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WeeBike 09-07-19 11:23 AM

Brompton Gearing and Brake Modifications
 
The Sturmey 8 speed

I have been running an 8 speed on a Brompton (16 inch 349) for 6 years and recently changed it up for another 8 speed but now with the drum brake. Works like a charm and I avoid the poor Brommie gears and worse brakes. The gears are a bit noisy but I always know what gear I am in by the unique sound from each gear. I use a J-Tek thumb gear changer (no space for a twist grip changer on a Brommie) which makes me think I have a Rohloff(what a dreamer) given how smooth it all works together. I use a Shimano Alfine 501 39 tooth crank set with the Sturmey 20T cog. This gives a range of 31 to 101 gear inches which works well in an environment without large hills (see Sheldon Brown's gear calculator if you wish to fine tune yours differently). On the Brommie you have to widen the rear triangle to 138mm (165mm initially to allow for spring back).

The Sturmey 8 speed gets a bad rap on the net, however, I have had great success with mine. I use Jagwire cable adjusters at the hub(for adjusting on the work stand)and at the gear selector(for adjusting on the fly) in case I need to fine tune the adjustment which really only is needed for the fist week of setup and riding.

I also installed a Sturmey front dynamo/ drum brake combo. The drum brakes are weather proof, dirt proof, stop on a dime and are nice and smooth. The dynamo has you lit up at all times for safety.

Next posting - Nuvinci hub with Shimano roller brakes on a Brompton (AKA - the heaviest non electric B ever)

Numerozero 09-07-19 12:15 PM

I also have a S-A 8 speed hub on my Downtube 8FH and love it. After I bought it I figured out why the previous owner sold it - poor shifting. The problem is that, first, the X-RF8 was a dog. Yan, owner of Downtube, told me he had many, many fail, and that is why S-A came out with the X-RF8(W). BTW, he still has a bunch of X-RF8s for sale for $125. Second, the S-A 8-speed hubs are basically geared for small wheels, such as folding bikes. Folding bikes have contorted shift cable paths to accommodate folding. etc. This increases friction. My solution was a Jagwire Elite Link Kit. Very expensive, but, two years later, it still shifts effortlessly. (I see that Jagwire knockoffs are for sale on AliExpress for much less. I'll be changing my cables on my Xootr with them).

2_i 09-07-19 01:24 PM


Originally Posted by WeeBike (Post 21112166)
The Sturmey 8 speed gets a bad rap on the net, however, I have had great success with mine. I use Jagwire cable adjusters at the hub(for adjusting on the work stand)and at the gear selector(for adjusting on the fly) in case I need to fine tune the adjustment which really only is needed for the fist week of setup and riding.

Good to hear that :thumb:.


Originally Posted by WeeBike (Post 21112166)
I avoid the poor Brommie gears and worse brakes. (...) This gives a range of 31 to 101 gear inches which works well in an environment without large hills


Horses for courses. Surely you must like one shifter better than two, but as to the gear selection I do not see much of a difference compared to a regular 6-speed Brompton. Maybe I should spell it out more on the personal level: not enough difference to make me move to modify a bike just for that.

As always photos would be good, but you may need to wait for enough posts.

Schwinnsta 09-07-19 05:45 PM


Originally Posted by WeeBike (Post 21112166)
On the Brommie you have to widen the rear triangle to 138mm (165mm initially to allow for spring back).

I am surprised that you can bend the stays that much without buckling deformation and that they spring back. I mean that is big bend for such short stays.

WeeBike 09-08-19 12:38 PM


Originally Posted by Schwinnsta (Post 21112530)
I am surprised that you can bend the stays that much without buckling deformation and that they spring back. I mean that is big bend for such short stays.

I was quite worried about this and read several posts about this before making my move. I used automotive spring compressors as clamps at the brazing points on the chain stay and on the down stays; then I used the same tool(total of three identical tools)in reverse to spread the rear triangle while running a small rod through the large holes on each side of the triangle to stop any slippage of the expanding tool as the whole triangle widened. You want to do this in one go, as the steel will harden and become brittle if you do the widen/ stretch over several iterations.

No buckling or deformations. The partial spring back is corroborating evidence of that.

WeeBike 09-08-19 12:48 PM

You do get a mildly greater ratio range than the original six, only one gear changer and only one gear changing/ pedaling sequence plus you get the drum bakes which have saved my life already more than once. This braking is no small deal. The small rims on folders are also at the peril of early failure by the extreme abrasion caused by the greater number of wheel revolutions passing by the friction points of the rubber/ grit brake shoes. A colleague of mine had his Tern wheel collapse under him for this very reason. Why do you think Brompton has fiddled with brake levers and rims over the years?

WeeBike 09-08-19 12:59 PM

Z

Glad to hear that you found the Jagwire solution. You correctly identified the poor shifting on a great hub and rectified it. I found also that the twist shifter is crap and that the newish Sturmey thumb shifter or the J-Tek shifter up the game considerably. Also the original Sturmey cables and casings also work better than after market cables and casings. I also found that cutting the after market casing with a band saw instead of with cutting pliers eliminates pinching and fraying at the ends and promotes better cable movements and ultimately way smoother shifting.

BromptonINrio 09-12-19 09:26 AM

sturmey archer 8 doesnt need to spread the frame, as its 116mm old. it will go in plug and play.
the ratios is the hard part, as brompton only aceppts 16 cog maximum due frama clearence.

so onlu aceptable gearing is 16 cog in back and 34 crankset on front.

Schwinnsta 09-12-19 10:25 AM

I am running 17 without any problem and I am confident 18 would go on without modification. I could put a 20 on but have to file a little off end of the chain stay but this would have no structural implications.

