Author Topic: Brompton gearing (Read 8993 times)

Zipperhead · « **on:** 27 February, 2015, 02:45:28 pm »

I previously had an SRAM equipped Brompton M6 (whose original frame I noticed was sitting in the corner at Phoenix cycles), and now I've got a SA BWR equipped M6.

Possibly with the SRAM hub, but definitely with the Sturmey hub I've always thought that the standard gearing was wrong (for me), 5th gear (low 3rd gear on the hub) was fine on the flat in still conditions, but the slightest headwind or rise and I'd have to change down which knocked off a lot of speed.

Looking for something the other day I came across a Brompton gear calculator so put in the details and got out the numbers. The top 3 rations are 64, 81 and 100 inches. Now teethgrinder of this parish may be happy to cruise around all day on 100 inches, but 81 inches on a very upright bike is a bit too much for me.

So I changed the numbers to look at the -12% option, 56, 71 and 88. 71 inches is about what my fixies have, so last night on went a new chainring, chain and sprockets. So much better, I can happily ride around in 5th, cope with inclines, regain lost speed all without changing down or making my knees hurt. 6th gear is doable in the right conditions on the flat. First is a bit low mind, even with a bag on the front and without trying the front wheel wasn't touching the road until about the 3rd pedal stroke - but I can live with that.

However, now I've got a quandary. While I had the wheel out I found a recently broken drive side spoke (they were all fine 10 days ago), so on the way to work I stopped off a Phoenix Cycles to get some spares, and given the price of single spokes a complete set made much more sense. Except now I've had another look at the wheel and the rim is worn - it's started to go concave by about 1mm.

So now I'm thinking that rather than taking the wheel out, sprockets off and putting one spoke in, it might be better if I went and picked up a new rim tomorrow and just rebuild the wheel. Choices, choices.

Kim · « **Reply #1 on:** 27 February, 2015, 03:15:55 pm »

Bromptons always seem very over-geared to me. Probably because they're designed for a) normal people who don't spin to ride in b) That London. I don't see the need for a 100" gear on a bike that's so slow.

I've gone for a gain ratio of 4.4^[1] in my cruising on the flat gear. That's gear 7 on the XRF8(w) with the 28T chainring, leaving me a jump up to 5.8 for the gentle downhills and tailwinds. There's a 39T ring to shift everything up a bit, mainly to make better use of the hub's closer ratio gears when there aren't any hills; I rarely use the resulting top gear of 8.1.

Bottom gear is therefore 1.8 (effectively about 24"), which is low enough for proper hills or really bad knee days.

I seem to spend a lot of time in 28T/3rd (gain of 2.6) and 28T/6th (gain of 3.9).

_{[1] 55.9 gear inches, but it's effectively higher than that because of the shorter cranks.}

Biggsy · « **Reply #2 on:** 27 February, 2015, 03:18:42 pm »

I'm glad I opted for the "Lowered" 6-speed gearing from the start.

Fab Foodie · « **Reply #3 on:** 27 February, 2015, 04:05:23 pm »

I shall be moving to the 44T chainring when funds allow (from 50).
Assume chain needs shortening and this can be done with a regular splitter?

Zipperhead · « **Reply #4 on:** 27 February, 2015, 04:08:04 pm »

Yes, it's a normal chain - 3/32's I think.

For a six speed 50T the chain is 100 links, for 44T it's 98.

LittleWheelsandBig · « **Reply #5 on:** 27 February, 2015, 04:46:52 pm »

I had the 44t ring on my T5 but use top (stock) gear on my M6L every day I ride it. Very useful on the local gentle downhills.

rogerzilla · « **Reply #6 on:** 27 February, 2015, 07:33:19 pm »

Assuming a nominal rolling diameter of 16.25", mine is geared at 48", 56" and 66" which is about right for flattish country. The 66" can be used on the flat if I'm in a hurry or chasing a bus.

