Changing the gearing on the Brompton

[a.twiddler]

Modifying the gearing on the Brompton
After my recent trip to Welshpool and back I'd been considering a quick way to get some lower gears. I’d already set up the original 50T Chainwheel with a 28T inner ring before I’d had all the fun and games with changing the FAG bottom bracket for the Shimano one. Unfortunately there was then just not quite enough clearance with the rear triangle when folded, so I refitted the single 44T ring. This had the virtue of simplicity with gears of 1st 38.4”, 2nd 51.2”, 3rd 68.3” (44X14T). To some of the Herculean thighed Titans out there 38.4” as a bottom gear is probably fine, and for the short sharp hills round here it’s manageable but for longer drags, for me at least, something lower is needed.

I had a go at the chainring bolts with a view to putting the smaller ring on the outside. Having read some of the vintage Brompton touring stuff, if using a greasy finger to change the chain between chainrings the system doesn’t care whether the smaller ring is on the inside or the outside unless a front derailleur is involved. The chainring bolts proved stubborn. When I assembled them I was determined that they wouldn’t vibrate loose so I used threadlock. Despite using solvents, heat and buying a humungous screwdriver as well as using a standard chainring tool, I only managed to shift one set of bolts. In the end, In a fit of exasperation, I ended up drilling the rest out. Fortunately I had enough spares to reassemble it again.

I set it up with the small ring on the outside, added two links to the chain and refitted the crank, pedal and chainwheel. It went on OK, and the chain runs freely on both rings. I thought it advisable to put the chain on the big ring before folding to avoid having too much slack, and there was adequate clearance with the rear subframe folded. Everything runs freely. I haven’t been up the road on it to try it out in anger yet but it looks promising. The gear range is 1st 24.66, 2nd 32.88, 3rd 43.47, 4th 43.6, 5th 58.2, 6th 77.6.

A smaller ring, such as a 26” would give 2 distinct ranges with no overlap but as it is with a 28 + 50 X14 the highest gear on the low range is virtually identical to the lowest on the high range. With no instant gear changer between rings, it would be difficult to make it work with the overlap as in the Brompton 6 speed system, so I’ve set it up to work as a separate high set of ratios and low set of ratios. Just got to suck it and see. Small ring:-

Big ring:-

Folded:-

 

[Schwinnsta]

What cogs on the back? I am not sure where your numbers are coming from? At present, I have 12, 14 and 19 T on the back. I have 44 in front. On my bike, by math I nave 14 x 44 yielding 50.5 gear inches. I am also contemplating expanding the range.

 

[gom]

I think that looks like a good setup. Using the "Bicycle Gear Calculator" http://ritzelrechner.de/?GR=SAAW&KB=28,50&RZ=14&UF=1330&TF=90&SL=2.6&UN=KMH&DV=gearInches helps me visualise what I think your gearing is.

I have a 26/46 setup (on a 5-speed, giving a lot of range overlap), using a TA chainset. It needed a BB change, but I just gave the bike to my LBS and asked them to find one.
I've recently managed to bodge on a front changer, which works if used with care. The small ring is on the inside. I think on the outside might make it easier to manhandle the chain back onto the big ring when needed?

 

[T4tomo]

Neat solution. I think your set up with equal gears 3 and 4 is fine as they are accessed from different chain rings, for more flexibility.

@Schwinnsta - it's got a 3spd hub at the rear

 

[a.twiddler]

Yes, the rear wheel has a normal range 3 speed hub gear. In middle gear on the hub the drive is direct so the 50 and 28 chainrings on the front give a gear that can be worked out using a gear table. The standard range three speed hub gear also gives a ratio of -25% in low gear and +33% in high gear related to the direct drive gear so potentially a total of six gears with the double chainring. There is plenty of capacity in the rear tensioner due to the need to accommodate the chain when folded so you can fit two chainrings of widely different sizes, 22 teeth in this case.

The hub changes gear with the handlebar lever, and the chainrings are swopped by using a finger to move the chain. It obviously needs some forward planning when on the road. The hub gear has a single 14T sprocket.

At some time in the future I will fit a wider bottom bracket and have a go at fitting a front changer with the small ring on the inside. I already have an adapter bracket from ebay in my bits box. Finding a suitable front changer would seem to be the biggest obstacle.

 

[gom]

I bought the cheapest changer I could get quickly on eBay. Not wide enough for the chain so I tried to deform it to a more suitable shape. Eventually the outer side broke off. But what’s left will shift down, and the inner side is strong enough to shift up.
When you have the small ring on the inside, I suggest get something cheap and see what can be bodged up.

 

[12boy]

I have a 58/38 combo on a 110BCD crankarms and having the 38 on the outside caused some rubbing under hard climbing. I tried a 24 on the inside of the 38 but it would not fold.

 

[u_i]

I bought the cheapest changer I could get quickly on eBay. Not wide enough for the chain so I tried to deform it to a more suitable shape. Eventually the outer side broke off. But what’s left will shift down, and the inner side is strong enough to shift up.
When you have the small ring on the inside, I suggest get something cheap and see what can be bodged up.

Lower level mechs seem to be made from softer steel that can be deformed with some impunity. I learned myself the hard way to avoid those shiny, but also brittle, top of the line ones, at least in the context of folders where quite a bit of tweaking is required. The cracked cage can still be repaired by soldering, if one has means for that.