Archive for May, 2011


May 23rd, 2011 Comments off

It looks like the reason for the insidious chain rub is the swingarm attitude. It’s flatter than in its original setup. Compare these pictures, starting with a detail of the donor bike.

A horizontal line from the pivot intersects the swingarm about 2/3 of the way down. On the Longshot the intersection is lower, passing almost through the dropout:

The effect is to raise the rear axle, lifting the chain line to the point where it hits the frame. So the question is how to push the axle back down? I could move the seat post back, but that would be an offensive amount of work and disruption. I could make new upper shock mounts, increasing the standoff to rotate the swingarm around the existing pivot. It would work but doesn’t feel right – that junction takes a lot of stress and with the increased length any alignment error would make it prone to bending.  I could move the main swingarm pivot closer to the seat post. The angle of the shock would be slightly steeper but I don’t think that’s a big deal.

There’s more thinking to do, but moving the main pivot seems like the best alternative.


May 21st, 2011 Comments off

That chain is turning out to be problematic.

Adding the second idler took up the slack and stopped the chain falling off. As a side effect it eliminated all the pedal bob. So far, so good. I set the suspension to compress about 25% under static load – the sag – and that pushes the pivot down a little. This brings the frame closer to the chain line, and when the chain is pulled tight it touches the frame:

Notice the small groove the chain has worn in the swingarm. On the smaller cogs there’s no problem, but riding in 1-3 causes some nasty rubbing.

Tightening the suspension so there’s no sag is a non-starter. That defeats the purpose of having suspension in the first place, and it won’t work: the shock isn’t long enough to prevent the rubbing. Pushing the idler and chain horizontally away from the frame won’t work either: the deviation needed to avoid that hefty swingarm is too great. I have a nasty feeling that some cutting and welding is going to be necessary, but first I have get my head around the geometry…


May 9th, 2011 Comments off

The bike is back in one piece. The recent modifications are not hugely obvious but constitute a fair amount of change.

  • Transmission: the addition of the second idler has made the chain much more manageable (i.e. it doesn’t fall off now). Shifting is not exactly crisp but it gets there. The rear derailleur needs realignment which will help it run quieter.
  • Cabling: the zap straps are gone. Housing runs are routed through guides and stops and all straight runs are exposed cable. Hooking up that front derailleur was interesting, requiring a recycled noodle and link arm to make the 90° bend. The brakes have lost that spongy feel and it all looks a lot neater.
  • Cranks: it’s hard to tell the cranks are shorter, though the effect may be more evident next time I head for that hill. I use clipless pedals on my regular bike but have been holding off fitting some on the recumbent. I’m riding fairly confidently now…might be time to try them.

There’s room for improvement. The seat doesn’t quite match the angle of the seatpost and flexes slightly under load. Firmer attachment would make the seat conform, but the current 4 woodscrews would strip out. I think I’ll just bolt it down. The suspension unit is barely adequate and jerks rather than smoothing out the ride. I’ll service and adjust it, and maybe keep my eye open for a better quality replacement.

More test riding to do, but construction is basically finished. Soon I’ll have to strip it all down, break out the wire brush and prepare for painting.