Re: Drive shaft

On Wed, 06 Jun 2007 13:05:55 -0400, Matt O'Toole
<mattotoole@xxxxxxxxxxxxxxx> wrote:

On Tue, 05 Jun 2007 14:08:38 +0000, Nexus7 wrote:

On Jun 4, 10:57 pm, jobst.bra...@xxxxxxxxxxxxxxxxxx wrote:
reasons why car manufacturers do not use torsion bar suspension, like
the ones I designed at Porsche, is that they require large torsional
anchors at both ends, ones that convert torsion to linear force. This
costs weight in levers and heavy bearings and money.

The comparison of torsion bars used in suspension to drive shafts is
meaningless. In any event, torsion bars are still used with certain
suspension/wheel geometries, three wheelers, for example. For most
cars, struts offer better control and economics, however this is
entirely unrelated to drive shafts. Cars, trucks, etc. still use drive
shafts in torsion. A long time algo a lot of cars used chains, and
actually some 3 wheelers still do, but shafts and gears have
displaced chains.

That they exist and that they don't command any significant market
share is an indication of design flaws, be they mechanical, economic
or a pain to change a flat tire.

By this reasoning, Lamborghinis must make really sucky cars.

As a former employee of two different Lambhorgini repair shops, I
concur.

Getting back to shaft drive -- for one thing, a shaft drive bicycle
requires two 90 degree geared joints in the drivetrain, each of
which is a lot less efficient than a chain, a lot more expensive to make,
and a lot heavier.

Perhaps a day-to-day mechanical engineer among us can produce some
figures. Some 90 degree gear drives I've been looking at
lately (for marine use) have barely 90% efficiency -- each. A typical
bicycle chain drive is 98-99% efficient, unless it's cross-chained and
dirty, when it drops to about 90%. So compared to a chain drive, a shaft
drive bicycle feels like pedaling through mud (I know, because I've tried
a couple). Plus, this doesn't even consider multiple gears. I
suppose a hub gear system could be used, but these are inefficient, heavy,
and expensive themselves.

Matt O.

Dear Matt,

Kyle and Berto measured a 27-speed Shimano's efficiency. The raw
figures (fig. 11) bounced around from 89.5% to 95%, with a theoretical
2.0% to 2.5% increase to account for the test equipment.

Theoretical (meaning corrected) efficiency in 44x16 (fig. 19) rose
from 94% to 98% as power rose from 50 to 300 watts.

http://www.ihpva.org/pubs/HP52.pdf

Spicer measured chain efficiency with a derailleur. His figures for
fairly low power (100 to 175 watts) bounce around 88.7% to 98.2%
(table 1). The higher the rpm for a given power, the lower the chain
tension, and the less efficient it is--spinning has its disadvantages.
For example, 100 watts at 50 rpm in 52-21 was 95.2% efficient, but
dropped to 92.0% at 70 rpm.

Table 2 shows such low-power differences even more clearly. At 100
watts with various lubes, 40 rpm in 52x15 was 94.0~95.6% efficient.
The same feeble 100 watts dropped to 86.5%~88.8% efficiency at 80 rpm.

More power increases chain efficiency, and there may be a correction
factor that would raise the efficiency, so things are probably not as
bad as they seem.

http://www.ihpva.org/HParchive/PDF/hp50-2000.pdf

I'd love to see a shaft drive and a no-idler fixie included in such
tests of chains and hub gears, with 60-90 rpm and 100 to 400 watts.

I suspect that this is the most efficient bicycle transmission:

http://www.jimlangley.net/ride/singerbritishchallenge.html

Cheers,

Carl Fogel
.