16 speed Brompton: Part 2

My SRAM gears have been having problems recently when back pedalling. The chain would go slack and then suddenly tighten up, or pull the chain off the chainwheel. It just didn’t feel right so I dismantled my Brompton and on checking the SRAM hub’s bearings I discovered I’d overtightened them. I loosened the bearings off and all was well again. However once the rear wheel was off I decided that I’d had enough of the Brompton 6 speed and today, right now was the time to start my Nexus 8 conversion.

I’d printed off this page from the AtoB site-link long since dead which gives reasonably clear instructions for modifying the Nexus hub and spreading the forks. I’d bought the bits to make the spreading jig already so it was just a case of fitting the bits together and bolting to the Brompton rear fork dropouts.

Brompton rear fork spreading jig.

My home made Brompton fork splayer bolted to the Brompton dropouts

Brompton rear fork splayer

Another view of the fork spreading tool

Then with just a little trepidation and with Damae standing by for moral support I started turning the inner pair of nuts. Slowly but surely the rear of the Brompton forks widened and at approximately 136 mm (under tension) I decided to check the width against the modified Nexus hub. I removed to the tool and the forks sprung back to around 120 mm width at the dropouts. The hub didn’t quite fit but I realised that I’d not filed down the outer lock nut on the left hand side yet as the instructions suggested. I took the hub into the garage and removed the lock nut and filed it down and re-fitted the locknut. A little to our surprise the Nexus then fitted in perfectly and I hadn’t managed to break the forks. It was very easy to do. (NOTE: I ended up using the tool again to open out the forks a little more. When I fitted the dropout washers it was quite hard to get the hub in and out. This time I wound the tool out such that the gap (under tension) was 138mm. The Nexus 8 fitted more easily after I did that.)

Nexus 8 in a Brompton rear fork

Nexus 8 in a Brompton

Nexus 8 hub gear in a Brompton

Nexus 8 in a Brompton. Note the absence of the black dust cover. This was replaced later after being modified a little.

Widening the forks had been my biggest worry. If the forks had broken then I’d not only have lost the use of my Brompton, but I’d would have needed to shell out for a new rear fork and then fit them. Now the forks had been widened I turned my attention to modifying the dust cover. This is made of two components and large plastic cover and a smaller metal cup that fits on the outside of the plastic cover. The central sleeve of the black plastic cover needed to be filed down by about a millimetre but the metal cap needed some more drastic surgery.

I fitted the cap to part of the jig and clamped the assembly in the vice. Taking an angle grinder with a thin cutting disc I carefully cut off the shoulder of the metal cap and dressed the cut edges. When I fitted the cap and dust cover back on the hub still just fitted in the forks. Although people do run Nexus hubs without this dust cover I didn’t want to do that as I’m aiming for maximum reliability.

Modifying the dust cover end cap

Modifying the dust cover end cap

I had to try to work out the offset of the rim and hub so I could work out what size spokes I’d need to order. With Damae’s help we measured the clearance between her Brompton rear wheel rim and the non-drive side of the rear forks. This turned out to be 13mm. We then placed the rim round the hub and bolted the hub back into the Brompton forks. With some careful measuring and re-measuring I came up with some figures that I would be able to put into an online spoke calculator. I’ll put the correct figures in the blog when I know what they are ie after building the wheel 🙂

I found that SJS Cycles do some short spokes that should be suitable. This means that it will take at least a few days for the spokes to get here. In the mean time I want to be able to use my Brompton. So I had to find a way to get my SRAM hub back in my Brompton forks. To do this I had to put some spacer washers on each side of the hub. This seemed to work fine and the wheel went back into the forks with only one real problem. As the forks have widened the two speed changer is now too far out to be able to pull the chain onto the larger inner cog. So my Brompton is now a three speed Brompton. Another minor issue is that the SRAM gear change cable and the chain tensioner now foul the front wheel when the bike is folded. This should not be a problem with the Nexus hub in place as the gear changing mechanism is on the inside of the forks and I will be using a different chain tensioner.

SRAM 6 speed in a widened Brompton fork.

The SRAM 6 speed in the widened rear fork. You can just see the spacer washers on the left hand side of the axle.

As luck would have it SJS Cycles had a 50/34 double chainring with steel rings at a very reasonable price. I was getting a bit fed up with the cost of the Brompton chainrings each time I needed to replace them. When I fitted this double chainring a few days ago I noticed that it felt considerably stiffer than the standard Brompton chainring. On accelerating it felt like the power was going straight to the rear wheels. I realised that there must be a lot of flex in the standard chainrings and cranks. I’m sure that any theoretical efficiency losses of the Nexus 8 over the SRAM three speed will be cancelled out by using a stiffer crank and chainrings.

