Center for International Rehabilitation

Building the Rear Wheels

Building a wheel that is light, perfectly round, doesn't wobble from side to side, and can withstand the stresses of curb climbing and dirt roads takes a lot of practice. It requires technical skills that can be learned from this chapter, and it requires a good deal of patience. This is a good job to tackle on a day when you are well rested and relaxed.

We prefer to use bicycle rims and tires instead of the solid rubber tires used on many conventional wheelchairs. Not only are bicycle tires less expensive and easier to find, but they give a much smoother ride and are easier to push. Before starting to build the wheel, be sure you know what size wheel your customer needs. Chapter 4 includes a discussion of the various wheel and tire sizes that are appropriate for farm and city use.

Each rear wheel is composed of a hub, a rim and rim strip, spokes and nipples, a handrim, and a pneumatic tire with its air filled inner tube. This chapter outlines the basic procedures involved in wheel building, beginning with the hub and working our way out to the tire.

THE HUB

The hub is the unit in the center of the wheel. It consists of an axle, two sets of sealed bearings that rotate around the axle, a spacer tube that holds the two sets of bearings apart, and a hub casing that fits over the bearings. All of this is held on the wheelchair by a nut that screws onto the axle bolt.

THE AXLE

The axle is a bolt around which the bearings turn. The axle also attaches the wheel to the frame of the chair. Many commercial wheelchairs use axles that are 1/2" (12 mm) or less in diameter. Since these axles must be made of hardened steel to prevent bending, when they fail, they often fail by cracking. Our chair is designed to use a 5/8" mild steel axle bolt (or a 16 mm bolt with a metric bearing). This bolt is thick enough that it does not need to be hardened. Using a soft (mild) steel bolt reduces the risk of axle fracture. In addition, 5/8" mild steel bolts are inexpensive and widely available. However, it may be difficult to find ball bearings that will fit these bolts.

The diameter of most bicycle axles is 3/8" (10 mm) or less. These axles are far too thin to be used on a wheelchair where they are supported only at one end. Wheelchairs that use bicycle axles must have a frame member outside the rear wheel to support the axle. This frame member can increase the weight of the chair, and worse yet, it always increases the width. In the Philippines, bicycle axles have been used on wheelchairs which had no frame member to support the outer end of the axle. As a result, the axles on hundreds of these wheelchairs are bent or broken.

BALL BEARINGS

Our wheelchair is designed to use sealed ball bearings. Second-hand bearings are sold by many repair shops at low prices. While they might be too old to use in an auto or in power tools, many can be perfectly adequate for a wheelchair. A well chosen used bearing can last for years in a wheelchair if it is kept properly adjusted and lubricated.

It is important that bearings be kept free from dirt and grit. The seals should keep your bearings clean; however, if a used bearing feels gritty when you turn it with your finger, pop out one of the seals with a knife, clean the bearings with kerosene, and add some new bearing grease. Do not smoke during this procedure. If you cannot replace the seal, pack the bearing with grease and put it back onto the axle with the broken seal facing the inside of the hub where there is less dirt.

BEARING SIZES - MATCHING THEM TO THE AXLE

Different sizes of bearings are sold in different countries. If possible, use bearings that will fit the 5/8" axle. Bearings with an inside diameter of both 5/8" and 16mm will fit the 5/8" axle. If these are not available, get a bearing with a 15mm inside diameter and have the diameter of a coarse threaded 5/8" axle bolt reduced to 15mm in a lathe. Make sure the bearings fit snugly, and be sure to use a locknut on the axle. The following table lists bearings that can be used in the rear hubs and caster barrels.

MAKING THE HUBS

MATERIALS

JIG FOR MAKING THE HUBS

• Hub Drilling Jig

Directions for making this jig are found in Appendix B. The jig is also available as part of the Basic Tool Kit.

DIRECTIONS FOR MAKING THE HUBS 1) Cut a 3" (7.6 cm) length of 1-1/2" O.D. tubing with a tubing cutter. 2) Scratch a straight line along one side of the hub casing. A short piece of angle bar makes a good ruler for drawing this line. Note: The hub drilling jig must have the same number of holes as the rim. 3) Slide the drilling jig over one end of the hub casing. Line the jig up so that the line scratched on the hub casing intersects one of the holes in the inner ring of the drilling jig.

4) Set the drill press to a very high speed. Position the jig and hub in the drill press vise so that the jig is furthest from you.

5) If you are using a hub drilling jig without drill bushings, use the holes in the drilling jig as a guide to drill all of the holes in one end of the hub. Use a 1/8" (3 mm) drill bit. If the spokes will fit through a smaller hole, use a 7/64" drill bit.

