Wednesday, January 30, 2008

Carbon lowracer production test

I made a couple of experiments last year to build (unsuccesfully) a carbon low racer. (A new high racer is on the way now)
I made the dummy frame out of MDF. This is easy to sand and easy to curve, very stable and cheap.

The MDF is then covered with a 125g/m2 CSM and polyester resin. A spray gun was then used to spay the polyester based filler (Gravispret). After some sanding and a final layer of paint, the frame was ready to make a female mould.

The final dummy frame can be seen below along the carbon layers required to make the frame. My first idea was to use carbon braids but my bladder pinched the braid in a couple of points and the first frame ended up with massive defects in way of the steering colomne.
I used Latex bicycle inner tube(green) for the first frame but it blew off when the resin attacked it. I then tried a nylon tube but I damaged it while slidding the wet socks over it and I never got more than 0.5bar of pressure in it. The last bladder was custom made but it was too short (my faut) and the valve was too short to fit a decent hose fitting. I need help to sort the bladder out.

The mould is ready to be closed (after folding the carbon covers). The inflatable bladder is visible. I used SICOMIN resin SR1700 and the frame has been cooked at 50degC. The resin was very runny and I hoped to get a better surface finish.

The frame is out of the mould. The excess of resin is visible where the mould splits.
This frame is strong enough but the surface quality is poor in a few area. After that third frame, I decided to make the next frame in prepreg and in three parts so if one part fails then the complete frame is not lost. I will only need a jig to bond everything together.

Tuesday, January 29, 2008

Old up-right frames

This is my very first carbon frame, built in 1998. The frame is built over an expended polystyrene foam core (15Kg/m3) with a hot wire and a couple of template. The foam has been bonded to a steel bottom bracket and a steel colomne then every things has been 'locked in place' on a big plywood sheet to keep the correct angles and avoid any warp. Hand lamination has been used.
I must admit that the very first time I tried the frame, I felt that the rear wheel was not always following the front wheel. That was a terrible feeling but it did not break. A layer of 400g at +/-45deg solved the problem. Final weight of the frame is about 1.5Kg, which is not that bad with a couple of steel inserts.
The rear lugs are all carbon and the aft derailleur is taped directly into the carbon.
The complete bike weighs about 9.55Kg as shown on the picture.
The bike is stiff vertically but can twist a lot if you push the aft wheel side ways with your hand. I can't feel it on the road unless I hammer it down.
Wheels are very bad below 40km/h, sensible to side wind and innacurate in corners. Not my best option.

This track frame wheighs 1.3Kg and is heavily reinforced.
I used ACG VTM264 low temperature prepreg to build this frame on an extruded foam core. (styrodur). I also made the oven with a couple of Styrodur sheets and a modified bathroom heater (I removed the safety switch). I used a car thermo sensor to keep 85deg in the oven. The vacuum pump is a modified fridge compressor.
Low temperature prepreg was a very good solution as I could laminate the complete fore triangle in one shot.
Unfortunately, while curing the rear wishbone, the oven temperature reached 110deg, damaging my oven (but no the frame's core)and wrapped the fore triangle which was not supported by any template. My first cure was probably not enough and the resin system reached its Tg again. If I have the opportunity, I will use an other cure to straighten the frame but I have little hope about this method. Try and see!