will just bottom out sooner. That can cause stability problems too.

[Intech]

The upgraded shocks are spendy, but worth every penny.

[silverback]

The upgraded shocks are spendy, but worth every penny.

I'm sure they are...if you have the pennies. When I can throw that much at my bike, I will upgrade to a 2017 model AND all the suspension bits. Oh yeah.

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[silverback]

I would be hesitant about putting a shim in. The set-able pre-load falls withing the compression range of the spring. I don't think you want to shorten that compression range, because then you will just bottom out sooner. That can cause stability problems too.

I may be wrong, and often am, but I don't think the spring will bind before the suspension bottoms out. Just looking at it, the stroke of the shock is considerably shorter than the free space between the coils on the longer (assuming hard) part of the spring.

Also, from what I can see, there is no preload adjustment of the stock shock. Meaning, throwing the lever does not change the compressed length of the spring(s). It only locks out the shorter spring on the hard (?) setting. At least, that's what I get from looking at the operation of it while messing around with it.

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[Name Etc.]

hyper pro makes a spring for that shock... cheaper than a shock upgrade. removes the hard soft bits and gives a progressive spring instead. Mine was sprung for me luggage and a passenger... worked decent.

[silverback]

I put the used shock in last weekend, quite a difference. I was unable to shim the spring due to time constraints and finding a suitable spring compressor. However, with some time, it wouldn't be difficult. I read an old article on fjr1300.info about shimming the spring. Seems to have worked OK for some. Maybe I can find a hyperpro spring at some point.

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[raYzerman]

Another option, get a GenIII A shock for ~$425. It has a 976 lbs/in. spring and revised damping........

[FeeJeR Fred]

Another option, get a GenIII A shock for ~$425. It has a 976 lbs/in. spring and revised damping........

If that is true, that must be a lot stiffer. AFAIK the linkage geometry is essentially unchanged between FJR generations other than the swingarm being slightly longer on 2nd and 3rds vs. the 1st. And the spring is a spring is a spring regardless of what shock it is mounted on.

I initially had a 950 lb/in spring on my 1st gen's Penske and even with minimal preload it was just too stiff unless the bike was fully loaded with passenger and full bags, and then it was good. But solo riding lightly loaded, even with my 225 lb weight, that 950 was just too much spring. I've since gone down to an 850 which is good for me solo and with a little extra preload is sufficient for our two up days.

[silverback]

Another option, get a GenIII A shock for ~$425. It has a 976 lbs/in. spring and revised damping........

If that is true, that must be a lot stiffer. AFAIK the linkage geometry is essentially unchanged between FJR generations other than the swingarm being slightly longer on 2nd and 3rds vs. the 1st. And the spring is a spring is a spring regardless of what shock it is mounted on.

I initially had a 950 lb/in spring on my 1st gen's Penske and even with minimal preload it was just too stiff unless the bike was fully loaded with passenger and full bags, and then it was good. But solo riding lightly loaded, even with my 225 lb weight, that 950 was just too much spring. I've since gone down to an 850 which is good for me solo and with a little extra preload is sufficient for our two up days.

Actually, I am pretty well confused about the spring conversations I have seen. Mostly because of units.

The spring equation is F=ks, where F is force, k is the spring constant, and s is distance. So, s should be in force per distance. Admittedly, 976 lb/in MIGHT be a force per inch, but it seems the front springs are always referred to as kg/mm. This is not correct. But, if you divide by the gravitational constant you can get a force, as long as the vector is vertical...which the forks aren't???

But, if the rear spring is in pounds mass, the suspension would move in feet instead of a couple inches over a bump.

And, what's with progressive springs on a swing arm? A torque on a radial arm compressing a linear spring doesn't equate to a linear force per distance anyway, so why complicate that situation? I could maybe see progressive springs on a cruiser on the forks to soften the ride but prevent bottoming at extreme deflections, but for controlling conditions at the wheel when the bike is tipped over 60° and accelerating after the apex it doesn't make much sense. Of course, I am not a suspension engineer and you don't see me tuning race bikes. But I would surely like to learn these things! Really!

And...

How much of the elasticity of the frame is a factor in the corners? Can a bike chassis be built too stiff?

Lots to ponder...

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[Name Etc.]

Send a guy to engineering school and he over analyzes everything instead of just buying penske like everyone knows he should. Unless there is cash for Ohlins....

[silverback]

Well, I don't think they give away either for the lint I have in my pocket...

Maybe when people start replacing the Gen3 shocks, I can pick up a used one from eBay.

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[Name Etc.]

Maybe when people start replacing the Gen3 shocks, I can pick up a used one from eBay.

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I bet it is already being done!

[khahn]

Another option, get a GenIII A shock for ~$425. It has a 976 lbs/in. spring and revised damping........