The hub drilling jig found in the Basic Tool Kit has two guide holes reinforced by drill bushings. The bushings prevent these guide holes from widening with use.

To use this type of jig, first drill two holes in the hub using the two holes reinforced with drill bushings as guides. Next rotate the hub to the right (clockwise) until the upper hole just drilled in the hub is lined up with the next hole in the upper ring of the jig. Pin the hub in place by inserting a nail.

Once the hub is pinned in place, drill the next two holes using the drill bushings as guides. Continue this process of drilling, rotating, and pinning until all of the holes in one end of the hub are drilled. To ensure an even spacing of the holes, always put the nail through the very first hole that you drilled in the hub.

6) Slide the drilling jig onto the other end of the hub casing. Position the jig and hub in the vise as before. Twist the hub in the jig until the scribed line on the hub casing points between two holes in the jig. The lower hole must be to the right of the scribed line (see diagram). It is important to position the jig this way because it will affect the wheel spoking pattern. Drill the rest of the holes in the hub, always inserting a nail in the first hole you drilled in this end of the hub.

7) Slide the bearings into the ends of the hub casing, putting the 1-1/2 inch spacer tube between them. If the bearings fit loosely, remove them and reduce the diameter of the hub casing with the indenting tool until the hub casing is small enough to grip the bearings. (See Chapter 6 for instructions on how to indent the tubing).

Don't indent the hub to make a stop for the bearings! The heads of the spokes will keep the bearings from falling out and the spacer tube will keep the bearings apart.

AN ALTERNATE HUB DESIGN

If the hub casing tubing (1-1/2" O.D.) is in limited supply, it is possible to make a split hub casing using half of the amount of 1-1/2" O.D. tubing. This design was invented by Omar Talavera of Nicaragua who split the hub on his own chair in order to have enough hub casing tubing to make another chair.

In addition to using less tubing, hub casings that are made according to this design will not need to be indented to hold the bearings. The disadvantage of this design is that the wheels will not turn quite as freely. This hub design adds about 1/4 of a pound (0.11 kg) of drag to the chair.

This hub can be made using the same materials as were used in the standard hub design; however, the spacer tube will not need to be exactly 1-1/2".

When drilling these hub casings, it is easiest to put a longer piece of tubing in the drilling jig, drill it, then cut it to size.

SPOKING THE WHEEL

Wheelchair wheels that use our hub design can be spoked with a three, four, or five- cross spoking pattern. Five-cross means that each spoke will cross five other spokes as it stretches from the hub to the rim. The more the spokes cross each other, the more flexible the wheel is, and the less likely it is for the spokes to break. We recommend using either a four-cross or five-cross spoking pattern on the rear wheels. The length of the spokes will depend upon the spoking pattern as well as the size of the wheel.

The above spoke lengths are correct for a wheelrim with 40 spoke holes. If your wheelrim has fewer spoke holes, increase the spoke length: For 36 spokes add 1/8" (3 mm); for 32 spokes add 1/4" (6 mm); for 28 spokes add 3/8" (10 mm).

The above spoke lengths are also correct for a wheelrim with a fairly flat inside surface. Some rims are indented where the spokes enter the rim; these rims need shorter spokes. To find out whether your rims need shorter spokes, measure the inside

diameter of the wheelrim. Divide this diameter by two to find the radius, then compare this radius with the "normal" radius in the chart below. If your radius is less than "normal", your spoke length should be reduced by the same amount.

Most bicycle spokes are 0.080" (2 mm) in diameter. If these spokes are kept tight, they are strong enough for most wheelchair use. For very active wheelchair riding, we recommend 0.105" (2.67 mm) diameter spokes.

MATERIALS AND JIGS NEEDED TO SPOKE THE WHEEL

• Drilled hub with bearings and spacer tube

• Bicycle wheel rim

• Spokes and nipples

• Spoking board

Instructions for making the spoking board are found in Appendix B; it can also be purchased as part of the Basic Tool Kit.

DIRECTIONS FOR SPOKING THE WHEEL 1) Set up the spoking board by inserting the 5/8" (or 15 or 16 mm) threaded rod into the center of the rim drilling jig. The rod should stick up about 2-3/4" (7 cm) beyond the nut on one side. 2) Place the hub casing containing the ball bearings and spacer tube on the spoking board. The end of the hub that has the scribed line passing through the hole should be on top.

3) Place the rim on the spoking board. Note that the holes on the rim alternate in a slightly up or down position.

4) Oil the threads of the spokes with light weight machine oil.

5) Put the first spoke through the hole on the upper end of the hub that was drilled on the scribed line. Connect this spoke to the rim through the lower hole on the rim next to the valve hole. Screw on the nipple two or three turns. Note that we are putting spokes from the upper end of the hub into the lower holes of the rim. This is the opposite of the spoking system used for bicycles. Our method pulls the upper and lower spokes closer together, giving more space for the rider's fingers under the handrim. Our spoking method also allows us to reduce the width of the chair by 1/4" to 1/2".

