Sunday, 12 June 2016

Converting a bike from 7-speed freewheel to 8-speed freehub

Just done this on Hoppy, my BTwin Hoprider 300.
Pretty easy, really.
You need:
1) a wheel with a freehub designed for 8+ speeds (on 11-speed freehub you also need a spacer to put behind the cassette, otherwise there will be a gap between the cassette and the locknut, and the gears will move about!). I'm using the 8-speed freehub wheel I retrofitted to Mermaid (my previous bike, that I stripped for parts). You could just take the wheel apart and change the hub, then relace the wheel, but if you have to think about it, you'd be better buying a new wheel with a freehub already fitted!
2) an 8-speed shifter. The spacing is different for 7-speed and 8-speed (8-speed gears are slightly closer together). I'm using the 8-speed SRAM TwistGrip shifter I retrofitted to Mermaid
3) an 8-speed cassette to suit your freehub (modern Shimano and Modern SRAM use the same standards, but Campagnolo is a law unto itself. There are no 8-speed Campagnolo groupsets currently available, though, so really if you want to go Campy, go for 9+ gears). The SRAM PG-850 11-32 I retrofitted to Mermaid for me!
4) an 8-speed compatible derailleur. It may be your 7-speed one will work, once you adjust the cable tension and the high and low stops. It did for me, because Hoppy came with a Shimano Altus rear mech. Looks like the M310 model, which is listed as being 7 and 8 speed.
5) An 8-speed chain. Again, your existing chain may be good enough. 8-speed chains have flatter ends on the links than 6-speed chains (the flatter end are so that the chain clears the slightly closer together gears sprockets). But the plates are the same size. Gear teeth are also the same thickness. For me, the same chain did the job. If your chain has much wear on it, now would be a good time to replace it, but my chain has very few miles on it, so I am keeping on.
1) Replace the shifter.
2) Feed the cable through (look at where the "old cable" went to get the routing right.
3) Undo/release brakes (if required). You'll never get a fat tyre past the pads on a v-brake!
4) Swap the "old" back wheel for the "new" one, popping the chain over
5) Do the wheel up properly!
6) Check, and adjust the rear brake (I put it here, rather than at the end, so that you don't forget to do it!)
7) Tighten the cable onto the cable clamp on the derailleur.
8) Try, carefully, going through the gears. Do this on a stand or by lifting the rear wheel a bit - NOT while riding it. If switching from a 7-speed to an 8-speed, the chain is unlikely to fall off the "inside", it do it gently, just in case.
9) Adjust the chain tension. The derailleur didn't move out of 8th gear when I moved the twist grip to 6th gear, and didn't get to 1st gear at all. Using the tensioner (essentially taking a bit of slack out of the cable) fixed this. You could also adjust the cable clamp if it is a larger amount you want to correct. If the chain tensioner won't go far enough in or out for what you want, then it will definitely be a cable clamp adjustment on the derailleur.
10) Adjust the high and low stops as appropriate. For me the chain was making a noise in the highest gear (the smallest cog). Moving behind the bike made it easy to see that the derailleur was going a bit past the 8th gear. So I adjusted the "high" stop so that the derailleur was directly below the smallest cog when the shifter was set to 8th gear. Similarly, I had to move the low stop out a bit, because the chain was a bit reluctant to change into 1st gear. Again, from behind, I lined up the derailleur so that the chain path was directly below the first gear.
11) Repeat 8, 9, and 10 as required. You should only have to do step 9 once, but if you didn't make a good job of it you may have to repeat it.
12) Check everything is done up properly, and no cable end are flapping about. You should really cut them and fit a cable end, but I tend to just tap then to a convenient chainstay or such like, so that if I am tinkering about, I have the whole cable length, and a "good" cable end to work with.

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