My DIY Carbon Fiber Recumbent Bike Odyssey

by Mark Rehorst

Update October 2006 - DISASTER STRIKES!

Ever since I first saw a recumbent bike (either an article in Scientific American magazine on human powered vehicles, or a TV show with a segment on HPV races) I wanted one.  That was almost 30 years ago. At the time I was a high school student and could not afford to get a recumbent bike, even if I could find one.  In the mid 80's recumbents starting to gain some marketing momentum.  I lived in San Diego and used to go to see the HPV grand prix in La Jolla and was bitten by the recumbent bug again.  I bought a Hypercycle frame from a company in LA and built up the bike.  It was a short wheel base (SWB) design that was fun to ride but not real good for long distance riding mainly because of the upright seating position.

I built the Hypercycle with a 26" rear wheel and very high pressure tire.  The rear wheel hub is a Sachs 2 speed internal gear plus six cog cluster giving me 12 gears all shifted at the rear wheel.  The hub also has a drum brake.  The bike originally used one long chain with an idler located close to the steering tube.  I did some experimenting with non-circular chainrings and even rode a rather extreme one (I called it "Mark-Pace" because Shimano Bio-Pace rings were big back then) on a Tecate to Ensenada ride, 75 miles through mountains, desert, vineyards, and finally coastal roads.

Here it is now!

That 75 mile ride felt like walking up stairs.  I found climbing hills to be much easier with this ring than a circular ring.  I retired that chainring when I replaced the idler with a mid-drive gear that you see on the bike in the photo below.  The mid-drive was quiet, smooth, and didn't let the chain throw itself off the chainring if I happened to pedal through a turn.


       

Here's my niece, Jenna, on the Hypercycle in 2004,

20 years after I built it.  My father is occasionally

seen riding around Milwaukee on it.


Fast forward to 2004, and there are lots of recumbent bikes available, but now that I can afford one, I am too cheap to buy one.  A commercially made recumbent bike with the stuff I want, such as the HP Velotechnik Street Machine costs over $2000.  I also like making things myself, as you may see from some of my other projects on this web site.  I decided to try designing and building my own recumbent bike.

My experience with the Hypercycle helped me decide what I wanted in a bike.  I liked the handling of the SWB design, but did not like the upright seat position.  On a recumbent you can't stand in the seat and let your arms and legs absorb the bumps, so suspension REALLY helps.  The Hypercycle used to bounce me around a lot on rough roads so full suspension was added to the list of must-haves.  I also wanted to be able to haul some cargo- I routinely need to carry 10-15 lbs of books to school and I'd like to make some long trips which will require the ability to carry some weight.

The final list of desired features looked like this:

Desired features and reasons:

I looked at several bike frame construction methods including welded/brazed metal tubing, wood, recycling old bikes by cutting the frames and joining them, and finally carbon fiber composite.  I ruled out welding/brazing and recycling old frames by welding/brazing because it requires a lot of expensive equipment and from what I read, a lot of skill.  I also felt that it would restrict me to using commercially available tubing and lock me into a sort of conventional design.

After a lot of reading at web sites and some trips to the library, I decided that carbon fiber composite was the way to go.

Frame Design

I chose carbon fiber (CF) composite construction because it required a minimum of tools that I didn't already own and CF allows a very wide range of design possibilities - not limited by commercially available tubing sizes.  The only tools I needed that I didn't already have were a vacuum pump and gauge.

I spent a lot of time hunting for good deals on parts, including making a trip to a big bike swap-meet in Madison, Wisconsin, in January.  You know someone's hard-core if they travel across two states to go to a bike swap-meet in Wisconsin in January!  After acquiring most of the parts I would need to complete the bike, I started making CAD models of them to allow me to design the bike on the computer.  This let me check chain clearance, and etc., before building the bike. Along the way I encountered several problems:

    Mechanical issues - how do I make it work?

    Bike fit issues- how do I design it to fit my body?

The solutions to the problems almost always led to additional problems.  Here is how I solved some of them.

The wide gear range without a front derailleur and using a 20" wheel was a tricky issue.  With a small wheel it's pretty easy to have "low" gears that will let you climb hills easily, but hard to get high gear ratios needed for high speed cruising and suicidal speed descents.  I thought about the range I wanted- on the low end I wanted it to be a little higher than the low gear on a typical mountain bike, and on the high side I wanted a gear that was about as high or even a little higher than the high gear on a typical road racing bike.  That is an overall range of about 5:1 to 6:1. 