6) On the upper end of the hub, insert spokes into all of the rest of the holes in the lower line. All of the bent ends must face in the direction shown in the diagram. 7) Count over four holes and connect the next spoke. Continue this process until all the spokes from this first row have been connected to the rim. Screw each nipple on two or three turns. 8) Twist the hub until the spokes are tight. Be sure that the heads of the nipples are pulled up against the rim, and that the spoke heads are tight against the hub. 9) Insert spokes into the top row of holes in the hub with the heads of the spokes facing in the opposite direction.

10) Choose any loose spoke; cross it over the top of the spokes that are already attached to the rim. The number of spokes each spoke crosses will depend on the length of the spoke as previously discussed. On the last cross, put your spoke underneath the other spoke (see diagram). This will help to prevent the spokes from loosening up later on. Connect the spoke to the hole in the rim halfway between two other spokes. 11) Attach all the rest of the spokes from the top row on the hub by counting over four holes in the rim for each one. Use the same crossing pattern, lacing each spoke under the last spoke it crosses. Screw on the nipples two or three turns.

12) Turn the wheel over, and replace it on the spoking board.

13) Find the scribed line on the hub. Look at the two holes it passes between. Put the next spoke through the lower of these two holes. (It should be to the right of the line as you face it.) Connect this spoke through the hole in the rim just in front of (to the left of or counterclockwise from) spoke #1. (See diagram.)

14) Insert spokes through the rest of the lower line of holes on the hub and attach them to the rim through every fourth hole starting with the spoke you connected in step #13. These spokes should face the same direction as the first row of spokes you connected.

15) Insert spokes in the remaining line of holes in the hub. The spokes should face in the opposite direction from the last line of spokes. Connect these spokes to the rim using the same crossing pattern that you used on the first side of the wheel.

16) Holding the wheel rim flat against the spoking board, tighten the spokes all the way around the wheel with your fingers. Continuing to hold the wheel rim flat against the spoking board, use a screwdriver to tighten all the spokes one more complete turn.

It is easier to know when you have tightened all the spokes if you begin tightening them on one side of the valve hole, then tighten them all the way around the wheel, and stop when you reach the other side of the valve hole. 17) Stretch the spokes by grasping two at a time and squeezing them hard. Then tighten each spoke an additional 1/2 turn. Repeat this stretching and tightening process until most of the spokes have begun to tighten up.

TRUING THE WHEEL Once the wheel is completely spoked, the challenge begins. By tightening and loosening selected spokes you can usually make the wheel perfectly round, the rim flat, and the spokes uniformly tight. Each spoke should be tightened only a little bit at a time. Be careful not to tighten the spokes too much. We've seen wheels that collapsed because the spokes were tightened too far before the wheel was true.

DIRECTIONS 1) Set up a truing stand. You can use an old bicycle fork with two nuts welded on. Bolts screwed into the nuts will serve as reference points when trying to judge how far out of alignment the wheel is (see diagram). Secure the bicycle fork in a vise. 2) Spin the wheel on the truing stand. Using the bolt as a reference point, notice when the rim moves out away from the hub and when it moves toward the hub. Tighten the spokes along the parts of the rim that move away from the hub. Remember to tighten the spokes at the edges of the bulge less than those in the middle. Keep tightening the spokes until the wheel is round. If the wheel gets too tight during this process, loosen the spokes around the wheel that are pulling the rim toward the hub. The spokes should only be moderately tight when this step is completed.

3) As a spoke becomes taut, the friction between the spoke threads and the nipple increases. If the friction is too great when the nipple is turned to tighten the spoke, the whole spoke can twist rather than becoming tighter as was intended. Stressing the wheel will put added tension on some spokes and it will allow some spokes to loosen and untwist. To stress the wheel, remove it from the truing stand, lay it on a flat surface, and press down with both hands on opposite sides of the rim (the hub will support it in the middle). As the spokes unwind you may hear some creaking sounds. Rotate the wheel slightly and press again. Repeat the process all the way around the wheel until you don't hear or feel the spokes unwinding. Turn the wheel over and repeat the stressing and turning on the other side until you can't hear or feel the spokes unwinding. Once all the spokes have been given a chance to untwist, you can tell how tight each spoke actually is.

