In article <[email hidden]>,
(Phil Brown) said:Quoted message said:Mass centralization makes a bicycle easier to lean and turn (less stable), not harder. Motorcycle
makers have been heavily touting mass centralization for several years as a design principle for
their fastest, most maneuverable bikes.
But since single track vehicles are inherently unstable this increased controlability means
they're easier to ride. There is no such thing as an inherently stable bicycle, hence anything
that makes it easier to control increases the stability of the rider/bicycle combo. Phil Brown
I think this reasoning is faulty. Bicycles are dynamically stable: most work just fine on their own,
as anyone who has "ghost-ridden" a bike can tell you.
Mass centralization means that the bike is able to turn faster. It's akin to shortening the
wheelbase of a vehicle, though not quite the same. Here is where the engineers would properly mutter
something about polar moments of inertia and pull out some nice maths to roughly describe the
numbers for a typical bike-rider system.
A vehicle which has a low polar moment etc. and mass centralization and rims made of dysprosium or
whatever is going to be twitchier, but that does not equate to more "controllability". You can
certainly understand that one could build a very short-wheelbase bike with a steep steering angle
and small wheels for "increased controllability." The problem is that at some very easily reached
point, the vehicle would require steering inputs faster and more accurately than most humans could
manage well, and it would be hard to ride.
The real world provides us with myriad examples: tandem bikes are clumsy to start, but once they
start rolling are ponderously stable. It's easy to control a rolling tandem because no mere bump in
the road is going to make the tandem behave badly.
Moving to the parallel world of Formula 1, in 1998 ('97? First year of the "skinny" cars) revised
regulations led to very different cars from the previous year's designs. At the start of the year,
all but one of the major teams (McLaren) started with a short wheelbase, near the limits of the
regulations. This conformed to your reasoning: the cars were more responsive to control inputs.
The McLaren was notably faster than every other team. By the end of the year Williams finally
regained some semblance of competitiveness by changing to a longer-wheelbase design like McLaren.
The cars were too twitchy, and the more stable McLaren was much easier to drive fast, even for the
all-out qualifying laps.
Another good parallel is motorcycle design. I have had the privilege of riding a great number of
different bikes, including everything from a BMW touring bike to a couple of rather extreme mass-
centralized sportbikes.
The most stable and controllable bike was surely the long, low Virago 1100 I rode. It had a turning
circle that was about 16' (yes, it could do a U-turn in little more than one lane-width), and
tremendous low-speed manners, all because it was really stable and responded relatively slowly to
inputs (wide handlebars contributed to these slower, but relatively finer inputs).
Meanwhile, a really fast bike (say, an FZR400 or a ZX-9R) would require more effort at slow or high
speeds. But its ultimate limits would be higher, and such a bike works well at speeds where the
steering becomes heavy due to the machine's inertia. And at such speeds the Virago's slack design
would start to betray it.
So unless your vehicle is so slow-responding and stable that it simply takes too long to make any
reasonable input change (think of a bus), then twitchier is not necessarily better.
--
Ryan Cousineau, [email hidden] sfu.ca~rcousineOpen ↗ President, Fabrizio Mazzoleni Fan Club