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27-Speed Brompton

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Old 01-23-19, 11:12 AM
  #26  
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Originally Posted by fietsbob View Post
My thought . because of the molded single piece, flanges & toothed pulley design, the chain oil dirt combined solids, packed in, have no place to go..
Wondering if a few holes thru the flanges .. a part of some of the CNC after market pulleys . gives some of that inevitable accumulation a way to sluff off and out..
3 speed , its no issue , backing disc gets a bit grimy , but it does not rotate in unison with the lower pulley
The side holes are there but indeed the flanges and plastic, rather than metal, likely contribute to the grime accumulation. When I looked at the aftermarket of pulleys and tensioners for Brompton, it seemed that you needed to pay a lot of money for a dubious gain. However, I may have another look.
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Old 01-24-19, 04:40 PM
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back of the envelope design ...

A scheme to add the flanges to a good commercially available sealed bearing pulley , like Tacx , sandwiched between them popped in my mind ,,
the flanges need not be part of the pulley to work as far as I can grasp the mechanics involved..

brass or oil bronze tubing can join the 3 pieces and slide along the pin in the chain tensioner.. as required.. & separate to do the occasional cleaning..

pulley can even turn faster than the flanges.. fixed to the bushing sleeve the bearing rolls around ..





.....

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Old 01-26-19, 03:09 PM
  #28  
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Originally Posted by pastorbobnlnh View Post
Ahhhhh! A clean transmission is always appreciated by changing gears! Glad this solved many of your shifting issues.

I wonder if the "skating" could be solved by a new chain?
The "skating" when riding with the largest ring turned out to be the residual problem with poor shifting to the middle cog from the smallest cog that I hoped to have solved with cleaning of the pulleys. (The latter helped but evidently not fully.) The problem that developed could be best illustrated with the diagram describing shifts: 12t -(pathologically hard)-> 14t -(pathologically easy)->16t and 12t <-(normal)- 14t <-(normal)- 16t. If tension in the shift cable was too low, the derailleur was failing to move the chain from 12t to 14t. This seemed tied to an insufficient stiffness in the plastic derailleur yielding hiccups in the derailleur operation. If I increased tension in the cable, the derailleur immediately started to overshift, jumping from 12t to 16t for the middle shifter position and then falling back to 14t, and so on, yielding the "skating".

I could not find a middle-ground cable tension that would solve the problem and I went with a nuclear option. Given that I had cassette pieces for all kinds of speeds, I reduced the gap between 12t and 14t, by using 10-sp 12t cog with built in spacer, and I increased the gap between 14t and 16t, by using a 9sp spacer there w/o filing. In my estimate one gap was reduced by just 0.15mm and the other increased by that amount. This nuclear option took care of the pathologies (or basically went along with them) and all shifts fine now for all gears in all combinations. In the back of my mind I worry though that, if 0.15mm matters, then with temperature changes or something, and the derailleur arm softening or stiffening, the problem might come back. If you do not try, though, you will never know .

The bike is now all ready for a trip in 2 weeks from now, where my survival crucially depends on this bike - my lodging location will be inaccessible by public transport for much of the week. The bike better perform there.
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Old 01-26-19, 03:43 PM
  #29  
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@2 I, where are you traveling to? Sounds like an interesting adventure. For the three rear sprockets and the Brompton RD, is the shifter friction or indexed? I've been assuming friction.
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Old 01-26-19, 04:14 PM
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Originally Posted by pastorbobnlnh View Post
@2 I, where are you traveling to? Sounds like an interesting adventure. For the three rear sprockets and the Brompton RD, is the shifter friction or indexed? I've been assuming friction.
It is just Germany, but I will be thrown into fields, with public transport there only to carry people to work and back and the nearest shop 20min away by bike, not even on the way of the public transport. There might be snow there, so I ponder taking along a studded tire for the Brompton, but I will check the forecast right before my travel. The shifter is an indexed SunRace M90. I have also a friction version of this shifter, that largely uses the same components. Friction can be easier when you are free to concentrate on the shifting, but I want to be ready when the surrounding is rough and I have to concentrate on making it through.
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Old 01-26-19, 05:54 PM
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That sounds like a great trip. I hope you have pleasant weather for the duration of your trip.

I'm using friction barend shifting on the 9 speed cassette of my Dahon. IMO, sometimes index is over rated. But I'm a vintage guy at heart and most of my bikes are 30-50+ years old.
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Old 01-26-19, 06:20 PM
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I spent one winter at the particular location in Germany and the snow was quite challenging there on occasions, sticking to a bike when riding over agricultural roads and making it look as if it belonged to a snowman . The brakes were obviously completely taken out of operation.

My leaning is towards indexed, but I am not dogmatic about that. I did offbeat things though for the cause in the past such as RHS on the left, Shiftmate and microstops with redundant stops filled with epoxy.
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