Cuda-W tub bike front sub frame part 2

Cuda-W tub bike subframe II

By Warren Beauchamp

The Cuda-W performed admirably at the 2005 World Human Powered Speed Championships, and I was able to crank it up to 64.4 MPH. Due to some compromises I had to make in order to get the bike finished enough to race, the drivetrain was not very efficient.

Inefficiencies and issues:
  •  The inherent drag in the new Rolhoff hub (which is supposed to decrease greatly at about 1000 miles).
  • The BB bearing dragged due to the fact that I clamped them in place.
  • There were two chain tension idlers plus the intermediary gear bearing to slow me down.
  • The 145mm cranks were way too short for me.
  • The cranks hit the tire when turning.

So now it's time to design another front subframe that cures many of these evils. I want to race the bike in HPRA races, which means cornering is a necessity. The plan is to move the BB back about 3/4 of an inch to allow clearance for 165mm cranks. I'll need to to a bunch of in-bike testing to see if they will clear, but I think I had more heel clearance than expected, so I am hopeful.  The headtube angle was way too relaxed, I had about 6 inches of trail because I built the subframe before I had the Cuda body to try it in. Changing it to about 3 inches of trail and moving the head tube back an inch will give me the crank to tire clearance I need, but it will reduce the wheelbase by 4 inches. Yeow! I'll use a big Fenner idler plus a 67T chainring with the Rohlhoff hub for the new drivetrain. That should improve the efficiency greatly and still allow HPRA speeds (44MPH at 100RPM).

Here's the old Cuda-W front subframe. I'll keep it around and build an entirely new one. I'm hoping to have it done for the June 2006 Waterford races.
The headtube and cranks arrived last week, and I had some quality time to figure out the new geometry. Here's the new Cuda-W front subframe, minus the smaller bracing tubes. I tacked it together enough to verify that the 165mm cranks do work in the bike, and that they clear the tire. As I had calculated, the wheelbase is now 4" shorter, at 53 inches. New headtube angle is 70 degrees, giving a bit over 3" trail.
The bike will also be about an inch lower now. It originally had 4" of ground clearance. 3" of ground clearance will still give me plenty of body clearance to lean past 45 degree in the corners. This does mean I'll have to re-work the rear wheel mounts and brakes. Erg.

As noted, clamping the BB bearings around the circumference caused some binding of the bearings in the old subframe. Instead of clamping the BB around the circumference, I'm clamping it side to side. Much better.

I think I brazed, un-brazed, fixed and re-brazed about half the joints I put together. Between forgetting that the rear mount is 4.5 inches higher than the front mount, and brazing parts on upside down (no instruction manual!), I had an annoying amount of rework. It was all worth while at the end of the day though, when I put the subframe in the bike, got in and did the pedal testing with everything clearing properly.

I made good progress last weekend. As usual, it was not without some trial and error. Tweaked the frame alignment one more time (un-braze tacks, stick frame in fairing, re-braze, repeat, Arg.) Added the lower support tubes to the subframe. Brazed everything together. Finally it's a frame. Well, almost, I ran out of gas and need to get a refill before I complete some of the tube fillets. The head tube I got from PowerOn Cycles was very tight and the bearing cups did not want to press in. While installing the steering bearing cups in the head tube, I bent up the upper cup slightly, and had to tweak it to get it back to a point where the bearing would fit in. Shortened the remote steer tube and reattached the handlebar. It steers! Installed the 67T chainring.

After brazing the lower brace tubes to the BB shell, it warped slightly making the bearings a press fit. Now I don't have to make a clamping mechanism. I hope I never have to remove them... The 6" idler (Fenner Drives Powermax v-belt idler - VA6001) from the fold-racer will work well on this FWD system. I'm ordering a couple more as it seems like an excellent general FWD power side idler. This weekend I will mount the power side and return/tension idlers. With luck I may be able to take a test ride! 
I built the power-side idler mount using a 3/8" tube pressed into a 1/2" tube. Here, I'm tested fitting the idler mount and reinforcement brace. The inside of the 3/8" tube was tapped to accept an 8mm bolt. As shown, a small chunk of 4130 steel was brazed at a 45 degree angle to reinforce the idler mount tubes.
Here's a close-up but blurry shot of the super narrow BB. You can see where I ground the inside of the crank's chainring web to provide clearance.
It's a drivetrain! After I brazed up the idler mounts, I decided it needed a better chain keeper on the Rohlhoff than is shown here.

Spinning the drivetrain shows that it is very smooth, but still a bit draggy. As noted before, the Rohlhoff hub still has pretty high resistance. I'll give it one year of racing to see if it gets any better.

