Re: The Low-Down on Bike ABS, Linked and EVO Brakes

["Steve Makohin" <wateredg@xxxxxxxxxxxx>]

Hello Paige,

From: <plpklt@xxxxxxx>

> As I understand it , ABS is activated when one wheel stops turning and the
> others ones are still turning.   It isn't just the difference of the rate
> of turning as you suggested.     It is possible to skid all 4 wheels of a
> car, since they all stop turning at the same time, the abs thinks you are
> stopped and doesn't activate.   I assume the same holds true with two
> wheels.   Has anyone ever tried to lock both wheels up totally?

I do not believe (but do not know for sure) that this information is
correct, as a wheel cannot instantly transition from a spinning state to a
stopped state. It must slow down to a stop, even though it may happen very
quickly. I don't know if ABS waits until a wheel is at a full stop before it
releases pressure, or if it cycles when it detects a "significant"
difference in spin between two wheels, or whether it uses some other
algorhythm.

Speaking only in terms of my own "reasoning", which may or may not apply to
the real world, I would imagine if a bike is doing 140 mph and the rider is
firmly applying his front brake with steadily increasing pressure, it would
be exceedingly difficult to balance right on that threshold in which the
wheel is spinning, say at 1/2 the rate of the road passing beneath it. I
imagine the tire would be slipping *very* slightly as it approached the
traction limit, and then when static friction between tire and road was
overcome, the wheel would very quickly come to a lock and a slide would
commence. But again, I stress that is only my guess, which could be wildly
incorrect.

I'd prefer to know for sure, rather than to be misled by "reasoning" that
sounds right. Genuine research is more likely to give us that answer than
debate and deductive reasoning.



> Has anyone ever tried to lock both wheels up totally?

When I first got my 2000 R1100S with unlinked ABS, I did a series of
progressively harsher tests to get a feel for what the ABS will do. In one
of these tests, I applied both front and rear brakes hard, and they never
locked. The ABS worked flawlessly.




> When ABS was first introduced they called them Anti-skid brakes...but then
> realized that is not true, and didn't want to give the impression that you
> couldn't skid out of control.

ABS is designed to help prevent skids, and good ABS does a really good job
at this, but they still cannot overcome physics. There are some scenarios
that, once started, cannot be undone no matter what the brakes do. Yet
some people will blame the brakes for not preventing the skid. As an
interesting exercise, apply your car's brakes over glare ice with ABS and
without. You may not notice the difference because traction is so incredibly
low, that the tire has a hard time spinning back up to speed when the ABS
releases the brake pressure. I once drove a car through an underpass that
had a lengthy patch of glare ice. The car was going straight when it
entered, and suddenly I felt a loss of traction as the car ever so slightly
started yawing. There was absolutely no response to any steering input. I
imagine that lack of traction would have affected brakes similarly. The car
slipped through the icy zone like an air-hockey puck, and fortunately was
pretty straight when it caught the first traction on the other side. Bike
ABS is similarly cursed in not being able to circumvent physics.

Bottom Line: Bike ABS is an awesome safety net, but it is still short of
being perfect. Sometimes, reality sucks.

- -Steve Makohin
 '01 R1100S/ABS
 Oakville, Ontario, Canada

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