Barracuda-W tub bike front sub frame

Barracuda-W tub bike subframe

By Warren Beauchamp

The Barracuda-W is intended to be a  monocoque or "tub" bike. This means that the streamliner's body does double duty as the frame. Because of this, the rider can sit on the floor of the body, which greatly decreases the overall fairing height. This of course requires that the bike be a front wheel drive (FWD) design. In a Short Wheelbase (SWB) configuration, the rear wheel can generally be mounted directly to the tub, and the front wheel is mounted to a subframe, which is bonded to the body in front of and behind the front wheel.  The rear wheel can also be suspended through the use of a small rear subframe. While many wheel combinations were considered, the 'Cuda-W will use a 406mm front wheel, and a 700c rear. The drivetrain will also be designed with the ability to use wide tires, such as the Tioga Comp Pools.
I'm planning on building a drivetrain similar to that of the 81MPH Varna Diablo, pictured here. It incorporates a road derailleur mounted directly to a gear cluster, which is located above the front wheel, and keeps the entire drivetrain on the right hand side of the bike. It uses a BMX style freewheel as the hub on the front wheel. While the Diablo looks like it only has 3 speeds, I'm planning on using and 8 speed cluster, to give me 7 gears, plus the gear which drives the front wheel.
Perhaps the hardest part of the design of this system, is to incorporate a suspension into the FWD front subframe. A conventional suspension would  undergo a lot of pogo action while a rider is pedaling hard, because the chain pulls from above while applying power to the bike. This pull from above compresses the suspension, causing the pogoing. The Varna Mephisto suspended the entire front subframe to get around this issue, though that is not an option for the 'Cuda-W as it would reduce the torsional rigidity to the point where the front subframe would become "floppy".
FWD Suspension Fork
The only way around this problem is to either use an extremely firm suspension, which certainly reduces it's functionality as a suspension, to use a trailing link suspension, with the pivot point of the suspension in line with the power side of the hubs drive gear, or to design a drive system where the mid drive is sprung. The drawing to the left represents the sprung mid drive approach. In this approach, the wheel is attached to the bottom of the sprung steer tube, and the mid drive is attached to the top. 

See Suspension Fork Designs for FWD HPVs for more info. 

2/25/02 - Narrow drive hub
Because of the very low Q-Factor of the cranks (3 inches),  the frame needs to be made from 1 inch by 2 inch square aluminum tubing. I noticed there was also only 3 inches of clearance between my legs, where the wheel, fork, chain, etc has to run. Another engineering challenge! As the top of the fork has no problems being narrow, the problem was to get the bottom of the fork narrow as well. The standard BMX hub I was considering using is 110mm, or over 4.3 inches. I figure I need something closer to 80mm (3.14 inches, cool!) to fit between my legs properly. After some research and consulting with folks on the streamliner list, I decided that I would need to use an older style road hub with a traditional threaded hub. The BMX freewheel screws on in place of the hub's gear cluster, and uses the same threading. After removal of the spacers, this gets the hub down to 100mm. The next step is to remove 20mm of the hub width. Apparently the only way to do this is to section 20mm out of the center of the hub. To do this you need to use a hub that has a straight section between the flanges. One method of fastening the hub back together is to find an aluminum tube with the proper diameter to fit snugly over the center section. This section can be machined to a uniform diameter to help facilitate this. After removing a 20mm section, fit the section of tubing over the sectioned hub and glue it with Locktite. Be sure to line up the spoke holes properly. If you can't get a tube to use as a sleeve over the sectioned hub, you can TIG weld the hub back together along the seam. Another alternative is to use an epoxy/flox mixture to glue it back together This should be used in a fairly thick layer to spread out the forces.
The photo to the left is of a "Sunshine" road hub, after I sectioned out 20mm of the center with a hacksaw. You can just barely see the cut line in the center of the hub. Since I know someone who has a TIG welder, and is not afraid to use it, I decided that I'd TIG weld it back together .

BMX Freewheel Note:
Something else to be aware of is that most BMX freewheels are designed for wider chains. They are spec'ed for 3/16", 1/8" or 3/32" wide chain. Only the 3/32" ones will work with derailleur chain!

