Anyone stuck an aftermarket shock in a DR650?

[Bass]

I'm still not clear on that. There is this weird(?) effect where if you are on the part of the curve with less slope, you potentially get larger amounts of movement for a given force. But that then means velocity is up, which means more damping = less travel . .

I know what you mean, but haven't you got it backwards? If you are on the flat part of the curve, (i.e. the shock is well extended,) then there is comparatively little shock movement for any given movement of the wheel and so less damping, not more.
It helps me to think of it (in as much as I can think at all) as a direct connection (without the linkage) but one where the bottom shock mount moves out along the swing arm towards the wheel as the shock compresses. At any point, it's a simple moment calculation then. If you had a linkage chart like the one you posted above you could do several calcs and plot your own curve for for any set up.

Fork springs are linear. Fork system is not; it has an air spring, this is what setting the oil level is all about. You are actually setting the size of the air chamber, effectively the rate of the air spring, which is very progressive (force per volume rises slowly at first, then increasingly rapidly - try it with a syringe). The air spring has little effect at full extension, starts to come in to play mid-stroke, and has a marked effect approaching full compression. It can be used to fine-tune bottoming resistance.

I thought about exactly that last night and I must have been tired as i concluded that the air spring is linear too. Of course it's not. It's asymptotic to zero volume.
However, that only affects the spring rate - the damping is still linear and so a damping adjustment should not be necessary with a preload change.

[warewolf]

I know what you mean, but haven't you got it backwards? If you are on the flat part of the curve, (i.e. the shock is well extended,) then there is comparatively little shock movement for any given movement of the wheel and so less damping, not more.

The thing that has me scratching my head though, is the force vs distance relationship. For a given force input, near the extended part of the travel you will expect a large distance change - even a small force since the curve is nearly flat. That same force input near the steep part of the curve will yield a smaller distance change. Is force the input? or distance? both? Suspension moves a lot more than we expect; something huge like 17,000 cycles during a lap of an MX track.

However, that only affects the spring rate - the damping is still linear and so a damping adjustment should not be necessary with a preload change.

Except that the damping is working against the spring... more preload, more stored energy, more rebound force. Thought we'd already covered this???

[Bass]

The thing that has me scratching my head though, is the force vs distance relationship. For a given force input, near the extended part of the travel you will expect a large distance change - even a small force since the curve is nearly flat. That same force input near the steep part of the curve will yield a smaller distance change. Is force the input? or distance? both? Suspension moves a lot more than we expect; something huge like 17,000 cycles during a lap of an MX track.:

Any of those inputs work fine. You could draw a whole family of curves which show them all simultaneously.

Except that the damping is working against the spring... more preload, more stored energy, more rebound force. Thought we'd already covered this???

We have covered it, but we are obviously still not in concert.
My point all along has been that (in the absense of a rate change linkage) increasing the preload changes only the ride height. There is no more stored energy, no extra rebound force and so no need for a damping adjustment.
We are obviously not going to sort this out on the net.
The next time I'm down your way, I will do a Ken MacIntosh on you. We will get out a spring, your drill press and your bathroom scales and i will prove it to you.
Make sure the Guinness is extra cold please.

[warewolf]

My point all along has been that (in the absense of a rate change linkage) increasing the preload changes only the ride height. There is no more stored energy, no extra rebound force and so no need for a damping adjustment.

Hang on; preload _IS_ the stored energy.

Didn't we agree that changing the preload changes the division of the spring force (which is constant at static sag) between the bike's weight and the preload mechanism? We agreed the bike's weight contribution is observed via the ride height. Since the total force on the spring hasn't changed, it's length has stayed constant, so the difference is the preload... stored energy. Compressing the spring further into its stroke -after applying preload- doesn't change that. It's still there and influencing the system.

Maybe try a static push test on a bike with easily adjusted preload. Watch the rebound speed between hi/lo preload.

I think what you are saying is that the total load on the spring is not changing -only the ride height, I agree- so once the spring is compressed further than the preload the shock must be acting the same regardless of the initial conditions. It's this disregard for the initial conditions that I'm challenging. Preload is the dividing line; it changes what forces are handled within the shock absorber system, and which are external. The initial conditions don't change when the spring is at mid-stroke/static sag. The differing internal components of the force affect the rebound damping. The difference between the internal/external forces is why we see different behaviour with different preload.

Gotta do some graphs for ya!

Mmmm... beer! Maybe you can demonstrate to Mr Ohlins, too?

[Bass]

One last attempt. Let's just relate this to forks for the moment because they don't have a rate change linkage complicating the issue

Hang on; preload _IS_ the stored energy.

Didn't we agree that changing the preload changes the division of the spring force (which is constant at static sag) between the bike's weight and the preload mechanism? We agreed the bike's weight contribution is observed via the ride height. Since the total force on the spring hasn't changed, it's length has stayed constant, so the difference is the preload... stored energy. Compressing the spring further into its stroke -after applying preload- doesn't change that. It's still there and influencing the system.

The preload is always there but all that it influences is the shock position where all the forces come to equilibrium, i.e. the sag - nothing else. There is no division of forces once the shock is off the stops. The spring will be as long as it needs to be to support the bikes weight - no more no less.

I think what you are saying is that the total load on the spring is not changing -only the ride height, I agree- so once the spring is compressed further than the preload the shock must be acting the same regardless of the initial conditions.

Correct. I am also saying that once the shock is off the stops, you can wind the preload up and down as you please and the shock will extend and retract accordingly, but the spring length will always be the same. The stored energy in the spring can be expressed as force x distance. The force required is determined by the bike's weight which doesn't change and so the force is constant. We now agree that the spring length won't change and so distance is always a constant too. Therfore force x distance is also constant as the preload is changed and therefore its contribution to the spring internal energy must be zero.

