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Cuda-W front subframe III

By Warren Beauchamp

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.
	10/05/2010 I have finally decided to restart this project. I'm building a new subframe that will be just for the derailleur system. In spring 2010 I ovalized some 1.625"x.049 wall tubing for the main tube, and I built and brazed in the "T" bracket that attaches the subframe to the front bulkhead. Last weekend I took measurements from the old drivetrain and cut the head tube holes in the main tube. This picture shows the tube being cut in the jig.
Here's the drivetrain mocked up on the new main tube. Of course I ran out of gas for my torch so I could not braze any of it together, but I got lots of parts cut out and ready to go.
	Here's the two sections of the main frame that will be brazed to the head tube. The mitering jig does a nice job.
Here's an image of the Neos derailleur on a Dahon folder that shows how it mounts forward of the dropout rather that behind and below like a standard unit.
	10/18/2010 I had a good day of brazing, and got the main components of the new drivetrain spot brazed together. After some mucking about I determined that due to chain alignment I could only use about 4 gears. Now I need to find more chain and then run it through the gears several times to ensure it runs properly. If these are not enough gears I may look into adding another gear to the hub, which currently uses a BMX freewheel.
10/31/2010 After another day of tweaking, I now have it to the point where it shifts properly through all 4 gears without derailing. I may even be able to add a 5th gear. I also added the shock mount, so now I can mount it in the 'liner and see if my knee bashes the derailleur. If my knee hits I can raise the whole mid-drive assembly up a bit. 11/03/2010 I put the drivetrain in the 'liner and spun it up. Miraculously, my knee (barely) clears the bulk of the derailleur, even when I'm flailing. The idler cage is also enough inboard that it's out of the way. I can still hit it if I try, but it's a soft hit so I think it's a go to finish brazing this thing up.
12/05/2010 I was a cold day to be outside brazing, and the quality of the beads certainly suffered, but now I have a brake mount for the new drivetrain!
	Here's the current drivetrain. The brake is mounted. Next I need to braze on the chainwheel chain keeper, a spring loaded idler for the drive side chain, and the bracket for the lateral movement inhibitor arm.
5/30/2011 I had another productive day of work on the drivetrain. It now has chain management and a power side idler. It's actually time to bolt it into the 'liner and see how it works around the block. I still need to see if it's possible to cram another gear it there to being it up to an amazing five gears! Currently it's set up for 70MPH top end, so it needs to be de-tuned it a bit for 40MPH top end if I want to have any chance of climbing the hill at the Waterford Hills race track in Michigan. I found a 15T BMX cog to replace the 18T. This will help with the top end speeds. With the 85T chainring, it's good for 82MPH at 100RPM cadance. Probably a bit high. With the 67T ring I can still go 65 MPH. HPRA - 67T chainring -> 28T -> 24T -> 18T = 15 MPH HPRA - 67T chainring -> 11T -> 24T -> 18T = 43 MPH WHPSC - 85T chainring -> 11T -> 24T -> 15T = 17 = 66 MPH WHPSC - 85T chainring -> 11T -> 28T -> 15T = 17 = 77 MPH
7/2011 I installed the new drivetrain into the streamliner. It was difficult to do. I had to remove the wheel and then it was tough to install it. Apparently I did not build it straight as the wheel was not in the middle of the wheel hole. I am disheartened and have no time to work on the streamliner now. I will need to cut it apart and fix the head tube angle. I have abandoned this drivetrain for a while...
9/22/2012 After racing the Cuda-W a few times with a new fork in the old Rohloff drivetrain, I have decided to start work on rebuilding the new drivetrain. As seen in this picture, which overlays the drivetrain3 onto drivetrain2, I really messed up. The headtube angle is about 15 degrees off and the BB is mounted an inch or two farther back than the old one. I'll need to cut the headtube and BB out of the subframe and remount them. Arg. Aside from that, this time I will build a crossover style drivetrain, and use a BMX left hand drive hub. This will allow me to have a couple more gears, and hopefully better efficiency.
	12/8/2012 I have cut apart the V3 drivetrain to rebuild it again. shown here are the major components, including the lefty drive wheel. Now I just need some quality time to build a jig I can use to ensure that this new drivetrain have the same geometry as the existing one.
12/30/2012 After building a jig, spending much time grinding, tack brazing, and doing several test fits, I once again have something resembling a front subframe. The geometry is now the same as my old Rohloff subframe. The next step is to build the crossover drivetrain.