Re: Front Sag Setting

[Ben Barkow <dr.ben@xxxxxxxxx>]

On 2006 Feb  21 , at 4:47 AM, oilheads-digest wrote:

>
> Date: Mon, 20 Feb 2006 10:27:24 -0600
> From: "Tom Brown" <tbrown@xxxxxxxxxxx>
> Subject: Front Sag Setting
>
> Tom is right again about the front preload affecting the bike's 
> geometry.
> This is about the only way to adjust front end height on Telelever 
> bikes
> because you can't really raise the front fork tubes like on a sport 
> bike with
> normal forks.  You'd raise them up (lower the front of the bike) for 
> turn-in,
> or drop them down (raise the bike up) for straight line stability.
>

Perhaps I can shed some light on the subject of "preload" (more 
correctly called "ride height" as it applies to BMWs).

First, yes, of course, obviously, if the bike is super low, you'll 
dislodge your muffler on the pavement on turns. Second, yes, of course, 
obviously, you can dial-in extreme settings of any suspension parameter 
and make a mess of the design geometry with consequences for handling, 
ride comfort, road clearance, spring bottoming, spring topping, wind 
blast on your bellybutton, etc.

Now let us get factual about the degree of influence the rear 
ride-height adjustment has on handling. And here I am only talking 
about you riding solo and thinking you want to take Tom B's advice 
about fine-tuning your handling by fiddling with the ride-height knob.

Please excuse me if it is too cold today to go out to my garage for 
accurate measurements. Let us say your wheelbase is about 60 inches. A 
big, if not quite extreme change in the ride-height adjuster on my bike 
might be 1/16 of an inch.  Or, with a leverage ratio of 1:6 (just 
guessing here), you'd be changing the rear wheel height by .38 inch.

There may well be people with "Golden Backsides" who can feel a change 
in handling of .38 inches over 60 inch wheelbase (AKA 0.6%). It is 
getting into the range that matters to racers whose world view is 
limited to a certain track on a given day. On the other hand, there may 
be many more riders who don't bother changing their ride-height 
adjustment after their passengers depart because they hardly notice the 
difference in geometry (which would be far larger).

Now, after-market shocks do give you far more room for adjustment and 
possibly extreme settings. They provide independent modification of 
spring rate (by buying different springs), ride height, pre-load, low- 
and high-speed compression damping, color, smell, where to glue their 
designer-label, etc. With that available, you can keep or you can 
depart as far as you like from the design geometry and in some cases, 
end up with better handling.

In summary, non-extreme changes in rear ride-height do alter geometry 
and might well be detectable by some riders. But for the most part, 
minor fiddling with the ride-height knob is mostly "placebo effect" for 
the rest of us. That is not to say you should be any less enthusiastic 
about making your suspension more to your liking and manner of 
riding... but by more effective measures. Hint: proper spring rate 
(which is not in any way influenced by "pre-load" even for progressive 
springs!) is where you start.

Ben Barkow
cool in Toronto

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