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AtomicZombie Extreme Machines Newsletter for December 18, 2014

winter bicycle

The Hammmerhead Winter Trike

Build a sturdy commuter bicycle


This fun and easy-to-build DIY project is designed to inspire you to build a bike that can conquer a typical winter climate with snow, ice and slush. The Hammerhead is a two-headed monster that eats snow and ice for breakfast and has no fear of Old Man Winter's icy wrath.

I designed this two-headed bike to give myself a way of staying in shape during the winter months when it is just too dangerous to ride a two wheeler. I used to take my fancy, overpriced mountain bike out for winter rides, but soon realized that it wasn’t suitable in deep snow or around icy corners.

A three-wheeled bike was necessary to maintain balance, so I rebuilt one of those old-style trikes (the kind with two wheels and a big basket in back) and tried to make it as light as possible by removing all parts that weren’t needed, finally switching those road tires to knobby tires for better traction. The results were very disappointing. Not only was this bike as heavy as a tank, but it also had no traction at all. Because delta style trikes (two wheels in the rear) only drive one of the rear wheels, it mainly spun around on most surfaces except bare pavement. Adding a differential (a gear system to spin both wheels and transfer power between them) was just too complicated and would have added even more weight, so I decided to scrap this type of approach.

My new plan was to have two front wheels for stability and one rear driving wheel for traction. The two wheels up front (tadpole style) design is popular on low-slung recumbent trikes, making them very fast and comfortable; however, these lowracer style trikes are not really a suitable design for winter riding for several reasons. First, you don’t want to be slung two inches from the slushy ground while winter riding because you will get very wet from the front wheel spray.

Second, people in motorized vehicles will not be expecting to see bicycles in the wintery conditions, so you want to be as visible as possible. A low recumbent trike is not very visible to drivers of motorized vehicles.

Third, is road salt. If you live in a community that routinely uses salt on roads and sidewalks, then this is a problem because salt will corrode metal. Why spend so much and money on something that will require many custom-made parts, and will end up rusted at the end of the year?

The Hammerhead is not only as high as a regular bike. To build it requires only regular bike parts and a little welding here and there. The design uses a regular mountain bike with two head tubes welded on each side of the frame to support two sets of front forks and wheels. Both wheels steer at the same time like skis on a snow machine. In fact, the steering linkage I used here is scavenged from a snow machine!

This capable winter trike is called the Hammerhead because the finished frame looked something like a hammerhead shark. You see it too, right?

Now that you have a plan and a desire to conquer winter, let's start by gathering some parts. As shown in Figure 1, you will need a complete mountain bike (stripped down to the frame), two front wheels, two head tubes (ground clean) and a matching pair of front forks. The critical requirement here is that both head tubes, forks, and front wheels be identical or very close in size. Even the tires should be the same, as any mismatch will cause the final bike to be uneven and wobbly.

recycle bike parts
Figure 1 - Gathering parts for the Hammerhead trike.

The first step is to create the two head tube extensions. Each head tube is welded to a pair of 12-inch lengths of one inch diameter thin walled electrical conduit, or similar bicycle frame tubing. These two tubes are then welded to each side of the original head tube on the frame. Both tubes are welded at exactly 90 degrees to the head tube, as shown at the top in Figure 2.

If the original head tube is not as tall as the two new head tubes, make sure that you position the new extension tubes so that they are able to mate to the original head tube. To make a good weldable joint, fish-mouth the ends of the tubing to conform to the round edge of the head tube as shown in the lower part of Figure 2. Weld carefully, tack welding only at first to ensure that the two tubes end up at 90 degrees to the head tube. Any error here will result in a front wheel misalignment, so check the angles with a 90 degree square as you work. Look ahead to see how the extension tubes will position the two new head tubes at the same angle as the original head tube and at 90 degrees to the frame tubing.

head tube extensions
Figure 2 - Creating the two head tube extensions.

When you have both head tubes welded to their two 12-inch extension tubes, it’s time to weld them to the original head tube on the donor frame.

As shown in Figure 3, the extension tubes are welded to the original head tube so that all head tubes are at the same angle and so that the extension tubes are at 90 degrees to the frame tubing. You want each head tube to end up at the exact same angle as the middle head tube so that the caster angle remains the same as it was on the original bike. If you imagine two identical bikes standing side-by side, then you can picture what we want here.

At this point, just make a few good tack welds around the joints to secure all of the pars together. A final alignment check will be made by installing the forks and front wheels to compare them with each other.

weld head tubes
Figure 3 - Weld the head tubes so that all three align.

To ensure that the two head tubes are aligned with each other, put the bearings, rings, and forks on both sides and then install the two front wheels. Remember that both front wheels must be the same diameter, which is why both tires should be the same type. When you have both wheels installed, stand up the bike and place both forks in the straight ahead position for a visual inspection. With the parts only tack welded, you can probably make slight alignment adjustments by tapping with a mallet.

