Mini Choppers

This page describes how you can build mini choppers and market and sell them anyway you like.  Be sure to visit the website for more photos and resources.

http://www.diygokarts.com/chopper-plans/mini-chopper.html

Chopped Up and Fast

Mini Chopper Plans and How To Build Info

This section is for our mini chopper information. Thus far, we've completed the build of a small chopper, and plan to make a larger chopper in the future. If you have build or design photos you'd like to share, please email us!

Our new chopper is probably small enough to be considered a "pocket chopper", for what that's worth.

We used a 3 HP B&S engine for the project. It is plenty of power for such a small bike. The frame was made from 3/4" pipe, bent with a Harbor Freight hydraulic pipe bender.

Chopper Parts

Our parts section has many of the general parts involved with building plans. And when you're ready to buy, there are many suppliers, but we've found that Northern Tool has some of the best prices around.

Chop That Frame Up!

How to Build a Mini Chopper Frame and Schematics

The frame design is one of the most rewarding parts of building a mini chopper, so if you are able, we strongly suggest building your frame rather than buying one. However, you must be sure of your welding ability. Either you, or someone you love will be supported by it, and traveling fast — you do not want this thing to fall apart.

Thinking About Frame Design

The first thing to do when considering the frame is what you want it to look like. If you

haven't already discovered it, there are many different styles of frame design, and subtle nuances to each. Surf our photos and others, and print a few out. This will help you sketch your own ideas better.

Using a Bender For the Frame Design

There are two main ways you can make your frame — with or without a bender. A frame that is square rather than rounded simply will not look as good. Our frame was made from 3/4" pipe, bent with a Harbor Freight hydraulic pipe bender.

[Learn about bending and selecting material].

How We Built the Frame

First, we cut and bent two sections of pipe to the same bend, roughly 100 to 110*. It's not too time consuming to bend them equally in the pipe bender, though it's not an exact science. We've heard that if you count the pumps you can make the same bend twice, so you may try that method.

Then we made two 90* pieces of material for the rear wheel assembly. Since the pipe bender won't make a 180* bend, you must make two pieces and then weld them together in the "U" shape. This "U" should be longer than necessary so you can cut the material to length when you mock it up.

The backbone tube was then welded to the "U" assembly. This will need to be very square. To accomplish this we laid the assembly on the concrete and traced the "U" shape. Using a carpenter's square we drew two parralell lines toward the backbone. Then we drew a 90* line through the two lines. From this line we were able to draw squared lines that outlined where the backbone was to be attached.

You must fishmouth the joint between the "U" and backbone. This can be done with a grinder.

Assembling the Upper and Lower Parts of the Frame

Take the two pieces making the lower assembly, and lay on the concrete. Take your upper assembly and mock up how you'll like the stretch of the frame to look. Since you cut all the materials longer than needed, you'll have some options.

Fabricate the Engine Mount

You can read more about these options in the parts section.

Engine mount was custom sized for the frame from a purchased engine mounting plate and 1/4" flat stock welded to form the correct size. When welding long welds like these, ensure that you tack first and use lots of short welds so you don't warp the plate or frame.

You can make your own mounting plate, but it's often cheaper to buy them, and you won't need cut the slots for the mounts.

Position The Engine

When designing your frame, consider where your engine will fit. You'll want the engine to be close to the rear wheel to minimize chain length and position the clutch farther from your legs.

Tack the plate in to your frame and place your engine on it, noting the adjustment travel, exhaust and carburetor clearances

All Raked Out

Mini Chopper Rake and Front End Forks

The angle that the front end forks are set at is known as the rake, and is the defining characteristic of a chopper.

How Much Rake For a Mini Chopper?

For our mini chopper project (Fig. 1), we used an extreme front end rake of around 50 degrees.

How Does Rake Effect Performance?

As rake increases, several things are effected. First, your turning ability diminishes. This isn't a major factor on a small bike, but shouldn't be overlooked.

Secondly, the longer you go with the forks, the more the material will flex on bumps. For a pocket sized chopper, the forks aren't too long. But the larger the mini bike, and the longer the forks, the farther the front wheel will be from the frame. This flex must be considered. Gussets and thick material are your solution, but at some point you'll reach "too long" — you are limited by material and welding ability, not imagination.

Rake and Safety

If you make poor welds or don't use gussets, you will have problems with your frame breaking. The amount of rake increases the stress on the frame, so reinforce adequately and be cautious.

Make The Forks

To make the forks without a large diameter drill involves a lot of welding. Ideally, one would cut two pieces of 1.5" flat stock, drill through them and then slide the fork tubes through the holes.

Without a drill bit large enough, we did not have this luxury. Instead two 3" tubes, boxed by 4"x1.5"x.25" flat stock create the top portion of the forks in Fig. 2. Holes were drilled top and bottom at 1/2" size for the frame neck to fork connection.

The two long tubes were then added to the top portion. (Fig. 3). A cross support was added near the bottom, high enough to clear the intended tire.

