New 5 speed hub for 90s SWB Brompton

[jhob]

So my wife's brompton currently has a sturmey archer 5 speed that dates from the 90s.  Advice I have had has been to get a new hub (& wheel) as these hubs had design issues anyway and the hub is 21 years old.

As standard brompton now do a 3 speed or a 6 speed, our other option is to get a new 5 speed hub and build a wheel around it.

My wife says she only uses, from lowest to highest, gears 1 to 4 which from Sheldon Brown's site indicates that the hub has percentage ratios of:

.667, .789, 1, 1.267, 1.5

Those indicated in bold are the range that we would be looking for from the new gearing, the current chainwheel and sprocket combination being 50-13.

The current favoured option on cost/simplicity grounds is to go for a 3 speed wheel but what I am trying to work out is how/if we can replicate, as close as possible, the gearing.

The brompton site shows a gear ratio table with the ratios in gear inches and I am not sure how to translate that between the percentage ratios given for the hub, so that I can then work out how they translate and from that what chainring/sprocket changes might need to be made to match the gearing.

Can anyone help?

[mrcharly-YHT]

(chainring/sprocket) x gear ratio x wheel diameter = gear inches

[Biggsy]

Before we begin the maths, I think we need to know the sprocket and chainring sizes of both Mrs J's bike and the relevant current Brompton's.

[jhob]

Before we begin the maths, I think we need to know the sprocket and chainring sizes of both Mrs J's bike and the relevant current Brompton's.

the current chainwheel and sprocket combination being 50-13.

I've managed to work it out now with the help of http://www.bikefix.co.uk/hub-gear-calculator

Our options
3 speed (easy option), 14t - slightly less range than before, but livable
5 speed (build new wheel, more faff, expense etc), 13t - greater range than before

All options with ratios can be viewed here.

I think in an ideal world I would go for a ready-built 5 speed but that doesn't exist so at the moment I think it's likely that a conversion to 3 speed is on the cards.

[ACyclingRooster]

So my wife's brompton currently has a sturmey archer 5 speed that dates from the 90s.  Advice I have had has been to get a new hub (& wheel) as these hubs had design issues anyway and the hub is 21 years old.

As standard brompton now do a 3 speed or a 6 speed, our other option is to get a new 5 speed hub and build a wheel around it.

My wife says she only uses, from lowest to highest, gears 1 to 4 which from Sheldon Brown's site indicates that the hub has percentage ratios of:

.667, .789, 1, 1.267, 1.5

Those indicated in bold are the range that we would be looking for from the new gearing, the current chainwheel and sprocket combination being 50-13.

The current favoured option on cost/simplicity grounds is to go for a 3 speed wheel but what I am trying to work out is how/if we can replicate, as close as possible, the gearing.

The brompton site shows a gear ratio table with the ratios in gear inches and I am not sure how to translate that between the percentage ratios given for the hub, so that I can then work out how they translate and from that what chainring/sprocket changes might need to be made to match the gearing.

Can anyone help?

Hi jhob. I have used this site for many,many,many years and have found it absolutely invaluable :-  http://www.jbarrm.com/cycal/cycal.html but I do not have a Brompton,I don't know whether it would make a difference.

[jhob]

OK, I think the preferred option is to go for the Sturmey Archer S-RF5 5 speed Hub which I will build a wheel around. Also going to change the sprocket to 14t as she would like a slightly lower gear (we live on a big hill!) and would never use the new top gear on a 13t anyway.

So my next question is:

Has anyone built a brompton wheel using the SA S-RF5 before, and if so is it a straight swap?  Would the 175mm axle need cutting down?

I've measured the current OLD and that matches up perfectly so I'm optimistic this should be relatively straight-forward but also have the feeling that it will not, because these things never are.

[LittleWheelsandBig]

The Brompton has a non-standard narrow rear OLD. Take the skinny LH lockring off your old hub to replace on the new hub and trim the axle when you finish building the wheel. The steel is quite hard.

Edit - I see that your link leads to the Brompton narrow version for the 28 hole version.

[chagzuki]

This 5 speed has different ratios to the old version that brompton used and the gears are spaced far apart in a way that I'm not keen on. I like the old close-ratio version a lot.

I built an older version of the S-RF5 into a wheel for my Brompton and I could only get the 175mm axle length that SJS stock. So I had to take a risk and shorten the non drive-side end of the axle: a risk because a certain amount (I couldn't find a diagram) is hollow for the ball-locking mechanism. I took off around 8mm and it worked OK. Ideally I'd have taken off a little more but I figured I'd play safe.

