ninnyCTSV

I have a trek 7.0fx with a 7 speed drive train. I got my hands on an ef51 8speed shifter and was wondering if it was as simple as switching out the rear gears to 8 speed.

Current set up is ef51 7speed shifter. Kmc z51 chain. Sunrace 14/34 7speed freewheel. Acera rd390 derailleur.

Also I got an alívio 430l crankset. It states it's 9speed, but will it work with my chain?

Homebrew01

I have heard there are freewheels with more than 7 gears, but your axle may not be long enough. Can you remove the rear wheel and take some pictures of the cogs ? It may speed up getting good answers.

HillRider

There are indeed 8-speed freewheels but they have a tendency to break axles.

How easy this change will be depends on the rear spacing of your hub and frame (126 mm, 130 mm or 135 mm) If it's 130 or 135 mm it's as simple as substituting the wider freewheel, respacing the hub, recentering the axle and redishing the rim. If it's 126 you need a new rear wheel and might as well buy a cassette hub while you are at it. Also you will have to force a 130 mm or 135 mm hub into the dropouts and, since the frame is aluminum, you can't cold set (i.e. permanently bend) it to the wider spacing.

ninnyCTSV

I can see about getting the measurements, but what do you mean by redishing the wheel? And how do they break axles? You have me wondering if it's worth the effort as I'm only doing it because I can get the parts free, and I wish I had an extra gear to speed up my commute

FastJake

An extra gear will not speed up your commute. What ratio are you missing? Get a different freewheel with that ratio. More speeds /=/ faster.

Since you have a freewheel you'll need an 8-speed cassette AND a new rear wheel to make this change. Is it worth it for one extra gear? That's for you to decide. 8-speed freewheels do exist but they're a terrible idea and never caught on because they'll bend or break rear axles if you look at them wrong.

RoadGuy

The wheel hub is offset to the left to make room as the freewheel or cassette gains additional gears. This offset is called the "dish". An 8-speed freewheel or cassette is wider than a 7-speed freewheel or cassette, so the wheel hub needs to be located farther to the left (in relationship to the wheel rim, to center the wheel rim in the dropouts and to provide enough room for the gears (freewheel or cassette) between the wheel hub and the right chainstay.

Are you sure that your rear wheel has a freewheel and not a cassette? As freewheels gain gears (from 5 to 6 to 7 to eight), and the rear dropout width increases, the length of the axle that is unsupported (because of the design of the freewheel hub) grows, and as a result, 8-speed freewheel hubs break axles more often than 7-speed freewheel hubs, which break axles more often than 6-speed freewheel hubs, etc.... Freehubs (which use cassettes) support the axle with bearing located farther out on the axles, and are less prone to break axles. Which is why most bikes with more speeds have cassettes and freehubs, and not freewheels.

Regardless of whether your bike has a freewheel or freehub and cassette, since your bike is a hybrid, most likely the rear dropout spacing in 135mm. You should drop the rear wheel out, and measure the space between the inside edges of the rear dropouts to confirm the rear dropout spacing. This becomes important if you decide to take the easiest way to convert your bike to 8-speeds, which is to get a 135mm spaced rear wheel set up for 8,9,10 speed cassettes (8,9,10-speed cassettes all use the same rear wheel). Choosing a rear wheel with a cassette will also make your bike less prone to breaking rear axles.

You bike has wide ratio 7-speed rear gearing. Changing to 8-speeds in the rear is not necessarily going to gain you a faster high gear, or a lower low gear. What it will do is make the jumps between gears smaller. This could potentially make it easier/more comfortably for you to pedal the bike, since you might find it easier to find the right gear that you feel comfortable pedaling. Are you finding yourself spinning out in the High gear (big chainring in the front, and smallest cog in the rear)? Do you know how fast you're pedalling (cadence/rpm)? Are you a spinner (keep rpms at 90-100 or higher), or a grinder (keep rpms at 70-85)?

How many teeth are on the chainrings of the spare crankset that you have? Changing the crankset usually means replacing the bottom bracket to get the chainline (driveline) to line up correctly. If you can get away with changing the chainrings on your crankset, you can bypass the bottom bracket replacement. Is the bolt spacing the same on your existing crankset and the spare crankset? Maybe you can swap the chainrings (you may or may not have to replace the chain) without replacing the crankset?

HillRider

This is almost but not quite correct. 6 and 7-speed freewheels are almost exactly the same overall width and both use 126 mm rear dropout spacing so the unsupported axle length is the same for both. 7-speed cogs were just closer spaced than 6-speeds (except for the relatively rare Ultra-6 closely spaced freewheels). As noted, 8-speed freewheels are indeed wider than 6 or 7-speed freewheels and required a longer unsupported axle length under them.

fietsbob

No, in short..

It may cost you too much, new rear wheel (cassette-freehub), spread the frame so it fits , and many etc, etc.