LittleWheelsandBig · « **Reply #7 on:** 27 February, 2015, 07:37:37 pm »

Whereas I use 4th for cruising and 5th when I'm in a hurry. 66" is lower than my fixed gear.

resus1uk · « **Reply #8 on:** 28 February, 2015, 08:08:27 pm »

As an old rider, I found my M3 over geared, so changed to 44 chainwheel with a 13 sprocket.
That gives 42/56/74, fine for city traffic.

quixoticgeek · « **Reply #9 on:** 01 March, 2015, 06:19:32 pm »

Quote from: Kim on 27 February, 2015, 03:15:55 pm

Bromptons always seem very over-geared to me. Probably because they're designed for a) normal people who don't spin to ride in b) That London. I don't see the need for a 100" gear on a bike that's so slow.

I've gone for a gain ratio of 4.4^[1] in my cruising on the flat gear. That's gear 7 on the XRF8(w) with the 28T chainring, leaving me a jump up to 5.8 for the gentle downhills and tailwinds. There's a 39T ring to shift everything up a bit, mainly to make better use of the hub's closer ratio gears when there aren't any hills; I rarely use the resulting top gear of 8.1.

Bottom gear is therefore 1.8 (effectively about 24"), which is low enough for proper hills or really bad knee days.

I seem to spend a lot of time in 28T/3rd (gain of 2.6) and 28T/6th (gain of 3.9).

_{[1] 55.9 gear inches, but it's effectively higher than that because of the shorter cranks.}

Do you have photos of your Brompton's drive train? Am curious of the layout of it all. Am seriously considering upgrading to the 28T front ring on my XRF8(W).

Julia

Kim · « **Reply #10 on:** 01 March, 2015, 07:21:19 pm »

Quote from: quixoticgeek on 01 March, 2015, 06:19:32 pm

Do you have photos of your Brompton's drive train? Am curious of the layout of it all. Am seriously considering upgrading to the 28T front ring on my XRF8(W).

Have a look in this thread: https://yacf.co.uk/forum/index.php?topic=88470.msg1818821#msg1818821

sg37409 · « **Reply #11 on:** 07 March, 2015, 06:20:51 pm »

Just ordered one today. 3 speed, -12% - Can anyone tell me what these are in inches ? I had a wee shot out side the shop and they felt ok, but would like to know the size.

Kim · « **Reply #12 on:** 07 March, 2015, 07:12:44 pm »

38.4" 51.2" 68.3"

If http://xldev.co.uk/bgc.html is to be believed. Looks like a good set of ratios to me.

sg37409 · « **Reply #13 on:** 08 March, 2015, 11:16:40 pm »

Thank you for the link. I usually like a 66 so for a small wheeler thought a high 50 might be good.no idea really, looking forward to trying it.

1nterceptor · « **Reply #14 on:** 22 June, 2015, 05:01:09 pm »

My 2013 S6L-X came with the lowered gearing(44T) when I bought it.
But I'm used to spinning with my roadbike and still had a hard time on the hills in my area.
I lowered the gearing further by swapping the factory 44T chainring/wheel to a Shimano
Dura Ace 39T ring:

https://www.youtube.com/watch?v=5AT6wI_J2MI&list=UUHyRS8bRu6zPoymgKaIoDLA&index=13

Fab Foodie · « **Reply #15 on:** 24 June, 2015, 12:17:32 pm »

Having now run the 44T on my SL6 for a while, I can say in general it makes the bike more useable, however I really do miss the top end that the bigger gear gave as I'm more grinder than spinner. A double chainset mod for the future seems on the cards .....

PAC · « **Reply #16 on:** 11 July, 2015, 11:19:02 am »

That all sounds too complicated...My S2L if fine for pretty much everything I use it for

rogerzilla · « **Reply #17 on:** 12 July, 2015, 09:55:48 pm »

I initially changed the rear sprocket for a 14T instead of a 13T and that was about right with the standard 50T ring, giving roughly a 60" middle gear. You do need a new chain for this, or at least a spare couple of links, but it's way cheaper than changing the chainring. I'm running 52 x 15 now that I have a proper crankset with a spider (new Bromptons have also dumped the easily-sheared all-in-one RH chainset) which is 56" normal gear and (ahem) 48" and 66" low and top gears on the AM hub. It's perfect most of the time as all three gears get full use, but you have to stand up for real hills. I also had 54 x 15 for a while, which is almost identical to 50 x 14 but is heavier, a bit smoother and wears less. Swings and roundabouts.