No interest in the SA 8. Right now I have 12, 14 and 17. If I was more confident that I could shift from 14 to 20, I would try that.

BromptonINrio 09-12-19 04:21 PM


Originally Posted by Schwinnsta (Post 21119498)
I am running 17 without any problem and I am confident 18 would go on without modification. I could put a 20 on but have to file a little off end of the chain stay but this would have no structural implications.

No interest in the SA 8. Right now I have 12, 14 and 17. If I was more confident that I could shift from 14 to 20, I would try that.

nice no know.
witch sprocket u are using? for 10v? or 11v?

Schwinnsta 09-12-19 05:16 PM


Originally Posted by BromptonINrio (Post 21120094)
witch sprocket u are using? for 10v? or 11v?

What is 10v or 11v? This on a modified 2 speed derailleur.

BromptonINrio 09-12-19 05:37 PM


Originally Posted by Schwinnsta (Post 21120195)
What is 10v or 11v? This on a modified 2 speed derailleur.

i just ask if you put a 10 speed sprocket or a 11 speed sprocket.
just regarding the tickness of sprocket

Schwinnsta 09-12-19 06:01 PM


Originally Posted by BromptonINrio (Post 21120215)
i just ask if you put a 10 speed sprocket or a 11 speed sprocket.
just regarding the tickness of sprocket

I got the Bikegang 3 speed kit or rather some of it. 10 speed sprockets and the spacer, and clip ring. I use SR 3 speed friction shifter.

dezzie 09-15-19 02:10 PM

Anyone work out my gearing as its beyond me, for my old F framed moulton i need to get finished building, i had the i had the 8 speed sturmey W hub built into a sun 16" 349 rim, i bought a 42t crankset and rear sprocket is 20t that cam with it, what will my gearing be and will i have to change sprocket sizes? Sorry to hijack the thread but never had help off anyone else when i asked before.

Winfried 09-16-19 03:31 AM


Originally Posted by WeeBike (Post 21112166)
I also installed a Sturmey front dynamo/ drum brake combo. The drum brakes are weather proof, dirt proof, stop on a dime and are nice and smooth. The dynamo has you lit up at all times for safety.



Which SA dynamo+drum brake is it? Did you need to enlarge the fork?


Originally Posted by Numerozero (Post 21112207)
Yan, owner of Downtube, told me he had many, many fail, and that is why S-A came out with the X-RF8(W).

According to the Brompton Gear Calculator, an S-A XRF8(W) even with a 34T crankset + 16T sprocket offers a low gear of 2.81m/35.2" which is still too high for hilly areas.

Are there ultra small 32/22T double cranksets?

BromptonINrio 09-16-19 04:00 AM


Originally Posted by Winfried (Post 21124131)

Which SA dynamo+drum brake is it? Did you need to enlarge the fork?



According to the Brompton Gear Calculator, an S-A XRF8(W) even with a 34T crankset + 16T sprocket offers a low gear of 2.81m/35.2" which is still too high for hilly areas.

Are there ultra small 32/22T double cranksets?

35" is not bad, its similar to road bike gearing.

to lower that you can put 17 or 18 sprocket on rear(and maybe sand the frame down a few mm.

the kinects shop feom scotland uses the sturmey archer srf8 with 18/33 sprocket crankset gear.

he adivices smaller than 33t the chain will rub on metal square cabble holder on chainstay.

Winfried 09-16-19 04:15 AM

To me, 35" is still too big when touring in mountains with a loaded Brompton.

An 18T sprocket required adding a spacer because the chain was rubbing slightly against the chainstay, and a 19T rubbed even more.

Even with an 18T cog and a 30T chainring, we're still at 2.20m/27.6", much higher than the 1.58m/19.8" I now have with a 46/30T crankset on a stock six-speeder.

>he adivices smaller than 33t the chain will rub on metal square cabble holder on chainstay.

I don't see what part it could be on my six-speeder: Is it specific to the single speed Brompton?

--
Edit:


Originally Posted by WeeBike (Post 21112166)
I also installed a Sturmey front dynamo/ drum brake combo. The drum brakes are weather proof, dirt proof, stop on a dime and are nice and smooth. The dynamo has you lit up at all times for safety.

A couple more questions about a SA drum brake+dynamo:
1. The OLD is 100mm: How did you fit it on a Brompton whose OLD is 74mm?
2. What about fitting the reaction arm on the fork?
3. Are you confident the Brompton fork can withstand the force from the reaction arm?

And about using a SA X-RF8(W) gear hub:
1. Is "X-RFR8" = "X-RF8(W)"?
2. Can a six-speed Brompton fit a X-RD8(W?) gear hub and enjoy its drum brake?
3. The web page says that the X-RF8/X-RD8 gear hubs can only take 20-23,25T sprockets: Aren't those too big to fit on a six-speed Brompton? Is a single speed Brompton required?

Finally, in case I wanted a wider gear range, are there subcompact double cranksets 30/20T with square taper?


Thank you.

WeeBike 09-22-19 01:28 PM

Sturmey 8 various OLD's
 

Originally Posted by BromptonINrio (Post 21119403)
sturmey archer 8 doesnt need to spread the frame, as its 116mm old. it will go in plug and play.
the ratios is the hard part, as brompton only aceppts 16 cog maximum due frama clearence.

so onlu aceptable gearing is 16 cog in back and 34 crankset on front.

The drum brake version requires about 138mm OLD whereas the small made for folding bikes version is 116mm but no drum brake. I use the Sturmey 20T cog and Shimano 39 T Crankset giving gear inch range of 32 to 102.
WB

WeeBike 09-22-19 01:35 PM

The front hub is a SA X-FDD 6 volt 3 watt and yes I did widen the fork to 100mm.


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