Anyone stuck an aftermarket shock in a DR650? [Archive]

Monstaman

10th February 2010, 07:16

Yep. I have and it's a common farkle as the standard rear really is underdamped in rebound - especially with a load on (e.g. luggage)
Ohlins front and rear in my case.
Seem to recall that the DR650 thread in the Aussie forum on ADVrider has buckets of info, although it's a huge thread and you may have to spend some time to find what you need.

Yes already waded through about 200 pages but nothing specific for what I wonna know unfortunately, some great info but no what I am after.

First try the cheaper way of filling the shock with 10w oil and recharging the reservoir to 150psi.
Cogent in the states also refurbish your old shock with adjustable guts.

Cheaper sounds better, how hard is it to do Nordie?, can Wanaka DR Engineering (my shed) do it or is is a Robert job?

Squiggles

10th February 2010, 09:47

Some put DRZ shocks in....

I'd like an ohlins plz

Monstaman

10th February 2010, 10:09

Sent an e to Gogent in the US, see what they say.

Having sold my MHe I have permission to 'upgrade' a few bits and pieces.

NordieBoy

10th February 2010, 15:50

Cheaper sounds better, how hard is it to do Nordie?, can Wanaka DR Engineering (my shed) do it or is is a Robert job?

I did mine.

And it still works.

NordieBoy

10th February 2010, 15:55

Sent an e to Gogent in the US, see what they say.

Having sold my MHe I have permission to 'upgrade' a few bits and pieces.

Cogent (Rick) will need your shock so that limits things...

But...
You could send him my spare shock and use yours in the meantime and gimme your old one when your new one arrives...

Or gimme your new one and keep using your old one...

zeRax

11th February 2010, 06:04

lol fran.

id probably go with a drz400e shock, cheap option, id check out the thumpertalk forums, lots of good seperate specific stuff in the dr forum there, no doubt find something, i thought about it, but im happy enough with the suspension, after all i weigh as much as a bag of peas

Monstaman

11th February 2010, 07:05

I did mine.

And it still works.

Do instructions come with this?, I am pretty good in my man space with all sorts of engineering goodies and oddities and knowledge from being an engine re conditioner and landrover owner (x5 over the years).

Cogent (Rick) will need your shock so that limits things...

But...
You could send him my spare shock and use yours in the meantime and gimme your old one when your new one arrives...

Or gimme your new one and keep using your old one...

I may as well just pay you direct and save myself all the bollies with NZ post :p

Will see what the e says from Cogent and if I choose that path I might take you up on that offer.

lol fran.

id probably go with a drz400e shock, cheap option, id check out the thumpertalk forums, lots of good seperate specific stuff in the dr forum there, no doubt find something, i thought about it, but im happy enough with the suspension, after all i weigh as much as a bag of peas

Another option which is cool, I am 74 kg so spring rate is pretty well ok for me, just need the rebound to act more sensibly, we are away to Mavoras next week with Toddy and we are two up so that will be interesting, already screwed the spring preload to near max which has worsened the rebound :eek5:

Bass

11th February 2010, 08:49

First try the cheaper way of filling the shock with 10w oil and recharging the reservoir to 150psi.
Cogent in the states also refurbish your old shock with adjustable guts.
I am interested in the theory behind this.
What viscosity is the standard oil in the shock and what is the standard reservoir pressure setting, please?
I have a near new standard shock in a corner downstairs and adjusting the viscosity to get the desired rebound damping (not externally adjustable) and then setting compression accordingly seems like a really good idea for starters.

I also note that MM has commented that increasing the preload has worsened the rebound and i am wondering just what he means by this.
I presume he means that he has a passenger on the back and has increased the preload to keep the ride height in the correct place. That would mean that for any given shock position, the spring compression load has increased and so the rebound damping effect has diminished - i.e. the normal situation when adding a passenger.
However, I was wondering about the effect on damping of increasing the preload without adding a passenger. Let me see if I can explain what I am getting at.

