Part of renovating my Brompton involved replacing the worn-out rim on the rear wheel. This is an opportunity to revisit the whole task of wheel-building. If you are looking to convert a Brompton to an 8-speed hub, this is a necessary first step.
You’ll need your hub (in my case a Sturmey Archer x-rf8), a rim, and spokes. I am using a 28-hole Sun-Rims rim. I have used cheaper rims in the past, but two wore out in as many years, so I’m hoping the Sun-rims version will be more robust.
Choose a rim with the same number of spoke-holes as there are holes drilled in the two flanges of your rim. A hub with 14 holes in each flange, will need a 28-hole rim. Sounds obvious, but easy to overlook! Rims sometimes come with a narrow air valve hole, and to fit a Schraeder inner-tube valve, you will need to drill the hole larger with an 8mm bit (plus a shade wider for wiggle-room).
The best way to gauge your spoke length is to use the Sapim Spoke Calculator. To use this you will need to measure your rim’s and hub’s dimensions very carefully, and enter them in to the calculator. You will also need to decide on the pattern of your spokes. In this example, I am crossing the spokes once on the drive side (the side with the sprocket), and not at all on the other side. The thinking here is that the drive side needs the torsional strength offered by angled spokes; the non-drive side does not.
Various wheel builders will be able to supply you with spokes cut to the required length. On the 16″ Brompton wheels, the spokes are so short, especially on a chunky hub, that you are unlikely to find the right size off-the-shelf. Many professional wheel builders will have invested in the magical Phil Wood Spoke Cutter, which is able to cut and thread a spoke in a single manual action. I myself source short spokes from YourSpokes in York.
OK, so time to get going on the wheel build. Place the hub and the rim on your knees with the drive side of the hub facing up. Drop a spoke down through one of the hub’s holes, and insert the other end through a rim hole to the right of the rim’s valve hole (see picture). Secure it with a spoke nipple. Screw the nipple on just enough to hold the spoke securely, and remember how may twists you gave it with your fingers. Working clockwise round the hub, skip one hole and drop a spoke into the next hub hole. On the rim, count four holes to the right of the first spoke, and insert the spoke into the fourth hole. There should now be three empty holes in the rim between the two spokes you have inserted. (See top picture.) Secure your second spoke with a nipple the same way as before, taking care to tighten it just the same amount.
When you have inserted spokes all the way round the wheel, it should look something like this.
Keeping the wheel the same way up, bring the next spoke upwards through the hub hole to the left of the hole where you started. Cross this spoke over your first spoke (and no other spokes), and insert it in the middle hole of the three vacant rim holes opposite. (See picture below.) Secure with nipple as before.
Continue clockwise round the rim, repeating this action, until all the hub holes on the upper side are used. Remember to always insert these crossed spokes from below, and ensure no two adjacent rim holes are occupied.
When all the crossed spokes have been inserted, the wheel should look like this picture to the left.
Now turn over the wheel, and drop the remaining spokes into the holes on the other side of the hub. Each spoke should be inserted into the rim hole opposite, and secured in the same way as before.
When you have inserted all the spokes, the wheel should look like the picture on the left.
The wheel is now assembled. Well done!
The next stage is equally important to get right, and involves tightening the spokes in such a way that the wheel is true.
We’re all familiar with the sight of a wobbly rim. This is one dimension in which we want the wheel to run true. There are two other dimensions to consider as well: the extent to which the hub is located in the dead centre of the rim; and whether the rim floats over the centre of the axle when the wheel on the bike is viewed from above (a dimension also known as ‘dishing’.) The centre of the axle is not the mid-point between the hub’s flanges, but the mid-point between the two locknuts (a distance also known as the over-lock-nut dimension, OLD for short.) If your spokes are the right length to begin with, on a wheel this size much of this balancing will work itself out.
Begin to gently tighten the spokes by working your way round the wheel, starting at the valve hole. Add a couple of twists each time with a screwdriver. Keep working round the rim, resisting the temptation to tighten any spoke too much at once. Your aim is to get each spoke to the place where the nipple is only just biting.
Once you reach this point, it’s time to check whether the hub axle is dead-centre in the rim. Measure accurately round the rim the distance from the hub flange to the rim: if one side is shorter, loosen an array of spokes on this side, and take up the slack on the other. This is a process of trial and error, but it should be possible to get the margin of error down to less than one millimetre.
The second adjustment is to do with the ‘dishing’. Your spoke length should help determine this; but it is worth checking visually that the rim is sitting half-way between the two lock-nuts. This is not an easy dimension to measure – if there is anything amiss it should show up when the wheel is put on the bike. The centre of the tyre tread should line up with the vertical frame tube (although beware – the section of the Brompton that contains the bottom-bracket is off-centre, and this can confuse the eye.) To adjust the dishing, loosen all the spokes on one side by a half-turn, and tighten all spokes on the other by the same amount. This will move the rim to the right or left of the hub. Repeat if necessary.
The third adjustment is to cut out any rim wobble. I usually do this after I have tightened up all the spokes. Begin by going round all the spokes, giving the nipples a half-turn. Continue this until all nipples are offering significant resistance to a screwdriver. It is generally easier to under-tension a wheel than the opposite. With a spoke key, give all the nipples a further half-turn. At this stage, it can be useful to go round the spokes, plucking each one, and listening for the pitch. Often, many spokes are at a very similar pitch – pick out any ones that are obviously higher and lower, and bring them in line with the pitch (spoke tension) of the others.
Now fit the wheel on the bike with the bike upturned. I use the brake-blocks as a guide (you can buy special equipment for this bit, but it’s not necessary.) Spin the wheel, and watch the gap between the rim and the block. If the rim wobbles, decide on which is the most common distance between the rim and the block. Start with any section of the rim that rubs on the block, and work with the spokes along this section. Say the rim rubs for a section eight spokes long: loosen the four spokes on the side of the rim that is rubbing, and tighten the four on the other side of the rim. This will pull the rim away from the block. However, when you loosen the four, make a smaller adjustment for the end two (say, a quarter-turn), compared to the middle two (say, a half-turn). Think of it that you are adjusting a bow shape. If this first adjustment does not stop the rub, repeat the process, until you achieve the rim-block distance you decided was most common to the wheel as a whole. Likewise, if the wheel bows away from the block, make the opposite adjustment. By this method, I have found that I can set a rim to spin with no visible wobble at all – say, maintaining a constant 1mm gap between rim and block. Very satisfying!
If you are fitting the Sturmey hub (x-rf8, short axle version) pictured here to a Brompton, be aware that the over-lock-nut dimension (OLD) of the hub is 120mm. However, the space between the rear dropouts of a Brompton is just 112mm. So how does it fit? The Sturmey hub comes with a double locknut on one side – one of these can safely be removed, which brings the OLD down to 116mm. With a little bit of flexing by hand, the rear dropouts can be opened by 4mm – just enough to allow the hub in. If you want to fit another hub, such as the Shimano Alfine 8, you’ll need to permanently widen the rear forks. There are pros and cons of fitting either 8-speed hub – I’ll cover this in another post.