Once you are certain that both head tubes are aligned, weld around all of the joints, following the same order of welding on both sides so that any distortions happen equally.

align forks
Figure 4 - Checking alignment using the front forks and wheels.

To make this frame structurally sound, a set of trusses will be added to the front in order to triangulate the frame. The frame is somewhat rigid as it sits, but any hard force to either front wheel could bend the frame at the joint between the head tube and the new extension tubing.

With some simple trussing, form a triangle on each side, making the frame extremely strong. Any tube with a diameter between half an inch and 1 inch will do for the trussing. I found some old lawn furniture with 3/4 inch tubing and cut a few pieces to make the trusses.

These trusses are welded from the top of each head tube to somewhere near the middle of the top tube on the main frame. The trussing should be installed a few inches ahead of where your knee will be when you are pedaling the bike. To find this spot, put on a crank arm and set the seat to your height, then mark it on the frame while you pedal. The main goal is to ensure that your knee does not hit the tube.

truss tubing
Figure 5 - The truss tubing makes the frame very strong.

The completed trussed frame is shown in Figure 6. With the two new truss tubes forming a triangle between the main frame and the 90 degree tubes, the completed frame is extremely durable. Without this added frame support tubing, any collision with a single front tire could cause the frame to bend where it is welded at the center head tube.

frame truss tubes
Figure 6 - The completed frame with truss tubes installed.

Now that you have your basic frame completed, it’s time to add all the steering parts.

Take the original forks from the mountain bike, or another set of forks that are compatible with the main head tube, and then cut off both legs at the fork stem area as shown in Figure 7. Once both fork legs have been removed, grind the fork stem area smooth. This fork stem will carry the gooseneck and handlebars just as it did on the original bike, transferring steering to both front wheels via steering linkages. The plate that will connect both control rods to the left and right forks will also be welded to the lower part of the stub.

remove fork legs
Figure 7 - Remove the fork legs and grind the crown area clean.

To make both wheels turn at the same time from a central set of handlebars, you must fabricate a linked steering system similar to that of any vehicle with two front wheels. You will need a few inches of 1/8 thick steel plate or flat bar, four small rod ends (ball joints), and two small diameter tubes to act as control rods about 12 inches long (see Figure 8). The rod ends can be salvaged from a trashed snowmobile, or purchased new from most bearing stores.

Rod ends are measured by their bolt hole diameter. The ones I used were ¼” in size. Any performance auto dealer should also have something like this in stock. The two 12 inch control rods will have the rod ends connected at each end, and can be any size in the area of 1/2 inch or so. The round tubes I used were the two halves of the seat stay cut from an old frame.

steering system
Figure 8 - The parts that make up the steering system.

Look ahead at the pictures in this tutorial to see that both front forks are connected to the rod ends which are in turn connected to the control rods and then to the main steering tube, so when you turn the handlebars both wheels turn. The two control arms that are welded to each of the outer front forks are installed at a slight angle as to allow the wheels to turn at a different rate depending on direction. This is called “Ackerman steering”. It makes one side turn at a different rate than the other. You may wonder why you would want this. Let me explain.

Let’s assume that you are driving around in a circle, turning to the right. The wheel on the right side (inside) is actually traveling less distance than the one on the left (outside) since the inner circle has a lesser circumference. Because the inside wheel travels less distance in a smaller circle, it has to make a sharper corner, so it needs to turn more to the right than the outer wheel. If both wheels turned at the same rate, one tire would have to drag in a turn; this would slow you down and cause one wheel to skid.

Making the inside wheel turn at a sharper angle is accomplished by setting the control arms that hold the rod ends to the forks on an inward angle. To determine this angle, draw a line from the center of the rear wheel straight through the center of each head tube, as shown in Figure 9 and you will get a fairly good Ackerman steering angle. If you really want to get into depth on the Ackerman steering concept, then search for “Ackerman steering” on the Internet and you will be able to read pages of information on the subject. If you just want to get up and running, use the method shown in Figure 9 to set your angles.

steering system
Figure 9 - Find the Ackerman steering angle by drawing a line to the back wheel.

Cut and drill the parts shown in the following drawing from some 1/8” thick or similar thickness steel plate or flat bar using the measurements shown in Figure 10. The rounded area ground out of the top of each of the smaller control arms is a fish-mouth that will conform to the round crown area on each outer fork. If your forks have a flat crown area, then you do not need to make this rounded area.

The holes drilled in the control arms should match the bolt holes or stubs on your rod ends. The holes are drilled in the control arms as follows: on the large one, the two holes are 1/2 inch from each corner, and on the two smaller control arms, the holes are ½ inch from the bottom and centered.

control arms
Figure 10 – Cut the control arms from some 1/8” thick flatbar or sheet.

