Rear swingarm, front wishbone

Thursday 13 Dec 2012, 6.30am

I haven’t been able to update this blog for a while because I haven’t had a camera. I’ve gotten quite a bit done over the last week, finishing the second swingarm and making my first lower wishbone for the front end.

The second swingarm caused some problems, as the resin didn’t penetrate into the inner layers of carbon, as I found when I drilled into it afterwards. So I had to do a second infusion, running resin through it underneath the outer skin. That seems to have fixed it. This is a pic of the completed swingarm before I cleaned it up and a pic of the finished swingarms –

I’ve sprayed them with gloss white enamel to protect the plastic from UV. I didn’t bog the surface and fair it, it just adds weight.

At the top are the axle mounts, in the middle is a mount for an idler cog to keep the chain tensioned. The tubes are the swingarm pivots, which ride on the rear axle bearing support tube on Teflon bushes, and at the bottom is the shocker mount. My original intention was to cut a hole in the carbon and mount the end of the shocker inside the arm, but I realised this wasn’t going to leave much carbon where it was most needed, so I’ve decided to mount it to the side instead.

I’ve also laced up both of the back wheels –

The wheel is 48 spoke 20”, with Velocity Triple V 1” rims (Queensland made) laced to Joytec BMX flip-flop hubs, allowing me to fit a left hand threaded sprocket on the right hand side and a right hand threaded sprocket on the left hand side. Having single speed freewheels on each side provides the differential for cornering – only the inside wheel drives. Pattern is triple cross. Tyres are Greenspeed Scorchers. Here’s where it mounts to the swingarm –

The wheel has been laced up with a 10mm offset to the inside, minimising the moment on the axle under load.

Now to the lower front wishbone. First I had to make a mould. I bent up this sheet of 3mm thick gal steel using the tray of my ute as an anvil.

The vacuum bag from the infusion is still attached, I’ll re-use it on the second wishbone. That yellow mastic is super sticky stuff, you have to cut it with a knife to separate it. The mould was polished four times with mould release wax, then coated with mould release PVA. The wishbone separated easily using wooden wedges. I’ll detail the manufacture process when I make the second wishbone. I also had to make another jig to ensure that the mounts were accurately located -

The two verticals hold tubes that locate the chassis pivots. Top centre is the shock mount and the nut at the bottom locates the rod end that attaches to the bottom of the kingpin. This is the finished piece –

For this part I used 19 layers of twill woven carbon cloth, in total about 1m x 1.5m, with a fillet of unidirectional fibres (preventing cavitation and adding strength) down each side of the central 16mm aluminium tube (primed with etch primer to prevent contact with the carbon and ensure good adhesion to the epoxy) and two layers of unidirectional tape across the tube to ensure a good bond to the main member. The tube was sheathed with 4 layers of tubular carbon, of two diameters. It’s massively rigid and only 4mm thick. This was the member that failed on Pospedal Mk IV, I don’t think it will be a problem again. Also shown are the shock and mounting bolt, as well as one of the stainless steel pivot bolts riding in a phosphor bronze bushing that will be mounted to the chassis. The locknut has been machined down to save weight.

You can see that the rod end is angled at 12 degrees. This is the castor angle. The shock mounts have stainless steel inserts and the chassis pivots are threaded to take the 10mm bolts. All metal parts were fixed with epoxy. The small gaps left around the edges of the chassis pivots after gluing were filled with epoxy thickened with talcum powder.

This is how the wishbone fits in the jig –

I had the same resin penetration problems with this part as I had with the second swingarm, and had to do a second infusion to finish a large dry patch on the part against the mould. It was OK after that, but next time I will be infusing it from the mould side as well to prevent this problem recurring. It was complicated trying to work out an angle of attack –

Next time – infusing the second lower wishbone.