The DR has a rising rate linkage and so the further the shock is extended then the less is the wheel movement relative to shock movement.
Under compression, when we hit a bump, the terrain has most control of the rate at which the the wheel rises and the shock compresses. Conversely, it is the spring/shock combination that has control of how quickly it extends again after the bump. Therefore, since increasing the preload with no extra weight on the bike increases the ride height, because of the variable rate linkage it should also decrease the effect of compression damping but increase the effect of rebound damping.

This of course completely ignores the effect on overall ride and the risks of the shock "topping out". However, I was wondering whether it might be a way to gain some small improvement in a setup like the DR which is chronically underdamped in rebound - i.e. ignore the normal setup guidelines and set the ride height a bit higher than would otherwise be the case.

My gut feel is that the improvement would be too small to be noticeable.

Monstaman

11th February 2010, 11:15

I also note that MM has commented that increasing the preload has worsened the rebound and i am wondering just what he means by this.

Heya Bass

All this means is that my damping is screwed right in to max and tightening the spring preload for MM and Mrs MM for two up at Mavoras has made the shock rebound to fast and allowing it to top out.

Saving grace is that two up we won't be going warp facter 2 anyway.

Keen on Nordies idea thos, appeals to me.

warewolf

11th February 2010, 13:06

I also note that MM has commented that increasing the preload has worsened the rebound and i am wondering just what he means by this.In simple terms, with more spring preload, the spring is exerting more force when rebounding so the rebound damping effect is reduced. Even without any extra load, rebound will be worse with increased preload.

Say you had an 8.0kg/mm spring. Normal preload might be 10mm = 8.0 x 10 = 80kg spring force at rest. Increase the preload to 15mm = 8.0 x 15 = 120kg force at rest. On rebound the spring is pushing an extra 50% or 40kgf against the rebound damping at top-out, and increasingly more as the suspension moves through the stroke due to the exponential curve of the rising rate.

The rising rate curve is fairly flat in the initial parts of travel, particularly around the 10% travel position for ideal static sag, even as far along as the 33% rider sag. So at rest the effects would probably be negligible. However, it's the effects a long way in to the travel that could be the make-or-break. I can't come up with a clear answer - maybe you should ask the question in the suspension forum and get an answer from RT?

Bass

11th February 2010, 14:36

In simple terms, with more spring preload, the spring is exerting more force when rebounding so the rebound damping effect is reduced. Even without any extra load, rebound will be worse with increased preload.

Sorry but this is incorrect and this is an argument (discussion) that I have had many times.
"Preload" is a complete misnomer. It does not change the load at all - it just changes the distance between the top of the spring and the top shock mount.
The swingarm and so the bottom of the shock also, are free to move. Therefore, all that happens when you increase the preload setting is that the swingarm moves down and the ride height increases. This is appropriate if you are putting more load on the bike, which then brings your ride height (rider sag) back to the original setting.
However, with no change in weight on the bike the spring force and so also the spring length according to Hookes law, are unchanged.
You have to change the spring length to change the load that it is exerting, but how can you change the length when the bottom end is free to move?
Take a tape measure out and check this if you doubt what I am saying.

I agree with your comments about the linkage curve and its effects however. I didn't know the curve shape and so this is good information.

NordieBoy

11th February 2010, 15:06

You have to change the spring length to change the load that it is exerting, but how can you change the length when the bottom end is free to move?

Because the bottom of the shock is a set length from the top. It's only free to move to that point.

WARNING!
Waffling and thinking out loud follows...

5kg/mm spring.

5kg pressure to move 1st mm.
10Kg = 2mm.
15kg = 3mm.
50kg = 10mm.

Preloading the spring by compressing it say 10mm means it's putting 50kg of force into trying to expand.

Preloading 1mm means 5kg of expansion force.

The preload changes the startpoint.

Sit 90kg on the spring and both will compress to the same length (slack-18mm).

Remove the weight (over a bump) and the 1mm preloaded spring will push back with a force of 90kg diminishing to 5kg.
The 10mm preloaded spring will push back with 90kg diminishing to 50kg.

