An alternative to gear lightening

[Antti Sukuvaara <antti.sukuvaara@xxxxxxxxxx>]

Dear Alfisti,

Short time ago here was a lot of discussion about lightening gears. I also
personally asked more precise information about how much gears could be cut
and also if somebody would have drawings about new shapes of the gears.

I got a lot of information that was mailed directly to me. Thank you
everyone. I also got great old article written by Rick Davis (from Sotto
Veloce) "Lightening the gears in 101 gearbox ends synchro problems". 


INERTIAS OF THE GEARBOX

The article had some interesting calculations that made me feel slightly
embarrassed. I was narrow mindly focused only to the gearbox itself and
forgot that there is always other rotating mass as a part of the total
inertia that the synchros has to fight against. In five speed gearbox the
clutch disk and the input shaft are together guilty for 67% of the all
inertia of the system! The disk has most of this inertia because it's
rotating mass is far away from rotating centre of the shaft. 

Here are the values of the inertia [lb sec^2] and the percentages per part
for 4- and 5-speed boxes:

			4 Speed Box		5 Speed Box

Disk/Input Shaft	.0735   76%		.0735   67%
Countershaft		.0087    9%		.0146   13%
First Gear		.0051    5%		.0051    5%
Second Gear		.0051    5%		.0051    5%
Third Gear		.0050    5%		.0050    5%
Fifth Gear		  -      -		.0055    5%

Total inertia		.0974			.1088
[lb sec^2]

(Source: Rick Davis, Sotto Veloce)

As the table shows, the difference of the inertia is not so big with 4- and
5-speed boxes, but important anyway - the difference is though +12% and
that tells us why the synchros are under so hard pressure.

In the example case of the article, inertia of the 5-speed box was reduced
from 0.1088 to 0,1050 [lb sec^2], so the reduction was 4%. Even with so
small change, results of the work were reported to be good. Drawings in the
article showed, that it could be possible to reduce weight of the gears
even more as were done.


ALTERNATIVE OR NOT?

Now, let's study about those inertias of the 5-speed box more. Disk/input
shaft are guilty 67% of the all inertia the system has. Let's make rough
estimation and suppose that Countershaft and Input shaft have about same
inertia 0.146 lb sec^2. That is 13% of the total inertia per shaft. This
leaves clutch disk to have 67% - 13% = 54% of the systems total inertia.
Updated table about inertia is now below.

5 Speed Box

Clutch Disk		0,0589   54 %
Input Shaft		0,0146   13 %
Countershaft		0,0146   13 %
First Gear		0,0051    5 %
Second Gear		0,0051    5 %
Third Gear		0,0050    5 %
Fifth Gear		0,0055    5 %
			
Total inertia		0,1088	
[lb sec^2]			

Now we see that the simplest way to quicken our gearshift and to make our
synchros last is to lighten clutch disk. At least, there is the most of
potential. Less than 1/10 reduction of the disks inertia would lead to the
same 4% result in total inertia as the lightening gears in the articles
case. 1/5 reduction of the disks inertia would nearly lead the 5-speed
gearbox system inertia to correspond to the 4-speed gearbox system inertia
with original clutch disk.

If one could find as light clutch disk as possible, it would be the final
and easiest answer to our problem. For racing purposes there are disks,
that are shaped to be 3-leafed stars and some of their body has been made
of kevlar. Those should be at least light weighted, but are those also
usable in the street car?

Now I am short of experience. If you have driven cars with those clutches
or other light weighted clutches, you could tell me and us all, if there is
any light disk clutch that could be used in street car. 

Materials of the clutches have improved all the time, so are we trying to
find relief to our synchros with too complicate way?

Antti S.
Finland

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