Front derailleur on a Bromptopn

[rualexander]

No problems on any of the three chainrings on mine.
Can't remember what bottom bracket axle length I have though.
But my cranks are Spa XD2.

 

[Fastpedaller]

No problems on any of the three chainrings on mine.
Can't remember what bottom bracket axle length I have though.
But my cranks are Spa XD2.

That's good to hear. I guess mine is shorter, as there is no way a granny ring would clear the fold.

 

[gom]

I have a Spa TD-2 double (outer ring replaced with a guard) on my 6-speed. See thread:
https://www.cyclechat.net/threads/spa-cycles-super-compact-double-on-a-brompton.280915/
I did have to change the BB from the standard 119mm to a 122mm (also from Spa) - the bolts from the inner chainring scratched the chain stay on folding. If you can get the size rings you want, you might find this gives a better chainline. I didn't really worry about that as I wanted the extra, low gears and was prepared to risk it. Seems OK.
I've not added a front changer, though I have the adapter/fitting generally used. The two spare changers I have won't move inboard far enough. And I'm happy with the manual change.

 

[Fastpedaller]

I have a Spa TD-2 double (outer ring replaced with a guard) on my 6-speed. See thread:
https://www.cyclechat.net/threads/spa-cycles-super-compact-double-on-a-brompton.280915/
I did have to change the BB from the standard 119mm to a 122mm (also from Spa) - the bolts from the inner chainring scratched the chain stay on folding. If you can get the size rings you want, you might find this gives a better chainline. I didn't really worry about that as I wanted the extra, low gears and was prepared to risk it. Seems OK.
I've not added a front changer, though I have the adapter/fitting generally used. The two spare changers I have won't move inboard far enough. And I'm happy with the manual change.

Thanks for the reply. Interesting to note the BB differences, as mine is an RPM 103mm and the granny ring castings would hit the chainstay on folding. When you say "the bolts from the inner chainring scratched the chain stay on folding" are you referring to the 74BCD bolts (I refer to a granny ring) ? I used the XD2 only because I wanted 160mm cranks not available in TD2. My concern was the chainring being too far out if I didn't achieve the same chainline as the original (1 piece) Brompton cranks. With the chainring on the inner side of the spider, the chainline is indeed dead in line with the middle sprocket of my now 3-speed DIY conversion. If the chain misses the dropout with the chainring on the outside of the spider I'll probably have no issues - after all, derailleurs are destined to be out-of-line a good deal of the time. My plan is to move the 50T ring to the outside of the spider and fit a 36T to the inside.Gear range will then be 38" to 69" which will please me.

 

[gom]

Oops - I see I got my XD & TD muddled, I assumed we were talking about the same thing, but with 2 or 3 rings. And I see the XD's recommended BB is narrower (110mm) than the TD's (119mm), with different crank lenghts available, so my comments may be less helpful than I'd hoped.
Yes, it was the 74BCD bolts that were the problem.

 

[Fastpedaller]

Oops - I see I got my XD & TD muddled, I assumed we were talking about the same thing, but with 2 or 3 rings. And I see the XD's recommended BB is narrower (110mm) than the TD's (119mm), with different crank lenghts available, so my comments may be less helpful than I'd hoped.
Yes, it was the 74BCD bolts that were the problem.

I reckon they are both the same (except for the hidden spider behind the crank arm) and suspect the difference is in the BB stickouts, but am willing to be corrected. Something which seems certain is that the forging for whichever model is the same if it's a double or a triple - When buying the triple there is an additional drilling/tapping operation and 5 screws & washers to fix the granny ring. If the screws (not the alloy of the mounts of yours) were clashing with the chainstays there may not be much difference compared to mine? When I tried mine without the screws it clashed, but if the 119-110 difference in BB widths =9mm is 'shared evenly' with both sides then the longer axle may well create an interference if the screws are fitted. I haven't explained very well, but hope that makes sense. Using my RPM 103 compared with Neco 110 gave a 2mm difference between the stickout on the drive side. I guess different manufacturers make BB's to different dimensions. If I got in a real problem I was ready to use a 73mm BB with a spacer on the driveside and an old-style lockring on the non drive side in order to shift the BB bearing towards the drive side.
All good experimentation

 

[gom]

I think there must be some difference. Looking for "Spa TD2" I see there are both 2- and 3-ring versions. The 2-ring just has a chain guard in place of the outer ring. Not using a front changer this suits me.
As you say, plenty of scope for experimenting/fettling :-)