It's this disregard for the initial conditions that I'm challenging. Preload is the dividing line; it changes what forces are handled within the shock absorber system, and which are external. The initial conditions don't change when the spring is at mid-stroke/static sag. The differing internal components of the force affect the rebound damping. The difference between the internal/external forces is why we see different behaviour with different preload.

I will agree that in a rising rate linkage, the effective length of the lever that the spring uses to support the bike's weight changes with suspension postion and so the spring length will be position sensitive, even with a constant load. I will also stipulate that because the incremental movement of the shock changes with suspension position that the damping rates are also position sensitive. But without that linkage e.g. in the forks, none of that applies. The incremental movement of the forks is linear with wheel movement and so the damping rate does not depend on fork position.
Therefore, if the damping rate of the forks is not position sensitive and if the preload influences only position, then how can the damping rate be preload sensitive?

Mmmm... beer! Maybe you can demonstrate to Mr Ohlins, too?

I will drink to that. This would all be much more fun over a beer.

[warewolf]

The preload is always there but all that it influences is the shock position where all the forces come to equilibrium, i.e. the sag - nothing else. There is no division of forces once the shock is off the stops. The spring will be as long as it needs to be to support the bikes weight - no more no less.

Correct. I am also saying that once the shock is off the stops, you can wind the preload up and down as you please and the shock will extend and retract accordingly, but the spring length will always be the same. The stored energy in the spring can be expressed as force x distance. The force required is determined by the bike's weight which doesn't change and so the force is constant. We now agree that the spring length won't change and so distance is always a constant too. Therfore force x distance is also constant as the preload is changed and therefore its contribution to the spring internal energy must be zero.

Ah yes, I think you are right on that point... As soon as you exceed the preload force, and as long as you don't top/bottom out, the spring is supporting all the weight and preload doesn't matter. Topped out it does, but not once the suspension is off the stops. Right!

I will agree that in a rising rate linkage, the effective length of the lever that the spring uses to support the bike's weight changes with suspension postion and so the spring length will be position sensitive, even with a constant load. I will also stipulate that because the incremental movement of the shock changes with suspension position that the damping rates are also position sensitive. But without that linkage e.g. in the forks, none of that applies. The incremental movement of the forks is linear with wheel movement and so the damping rate does not depend on fork position.
Therefore, if the damping rate of the forks is not position sensitive and if the preload influences only position, then how can the damping rate be preload sensitive?

I'm not sure the rising rate has much to do with this point, it just amplifies the numbers at various positions.

Damping does not depend on position - it depends on velocity, and velocity is a result of the force, luke. At rest, there is no damping (unless you have one of the newfangled Ohlins shocks). But we are talking about preload's relationship to rebound damping, which is not a static condition but a dynamic one, ie with some velocity. I will run some numbers before I write more.

[Gizzit]

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I think the original question was something like ....... has anyone stuck an aftermarket shock on a DR650 ?????

[hondav2]

Yes and no, had the bike approx 8-9 years, so tears and ahppy, not sure which.

Down to one bike at present, Hypermotard has lost hardening on gear and sent stuff through the motor, Casbolts looking at it now but it is a full engine out and case split, total dismantlement .

Bugga thats no good , any reason y it happened ?????? now u really need a 1098 , would look good in yr lounge Cheers Toddy

[NordieBoy]

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I think the original question was something like ....... has anyone stuck an aftermarket shock on a DR650 ?????

in a DR650...

[Bass]

Ah yes, I think you are right on that point... As soon as you exceed the preload force, and as long as you don't top/bottom out, the spring is supporting all the weight and preload doesn't matter. Topped out it does, but not once the suspension is off the stops. Right!

Ace! So far, so good.

I'm not sure the rising rate has much to do with this point, it just amplifies the numbers at various positions.

Damping does not depend on position - it depends on velocity, and velocity is a result of the force, luke.

Agreed, spot on. That's why I used the term "incremental movement".
I apologise. I was thinking in differential calculus terms and should have said so.

At rest, there is no damping (unless you have one of the newfangled Ohlins shocks). But we are talking about preload's relationship to rebound damping, which is not a static condition but a dynamic one, ie with some velocity. I will run some numbers before I write more.

Can I suggest please, that when you first run some numbers, you leave the linkage out. Just deal with the forks to begin with.
What I was trying to say there (and probably doing it badly) was that if you impart some velocity to the suspension at the front wheel, then the damper will have that same velocity. Further, that this will be true regardless of where the suspension happens to be in its stroke or in other words that the damping rate is independant of stroke position and depends only on the velocity of the assembly.
None of that is true if you have a rate linkage in there.
Indeed, (working from memory), the linkage curve graph that you posted had a multiplier of about 0.5 at the extended end which increased to about 4.5 at the compressed end. That is a massive change and much more than I expected. I had thought maybe 2 or 3 to 1 as the max to min ratio. Instead, it's more like 8 or 9 to 1. That means the shock response is very greatly dependent on position within the stroke because the velocity depends on it.
IMHO, that is major complication and so is best discussed once we are in agreement on the simpler case.

[Bass]

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I think the original question was something like ....... has anyone stuck an aftermarket shock on a DR650 ?????

What's the matter?
Can't you recognise a total hijack when you see one?

[NordieBoy]

So what sort of oil should you use?

[warewolf]

So what sort of oil should you use?

The penis you have entered is too short. Please lengthen your penis to at least 10 characters.

[ducatijim]

I think I'll just walk, thanks.

[NordieBoy]

I think I'll just walk, thanks.

And with the right oil you won't be walking funny.