Anyone stuck an aftermarket shock in a DR650?

[NordieBoy]

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]

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]

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]

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]

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]

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]

[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]

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]

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 .

[Bass]

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]

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! 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.

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 ). 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 I'm going to see if I can't whip up some charts to show the effects.


All discussion is good, if it leads to greater understanding


PS Linkage chart for 640A:

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]

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]

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]

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]

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 .

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.