There are wheel hubs that have 2, 3, 7, 8, and 14 speeds all accomplished by internal gearing- no derailleurs required!  I considered using one of those but the range of gears available was not wide enough.  There are also hubs that combine a 2 or 3 speed internal gearing system with a multi-cog gear cluster so you have have as many as 24 or 27 gears all accessible at the rear wheel without any front derailleur.  Unfortunately, all those options are pretty expensive.  They require building a custom wheel, and the hubs are usually not cheap (the 14 speed hub costs about $1000!).  I decided to stick with a conventional rear derailleur system and work out something with a mid-drive.

If you use a front derailleur with a single mid-drive gear you need to add some sort of chain tensioner.  When people do that they usually use an old rear derailleur to take up the slack in the chain.  I figured that if I was going to use a rear derailleur to take up chain tension, I may as well just shift gears at the mid-drive  and eliminate the front derailleur.  I crunched the gear ratio numbers and realized that this was the exact solution I needed- by shifting between four gears at the mid-drive and 8 at the rear wheel I could achieve a 6:1 gear ratio range using two standard rear derailleurs with a 20" drive wheel.

Full suspension usually requires that the bike frame be made in two pieces that are then connected by a hinge.  A shock absorber mounts between the two parts of the frame.  I needed to figure out what sort of parts to use to make the suspension pivot.  I looked at my MTB, and several others in local bike racks to see what was done.  Expensive bikes use ball or needle bearings, cheaper bikes have bronze bushings and steel shafts.  I didn't want too many pieces because I was afraid I would have difficulty aligning them when it was time to build the frame. 

While doing some research into suspension frame geometry I found this site.  What I learned from it was that if you select the right pivot location relative to the chain line you can minimize the bobbing of the frame that occurs when you pedal hard.  The key is keeping the chain as close to the pivot as possible. 

This gave me an idea.  I was going to use a mid-drive to get the gear ratios and range I wanted, and I need a suspension pivot, so I thought, why not use a rear wheel hub with a free-hub to mount the mid-drive gears?  Then I could use the axle as the suspension pivot.  This minimizes the "bob", and simplifies construction by allowing use of relatively cheap standard bike parts.  Another nice thing fell out of this decision- the rear swing arm geometry was simplified to two parallel stays, an easy to build structure.  Here is the CAD drawing I made to check sizes and clearances:

Using a rear wheel hub for the mid-drive created another problem.  I had to figure out how to make a rear derailleur mount for it.  I already needed to make the dropouts for the rear wheel.  Adding a mid-drive with selectable gears meant I needed to make an extra set of dropouts for the suspension pivot, one of which also needed to include the mid-drive derailleur mount.

The shock absorber mount, seat mount, and cargo carrier all worked themselves out at once.  The only place I could figure out to carry cargo was behind the seat, over, and along side the rear wheel.  That meant I needed some sort of projection from the frame to bolt on the carrier, so I just made a "tail fin".  It turn out the tail fin was also a convenient spot to attach the shock absorber and the upper part of the seat mount.

Accommodating multiple rear wheel sizes turned out to be easy.  I just had to make a bolt-on brake mount that could be moved to allow a larger wheel to fit in the rear swing-arm.  As built, the bike will accommodate any rear wheel up to 27" diameter.  The front wheel is another story.  As long as I use a suspension fork up front, I can't accommodate anything bigger than a 20" wheel because a bigger wheel will lift the seat bottom too high.  If I use an unsuspended fork I might be able to go to a 24" wheel up front.

One of the biggest problems was figuring out how to make the bike the right size to fit my 6'2" tall body.  I dug up a few human body drawings and models but couldn't find anything that I could use easily.  I finally settled on making a few simple measurements of my body and hoped that the seat adjustability would take care of any errors.  I measured the distance from my back to my heels with my legs straight, and the distance from my shoulders to my palms, and the length of my back from my shoulders to my butt.  It worked out well.

