Christiaan

I'm fixing up an old Marin Larkspur. I need a single chainring because I'm getting rid of the front gears, so I only have 7 gears.

My rear cassette = Shimano HG30 7 Speed
And my chain = SRAM PC-890

jimc101

Why are you getting rid of the chain rings? guessing if you stayed in the middle all of the time, which is probably a 32T, thats what you want to get, you will probably also want to get a chain guide if you are staying with a cassette as well.

HillRider

Are you replacing the entire crankset, just removing existing chainrings or replacing the current chainrings with a single one of a different tooth count?

If you are just replacing the current chainring(s) you need to match the bolt circle diameter (BCD) and the number of bolts (3, 4 or 5 with 4 and 5 the most common). An older "flat" type chainring without shifting enhancements will work best in a single ring set-up.

If you are replacing the entire crank set you will need a compatible bottom bracket to match it.

Christiaan

Thanks guys.

The rings are knackered. I have a new chain and rear cassette too so really need new rings as well right? I could conceivably reuse the cranks but I can't see how to separate em from the rings. See photos.

I'd like to have one ring to simplify things. It's just for riding around London and I don't need 21 gears. I'm pretty sure it's a 1999 Larkspur and this page says the existing crankset is a Shimano Nexave, 28/38/48. Would I be better with 32 teeth for a single ring?

What does the chain guide do, keep the chain on the ring? Is that because a new crankset and single ring would likely be off centre in comparison to the existing middle ring? What about a crankset with bash guards? I'd like to keep it simple.

Righto.

The bottom bracket is a Shimano UN54 68/122.5 mm. Box says "For bracket type front derailleur (E)". What does that mean? I didn't fully appreciate the effect the bottom bracket type would have on what kind of crankset I could use. I just replaced it with the same as the existing one.

gyozadude

Ahh... a picture is worth a 1000 words. Yes, riveted and press fit crankset.

Unfortunately, each model of crankset may have a slightly different depth to which it slides onto the BB spindle. And in the old days, there were manuals that told us roughly which spindles went with which crank sets for 2 chainrings and 3 chainrings. You may still find such tables on the web and at Sheldon Brown's site. But for new cranks, the question is a bit tougher if it doesn't come with a manual. The better ones have recommended spindle references based on BB width. So you need to get the crank and then get the BB and spindle. A good LBS would know and a good online retailer might be able to answer such questions. In the old days, with 4sided tapered cranks, I had a bunch of different sizes with the same 68mm spacing and I typically used a process of trial and error on a crankset and spindle until I had a good chainline. Typically, it take 3 attempts and I'd nail it on the 3rd spindle.

HillRider

The picture does explain a lot. That is a very low-line crank and the chainrings are riveted in place and cannot be either removed or replaced so you are in for a complete new crank and, most likely, a compatible bottom bracket.

The "For bracket type front derailleur (E)" means the bottom bracket is intended for use with a front derailleur mounted on a bracket that is clamped between the drive-side bottom bracket cup and the frame's bottom bracket shell. This is the configuration used on some MTBs. Most bikes fasten the front derailleur with a clamp or a tab on the frame's seat tube.

Christiaan

I see, thanks. If I had to get a new BB it would be a costly mistake on this little project (I had it professionally fitted too). Would I be asking for trouble if I re-used this crankset (given that I have a new chain and cassette)?

And if I were to try and find a crankset compatible with the BB I've used, what should I be looking out for/asking for in terms of specification/measurements?

HillRider

Cranks are very tollerant of wear until the tooth profile is really bad and yours doesn't look too terrible so it should be ok with a new chain and cassette.

A new crank using your current bottom bracket would be a JIS square taper type and would have to be compatible with the length of your current spindle. That is, it couldn't sit too far from or too close to the frame.

Christiaan

Okay, thanks again, appreciated.

Christiaan

Do you think these might do the trick?

Brev.M Fixie Crankarm
https://www.evanscycles.com/products/...nkarm-ec026213
It says "Square taper / JIS bottom bracket interface 103mm recommended width". Does that mean it might sit too far out from the frame on a 122.5 mm spindle?

And a 42T one of these (will 42 be too many I wonder?):
https://www.evanscycles.com/products/...-ring-ec026212

The chainline of my existing crankset looks to be 47.5 mm (pushed on by hand without tightening bolt).

P.S. on second thoughts will these be compatible with my SRAM PC-890 8-speed chain? How can I tell?

vredstein

I believe "103mm recommended width" means that if you had just a single speed or fixed cog in back, a 103 mm bottom bracket spindle would give you the correct chain line. With your 7-speed cassette, what you're hoping for is a chainline that would put your single front ring in a straight line with your middle (4th) cog.
You could order the crank, install it on your existing bottom bracket, and see how far off your chain line is, then order a new bottom bracket with the appropriate spindle length to achieve a chain line that puts your front ring and 4th cog in a straight line.
Just be aware that these cranks only appear to be available in a 165mm length.
And find out if the ring you're looking at is 1/8" width. This would cause problems if you used a narrower 7-speed chain.

Christiaan

Thanks.

Do you think 48T would be too much?

Christiaan

Went with these:
https://surlybikes.com/parts/stainless_steel_chainrings
https://www.brevm.com/parts/cranks/crank-arms

vredstein

Looks like it should work. The number of teeth really depends on your riding style and what you found yourself using most on the previous setup.