Introduction: Carbon Disc Wheel for a Track Bike Without Custom Tooling
This instructable describes how to make a one-off carbon track disc wheel without having to fabricate jigs, tools or moulds. The materials can be ordered from ideas2cycles or you can make them yourself using these instructions. A lathe and a drill press/milling machine is needed as well as some common composite lay-up supplies. Basic knowledge of machining and laminating is required.
The wheel is made up of a rim, two parallel side plates and a custom hub. They are joined using adhesive bonding. The flat side plates can be customised with, for example, patterns and text either by modifying the mould before laminating or by attaching decals etc. afterwards.
Step 1: Design
Selecting a rim
Almost any rim can be chosen as long as it has a two planar and parallel surfaces that the side plates can be bonded to. Features such as steel eyelets and double wall construction, deep cross section etc. only add weight. A cheap and light rim is recommended. Spoke hole count does not matter. Clincher and tubular rims both work. An aluminium clincher rim with machined side walls is used in this example.
Selecting material and manufacturing method for side plates
The side plates could be made from thin alloy (steel, titanium, aluminium, magnesium) sheet without a terrible weight penalty and with several benefits. However, using a low density composite laminate gives a better stiffness-to-weight ratio. Any reinforcement and resin system will work, but the best options are either aramid fiber ("Kevlar") or carbon fiber in an epoxy matrix. A laminate thickness of 0,6-0,9 mm is sufficient when using carbon fiber as reinforcement. Even thinner side plates are possible, but a core material may have to be added to ensure rigidity of the wheel and stability of the planar surfaces. A couple of plies of bi-directional carbon fiber weave works nicely. For the unidirectional pre-preg lay-up shown here, a [0/90/-45/+45]SE laminate was used. With eight plies the total thickness was 1,0 mm.
The laminates can be made in several ways depending on available equipment. A regular wet lay-up requires minimal supplies, but a higher fiber volume fraction can be obtained by applying pressure on the laminate either mechanically or using a pressure difference (vacuum bagging, autoclave). No tooling or moulding is necessary since the side plates are flat. A non-porous planar surface, such as a sheet of metal, is sufficient. A table top could be used as well, but glass will result in the best surface finish. Mould preparation is the same regardless of material and manufacturing method and is described in the next step. Debossed features can be made into the laminate easily by placing thin two dimensional shapes on the mould before applying resin and reinforcement. A water/laser cutter or vinyl cutter is very useful for making debossed text, for example.
Designing the hub
A custom hub is required. If you don't want to machine it yourself one can be ordered from ideas2cycles. The side plates are bonded to the hub and therefore special flanges are needed. The hub flanges must be as far apart as the selected rim is wide. If using, for example, a rim that is 20 mm wide, the outside distance between flanges must also be 20 mm. This way the flat side plates can be bonded using a uniform bond thickness. A three-piece hub was designed for a couple reasons. Firstly, the large diameter flanges can be cut from bigger stock, while the smaller diameter features can be cut using smaller stock resulting in less material wasted. Secondly, the side plates can be joined to the hub not just by adhesive bonding, but also by clamping them between the hub bodies. Thirdly, the whole wheel is not ruined if the threads for the cog are stripped, because the threaded part can be detached and replaced. The only difference to a regular track hub design is the flanges. Spoke hole count should match the hole count in the selected rim, but not every hole on the rim needs to be populated. For example, a 32h rim can be laced with sufficient accuracy using 16 spokes, which means 8 holes on each hub flange. A hard aluminium alloy is recommended for hub material.
Step 2: Fabricating the Parts
Fabricating the side plates
Whatever surface is used as a mould, it must be cleaned and preferably sealed using a sealing agent or similar. If debossed features are wanted on the side plates, they should be attached to the mould at this point. Vinyl works nicely because it has an adhesive surface already and does not bond easily to the resin. The text should be cut mirrored so that it comes out the right way (left to right) in the final product. Sealing tape needs to be laid down (for vacuum bag processes) before waxing the mould. In addition, all required plies should be cut before waxing the mould, especially the reinforcement. Mould release agents can be substituted with carnauba wax or some automotive wax. A paper template outlining the wheel is useful for keeping track of how much area is needed.
The steps for vacuum assisted resin infusion (VARI) and pre-preg lay-up are described below.
1. Clean mould surface with acetone or MEK.
2. Seal material using sealing agent (optional).
3. Lay down sealant tape leaving a large margin.
4. Cut reinforcement fabric, peel ply, flow medium and vacuum bag.
5. Wax the mould surface thoroughly twice and polish after each application.
6. Position reinforcement fabric and tape down stretched-out spiral wrap around the perimeter to ensure evenly distributed pressure.
