Spa Cycles super-compact double on a Brompton

[Bee]

Has anyone used one of these Spa Cycles super-compact doubles on their Brommie? I've just changed the BB for a Shimano UN55 118mm so would prefer to not have to replace it again -- but how to know if the Spa double will work out ok (ie not interfere with folding etc)?

N.

 

[u_i]

I may be able to offer advice, though my details could have differed. I managed to refrain from extending the length of BB axle by putting a spacer onto the BB on the drive side and keeping the original Brompton crank on the left non-drive side. A cassette spacer can be used as a BB spacer. Keeping the fold neat may require reshaping the left fender/mudguard stay. By now, I already upgraded to a triple, all with a 119mm BB axle.

 

[Cycleops]

@chriscross1966 is our resident expert on all things modded on Bromptons so I’m sure he’ll be able to tell you. Seem to remember a few people doing this, including @Fab Foodie ?

 

[TheDoctor]

I've got a compact on my M12R and it didn't affect the fold at all. It's a fairly common thing for touring Brommies.

 

[Fab Foodie]

I have a Stronglight Compact double on my 6 speed 50/26 or 50/34 can’t remember. Works a treat except when folding on the small chainring the chain falls off.
As @TheDoctor states, it’s pretty common.

 

[TheDoctor]

50/44 for me. Gives me basically a laden set of gears on the 44, and an unladen in the 50.

 

[Bee]

Thanks for the input everyone... still a head scratch as to whether the Spa Cycles double will be a hassle-free fit. I could live with the chain falling off if folded on the small ring, but am worried about the inner fouling the fold or the chain line being horribly compromised.

Browsing further at Spa I've come up with an alternative to the Spa double... what about one of these Stronglight adapter chainrings? It'll fit the stock 130BCD Brompton crank and has integral mounting for a 74BCD inner. Thoughts anyone?

N.

 

[u_i]

I used such an adapter chainring too at some point. It worked. There are many ways to get there. It is a tad less rigid solution. Think also about giving up on a chain guard now, not only after finding out that you are staining your clothes.

 

[12boy]

I have a Sugino 110 BCD with a 58 and a 38. I actually tried a 28 as a 3rd chainring but the chainring prevented a proper fold. At the moment I have a 13 tooth sprocket on the back but also a 177% SA IGH wheel so I can switch between the 2 rear wheels fairly quickly. I will set up the single sprocket wheel with a studded Schwalbe tire and have an extra front wheel with a studded as well.
Bikegang gave me a trial 3 sprocket setup with 11, 14 and 17 tooth sprockets which works very well but I was concerned the winter salt and sand would wear out the 11 tooth sprocket so I'm back to a single sprocket for now. The 13 tooth sprocket with the 58 tooth chainring yields about 72 gear inches and the 38 about 50 for hills. If dry weather will be on hand for a while I will return to the 3 speed SA setup with Marathon Racers.
Hopefully all this isn't too confusing.

 

[u_i]

I have 3 rings on some rare Shimano crank. The fold is slightly bulkier than for a single ring, but you need an A/B comparison to really see it. I had to modify the mudguard stay that I mentioned, and that was it, in my memory.

 

[Bee]

This is all very encouraging, and at the same time not very conclusive with regards the particular adaptations highlighted at Spa. Will just have to take the plunge and see how it turns out.
The idea being to swap a double on and off as and when required (ie very long and/or particularly hilly rides) -- so no worry about oily clothing due lack of chain guard as I'll be wearing shorts or tights when the double is on.
The adapter ring with a 28 inner appeals as it allows me to keep the stock cranks... the Spa Cycles double as it'll probably be quicker to swap cranks on and off than chainrings.

 

[gom]

Owing to a concat. of circs. I have recently become the owner of a new (2023) 6-speed Brompton (an M6R in old money). It came with a 50T chainring, which was to much for me, so based on a vague memory of this thread I bought the Spa Chainset mentioned in the OP's first post (44/28 T).

As a simple swap with the original it does work as a double, I was pedalling round locally with a very high cadence in bottom gear. However…
The inner chainring is just too close, and its bolts have nicely scratched the chain stay. (If it had been a mm further out all would have been OK, and a mm further in I think it would have bumped the stay & I would have noticed. I’d put tape on the stay, but the bolt heads cut right through that. )

So I have now replaced the 119mm bottom bracket with this 122mm BB from Spa https://www.spacycles.co.uk/m16b0s110p1933/STRONGLIGHT-Bottom-Bracket-JP400-Plastic (using no additional spacers). This seems to be all OK. I’ve done one 10 mile ride, including a good hill that needed the bottom gear.

Calculated gears: http://ritzelrechner.de/?GR=BBWR&KB=28,44&RZ=13,16&UF=1330&TF=90&SL=2.6&UN=MPH&DV=gearInches

🤞

 

[u_i]

As a simple swap with the original it does work as a double, I was pedalling round locally with a very high cadence in bottom gear. However…
The inner chainring is just too close, and its bolts have nicely scratched the chain stay. (If it had been a mm further out all would have been OK, and a mm further in I think it would have bumped the stay & I would have noticed. I’d put tape on the stay, but the bolt heads cut right through that. )

So I have now replaced the 119mm bottom bracket with this 122mm BB from Spa https://www.spacycles.co.uk/m16b0s110p1933/STRONGLIGHT-Bottom-Bracket-JP400-Plastic (using no additional spacers). This seems to be all OK. I’ve done one 10 mile ride, including a good hill that needed the bottom gear.

A spacer in the 1-2mm range on the drive side would have done the same. Spacers from worn out cassettes can be used here.

 

[gom]

A spacer in the 1-2mm range on the drive side would have done the same. Spacers from worn out cassettes can be used here.

As far as I could tell the two plastic cups holding the Brompton BB in place are fully screwed into the BB shell. Surely adding spacers would leave the BB loose, which I take to a Bad Thing? I did think about cutting the collar off the LH cup, but decided I’d rather not damage it in case I wanted to go back.
The new BB has no collar on the LH cup, so if the Brompton one had been like that I could have tried adding spacers.

 

[u_i]

As far as I could tell the two plastic cups holding the Brompton BB in place are fully screwed into the BB shell. Surely adding spacers would leave the BB loose, which I take to a Bad Thing? I did think about cutting the collar off the LH cup, but decided I’d rather not damage it in case I wanted to go back.
The new BB has no collar on the LH cup, so if the Brompton one had been like that I could have tried adding spacers.

Admittedly I do not remember the details of the original Brompton BB and am away from any Brompton to check. However, for the cases where the BB is not permanently attached to the drive side cup, I cut off rings out of a smaller diameter tube, to stuff them to the inside of a BB shell when needing to keep the BB in place when a space develops inside. Of course, by now your solution works, but the advantage of the spacers is that you can try out and settle on a solution without any essential financial investments, including determination of longer axle length, if the spacers do not suffice.