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.
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.)
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.
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.
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.