I took a trip to Bill Murphy's to weld the hub back together. After spinning the hub a little it was clear that the edges of the cuts weren't 90 degrees to the body, so we stuck it on the lathe to clean up the faces of the cuts. Bill then TIG welded the hub back together. Spinning the welded hub in the lathe proved that the flanges were parallel. I also recently picked up a 3/32" Shimano 18 tooth BMX freewheel, and confirmed that it would work properly with my derailleur chain, yay! 
Minor Digression - Left hand drive options
While I am designing my hub for right hand drive, some people prefer to have the hub drive chain on the left side of the wheel. This is normally accomplished by using a track hub, which is designed to be driven in either direction. The caveat with this solution is that a track hub has no freewheeling capability, and when you are coasting at 40MPH, the chain is buzzing along in close proximity to your leg. One way around this is to use a BMX left hand drive freewheel (about $17, You can either use the lefty freewheel with a lefty BMX hub (about $300!), or you can have left hand threads machined into the left side of a beefy road hub. For instance, in the photo above, the material on the left side of the left flange is able to be machined to accommodate this oddity. Len Brunkalla is willing to perform this task for a modest fee.
Fortunately I'm using a normal right hand drive hub, so no lefty hubs for me. After grinding a couple of mm off of the left hand bearing race nut, and chopping the excess axle off each side, I was left with a very compact hub, at 83mm nut to nut. This comes out to  3.26 inches, which should be narrow enough.  In the picture below, you can see that the hub is not too much wider than the tire.
As this will be a drive wheel, I laced up the Sun CR18 406mm wheel in a 3 cross pattern. I mounted a Comp Pool 1.75" wide tire on the wheel. This will allow me to build the fork around the max tire size that I may use. When figuring out the spoke lengths and dishing, I discovered that the modifications I made allowed for a dishless wheel, so the wheel is centered between the hub flanges. This means that even though I narrowed the hub by 20 mm, it should still be quite strong.  
Now that Reg Rodero is working on the Cuda-W fairing, it's time to look at the drivetrain again. I'm planning on simplifying the drivetrain even further by using a single power side pulley, and a Rohloff 14 speed hub for the gearing. This combined with a 95T chainring cranking at 110RPM is enough to get to 70MPH with no intermediary gears! 
The front subframe will be mounted to the tub bike frame at two locations. In the front on a pivoting mount, and at the rear on a composite hoop that is positioned just behind the riders knees. The rear mount would connect to the subframe via an elastomer, air shock, or just solidly depending on the amount of suspension travel required. Suspension travel at the wheel will be roughly half the distance of the travel at the rear mount. The remote steered handlebars will also be hung below this composite hoop. 
The latest update to the moving target of the Cuda-W drivetrain, is that I have obtained the Rohloff Speedhub (Thanks Rohloff!), so I'll definitely be using that. Instead of using a huge chainring though, I'm going to use an intermediate step up hub. According to the gear inch calculator, with an 18 inch wheel at 110RPM, if I use a standard 62T chainring, A 16 tooth primary and 32 tooth secondary on the intermediate sprocket, with the 15 tooth gear on the Rohloff hub when it's in it's highest gear of 1.462:1. I can go 71MPH. As breaking 70MPH is my current goal, this sounds great. With the Rohloff hub at it's most efficient setting 1:1 (12th gear) I could go 48MPH. This sound like the best setup as it would be relatively painless to swap out the intermediary gears to adjust for the different top speeds at different racing venues.

Now that Reg is nearing completion of the Cuda-W body, it's time to start building the front sub-frame. I built a rude test fixture to hold the narrow Q cranks in place, and measured the available distance between my knees at  4 - 1/2 inches. That isn't much, but the frame will only be an inch wide. 
The big issue is how narrow can I make an intermediary drive? 

Fortunately, Rick Wianecki has volunteered to build a custom machined, narrow intermediary drive that will support two cogs, and allow them to be interchanged. He machined the drive from 6061 aluminum, and it will use skate wheel bearings inside the hub. Total width? 3/4". Cool. 

Pictured are the parts for Rick's intermediary drive.

I finally have my Rohlhoff hub front wheel all built up. It looks very cool, and should be heavy duty. 

The latest setback is that the Barcroft aluminum FWD fork isn't going to work out as I had hoped it would.  It has a very large gap (4"?) between the rim and the fork crown.  I was planning on using a caliper brake, but that would never reach. It also has brackets for Canti brakes (Too wide), and disk brakes. (I don't have the disk brake hub). 

Because of that I'm resurrecting the steel fork that had originally intended as an offset fork for the ti-Cuda. I spent some time last weekend making the new dropouts for it, one is a derailleur bracket dropout, and the other is a special extra long dropout for the Rohlhoff OEM torque tab. I still need to do some work to fit the dropouts to the fork legs but hopefully I'll be ready to braze it together by next weekend. 

Here's the fork kit. It comes in 4 pieces, head tube, crown and two legs. I shortened the legs and re-bent the crown flanges.

Making the dropouts is easier than it sounds. I started with an 1/4" thick chunk of 4130 steel, then used my hack saw to cut out the rough shape. 

I think dropouts are normally 3/16" thick, and they normally use plain steel, but this fork is going to see some rough duty.

Then it was time to file, file, file. A good assortment of files helps. I have 3. One flat bastard, one with one side rounded, and another is a rat tail file.

Here's the derailleur side dropout.

