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-   -   27-Speed Brompton (https://www.bikeforums.net/showthread.php?t=1164607)

2_i 08-30-19 03:48 PM


Originally Posted by Winfried (Post 21100366)
Turns out the inner bolts on the triple chainset I installed to get a 46/30T rub against the chainstay when folding the bike :-/

I currently use a Shimano BB-UN55 118x68mm bottom bracket, which is also available in 122.5mm.

Before I order it, is there a simple way to move the crankset away from the frame by 1 or 2mm?

Yes, it is possible that you do not need to order a new BB at all. The BB can be shifted to the right by few mm by placing a spacer or spacers under the drive side BB cup. These spacers have the same size as for the sprocket cassettes, so if you have an old one, you can pull a spacer from there, possibly just filing out the protrusions that may be there to prevent a spacer from rotating. In any case, a bike shop will normally have such spacers and they range in thickness from 0.5mm to 3mm. I usually have such spacers at hand and use them to tweak the protrusion of the crank to the right side. Incidentally, in my case I use the original Brompton crank on the left as it did pot protrude too far to the left but still left a good 5mm for eventual movement of BB to the right. To guide you, here are such spacers on Amazon.

Winfried 08-30-19 04:07 PM

Thanks much for the idea. I didn't think of adding a ring between the BB and the frame.

The snap ring sold with the Brompton 13/16T cassette is 2mm thick, and fits the BB cup to the T in terms of diameter. OTOH, the spacer is 2,15mm and slighly wider, although it can still sit on the cup, just a bit on the edge.

Do you think the snap ring will do?

https://cimg7.ibsrv.net/gimg/bikefor...e915c3b0d8.png

--
Edit: No go.

To move the crankset away from the frame, adding a ring will do nothing: For this, I need to a tiny spacer at the end of the axle.
https://cimg0.ibsrv.net/gimg/bikefor...8c22f3718a.png

2_i 08-30-19 06:41 PM


Originally Posted by Winfried (Post 21100460)
Edit: No go.

To move the crankset away from the frame, adding a ring will do nothing: For this, I need to a tiny spacer at the end of the axle.
https://cimg0.ibsrv.net/gimg/bikefor...8c22f3718a.png

DON'T DO THIS axle thing. The good outcome would be that it does nothing, the bad that it ruins your crank. For one you are pointing to the wrong side of the BB, but maybe you just did not pay the attention. Otherwise the spacer I suggested must work - it is the standard way of dealing with the issue. The ring would presumably work for a while but the BB is held in place by friction. Putting something flat under its lip yields more surface for the friction to work than a ring. Overall 0.15mm gain in your situation is insignificant compared to the issue - you must leave extra room for wobbling of the rings, crank catch on the BB axle getting worn out over time etc. Maybe you should just get the BB with a longer axle. Still the benefit of playing with the spacers is that 1) maybe you get away with the existing BB, 2) you develop a better judgment what axle length you actually need, 3) if the new BB comes in and is still too short, you can ix it with a spacer, 4) you get away with a shorter axle with spacers than without, limiting the folded size.

bikeforumKEN 08-30-19 08:12 PM

The bromopton lifestyle on youtube PART 5: DOUBLE CHIANSET AND FRONT DERAILLEUR INSTALLATION has good spacer info and under the comments is says I used a Shimano BB-UN55 bottom bracket (118x68mm) and a 50/34T compact crankset, and had no need for spacers.

2_i 08-30-19 09:45 PM


Originally Posted by bikeforumKEN (Post 21100788)
under the comments is says I used a Shimano BB-UN55 bottom bracket (118x68mm) and a 50/34T compact crankset, and had no need for spacers.

Different cranksets can stop at different depths on the same BB axle, so for one brand/age of a crankset you may get away without spacers and for other you might not. The overreaching rule is to have some spacers at hand and cope with the situation as it develops.

Winfried 08-31-19 12:17 PM


Originally Posted by 2_i (Post 21100670)
Otherwise the spacer I suggested must work - it is the standard way of dealing with the issue.

Stopped by the local DIY bike workshop.

Solved.

Thanks!

https://cimg4.ibsrv.net/gimg/bikefor...6d5bf2fb85.jpg

https://cimg3.ibsrv.net/gimg/bikefor...9fcdc9161a.jpg

2_i 11-18-19 07:49 PM

@Schwinnsta, I cannot answer your PM as your settings apparently do not allow for receiving a PM.

As to the question of the bikegang spring being worthwhile, the short answer is yes. It makes the shift more precise when the cable is at its most loose.

The reason for there being a longer version of the answer is that the spring had a tad too large diameter for the chain stay side and I had to wind it a tad tighter on that end to make it fit. In the end it was not a big deal. Also it is likely that one can buy a stock spring and just cut it to size. I doubt that bikegang does anything else.


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