Wednesday, August 31, 2011

Building my own carbon mountain bike

I’ve been toying with idea of making my own carbon mountain bike for nearly a year now and finally committed to it back in April by ordering the carbon fibre and resins.  This blog contains the steps I followed to build my own carbon 29er single speed bike. 

The concept
I have thought about building a bike for a while but only wanted to do so if I had a unique design. So I started by placing the key geometry points of a bike onto paper.  I wasn’t interested in this stage in designing the geometry, so I simply took the geometry of my Ventana El Commandante, as I really enjoy riding this bike and I know the geometry works well for me. Having the key points: head tube, bottom bracket, drop outs and top of seat tube located on paper allowed me to sketch frame designs linking all of these points together.  I quickly came to the likings of the simplicity of a combine top/down tube and only enlarged chain stays to connect the rear dropouts.  This gave the appearance of a sleek clean bike and would also be easy to manufacture using carbon fibre as there are less tubes and joints to deal with. Playing around with different configurations I finally settled on a design.  

The construction concept
Now I need to know how to make my own bike.  Luckily I had all ready thought about this before designing the bike. My fabrication plan consists of making a polystyrene core of the frame that would allow me lay the carbon fibre over it, as per a standard mould-less wet layup. For the key features of: bottom bracket tube, head tube and seat tube, which had to accurate internal diameters, I decided I would make these tubes from carbon to the right size by using aluminium mandrels, as moulds to wrap the carbon fibre around. Collin a friend of mine machined three mandrels to the sizes I needed for my tubes (head tube upper and lower limits ID = 49.62, 49.57mm; bottom bracket upper and lower limits ID = 54.0, 53.8mm; and seat tube upper and lower limits ID = 27.25, 27.10). I had the luxury of having the rear drop outs profile cut on a CNC router, cheers to Bill for this. The LH dropout also had the addition of a brake mount holes machined into it to allow the brake as the brake needs to go somewhere.  This saved the hassle of having to add mounts for the rear disc brake onto the frame.  Much better to combined tight tolerance items together so they can be located from a single point, such as the dropout centre. The other major issue to address in making a bike is to keep everything aligned and square, else your bike will feel weird to ride and will not ride in a straight line and favour turning to one side, which could be good for a Nascar style bike. 

Building the jig
Again I exploited the use of the CNC router that my work has.  I designed up an interlocking jig on Solidworks and then produced G code from the models using Rino CAM. The CNC router then did the rest accurately cutting out all the jig items.  

Sourcing the materials
Next step in the project is to get all of my materials for my frame.
  • Resin,, these guys are really helpful at providing the right resins for the right job.  I used ARD270 resin and ADH28 hardener.
  • I ordered the carbon firbre from High Modulus located in Auckland. These guys were also very helpful, in providing me information of what fibres to use and even gave me an indication of how much fibre to use.
  • Polystyrene for the core I got from Ploycut Christchurch. These guys cut up polystyrene blocks for insulation, signage and other polystyrene foam products.
  • Aluminium for drop outs were just off cuts from work.   
  • MDF, screws, threads, etc were all purchased from a hard ware store. 

Let the building commence
I choose to make the bottom bracket tube first as I was going to use a Bushnell bottom bracket  ebb (eccentric bottom bracket) to tension my chain.  The Bushnell bottom bracket expands its diameter as one tightens the adjusting screw. The ebb locates in the desired place by friction force created by high compression force of the ebb expanding into the bottom bracket tube.  So using the ebb, which I have from my Ventana bike, I could test the strength and stiffness of the carbon bottom bracket tube by inserting the ebb into the bottom bracket and tightening the adjuster screw on the ebb.  I tightened the screw until it went as tight as I could get it using a standard size 4mm allen key. I was pleased with the result as the ebb was securely locked into the bottom tube and the tube remained rigidly intact.  To make the bottom bracket tube work it must have a high hoop stiffness to ensure that the expanding ebb will locate in place.    Knowing that the ebb is mostly under hoop stress, uni direction carbon fibre was used wrapped around the bottom bracket mandrel with most of the fibres aligned circumferential. Using 200g/m^2 uni carbon fibre with the following stacking sequence. Once the carbon was wrapped around the mandrel a layer of peel ply and breather material was also wrapped around. Vacuum bagging was used to suck out the air and apply atmospheric pressure to tightly hold the carbon fibre in place during curing. 

The seat and head tubes were made in a similar way as per the bottom bracket tube but had different stacking sequences that I determined would be adequate for these tubes. 

Shaping the foam core
This was one of the messy but equally one of the funnest tasks in the project. I began with a block of polystyrene foam and drew the side profile of my bike on it. I then cut out the shape using a wood saw and sanded the polystyrene away using 80 and 150 grit sand paper.  Cutting and sanding polystyrene is a quite a rapid process as it is very easy to cut.  I took great care in making sure I didn’t cut too much away.  Over the space of a few hours spread across a few evenings I had a nice looking foam core of my bike.    I used the jig as a guide to help make sure I was sanding the foam away from the correct places.

The first layers
I put the tubes and drop outs into their respective jigs and positioned the foam core. After double checking the alignments I was ready to begin adding the carbon fibre.  The first layers I put on wrapped around the bottom bracket from the down tube. I then put additional layers from the bottom of down tube to the beginning of the rear stays.
Next session I wrapped uni directional fibres around the head tube, seat tube and rear faces of the seat and down tube, and also along the full length of the underside of the down tube.
Third session, concentrated on the rear stays, here I put uni-directional fibre across the top and bottom of the stays and wrapped the ends of the stays tightly with carbon yarn. To prevent any aluminium carbon corrosion I put a layer of fibre glass around the aluminium to insulate the aluminium from the carbon. I also placed carbon unis along the top side of the downtube and seat tubes. 
Fourth session this was a tricky one due to having to place carbon from underneath, luckily I had a helping hand for this. Here I placed carbon unis along the sides of all the tubes for lateral bending stiffness.  Before I placed the layers I added a plastic tube to allow for internal cable routing for the rear brake.   Again once all fibre was in place I wrapped the bike in PVC tape to keep everything tightly in place.

The remanding sessions, up to 8 or 10, were spent on adding layers of carbon twill and additional unis.  Once I thought there were enough layers on the frame and it felt stiff enough from standing on it I began assembling the bike. I pinched all of the components, forks, stream, handle bars, wheels etc from my Ventana bike. The only difficult arose when I tried to spin the cranks and noticed that the drive side crank rubbed on the chain stay. This was soon fixed with a few spacer rings on the drive side of the bottom bracket to space out the drive side crank.    
Look at the carbon hotness

Taking it for a spin up and down the drive was really exciting and rewarding.  The bike held up and didn't explode into a million bits of sharp carbon fibre splinters.  This gave me the confidence that I must be on the right path, so I painted my bike with some old yellow paint, Sun Yellow, left over from Canadian canoe.  
Sun Yellow what was I thinking....

With the bike painted it was time for its maiden voyage.  Up Worsleys, down the Flying Nun and across the Traverse and back down Huntsbury. The bike held up and felt really stiff in the back end.  
Just been out riding it tonight and can happily report that the bike can with stand the abuse of Victora Park.

Oh for those that are interested the carbon frame weigh in at 2.1kg, a little on the porky side.

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