The Axle Tabs

For the axle tabs, a 1.5"x1.5"x.25" piece was welded to a 4"x.75"x.25" piece of flat stock.

A slit was cut into the back side of the forks (to minimize asthetic damage).

The axle tabs were then inserted into the forks (Fig. 4). Some material was ground off the inserted portion and the fit was very tight. The tabs were then welded in place at the bottom and up the back at the slit for maximum safety.

So far, this method worked very well for a small bike. Your results may vary.

Adding the Handlebars

The forks were installed, and again it was ensured that the frame sat level to the ground.

Risers were welded in place, and then the handlebar was welded on. We opted for a swept back, simple design on this bike instead of ape hangers. They were made from 3/4" pipe so they could be bent to shape.

Strengthen the Frame

After mocking up the forks, wheels, and frame, and ensuring that all the toleranaces were correct, a strengthening gusset was welded to the neck tube (Fig. 8). This is critical for adding needed strength, as the long rake will put serious stress on the neck tube.

Burning Rubber

Mini Chopper Wheels and Tires

You can use all kinds of wheels and tires on your chopper.

Some people use go kart style wheels. Some use ATV style. Some use bicycle wheels for up front to get the skinny look. Used a cool wheel? Send us a photo and we'll publish them!

For our small mini chopper, we elected to use 4" go kart style wheels and tires.

This turned out to be an adequate choice for the front, but the rear needed a 5" wheel to provide clearance for the sprocket.

Make Wheel Bushings to Center the Wheel

We used a solid axle, which our 5/8" bearings turn on. Because the wheel is therefore free floating, you'll need bushings to keep you wheels in the center of the axle.

Depending on your wheel hub design, this might become tricky.

Fig. 2 shows one side of the steel wheel and hub assembly that was used. As you can see, the bearings stick out about 1/16" from the hub. A 5/8" flat washer provides a surface for the bearing edge to ride against, and allows the wheel to spin freely. Measure the distance to the axle tabs and weld your washer to a sized piece of cut pipe.

The opposite side of this two-piece wheel made things a little more complicated. Because the hub doesn't extend through the second half of the wheel, we needed a washer small enough to not rub against the wheel. We found the solution (Fig. 3) in a piece of an old bearing race, welded to the appropriated sized pipe. This fit perfectly (Fig. 4), and we were able to install the wheels.

Use plenty of Loctite or some other method to ensure the axle nuts will not come off. Careful not to over-tighten the wheels. This will reduce bearing life, and either spin one of the nuts off, or overtighten the axle bringing you to a quick halt.

Blat, Blat, Blat

Mini Chopper Custom Exhaust and Engines

For our smaller mini chopper project, we elected to use a 3 HP Briggs and Stratton engine. This produced plenty of power for a 200 pound rider and the bike.

To get even more power and response, a torque converter may be used to achieve a variable speed transmission.

The Engine

We used a 3 hp Briggs and Stratton engine for the small mini chopper. This is a horizontal shaft engine, found on tillers, go karts, and the like.

[Read more about engine choices].

The Exhaust

You may certainly use a standard engine exhaust for your project. This will accomplish the quietest ride, and the high-flow mufflers that are made shouldn't effect your performance in any large way.

However, if you've already purchased a pipe bender, we strongly suggest making your own custom exhaust.

All horizontal mount engines are generally threaded with NPT pipe threads on the block. Our 3 hp engine accepts a 1/2" NPT thread.

Open Pipes Custom Exhaust

Black pipe was purchased at the local Home Depot. After measurements were made, the 1/2" pipe was bent at roughly a 60* angle.

This bend was measured, and the next bend was selected. An opposite bend was made in the opposing direction finished our exhaust pipe, which fit the profile of the frame very well.

Installing the Custom Exhaust

The engine was removed to install the exhaust, since it had to be screwed into the block. The result was phenomenal. The open pipe completely changed the tone of the engine. Rather than the predictable "lawnmower" sound, a deeper, satisfying "blat, blat blat" emanates from the pipes.

Power To The Wheels

Drive train, Sprocket, and Rear Assembly for Mini Choppers

The drive train is a simple system on karts and mini bikes. The cheapest option to get the power from the engine is the centrifugal clutch. For a much better option, a torque converter can be used which effectively gives the project an automatic transmission.

[Read more about torque converters and centrifugal clutches].

Our design incorporates a centrifugal clutch with direct drive to the rear sprocket — however a jackshaft could also have been installed to increase speed, increase torque, or to simply have two runs of shorter chain instead of one long run.

Clutch

Our clutch is an 11 tooth Max-Torque centrifugal clutch. The #35 chain was very long (Fig. 1), and we've since added a chain guard to protect the rider if there is a breakage.

Chain Guard

Even for a short chain, a guard is a good idea. If the chain breaks, the guide will ensure that it doesn't wrap around the rear sprocket and lock up the back wheel.

Our chain guard was made from 16 gauge material found at a scrap yard. Two 3" pieces were welded together, and then the profile was cut with the grinder. Finally, another 3" piece was tacked to the profile and finish welded after the shape was acceptable.

