Brompton bottom bracket upgrade
 

Hi all,
I didn't see anything too recent on this topic, so here goes...
I have a 2 year-old Brompton S2EX with lots of upgrades (it's down to 20.9 lbs now and the ride is phenomenal). Was thinking of upgrading the last few bits, including the bottom bracket. Is it worth upgrading from the stock Brompton one to the Shimano UN55 (about $40) or would you just go all out and go with the Phil Wood one ($200+)? Would I even feel a difference? The stock bottom bracket feels a little too mushy for me; however, I'm used to nice road bikes... Let's try to avoid the "it's not worth the money" opinions, if possible :) Thanks in advance...

The Brompton bottom bracket is a special one, its asymmetric with the right axle 5mm longer than the left axle to compensate for the frame bottom bracket box asymmetry.

To my knowledge, there are only two possibilities for an asymmetric 118.5mm square taper JIS bottom bracket :
- either the Ti Parts Workshop one made specially for the Brompton (exists with steel bearings or ceramic bearings) it has a titanium axle and save about 100g vs. the original BB.
- either the Phil Wood BB that exists with titanium axle (similar weight saving as for the TPW) or steel axle.

If you want to further reduce the weight of your Brompton, you can also change the crankset, the original one is quite heavy and replace the chainring+cogs+chain with 11s ones that weight less (a KMX X11SL chain last much longer and weight significantly less than the original chain).

If you want to change everything, Ridea has a beautiful low weight crankset with integrated axle and external bearings specially made for the Brompton.

Thanks for the info... I'll look into the Ti Parts Workshop one too.

Reviving my thread from last year... finally about to splurge on a Ti Parts Workshop bottom bracket with stainless steel bearings. Anyone have experience with this bottom bracket?

I only have experience with a Ti Parts Ti Tensioner Wheel for Brompton. At 6x the price of OEM it performed miserably, making the bike dysfunctional, and had to be taken off. Phil Wood BB cups provide no sealing. Good luck there!

In my measurement, the Brompton's BB spindle sticks out just by 2mm farther on the right side than on the left, something that can be taken care of with a spacer if anyone cares. I think I rode with a symmetric TA Axix Light for a while without any problem. My current spindle is asymmetric, but this has a different background than here at play.

Yikes! Ok, maybe I should just look at the Shimano one... I think it's the UN55, correct? Does anyone know where I can find the one that fits the Brompton?

In 2017, when Brompton had the bottom bracket recall*, I replaced my stock bottom bracket with a UN55. The SKU was 4524667778790, and it's listed as part number BBUN55B18. I've been riding it with this bottom bracket for the past 6 years with no issues.

This is a 68 x 118 British thread square taper. I don't recall having to do anything special, such as add a spacer, to make it work.

*I bought it from Clever Cycles in Portland, but live 150 miles from there. I wasn't about to drive it to Portland to have a free replacement installed.

Are you saying that you had to take it back to the dealer that you bought it to get the upgrade free?

What I recall is that the recall fix had to be done by the dealer. I probably had the option to do it at a local-ish dealer, but that would have been 40 miles away from me and it would have been on their schedule.

The main point is that bottom bracket I mentioned does fit in the Brompton.

I just wondered if any Brompton dealer would have done it free? I got mine done on a recall and wondered if any Brompton dealer would do it at their cost. I had to take the Brompton supplied BB they put it in. No issues with it.

A real upgrade would be TA Axix Light Pro, in 119mm: https://www.starbike.com/en/speciali...t-bsa-ti-axle/. The bearings are replaceable, so that would be a bottom bracket lasting forever.

I am using SRAM Dub BSA BB, with SRAM RED AXS crank, with 50TChainring(4mm offset), It works perfectly.

OP here with an update (yes, it's been a while!). I ordered and just received this bottom bracket (price was too good to pass up):

https://curbsidecycle.com/en-us/coll...46780219818299

https://curbsidecycle.com/cdn/shop/p...196&width=1800

Is it worth it to consider upgrading the crankset as well or just use the standard Brompton one on there? Thanks.

