Thursday, August 17, 2017

Brompton M6R: Shimano Crankset and Bottom Bracket

Today, I am going to upgrade the stock Brompton crankset. The stock crankset is a square taper crankset, with a 5 arm design and 130mm BCD for the chain ring. What I found was that if you point the crankarm downwards and step hard on the pedal, you can actually see and feel the crankarm flex inwards. To me, this is not acceptable as the excessive flex is unsettling, and also means less efficient pedaling.

I plan to change to a Hollowtech II crankset, where the spindle is integrated with the right side crankarm, and mounted through an external bottom bracket. This setup will be stiffer than a square taper system.

After consulting some friends, I was advised that it will not be a straightforward swap, as the clearances around the crankset and chainring are tight. If the chainline is too inwards, the chainring will interfere with the frame. If it is too outwards, it will interfere with the folding.

This means some trial and error is needed to adjust the chainline and avoid interference. Before installing the new crankset, let's take a look at the original setup.

Original setup has a gap of about 3mm between the chain and chainstay.

After removing the stock crankset and bottom bracket, the bottom bracket shell is confirmed to be standard 68mm English threaded.

Threads are cleaned and ready to accept a new Hollowtech II bottom bracket

Original 130mm BCD, 44T chainring + chain guard is 89 grams

Original square taper bottom bracket is FAG branded, and weighs 242 grams.

Original Brompton left side crankarm is 213 grams...

...while the original Brompton right side crankarm is 300 grams.

Together, the original crankset (without chainring) is 513 grams.

In order to match the colour scheme of the Brompton, I wanted to get a crankset with polished silver colour to match the rims. A black crankset is nice on its own, but it does not match this Brompton. I tried to get a Dura-Ace 7800 crankset, as the polished finishing is really nice, but it is hard to get a suitable one in good condition as it has already been out of production for almost 10 years, having been replaced by the newer 7900, 9000 and R9100 series.

The closest one that has a similar finishing would be the Tiagra 4600 crankset, which is also in polished silver. Of course it is not as lightweight as the Dura-Ace crankset, but it is OK as weight is not important for this bike.

Tiagra left side crankarm, 250 grams. Heavier than the Brompton left side crankarm as I guess it has more material for stiffness.

Tiagra right side crankarm, 424 grams. Also heavier than the Brompton right side crankarm, as it has more material for stiffness and also includes the weight of the spindle.

Normally, for a road crankset such as Tiagra, if it is installed in a 68mm wide bottom bracket shell, a road bottom bracket will be used, with no spacers required under the bottom bracket cups.

However, with advice from friends who have installed a Hollowtech II crankset on a Brompton before, a spacer of 3 to 4mm is required to push out the right side chainring to clear the frame.

From my experience, it is possible to fit a spacer with maximum thickness of around 2mm under the bottom bracket cups, before the left side crankarm runs out of sufficient spindle length to be safely fixed on. In this case, since I needed more than 2mm, some creative modification is required.

From a modification done previously on the Avanti Inc 3, I know that the MTB Hollowtech II bottom bracket is 1mm slimmer on each side, compared to the road type.

Therefore, I decided to use a MTB Hollowtech II bottom bracket, instead of a road bottom bracket. The slimmer bottom bracket cups will allow me to put an additional 2mm of spacers, enabling a maximum spacer thickness of 4mm.

Summary of crankset + bottom bracket weights (excluding chainring):
Brompton crankset + FAG BB: 752 grams
Tiagra crankset + Hollowtech II BB: 764 grams

The new setup will be a little bit heavier than the stock setup, as the Tiagra crankset has more material for stiffness, and is just a mid range crankset that is not lightweight. As already stated earlier, weight is not important for this bike, so this is no issue at all. If your objective is to reduce weight, you will need to get the higher end cranksets, such as Ultegra or Dura-Ace.

As I have read that the plastic chain guard on the chainring is an integral part of the Brompton folding, I plan to use the original 44T Brompton chainring + chain guard. Installation of this chainring onto the Tiagra crankset is no issue, although the appropriate length of chainring bolts are required.