Next I had to figure out how to make the steering work.  This site had a lot of info on steering geometry that helped a lot.  I decided on the head tube angle to use and then tried to figure out how to attach handlebars (and what handle bars to use).  This was one of the biggest unknowns in the whole frame design.  Would I be able to reach the handlebars when my feet were on the pedals?  I eventually found some handle bars from a commercial recumbent bike that worked well.  I later realized that the head tube angle of this bike was quite adjustable by simply moving the rear shock attachment point on the tail fin.  This allows a wide range tuning of the steering performance.  I am really glad I decided to make that tail-fin!

I looked into several techniques for making the seat but then lucked into a very good deal on a used, commercially made CF M5 seat.  That solved that problem!  I left the final details of how to mount the seat until the frame construction was just about finished.  Compared to most of the other problems this was easy.

Construction details

The first thing I did was make the dropouts.  I had designed them using a CAD program and plotted them at life size.  Then I sprayed the back of the paper with adhesive and stuck it to the aluminum plate.  Next I punched all the hole locations.  Finally, I used my electric saber saw to cut the aluminum.  It only took about 20 minutes to cut each dropout and file it to its final shape.  I drilled the holes, tapped those that need it, and the dropouts were done.

The frame was made by wrapping CF cloth wetted with epoxy over a dense foam core, then vacuum bagging to squeeze out the excess epoxy.  I used house insulation boards for the foam.  I simply drew an outline of the shape I wanted and cut it with a saw.  Then I shaped the foam pieces using sandpaper.

Since I had never done any sort of composite lay-up before I decided to start with the rear swing-arm so that if I made a mistake it wouldn't cost too much if I had to start over.  The rear swing-arm is just two parallel stays with a single, "V" shaped bridge piece between them to provide rigidity.  I started with the bridge piece and quickly ran into problems.

I could not get the cloth to stay put on the foam core, and when I vacuum bagged it, the pressure caused the "V" to close up.  I sent a few emails and posted a few pleas for help on some internet forums and quickly found out the right way to do the job.  From then on I had no problems.

The right way to do it is to spray the cloth with adhesive just before cutting it, then attach it to the foam core or work piece, then apply epoxy, then vacuum bag it.  When cutting the cloth you have to plan the wrap around the work piece so that the tows bend at 45 degrees instead of 90 degrees - i.e. the axis of the work piece must be oriented at 45 degrees to the weave of the cloth.

On my second attempt at making the bridge piece I did not cut the "V" into the foam.  First I put a couple layers of CF on the rectangular foam core and vacuum bagged it.  After the epoxy cured, I cut the "V" notch into it and then applied several more layers of CF and epoxy.

The stays had to be made perfectly straight, and needed to have flat surfaces at their ends to allow attachment of the drop-outs.  I built a simple jig from a piece of plywood and a 2x4 both covered with polyethylene sheet so the epoxy would not stick.  I cut some foam cores and notched the ends where I then installed pieces of 1/4" plywood.  The plywood pieces gave me the flat surfaces I needed to mount the dropouts.  The rest of the jig ensured that the surfaces where the bridge piece would mount were flat.  I wrapped the stays and vacuum bagged them on the jig.  They came out light, strong, and stiff. 

Next I needed to attach the dropouts.  Alignment was critical here.  Any error would cause the front and rear wheels of the bike to mistrack.  I built a jig out of 2x4s and threaded rods.  Nuts on the rods held the dropouts at the correct spacing and parallel to each other.  The stays (and all other aluminum that was to be epoxied to the CF) were treated with West Epoxy System Aluminum cleaner kit.  A layer of glass cloth was put between the aluminum and CF to isolate it electrically and prevent corrosion.  Once the dropouts were positioned, I applied some epoxy to the stays and used clamps to hold them to the dropouts.  Finally I applied some epoxy to the bridge piece and put it into position on the stays.  When the epoxy cured I had a nearly perfect rear swing-arm.

Later, I wrapped the whole assembly with another layer of CF to hide the seam between the stays and the bridge piece.  I added brake mount rails and I drilled the stays and dropouts so I could put safety bolts through them in case the epoxy ever let go.  I also added the lower shock absorber mount made from two pieces of aluminum corner stock.