Ok, the new subframe is "done"! After taking it all apart, painting it, and putting it back together, it was time for the beauty shot (such as it is). I didn't have enough black paint for the whole frame, so I used some old "chrome" paint I had laying around. Not too bad...
Here's a shot of the Rohlhoff  hub cable adjustment. The way the hub shifts is the left cable pulls to shift up, and the right cable pulls to shift down. It takes a little tweaking around to get the cables all adjusted properly.

Here's the back side of the power idler. You can't see the reinforcement tab in this picture, but it's there. Really.

Yes, I figured out how to take close-ups that are actually in focus...

Here's the head-tube side of the remote steering. It's a 1" aluminum split clamp clamping the head tube. A 3/8" aluminum rod is threaded and screwed into a 3/8" threaded hole that has been drilled and tapped into the side of the clamp. An aircraft quality rod end bearing is screwed into the top side of the 3/8" rod. The rod end bearing is inserted into a 3/8" OD aluminum tube, and it's pinned and epoxied into place.
This shot shows the underside of the handlebars, where the remote steering attaches, as well as the handlebar mount, which was a 3/8" steel rod brazed into the handlebar tube. The rod captures a pair of hardware store 3/8" ID radial bearings and was threaded on the end to allow it to be held in place with a locking nut.
I mounted the subframe in the Cuda-W, got in, and spun up the drivetrain for the first time. Nothing hits! You don't know how happy that makes me. As the front of the bike now sits 1.5" lower, I modified the rear wheel mounts so it's 1.5" lower too.
After a few test rides I found there is more stuff to tweak. The pressed in bearings loosened up, so I added a bearing retainer system as shown in this picture, it clamps the BB bearings from the side. Also my heel hits the chain keeper down by the wheel hub, so I need to relocate that. Fortunately the list of things to fix keeps getting shorter and every test ride is more enjoyable.

I moved the chain guide that keeps the chain on the Rohlhoff's hub up a bit higher to provide heel clearance.

 Also I stuck a 7/8" aluminum tube into the top of the steerer tube to stabilize the subframe. The remote steering clamp does a nice job of holding it in place. The top of this tube will fit into a U channel made from carbon fiber that I will put into the top of the fairing. Shown here is a "cutaway view" showing the subframe in the fairing. 

The bulkhead that the frame pivots on is to the left of this picture.

The shock is blue. The red thing at the top is the stabilizer tube. The stabilizer tube fits into a U carbon channel in the top of the fairing will allow the subframe to bounce up and down inside the fairing, but will prevent the frame from twisting. The red line coming from the back of the subframe is another stabilizer bar, which prevents the frame from moving side to side.
The Cuda-W behaved well at the Northbrook 100, but the subframe could still be stiffer. I covered the left half with a layer of 1/4" foam, epoxied it in place, and then covered it with a couple layers of carbon fiber (currently curing). Next will be the right side, but that side will need some cutouts for the moving bits.
It has been exceedingly hot the last few weeks, so the only days I can work with epoxy are weekend days when I can get out to the barn in the morning when it's still relatively cool. On the plus side, when I lay up the carbon, it cures really fast! The frame is now completely carbon covered on both sides, and I have reinstalled all the drivetrain parts. I'll try to get the drivetrain reinstalled into the 'liner this weekend for a test ride. Also you can see the drivetrain stabilization tube sticking out of the top of the steerer tube.
The drivetrain has been fairly reliable over the past season of racing. I kicked the chain guide for the drive cog last year during the Northbrook 100 causing the chain to derail and me to have to drop out of the race, but otherwise no issues. The Rohloff is leaky, noisy, and I have doubts about it's efficiency, but it's very hard to replace in this narrow 'liner.
Over the past year, I padded the power side idler to prevent leg abrasions on windy days, added an 80 tooth chain ring, and replaced the small non-power side idler with the larger idler from my NoCom.

The 80T chain ring combined with the 13T cog on the Rohloff and the 18.5" tires, will give me:
A speed of 33 MPH in the 1:1 Rohloff gear 11 at 100 RPM. This should be good for road races.
A speed of 40 MPH in the 1:1 Rohloff gear 11 at 119 RPM. This should be good for sprinting on velodromes.
A speed of 55 MPH at 100 RPM in 14th gear.
A speed of 70 MPH at 126 RPM in 14th gear, which is my goal for this year's Battle Mountain event.

This drivetrain has had issues with frame rubbing and crank interference ever since I built it. When I built it it did not have the issues, but then after racing it did. After several frame modifications and going to shorter cranks I determined that the fork was not well connected where the steerer tuber connected to the fork crown. I have now discarded that fork, and built an entirely new fork based on an MTB fork. This one should be plenty sturdy.

At Battle Mountain in 2008, George Georgiev brazed an 11 tooth cog to the Rohloff 13 tooth cog, to give me the ability to go faster. This allows me to use a smaller 67 tooth (as in 7/31/2006 picture) chainring for HPRA racing.  At 80 RPM, I can go 40 MPH in top gear.


Now back to the Cuda-W main page for more body work

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