I've recently bought a second hand Brompton for my commute and day to day riding around town, but the gearing feels a bit off to me. I'm used to riding a 700c fixed gear bike, running 48t x 17t, and I guess ideally I'd want the middle gear on my Brompton to be roughly equivalent to the gear I run on my fixed gear bike. The brompton has a sturmey archer 3 speed hub, which is labelled 'SRF3', and is currently running 50t x 14t. Is this going to be possible? What sort of sizes of sprocket/chainring do I need to get my middle gear roughly equivalent to the 48t x17t on a 700c?

basic math ..

48/17x26=73.41..
I'm using a 54/15, top gear is about 77"

53/13x16=65.2 x1.33=86.7, high gear

a math help program https://sheldonbrown.com/gears/internal.html

Thanks Bob.

A quick check of Sheldons site suggests that my fixed is running 74.5", and my Brompton is running 43.0/57.4/76.5 which makes sense. I spin out too quickly on flat in middle gear, but can rarely get into top gear unless there's a downhill. I could (I think) go to 54 x 13 on the brompton, which would give 50.1/66.8/89.0, which might help a bit with the middle gear, but it might still be too low, and the top gear I think would be pretty much useless. 50 x 15 would give me 40.2/53.6/71.4, so I could probably use top gear on the flat instead. I wouldn't have anything extra for downhill sections though, and in London I can hardly imagine using the lowest gear, but I guess it might have some use if I ever go somewhere hilly.

Am I right that the biggest I can easily go at the front would be 54 with brompton cranks? And that the rear sprocket can be 13-15 on the SRF3?

Has anyone got any thoughts on what might be the best option? What gearing are others running on their Brompton?

I ran a 3sp for a while, and found I liked it best to have the middle gear right for cruising flats, low for climbing and high for downhills. Middle gear was somewhere in the 70s.

This also make best use of the efficiency of the gearing, middle gear is direct drive, and best efficiency.

13-24, actually.

I have the top gear for cruising and two gears for climbing/headwinds, and if I spin out I can coast down hills. I like this better than powering downhill and walking up. I'm not the only one who sets three-speeds up this way.

Don't worry about efficiency in the gear ratios - a broken-in and properly lubricated three-speed hub is amazingly efficient - on average, better than a high end derailleur gear train, and arguably the most efficient multi-gear bicycle drivetrain available.

On my tikit, with the same wheel size as your Brompton, I use a SRAM iMotion3 hub, 53x16, giving 39/53/73g.i.

I'm sure I remember reading something about the maximum rear sprocket size being 15t, but maybe that was to do with the brompton chain tensioner rather than the hub?

My initial thoughts were to set up the middle gear for cruising on flat, but unless my maths is wrong, it seems that I would need to run a 58t chainring, which seems huge, and would require a whole new crankset I think. Unless I'm missing something, the easiest and cheapest option seems to be a 15t sprocket, to get top gear a little lower for cruising, at the expense of never really needing the lowest gear.

That might be the largest that Brompton offers -- I have not looked in years -- but I've seen people with bigger sprockets than a 15.

On a 50x14 Brompton with SRF 3 you have ratios of 44.3,59.1 and 78.8 with a 13 sprocket you have 47.7,63.7 and 84.9

Have a look at https://xldev.co.uk/bgc.html for calculating Brompton gear ratios..Much better than the Sheldons..This is just for Bromptons alone..

If you stick to the ones Stronglight makes for Brompton, then that is true.

If the hub was a BWR, the cog could be a 12t, as the spline is smaller.

I could not fit larger than a 15 on my Mk2 ,[UK AW3 hub]
I had the sprocket teeth into the rear frame tube, with a 16t.
I have no experience with Mk3[sachs/sram hub] or the current Mk4
The end of the right rear triangle could be made to provide more clearance.

my gear : 54:15 , I use a Schlumpf mountain drive 2 speed crankset,
it's planetary low is a '/, 2.5 reduction gear or a 54/21.6t.
6 gear ratios, high in low range is #3, low in high range is #4.

Because the town is built on a Hill.

Looks like I'll be going for the 15t. This, and maybe a new chain, should be all I need right?: https://www.sjscycles.co.uk/sturmey-a...850-prod19351/

Can anyone confirm that a 15t definitely fits in a Brompton? I've read that 14 does and 16 doesn't, but is that in an old model? What about 15?

I have the 2 speed S-Type Titanium with 54 - 12 + 16 cogs so 55.9 & 74.5 gear inches.
I did get a modified sprocket from Steve Parry soon after buying the bike.... but to be honest I just couldn't bring myself to grind a section out of my beautiful Titanium rear triangle I would like a like a Schlumpf speed drive but I'm just not sure I would like to add weight to my Lightweight Brompton

My 15t came from spares for the Sachs/SRAM 3x2, a 13/15 pair 3/32 wide.
they made the short cassette using the same 3 tab spline..
newer Sturmey BWR adopted the Shimano BMX hub spline,
haven't laid hands on them , spline for the derailleur cassettes has 1 wider than the others.
to align the cogs shifting details to each other ..
perhap all splines are the same when a single cog Bmx setup..