4) Mount the wheel back on the truing stand. This time, check for side to side motion as the wheel spins. Holding a pen or piece of chalk next to the wheel may help you to find the bulges. The chalk will leave a mark on the rim on the spots that bulge out. Where the wheel bulges to one side, tighten the spokes that will pull it to the other side. Remember to tighten the spokes on the ends of the bulge less than those in the middle. It is difficult to separate steps 2 and 4 above. Whenever you tighten a spoke it will affect both the side to side and up and down alignment of the wheel; consequently, it is important to work with the wheel as a whole. 5) Continue to make adjustments on the wheel, until all of the spokes are fairly tight. Once the wheel looks like it is round and doesn't wobble from side to side, remove it from the truing stand and repeat the stressing and turning procedure on both sides of the wheel. Place the wheel back on the truing stand and make final adjustments. 6) Finish this project by tightening each spoke another 1/2 turn (unless the spokes are already very tight).

ADDING THE HANDRIMS

Once the wheels have been built and trued, the next step is to attach the handrims. The easiest method of attaching a handrim to a wheel uses brackets made out of short pieces of flat steel bar. There are two different styles of brackets that will work well. One style is best for a handrim that is almost as large as the wheel, the other is used for a handrim that is smaller than the wheel. Connect the handrim to the wheelrim in four points equally spaced around the wheel. If the wheel rim has 36 spoke holes, three connecting points are preferable; it will save you some work and will result in a lighter wheel. The illustrations show the two different methods of using brackets made of steel bar to connect the handrim to the wheel.

MATERIALS FOR ADDING THE HANDRIMS TO THE WHEELS

JIGS FOR ATTACHING THE HANDRIMS TO THE WHEELS

• Handrim Drilling Jig

This jig is an adapted spoking board. Directions for making one can be found in Appendix B; it can also be purchased as part of the Basic Tool Kit.

DIRECTIONS

The directions that follow describe how to mount an 18" diameter handrim on a 24" x 1- 3/8" wheelrim using method B. If the handrim is nearly the same diameter as the inside of the wheelrim, use straight pieces of bar for the handrim brackets.

1) Cut 1/8" x 1/2" (2 or 3 mm x 10 mm) flat steel bar into three or four 3" pieces (three or four pieces if your wheel rims have 36 holes, four pieces for all others). 2) Scribe a line 1/4" (6 mm) from one end of a piece of bar. Center and drill a 7/32" (6 mm) hole on the line. Use this piece as a jig to drill identical holes in the other pieces of bar.

3) Set up the Hossfeld type bender to bend bar. Mount the "U" Pin Roller on the center pin. It will serve as the bending die. (A detailed description of how to set up the bender to bend bar can be found in Chapter 6). 4) Start the bend as close to the hole as possible. Using the drawing as a sample, bend the bar to the correct shape. It will probably take a bit of trial and error to get just the right bend.

5) Place the wheelrim in the rim drilling jig and drill four 7/32" (6 mm) evenly spaced holes. (Drill three or four holes if you are using a 36 spoke wheelrim.) If you are not using a jig, please note that the holes are not drilled in the middle of the rim, but are drilled closer to the outside edge.

6) Bolt each handrim bracket to the wheelrim using 3/16" (5 mm) diameter bolts and lock nuts (or lock washers). The bolts should be 5/8" (16 mm) long.

7) Balance the handrim on top of the handrim brackets. Check to be sure that the handrim is level in all directions and that the brackets are centered under the handrim. If the handrim isn't level, or if the brackets are not centered, bend the handrim brackets until the problem is corrected. The brackets must be centered under the handrim or there will not be enough clearance for the rider's fingers.

8) Weld the handrim to the handrim brackets with a steel or brass welding rod. Since the handrims will not be painted or plated, the welds must be filed so that the discolored metal is not too obvious.

9) Wax the handrims to prevent them from getting rusty before they are sold. Plating the handrims should not be necessary if they are to be used on a daily basis; oil from the rider's hands usually protects the handrims from rust. If the handrims are plated with chrome, the plating often peels and can cut the rider's hands badly. Because of this problem we prefer less expensive nickel plating.

ADDING THE TIRES AND ATTACHING THE WHEELS TO THE FRAME

1) Check to be sure that none of the spoke ends are protruding past the heads of the nipples. If any do, cut and file them until they are flush with the nipples.

2) Put a rubber rim strip on each rim. These can often be purchased from a bicycle parts dealer. If you can't find any you can make your own out of a strip of old inner tube.

3) Put the inner tubes and tires onto the rims. Inflate the tires to about 20 pounds of pressure. Check both sides of the wheel to be sure that the tire is seated all the way into the rim. Then inflate the tire to its rated pressure.

4) Using the axle bolts, attach the wheels to the frame through the axle socket. Use locknuts or nuts that you have made into locknuts (a description of how to do this can be found in Chapter 6). The nuts should be very tight.

5) Spin the wheels to make sure they are true and spin freely without too much friction.

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