Re: T. Brown/shock advice

[Ben Barkow <dr.ben@xxxxxxxxx>]

On 2006 Feb  11 , at 4:47 AM, oilheads-digest wrote:
>
> If there were a Wikipedia for BMW suspension philosophy and advice,
> THAT post would be the only chapter we need.
> Kudos.

Again, Tom Brown has posted an interesting, even inspiring discussion 
of suspension. Because of his detailed post, I hope many riders will be 
interested in exploring this subtle frontier of modification of an 
already fairly good family of bikes. For sure, I have found working on 
my suspensions very gratifying intellectually, shopworkwise, and on the 
road.

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

BTW, it is easy to reduce spring force by simply grinding down the 
outer circumference of the spring. Like anything related to safety, 
don't do it if all you know about it is my single permissive sentence. 
Also, the spring rate and the shock damping have to be matched... by 
grinding without proper pre-analysis you may be increasing or 
decreasing that match.

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

Rebound damping is just your constant-springs trying to restore your 
constant-weight to ride high "by keeping the rubber side down" on the 
road. That's why the shock mechanism for rebound damping is typically 
just a hole with a tapered stopper stuck in it and that's why it is 
cheap and easy for BMW/Showa to provide rebound adjustment. 
Conceptually, it is easy to intuit your bike hitting a bump and how 
unsprung (wheel) and sprung (chassis/rider) weight react to the 
compression force. But is it a far different matter to conceptualize 
how fast you want the rebound to take place.

Proper rebound keeps the wheels on the ground and you at ride height. 
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). In other words, the rebound damping is 
not just "more is better" adjustment. 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.


Last week, with the help of a speed shop that owns a spring-rate scale, 
I ground down my R1100S front spring by about 12%. When spring returns 
to Toronto, I'll know the results (I suspect the shock is kind of stiff 
for compression... but fun to try stuff). A bathroom scale on a 
drillpress can be used to measure spring rate if it goes high enough or 
a see-saw gizmo and concrete blocks.

Ben
Toronto

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