From: John De Armond
Subject: Re: Driving a 5 speed to get the most MPG's
Date: Fri, 30 Sep 2005 14:36:06 -0400
On Fri, 30 Sep 2005 11:16:39 GMT, Logan Shaw
>Use Your Head wrote:
>> I'm not talking about semi-automatic trannys.
>> When you've got 12 or more speeds (or less, too), you use the clutch
>> once. Period.
>> You just have to learn to time your shifts. You can do it on your
>> passenger car, too. It ain't rocket science.
>> Think about it. A 6 speed truck with a split rear end means 12 or 18
>> gears. Your left leg would fall off.
>Is there any advantage that you can give for this technique other than
>that it's easier for the driver? I cannot see how it would save fuel,
>and it seems that you have reduced the margin of safety against
>damaging the vehicle.
I doubt it has much effect on fuel mileage but it doesn't damage the
tranny either. While clutchless shifting works fine on my truck
(GMC/Izuzu cab-over), I've never been able to get it to work very well
in a car. The reasons are many.
First, consider the difference in RPM range. My diesel engine starts
making useful power at about 900 RPM, redlines at 2800 and normally
needs to be shifted at about 2000. If I shift at 2000, the next gear
is going to put the engine at about 1500 rpm. That's only a 500 RPM
difference. The ratios are narrow to accommodate the engine's narrow
RPM band. All I have to do to shift is put gentle pressure on the
shifter and lift slightly on the pedal. When the engine RPM matches
the gearbox, the next gear just sorta snicks into position. No
grinding and no wear on the syncros because they are not engaged. The
only potential wear is on the forks but since only very gentle
pressure is used and only for a second at a time, no big deal.
The long shift cable necessary to allow the cab to tilt forward helps
to, in that it stores some energy as the cable is compressed and emits
that energy when the fork starts to move.
Consider a small car with a manual tranny. The engine might start
pulling at 1500 RPM and redline at 5500, a much larger spread. One
might shift at 3500 during normal driving, 4500 during spirited
driving and near the redline when hauling ass. The ratios are far
apart to cover the much larger speed range - perhaps 120 mph tops vs
about 85 at the redline for my truck. Therefore the engine has to
make a much greater speed change between gears. If the delta-speed is
great enough to shift gears in a reasonable interval it is too fast to
allow the gears to move from one gear to the other without the clutch.
I used to drive >70 miles one way to work, much of that in stop-and-go
traffic. I played around a great deal with my manual gearbox trying
to figure out how to shift without the clutch. I could usually do it
by gently feathering the throttle until I felt the pressure lifting
from the shifter. But I missed often enough not to do it very often.
This is a place where a little computer assistance would help a great
deal. Imagine a sensor on the stick that detects the desire to shift,
perhaps from touching the handle or stress on the shaft. The PCM cuts
power so that the engine RPM is matched to the new gear, even as the
car slows between gears.
That would be quite simple to do but I don't know of any mfr that is
doing it at the present.