Everyone always wants a build list well here it is, start building!
The build is a motorized bicycle. It uses a Honda xr50 or chinese clone engine and downhill mountain bike parts mounted to a custom frame and swing arm.
Why a Honda 4-stroke?
The Honda 4-stroke horizontal OHV motor is the most popular and most copied engine in the world. It was first introduced in the Honda mini-trail 50 in 1969 and is still being used today almost unchanged in the crf50. The Chinese have been making copies of the Honda for years now, with all kinds of variations in design and displacement, but all having the same motor mounts as the Honda. The Chinese copies of the Honda xr50 has spawned a whole group of minibikes called "pitbikes". The amount of aftermarket performance parts for the Honda xr50 and it's pitbike clones is endless.
Yes, you could make a lighter and faster downhill motorized bicycle with a two-stroke engine, but 2-strokes are polluting and noisy, that's why they've been almost completely outlawed. The goal is to make a good reliable motorized mountain bike that uses a 4-stroke engine and the most readily available components from the motorcycle and mountain bike industries, with performance being the second goal.
What do I need to build my own?
Here is the detailed build list of off-the-shelf motorcycle and mountain bike parts that you will need to complete the build. The basics are a Honda xr50/pit bike 50 - 150cc motor, carburetor and throttle, gas tank and a few other parts for the motorcycle, and forks, brakes, rims, tires, spokes, cranks, and pedals for the mountain bike. Everything custom such as the swing arm, jack shaft, bottom bracket, rear hub, etc has to be custom fabricated.
What good is it? It's not a motorcycle or a mountain bike.
The build is extremely light and uses the latest in mountain bike suspension. This makes it a versatile bike with the same foot-print on the ground as most downhill mountain bikes. Riding it with the motor off on a steep trail feels like a heavy mountain bike. When the rider reaches the bottom of the trail, the motor can be turned on and the bike ridden on the road. NEVER ride bicycle trails with the motor on.
This build is also a blast to ride on motorcycle trails. It can be ridden surprisingly fast and is a lot of fun.
You can also get around town with maximum efficiency. This build achieves 80-100 mpg.
How fast is it?
Any of the Honda or Import(Chinese) 50cc to 150cc horizontal ohc motors can be mounted to the frame. With a stock 50cc engine, it'll go about 30mph, when a 88cc big bore kit is used on these motors, they reach about 45 mph on the road. A 110cc-140cc engine would get you up to the 65mph range. The 4 speed transmission should be used for best results.
How does it work?
When the motor is on, the rider can either choose to pedal or not. Whatever is sending the most power to the jack shaft will turn the rear wheel. So the rider can also help the motor by pedaling.
Is it legal to ride on the street with the motor on?
Laws for motorized bicycles vary by state so It's recommended reading up on your local area before hitting the road. However, in some states, motorized bicycles that are 50cc or under, automatic transmission, and is capable of pedal propulsion do not require registration. Also, there is usually a 30mph speed limit on motorized bikes. Some other requirements like headlight, taillight and horn maybe needed, along with proper safety equipment.
How much does a complete bike cost?
All custom components cost money. Depending on whether you have an old pit bike and mountain bike sitting around, a complete build of this type can typically be built for under $1000 through some resourceful brainstorming.
Heres the spec's
Engine
Engine Type Honda 50/Import 4-stroke automatic clutch
Ignition CDI
Chassis/Suspension / Brakes
Frame 4130 Chromoly Honda crf50/Pit Bike Style frame
Swing Arm 6061 Aluminum hand TIG welded
Body/Seat /Fenders Honda CRF50/Pit Bike
Front Suspension 8" travel, Downhill mountain bike 1-1/8" headset
Rear Suspension 8" travel, 10.5"-11" ItoI Pitbike shock
Front Brake Downhill mountain bike hydraulic Disc 8" rotor
Rear Brake Downhill mountain bike hydraulic Disc 8" rotor
Front Rim & Tire 26" downhill mountain bike
Rear Rim & Tire 24" downhill mountain bike
Dimensions
Rake 24 degrees
Wheelbase 54 inches
Seat Height 36 inches
Bottom Bracket Height 14.75 inches
Ground Clearance 12 inches
Curb Weight 107 pounds-Dry, 115-ready to ride(depending on components used)
Fuel Capacity .8 gallons
Frame
The frame is a modified 4130 Chromoly Honda CRF50/Pit Bike
Body
The tank, radiator shrouds, seat, and rear fender are 2000+ Honda XR/CRF50 or Chinese import/pit bike.