For a start, I installed a 2.5mm spacer under the right side bottom bracket cup, and installed the right side crankarm to test out the chainline and frame clearance.

With a 2.5mm spacer, there is practically no gap between the chainring bolt and the chain stay when the Brompton rear triangle is folded.

To be safe, a clearance of at least 1mm is necessary. Mixing and matching my stash of spacers generated a spacer of 3.6mm thickness.

Note that this spacer thickness is only possible as I am using MTB bottom bracket cups that are slimmer than road bottom bracket. If not, using this spacer thickness will make it impossible to install the left side crankarm safely on the spindle.

After using 3.6mm spacer, there is a minimum clearance of 1mm

No clearance between the end of the crankarm and the plastic chain tensioner!

Although there is now clearance between the frame and the chainring bolt, I found that there is no clearance between the crankarm and the plastic chain tensioner when folded (as shown above). The crankarm only just manages to slide past the plastic surface. As this condition only occurs when folded and not during pedaling, it is acceptable. Just need to take note of this if you plan to do the same modification. Final spacer thickness installed under the right side bottom bracket cup is 3.6mm.

During installation of the left side bottom bracket cup, I found that the cup did not rest flat against the frame when fully tightened.

As I was afraid that I had cross threaded the left side cup, I reinstalled it, trying to align the cup properly while screwing it in. This time, I ended up really cross threading, causing damage to the threads on the cup.

Damaged threads on the aluminium left side cup due to cross threading

Luckily, this cup is made of aluminium, while the frame is made of steel, so no damage was done to the steel thread on the frame. All I needed to do was to get a replacement left side cup and reinstall it. Although the left side cup did not rest fully flat against the frame, there was no problem installing the crankset, so I left it as it is. The conclusion is that the bottom bracket shell on the Brompton frame is not faced properly to be parallel.

Tiagra crankset with original Brompton chainring installed! Silver colour matches better than a black crankset.

The plastic chainguard allows the front fender stays to rest against it when folded, preventing excessive movement of the front wheel.

This upgrade was a little tougher due to the tight clearances around the crankset, making it more technically challenging. Nevertheless, the crankset has been successfully upgraded to a stiffer Hollowtech II crankset, which provides stiff and efficient pedaling.

In summary, what you need to install a Hollowtech II crankset on the Brompton are as shown:
1) Road Hollowtech II crankset, double (not triple, as the spindle length and chainline are different)
2) MTB Hollowtech II bottom bracket cups
3) Spacer of approximately 3.6mm
4) 5 x chainring bolts for single chainrings
5) The right tools to remove the original crankset and BB, plus tools to install the new BB and crankset


  1. Thank you soo much! about to do the same upgrades! btw, is XTR BB also has a slimmer fit than the road BB?


    1. Not sure about XTR, as it may use a different bb as other series.

  2. Shimano Alfine single ring crank and BB also work straight out of the box. (Though they are only black) I've had one on my SL2 for three years.

  3. Hi. I'm looking to do the same as I have an old 105 chainset in the garage. Did you put any spacers one non-drive side? I'm assuming not.

    1. No spacer on the non drive side.

    2. Thanks. Parts arrive tomorrow i have 3.5mm of spacers ready. Fingers crossed

    3. What is the length of the spindle? 113mm or 116mm?

  4. May I know how much difference compare to stock crankset in term of performance?

  5. By offsetting the crank on the DS you now have the cranks unevenly spaced on the right vs left, ie pedals closer to the centre line on one side than the other. Only a few mm that I wouldn't notice, but worth noting.

    1. You have a valid point, and I actually checked that in the next post when I installed new pedals. Surprisingly the offset is not that much.

  6. Hi, if you were to install the tiagra crank as a 53-39 (or 50-34) chainset, then when the bike is folded with the chain on the 53t chainring, it should clear the brompton frame without needing 3.6mm of spacers right? Since the 53t chainring is spaced more outwards?

    1. The 53T or 50T chainring will have the same chainline as the 44T stock Brompton chainring, since the mounting location on the crankarm is the same. There will be the same interference issue.