[jhob]

This 5 speed has different ratios to the old version that brompton used and the gears are spaced far apart in a way that I'm not keen on. I like the old close-ratio version a lot.

I built an older version of the S-RF5 into a wheel for my Brompton and I could only get the 175mm axle length that SJS stock. So I had to take a risk and shorten the non drive-side end of the axle: a risk because a certain amount (I couldn't find a diagram) is hollow for the ball-locking mechanism. I took off around 8mm and it worked OK. Ideally I'd have taken off a little more but I figured I'd play safe.

I agree, the slightly narrower ratio, for commuting purposes, would be preferable. 

That's great to hear the axle will work ok with a little bit of sawing.  Did a hacksaw get through it ok?

[chagzuki]

I used a cheap dremel which took ages, but allowed me to round the edges nicely. A angle grinder would be better. Like I say, I don't know how much axle you can safely remove.

[jhob]

Well I've ordered the bits from SJS so we'll see what happens next week once I've built up the wheel!

I'm hoping that I won't have to get the dremel or hacksaw out...

[jhob]

Built up the wheel just fine.  Seemed a little odd that the standard brompton spokes were all the same length despite the dishing but the wheel came together no problem.

Putting the wheel back on the bike was ok too, just took quite a long time with much swearing at circlips, grazed knuckles and getting the schwalbe marathon tyre off the rim.  I had to use a bit of a blend of old and new parts to get it work properly.  e.g. old hub dustcap and chain guard, old gear cable clamp and hub chain roller guide. 

Only major issue I had was that even though the overall axle length is the same on the new hub it is distributed evenly on both sides whereas on the old hub it had more length on the drive side for double nuts, chain tensioner etc that needed to go on too. 

As it was I had just enough thread left to get a few turns on the second nut that keeps the chain tensioner assembly in place.  It's not ideal but as it is not subject to any high stresses I can't see it being a problem. 

I'm not entirely happy with this and there is a washer that I think I could safely remove from the axle to get another couple of turns on there so I will probably do that.  I might also put a bit of threadlock on there too.

Also I couldn't use the slightly larger 14t sprocket that we had planned to use to lower the gearing a touch as it fouled the first pulley on the tensioner.  That was no biggie and we reverted to whatever smaller sprocket it did have on (I didn't count the teeth).

[SA_SA_SA_SA]

http://www.atob.org.uk/technical/brompton-5-speed-upgrade/

The Brompton used thinner locknuts etc which can be salvaged from the old hub as described in above article.

[jhob]

http://www.atob.org.uk/technical/brompton-5-speed-upgrade/

The Brompton used thinner locknuts etc which can be salvaged from the old hub as described in above article.

Already using the old locknuts unfortunately    Thanks for the suggestion though.

I'm going to omit the last washer that goes on before the final locknut so that I can get a couple of extra turns on there:

[rogerzilla]

I find that a normal SA axle (5 3/4") on a traditional hub like an AW, AM or even (if you like tinkering) FW fits a Brompton just fine.    With non-Brompton-specific modern SA hubs like the SRF3, there can be rather a lot of excess axle sticking out of the left-hand side, but that's not an issue since the Brompton LH axle nut is open-ended (most traditional SA bikes have a closed-ended nut, unless they have a two-piece indicator spindle).

I think the big washer before the chain tensioner retaining nut is quite important for centring the tensioner as well as stopping the nut easting into the plastic, so I wouldn't leave it out.

What I think you need to do is to check the OLD spacing of your new hub vs the frame.  Bromptons are very narrow, about 112mm, although they probably vary a little from bike to bike.  The hub as shipped may be a little wide (they have no idea what bike it's going to end up fitted to) and therefore needs spacers removing, if there are any, from between the RH cone locknut and the cone.  This entails resetting the hub internal clearance (done with the RH cone) and the bearing adjustment (done with the LH cone) afterwards, and you'll have to redish the wheel unless you removed the same amount from the LH side.  It's all on Sheldon.

For the record, the spacing of a 5 3/4" axle SA hub can be varied from about 110mm up to 118mm (if you're careful, with pressed steel dropouts).  The 6 1/4" ones are good for over 120mm.  I have a stock of 1mm axle washers which are useful for fine tuning to match a particular frame.