The non-drive side dropout was made with an extra long slot to accommodate the Rohloff OEM torque tab.

After cutting slots into the ends of the fork legs and shaping the fork ends to have nice round ends, I brazed the fork together. Next step is to braze on the cable stops for the Rohlhoff hub, and find a 6mm fine tap to tap out the derailleur mount.

Rummaging through my old parts turned up the old 1" head tube from the steel 'Cuda and matching headset. 

Dana Barlow had a great idea, which was to move the front pivot point for the subframe to a location in line with the pedaling force. This will help prevent any pedaling induced suspension hop. 

Here's the latest subframe design. This shows the mounting brackets on the front bulkhead. This new design allows a "quick disconnect" of the front subframe to allow easy installation and removal. The front tube extending downward from the front pivot is trapped between the two small tubes that extend diagonally from the front bulkhead to the 'liner floor. This prevents side to side movement. The angle of the clip will prevent any front to back motion. The frame will be kept in the pivot with a cable. This gives an added benefit of allowing the cable to be used to guide the sub-frame into the clip.
The front half of the front sub-frame is now brazed together. It's amazing how long it takes to build every part of this bike, since everything is custom. For example, just fitting the 62 T chain ring to the Black Widow crank set was a job in in itself, because all the chain ring bolts had to be shortened, and then the nuts had to be filed down too.  I need to shave down the back of the nuts a little more, as they just barely kiss the frame.

Clearances are tight, but that was the intention!

Next I need to finish the BB clamp, mount the headset, and work on the idlers. 

I did some testing, and found that the heels of my shoes get caught on the derailleur and skewers. Because of that I ripped the sub-frame back apart and moved the BB about an inch forward. Fortunately I had just tacked it all together before. Garrie Hill will be donating some low profile skewers. I need to clamp this portion of the frame to something solid so that I can test the clearance with have to balance the sub-frame between my legs while pedaling. It's like trying to ride a unicycle with no seat... 

I'm going on a road trip to pick up the body from Reg, and the Intermediary drive from Rick on the weekend of the 19th. I hope the weather cooperates. 

I now have the Cuda-W body, and have been continuing with the drivetrain construction as time permits. As of last weekend, the intermediary gear  is mounted, and the drive-train between the cranks and the intermediary gear is done. I mounted a spring loaded idler to take up the chain slack. Thanks to Rick Wianecki for the idlers!

Here's how I held the spring tab in place while brazing it. The white thing is a rare earth magnet from a hard drive. The washer is just a shim.

Since the chain-line between the intermediary idler and the hub is not straight (hub sticks out farther than the idler), I had to round off the teeth on the idler gears to be sure the chain stays on. Those gears are made to make the chain pop off! Also I'll need to add chain keepers everywhere just as insurance.

Here's the finished idler from the back so you can see the spring and the spring mounting tab.

Here's my latest bit of wall sculpture. If you look close on the right side, you can see the bracket to hold the subframe to the front bulkhead. 

The fork needs to have the bottom bearing race area cut down to fit the bearing race, then I can finish the idler to hub chain-line. Earl Russell is performing that feat for me. Once I get it back I'll be finishing the intermediary gear to wheel hub drive system.

Once the drive-train has been finished, I'll be working to fit the drive-train into the fairing. I did verify that the drive-train can be inserted through the cockpit area of the fairing. It's close but I can slide it in without removing the wheel. This will make it easier to pop the drive-train out to work on it. I'm building a frame jig to ensure that everything is lined up properly.

Once I have the approximate location of the drive-train worked out in the fairing, I'll be able to add the frame parts behind the head tube. 

I picked up the fork last weekend (Thanks Earl it looks great!), and have started work on the lower part of the drivetrain. Now before you start thinking that this whole thing is going together like a finely design piece of engineering, let me assure you that it is not! I have a plan, but much of the drivetrain construction is trial and error. I have seen a bunch of FWD drivetrains, but of course this one is a bit different with the super narrow BB, and it's the first one I have built myself. Last night I though I had it about together but since the intermediary cogs are so much further inboard than the Rohlhoff hub's drive gear, the chain would skip off under a mild spin. Not good. On top of that, I had planned on mounting the idler to the fork. Whenever I did that the chain tension would pull the wheel to the left. Not super bad but a PITA. I was disgusted, and started to think about scrapping the Rohlhoff, using a conventional narrowed hub, building a new fork, etc... Noooo... Sleeping on it helped. I spaced the intermediary cog out about 1/2 inch, that seems to be enough to straighten out the chainline. I'll have to round off the cogs on the other intermediary gear now, but that's a small price to pay. I'll mount the lower idler to the frame instead of the fork, which fixes the pull to the left syndrome, and the lower idler arm will server as a keeper to prevent the upper chain from derailing off the mid drive gear. Much better. Back to the saws, drills, torch and files...