7. Lay down peel ply, flow medium and feed through for vacuum hose.
8. Seal the bag and attach feed through for the resin hose in the middle.
9. Pull a vacuum and check for leaks. Make sure there is suction in the resin hose.
10. Mix resin components and submerge hose into resin. Make sure no air is fed into the bag along with the resin. Resin flow rate should be controlled by adjusting the pressure in the vacuum hose or clamping the resin hose. A slower flow rate will ensure that the plies are wet uniformly. A thin resin system will make the process faster and wetting easier. The lamination shown here took about 45 minutes because the carbon nanotube doped resin system had a high viscosity.
11. Clamp the resin hose when the required area of reinforcement is wet out and keep the bag under vacuum until the resin has cured.
There are several ways of setting up a pre-preg lamination. The required steps are similar as in the VARI process, except no additional resin is needed. In addition, the materials used need to withstand the flow and cure temperature of the pre-preg. The lay-up is as follows: mould surface, mould release agent, pre-preg plies, peel ply (optional), release film, breather and vacuum bag. The whole package is placed in a circulating air oven or autoclave. Heating ramps and holding times are programmed using the control unit of the oven or simply adjusted manually. Once the resin has completely reached its flow temperature, full vacuum can be pulled for consolidating the plies. After the laminate has been debulked, the temperature is raised to the curing temperature. The laminate can be cooled down with the oven or a cooling rate of approximately one Kelvin per minute can be programmed to avoid residual stresses. A carefully laid symmetric laminate should come out fairly planar.
A regular wet lay-up should work fine, too. For a better fiber volume fraction some mechanical pressure can be added on the laminate. Any flat piece is good, for example an MDF panel is fine with some weights distributed on top. In that case the lay-up would be as follows: mould surface, wax, reinforcement wetted with resin (a mohair roller is perfect for wetting), peel ply (optional), perforated release film, breather for soaking up excess resin, painters's plastic or similar to prevent weights above from bonding to the breather, panel with weights on it.
If you don't want to laminate the side plates yourself, they can be ordered from ideas2cycles.
Step 3: Assembly
Preparing the sideplates
The side plates are prepared for assembly by making a hole in the middle for the hub to go through. A diamond tipped hole saw used for cutting tiles does the job cheaply. Available sizes should be checked before machining the hub. A tool diameter of 40 mm or 1 5/8 inches (41 mm) should be easy to find. Some lubricant such as water or ethanol (which evaporates faster) can be used when abrading the hole. Holes should also be drilled for the bolt pattern if a three-piece hub is used. Normal metal cutting drills should suffice, but some pre-preg laminates can be so hard that something else, such as a conical carbide burr tool, has to be used.
Bonding the parts
The hub is laced to the rim in a normal fashion, except the spokes do not need to be tensioned very much. This gives the concentric tolerance of the hub relative to the rim. Any old spokes can be used and the length of the spokes doesn't matter if using a radial pattern where one side of the hub has only leading spokes and the other only trailing spokes. Once the wheel is true, one of the side plates can be bonded to the rim and hub. Surfaces should be degreased, roughened and cleaned again just prior to applying the adhesive. Acetone/MEK+ScotchBrite+Acetone/MEK is recommended before bonding with a structural adhesive, such as a two-component epoxy that cures at room temperature. When the adhesive has cured, the side plate can be trimmed using a diamond tipped cut-off wheel or a Dremel type of tool will work, too. The spokes can be cut at this point and the wheel should stay true and the other side plate can be bonded to the hub. If thin laminates were made, then it may be a good choice to include some core material in the construction. The prototype wheel shown here has four strips of PET foam that were bonded between the side plates using epoxy resin filled with glass microspheres.
Once the second side plate is trimmed to match the rim, the hub can be fully assembled. In the design shown here, the final assembly includes pressing deep groove ball bearings into the hub and installing a 10 mm axle with M10x1 mm threads together with jam nuts and spacers to achieve the desired hub spacing (120 mm on track bikes). A slot for the valve can be cut at this point using the same hole saw that was used for making the center hole in the side plates. Two 40 mm holes spaced apart and then connected by cutting with a Dremel or hacksaw blade give plenty of room for most pump heads.
The surface finish straight out of the mould should be quite good, but a clear coat can be added if so desired. A pour-on urethane gives a nice depth to carbon but with a significant weight penalty. Boat varnishes work well, too. Decals and vinyl are easy to apply due to the flatness of the side plates.
Checklist before riding:
-Is the wheel true?
-Are the adhesive bonds continuous?
-Any visible cracks?
-Do the bearings roll smoothly?
-Does the cog tighten properly?
Any imbalance will quickly be noticed on a velodrome or other smooth surface. Unfortunately there is not much that can be done afterwards if the wheel is not true. Any cracks in the laminate can be fixed fairly reliably by laminating a patch on top.
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