After some finish grinding, and a few holes drilled for heat dispersion, it looks pretty good!

Though you can purchase small chain guards from parts suppliers, the positioning of the mini-chopper's clutch is dangerously close to pants legs. An enclosed guard is much preferred.

Sprocket Choices

Sprocket is a 60 tooth #35 pitch. This size sprocket needs a 5" wheel for proper clearance. The example photo shows a 4" wheel. Before selling the chopper we switched it to a 5" wheel. The ground clearance on a 4" wheel is not adequate because on a two wheeled chopper you make sharp leans when cornering. When doing this, the sprocket will scrape the ground, damaging it and the chain.

During the test drive, we over-inflated the 4" tire to compensate for the lack of clearance; it blew out! Do not do this, get a 5" or larger rear tire for this sized sprocket.

Brakes and Throttle

Braking Systems and Throttle Control For Chopper

For this project we chose a disc brake for the rear. There are many different styles of brakes available: band, drum, and disc. Read here for more info on mini bike brakes.

The Disc Brake Installation

A parts supplier informed us that many people are using a disc brake without a dedicated braking disc, instead opting to use the drive sprocket as the disc. We decided to give it a shot (Fig. 1) and so far have had good results. It is worth noting that you should be careful of over-oiling the chain and getting excess oil on the sprocket, as this would hamper good braking response.

Hand Brake, or Foot Brake?

Both are adequate choices, and both are readily available from parts suppliers. We were looking for clean lines on the handlebars, and therefore went with a foot brake (Fig 2 &3).

If going this direction again, we'd run the cable through the frame. With a little planning it would be very easy, and give a further clean look to the frame.

To provide brake stops, we welded a small nut to a riser just before the foot pedal. This ensures that the cable sheathing can not pull itself forward.

The brake line was attached at the disc brake with common wire stops. We found that the easiest way to cut the sheathing and the wire line was with a cut-off grinding wheel.

Throttle Installation

Throttles are generally sold in 7/8" and 1" ID. This would work fine if you're using tubing, but because we wanted a bent handlebar, we had used pipe (OD = 1.05"). [Read more about tubing vs pipe].

Therefore we simply marked where the throttle assembly would end on the pipe, and ground down the material up to that point. (Fig 4). This isn't the most elegant solution because invariably you grind farther up the bar than intended and it will show after the installation. Perhaps using masking tape to make the edge would help.

Throttle Cable

The throttle cable runs from the grip back to the carb. We had removed the governor on the Briggs and Stratton engine, therefore we hooked it up to the throttle butterfly directly. This took a little engineering (Fig. 5).

Sitting in Comfort

How to Make a Mini Chopper Seat

After you're done with the chopper's frame and other doodads, you'll still need to make a comfortable seat. Though a small part, it's design might be on of the most important parts to enjoying riding the chopper.

What Material to Use?

We've used vinyl, which is purchased at any fabric store. Look in the remnants section, and you'll find a yard or two of all the various colors for at least half the price.

For a classier seat, go to your local leather dealer. They will also have remnants from upholstery jobs. We found a suitable piece of leather for our next project for only $8. This small addition will make for an awesome looking chopper.

Foam can also be purchased at a fabric store.

For a low-cost idea, consider checking directly at an upholstery shop. They're certain to have all matter of material lying about, and would probably be tickled to help, especially if you can find a small operation and bring your project with you to show them.

Preparing the Job

After you have your materials, it's time to get down to work. We used a piece of 1/2" plywood as the base of the seat. First the plywood is cut to the desired shape of the seat.

Then you'll cut the material at least 2" over the edges of the seat pan. This is so the seat can be overstuffed.

Begin the shaping process with some common tape. This wrap makes sure that the stuffing stays the right shape, and doesn't shift during the upholstering process. (Fig. 1).

Upholstering the Seat

Using a staple gun with short staples (we uses 1/4"), attach the back of the vinyl to the board. (Fig. 2). Make sharp creases by pulling the side of the material tight (Fig. 3) before folding it over and securing (Fig. 4). Repeat for the opposite side.

The front is the folded for both sides. (Figs. 5-6).

Do not use too many staples at this point. If you mess something up and have to start over, you'll appreciate not having to pull many staples out, which will invariably ruin the entire piece of material.

Inspecting Your Work

Turning the seat over, inspect the finished product. If acceptable, finish staple it around the edges. (Fig. 8).

You'll then use a razor to cut away this material so that you can mount the seat properly.

Mounting Methods

You can either use bolts, with the special wood nuts that should be embedded in the top side of the board, or use large wood screws. Since we used the special nuts on the go kart project, we used large wood screws to test their capacity.

Drilling a pilot hole that is just smaller than the screw, we installed the seat (Fig. 9) and have had no problems.

A mounting bar (Fig. 8) was added to the frame and two mounting holes were drilled. This is the only mounting on our seat.

The mounting bar is 1/8" below the profile of the seat portion of the frame. This ensures that the seat is sucked down toward the mounting bar, and against the frame members for a tight fit with no wobble.