NordieBoy

11th February 2010, 15:10

I am interested in the theory behind this.
What viscosity is the standard oil in the shock and what is the standard reservoir pressure setting, please?
I have a near new standard shock in a corner downstairs and adjusting the viscosity to get the desired rebound damping (not externally adjustable) and then setting compression accordingly seems like a really good idea for starters.

5w and about 150psi standard.

Especially with a heavier spring going to 10w brings the compression damping back to the middle of the adjuster and rebound damping isn't overpowered by the spring anymore.

I don't get it bottoming out anymore and it still behaves over the smaller bumps/rocks.
Before with 5w and heavier springs it'd bottom out far more easily and pogo too much.

Standard spring and I'd try a 7.5w mix first as 10w may make it a bit overdamped.

Of course this is nowhere near as good as a properly adjusted shim stack or gold valve.

But it's nowhere near the price either.

Bass

11th February 2010, 15:24

Because the bottom of the shock is a set length from the top. It's only free to move to that point.

Preloading the spring by compressing it say 10mm means it's putting 50kg of force into trying to expand.

=.

See mate, this is where you have got it wrong too.
Assuming the shock is NOT fully extended, then increasing the preload does NOT compress the spring - it just increases the ride height. Get out in the shed with a spanner and a tape measure if you don't believe me.

You have most of the information that you need to understand what I am saying.
You understand that spring load and spring length are proportional to one another.
Assume that the shock is in mid-stroke somewhere so that the rear suspension is free to move. If you don't change the load on the bike then you don't change the load on the spring and so its length must stay the same. So if you move the top of the spring away from the top shock mount (increase the preload (I hate that word) setting), you also move the bottom of the spring away from the top mount by the same distance i.e. the shock extends and the ride height increases.
The spring length stays the same.

In a static situation, the only thing that compresses the spring is adding more weight to the bike.

Preload is all about keeping the ride height correct with varying loads on the bike.

If the shock is fully extended, then a mechanical stop cones into play, in which case, all bets are off.

NordieBoy

11th February 2010, 15:43

If the shock is fully extended, then a mechanical stop cones into play, in which case, all bets are off.

Ah. That was where I was basing my thoughts.

Sooo...
For sag, 5mm more preload (spit) lengthens the shock 5mm and depending on the linkage raises the sag a proportional amount.
Makes sense as long as bump stops are not involved.

On my DR (7.6kg Eibach) 1 turn of the "adjustable top spring retainer" is 3mm of static sag.

Bass

11th February 2010, 15:57

Ah. That was where I was basing my thoughts.

Sooo...
For sag, 5mm more preload (spit) lengthens the shock 5mm and depending on the linkage raises the sag a proportional amount.
Makes sense as long as bump stops are not involved.

On my DR (7.6kg Eibach) 1 turn of the "adjustable top spring retainer" is 3mm of static sag.

Bingo!!!!!!!!!!!!!

ATSR - I like that

I know I harp on about this, but so often I see people trying to stiffen up their suspension by adding preload and as you now understand - it doesn't. In fact with a rising rate linkage, extending the shock actually reduces the compression damping effect and so softens the suspension.

warewolf

11th February 2010, 16:53

Sorry but this is incorrect and this is an argument (discussion) that I have had many times.
"Preload" is a complete misnomer. It does not change the load at all - it just changes the distance between the top of the spring and the top shock mount.Ummm, yes it is correct! :) I think we are talking in circles a bit, but here's why it is correct to call it preload; heck it's not even complex nor counterintuitive.

Question: how are you going to change the distance between the top of the spring and the top of the shock, if not by compressing the spring?? That compression takes some force or load, which we call preload. Hookes law is great for free springs... which we don't have... because the preload mechanism stops the spring expanding to its free length.

Okay... remove shock from bike - stop thinking about ride heights, linkages, sag, mid-stroke yada yada.