If that is true, that must be a lot stiffer. AFAIK the linkage geometry is essentially unchanged between FJR generations other than the swingarm being slightly longer on 2nd and 3rds vs. the 1st. And the spring is a spring is a spring regardless of what shock it is mounted on.

I initially had a 950 lb/in spring on my 1st gen's Penske and even with minimal preload it was just too stiff unless the bike was fully loaded with passenger and full bags, and then it was good. But solo riding lightly loaded, even with my 225 lb weight, that 950 was just too much spring. I've since gone down to an 850 which is good for me solo and with a little extra preload is sufficient for our two up days.

Actually, I am pretty well confused about the spring conversations I have seen. Mostly because of units.

The spring equation is F=ks, where F is force, k is the spring constant, and s is distance. So, s should be in force per distance. Admittedly, 976 lb/in MIGHT be a force per inch, but it seems the front springs are always referred to as kg/mm. This is not correct. But, if you divide by the gravitational constant you can get a force, as long as the vector is vertical...which the forks aren't???

But, if the rear spring is in pounds mass, the suspension would move in feet instead of a couple inches over a bump.
Don't forget the angle of the dangle.
And, what's with progressive springs on a swing arm? A torque on a radial arm compressing a linear spring doesn't equate to a linear force per distance anyway, so why complicate that situation? I could maybe see progressive springs on a cruiser on the forks to soften the ride but prevent bottoming at extreme deflections, but for controlling conditions at the wheel when the bike is tipped over 60° and accelerating after the apex it doesn't make much sense. Of course, I am not a suspension engineer and you don't see me tuning race bikes. But I would surely like to learn these things! Really!

And...

How much of the elasticity of the frame is a factor in the corners? Can a bike chassis be built too stiff?

Lots to ponder...

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Don't forget the angle of the dangle.

[raYzerman]

I don't have the exact formula, but if you go to RaceTech.com site and do the spring rate calculator, front and rear spring rates are listed. For the rear springs, the last three digits are the lbs/in. equivalents......
So, 976 lbs/in is 17.4 kg./mm..... for a 200 lb rider without gear, RaceTech recommends 18.5 kg/mm or ~1050 lbs/in. It is the opinion of several that RaceTech's rear spring suggestions are a bit on the harsh side..... and my RaceTech rear shock was, as I had to back off their preset preload a fair bit. I'd say 976 sounds pretty good for a 200 pounder.......

GenII stock is 708 lbs/in or ~12.6 kg/mm.........

[FeeJeR Fred]

I don't have the exact formula, but if you go to RaceTech.com site and do the spring rate calculator, front and rear spring rates are listed. For the rear springs, the last three digits are the lbs/in. equivalents......
So, 976 lbs/in is 17.4 kg./mm..... for a 200 lb rider without gear, RaceTech recommends 18.5 kg/mm or ~1050 lbs/in. It is the opinion of several that RaceTech's rear spring suggestions are a bit on the harsh side..... and my RaceTech rear shock was, as I had to back off their preset preload a fair bit. I'd say 976 sounds pretty good for a 200 pounder.......

GenII stock is 708 lbs/in or ~12.6 kg/mm.........

Yeah, for whatever reason, they generally spec fork springs in kg/mm and spec rear springs in lbs/in. but those are the spring's deflection characteristics. At the front end it is a fairly simple correlation since the spring is being used to directly suspend the front of the bike, albeit at a slight forward inclination.

On the other hand, the rear suspension's effective spring rates get a bit more complicated to figure out because the spring on the shock gets weight applied to it through two levers: The swingarm which is more than a 3.5:1 lever (21": 6" on my 1st gen) every inch of wheel travel only generates ~1/3" of dogbone motion. (the length of the dog bones do not matter in the spring rate since they merely couple the swingarm to the relay arm)

The second lever is the relay arm which is roughly a 1:2 lever, where every 1/3" of dogbone travel is amplified to 2/3 inch of shock travel. So the overall mechanical advantage is 3.5/2 or roughly 1.75:1, meaning for every 1.75" of wheel travel the shock will compress 1", or the inverse, for every inch of rear wheel travel the shock will only compress 0.57".

These ratios should also apply to the effective spring rate felt at the axle, so using round numbers an 800 lb/in spring on the shock will have an effective springing of ~ 456 lbs/in travel at the rear axle.

Remember that each wheel supports roughly half the total weight, biased somewhat towards the rear, so a first gen FJR (604 lbs wet) plus a 235 lb rider with riding gear we are talking about 839 lbs total or about 425 lbs static load on the rear axle.

Add another 150 lbs for a "nominally sized passenger and another 50 lbs total for full loaded luggage making the grand total weight is a bit over a half ton (1039 lbs) BTW - the GVWR of the bike is 1049 lbs. And most of that added 200 lb load will be over the rear wheel, you'd be looking at around 600 lbs on the rear axle or the need to have about an 1050 lb/in spring to get the same effective springing.

[silverback]

My question is how much force is a kilogram? Spring rates are based off of force.

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