Rear shocks

asphalt900 wrote:

Might also check the fully compressed length of those shock you'd purchased too. The damper rod controls the "static/resting" height and then spring rate controls "sag", usually around 20ish percent when sitting on the bike. The damper assy will bottom out before coil bind or should. So if as suggested changing springs around could create this unwanted situation. If "whatever" shock you use check that the tire doesn't rub on the inner/rear fender or the metal frame bit where these fenders meet. Seems like a lot of unnecessary work though, unless you like to fiddle!

Clay I’m going to disagree with you on this one, the spring controls the (static or otherwise) ride height hence “height adjustable spring platforms” the stop on the damper rod simply limits travel when the wheel is not in contact with the ground (as soon as the bike's weight is on the suspension there is a degree of compression at which point the stops on the damper rods are no longer in contact with anything). Changing the spring will not effect clearance as the bump stop limits travel - fair point ref spring binding if you went crazy short on the spring but I believe this guy is only looking for a mild “drop”

Damper rod dictates seat height.  Thats why when you lower suspension its done internally, not with the spring. Those "spring adjusters" only change the preload, not the height or the spring rate.

OP could have just bought a set of those lowering brackets for the lower shock mount.

Kidkawie wrote:

Damper rod dictates seat height.  Thats why when you lower suspension its done internally, not with the spring. Those "spring adjusters" only change the preload, not the height or the spring rate.

OP could have just bought a set of those lowering brackets for the lower shock mount.

So are you saying that when the weight of the bike is placed on the suspension (without the rider) there is zero compression (sag) of the suspension (so fork length and wheel to rear fender clearance is the same when the bike is suspended wheel free and when it's sitting unsupported on the wheels) so therefore even when loaded the damper rod limiter is hard against the stop ?? It is my understanding you could achieve zero static sag by winding up the "pre-load" adjuster until the stored energy in the spring was great enough to overcome "sag" so you could in effect "raise" the seat height this way but not lower it further than is determined by static sag. See the below link from Maxton
Microsoft Word - 6.STATIC SAG.doc (maxtonsuspension.co.uk)

Spring preload will change the static sag.  Changing the spring will alter the rider sag.  Doesnt matter what preload you have, the shock will compress the same with the same spring. That is why you shorten shocks internally. 
I've been modding both vintage bike and modern MX suspension for over 10 years.

Thinking we're all saying the same thing. I do quite a few of the Kayaba and Mulholland shocks on the Z1's to the single/duel spring 1977-1980 LTD and Z1R's. Yes!! Find some lowering blocks would save time if you could find some vintage ones. I never cared for them because they bite into the swingarm and would "clank" over bumps. And "sag" is an element of damper operation required because you want the piston somewhat within it's dampening travel otherwise you will end up with a harsh ride and sort of a dangerous ride of you hit bumps. Might as well be using solid struts!  

^^^ Sag is all about the spring, not the damper.

If you need to adjust your sag you do it with the spring.  

There's lots of suspension misinformation on this site, been going on for years.  Most of the time, the technical end has zero to do with stock 70s bikes.

Kidkawie wrote:

Damper rod dictates seat height.  Thats why when you lower suspension its done internally, not with the spring. Those "spring adjusters" only change the preload, not the height or the spring rate.

OP could have just bought a set of those lowering brackets for the lower shock mount.

Adjusters on top of the spring alter pre-load, adjuster below the spring alter "ride height" unless you wind it up so far it removes static sag, then it will start to increase pre-load

Kidkawie wrote:

Spring preload will change the static sag.  Changing the spring will alter the rider sag.  Doesnt matter what preload you have, the shock will compress the same with the same spring. That is why you shorten shocks internally. 
I've been modding both vintage bike and modern MX suspension for over 10 years.

Happy to debate the original point which was "what controls ride height" but not interested in getting into a "pissing contest" about who has the most experience !

Maybe I haven't made myself clear regarding this topic, hopefully the below diagram and the Maxton link I provided will clarify (Maxton talk about lifting the bike until the shock "tops out" which is when the damper limiter comes into effect) I am happy to be educated if you can give a rational explanation to the question in red text

 

Take a 12" shock, with the preload set so static sag is 2".

Shorten the damper rod so the shock is now 10" full extension.  Adjust the preload as before the shock will have the same 2" static sag.

Both shocks, same weight applied, same spring rate. Preload adjusted for the new damper length.

Kidkawie wrote:

Take a 12" shock, with the preload set so static sag is 2".

Shorten the damper rod so the shock is now 10" full extension.  Adjust the preload as before the shock will have the same 2" static sag.

Both shocks, same weight applied, same spring rate. Preload adjusted for the new damper length.

OK so you need to bear with me on this as I get a bit OCD around understanding things !! I get what you are saying so I guess the first thing is
Do you agree once static sag is set that the limiter on the damper rod is no longer effecting the rest position (height) but that it is the stored energy in the spring that is determining this ?
therefore if you wound off the pre-load adjuster to the same position on the shock body of the standard damper the rest positing determined by the stored energy would be the same but the static sag would be excessive ?
That being the case surely the purpose of shortening the damper rod is so that at full extension with the pre-load adjuster in this raised position that the spring doesn't become dislodged as it would no longer be in contact with the adjuster ?
It kind of all comes back to "the spring determines the rest position"

 

I dont know how to explain it any differently (easier, lol).  Major adjustments on spring preload to try to gain height or sag is not how you adjust suspension.
99% of the 70s bikes have ZERO static sag, maybe a hair fork sag.  Static sag on a 70s twin shocker is a non issue.  You're only adjusting preload. To put it bluntly, twin shock bike suspension is shit.  I'm arguing suspension theory, not vintage twin shockers.

I spent a good two years studying suspension before I dove into it.  I'm not a teacher.  There's tons of info online to learn from.  I think I explained how reducing damper length affects ride height fairly well in the last post.

I guess we are just going to have to agree to disagree on this one (the original point was any sag at all means the spring not the damper is controlling ride height but that's kind of gotten lost)
Be well