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

["Steve Makohin" <wateredg@xxxxxxxxxxxx>]

Hello Wayne,

From: "Wayne Woodruff" <wayne@xxxxxxxxx>

> You state the following:
>
> "Maximum stopping power, that is the greatest rate of deceleration, can
> be attained just *before* a wheel locks up."
>
> I would argue that the maximum coefficient of friction is when the
> wheel *is* locked up, therefore, the greatest rate of deceleration.
>
> Comments?

(1) On a motorcycle, it's a moot point, as the front wheel provides most of, 
and up to 100% of the stopping power, and a locked front wheel spells a high 
dropability of a dropped bike. (Most folks will want to hit the delete 
button now)

(2 ) I can provide only a layman's response. Those seeking one in terms of 
physics should run off to the library and start reading.

(3) People will have observed apparent contractions to this, such as

   Q:: Does a car stop quicker on snow with a lockup? Why?
   A: Yes, because the locked tire collects snow in front of the tire, which
       packs, and acts like a mini chock.

   Q: All other things being equal, can a car in full lockup stop quicker
        than a car with ABS? Why?
   A: Yes, because ABS is not designed to stop a car quicker. It is designed
        to prevent loss of vehicular control, its primary function. ABS does 
not
        attempt to keep the brakes at threshold, but rather, it works to 
keep
        the car controllable, which sacrifices ultimate braking power that 
could
        be attained through threshold braking.

So why is ABS on a bike *apparently* different than on a car? Because a 
front wheel lockup in a bike results in the virtually certain, virtually 
immediate loss of control, followed by a dropped bike, which increases the 
risks of personal injury. Riders fearing this typically do not apply their 
brakes anywhere close to the traction limits, therefore not making the best 
use of their brakes and needlessly increasing stopping distances. Other 
riders err by over-applying brakes, locking, and dropping the bike.

In your search for why the best stopping power is attained just before wheel 
lockup as opposed to during fully locked wheels (i.e., a skid), realize this 
is not an "ABS vs no ABS" comparison, because ABS is well short of keeping 
the wheels at the threshold of lockup. That's the reason why there are  some 
exceptional scenarios in which a really good braker can get shorter stopping 
distances on a non-ABS bike than with an ABS bike. But back to your 
statement: When you hit the text books in your research, pay particular 
attention to "static" friction. That's for the theory, at least.

In practice, other factors play a part, such as how much grip the rubber 
provides in relation to temperature. Most riders know that cold tires offer 
less grip than hot tires. The same applies in a stop. During a skid, a tire 
heats up more to provide potentially more stopping power as compares to a 
much cooler tire which is not skidding. But again, this is a moot point on a 
bike where a skid is bad news.

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

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