How To Build A Home Grown, Low-Buck Pit Bike!!!...

[Fat Hack]

Okay, I know EVERYONE is sitting on the edge of their seats eagerly wanting to know how to transform a $10 flea market kids' bicycle into a speeding, noisey, bucks-down, two wheeled DEATH ROD, so here's the run-down!!!

I started with a completely bare Schwinn Fairlady Junior frame (the girls' version of the Stingray Junior) and built the bike itself from my stack of parts, but starting with a whole bike would be even easier!

The project began with the rear motor mount, which bolts to the rear dropout on the frame where the original sissy bar clamp would have bolted up. Most older bikes have a hole in the rear dropout that can be utilized, or enough 'meat' back there to drill a 1/4" hole for this purpose.

Note: ALL fasteners used to mount the engine are 1/4-20 coarse thread nuts and bolts.

The rear motor mount is a simple bracket made from steel strip obtained at a local hardware store. Actual dimensions are open to individual application, but I made mine "short" to allow for the use of long upright bolts to make adjusting the engine's tilt and height, as well as tension on the tire.

By using nuts to fasten the long bolts to the bracket, you can thread another set of nuts onto these bolts below that bar pictured. Once you place the engine over the bolts, those nuts below the bar (or 'adjusting plate' as I call it) can be threaded up or down to position the engine. Flat washers on top of the engine base with another set of 1/4-20 nuts tighten the motor down to the rear mount in the finished installation.

Below is a crude drawing to illustrate the concept. It IS NOT TO SCALE, but it shows the 90 degree bracket and the gusset that gives it support. I will include a few detail shots of the real parts on the bike in subsequent posts as well so you can see the actual finished product.

(More to follow very soon...read along as we go from pedal power to GAS power! http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif)

[Fat Hack]

Okay, now on to the front mount.

To get the strength I wanted, I was reluctant to twist the extension piece coming straight off of the top of the frame and out to where you see the two holes drilled. So, I bought a steel hinge from the hardware store with undrilled flats and popped a couple of 1/4" holes in each side. Bolted to the extension piece, it flops freely, so that the angle on the extension piece and the angle of the front mount can be different without needing to twist the steel and compromise strength. This also allows for further adjustability should you need it!

Again, like the rear mount, I utilized a plate with nuts below it to allow for adjustments. On my particular bike, the fastening nuts and adjusting nuts are nearly touching each other, but that may be different for you. This design just allows for all sorts of variables, to make it as foolproof and universal as possible.

The drop piece that comes off of the extension piece and goes down to a simple bracket near where the pedals would have gone is your most critical piece...as most the strength of the finished mounts falls largely to this one brace! You want it to fit PERFECTLY, and I had to tweak the ends of mine a couple of times to get it just right. Most importantly, you want NO BENDS in it aside from the very ends where the bolt holes are...it has to run straight up to the outer hinge bolt to provide the proper amount of support!

The two-piece lower bracket that the drop bar bolts to goes over the top of the frame in front of the rear wheel a ways and behind the old pedal area. I had to add a little extra piece of flat steel above and below the two you see drawn in for more support. A central nut and bolt passes through all of them and goes between the frame tubes. Tightening the through-bolt clamps the mount to the frame, just like the upper mount is attatched in front of the tire there.

I made a cardboard rectangualr template from the bottom of my engine with the four mounting holes in place to aid the mock-up process...WAY easier than trying to wrangle the whole engine on and off a billion times as your making your mounts!

With the rear mount in place, and the front mount located by using the template attatched to the rear mount, I made the drop bracket and fit it to the edge of the front mount perfectly. The template provides proper spacing between the front and rear mounts, and is held in suspension, just as your engine will be.

Once the drop bracket is fabbed up and installed, the mounts can support the weight of the engine for the first time if you want to bolt it on and check your progress.