Spring on my 640A can be removed by hand when preload collars backed off all the way. Spring is only just free, has some free length measure between spring mounting base and upper contact point with preload collars. Spring rate is 7.0N/mm say 0.7kg/mm. Wind the preload collars 10mm down the threads, thereby increasing the distance between the top of the shock mount and the top of the spring, and reducing the distance between the top of the spring and the base of the spring, but not changing the overall eye-to-eye length of the shock... whew!... the spring at a fixed rate of 0.7kg/mm is now 10mm shorter... no longer free... and it takes 10mm x 0.7kg/mm = 7.0kg force to get there. We say the spring is pre-loaded by 10mm or 7.0kg.

Stick it in the bike, and when the suspension is topped out, the spring will have no load on it apart from the PRE-load.

Now, once the bike's (and optionally rider's) weight is applied to the spring, via the linkage, then it gets more complicated. Force vs length calculations have to factor in the preload. Specifically, the spring will not compress further until the applied force exceeds the preload.

Something to think about: the force on the spring (as you say, constant at static sag) is divided between the bike's weight, and the preload. As you add more preload, less of the force on the spring is a result of the bike's weight, so the ride height increases. At top out, there is 0kg on the spring from the bike's weight - all the pre-existing load comes from the preload.

More later..

Bass

11th February 2010, 17:06

Ummm, yes it is correct! :) I think we are talking in circles a bit, but here's why it is correct to call it preload; heck it's not even complex nor counterintuitive.

Question: how are you going to change the distance between the top of the spring and the top of the shock, if not by compressing the spring?? That compression takes some force or load, which we call preload. Hookes law is great for free springs... which we don't have... because the preload mechanism stops the spring expanding to its free length.

..

Sorry Colin, but I still disagree with SOME of what you say
As soon as you take the shock off the bike it fully extends. You therefore have a mechanical stop in play which you do your very damndest to avoid when setting it up for normal use. All your settings and adjustments etc are for mid stroke and they all go away when you top out or bottom out the shock.
Everything I have been saying is predicated on this.

When the bike is on its wheels and so the shock is in mid stroke somewhere (i.e. Hookes Law DOES apply), if you increase the distance between the top mount and the top of the spring, the shock extends, the ride height increases but the spring length stays constant. It's easily tested! I'll put a Guiness or 6 on it for the first time that we meet.

I don't know how to put it any simpler than that.

(Are there 2 n's in Guinness?)

Fran has got it - perhaps he can help.

Your last 2 paragraphs are almost correct. At full extension, any residual load on the spring is indeed due to whatever the preload adjustment is and the bike has to exert at least that load to get the shock to move at all. I think we are close but to understand how the suspension really works, the distinction I am trying to draw is an important one.

I guess the best thing I can say here is to forget / ignore / disregard anything that is affected by the shock either being fully extended or fully compressed.
This is all about movement under control of springs and damping. Considering mechanical stops is not helpful.

IMHO, the key to this is that the whole purpose of preload is to get the correct ride height with changing loads on the machine. It is a compromise - the only pure way of compensating is to fit a different rate spring. Ideally, of course, with every load change the damping rates also should change.
I am sure you can see us all changing the spring every time we put some luggage or a passenger aboard - hence preload and adjustable dampers.

Remember that what I am disputing is your claim that increasing preload will worsen an already inadequate rebound response with no load change on the bike. I contend that (rising rate linkage aside) it will have no effect other than increasing the ride height, because in mid stroke it doesn't actually affect the spring load at all.

hondav2

11th February 2010, 18:09

[QUOTE=Monstaman;

Having sold my MHe I have permission to 'upgrade' a few bits and pieces.[/QUOTE]
U will be happy the the MHe has gone, I can slot u into a mint 1098 if yr interested

NordieBoy

11th February 2010, 18:40

Sorry Colin, but I still disagree with SOME of what you say
As soon as you take the shock off the bike it fully extends. You therefore have a mechanical stop in play which you do your very damndest to avoid when setting it up for normal use. All your settings and adjustments etc are for mid stroke and they all go away when you top out or bottom out the shock.
Everything I have been saying is predicated on this.

When the bike is on its wheels and so the shock is in mid stroke somewhere (i.e. Hookes Law DOES apply), if you increase the distance between the top mount and the top of the spring, the shock extends, the ride height increases but the spring length stays constant. It's easily tested! I'll put a Guiness or 6 on it for the first time that we meet.

