Take a small kart engine, add a slick variable-ratiobelt transmission and you'll have a real performance'bear' for half the cost of a commercial motor scooter Build

FLAT OUT, this scrappy little motorscooter will hit up to 32 mph on a

straightaway. When you come to a grade,it automatically downshifts to a lowerdrive ratio to give you rugged hill-climb-ing power. Although designed around amodest 2-1/2-hp engine, its variable-speeddrive enables the PM minibike to equalor exceed the performance of many com-mercial scooters selling for twice as much.In most states, it can be licensed for roaduse with no difficulty.

The best part is the machine's light-weight, low-cost design. The basic framecomes from a discarded 26-in. boy's bi-cycle, which you can usually pick up for afew dollars at a bike shop or junk yard.Most of the parts are stock bike or kartitems, easy to come by and easy to assem-ble. Even if you start with a new engine,the entire scooter can be built for $100 to

$120. With a used engine, the cost is con-siderably less.

Special features include a two-passengercushioned seat, a motorcycle-type twist-grip throttle and a novel, spring-actionfront fork made by modifying the conven-tional bicycle fork. The automatic belttransmission includes not only a cen-trifugal clutch but also a variable-diam-eter pulley system that adjusts drive ratioto load. For road use, lights and a horncan be added.

Start by cutting the bike frame so youcome out with the portion shown in colorin the drawing above. It's necessary tobend the "gooseneck'' section of the frameforward to make room for the engine. Thiscan be done with a bumper jack as shown.Bend carefully to avoid flattening the tub-ing. Heating with a propane torch willmake the job easier.

Yourself a Minibike

Remove the rear fork as shown, butdon't throw it away. It becomes the rearsupport for the double seat. A f te r bendingthe gooseneck forward, cut it and the seatpost so they are both 12-1/2 in. long asmeasured f rom the top bar. Their endsshould be spaced 14 in . apart.

The next step is to make the simplewood welding j ig shown above. This holdsal l the parts in correct al ignment andmakes assembly easy. It 's especially handyif you don't do your own welding sinceyou can bundle the entire jig" off to a localwelding shop and have al l the parts setup and ready for a quick job.

The lower part of the minibike's frameis made f rom 1/2-in. pipe. You ' l l need two22-in. lengths for the side rails, eachthreaded on one end, one 6-3/4-in. lengththreaded on both ends for the f ront cross-bar, and two 4-7/8-in. lengths threaded on

one end for the footrests. Cut the rearforks f rom 1/4x1-in. bar stock, flatten theends of the pipe side rails, and insert theforks in the flattened ends.

The spring-action f ront wheel is madeby cutt ing the legs off a regular bike forkjust below the post, leaving a stub fork.Bol t the legs to this stub fork as shown,w i t h the spreader bar inserted betweenthem. The upper ends of the legs, whichnow extend above the fork, are anchoredto a spring that bears against a plate onthe steering column. As the legs pivot onthe fork, they compress the spring, givingthe f ront wheel a soft, cushiony suspension.

Fasten the f ront and rear forks to theblocks on the j ig , and you are now readyfor welding. Shape the crossties as shownfor a close f i t inside the side rails. Forclean welds, remove the paint from allweld points. Tack-weld al l joints to hold

EXPLODED VIEWS above and across the page show the major parts in the mini-bike assembly. In the first two photos on the opposite page you can see how the jack-shaft is mounted behind the engine with the drive chain running from the smallsprocket on the brake drum to a large sprocket on the rear wheel. A V-belt runningfrom a centrifugal clutch on the engine shaft to a spring-loaded split pulley on thejackshaft provides a self-adjusting variable-ratio drive. The photo at far right showshow the original front-wheel fork is altered to improve the suspension. The upperends of the pivoted fork legs bear against a spring on the steering column.As the wheel rocks back and forth, it compresses the spring, cushioning road shocks

MINIBIKEthe assembly rigid, then remove the framefrom the j ig to complete the welding.Tack-weld the threaded joints at the teesto prevent their twisting.

Wheels and axles

Recommended wheels are the pneumat-ic kart type 4.10/3.50x5 in. (11-1/2 in. indiameter). You could also use 4.10/3.50x4-in. wheels. The front wheel has a 4-in.centered hub. The rear wheel should be ofthe type that has an extended hub andmounting flange on one side to attach adrive sprocket.

The axles are 3/4-in. cold-rolled steel.The front is 5-13/16 in. long overall, and therear is 9-3/4 in. long. The ends on both areturned down for a length of 7/8 in. andthreaded 3/8-24 for mounting in the forkswi th 3/8 in. nuts. Use 3/4-in.-diameter shaftcollars to position the wheels on the axles.(For convenience, if you'd prefer not tomake the axles yourself, they can be pur-chased ready to install from Gill iom Mfg.Co., 1109 N. 2nd St., St. Charles, Mo. 63301.Also available from Gill iom are the partsfor the spring-action front-wheel fork.)

Engine and jackshaft

The engine-support rails are formedfrom 18-gauge sheet metal as shown. Anyfurnace or airconditioing sheet-metal shopcan bend these up for you. Dr i l l 3/8-in.holes at the ends of the rails to match theholes in the crossties. Do this after theframe has been welded to insure precisepositioning. The rails are mounted on 3/4-in.-thick rubber pads that serve as shockmounts to cushion engine vibration. Youcan purchase commercial mounts or runheadless bolts through thick rubber pads.

