Brompton higher gearing option

[lyndsayp]

Thank you both @rogerzilla and @12boy . I've replaced the folding pedal with SPDs, so should be okay there. I'll probably try a 58T first, before going up to 60T. I'm curious why a single speed chainring, with no ramps or cut outs would be better though (I don't know much about this area)?

 

[chriscross1966]

@chriscross1966 - I've got a 2 speed Brompton that I'd like to increase the gearing off. I'm considering replacing it's 54T chainring with a 60T one, but unsure of what other specs to look for (other than 130BCD). Is this suitable? https://www.ebay.co.uk/itm/Fast-Shi...075-Alloy-10-11-Speed-Chainring-60T-BCD-130mm

Thanks

Er.. the link isnt working for me.

 

[chriscross1966]

Thank you both @rogerzilla and @12boy . I've replaced the folding pedal with SPDs, so should be okay there. I'll probably try a 58T first, before going up to 60T. I'm curious why a single speed chainring, with no ramps or cut outs would be better though (I don't know much about this area)?

The ramps are just missing material in our use, it is effectively just getting a ring that already has wear...

 

[chriscross1966]

58t is about half the price of 60t and has an integrated guard. You can also get different colours.

www.ebay.co.uk/itm/263793828868

 

[rogerzilla]

A ring with full height teeth is less likely to unship the chain over bumps, especially if it's a large ring (conversely, those new 1 x 11 setups with a tiny ring need a special clutched rear mech to maintain chain tension). And yes, it'll last longer.

 

[12boy]

So yesterday I am going up a hill which required I get off the 58 and onto the 38. But as I start grinding away the chain slips and does that whenever the effort goes up. I stop and examine this and can see that either the 38 or 58 has a warp which has the 58 nudging the chain as it travels back to the tensioner. Perhaps a 130 would be better than a 110 as there is more crank spider and less chainring. Probably is the 58 which is warped and which I can try to straighten or ditch the 38 and just go with the 58. Once going up a hill with a headwind the extra gear inches don't appeal quite so much. Has anyone else encountered chainnring warpage?

 

[rogerzilla]

It's easy to straighten chainrings with a big adjustable spanner. if it's flex rather than warp, that's (as you say) a case for a bigger BCD.

 

[12boy]

I spent some time looking at this closely and the problem is different than I had thought. I like the big chainring on the inside which gives a straight chainline to the rear sprocket and makes it easy to shift from one to the other. But the 38 tooth chainline is out far enough that the chainline is far from straight and the chain hits the 58 tooth chainring just a little. I never had this issue with a 54 or 53 tooth and a 38, so I can go back to that, get a shorter BB, say 117 mm, or put some spacers between the 58 and the crank spider so it is closer to the frame a bit. Other than the rubbing issue, the chainline with the 38 has seemed to work very well with no skipping. I would like to have both chainrings as close to correct as possible so probably the shorter BB will be the best option.

 

[chriscross1966]

So yesterday I am going up a hill which required I get off the 58 and onto the 38. But as I start grinding away the chain slips and does that whenever the effort goes up. I stop and examine this and can see that either the 38 or 58 has a warp which has the 58 nudging the chain as it travels back to the tensioner. Perhaps a 130 would be better than a 110 as there is more crank spider and less chainring. Probably is the 58 which is warped and which I can try to straighten or ditch the 38 and just go with the 58. Once going up a hill with a headwind the extra gear inches don't appeal quite so much. Has anyone else encountered chainnring warpage?

I wonder if the chain tensioner isnt causing issues trying to take up 20 teeth of difference and still work normally... remember it isnt so much a tensioner as a take-up device to stop the chain unshipping when folded.

 

[12boy]

You may be right that the span is just too much. The chainline appears to be 43.5 mm with the 58 on the inner part of the crank. The little one is the thickness of the spider further out and definitely looks crossway a bit. Since I primarily use the big one and reserve the 38 as a granny gear it seems best to have the correct chainline on the 58. I was toying with getting a BB 3 mm shorter than the current one as there is a 5mm gap between the ring and frame and mount the 38 on the inside. It does seem better with a 53 and 38. I have tried to find out what the standard chainline is supposed to be but haven't found that. It is getting to be winter here and I have studded tires but the 58 will definitely be too high for me to pedal with the extra work they require. Perhaps next year I will just go with the 58 alone and try to get stronger.

 

[rogerzilla]

Bromptons are quite hard to modify because of the fold. If you fit an alternative crank, the ring normally has to go on the inside of the spider; Bromptons have a large Q-factor to allow the crank to clear the rear triangle during folding. The LH pedal is amother possible complication, as I mentioned above. The headset needs to be a pretty low stack height - a Chris King 2Nut will fit if you are aggressive with the facing tool and grab 1mm extra stack height. Anything bar-related has to clear the front wheel when folded. It's an interesting challenge.

 

[12boy]

Rogerzilla, do you know what the chain line is exactly? I can figure out the front fairly easily but the back.is harder. I am thinking if I can get the front crank out far enough so the chain line falls on the middle of the crank arm between the two chain rings I can avoid any rubbing of the chainrings
Thanks, Ron

 

[rogerzilla]

I used to but I've forgotten! I think about 46mm. You can adjust it a little at the back by moving sprocket spacers and also the washers under the jockey wheels of the chain tensioner. I initially tried a Sugino XD2 double with 107mm BB and the ring on the outside, but the crank hits the rear triangle when folded. So I used a 110mm with the ring on rhe inside, and got the sprocket a bit more inboard at the back to.match. My chainline is about 43mm now. I had the 110mm anyway; I could have bought a 117mm and it should have Just Worked with the ring on the inside. Disclaimer: I'm not using the Brompton black plastic spacer/dust shield under the sprocket as mine has AM hub internals with the usual metal labyrinth seal and two 1/16" spacers.