Aside from a chain-derailment during the 100 lap race in Northbrook, the Cuda-W
performed well in 2007. At the 2007 Hawkeye downs races, conditions were
good and the track was plenty big enough to be safe at the near 40MPH
speeds I ran at, but I was still unable to beat Dennis Grelk in my old
Barracuda 'liner. He was consistently about 1/2 lap ahead of me. Curses!
So 2008 is here and I need to go faster
this year. How do I do it?
Training in the 'liner position.
The streamliner is much more upright than the Nocom which I normally
train on, so I really need to train in that position to be competitive.
Better aerodynamics.
Hmm. All I can do there is add the little door for the landing gear,
finish fairing in the rear wheel disk, and see if I can close in the
front wheel a little. But not too much! I need some air in that thing!
Improve the drivetrain efficiency.
While the Rohloff hub works nicely, offers a very wide range of gearing,
and makes for a very simple drivetrain, I can't help but think I am
losing a bunch of precious watts making all those little tiny gears spin
around. I will build a new derailleur based drivetrain to replace the
trusty Rohloff.
It looks like if I use the 85T chainring
that Dennis made for me at the high speed events, and the 67T chainring
I am using now at the HPRA events, in conjunction with step up gearing
and a BMX hub, I will have a versatile and high efficiency drivetrain.
| I found this very interesting
article from
IHPVA Human Power #52, detailing testing of internal hubs vs
a standard 27 speed derailleur system.
Basically it says that at 200
watts, the derailleur system is on the average 2% more efficient
than the Rohloff 14 speed hub. 2% of 200 watts is 4 watts, which
doesn't seem like much.
But... |
 |
 |
When you look at the Rohloff hub
efficiency for each of the gears, you can see that the upper 3
gears were only about 89% efficient. I spend all my time in
those gears while blasting around the track!
The top 3 gears of the derailleur
system in this graph seem to be more variable in efficiency than
I can believe, so I'm going to arbitrarily throw out the 90%
data point and say that that system is around 93.5% efficient
for top gears on the derailleur system. |
| This gives 4.5% difference in
efficiency. I can work with that!
4.5% of 200 watts is 9 watts,
which still doesn't seem like much, but...
According to the
HPV speed simulator, the
Cuda-W goes 39.942 MPH with 200 watts, and goes 40.77
MPH with 209 watts. That's .828 MPH, which also doesn't seem
like much until you realize that during a 1 hour time trial on
the Hawkeye downs track, that makes me 2 laps faster! That may
be all I need! |
 |
This 2% efficiency gain may be further
mitigated by the multi-chain step up drive needed to construct the
derailleur gearing system. Because of this I'm going to keep the old
drive system around, just in case the new one doesn't seem any faster.
Gearing
According to the gear inch calculator:
Battle Mountain drivetrain
85T chainring + 11T -> 30T step up gearing + 18T BMX hub gear =
68MPH at 100RPM with a 406mm (18") Stelvio drive tire (75MPH at 110RPM).
HPRA drivetrain
Changing to the to the 67T chainring gets me to a speed of 54MPH at
100RPM. That's a bit high for HPRA racing. I will need to change the
intermediary gearing as well. Looks like I will be using a custom
mid-drive cluster to allow for track specific gearing changes. I always
was a gearhead...
67T chainring + 11T -> 24T step up gearing + 18T BMX hub gear =
39MPH at 90RPM with a 406mm (18") Stelvio drive tire (47MPH at 110RPM).
Perfect.
|
2/02/08
I brazed up the fork for the narrowed hub with BMX freewheel.
This makes a nice narrow package. |
 |
 |
I temporarily attached a
cog set and derailleur to the top of the Cuda-W subframe. This
position would allow the large cog to be used to drive the BMX
Freewheel. The derailleur
is attached directly to the axle shaft. The derailleur bolt is
the same size as the axle shaft, so a long axle nut attaches
them nicely.
Unfortunately, my knee hits the
derailleur. I tried everything I could to work around the issue,
but apparently this type of drive only works if you have short
legs. |
| Unless the many fine people that
have suggested possible solutions to this dilemma can figure out
a workaround,
it will be back to the old drivetrain.
Along those lines, I changed the
old skate wheel idler to the larger NoCom Idler. That should be slightly more
efficient plus it will give me tire clearance so I can use the 85T
Grelk chain ring. |
 |
 |
2/05/08
Larry Lem supplied suggested moving the mid
drive forward as far as possible. This way the chain ring drives
a small cog on the mid-drive, with no idlers needed due to the
close proximity. Also My knee won't hit it. On the hub
side of the mid-drive, the chain would pass through several
idlers as well as the derailleur guides. I still think this
would be more efficient than the Rohlhoff, especially while
coasting which seems to be particularly draggy. Rube Goldberg,
here I come. |
2/18/2008
After some thinking and drawing I noticed that the
derailleur cluster cannot be mounted that close to the chainring
unless I invent a new power side derailleur for the cluster.
Because I don't really want to do that, it's back to the drawing
board.
Eric Ball suggested I try the
Dahon
Neos (Suntour) derailleur. It mounts forward of the axle,
routes the cable forward, and is very low profile. I ordered one
from the LBS.
|
|
Here's the latest drivetrain mockup.
After some pedaling with the drivetrain screwed to my workbench,
it appears that this configuration may work with the low profile
derailleur.
I will need to raise the cluster
about another 1/2", and be sure to add a knee guard to prevent
accidental bumps into the cluster from getting bloody when
getting bounced around at speed. I'm hoping to be able to add
the mount for the cluster to the existing frame. |
 |
3/02/08
I picked up the Neos derailleur and
though nothing fancy, it looked good. Over the weekend I mounted
the derailleur on the mocked up drive-train, and raised the
cassette about a half inch. My knee now clears the cluster, and
it appears it will only rub on the derailleur when my knee gets
bounced to the inside during a wind buffet or while flailing. It
should be a soft hit on a rounded portion of the derailleur. I
cut a couple parts for the actual mid-drive mount. Initially Because the chain has to flex
over an inch to the outside when I'm in the top gears, I need to
make some different spacers for the chain-ring, to ensure it is
mounted as far out toward the crank as possible. |
 |
3/08/08
Here's the new derailleur mount, ready for brazing, but it's too
cold outside today for brazing, so instead I'm typing away on
the computer.
It stands about 1.5" above the
top frame tube. I mount this assembly to the frame temporarily,
then mount the sub-frame back into the 'liner and do some more
testing before I make it permanent. I don't want to burn any
bridges and will keep the capability of using the old drive-train on the same sub-frame.
Once I have determined it's all good, I'll slather it in epoxy
and CF. |
|
Here's the wide range cluster I will
be using, with the Neos derailleur mounted to the axle bolt. The
silver tab below the mounting bolt will be connected to an
aluminum strap to keep the derailleur in place and provide a
guard to keep my knee out of the gears. |
 |
4/12/08
Work has stalled on the new Cuda-W drivetrain. The chain angle
between the chainring and the cog set is so severe that when I'm
in the smaller gears the chain rubs the derailleur cage on the
non-power side, and rubs the chainring guard on the power side.
In addition to that, the drive chain that goes to the wheel is
so close to the frame that it will rub with the slightest turn.
Because of this I have decided I will need to create a crossover
drivetrain where the cogset is mounted to a narrow BB, which
transfers the power to a cog on the left side of the bike, then
down to a lefty drive on the wheel. Because I don't have time to
do all that now, I just repainted the old drivetrain and put it
back together with the Rohloff hub. |
|
|
|
|