 

[Fastpedaller]

The mount for a front derailleur arrived today
https://www.ebay.co.uk/itm/315754560971?var=614182664648
I thought I'd do a 'trial fit' prior to the arrival of the Shimano front derailleur which despite a '48 hour' delivery seems to be lost in the Royal Mail network . I haven't been able to win an auction bid for the 36T chainring I intend to use either. That aside I could experiment with the mount and an old Suntour braze-on front derailleur I have in my spares. The mount size is 40mm as expected, so I have 1mm thick double-glazing packers at the ready to take up the slack around the 38mm seat tube. I guess if I heat them in boiling water I can 'mould' them around an odd piece of tube so they'll be an easy fit around the seat tube. This mount isn't perfect, but it's better than I can source from a UK manufacturer (thank you China..... whatever happened to UK manufacturing?). All is not lost, so using a file I intend to make the shape of the upright a better match to the front mech, and where the upright attaches to the band could also be improved to give it more stability it so it isn't totally reliant on the clamp screw. The joys of modification

 

[u_i]

One problem with this mount is that it does not adequately prevent the rotation of the holding finger. The second problem is that the bulky band is eating up the space for the derailleur cage. Still, it is better than nothing.

 

[Fastpedaller]

One problem with this mount is that it does not adequately prevent the rotation of the holding finger. The second problem is that the bulky band is eating up the space for the derailleur cage. Still, it is better than nothing.

The Shimano front derailleur arrived today and the shape is an exact match to the 'finger' (your good description btw) of the bolt-on mount. The Suntour front mech I tried yesterday was different, and I wonder why? Maybe the braze-on mount was different on the bike it came off (a Rudge Montague folder)? I can file the lower part of the finger so it goes into the slotted part of the mount - If I do it very carefully it can be a close fit which will prohibit rotation without being totally reliant on the screw. I can appreciate the issues with a triple front changer - there is certainly room for the double, but yes, a triple can't be easy to fit. One thing I measured was the distance from the rear triangle hinge to the BB centre as 95mm, so when the rear is folded under the chain is effectively 190mm too long - disregarding small angles, this meant 190mm (7.5 inches) of chain slack, equivalent to 15 half link of a chain which suggests whilst using the bike there can be 30T difference of the chainrings without a problem. Clearly folding the bike with the chain on the largest ring is probably wise. I note the standard Brompton tensioner is about 100mm centre-centre.

 

[u_i]

The Shimano front derailleur arrived today and the shape is an exact match to the 'finger' (your good description btw) of the bolt-on mount. The Suntour front mech I tried yesterday was different, and I wonder why? Maybe the braze-on mount was different on the bike it came off (a Rudge Montague folder)? I can file the lower part of the finger so it goes into the slotted part of the mount - If I do it very carefully it can be a close fit which will prohibit rotation without being totally reliant on the screw. I can appreciate the issues with a triple front changer - there is certainly room for the double, but yes, a triple can't be easy to fit. One thing I measured was the distance from the rear triangle hinge to the BB centre as 95mm, so when the rear is folded under the chain is effectively 190mm too long - disregarding small angles, this meant 190mm (7.5 inches) of chain slack, equivalent to 15 half link of a chain which suggests whilst using the bike there can be 30T difference of the chainrings without a problem. Clearly folding the bike with the chain on the largest ring is probably wise. I note the standard Brompton tensioner is about 100mm centre-centre.

Suntour front mechs expect a shallow cavity in the mounts. Maybe that was usually the case with the braze-on mounts early on.

When you fold a Birdy, you are supposed to shift first to the highest gear for chain management. Thus, the envisioned regiment would not be an isolated situation. I use a pin to gather slack upon folding my Brompton with a large chainring difference, but before that pin, I often left some chain slack in the folded bike, and incidents when that caused trouble were rare.