On a normal bike I aim for a 60" most-used gear but the Brompton benefits from a slightly lower one as cruising speed is slightly less. A lot less on coarse tarmac!

Incidentally, a Stronglight XD double crank and a Shimano 113mm square taper BB give the correct chainline but you have to have the ring on the inside of the spider. There is no technical disadvantage to doing so but it looks slightly odd. You can't use a shorter BB as the cranks will hit the folded rear triangle; there is a fairly high minimum Q factor.

Kim · « **Reply #18 on:** 12 July, 2015, 10:30:43 pm »

Quote from: rogerzilla on 12 July, 2015, 09:55:48 pm

Incidentally, a Stronglight XD double crank and a Shimano 113mm square taper BB give the correct chainline but you have to have the ring on the inside of the spider. There is no technical disadvantage to doing so but it looks slightly odd.

You can reasonably stick a bash ring in the outer position to keep the oil off your leg^[1]. Or indeed a second chainring for foot and finger shifting if you're careful about the chain length (there's about 11t of capacity on the dérailleur version of the chain tensioner after allowing for the fold, not counting whatever's used by a second rear sprocket).

LEE · « **Reply #19 on:** 04 April, 2016, 10:22:24 pm »

My M3 is too low-geared for my ideal purposes, I spin out in top gear too quickly (I'm used to riding single speeds and grinding rather than spinning). Assuming it's mid 60"s I'd prefer 70" in top gear then the other 2 gears would fall into useful places as well.

A new front chainring/crank is expensive, what is the smallest freewheel I can get?

LittleWheelsandBig · « **Reply #20 on:** 04 April, 2016, 10:26:09 pm »

There was a rare 12t SA cog that tended to crack eventually. The 13t SA-compatible cog is reliable and easily available from Brompton. With a 3 sp hub and any reasonably sized chainring, that should give a decent top gear.

LEE · « **Reply #21 on:** 11 July, 2016, 10:59:12 am »

Managed to source a one-piece 50T Chain/crankset (a lucky find whilst browsing in Stonehenge Cycles at the weekend).

It now feels about perfect. 50T x 14T 3 Speed.

rogerzilla · « **Reply #22 on:** 11 July, 2016, 01:54:52 pm »

I could never understand why Brompton didn't offer 50 x 14 as a factory option. If you wanted slightly lower gearing you had to spring for a smaller chainring. OK, a smaller chainring is a lighter option (especially with a couple of links lost from the chain) but we're talking about tiny savings on a bike that will always be pretty lardy.

LEE · « **Reply #23 on:** 12 July, 2016, 10:18:44 am »

I managed a 29:56 over the Andover Wheelers 8.5 mile TT course last night (26.5km/Hr according to Strava).

That's not too bad on a Brommie into some headwinds with M-Bars. It's confirmed that the 3-spd gearing is about as good as it's possible to get for me.

I rode to and from the start. 20 miles in all. It was really enjoyable, sitting up, pootling along and taking in the scenery.

Fab Foodie · « **Reply #24 on:** 14 July, 2016, 07:45:12 pm »

After earlier posts where I started with the Std 50T S6L. I'd found it great until touring or in hillier regions I then swapped for the 44T. This was much better when load lugging or going up hills, but I missed the top gears as I'm not a very spinny rider. So for £60 I've gone for a Stronglight 50/34 Compact double which gives me back the lovely big ring back but with a granny geared system that is great for touring and lumpy terrain. OK it's manual shift but the 34 is more usable than I expected.
Didn't need to change the BB and took the opportunity for a new chain and rear sprockets. Fab. Got an S12L now