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#95652
I fitted a pair of 345mm shocks to my trials bullet (off a YBR125 as it happens). This is a good 45mm over standard. Excellent for ground clearance. Disastrous for chain alignment. So with this setup, if the chain tension is set to fully tight at full compression, the chain is dangerously loose at full extension.

I've seen the trials sprung tensioners and frankly, they look like a recipe for getting something stuck in your spokes at 50mph.

What I did notice is that the rear brake pivot on the Indian Bullet frames is located handily near the chainline so I had this turned up out of EN8.
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So it's like a standard brake pivot but with an extension towards the centre of the bike that the pictured industrial nylon roller/bearing (think skateboard wheel) will run on.

You land up with something like this:
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You can see how much tension it's adding to the chain with the bike resting on its wheels. However when the suspension is fully compressed, the chain line straightens out and nearly lifts up off the roller:
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Brake lever loosely attached:
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Not road tested it yet but it seems to be able to keep sufficient tension in place when he supension is unloaded to keep the chain on while not going over-tight at full compression. Small mod for a significant gain in rear ride height.

It's a little more than the gain from fitting a 21" rim on the front giving me overall just over an inch of ride heigh while at the same time tightening the rake angle. I make it 26.5 degrees. Still slack for a trials bike but a lot less slack than using a 21" rim on a 300 or 310mm shock.
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#95654
It's 8mm. I could have gone for 10mm (the thread on the left of the picture is M10) but it wouldn't have left me much "meat" of nylon roller and you need enough for the chain side plates to sink into. I also have the option of fitting larger diameter rollers fairly inexpensively.

It shouldn't see much load overall, the majority of the engine power is transmitted along the top run of the chain. The roller should only see mechanical load from the changing tension as the suspension compresses or when engine braking.

The peak load is about 25mm from the nut. It's going to take a lot of force to bend a piece of 8mm diameter EN8 that long.

Time and testing will tell. Statically, the idea works, dynamically, remains to be seen. If it needs M10 and a larger roller, that's not impossible.
#95656
vince wrote:
Fri Mar 12, 2021 7:55 am
Hi, with the lengthened shocks does the chain rub on the top or the swingarm pivot? Vince :)
Yes. I've made a chain slider out of a piece of nylon chopping board. I'm less confident that's going to be robust enough, it turned out to be really difficult stuff to work with. I eventually folded up a little "tray" out of thin aluminium sheet, sat a section of chopping board in it and put it in the over until it heated sufficiently to bend to conform with the tray. There's a fine line between hot enough to bend without snapping and a puddle.

Plan #2 if that doesn't work is a section of of wide bore blue alkethene water pipe split and jubilee clipped in place.

The swingarms are a pain to fit conventional sliders to because the chainstays don't meet the pivot at anything close to a right angle.
#95658
This is of course a problem with the design of the Bullet back in the late forties when ANY rear suspension was a bonus.

Maybe the answer is a customised swinging arm with the end plates modified to mount the rear axle lower down. Might that be feasible?

A.
#95723
This is one of a few different diameters I made a number of years ago. There's a bearing in the other side (not visible in the photo)
Works perfectly to keep longer chains off the lengthened swing arm.
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