I made the changes as above,  and  have finished the bottom half of the drivetrain including the bottom chain idler. MUCH BETTER! The drive system is much quieter now and the chain stays on when pedaling forward and backward. I still need to add some chain guides in front of the "on" side of all the gears to bulletproof the system. This will ensure that the chain does not pop off when slamming into road irregularities or getting hammered by gusts of wind at high speeds. It's hard to test for those conditions on the bench, but easy to prevent their effects. I'm hoping to test fit the drivetrain in the body this weekend to get the measurements I will need to allow me to add the back half of the front subframe.

After screwing around with it for way too long, I gave up on the "quick release" for the front subframe. I'm using a more conventional "T" bar on the front of the frame, and 4 bolts to hold it to the front bulkhead. Pictured are some of the tube clamps that fasten the "T" tube to the bulkhead. These items are sitting on the frame jig that I just finished building to insure the Cuda-W frame is actually straight. The tube clamp is sitting on a rare earth magnet from a hard drive. Strong magnets like these are great to use to hold things together while brazing. To get to the bolts on the front bulkhead, I had to build a 3.5 foot long extension for my socket wrench.
Here's the front subframe in the body. Yes, it's not just an optical illusion, it's pretty narrow up in front. You can see the "T" bar attached to the bolts mounted into the front bulkhead (the reason for the long extension on the wrench). You can also see the reinforced "hoop" and horizontal stiffeners. They are the black areas. The red area is the Kevlar in the carbon/Kevlar weave material that Larry Zenger donated. Jack from TrikeTrails hooked me up with a nice shock unit which will be mounted between the top of the frame tube in the middle of the picture, and the top side of the hoop.
This picture is a good example of why I haven't been taking more pictures lately. They would all have a very high "what the heck is that??" factor. This is the rear wheel mounting bulkhead, with one of the wheel mounting brackets installed. The center of the bulkhead where the wheel will go is not yet cut out. The red lines represent where it will be cut. I just finished glassing in the right side in this picture. I used some broken Popsicle sticks to mark where the T-nuts are mounted so I can find them again. In a couple days I'll be able to cut out the center of the bulkhead. The white around the outside is the hole in the seat back that I took the picture through. The wheel goes through this hole to install it..
Here's the front subframe outside of the bike. It's almost done. I just braised the shock mount tabs on the rear of the subframe.
Here's the shock unit mounted to the subframe. The aluminum brackets on the top of the shock will be bolted into a piece of plywood using T-nuts. The plywood will be glassed into the top of the hoop.

The next step is to build the handlebar mount and handlebars. The good folks at Aurora Bearing donated a set of high quality aircraft grade rod end bearings to use in the remote steering, and I found some radial bearings at the hardware store to use for the steering pivot. Now I need to stick the subframe back into the fairing and climb in with it to find out how much of the tube behind the shock mount I can cut off. The fun never ends! 

Only 10 days until the Northbrook velodrome races. Will it be ready? I really would like to be able to try the bike out there!

Steering metal bits all cut and mitered and ready to be brazed together. The radial bearings are mounted in the cylinder under the handlebar. The cylinder in brazed to the rear of the frame, behind the shock mount. The aluminum bar ends are clamped onto the handebar with the tube stubs on either end of the handlebar. This allows them to be very adjustable. This us important as right now I don't know exactly how I'm going to mount the brakes to both leg and fairing clearance.

The handlebars are now mounted to the subframe, and the remote steering is completed. The subframe has been in and out of the body at least 100 times (or so it seems), checking clearances,  tweaking, etc. I filled the Rohlhoff hub with oil, and hooked up it's gear shifter and cables. Hooked up the front brake. In this picture the brake is mounted upside down and behind the handlebar. This is "maximum clearance" position.
Here's the subframe back in the bike again. For the final time? OF COURSE NOT! The Brakes cleared the fairing mounted to the top of the handlebars, y. which I had hoped, as that's best ergonomically. You can se art of the hourglass shaped hole in the fairing to allow the wheel to turn a bit.  

Also visible is the front edge of the seat. It's made of 1/8 luan plywood, carbon fibered to the frame. The are holes under it to run the brake cable, landing gear cable, and the fuel lines for the retro rockets. Still needs a little padding...

Here's another shot of the frame in the bike. The pedals clear the front mounting brackets by about 1/2". Oddly, when pedaling normally, my toes don't hit the front brackets. I don't point my toes too much when pedaling...

The rod end bearings from Aurora Bearing worked out nicely. I used some JB-Weld epoxy to glue them into the remote steering rod, then drilled and pinned them just for safety's sake.

The frame is now done, except for some tweaking, so it's time to go to the Cuda-W Final Construction Page to see the rest of the building process.

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