I don't know how to put it any simpler than that.

(Are there 2 n's in Guinness?)

Fran has got it - perhaps he can help.

Your last 2 paragraphs are almost correct. At full extension, any residual load on the spring is indeed due to whatever the preload adjustment is and the bike has to exert at least that load to get the shock to move at all. I think we are close but to understand how the suspension really works, the distinction I am trying to draw is an important one.

I guess the best thing I can say here is to forget / ignore / disregard anything that is affected by the shock either being fully extended or fully compressed.
This is all about movement under control of springs and damping. Considering mechanical stops is not helpful.

IMHO, the key to this is that the whole purpose of preload is to get the correct ride height with changing loads on the machine. It is a compromise - the only pure way of compensating is to fit a different rate spring. Ideally, of course, with every load change the damping rates also should change.
I am sure you can see us all changing the spring every time we put some luggage or a passenger aboard - hence preload and adjustable dampers.

Remember that what I am disputing is your claim that increasing preload will worsen an already inadequate rebound response with no load change on the bike. I contend that (rising rate linkage aside) it will have no effect other than increasing the ride height, because in mid stroke it doesn't actually affect the spring load at all.

Yes.

There are 2 n's.

This is almost as bad as the one we did about shortening springs and the effect it'd have...

Monstaman

11th February 2010, 18:43

U will be happy the the MHe has gone, I can slot u into a mint 1098 if yr interested

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 .:angry2:

Bass

11th February 2010, 18:54

Yes.

There are 2 n's.

This is almost as bad as the one we did about shortening springs and the effect it'd have...

I was wrong

He can't help

warewolf

11th February 2010, 19:49

As soon as you take the shock off the bike it fully extends. You therefore have a mechanical stop in play which you do your very damndest to avoid when setting it up for normal use. All your settings and adjustments etc are for mid stroke and they all go away when you top out or bottom out the shock.
Everything I have been saying is predicated on this.

When the bike is on its wheels and so the shock is in mid stroke somewhere (i.e. Hookes Law DOES apply), if you increase the distance between the top mount and the top of the spring, the shock extends, the ride height increases but the spring length stays constant. It's easily tested! I'll put a Guiness or 6 on it for the first time that we meet.Yes, you are right about Hookes Law in that case as it is not mechanically stopped. I'm not at all doubting what you are saying about the observed effects of changing preload while the shock is mid-stroke - perhaps your understanding of how & why?

Yes, you are setting up the shock for mid-stroke action; you don't want it to top out or bottom (hard) while in use. However the fully extended condition (ie preload) does not "go away" when the shock is compressed; quite the contrary, it is the defining "initial conditions" and an influencing factor on mid-stroke (and either end) characteristics - which you keep pointing out :) - and thus the reason we are having this discussion! :D If it did go away, then it would have no effect on the ride... and shocks would not have a pointless adjuster built-in.

Remember that what I am disputing is your claim that increasing preload will worsen an already inadequate rebound response with no load change on the bike. I contend that (rising rate linkage aside) it will have no effect other than increasing the ride height, because in mid stroke it doesn't actually affect the spring load at all.Mid-stroke at static load it doesn't. Move the suspension, and it will, particularly when the shock rebounds past the rider sag position and gets close to top-out, ie at the point that wouldn't be reached with different preload. Possibly, eliminating the rising rate is key to the difference of opinion... maybe. :scratch:

The thing is, the linkage curve is exponential, approaching horizontal at top-out, and approaching vertical at bottom-out (except lowering links which are much flatter :angry2:). Some of the subtle configuration changes can have dramatic results because it is a complex non-linear system. A couple of further complications is that the spring is position sensitive, the damping is velocity sensitive, and the damping has a delayed effect, too - must be some movement before you get any damping at all.

