435 NE Spruce Ct  *  Redmond, OR 97756  *  U.S.A.
Phone:  (541) 923-4009

Email: carrprecision@centurylink.net

Conversion Kit 2
Home Up



(available plans only)

This is the Trimmer-1 Power Unit mounted on a 20" bicycle.

403.2 mpg *
Weedeater (Featherlite) 18cc engine
Lightweight only 6 lbs.

Aluminum Folding Bike: 20", 31 lbs.
Folded size is 32" X 32" X 12"
Ryobi 26cc 4-cycle complete unit weighs 9 lbs.




1 pint
Fuel Mix
Unit Weight
Weedeater 2-cycle  18  1. 0 21 403 50 Yes 6 lbs.
Ryobi 4-cycle  26  1. 2 25 358 44 regular gas 9 lbs.

* Actual MPG was measured at 12-14 mph speed.


The two fixed mounting points directly under the engine go to the axle or frame point. The third point forward attaches to the seat post. A left hand thumb clutch control pushes down on the front of the unit and engages the drive roller to the tire. The right hand thumb controls the throttle. The unit has wide spacing to accommodate wider tires.

   Rear View of Trimmer-1 Unit
BMX style
Weedeater (Featherlite) 18cc

The center of gravity of the engine is in the center of the bike directly over the axle so the balance of the bike feels normal.

The design of the unit is to power assist the bicycle and be as light and simple as possible while utilizing the world's lowest cost and readily available small gasoline engines.

Because of it's 3-point mounting, Trimmer-1 unit can be easily mounted on any size bicycle. The gear ratio can be easily changed to suit different conditions.

The unit have high quality sealed ball bearings and all stainless steel hardware.
My dream of building my own motor bike started in Barnesville, Minnesota about 1952. I had my nose pressed against the front window of a coast to coast hardware store looking at a brand new "doodle bug" scooter. Just down the street there was a Cushman Eagle with a wonderful sounding 4-cycle engine. Then there were a couple of Whizzer motorbikes that were the ultimate machines. But the cost of those machines was way beyond my pocketbook.

For 30 years I have raced and rode motorcycles and covered thousands of miles of trails with my son while he was growing up. Although this satisfied my need for speed I have never lost my desire to have and ride a motorized bicycle.

Several years ago, while converting string trimmer engines for radio control model aircraft engines, I realized the engines were perfect for powering a bicycle.

1. Their horsepower was more than adequate to push a bicycle past 25mph.
2. They are very lightweight. Approximately 5 lbs. (build mostly of aluminum).
3. Air-cooled with solid state magneto ignition (no points to adjust).
4. Sealed fuel system so there is no gas odor in storage.
5. They have a cantilever crankshaft like a model airplane motor so they are very narrow
    which is ideal for a bicycle (no overhang).
6. They have very good mufflers and because of the lower rpm used on the bicycles; they
    have quite reasonable sound.
7. They are manufactured in the USA in huge numbers and parts are low cost and
    available at any lawn and garden store or repair shop.
8. Almost every home in America has a string trimmer hanging in their garage. So there
    will be parts available for a long, long time.

The  Design  Advantages  of  the  Trimmer   Bike  Unit
The engine and the frame parts work together to form a solid unit. The drive roller shaft is supported on both ends with sealed bearings so all the tire and road abuse is absorbed by the counter shaft and not the crankshaft of the engine, insuring long life and durability.

The speed/torque ratio of the unit is controlled by simply changing the size of counter shaft pulley, which allows you to adjust the gearing ratio to suit your weight and the terrain you will be riding on (i.e. hilly or flat). A large diameter roller provides excellent contact area to the tire.

The roller is engaged or disengaged from the tire by a clutch control cable to the left-hand handle bar. When the clutch is disengaged and the drive roller removed from the tire, the bike is totally free- wheeling with no drag.

The entire unit is very light and can quickly be removed from the bike with 3 Allen screws. Being able to remove the unit easily and leave it at home frees the bike up for competition work. The main advantage of the roller on the tire type drive is, it's the lightest and simplest way to power assist a bicycle. Power assist is were you are the main power source for the bicycle and the bicycle's efficiency is not decreased by a lot of machinery or mechanism (i.e. transmission, gears, belts, pulleys) permanently attached to the bike. Like a moped which is a very inefficient bicycle relying on the engine to be it's primary power source.
Advantages  of  a  small  engine:

1. They are lighter
2. Quieter
3. Easier to bump start (less compression to overcome).
4. They are the most efficient in MPG and even the 18cc has more than enough power
    for power assisting a bicycle.

Do  It  Yourselfers
Kit Builders

Since the engines are originally designed to operate lawn and garden equipment (i.e. string trimmers), there is some conversion or modifying that has to be done to adapt them to the bike unit.

The bike unit is designed for a specific engine.

As a string trimmer engine, the power is transferred to the grass cutting head by way of a long flexible rotating cable.

To power the bike unit, remove the cable drive coupler from the crankshaft and replace it with a belt drive pulley that is included in the kit.

The centrifugal clutch on the string trimmer is designed for very light loads and will not work well for powering a bike. The centrifugal clutch and rope pull start mechanism is removed and a drive belt pulley is installed on the engine for direct drive. To start the engine you simply engage the clutch to turn the engine over. This is called a bump start.

The plastic housing that covers the flywheel has to have a slot cut in it so the drive belt can exit the engine and go to the countershaft. It will take some time to saw and file or grind the slot to clear the belt.

Do  It  Yourselfers

Plan Builders

A set of dimensional drawings for Trimmer-1 is available for each type of engine and covers all the parts in a kit.

A complete price list of all the parts and hardware in the unit is provided so if you have difficulty making a specific part it can be purchased from Carr Precision.

I recommend that you have access to some metal cutting tools (bandsaw, belt sander, drill press, lathe, and milling machine) or someone who does.

Several units have been fabricated out of marine plywood instead of aluminum.

Trimmer-1 bike unit plans, $25.00 plus $5.00 shipping and handling. The plans are available for the following engines:
Homelite 25cc, 30cc
McCulloch 28cc, 32cc
Weedeater (Featherlite Series) 18cc, 21cc
Ryobi 26cc 4-cycle.

We accept cash, check, or money order.

                                Terms:  Prices and specifications subject to change without notice.

Carr Precision
435 NE Spruce Ct  *  Redmond, OR 97756   *  U.S.A.
Phone: (541) 923-4009
Email: carrprecision@centurylink.net


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Copyright 2002 Carr Precision
Last modified: 1/6/2012