Downshifting to Brake

["Al Powell" <APOWELL@agcom.tamu.edu>]

Couple of things about downshifting....

If you're going to downshift to increase braking power, be aware 
that there is *some* increased wear on the engine.  Why?  Because 
**IMHO** (note qualifier) the correct way to downshift is to 
double-clutch, which implies matching the revs, as this is required 
to properly double-clutch.  More revs means more total engine work, 
meaning slightly more wear over the long run.  However, I personally 
consider the very slight increased wear to be worth the benefit!

Why double-clutch?  Because doing so matches the engine and clutch 
speed to the speed of the driveline, allowing a smooth downshift and 
smooth clutch engagement without a jerk.  

If one does a proper double-clutch, (although it is not technically required 
in syncro transmissions to avoid clashing gears) you will slow much more 
smoothly and your control of the vehicle will be better.  I suspect 
that a jerky downshift results in much faster clutch wear because of 
the violent engagement, and it d*mnsure is not as comfortable.

Most double-clutching also involves heel and toe technique.  In my 
280ZXT all I have to do to heel and toe is put my right foot half on 
the brake, half on the accelerator and depress both.  Rocking 
slightly left-to-right fine tunes the pressure on each.  Audis also 
have a very nice relationship of brake and accelerator pedal.  (GM 
cars are famous for being screwed beyond belief in this regard - it 
takes a contortionist to heel and toe them!)

Of course, if one is ONLY shifting, not braking, then no heel and toe 
is required.  but shifting down is usually done when one wishes to 
slow rapidly or to select a lower gear for a corner, also involving 
braking.  Also IMHO, if you're slowing down for a full stop at a 
stoplight, downshifting is unnecessary and not called for.

To double-clutch (easier to do, once learned, than to describe):

1) Disengage clutch;
2) Accelerate engine slightly;
3) As engine accelerates, shift into NEUTRAL and engage clutch - purpose 
is to spin the clutch assembly, but not engage the driveline;
4) Disengage clutch, shift into lower gear, keep throttle partially open 
to keep engine speed at approx. the speed it NEEDS to run when the 
lower gear is engaged;
5) Immediately re-engage clutch.

Done correctly, this requires perhaps one second for a driver of 
average skills.  Experts can do it faster.

The result should be a smooth downshift in which the engine speed in 
the lower gear is already matched to the driveline speed.  IMHO, the 
double-clutch is desirable because it matches the clutch assembly 
speedn and results in a smoother, better controlled downshift.

If I have not described this accurately, I'm sure other members of 
the list will point out any careless omissions.

Cheers....


*.......................................................*
Al Powell, Ph.D.                 Voice:  409/845-2807
107 Reed McDonald Bldg.          Fax:    409/862-1202
College Station, TX 77843      
Http://agcomwww.tamu.edu/agcom/satellit/alpage.htm
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