From: ericm@microunity.com (Eric Murray)
Subject: Re: Engine Theory 101
Organization: MicroUnity Systems Engineering, Inc., Sunnyvale, CA
Date: Fri, 8 Jan 1993 18:02:55 GMT

In article <moseley.158.726511625@u.washington.edu> moseley@u.washington.edu (Steve L. Moseley) writes:
>In article <1993Jan8.141506.10292@spdc.ti.com>
> achenbac@epcot.spdc.ti.com (Jeff Achenbach) writes:
>
>>Dirtbikes are not expected to reach 50-100,000 miles. The frames are often
>>broken, the suspensions worn out, and in general they're abandoned for a 
>>newer model in only a couple years. Combined with the expectation of a 
>>rebuild when racing, tenths of a second are probably more important than
>>extended transmission life.  For everyday street riding, I expect most people
>>would opt for longer service.
>
>
>I shift the Concours clutchless about 75% of the time, and can usually shift 
>smoother without the clutch than with. 1st to 2nd upshifts are a problem, 
>but downshifts are pretty smooth in all gears. I cannot shift cage 
>transmissions smoothly without the clutch, and I thought that had something 
>to do with helical-cut gears on a car versus straight-cut gears on a bike. 

In all the posts on this subject that I've seen, no one has mentioned
that bike transmissions are of a different design than car and truck
trannies.  Bikes have what's known as contrant-mesh transmissions, where
all the gears are in mesh, all the time.  The pair(s) of gears that
have power to them are connected to their shaft(s) via dogs on the sides
of the gears.  The gears never have to mesh when you're shifting, because
they're already meshed!  The dogs DO have to mesh, however.  That's
why you have to unload the gears for a moment, in order for the
dogs to mesh.  You can unlead them by using the clutch, or by rolling
off the throttle, or by cutting the ignition for a moment (drag
bikes with air shifters do this).

Of course, if you have the engine speed wrong in the new gear, you'll
send a skock through the drive line, and maybe break loose the rear wheel.
You might also damage the drive line.  Dirt bikes have less problem
with that, since there ain't much traction on dirt.  Street bikes
have lots more traction, so there are shock dampers in the driveline
on most of them.

On my bikes, the quality of the transmission affects the number of
clutchless shifts I'll do.  On the BMW twin, with it's "box of rocks"
transmission, I won't ever shift without the clutch.  The Seca 750
I used to have got lots of clutchless upshifts, because it had a
decent transmission and driveline.

Cars and trucks have transmissions who's gears slide in and out of
mesh with each other.  They also have synchros, which are gears that
don't quite mesh, but will spin each other using oil pressure as the "real"
gears are about to mesh.  I've shifted lots of trucks without using
the clutch, but I think that even if you match the engine speed correctly
you'll cause more wear on the synchros than if you use the clutch.
Two-speed rear ends are supposed to be shifted without the clutch.
I think that they're built like bike transmissions, but I don't know
for sure.   I do know that they can get stuck in false neutrals worse
than any BMW twin transmission- there's not much that's scarier than being
in a speeding tanker that's carrying 2 1/2 tons of water, has bad
brakes, and has it's 2-speed rear end firmly stuck between gears.  Yow!



--
  Eric Murray                                             ericm@microunity.com
      "Archiving the net is like washing toilet paper!"- Leader Kibo
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