I have a Ti Parts Workshop bottom bracket in my Brompton since several years.

It work fine but you should avoid titanium bolt to mount the crank.

It weight much less than the original one.

If weight is important for you, for 75USD its a good deal.

I bought an expensive Ti Parts tensioner wheel for Brompton from the same source, and it worked much worse than the original plastic from Brompton. This makes me skeptical of Ti Parts engineering. In terms of weight, a Ti BB is undoubtedly a gain, and it is presumably a class above an Ace Ti BB.

I also bought several components from Ti Parts Workshop.

Some where bad like the tensionner wheels with double bearings that indeed are a bad design (its a misunderstanding of the way of working the Brompton derailleur fork that require to work wheel that the whole wheel turns), some work well like the titanium-carbon rear rack.

For the bottom bracket, its reliable, I had no issue with moisture or water entering in it and my TPW bootom bracket is now about 8 years old.

The only issue is a well known one with titanium BB axle: titanium bolt are very difficult to be enough tightened in a titanium female thread, therefore that its better to use long stainless steel bolt.

Another issue that was encountered is that some Brompton had a badly threaded bottom bracket box, i.e. the threads of the two sides were not perfectly aligned what make he mounting of bottom brackets like the TPW with two separated bearings difficult. But this issue is not due to TPW and any bottom bracket with separated bearings will have problems.

About the asymmetry of the Brompton bottom bracket box, its 5mm to the left of the bike to have more room on the right side for the crankset and chainring, so the right axle is 5mm longer than the left axle to compensate and have symmetric pedals. With a symmetric bottom bracket, the pedals will be slightly asymmetric (which is not a major problem).

I bought a similarly asymmetric BB with a Ti axle on eBay in the past. I think it was the World Class brand, and the seller did not declare it asymmetric. I noticed the asymmetry when browsing my stock for something that would fit Brompton, and it was a perfect fit. Moving a symmetric BB by 3mm or so to the drive side with spacers may be possible.

As for tensioner wheels, I made some with bearings useful by pouring superglue into and around the bearings and seizing them.

From the specs and my measurements, its 5mm asymmetricity.

I have no reason to distrust this, but in my experience, there is quite a bit of leeway. I got away with a 119 mm axle length, the same as in the original BB, while accommodating a triple crank.

(bold above) Seems to me, that a 2-piece (steel) hollow-spindle crank is going to be weight competitive with a solid titanium taper spindle, but at way lower total cost. Plus, the hollow spindle is stiffer, and puts the bearings closer to the crank arms so less bend in the spindle, and the external bearings are superior in being able to adjust out any slack over time. That would be my choice. Plus, generic 2-piece hollow spindle cranks with chainrings and bearings are dirt cheap on amazon, about the same cost as just two chainrings.

Some folks in my area touring on Brompton M6s, also had Schlumpf drive in the bottom bracket for true touring lows.

Using an IRD bottom bracket in my C-line "Explore" (6-speed). Solid BB, no plastic, good cartridge bearings. 115mm spindle if I recall correctly, symmetrical. Don't sweat it if the chainline runs +/- a couple millimeters either way, that's nuthin'.

The bike shop mounted the Ti Parts Bottom bracket with no fuss. It feels so much better than the stock Brompton one. Much less flex/springy-ness. So far, so good!

Titanium is about half the density of steel, so lighter for the same dimensions, but it also has half the elastic (Young's) modulus of steel, so twice as flexy (though still 50% more stiff than aluminum). This is all elastic flex, no permanent yielding of any of the materials. If the Ti BB feels stiffer, it's because a) (if) the center portion is larger diameter than the steel spindle, and/or b) (if) the bearings are more radially rigid, or impart some bending stiffness to the spindle.

Quick update: So far, the Ti PartS Workshop bottom bracket feels really good, spins smoothly, and most importantly, I'm not thinking about it on a ride. Stand-up pedaling feels much better than the stock Brompton bottom bracket. Plus, my bike lost even some more weight!