The main frame was also made using a foam core.  I used a large cross section area to ensure that the final frame would be very stiff.  After I cut and shaped the foam I used the head tube like a drill and manually twisted/pushed it through the foam core.  I installed the bearing cups before applying the CF and epoxy, which was a mistake that required a lot of careful labor to clean up after the frame was finished.  Next time, if there is one, I will insert the head tube into the core, but I won't install the headset bearing cups until after the frame is done.

The design included a tail-fin to allow for mounting the seat, the shock absorber and some form of cargo carrier.  I made the tail fin from 3/8" plywood which I glued to the foam core before applying any CF cloth.  By using a plywood core I could drill and bolt through it at will without having to worry about crushing the composite.  It added a bit of weight to the frame, but I think light weight is grossly over-rated anyway. 

I started to run out of CF cloth after having put several layers of cloth and epoxy on the foam core (a total of about 6 yards, minus the stuff I used in the failed bridge piece for the rear swing-arm).  I wasn't sure if I should order more cloth so I decided to test it.  I placed a couple 2x4s on the floor, set the frame across them, then stood on it.  Nothing broke, so I tried bouncing on it.  Still no snap, crackle, or pop, (not even any detectable flex!) so I decided it was strong enough and I didn't need to order any more CF cloth.

The next step was to install the rear wheel hub that was going to be the mid-drive and suspension pivot.  I needed perfect alignment so I made another jig from some straight 2x4s.  The jig clamped the wheels in alignment.  I installed the headset bearings and front fork into the frame and put the fork on the front wheel.  This held the frame in final alignment position.  I bolted the rear swing arm to the rear wheel, and bolted the mid-drive hub to its dropouts.  All that was left to do was to epoxy the mid-drive hub to the main frame and the frame alignment would be perfect.  I mixed some strengthening filler with epoxy, put a few dollops on the mid-drive and clamped it to the main frame.  Once the epoxy set I unbolted the mid-drive from the rear swing-arm, removed the front fork, and turned the frame upside down so I could fill in the gaps with epoxy and add a couple layers of CF cloth over the joint. 

 

The last thing to go on the frame was the bottom bracket shell.  I installed a bottom bracket in the shell and bolted pieces of aluminum to each side using crank bolts.  One aluminum piece was positioned to run parallel to the main frame and the other hung down vertically.  I clamped a bubble level to the vertical piece and another to the front wheel rim.  I used rubber bands and Velcro straps to hold the other aluminum piece in alignment with the frame.  Once everything was set I applied a few drops of epoxy to the BB shell to lock it in place.  After the epoxy cured, I filled the gaps between the frame and the shell with more epoxy and applied a few layers of CF cloth over the whole thing to make it look nice.

In the end the frame/rear swing-arm and frame/crankset alignments turned out perfect.  I can unbolt one end of the rear shock and fold the bike and the two tires meet each other dead center.

Time to mount the seat.  The back of the seat was going to be bolted to the tail-fin but I had yet to work out a way to mount the bottom of the seat.  I had some rectangular aluminum pipe from an old project so I decided to use it.  I drilled some holes in the sides then epoxied it to the top of the bike frame.  I cut some small pieces of corner stock and drilled them so they could be bolted to the seat bottom.  Then I lined the holes up with the rectangular pipe and put a long bolt through it.  That was easy!

A few days after the last bit of CF cloth was applied to the frame I built the bike up and took a very short, very nervous test ride.  Other than needing an adjustment to the seat position, it handled very well.  The next day I was making a trip to visit family in Wisconsin so I took the bike with me (along with a lot of tools and some small parts, just in case).  Before I really had a chance to ride it, I let my nephew try it out.  It turned out to be a good test of the frame's strength- he weighs 230 lbs compared to my 160.  Nothing broke, so I was very happy (he was too!).  The next day I got to actually spend some time making adjustments and riding it around.  I ran into just one problem.

The mid-drive was a standard rear wheel cassette that I restacked to get the gears I wanted.  The largest cog was driving the rear wheel while smaller cogs were driven by the chain from the pedals.  When I put a lot of pressure on the pedals, the chain tended to slip over the teeth on the cog that was driving the rear wheel.  The solution was to take that cog off and turn it around.  I had to file out one of the little indexing tabs so it would go back in, a 2 minute job with a hand file, and from then on it has worked perfectly.

