Go Back  Bike Forums > Bike Forums > Folding Bikes
Reload this Page >

Brompton- Switching to 39T, noob questions

Notices
Folding Bikes Discuss the unique features and issues of folding bikes. Also a great place to learn what folding bike will work best for your needs.

Brompton- Switching to 39T, noob questions

Old 10-09-19, 10:33 AM
  #1  
fishgirlcycles
Junior Member
Thread Starter
 
Join Date: Jul 2018
Location: East side of the Emerald City
Posts: 20

Bikes: Liv BeLiv 1 (2018), Mosaic GT-2 (custom, 2019), Brompton S6L (2018)

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked 0 Times in 0 Posts
Brompton- Switching to 39T, noob questions

Hello all. I currently have a 2018 S6L with the -12% gearing. I'd like to try changing to the 39T chainring since I live in a hilly area (13% just to get out of the neighborhood), plus I find that I hardly ever use it in the third gear right now because the resistance is too high for the cadence I like to keep.

Thank you to everyone who's done posts and videos showing this process already--they've been really helpful. But, I'm a total noob when it comes to wrenching. I have two questions:
1) For the chainring, the important numbers are that it needs to be a 39T, 5-bolt style, 130mm BCD?
2) The chain for Bromptons is a 3/32" size? I'm confused on what speed of chain I need. If I go with for example, a Shimano Ultegra 39T 130 BCD chainring that says it's for a 10-speed, can I get a 10-speed chain or do I want an 8-speed chain?

Thanks!
fishgirlcycles is offline  
Old 10-09-19, 01:52 PM
  #2  
2_i 
Senior Member
 
Join Date: Sep 2005
Location: Michigan
Posts: 2,739

Bikes: Trek 730, Bike Friday NWT, Brompton M6R*2, Trek 830, Trek 720, Dahon HAT060, Dahon HT060,...

Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 431 Post(s)
Liked 64 Times in 49 Posts
Look whether you can find a ring with a chainguard if you ever want to ride in long pants. Yes, you need 39T, 5-bolt, unless you decide to change the right crank too. It does not matter in practice what speed chainring you get. The chain still has to work with the rear sprockets, so you should not turn to a more narrow chain. If the chain is wider than the most narrow chain that the front ring can accommodate, you are fine.
2_i is offline  
Likes For 2_i:
Old 10-10-19, 07:01 PM
  #3  
1nterceptor
LET'S ROLL
 
1nterceptor's Avatar
 
Join Date: Nov 2009
Location: NEW YORK, NY - USA
Posts: 4,773

Bikes: 2014 BMC Gran Fondo, 2013 Brompton S6L-X

Mentioned: 18 Post(s)
Tagged: 0 Thread(s)
Quoted: 304 Post(s)
Liked 36 Times in 26 Posts
1. Correct.
2. I try to match the chain w/ the chainring(10 speed to 10 speed, 9 speed to 9 speed, etc.)
If your chain is not too old and want to reuse it; I'd try to find a 8 speed ring.

I went w/ a 10 speed ring(Shimano Dura Ace 39T) and 10 speed chain(Shimano Ultegra):

2013 Brompton DA chainring Ultegra chain by 1nterceptor, on Flickr
1nterceptor is offline  
Old 10-10-19, 10:22 PM
  #4  
2_i 
Senior Member
 
Join Date: Sep 2005
Location: Michigan
Posts: 2,739

Bikes: Trek 730, Bike Friday NWT, Brompton M6R*2, Trek 830, Trek 720, Dahon HAT060, Dahon HT060,...

Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 431 Post(s)
Liked 64 Times in 49 Posts
Originally Posted by 1nterceptor View Post
2. I try to match the chain w/ the chainring(10 speed to 10 speed, 9 speed to 9 speed, etc.)
If your chain is not too old and want to reuse it; I'd try to find a 8 speed ring.
No, the rear sprockets on Brompton are fairly thick, while having a wider chain than the minimal for a ring goes with impunity. Sure 8sp ring would be better, but once you put too many specifications into your search you come up with an empty list.
2_i is offline  
Old 10-10-19, 11:57 PM
  #5  
fishgirlcycles
Junior Member
Thread Starter
 
Join Date: Jul 2018
Location: East side of the Emerald City
Posts: 20

Bikes: Liv BeLiv 1 (2018), Mosaic GT-2 (custom, 2019), Brompton S6L (2018)

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked 0 Times in 0 Posts
Originally Posted by 1nterceptor View Post
1. Correct.
2. I try to match the chain w/ the chainring(10 speed to 10 speed, 9 speed to 9 speed, etc.)
If your chain is not too old and want to reuse it; I'd try to find a 8 speed ring.

I went w/ a 10 speed ring(Shimano Dura Ace 39T) and 10 speed chain(Shimano Ultegra):
https://www.youtube.com/watch?v=5AT6...IoDLA&index=33

2013 Brompton DA chainring Ultegra chain by 1nterceptor, on Flickr
Thank you! Your videos are so helpful (and better than the official Brompton ones)

Originally Posted by 2_i View Post
No, the rear sprockets on Brompton are fairly thick, while having a wider chain than the minimal for a ring goes with impunity. Sure 8sp ring would be better, but once you put too many specifications into your search you come up with an empty list.
So, 8-speed chain so long as I'm using the stock Brompton rear sprockets. Got it. Thanks!

