Cycling UK Forum

Has anybody got the telephone number and email address or website so that I can obtain a clip to fit a front deraillurer on a Brompton?

Regards

Mary

SP Cycles (Steve Parry) did one
http://www.ebay.co.uk/itm/Taiwan-made-Braze-On-Front-Derailleur-Adapter-For-Folding-Bike-Red/161550237171?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649
NB I have neither

I'd be concerned about an FD mount that says "inner diameter around 40mm"

Hi, Brompton say that the chain lengths on their bikes are extremely critical for proper functioning of the rear changer, which is not a true derailleur and the tension arm has a much more limited range than the proper derailleur type changers you find on many other types of folding bikes. If you change the size of the chainwheel, say from the standard, to the low gear Brompton chainwheel, you have to actually trim the chain, even though this is only a matter of a few teeth. I have heard of people making this kind of modification to other folding bikes which have a more conventional rear derailleur set up, but I've never heard of anyone doing it on a Brompton. I f you know of anyone who has done this, I'd be interested to hear about it. Hope this helps.

Mark of Leeds wrote:Hi, Brompton say that the chain lengths on their bikes are extremely critical for proper functioning of the rear changer, which is not a true derailleur and the tension arm has a much more limited range than the proper derailleur type changers you find on many other types of folding bikes. If you change the size of the chainwheel, say from the standard, to the low gear Brompton chainwheel, you have to actually trim the chain, even though this is only a matter of a few teeth. I have heard of people making this kind of modification to other folding bikes which have a more conventional rear derailleur set up, but I've never heard of anyone doing it on a Brompton. I f you know of anyone who has done this, I'd be interested to hear about it. Hope this helps.

To be fair, I've an S2 that is running 12 & 18t sprockets with 54t standard front chainring. No mods on the rear changer, only had to add 2 links on the chain and a small dimple inside the Rh chainstay.
The shifting of the (Shimano hg 8sp) 18t is actually better than the original 16t sprocket
No problems at all while folding.

Since the late 90's we have been occasionally touring on a couple of Brompton T5's. We found the original gears to be not appropriate quite quickly. We have had several set ups over the years.

I had a chat with Mr Parry on the phone a while back about Brompton front derailleurs. He is a man who knows a lot but seems nowadays to be more of a semi-retired hobbyist, rather than a commercial operation. He said that nowadays he prefers to leave off the front derailleur but mount twin chainrings with the smallest on the outside, changing with a push of the foot; I did try that for a while but found it a bit frustrating.

Our current set up is a pre-NIG AW 3 speed hub (28 spoke for hers and 36 spoke for his) on the back, a standard Brompton chain tensioner, a Stronglight 86mm bcd twin chainwheel crank (obsolete but still findable second hand, and Spa still sell replacement rings: https://spacycles.co.uk/m2b0s149p406/ST ... -38t-inner ), one of those Chinese derailleur mounts that SA_SA_SA supplied the link to and a Shimano CX70 bolt-on cyclocross front mech.

Our chainrings are 28t and 46t. Smaller than 28t led to the chain fouling the frame and 46t is the max recommended for the mech. I expect that other modern mechs would work, but I haven't tried them. Controls are a simple Suntour friction lever on the left and a modern Sturmey plastic 3 speed lever on the right.

The Brompton seat tube is 38mm dia, whereas the derailleur mount is 40mm internal dia; we shimmed that up with a couple of wraps of duct tape and it works fine.

Here's some pictures:







Our SA 3 speeds use a 13t 3/32 sprocket (Bromton part No QRSPR13DR); 13t is a prime number, and so extends chain life by eliminating chain harmonics. We had to adjust the chainline slightly on the inner jockey wheel of the chain tensioner by fitting a slightly longer spindle bolt and a couple of washers to move it to the left. The chain is a 6/7/8 speed one.

