Shocks etc.

["Tom Brown" <tbrown@xxxxxxxxxxx>]

Ben:

>I think it is important (before readers fall asleep) to mention that
his detailed and profound-appearing discussion of "pre-load" adjustment
is out to lunch (and I cover rebound adjustment below).

 Ben? not this AGAIN!


>Alas, the word pre-load brings connotations of macho more-is-better and 
>that you are
doing something real important in wrangling those strong springs.

Who ever implied that????


>In
fact, pre-load does little for handling but adjust ride-height and keep
the shocks centered.

Gee, that's sound kind of important to me!  Especially on a bike with 
variable geometry on the suspension.   The lower the suspension is 
compressed the more leverage the springs and shocks get making each 
effectively stiffer.   This is one of the three reasons why having the 
suspension working at the correct part of the suspension stroke IS 
important.   I'll let you guess the other two.

>How could it... you sit on the bike and the
springs sag till they are pushing up with the same force (and same
rate-of-change-of-force) as you are pushing down. The spring rate (in
kg per mm of compression) is pretty much the only parameter of the
spring that matters.

Well...No, it isn't.  You yourself go on to say....

>At the extremes, moving pre-load far from the
design value can change suspension geometry subtly; and not so subtly,
can cause "bottoming" as the bike suspension prematurely yet with extra
force rebounds to the beginning of travel with a thump.

Setting the preload too low really changes things as well.  Especially when 
a lot of rebound damping has been dialed up.   Ride gets very harsh even 
before bottoming.

>BTW, it is easy to reduce spring force by simply grinding down...

Golly, Mr. Wizard!

>Tom talks mostly about rebound damping since that is the only
adjustment we have in the stock struts.

Well, the guy doesn't have aftermarket shocks yet!!!  He can't adjust 
anything but preload and rebound damping!

>However, compression damping
(when you hit a bump or the far side of a pothole) is a far, far more
important issue and far more subtle design question.

"Far, far more important"?  Really??

>Compression
damping is a reaction to a vast range of slow and sharp, low and high
bumps and the mechanisms used today in shocks are very subtle indeed
(and can be re-configured if you rebuild our non-rebuildable shocks).

This is the real reason why compression damping is not adjustable on many 
stock bikes...Because it is a complicated "reaction to a vast range of slow 
and sharp, low and high bumps"....BMW chooses set it for the rider.  One 
thing about compression damping, it doesn't need to change much when you 
change the spring preload like rebound damping does.   Maybe this is why BMW 
give us the rebound adjustment?

>...If there is too little damping (AKA soft setting, that is lotsa oil
flowing through the stoppered hole), the wheel rebounds fast and if too
fast, it feels harsh (or hard).

Errr...not on my motorcycles!   To little rebound damping makes it ride like 
a 60's Cadillac.  Not harsh at all, but you get seasick and the tire can 
bounce over bumps and feel unconnected to the bike if it's underdamped 
enough.

>In other words, the rebound damping is
not just "more is better" adjustment.

Ben, I don't recall saying anything like that.   The rebound damping on many 
stock shocks is inadequate for the suspension.  The full hard setting is 
sometimes almost enough.    This is true on my new RT.   I got some Wilbers 
and can report that they have plenty of adjustment on all parameters.

>Got that convoluted rebound logic
and funny use of "more damping" meaning "less oil flow," "slower wheel
restoration," and "hard"? I hope you do because I get it backwards
about 50% of the time.

I don't understand the above at all.  When you have too much rebound damping 
(not enough oil flow; too slow recovery) for the amount of spring or spring 
preload, the suspension "packs up" over a series of bumps and gets into the 
high leverage area of the suspension near the bottom.  If it actually 
bottoms, you get a very very uncomfortable ride, if it just rides near the 
bottom you get a mearly lousy ride.   If I ever notice a harsh feeling ride 
on a trip, the first thing I do is increase preload a little (1/2 to 1 
turn).  It usually takes it right out, sometimes with a penalty of a floaty 
ride...in which case, I either back the spring down 1/2 the distance I just 
adjusted it or I put one click more rebound damping on (slowing down 
recovery a little to stabilize the float).

Too near the top (from too much preload and not enough weight) will cause 
the suspension to hit the top.  This lifts the wheel off the ground when 
rebounding from bumps.   Neither condition is a good thing.  As you know, 
the goal is to keep the tire connected to the pavement as much as possible 
during the bump sequence.    This and a steady bike with handlebars that 
aren't boucing all over the place, combine to give the rider (me) the most 
confidence.

On BMW rear suspensions, you want to set the preload where I said to set 
it....about 1 3/4" to 2" of sag loaded...and you do that first.   Then go 
and set rebound, finally, set compression.  ALL are "important".   You set 
in this order not because of "importance", you do it this way so you don't 
correct for the wrong things with the wrong adjustments.

Preload can be set independent of shock settings.  In fact, you don't even 
need any damping in either direction to set the spring correctly.   Rebound 
is dependent on your preload setting, so you want to set rebound after 
preload.   If compression is already too strong, you won't be able to tell 
what rebound is doing and you'll get lost.   So, loosen the compression up 
(if you have a compression adjustment) before setting rebound damping.

Compression damping is "more important" only because even a click or two too 
much will really horse up the ride and the ability of the bike to handle 
bumpy curves.   That's why you dial it in carefully after the other 
adjustments are correct.

- -TB

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