Note: The little bit of slop in the hinge, along with the natural flexibility of the rear dropout on your frame and the rear mount will make the engine appear to be REALLY rickety until you have the clutch in contact with the rear tire to triangulate this system. It will still move a little with engine vibration and harmonics, but this IS NOT A PROBLEM...it HAS to have some give, and no bike frame is THAT rigid anyway...trust me, you need not worry...just keep a decent amount of tension on the rear tire with the clutch by adjusting your four mounting nuts and the plates below the front and rear of the engine.

[Fat Hack]

Let's try a couple of black and white photos of the actual bike and engine assembly so you can see what you're shooting for.

(Color pics distract you with various colors, I've always thought that b&w photos illustrate most "how-to" articles better...but that may be because I'm used to reading old books from the 50s and stuff!

[Fat Hack]

This shows the assembly from the right side. You can see how the clutch contacts the center of the tire, and you can see the mounts as well.

The clutch can be positioned further in or out to some degree if needed. Two set screws hold it onto the output shaft of the engine. (Some engines have a threaded hole in the end of the crank to allow for a retaining bolt, mine does not...but with the tension adjusted on the tire, the clutch CAN'T 'walk' off the end of the crank.)

[Fat Hack]

Backing up for a better view.

(The foot pegs are made from a 2" piece of exhaust tubing pounded through the old crank area)

[Buick59]

Im gonna print this one!!!!!!!!!

[Fat Hack]

Buttoning up this project called for some sort of throttle control and a heat shield.

The throttle is operated by a standard handlebar mounted brake lever and cable. I fabricated the carburetor end to hold the cable in place (by a bracket bolted to one of the old air cleaner mounting holes) and an arm with bent rods to work the throttle. It works well with the return springs run up to a bracket that is held in place by one of the cylinder head bolts.

The heat shield is made from thin sheet aluminum and helps keep the seat cool enough so that you can stand to ride this thing more than a block at a time! (Those first few test rides sure roasted my buns!!)

This design has proven itself sturdy after a Summer spent blasting around town on lust Michigan roads! I've hit potholes that damn near through me off, but nothing has bent, broken, come loose, or fallen off!

(The only trouble spot has been the tempermental updraft carb, hence the can of starting fluid that travels with me in that 'holster' you see in the above pictures.)

Standard bicycle brakes could be added to your own version if desired, and the pedals and chain could also be left in place to give you "alternative power" in the event of an engine failure, or an encounter with a less-than-understanding traffic cop!

(I have had NO trouble from local cops with this thing...they view it as a motorized bicycle like an old Solex...which is cool because that means I don't need a helmet, and the bike doesn't even need a moped sticker! Check the laws where you live to see what's up, though!)

This is a fun little distraction...something you can build to use as a pit bike, a kid's toy....or even as transportation when you got your hot rod apart!!! I jump on and take mine for a spin just to unwind now and then!!

I have less than $100 invested in this bike...including the $10 purchase price of the engine and the parts used to rebuild it. The clutch is a standard mini-bike or go-kart 4" centrifugcal job that cost $30 at the local hardware store. On a 20" tire, it gives you a 5:1 drive ratio. I push off a little with my feet to get it rolling...and this thing will CRUISE with MY fat ass on it...oughtta REALLY haul with a lighter rider!!!

Feel free to ask any questions you might have...I'm sure I missed something along the way here!

[wingnutz]

With all that Horse Power and speed I wouldnt trust the coaster brake! http://www.jalopyjournal.com/ubbthreads/images/graemlins/shocked.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/tongue.gif

I'd think about additional braking ..., Hand brakes, disk brakes, and drag chute...! http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

Mark

[prime mover]

is that the "engines of creation" album on your profile?

[RileyRacing]

Don't forget about 96 MPG!! http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

Jay

[dixiedog]

How does that set up work with a wet tire? Is the loss of friction enough to cause problems?

[Flat Ernie]

I only got one question - whydya use a gurls bike??

http://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif

[prime mover]

picked this up yesterday at the pomona swap meet, any ideas?

[burndup]

yeah, prime, sell it to me!

http://www.jalopyjournal.com/ubbthreads/images/graemlins/laugh.gif