Cheapie 50/34 crank on a Brompton

50/34 spider and chainrings on a Brompton

Modified dust cover on a Nexus 8 speed hub

Here is the narrowed Nexus 8 speed with modified dust cover ready to go into my Brompton

8 Responses to “16 speed Brompton: Part 2”

  1. Julian Says:

    Hi,
    I have a simple problem of replacing a spoke on the right hand drive side of a Brompton rear wheel with two cogs. My question is how do I get the cogs off in order to thread through a new spoke?
    Thanks

  2. syklist Says:

    Replied by email a few days ago. I hope your wheel is now as good as new.

  3. Robert Lawson Says:

    I just got a Brompton and I have started preparing it for my red line nexus 8 speed hub. Fork spreading seems fairly straight forward. However, spoke length calculation is difficult. What length spokes did you use?

  4. syklist Says:

    Hi Robert,

    That’s a “How many balls of string would you need to connect the Earth to the Moon?” sort of question 😉

    The answer is, it depends on the dimensions of your Nexus 8, (have narrowed it for instance?) and it also depends on which rim you are planning on using. So I am a bit wary of giving you the spoke lengths I used in case your Nexus hub is a little different to mine and you are using a different rim. So use this information with care, see my earlier post http://mistymornings.net/blogs/mm/?p=119 for spoke lengths.

    I used the Edd spoke length calculator to find the correct spoke lengths. It is worth playing around with it to see what figures you come up with. If you do try using Edd to calculate your own spoke lengths yourself I would suggest that you note down which cross pattern you have chosen when using Edd. It makes the build a little easier to start if you don’t forget that important detail.

    Edit : corrected link to Edd

  5. Robert Lawson Says:

    Thanks syklist for all this information. I am able to do all this in part of an afternoon using your posts. It would have taken untold hours if you had not shown exactly how to do it.

    I narrowed my hub down to 124mm yesterday and went into the shop to get a lock nut that would fit inside the dust cap and then get some custom length spokes made. The mechanic there was excellent. She was entertained by my project and wanted to help. First she said she absolutely would not make spokes until she actually had the rim in hand to measure it- ERD 349 was not enough for her. Since this was not a normal project, she also wanted to see the frame too. I mentioned my problem with custom spokes on a modified hub like this that is dished was having to take half the spokes off that are a little too long and grind them down a little bit. We talked some more and she said if I paid her $60 plus the price of the spokes to build the wheel, then it would all be her problem. I think I’m in love. Her coworker told me her favorite beer flavor and I walked down and bought her a 6 pack of Sierra Nevada IPA.

  6. syklist Says:

    Let me know how it works out with your Brompton.

  7. Robert Lawson Says:

    I am assembling everything now and I am having 3 problems with the chain tensioner.

    1- The tensioner frame hits the lock ring that holds the small part kit shifting mechanism together before it seats on the drop out. I have an older used Brompton and my dropouts look different from yours.

    2 the 18 tooth cog on the nexus is too large a diameter and it interferes with the tensioner pulley. I see you used a 16 tooth and it worked- this is solved.

    3 the chainline is off with the nexus cog about 1/2 chain width closer to the midline of the bike than the chain ring- this is easy to fix with washers between the pulleys and the tensioner and longer screws to move the pulleys in. I’ll probably get a cheapo right crank arm with a spider and mess around with it to get the chain ring right.

  8. syklist Says:

    1) I used a 2 speed tensioner as our bikes were six speeds. Although I believe that Junik HPV recommend using a 3 speed tensioner instead of a six speed so it shouldn’t make a difference. The advantage of the six speed is that it is more tolerant of chain line problems, the pulleys being able to slide in and out as needed.
    2) I used a 16t sprocket as it gave me a suitable range with the 50/34t double at the front. Not for any other reason, although I later discovered that a 16t sprocket was the biggest one that cleared the rear forks on our bikes.
    3) I seem to remember fitting longer bolts to our tensioners to move the pulleys in. You could try swapping the sprocket round to correct the chain line as well as the standard sprockets are dished. Or, buy a non-dished sprocket, Sturmey Archer 3 speed ones fit as well. SJS Cycles in the UK have a big range of such sprockets. As you might have read I bought the cheapest generic two ring chain sets with steel rings that I could find.

    Things like the chain line have to be fettled after the forks have been spread and the hub narrowed. There is no guarantee as to which side moves most when you spread forks in this way. I noted small differences in clearance between our two Bromptons.

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