Monthly Archives: May 2010

16 speed Brompton: Part 2

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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.

16 speed Brompton: Part 1

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Bromptons are beautifully styled bikes with an amazingly simple and compact fold and generally enjoyable cycling characteristics. A Brompton makes me think of the original Austin Mini. Both were designed around a brilliant idea and set new standards in their respective markets. Both are examples of the tradition of inspired British engineering. However, outstanding packaging apart, the Brompton like the old Mini, suffers from a lack of gears and some truly awful detail design, particularly where gears and brakes are concerned. Here in the Netherlands where the terrain is mostly flat you can live with three or six gears, and good brakes are not that important, except if you load your Brompton up with camping gear and go touring.

Since the arrival of our Rohloff Speedhub equipped VSF T400 bikes with Magura rim brakes, our six speed Bromptons have taken a large step backwards in the perfection stakes. Whilst the Brompton brakes work they do so eventually and with considerably higher brake lever pressure than even cheap V-brakes. The six speed gears consisting of an SRAM three speed hub and the Brompton 2 speed derailleur changer have become an annoying distraction rather than just mildly irritating. The two speed changer is the least appealing feature of our Bromptons. The changers which seem to have a mind of their own, are very sensitive to chain wear and grit. They can work for weeks on end without problems and then decide not to shift at inopportune moments. Not what you want whilst racing through town on your way to work.

Our life plans now involve a move to Norway, a country not known for its flatness. We’ve cycled through lots of it and know that, if we are ever going to our Bromptons again, that they are going to need a gear range from 18-80 gear inches. So is it time to give up on our Bromptons and get used to using our 24 speed Dahon folders? Well after customising the Bromptons so that we can carry lots of luggage and fitting more adjustable handlebars we’re not going to give up just yet. But fitting any hub gears other than the Rohloff will not give the range we need and the only 8 speed hub that will fit the Brompton rear fork without modifying it is the Sturmey Archer 8 speed.

A while back I ordered a Kinetics Sturmey Archer 8 speed kit, well more than a year ago actually. At the time of writing the order had not been fulfilled so I decided to cancel it. However this is probably a good thing, as in the intervening period I have discovered that even the current SA 8 speed hubs still have significant reliability problems. I’ve been looking for a more reliable solution but after a chat with Steve Parry about putting Rohloffs in Bromptons we’ve decided that our Bromptons aren’t worth throwing that much money at. In the meantime I stumbled across this page on the AtoB website detailing how to squeeze a Nexus 8 speed hub into a Brompton.

Now whilst the Nexus 8 speed has had some reliability issues in the past, the newer Premium models have better seals on them. This has apparently improved reliability according to information I have managed to glean from internet forum postings. Two other things are in the favour of the Nexus 8 speed are 1) availability and 2) price. A Premium Nexus 8 hub can be picked up from about Eur 115. So should the hub go pop after two years replacing it with a new one doesn’t really break the bank. Eur 115 every two years compares quite favourably to the cost of changing the cogs, chain and chainwheel on a 6 speed Brompton in the same period.

So is this the way to go?

Well I think it might be. A simple 8 speed will not give us the gear range we will need. However, a Nexus 8 speed when combined with a double chainwheel would. See the chart below (thanks to The HPV Drivetrain Analyzer). Using a 16 tooth sprocket and a 34/50 double chainring give an 18-80 gear inch range without exceeding the Shimano recommended minimum sprocket/chainring ratio.

After a considerable period of deliberation I’ve decided that this is the best method to get the range of gears we want. So I have started a new Brompton project, the 16 speed Brompton. I ordered the Nexus 8 speed hub, from Roseversand and a double chainring and 36 spoke 349 rim from SJS Cycles a few weeks ago. All I have to do is order some spokes when I know the relative position of hub and rims in the Brompton rear fork.

This is the most expensive Brompton modification I have made to date, and will also include my first wheel build. At the time I writing I have take the first steps of modifying the Nexus hub and spreading my Brompton rear forks.

Watch this space for some scintillating updates.

Avoiding the Door Zone

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Avoiding the Door Zone

This has been reasonably well publicised on forums but is a good introduction to the problems of the ‘Door Zone’ for cyclists. This is the area of the road where the doors of parked cars will open into.

Although cars and hence car doors are generally smaller here in Europe, so are roads and cycle tracks. Many cycle paths here in Utrecht consist of a differently coloured strip of tarmac on the side of the road, between where cars drive and cars park. Drivers assume that you should stay on this narrow strip at the side of the road but it is actually a dangerous place to be as a cyclist. Whilst a car pulling out on you is not a good thing, and can throw you onto the ground in the path of oncoming traffic, being ‘doored’ is rarely a trivial event and in some respects more dangerous.

I’ve only ever been ‘doored’ once a very long time ago and luckily I was cycling into a stiff headwind at the time on a quiet residential street. Thus when I hit the car door that someone had carelessly opened, I wasn’t going so fast and there was no traffic coming in either direction. Other cyclists are not so lucky.

So car drivers, before you open your door in haste, remember to take a good look behind you. It might be your son, daughter, grandchild or grandparent on that bike you didn’t see.
http://www.youtube.com/watch?v=1TQ7aID1jHs

Thanks to YouTube user Wpreston3 for making this material available to us all.