Re: synchro problems and brake pedal height

[Einar Sjaavik <einar@ski.mail.telia.com>]

costel wrote:

> double-declutching  and tapping the throttle.  Or maybe some people call
> the scenario described in the previous sentence "double-declutching"?

Previous (long time ago) postings in Italian Cars Digest indicated that
double-declutching means different things to different people in USA.
Maybe because a clutch is a fairly exotic device there :-)

Around here it means:

1. Depress clutch, and lift off throttle.
2. Select neutral.
3. Let out clutch, and blip throttle.
4. Depress clutch and lift off throttle.
5. Select lower gear.
6. Release clutch and match throttle like a single-declutch change.

The single-declutch syncro saving procedure is 1,2,5,6 from above while
the more usual "crash-change" is 1,5,6.

> I'm not sure why you bring the brake pedal in the picture, so I'm not the
> one to answer the question.

Because a double-declutch only makes sense for downshifts, I usually
need to brake at the same time. While this can be done with the left
foot, I like to use one foot for both if the pedal heigths allow this.
 
> Do you think the driveline has a high momentum?  It's long indeed, but not
> too much mass away from the axis of rotation, and that's what increases the
> momentum.  It might not add much compared to the flyweel.

It's VERY high compared to my other cars. Mainly because it has 2
flywheels.
One to carry the starter ring and dampen torque pulses before they get
to
the donuts, then one in the rear that is the clutch carrier. Both are
somewhat
lighter than the normally front mounted dual-purpose flywheel/clutch,
but only
slightly. So I was somewhat amused with the recent thread on carbon
fibre
propshafts. With it's small diameter, and being mostly a tube, that is
peanuts compared to the double flywheels. My bet would be on a lighter
clutch
assembly. I know there are 5.5" multidisk clutches available, but they
cost
a lot plus taxes :-(
 
> And since we are talking synchros, can somebody explain why Alfas have
> problems only with 1st and 2nd gear synchros.  Why not 3rd and 4th, since
> they have exactly the same synchros?

The synchros work the opposite way of the clutch. They are driven by the
rear
wheels, and modify the speed of the input shaft. This results in more
torque
transfer in lower gears. To envision this turn engine off, try to put
the car
in 5'th and then push it. Then do the same in 1'st. You probably don't
even need
to try it to know when it's hardest to push. Admittedly the resistance
will be
much lower when the clutch is pressed, but still the same relationship
applies.
This is simplified, but I try to get the idea rather than the details
through.

One detail that seems to have escaped many is that the whole gearwheel
pack has
to change speeds when selecting another gear. So just making one gear
wheel
lighter will only bring some benfit, but it will benefit entry to any
gear,
not just the one that is lightened.

>  Is it something intrinsic to the way
> a gearbox works (i.e. 1st and 2nd harder to synchronize)

Yes.

> or is it just the Alfa design causing the problem?

Naah. The Alfa just has underdesigned synchros. But the difference is
valid
for all cars. I have indeed seen cars where the dimensioning of synchro
rings are different for different gears, although this is uncommon.

I am mostly used to Opel which can be thrown into any gear with no
problem.
I has fairly small moment of inertia of the gears, and well dimensioned
synchros. Can burp, but not likely.
Then I have the Maserati (ZF box) with hefty gears (1st gear=1600grams!)
and
hefty synchros. I am just not brave enough to force past the synchro
action.
It simply will not go into a lower gear until the syncros have finished
their
job. When cold, this means seconds to get 2'nd gear, but never any
toothbrushing.

In theory lightening the gears, and using a light oil should help.
I talked to my local "Alfa guru" today about remedies. He said that this
theory
also held up in practice. Lightening the gears costs a fortune though
(for me
it does'nt), and would yield a recognizeable improvement. Due to the
cost,
he did not see this as an alternative for road use.
But he recommended a synthetic oil after a gearbox overhaul. I did not
ask
him why I should not put the synth in the box without an overhaul. I'll
do
that next time I talk to him. Maybe because it will piss oil?

So I'll plan for a rebuild with new synchros and swiss cheese
gearwheels.

-- 
Einar Sjaavik, einar_sjavik@login.eunet.no,
http://www.login.eunet.no/~einar/
1986 Alfa GTV 2.0, 1986 Maserati Spyder 2.5, 1974 Mini 1000, 1952
Ferguson TEA