Geometry Lesson

[RockyRaccoon]

I recently got an 1100GS from an inseam-challenged friend who had lowered it about an inch using a set of special-order Works Performance shocks. Since he also gave me the old stock shocks, and since I have another 1100 set up with regular Works Performance shocks, I was able to compare the difference in handling caused by the lowering.

The front end felt quite a bit different. It didn't feel nearly as "planted" on the line as it is with the regular set-up. The front end just felt looser and it wanted to turn into the corners a lot easier. This must have been caused by the shortened forks, which would place the front wheel just a little farther under the bike.

The effect was particularly noticeable when going downhill and braking into a corner on bumpy pavement.

My friend actually liked the handling better with the bike lowered. There were some situations where I felt it may have been an advantage; it seemed more agile at very slow speeds, for example, but on the whole I like the regular set-up.

I'm not sure what the point of this is except to note that small changes in a bike's geometry can make real differences in the handling.

[xrmattaz]

Ya think?

[2Laud 2Bro]

Thanks for the lesson.

Currently using the lowest preload setting on the front shock for tar running. Rear shock 8 clicks from the highest. 45000 miles.

On new shocks I intended to shorten the front shock 1" (1/2" may be a wiser choice), hopefully getting in the adjustment range. Leaving the rear shock the same length.

[Stephen]

I'd hesitate to generalize from a specific instance such as this to a broad characterization of the effects of lowering.

Let's work backwards for a minute. If steering angle is held constant, lowering a bike using shortened shocks will primarily alter handling by lowering the center of gravity. Secondary effects depend on the specific geometry of the bike; for example, as the rear swingarm typically droops in running position and the forks have positive rake, shortening the shocks moves the axles back relative to the CG, effectively biasing the weight distribution to the front wheel. Because front rake is typically steeper than the arcing travel of the rear, the wheelbase is also shortened.

A bike with a lowered CG, absent any effects on geometry, will have to be leaned over farther to make the same radius turn. This effect may--may--be perceived as "more stable" handling, as the bike has to be moved through more degrees of arc to turn; small-amplitude perturbations will have less effect. The lean effort is also increased because the lever arm working on the roll axis of the bike has been shortened. Some of the perception of "heavier steering" is mitigated by the reduced distance of arc that results from the shortened radius of lean of the lowered CG.

Note that on a typical bike, the secondary effect of biasing weight distribution to the front will make the bike turn quicker, as will reducing the wheelbase. The overall result is a bike that may feel more stable, with a "planted" front, yet will carve a turn better.

Now to the GS. The primary CG effects remain the same as above. The telelever causes the wheel to travel much more vertically as the suspension compresses than a telescopic fork would, resulting in very little or no reduction in wheelbase on a lowered bike and little or no change in weight distribution. We would expect the change in CG to govern any differences in the feel of the bike; greater effort to turn, some heavier steering, though perhaps better flickability, without quicker steering or a planted front.

Kurt reports that the bike "felt looser" especially downhill and braking on bumpy pavement. Perhaps the specific geometry of the telelever has actually moved the CG back, unweighting the front wheel. It is also possible that the lowering is not even front-and-back, thus changing the fork rake. A third factor is the specific spring rates, preload, and damping. Identical settings may not be optimal for the lowered bike; the actual settings may or may not be identical to original bike or optimal for the lowered bike. My gut feeling is that all three of these are at work. As near as I can tell, the front sounds light and the back sounds soft. But I'm guessing.

We don't know enough about the two bikes to go further, though we're probably safe in assuming that bearings, pivots, bushings and balljoints are all in spec--or similarly worn on both bikes. And I don't know enough theory or have enough numbers to say much more without totally blowin' smoke out my ass (as opposed to the partial blowin' I've done so far). But boy oh boy could we have some fun with those two bikes, some big rulers, and an afternoon at a track! Of course, any of us could have fun with two bikes at a track, ya reckon?

[R-dubb]

Said Stephen:
I'd hesitate to generalize from a specific instance such as this to a broad characterization of the effects of lowering.

jeez Stephen,

Let's not go all brainy on Kurt's very plausible observation. Yup, there are lot's of things that can affect the geometry from one example to the next. And no, we don't have enough data here to speculate. But, what stands to reason is that lowering the bike without a very careful re-evaluation of ride height and other factors is much more likely to adversely affect handling than it is to benefit. But if the owner is short and willing to pay the price, so be it.


R-dubb

[Stephen]

Said R-dubb:
jeez Stephen,

Let's not go all brainy on Kurt's very plausible observation.

Ooops. Sorry. I can't help it. I hardly ever do jack-diddly to increase engine performance, but I've spent years trying to figure out frame and suspension. So whenever it comes up, I kinda go kablooey, and just hope some other folks enjoy it as much as I do. I certainly weren't dumpin' on Kurt. I was overjoyed to hear his impressions--that kind of stuff fascinates me, and here is a rare side-by-side comparison. But I am a brainy guy, no brag, just how I am. It's important to note that "brainy" is not all that useful a trait for some things, say going fast in dirt for example; "apeshit" seems to work a whole lot better. Brainy is not too bad when evaluating suspenson performance, though.

...lowering the bike without a very careful re-evaluation of ride height and other factors is much more likely to adversely affect handling than it is to benefit.

Well, I'd be more comfortable saying the results would be unpredictable, but then, I took a lot of economics in college, and economists say stupid shit like that.:): You are in essence correct, since hearing "unpredictable" and "handling" in the same paragraph tends to make us feel queasy.

You're right, lowering a bike means starting over with the suspension setup.

But if the owner is short...

Lowering is not just for the half-pint crowd. If you want to run knobbies, or if you want a light bike without all the Useless Crap (TM), or some single-cylinder machine with stout suspension instead of commuter trash, but have no need for enormous clearance or suspension travel, or just want the bike to handle with precision on pavement, then as long as mfgrs fail to make such bikes available, lowering a dualsport makes good sense. I flop my 36" inseams over a lowered KLR. It works good. Lowering an 1150 GS is easier than getting knobbies to fit an 1150R. Lowering a KLR is a shitload cheaper, and comfier, than a KTM Duke (not that the KLR could become a Duke, just that there are few low singles around).

Okay, enough with the brainy bullshit, somebody gimme a beer.

[cRAsH]

Said Stephen:
It's important to note that "brainy" is not all that useful a trait for some things, say going fast in dirt for example; "apeshit" seems to work a whole lot better.

[RockyRaccoon]

Said Stephen:
Kurt reports that the bike "felt looser" especially downhill and braking on bumpy pavement.

Perhaps I didn't explain it very well, but the sensation was of losing that "force" that makes the front wheel want to track straight. It felt a little like the wheel would have been perfectly happy going off in any direction.