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Wednesday, December 9, 2009

Suspension Not Quite Going According To Plan

Having tried a few variations of the last idea in this post
I ended up breaking the frame with an incredibly stupid attempt, because I tested it without putting any support between the stays:

As you can see, that left me very unhappy.

On the righthand stay is the prior attempt, which was not long enough; I decided to try one a little farther forward and longer, but being farther forward it had zero support from the dropouts and just crushed the tube as soon as I put the suspension together and let the bike's weight on it, and pushed down just a little.

The repair required replacing some of the tubing. Fortunately I had a frame already being parted out with some tubing just larger than what I had to replace, so it could be slipped over the damaged area and welded on.

I cut out the damaged area first, then straightened the dropouts and other stays.

Then I cut off a piece from the other frame large enough to completely cover the removed area plus as much as I could get from it. I left the curved end on because it comes close to matching the curve on the back of the stay to be fixed.

I left the brake stud on there as a way to manipulate the tubing while I did the repair; it will be removed later once I am sure I'm done.

I slipped it over the front of the stay first, then aligned the stays and slipped it over the back end.

so it ends up like this:

Once alignment was certain, with a wheel in the dropouts instead of the triangle above, I welded it in place. It seems as strong as ever, but I am sure it is not quite as good as it used to be.

Now I have a better plan for doing this, based on this idea:
but modified to be adjustable, based on Drunkskunk's seatpost idea:

So that the cut-off part of the seattube will end up rewelded pointing forward, so a shortened seatpost can be clamped in as an adjuster for the ride height.

Now, I still have to work out the chainline issues, as the chain currently wants to pass directly thru the space the pivot BB occupies. I must install something to force it to go around that, yet still shift across it for gear changes.

I have a nylon or teflon cutting board that I may try to cut a strip from that will be used to deflect the chain across, and clamp it to the BB. It will wear, and it will add noise, but it should let me test the feasibility of the suspension while I think up other ways to get the chain around it (eventually redesigning the rear end to eliminate the problem).

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