northerntier

I've got a slightly bent spider. I say "slightly", but if I put a 50t chainring on it the chain hops off. A 38t works, but I don't have any 20% hills near me.

I assume to get a new spider I need to buy a whole new crankset. If so, is there anything specific to using an eccentric bottom bracket (EBB) to guide my choices? I have a Thorn Nomad 2 w/ Rohloff, EBB, and only a single chainring.

And, finally, I'm assuming it's the spider. I guess it's possible that the there's a problem in the BB, but then wouldn't the chainring rotate smoothly in a slightly off-kilter direction. Both (new) chainrings I put on show a wobble, w/ the 50t wobble being quite pronounced.

Depressed in Takoma (Park)

fietsbob

I find the Stainless steel chainrings QBP sells under the Surly brand excellent..
[up to 50t.]

you may be able to bend your existing chainring,
if a 38t works, spider may be fine
then you may have just a bent chainring,
have you re bolted on the same chainring
rotated in 20% increments (5 arm spider)..

HillRider

Try to determine if it's the spider or the chainring that "bent". A chainring can be straightened by the judicious application of an adjustable wrench closed down to clamp the out-of-line area. If it's truly the spider that's bent, these can be straightened too but it's more difficult and a replacement crank would be worth considering.

As far as the EBB is concerned, I assume it's threaded with standard English threading so any type of English bb should work.

northerntier

It's definitely the spider that's bent, as a chain hops off a brand new surly 50t if I screw it on tightly. The brand new 38t "works", but it still has a small wobble (much smaller diameter).

Aha, looking through the sales slip :-), I do see:

Shimano UN54 Bottom Bracket 73 mm Shell English Thread 110 mm

However, I'm still not sure where to go from here.

Searching for "crankset english threaded" on amazon gives me one match: SRAM S300 1.1 175mm 48T Black GXP Cranks English Threaded.

Unfortunately, this is a 130mm spider, vs the 110 I currently have.

HillRider

A 130 mm bolt circle will work fine and will accept down to a 38T chainring. Also, all "compact" road cranks (except some Campy models) use a 110 mm bolt circle so you could use any of them too.

Cranks aren't threaded, the bottom brackets are. The crank arms do have to match the spindle type such as square taper, Octalink, ISIS or you could use one of the newer external cup bottom brackets and it's accompanying crank with a built in spindle such as the Shimano Hollowtech II designs.

So, you are not limited to a square taper bottom bracket although they are still widely available in various spindle widths if you find a compatible crank.

Frankly, searching Amazon (or e-bay, etc.) for an appropriate crank isn't where you should be. A good LBS may be able to straighten your current spider or provide a suitable replacement crank and/or bottom bracket

FBinNY 

If it's an alloy crank you can take a shot at straightening the spider, but before you do so do this quick analysis.

Drop the chain to the BB to get it out of the way. Slowly turn the crank and observe how the chairing wobbles. If it's a long continuous movement out and back again, odds it's the entire crank, but if it's sort of steady with a local wobble then it's the spider.

The crank induced wobble might mean that the crank is a bit askew on the spindle, and sometimes resolves if the crank is pulled, and re-installed in a different position (yes, you'll also have to move the left). If the crank has a pre-fixed spindle, it usually cannot be fixed.

A locally bent spider, can be straightened, using a lever such as a long screwdriver blade, with the base as a fulcrum. Folks good with hammers can also straighten cranks with a rubber mallet, but this requires a bit of touch.

northerntier

It's kind of long and continuous and then one very pronounced wobble :-). I think it's the spider. I haven't had really good experiences with any of the LBS's, but I guess I'll try one.

northerntier

Okay, so go ahead and tell me how stupid this is. It's definitely one arm of the spider, somehow bent back a bit. I stuck a washer between the spider and the chainring, and now it's pretty smooth. Went for a 30-mile ride w/ no ill effect yesterday.

FBinNY 

Not stupid at all. You solved the problem so the chainrings spin true without risking the possible consequences of bending the arm. Good fix.