Bicycle Mechanics - replacing 11 cog with a 34

Hi Everyone

I am updating my gearing for a cross-country tour, and I am thinking of tinkering with my cassette to make it more hill-friendly. I have a trek 520 with an 11-30 8 speed hg-50 cassette with a 26-36-46 crank. I want to remove the 11 tooth sprocket and add a 34 tooth sprocket on the upper end of the cassette, making it 13-34 climbing machine. Is this a doable operation or do I need to get a new cassette. Thanks for any suggestions.

Sasha

Why not get an 11-34 cassette?

Sasha, you would not be able to do what you are describing. The lockring on the cassette interfaces with the 11 tooth cog. If you take the lockring off, you will be able to see the ribbed (for lack of a better term) surface on the cog which engages and locks the lockring on. CdCf's suggestion would be the way to go.

Yeah, you'll get from 21 to 114 gear inches.

Yes it's do-able. You will need another spacer, for the 30-34 step, because the one for the 11 is part of the 11t sprocket.

I haven't had any problems on mine, but you may want a new 13t that is meant for use as the last cog, because there isn't much engagement of the splines with the stock second-to-last cog, due to the splines ending a touch early so that an 11tcog will fit on the hub. The 13t that is on there does have a built in splined spacer just like the 11t but it is a slightly narrower spacer because it must be used with the thin wavy spacer and thus dosen't engage quite as much. The thin wavy spacer is there to make up for the heads of the rivets that hold the rest of the cassette together.
[edit:add] Also the last sprocket has ridges on its side that lock with the lockring, the second cog does not. (On a 7 speed cassette the last cog also has the lock ridges, but it rather than the second ring, is for use with a wavy spacer. I mention this because 12/13/and 14t top cogs were very common in the 7 speed days, rather than the 11 tooth fits all thing that you find on the 8 and 9 speeds, and a 7 speed top cog will work on an 8 speed cassette.)

Let's say you have what's needed: one complete HG 11-30 cassette, a 7 or 8speed HG 34t sprocket, an 8 speed HG spacer, access to a lockring tool(with handle or the right size wrench), one chainwhip, and a light grinder or a good metal file, some solvant/degreaser cleaner, and WD-40. Lets also say that you go with a salvaged 7 speed, 13t, ridged, end-cog with it's wavy spacer.

First remove the lockring using the chainwhip and lockring tool.
Slide cassette off, the 11 and 13 tooth sprockets and a thin wavy spacer are loose, note the order they come off and you will see what I was rambling about above. It should go 11t-13t-thin wavy spacer-the rest of the cassette.

Three rivets hold the cassette together, pick a side either side and carefully grind or file off the rivet heads, all the way flush. then use a medium-small nail and small hammer to knock the rivets out, it won't take much.

younow have a bunch of loose sprockets and spacers, use the solvant or degreaser to get all the grunge off of everything, dry the lot, then lightly coat with WD-40, to help prevent rust and what not, and pat off the excess with a paper towel.

Now just slide the cogs and spacers back on the hub in the order you wish, they will only slide on one way. To make it easier there is a small triangle on each sprocket near the thin/wide/odd spline. 34-30-26-13-20-17-15- wavyspacer 7speed 13 (or an 8 speed last-cog 13 without the wavy spacer).
Note: if using the stock wavy-spacer13 it will feel like it is barely engaging the splines, it will engage much better once the lockring has compressed all the spacers slightly.

Thread the lockring on all the way by hand, make sure the last cog is still properly engageing the slpines(didn't pop up and shift or something) then finish tightening the lockring with the lockring tool and wrench.
Voila!

[EDIT] Oops I forgot about the tooths on the lockring and last sprocket.

Thank you all for the suggestions. The reasons I wanted to go from 13-34 is that I did not want to switch to a 9 speed cassette and I never use the 11 anyway. In fact, I hardly ever use anything above 90 gear inches. My main priority is to have as many low gear inch choices as possible to fine tune steep climbs. I'll try to follow capsicum very throrough directions. If all fails, it'll be back to Sheldon Brown for his custom cassettes.

Sasha

I don't believe you can buy a single 34 cog. Suggest you buy a 12-34. If you like you can replace the 12 with a first position 13 (with built-in spacer).

Al

Read http://sheldonbrown.com/k7.html

Basically, the first cog is different, so if you want to start with 13, you need a new 13.
Then you need the largest cog.

But while you are at it, you may get other custom configurations. For instance, my tourer now has : 12-14-15-16-17-19-21-26-34 and my tandem has 12-14-15-16-17-19-21-25-32 ... all made by combining a few 12-25, 11-32 and 11-34 cassettes (+ leftovers on the city bike, on the trailercycle...)

You are running a smallish triple already ... you dont need bigger cogs. Train hard and ride on. :)

Since you are using old cassettes, How do you avoid skipping problems and alike?

Since you are using old cassettes, How do you avoid skipping problems and alike?
Old, but not very worn.