That's me, on the bike, a few days after completion.  Note the look of sublime satisfaction that only comes from having successfully built a full suspension, CF, recumbent bike on the first attempt.  Since completing it in August 2005, I have put 600 miles on it, mainly commuting to school.  After a couple initial adjustments I have had no problems with the bike whatsoever.  The longest ride I made was a 50 mile charity fundraiser.  The bike is so comfortable I could have easily done another 50 miles.

I've done a few things to the bike since the picture was taken. 

Here's what it looks like today:

Note the liberal use of reflective tape.  I live in a rural area with no street lighting at all.  I like to be visible.  The large space behind the seat will lend itself to a hydration pack and a tube or bag for carrying small stuff.  To be added...

Here are some photos of some of the details:

Seat bottom mounting detail.  The rectangular tube is epoxied to the bike frame.  A single bolt goes all the way through the tube.  I intend to replace the wood spacers when I have finished playing with the seat position.

 

Here is the back of the seat.  Again, wood spacers are temporary.

 

Detail of the rear brake mount.  The rails are epoxied to the bike.  1/4" x 20 tpi SS bolts hold the 1/4" thick aluminum bridge to the rails.  The chain clears the brake shoe by about 5 mm.

 

Shock absorber mount.  The bottom is attached by two pieces of 1/8" thick corner stock epoxied to the rear swing-arm.  They seem to be thick enough, and the epoxy seems to be holding up OK.  As built, I have 6" of travel on the rear end of the bike.  I have ridden the bike down curbs with 15 lbs of books in the trunk and it handles very nicely.

 

This plastic disc was added to the mid-drive to prevent the rear chain from auto-shifting when I pedal backwards.  I will soon be modifying the mid drive set-up and may be able to eliminate the disc.  You can see the SS safety bolts I put through the dropout in case the epoxy ever lets go.

 

Detail of the trunk mount.  I have decided it is too high up for anything more than about a 15 lb load.  With the cover on it is rain-proof.  Without the cover I just throw my backpack in and go.  No need for straps or elastic cords.

 

Vital Statistics

 Top Gear  196 gear inches (too high, mon!)
 Bottom Gear  32.5 gear inches

 Fork

 RST 200K  2" travel

 Shock

 Cane Creek AD-5, gives 6" travel

 Tires

 Schwalbe Marathon Slicks 20x1.35

 Front Brake

 Avid Single Digit 7 V-brake

 Rear Brake

 Nashbar road brake

 Brake Levers

 Avid Speed Dial 7

 Mid-drive shifter

 SRAM ESP 9.0 front shifter

 Rear shifter

 SRAM ESP 9.0 rear shifter

 Mid-drive derailleur

 Shimano Nexave rear derailleur

 Rear Derailleur

 SRAM 7.0

 Seat

 M5 carbon fiber

 Crankset

 Bontrager Race Lite

 Wheel Base

 42"

 Steerer tube angle

 72 degrees

 Trail

 1.3"

 Weight

 34 lbs (incl. fenders, trunk, etc.)

Now that it's built and I've got some miles on it, I have found that there are a few things that need to be changed. 

I normally use only the smaller chain ring (the other one is there because the bolts are too long to hold just one chain ring).  When the chain is on the smaller cogs of the mid-drive it drags on the idler that lifts the bottom of the chain over the fork.  This could easily be solved in two ways.  I could use the larger chain ring, as shown in the photo above, with appropriate changes to gearing at the mid-drive and/or the rear wheel, or raise the bottom bracket a few inches.

Right now, the high end of the gears is too high and I can't really use it, even if I'm going down hill with a tail wind.  I am planning to make some changes to the mid-drive gearing to fix this.  The bottom gear could stand to be just a little lower, too.

I have added a trunk (well, actually a box) on the tail fin to carry stuff.  I find that when I load it to 20lb or so, the weight tends to flop a bit due to flex in the tail-fin.  It also tends to make the handling feel a little wobbly, since the extra weight is up so high.  If I were to make another frame, I'd try to work out some other way to mount a cargo trunk.  One idea that recently came to mind was to make a new rear swing-arm that is longer than the one I built.  I could then add a lot of cargo capacity to it just in front of the rear wheel.  The cargo would not benefit from the suspension like it does now, however.

 

One last thing:

 

Update February 2006

Update October 2006 - Disaster Strikes!

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