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Old 02-19-08, 06:46 PM
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Join Date: May 2005
Location: Melbourne
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Downtube Mini suspension upgrades

As you may know, I have been experimenting with elastomers instead of the stock spring in my Mini to cure bounciness. I have had quite good results but not a complete cure.

Further experiments have made me suspect that perhaps the seatpost is to blame for a large portion of the bounciness. The most recent experiment was to put in the hardest elastomer to check the effect on bounciness.

Yesterday I decided to ride my Downtube Mini to work. I found that it still bounced badly at a particular cadence. So the seatpost is now a prime suspect.

At one point there is a steep climb which I usually do out of the saddle. As I was climbing I noticed a clunky feeling coming through the cranks at every revolution. I looked down and what the... am I imagining it or is the tail end fishtailing...? So I stopped and pushed with my foot sideways against the suspension joint, and yes it has a fair amount of play. I did notice the play before but hadn't noticed it before during pedalling out of the saddle. I noticed it as an instability when taking corners at speed. A bit disconcerting, that.

Since I am at the moment scrutinising the Mini with a view to making it perfect, I decided to dis-assemble the suspension joint. It is a very simple and inexpensive setup, what you'd expect of a bike at this price point. It consists of a plastic bush inside the short tube which is at the front of the rear triangle, actually 2, one from each side, and a steel spindle which rotates inside the plastic bush (again, 2 actually). The spindles insert from the sides of the brackets which are welded to the bottom bracket shell. A screw holds the spindles together inside the bushes.

What I found was that especially one of the spindles has a lot of slop inside the bush. Also evident were wear marks from rotation of the spindles against the bushes, and no lubrication. Again, all par for the course for this bike I suppose.

So I made 2 new bushes in which the existing spindles fit snugly/tightly. Free rotation is not needed, in fact some binding is OK since it would provide damping, as long as it is not too much. I used Delrin which is easy to machine and has a low friction coefficient. I will be including grease between the spindles and bushes anyway, to limit wear.

The joint is not dustproof or waterproof, unfortunately. I did come up with a design for bushes which would be water/dustproof, but maybe for later. Right now I want as little as possible fishtailing to see the effect on handling.

Perhaps Delrin is not suitable for this job, so I also came up with a design using sealed ball bearings. If the Delrin bushes wear out anytime soon, I'll be getting some sealed ball bearings.

I also made another suspension 'elastomer' from nylon, effectively locking the suspension out. I want to see if the bobbing comes from the seat post as I now suspect.

I'll keep you posted and try to snap a few pics before assembling it all tonight.
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