Once you have the three control arms cut, weld the larger one (the one with two holes) to the bottom of the crown on the center fork as shown in Figure 11. Once the main control arm has been welded to the fork crown, install all of the bearing hardware for the center head tube. Also, connect one rod end to the end of each rod and then bolt them to the control arm. If your rod ends do not thread into the rod, or you choose to use salvaged bicycle tubes like I did, you will need to weld the rod ends to the end of the tube. Only secure or weld one rod end to the control rods at this point since you need to figure out the optimal length for the control rods by trial and error.

control arm welded
Figure 11 – The main control arm welded to the center fork.

If you have not already done so, weld the two smaller control arms to each of the outer forks based on the image shown in Figure 9. The control arms are welded to the crown area on the two outer forks as shown in the next few steps. To find these angles, place a long rod or the edge of some angle iron through both fork dropouts to hold both forks in the straight ahead position. You can also clamp a tube along the four fork legs to hold them all straight in alignment.

Once you are certain that both pairs of forks are facing straight ahead, draw a line from the head tube centers to the back wheel hub (refer back to Figure 9) using a string or tape to align the two outer control arms and then weld them in place as shown in Figure 12.

Once you have all three control arms welded and set to the straight ahead position, you can determine the proper length for the control rods by measuring between the holes and making the rods the correct length. Figure 12 shows both control rods and rod ends installed so that the two outer front forks are in the straight ahead position along with the large control arm installed on the center fork stem.

completed steering
Figure 12 – The completed steering system.

Once you have all the steering components installed, place a gooseneck and handlebars in the center fork for a steering test. Steering from left to right, you will notice that the fork on the inside of the turn will rotate much more than the other; this is Ackerman steering at work.

If the steering system allows you to “over steer” causing one set of forks to lock up or spin right around, you may need to add a lockout system to stop this. I designed the steering so that the tie rod would hit against the middle fork stub if over-steered, but this depends upon the thickness of the rod. You may need to weld a bolt or piece of steel in there to make this happen. Forty-five degrees of turn to the outside fork is plenty of steer to allow you to make circles on a narrow street.

For winter riding, a tall set of handlebars like the BMX style shown here work well, placing you in a more upright position for visibility and control. At this point, you are ready to disassemble the frame and start painting.

homemade bike frame
Figure 13 - The completed frame ready for painting.

When painting a new frame, you don’t need to remove all of the old paint, only the stickers and rusted spots. Stickers can be removed with a sanding disc and the rest of the frame can be lightly sanded with emery cloth so that the primer will adhere properly to the tubing. If you take your time and follow the manufacturer’s instructions, spray primer and paint will yield a nice paint job. Remember to let primer and paint cure for a day before handling the frame or it will scratch easily.

painted bike frame
Figure 14 – The painted frame sitting for a day to cure.

Once the paint has cured, you can re-assemble your new two-headed trike! Start with the bearing components, adding a fresh coat of grease after cleaning all of the bearing parts. For winter riding, an aggressively knobby tire (Figure 15) on the rear will help with traction and narrow front tires will help cut through the snow for better control. Since you have three wheels and can only install two brake levers, you have the option of just using a single rear brake or to use a cable splitter and run two front brakes from a single lever for full wheel braking. If you are just planning to ride around on level ground in the winter, a single rear brake will be fine.

winter tires
Figure 15 – An aggressive rear tread will offer good driving grip.

Riding the Hammerhead is easy because it does all of the balancing for you. Your only concern will be how fast you can go around corners before lifting a wheel. Of course, riding on two wheels is fun as well once you get the hang of it!

When you are tackling deep snow, set your gears very low and spin the cranks to overcome the friction. For icy hard surfaces, ride normally, being mindful of your top speed and the grip you get when making turns. Locking up the rear brake around corners is also great fun!

winter bike riding
Figure 16 – Riding on a frozen river over the snow.

The Hammerhead will traverse many types of wintery terrain, including snow banks, slush, steep, icy roads and hard packed trails. Deep snow is certainly a heavy workout, but that’s one of the main goals of this project, keeping in shape during the winter months. If you are serious about winter commuting, the HammerHead would be a great base vehicle for a rear hub motor and some kind of front fairing or windshield to keep you out of the elements. Add some lights and reflectors and you have yourself a decent winter commuter.

riding a bike in winter
Figure 17 – Hammerhead fears no winter terrain!

Thanks for stopping by our website! If you plan to jump into this great hobby, please consider joining our forum, and have a look at our global bike builder’s gallery for more inspiration! 

Have you ever built a winter bicycle? Tell us about it to share in an upcoming newsletter.

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