Aside from any theories; in every shock/forks I've had with effective adjustment, increasing the preload has necessitated an increase in rebound damping to compensate. And I'm pretty sure that this factor is noted in many suspension tuning guidelines. Go find a bike with easily-adjusted (eg hydraulic) preload, try hi/low preload with hi/low rebound damping :scooter: I'm going to see if I can't whip up some charts to show the effects. :crazy:

:drinknsin All discussion is good, if it leads to greater understanding ;)

PS Linkage chart for 640A:
<img src="http://i116.photobucket.com/albums/o20/warewolf885/mc/bikes/ktm/lc4/loweringLinkLeverage.jpg" />
Bottom scale shows axle position versus swingarm horiztonal; ie it swings up 150mm and down 150mm for 300mm travel. Left graph shows the 2" lowering link starting at -100 not -150, and bottoming force reduced from >1900lb to <1000lb. The green marks bracket the ideal preload ranges. Right graph shows the lowering link's lack of rising rate.

warewolf

11th February 2010, 19:56

Ahem.

NOTE!
Ensure that the springs are properly pre-loaded before attempting to make any adjustments. A simple rule is that increased pre-load of the spring should be followed by an increase of rebound damping by two steps.

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

warewolf

11th February 2010, 20:38

Also, your pet peeve (and mine):

NOTE!
The spring pre-load affects the ride height, it does not affect the spring stiffness. Therefore, on models with a linkage to the shock absorber, the suspension may actually feel harder when you reduce the pre-load and the shock absorber gets into the harder range of the link system.

Bass

11th February 2010, 22:11

This is all good stuff and you are filling in some gaps in my knowledge. Specifically, the change in rate shown on the linkage curve you posted, is much more pronounced than I expected i.e the linkages are more effective. In fact it is so much so that I strongly doubt that its effect can be neglected for any truly meaningful discussion. So yes, such a linkage will have a pronounced effect on the damping rates which will change quite markedly with shock position. However the change will be much more pronounced around the compressed end of the travel rather than around the extended end where we are making changes.
Even so, that change in rate is is entirely capable of explaining Mr Ohlin's comments and your experiences.
I am also intrigued by him suggesting that you give it another couple of clicks regardless of how much the preload was increased. Sort of "suck it and see", eh?

I also have to admit to one fuck up. I said that the rebound rate would improve as the shock extends. It is of course the other way round and a rising rate linkage means that both rebound and compression damping increase with shock compression.

However, what I said about the preload not compressing the spring remains true, so do I get my Guinness (with 2 n's)

Strictly speaking of course, its only true if the effect of the linkage is neglected, but as you pointed out, we are working close to the fully extended end of the curve and it's pretty flat down there.
So any change in spring length is likely to be so small as to be unmeasurable.

I also note that your Mr Ohlins said that the preload affected the ride height not spring stiffness......................

Lastly, I can understand this at the back end where there is a linkage but it still defies reason for the forks which are linear (unless you are about to fill another gap in my knowledge)

warewolf

11th February 2010, 23:56

I am also intrigued by him suggesting that you give it another couple of clicks regardless of how much the preload was increased. Sort of "suck it and see", eh?Yes, but they are guidelines... starting points... you must fine-tune from there.

I also have to admit to one fuck up. I said that the rebound rate would improve as the shock extends. It is of course the other way round and a rising rate linkage means that both rebound and compression damping increase with shock compression.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 :scratch:.

However, what I said about the preload not compressing the spring remains true, so do I get my Guinness (with 2 n's)Yes. I'll have another while you're up.

Lastly, I can understand this at the back end where there is a linkage but it still defies reason for the forks which are linear (unless you are about to fill another gap in my knowledge)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.

Bass

12th February 2010, 06:34

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 :scratch:. .

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

12th February 2010, 07:29

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??? :crazy:

Bass

12th February 2010, 07:50

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??? :crazy:
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

12th February 2010, 09:02

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

12th February 2010, 09:51

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

12th February 2010, 15:16

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

12th February 2010, 17:42

......................................... ......................................... .....................................

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

hondav2

12th February 2010, 19:43

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 .:angry2:

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

NordieBoy

12th February 2010, 19:44

Bass

12th February 2010, 21:52

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.