A look at Sheldon Brown's on line gear calculator (http://www.sheldonbrown.com/gear-calc.html) will show that our set up is fairly low geared, but it suits our style of slow touring very well; I don't think that it would satisfy speedier people. That gets me round to "would I do that today", and the answer is "no". Our bikes are old ones which pre-date the Brompton rear derailleur, which we can't retro fit. If I was starting off touring today with a new bike I would go for the Brompton BWR 6 speed and 13t and 17t rear sprockets (both prime numbers) and 40t chainwheel. Brompton don't offer a 17t cog, but the BWR hubs have standard Shimano splines. Another squint at Sheldon Brown shows a similar gear range to our front derailleurs.

Anyway, I hope that this is some help.

Gattonero wrote:
To be fair, I've an S2 that is running 12 & 18t sprockets with 54t standard front chainring. No mods on the rear changer, only had to add 2 links on the chain and a small dimple inside the Rh chainstay.
The shifting of the (Shimano hg 8sp) 18t is actually better than the original 16t sprocket
No problems at all while folding.

Thread resurrection... so the Shimano splines are the same as the Splines on the 2 speed hub?

Will this work?
https://www.wiggle.co.uk/shimano-dx-sin ... ljEALw_wcB

DX sprockets will work but they are not derailleur sprockets, so don't shift very slickly. Sturmey archer S3X sprockets are similar but a fair bit cheaper usually.

cheers

Brucey wrote:DX sprockets will work but they are not derailleur sprockets, so don't shift very slickly. Sturmey archer S3X sprockets are similar but a fair bit cheaper usually.

cheers

Thanks!

I had a look at the design of the freehub splines and it seems to me sprocket thickness is critical. If I understand it correctly there is a set distance covered by sprocket + spacer + sprocket and then locked in place with a circlip. If the new sprocket is thinner the assembly will have play, if thicker, then the circlip won't fit...

But maybe I got it all wrong

Also, Brompton site suggest this is a workshop job, but it seems to me a case of removing a circlip with a screwdriver or a pair of suitable circlip pliers and that's pretty much all there is to it...

you can both remove and refit the snap ring using small screwdriver, if you have reasonably strong fingers. Yes you need to be sure that the assembly retains the same length (within about 0.2mm), if necessary by using spacers of different thicknesses (the ones that are sold to fit 10s cassettes onto 11s freehubs etc are useful).

Needless to say you can't use 1/8" sprockets but 6/7/8s derailleur sprockets (nominally 3/32") vary from about 1.8mm to over 2.0mm.

cheers

Brucey wrote:you can both remove and refit the snap ring using small screwdriver, if you have reasonably strong fingers. Yes you need to be sure that the assembly retains the same length (within about 0.2mm), if necessary by using spacers of different thicknesses (the ones that are sold to fit 10s cassettes onto 11s freehubs etc are useful).

Needless to say you can't use 1/8" sprockets but 6/7/8s derailleur sprockets (nominally 3/32") vary from about 1.8mm to over 2.0mm.

cheers

Thanks again... I think I have a 16T DX sprocket in the garage that I can use as a dummy before I order an 18 T one. I also have some spacers of various thicknesses

Update:

I found a 17T and a 19T as part of a 5700 10 speed cassette. I tried to fit the 19 T first, but it's too ambitious and fouls the stay. I resisted the temptation to file off 1 mm of the hollow stay and fitted the 17T, which works fine. Shifting is not as crisp as with the original 16T, not sure if it's the size of the sprocket or it's because it's a 10 speed one. I'll see if I can source an 18T from a cassette (ideally 8 or 9 speed) and try it...

Overall I am very pleased it only takes 5 minutes to do the job

Current gear should be 52 inch, which is the same I had in my youth with a 42 x 21... I used to climb fairly big mountains with that, so I should be fine, right?

Find a sprocket from an 8 or 9sp cassette, also you need to indent the Rh chainstay for anything over 17. It's not rocket science but if done wrong it may wreck the rear triangle...

Gattonero wrote:Find a sprocket from an 8 or 9sp cassette, also you need to indent the Rh chainstay for anything over 17. It's not rocket science but if done wrong it may wreck the rear triangle...

I suspect that'd be the end of any warranty claim... not to speak about resale value... I'll probably stay with the 17, seems to be OK...

Brucey’s point about the snap ring should be noted. Getting it off is easy when replacing sprockets but I could not get it back on. Ended up going to my local garage and one of the mechanics got it on immediately with no problem.