Engine
The engine can be any 1969 and up Honda mini trail-50 ,CT-70, SL-70, trail-90&110, xr50,crf50, or any of the Chinese import pit bike 50 to 150cc horizontal ohc motors. Semi-auto or auto transmission is best - manual clutch is difficult to use on a motorized bicycle because of no foot brake, however you can always custom design a double barrel single pull brake lever.
Front fork
The front fork should be a dual-crown fork with at least 7" travel. The head tube is 1 1/8 and is 5.5" long. The fork should have a tall crown and a steerer tube at least 8.5" long with standard stem or about 7.5" long with direct mount stem.
Brakes
Brakes are mountain bike - 51mm International Standard disc brake mounts. You should use at least an 8" rotor.
Hubs and Wheels
The rear hub is custom made. The rear hub is 48 hole, and the rear rim is 24". Halo and Arrow Racing make a 24" 48h rim. The front rim is standard 26" mtb
Cranks and bottom bracket
The bottom bracket is custom made, it's wider than a standard mountain bike. The cranks are mtb or bmx with square taper JIS interface.
Rear shock
The rear shock is a 10.5" - 11.0" eye to eye, 3" stroke pit bike shock. Spring rate is around 700 lb for a 250 lb rider.
Exhaust
The exhaust system is custom made by us. If you try to use a standard pit bike(xr50) exhaust system the muffler winds up hanging down about 8" below the seat, so you have to do some rebending of the pipe and then make your own muffler hanger. It's highly recommend using a custom exhaust system, it's tucked tight to the frame and gets routed through the subframe with the muffler under the rear fender, a must when riding with shorts on.
Engine options
The Motorized bicycle can use any of the Honda mini trail 50, CT-70, SL-70, trail 90&110, xr50,xr70, crf50,crf70 from 1969 to 2010. Also any of the Chinese clone(pitbike) engines - Lifan, Jialing, YX Works, GPX, SSR, etc. Which motor you choose depends on what type of riding your going to be doing and how much you want to spend. If your going to ride this as a motorized bicycle you'll need to choose an engine based on the requirements of your state. If it's for off-road, you can use anything you want.
Manual or semi-auto? The manual clutch would be hard to run on this motorized bicycle. Because the motorized bicycle has pedals, you can't run a foot brake, so you would have to run both brakes and a clutch on the bars. Trying to work a clutch and a brake with one hand and a throttle and brake with the other might be tricky. The other consideration is legality. If you are trying to ride a bike on the street as a motorized bicycle, some state laws say it must have a fully-auto transmission, meaning no manual clutching and no manual shifting. Lifan makes a 50cc fully-auto single speed, if you want to be totally legal as a motorized bicycle, but it is slow. With a semi-auto it is possible to fake a fully-auto(single speed) by leaving it in one gear(2nd or 3rd) and not shifting while your on the street.
Also, most state laws have a cut-off at 50cc for motorized bikes, so you should have a cylinder that is marked 50cc or less.
Lifan engines are almost exact copies of Honda engines and are meant to be direct replacements for Hondas. They are decent quality for chinese, and you probably get the most for your money with these engines. A 50cc Lifan is about $225, a 70cc is about $280 and a 110 is about $320. GPX,Thumpstar,SSR, YX Works etc. are performance chinese engines and run in the $400 - $800 range. The GPX 140cc is $450. If you want higher quality, then start with a Honda motor and build it up with Takegawa(Japanese) or Kitaco parts. If cost is no object, a new complete Takegawa 124cc motor is about $4500. Then if you want to go real fast there's the Takegawa and Daytona 150cc 4-valve DOHC motors at about $2000 for the top end kits.
There is a huge selection of after market parts for these motors. Some of the parts are interchangeable between the Honda and Chinese motors and some are not. With a Honda motor you have a better selection of after market parts because you know what will work and what won't. With a Chinese motor, it's hard to tell what will work and what won't until you buy it and try it. A big bore kit may be useful on a motorized bike(for off-road use only, of course).
Peace Crazy Horse.
P.S. I'm researching the possibility of being a distributor for these Motorized Mountain Bicycles.