I'm waiting to hear back from a company selling the Stronglight chainset--130 BCD (I think) 39T, has a chainguard, and 165mm cranks which would match the length on my full-size bike. I think the numbers all still work out, but hoping they can confirm.
fishgirlcycles is offline  
Old 10-20-19, 01:48 PM
  #6  
tomtomtom123
Senior Member
 
Join Date: Jan 2017
Posts: 945
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 306 Post(s)
Likes: 0
Liked 98 Times in 70 Posts
You don't need to replace the entire crank if you want to simply use a different size chainring and keep the same crank, unless you're also wanting the shorter 165mm arms, which might be useful if your legs are short. I used to have stock 170mm cranks on my old Dahon bike, and installed 165mm on my new Dahon bike. I don't know if I feel much difference in performance, but I do seem to pedal at a slightly higher RPM due to the lower torque from the shorter length and probably at a lower gear to maintain an equivalent speed and power output.

If you change the crank, assuming it uses a square tapered BB, different models may need different length BB spindles to maintain the same chainline, since different cranks will have different dimensions. If the difference in chainline is small, then it won't really make a difference in performance, but it's theoretically better to keep the middle of the chainring teeth centered in between the 2 cogs at the rear in order to keep the chain as straight as possible. You may want to measure the chainline of your existing crank/chainring and then measure again if you replace the crank with a different one.

Chainring with built in guard would be convenient. But if you are switching to a different crank with a flat outer surface on the bolt flange/arm, you could also simply mount a bashguard to the outside of the crank. You might need to cut off an arc of the bashguard to allow the crank arm to pass through. If the bolt flange/arm is tapered, you could also stick into the bolt hole counterbore a 10mm ID x 12mm OD spacer. The length of the spacer depends on how much clearance you need between the tapered arm and the bashguard to clear the tapered surface and between the chainring and bashguard to clear the chain. You then use longer chainring bolts. I use Truvativ chainring bolts which have a 5mm hex on the male side and 6mm hex on the nut side, so that I can avoid using the sometimes extremely difficult to use standard slots that are on the heads of most other chainring bolt nuts.

On my Dahon bike I used spacers from Misumi $2 each, on my Sugino cranks since it has a tapered surface. I selected 5mm long spacers because that's what I needed to get the right amount of protrusion of the chainring nuts for the bashguard to bear onto. It's more space than needed and risks my chain dropping in between the outer chainring and bashguard for my double setup, but I have tuned the high limit screw on my FD to try to prevent this from happening.
https://us.misumi-ec.com/vona2/detai...de=WSSS12-10-5
https://uk.misumi-ec.com/vona2/detai...de=WSSS12-10-5









Also, one trick if you're wanting a narrower Q-factor to bring in your feet closer together, since you have a single chainring, if you have a crank with a flat outer surface on the bolt arms, you could mount the chainring to the outside of the arms, then use a shorter BB to bring the cranks in closer to maintain the same chainline. But you'll need to check the frame clearance.

Last edited by tomtomtom123; 10-20-19 at 01:58 PM.
tomtomtom123 is offline  
Likes For tomtomtom123:
Old 10-20-19, 02:20 PM
  #7  
tomtomtom123
Senior Member
 
Join Date: Jan 2017
Posts: 945
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 306 Post(s)
Likes: 0
Liked 98 Times in 70 Posts
I looked around Google and found this video about changing the BB on a Brompton.

It seems old models use ISO square tapered BB while new models use JIS square tapered BB. So whichever new crank you're going to use should match the same standard as the BB. The guy in the video used a new JIS BB but an old ISO crank, which seems wrong. The tapers are at different angles, so the crank won't fit right with the spindle.

If you plan to switch to cranks with integrated spindles, this blog site documented such a modification. You have to be careful with the chainline and frame clearance, as described in the blog. One thing the person didn't seem to mention is shaving the water sleeve for the Shimano MTB BB to fit in between the MTB cups. I did a similar modification for my Sugino cranks on my Dahon bike because I needed to move my chainline further to the right. Since the Shimano MTB BB cups are 1mm thinner than the road BB cups I could shim the right cup further out while conserving a little bit of spindle length. I shaved a road water sleeve to fit the MTB BB cups, but I guess you could also shave the MTB water sleeve too if you shave both sides.
https://handsonbike.blogspot.com/201...nkset-and.html
tomtomtom123 is offline  
Likes For tomtomtom123:
Old 10-20-19, 05:33 PM
  #8  
fishgirlcycles
Junior Member
Thread Starter
 
Join Date: Jul 2018
Location: East side of the Emerald City
Posts: 20

Bikes: Liv BeLiv 1 (2018), Mosaic GT-2 (custom, 2019), Brompton S6L (2018)

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 7 Post(s)
Liked 0 Times in 0 Posts
I did decide to go with shorter cranks (I'm short and use 165mm cranks on my full-size bikes. Could probably go even shorter but wanted to stick with what I know). Did some poking around online and found a guy who did a lot of similar mods to what I'm looking for. Will need to use spacers as I don't want to change the BB on my Brompton and the cranks are a 113mm spindle versus my bike's 119.

But, I don't have a crank removal tool so I couldn't get my left crank off. Got the chainring swapped out fine (again, thank you all for the videos and posts you've done documenting this!) and the new drive-side crank on, but there are enough fiddly bits left (shorten chain, would like to recover the bolts on my old crankset because they're an upgrade and nicer than the stock ones with the new cranks but I can't get them out, get the left crank switched out, need different spacers for proper chainline) that I'll just take it to my bike mech and have him do it. I'm not very mechanical and am worried about stripping bolts or threads.
fishgirlcycles is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.