 

[Fastpedaller]

After receiving the 36T inner ring a few days ago, I've been fettling parts today. The mount for the front derailleur has a couple of issue, but they have been overcome. It changes between the ring in the workstand, and a road test will follow in due course. With the chain on the big ring and small sprocket the chain causes no issues when the bike is folded or unfolded. I fitted a suspension lock-out control for cable control of the front derailleur, and although this feels tough to move when the bike is in the stand it may be ok when I'm on the bike - I may change it to a Sunrace shifter. Some photos follow. 1) There is an angled face to the finger which means the clamp bolt/washer of the front mech isn't flat to the surface. 2) I cut and filed 2 grooves so the finger goes into the slot of the mount - the slots prevent it from turning and give more security. 3) Using a dremel I tried to spot-face the angled area of the finger to make it parallel - this was a mistake, so I then used a file. Not pretty, but it did the job. 4) two pieces of window-fitters plastic shims were cut to length (60mm), put in boiling water and then moulded around a 30mm tube to give them a curve which made fitting easier. 5) Because the clamp is 40mm (tube is 38mm) the 1mm shims are ideal, don't damage the frame and the mount is secure. 6) a view of the set -up 52T and 36T rings. 7) with the Brompton folded the chain mostly stays on the big ring, and doesn't fall off when the bike is unfolded. Good points about the derailleur mount..... a) It's available. b) Relatively cheap. c) there is good adjustment up-down fore and aft of the finger and the derailleur clamp position, and of course it's possible to move the clamp up and down on the frame tube. This is probably better than having a braze-on already on the tube (unless the position is decided prior to spraying)

 

[u_i]

After receiving the 36T inner ring a few days ago, I've been fettling parts today. The mount for the front derailleur has a couple of issue, but they have been overcome. 2) I cut and filed 2 grooves so the finger goes into the slot of the mount - the slots prevent it from turning and give more security. 4) two pieces of window-fitters plastic shims were cut to length (60mm), put in boiling water and then moulded around a 30mm tube to give them a curve which made fitting easier

Smart!

 

[a.twiddler]

I've been watching this thread with interest. Like most Brompton owners, I can't resist meddling with it. Mine is an older M3R which originally came with a 50T (one piece) front, 13T rear giving a choice of high, even higher and absolutely ridiculously high ratios. I went the usual route of fitting a 14T on the back, then came across a budget 44T on the internet which I put on the front, and which made a difference. After printing out Sheldon Brown's BCD measurer I found that one set of lightening holes in the original 50T chainwheel corresponded to 86 BCD.

About two years ago, after reading about what might be possible on various forums I picked up a 28T Stronglight ring, a 99 I think. I'd recently changed my bottom bracket from a FAG ISO taper one to a Shimano JIS one, as far as I can recall 118mm but the cranks fitted fine. There was a slight tecnical hitch in that the small ring just skimmed the paint on the rear triangle when fitted on the inside. This seems to be because the amount of spindle protruding each side is symmetrical with Shimano, but longer on the chain side with the original one. This was something that I hadn't realised when I bought it, even though I'd been thinking about possibly adding a ring.

So...I fitted it on the outside! Although I'd previously bought a front derailleur clamp with an idea that I might fit a front derailleur at some time, I wasn't averse to changing with a finger to see if the principle worked.

It did work, but with the combination of short chainstay and big difference in ring size, the chain rubbed and occasionally snagged the teeth of the big ring. I put the 44T chainwheel back on for a while, until I could work out a solution.

What if...I took the BB out and put a spacer in on the chainwheel side? Would that do the job? In my bits box I found a BB adjusting ring for a cup and cone BB. I filed the threads off the inside. It was about 4mm thick. I reassembled the BB with that in place as a spacer, fitted the 28T ring to the inside of the 50T chainwheel and reassembled it to the bike. Everything fitted solidly. A test ride proved that it all worked quite well, with no rubbing or odd noises apart from the usual Brompton ones. It's just possible to push the chain from the big ring to the small with my foot when moving, but it's necessary to stop and use a finger or a tool to get it back on the big ring.

The ratios are:-
1st 24.66, 2nd 32.88, 3rd 43.47, 4th 43.6, 5th 58.2, 6th 77.6. 3rd and 4th are similar, giving a good overlap since it's possible that I might have to ride for a bit before being able to stop and change ratios.

New things that I have to remember. 1) make sure that the chain is on the big ring when folding it. 2) When riding on the small ring, it doesn't like it if you backpedal!

I've been putting off trying to fit the clamp and a suitable derailleur to the front for the last couple of years, as it works quite well as it is. I don't really know which derailleur to go for. It will need to cope with 22 teeth difference, and the range of movement available will need to be generous. Also it will have to fit within the fold. A front derailleur that complies with those restrictions would be a nice finishing touch, though.

 

[u_i]

To push the ring out, you can get the newer Brompton JIS, which should be asymmetric. You could also put a spacer under the lip of the bottom bracket on the drive side. While you can find dedicated BB spacers in the market, the cassette spacers are the right size and may be used for this purpose.