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Replacing transmission – get more clicks

Dissatisfied with your sprockets? Suffering gear envy? Existing transmission shot to pieces? Now may be the time for an upgrade to n+1 speeds… We’ve covered replacing most of the bits of the transmission, so here we take a look at some of the issues you’re likely to encounter doing the full replacement monty.


This is the heart of the upgrade. The most obvious difference is the number of sprockets, but there’s a host of other harder-to-see differences that may be important depending on where you’re starting from.

The first is the overall width of the cassette. If you’re going from eight speed to nine speed there’s no problem – the two cassettes are the same width so you can simply remove the eight speed one and slide on the nine. Moving from seven to eight (or seven to nine) is more problematic. Eight and nine speed cassettes are both wider than seven, so you need a wider freehub body. Occasionally you’ll get lucky – if your bike is recent enough that bikes above it in the range had eight speed cassettes you may find that you’ve actually got an eight speed hub with a spacer behind the seven speed cassette. This is quite unusual, though.

A more likely scenario is that you’ll need to upgrade the hub too. In theory you can just replace the freehub body, fiddle with the spacers on the axle and redish the wheel, but that’s nearly as much of a faff as it sounds. It is the cheapest option, though. If you can build wheels, next cheapest is to get a new hub and build you old rim onto it. You ought to get new spokes if you’re doing this, though, and if the rim’s a bit worn it’s possibly not worth it. Which leaves you with the pricier option of building a whole new wheel or the priciest option of getting someone else to do it for you.

The other important differences between cassettes are connected to the overall width – the thickness of the sprockets themselves and the distance between them:

Speeds Spacing Thickness
Seven 5.0mm 1.9mm
Eight 4.8mm 1.8mm
Nine 4.3mm 1.8mm

Shimano nine speed sprockets are actually a whisker thinner than eight speed ones, but the main difference between eight and nine speed is the distance between them. Seven speed sprockets are both thicker and further apart. These differences are where the transmission upgrade domino effect begins to happen…

Cassette replacement: Walkthrough >>


As you can imagine, having the sprockets closer together means you need a narrower chain. Or at least, you may do. Chains are pretty much interchangeable between seven and eight speed, but you’ll need a narrower one for nine speed. If you’re using Shimano chains don’t forget to drop a couple of narrower joining pins in your trail pack for emergencies…

Chainrings and front mech

The concept of nine speed chainrings may seem like a bizarre one, but all it means is that the chainrings are narrower to work with the narrower chain. This is one of those areas where you might get away with not changing anything. Plenty of people run nine speed chains on eight speed rings or vice versa without any problems. But then again, plenty of people have nightmares with it. You can’t go wrong with having the right ones, though.

The front mech has a narrower cage to go with the narrower chain. Again, you might get away with it but the shifting is unlikely to be spot on. This is an area where Gripshift scores, though – if you’ve got a front Gripshift with lots of clicks you can tune the front mech to avoid rubbing, something that’s impossible with a RapidFire unit that only has three clicks.

Setting up your front mech: Walkthrough >>

Rear mech

Clearly if your sprockets are closer together, your mech needs to move less far per click to shift from one to the next. The good news is that all (well, nearly all – Dura-Ace is different) Shimano mechs move the same amount for a given amount of cable pull (the ‘actuation ratio’). So the bit that makes them move less far is the shifter, not the mech. So far so good, but you might still need to change it. Why? Well, nine speed cassettes are available in some very wide ranges up to 11-34, so Shimano tweaked the slant angle of the mech so that the top jockey wheel drops further as you shift to lower gears to stop it colliding with the bigger sprockets. If you’re sticking to cassettes with 28 or 30 tooth bottom sprockets you’ll be OK. Except that the jockey wheels in nine speed mechs are narrower to work with narrower chains. You might get lucky and have it work fine, or you may have to change the jockey wheels.

SRAM’s Shimano-compatible mech range have the same actuation ration (otherwise they wouldn’t be compatible), but their ESP stuff uses a different ratio to make the system less susceptible to cable drag. The upshot of this is that you have to use ESP mechs and shifters together.

Setting up your rear mech: Walkthrough >>


More gears = more clicks. The shifters are the bit that make the mechs move different amounts to cater for different sprocket spacing, so you’re definitely not going to get away without replacing these. Having said that, the difference between seven and eight speed spacing is sufficiently slight that lots of riders happily use old seven speed thumbshifters to shift across eight speed cassettes (the end stop on the shifter acts as an ‘extra click’). The small differences are cumulative, so the trick is to set the indexing in the middle of the cassette so they don’t add up too much by the time you get to the ends. That said, we’ve never quite been able to make it work to our satisfaction.

Replacing your shifters: Walkthrough >>


You’ll have noticed that you’re potentially into a lot of new stuff here. While all sorts of combinations of eight and nine speed stuff have been known to work, it’s far from guaranteed. Just because someone else has managed to get something to work acceptably doesn’t mean it’ll work for you, and that’s before you consider the subjective nature of the term ‘work acceptably’. The BM threshold of acceptable is pretty high, and we’ve never really been satisfied without having all the bits properly matched.

If nearly every transmission component on your bike is hammered then there’s no very good reason not to replace them all with nine speed gear. If it’s just the chain and cassette that are a bit weary you might as well just replace those…


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