The two jackshaft supports are eachmade from one piece of 1/8x1x1-in. steelangle and one piece of 1/8x1-1/2x1-1/2-in. an-gle welded together as shown in the detaildrawings. Note that the two sizes of angleare joined to form a channel for the r ight-hand support and to form a Z-shape forthe left-hand support. The Z-shaped leftsupport also requires a 1/8x3/4-in.-wideextension welded to the base. A 3/8-in. boltwelded into the support forms a stud toanchor the brake band. A cotter pin andwasher hold the brake band on the stud.

The two jackshaft supports are posi-tioned flush wi th the rear ends of the

BUILD YOURSELF A MINIBIKE

engine rails. The rearmost holes in theirflanges fit over the same shock-mountbolts that hold the engine rails to thecrossties. The remaining holes are for shortbolts directly through the rails.

The jackshaft is held in two 3/4-in.-borepillow blocks bolted to the supports. Makethe shaft from 3/4-in. cold-rolled steel rodwith 3/16-in. milled keyways as shown.Mounted on the shaft between the two p i l -low blocks is a combined brake drum anddrive sprocket assembly. The drum itselfis merely a narrow ring sliced from a pieceof 3-1/2-in. iron pipe. A rectangular metalplate is welded inside the ring to forma hub.

Dr i l l the 3/4-in. shaft hole in the hubonly after welding to be sure of accuratecentering. If a metal lathe is available,it's a good idea to chuck the shaft wi th thedrum on it and turn the r im true. This isnot a must, however, as exact roundness isnot critical in this type of brake drum.

The 22-tooth sprocket is bolted to thehub of the brake drum wi th two 1/4-in.bolts. Lock the drum-sprocket assemblyto the jackshaft wi th a 3/16x3/16x1-in. steelkey. Al ign the sprocket wi th the 60-toothsprocket on the rear wheel and tighten thesetscrew. With the jackshaft installed,temporarily set the engine in place to besure there's adequate clearance. If there'sany problem, the sheet-metal part of themuffler can be trimmed back slightly.

While you can make your own brakeband, it's simpler to purchase a commercialband made for the purpose. A band l ikethe one shown can be obtained from Auto-matic Steel Products Co., 1201 CowdenAve. S. W., Canton, Ohio (part No. 4557).

One end loop on the band slips over theanchor stud on the left jackshaft support.A standard bicycle hand-brake cable isattached to the other end loop. Notch theloop as shown, insert a cross pin wi th ahole in the middle, and run the brakecable through the notch and the pin. Acable stop can be adjusted to lock thecable at the desired tension.

An anchor for the brake-cable sleeve ismade from a thumb screw and mountedon the left jackshaft support. The cableruns to a bicycle hand brake on the han-dlebars. When the hand lever is squeezed,the cable pulls the brake band tightlyaround the brake drum.

Engine and transmission

The drive is by V-belt from the engineto the jackshaft and by No. 35 roller chainfrom the jackshaft sprocket to the rear-wheel sprocket. The engine used here is

a 2-3/4-hp Clinton Model A-500 with clock-wise rotation. The clockwise rotation isimportant and is available on order, butbe sure to ask for it specifically.

The variable-speed drive consists of apair of split-sheave V-bel t pulleys, oneon the engine shaft and one on the jack-shaft. As engine speed increases, the cen-trifugally operated drive pulley forces thebelt out toward the r im, increasing thepulley's effective diameter. At the sametime, the belt is forced closer to the hubbetween the spring-loaded halves of thepulley on the jackshaft. This lowers thedrive ratio and increases the scooter'sspeed. When the scooter slows down undera load, as in climbing a hi l l , the reverseoccurs. The belt drops to the hub on theengine pulley and rides up the r im on thejackshaft pulley. This automatically i n -creases the drive ratio to provide greatertorque.

The drive specified here is made by theV-Plex Clutch Corp., Hagerstown, Ind.(part No. 66-535). It gives a low-gear ratioof about 8.5 to 1 and a high-gear ratio of4.3 to 1. This produces a top speed of about32 mph at 3.800 rpm. Alternate drive ratioscan be obtained by changing the sprockets.

Engine speed is controlled with a motor-cycle-type twist-grip throttle. These areavailable to fit on standard 7/8-in handle-bars. The A-500's throttle linkage has abui l t - in spring return to the idle position.On engines not having this feature, it isnecessary to install a separate returnspring to pul l the cable back when you letgo of the twist-grip.

Final details

A two-passenger seat can be made asshown, or you can purchase a long "ba-nana" seat from a bike shop. The rear sup-port is made from the rear fork originallycut off the bike frame. A 90° seat post, astock bike item, is needed to move thefront seat support farther forward. High-rise handlebars, another standard bike ac-cessory, complete the job.

If you're planning to fit out the scooterfor road use, check wi th your local licensebureau to see what extras are required bylaw. Fenders are optional, but are advis-able for road travel. (In addition to theitems already noted, the Gill iom Mfg.Co. can supply fenders, engine rails, en-gine shock mounts, twist-grip thottle,jackshaft, axles and a combined brake-drum-sprocket assembly—all designed tofit the PM minibike. Mail-order houseslike Sears and Wards are sources for en-gines, -wheels, sprockets, clutches, bear-ings, belts and chains. Kart-supply centersand motorcycle shops can also providemany of the items listed. * * *