b1983

Hi,


I recently bought a cheap fixed gear bike (Betty Leeds 2020 model from Bikester) and have been changing a few parts for the fun of it(and to learn)


I put a new chain on it, changed the brake pads and tyres which was successful and easy enough.


The next thing I tried was changing the sprocket from 16T to 15T (Halo Fixed Gear Track Cog) but I'm having issues with fitting it correctly.


I don't have a maintenance stand so turn the bike upside down when working on it. The first issue I noticed after fitting the new sprocket is when I spin the pedals the chain bounces about a bit.


I took it out for a ride and when pedalling forward it seems ok but if I try and brake with my legs it makes a nasty crunching which sound like its going to throw off the chain.


I'm assuming there must be something different between the 2 sprockets which is causing this but I'm not sure what could be or how to fix it?


In the meantime the old sprocket has been put back on and it works without any issues (no bouncy chain and braking is OK)


Any help would be much appreciated.

mprince

Did you adjust the wheel position in the dropouts when you changed to the smaller cog to retension the chain? This sounds like you lost some tension in the chain, which other than proper chainline is about the only adjustment needed on a fixed gear setup.

b1983

Yes, after changing the sprocket I am realigning the wheel and retensioning the chain.

I've played about with this quite a bit and was also thinking there is something wrong with the chainline but don't understand why I am able to get the chainline right with the old sprocket but not the new one (I've swapped them back and forth several times now)

mprince

Are the cogs the same pitch?

b1983

I don't know what that means how can I check?

I'm not sure if this relates but the new one has "BC1 37x24tpi 1/8 - 15T" marked on it but the old one has nothing.

HillRider

the "1/8" engraved on the 15T cog means it's for use with a 1/8" wide chain. If your old cog and chain were the more common road/MTB width of 3/32" then the new cog is incompatible.

b1983

I think all is well regarding 1/8.

I previously swapped with chain that came with the bike with a 'Izumi Standard Track/Fixed Chain 1/8 x1/2' chain and this worked OK with the existing parts.

b1983

I have it running much better now. What I did was put the chain that originally came with the bike back on and it fixed the issue.

I'm not really sure why though. The reason I tried the old chain as I was having difficulty getting the chain tension right with the new one. It would go from too slack to too tight with a millimetre of movement.

I'm not sure if chain wear was the issue as neither of them had been used much

Old chain that works - probably did less than 50 miles
New chain that does not work - done less than 200 miles

Also I measured them side by side and they are both exactly the same length.

Does anyone know why it did not like the newer chain with the new sprocket?

icemilkcoffee 

A wider chain will work with the narrower cog, but the narrower chain will not work with the wider cog. That's my guess.

79pmooney

The Izume chains are excellent but rather "square"; ie no shaping of sideplates to get a smooth, silent run. On similar "square " cogs, the result is a combo that is a little noisy but rock-solid reliable. (I run the ECO chain which I believe is what you have (~$20) and EurAsian cogs (which despite the name are made in the US).

I've never heard of the Halo brand. I wonder if it has quality issues that affect the tooth shape so perhaps the Izume chain doesn't sit down fully on the teeth and the more shaped and rounded edge stock chain does.

I take chain-cog fit very seriously as I have ridden my fix gear up mountains and down (using a bike set up to run three different cogs. I'm not superman.) I've done multi-mile 3000' descents at very high speed on a 42-12. The bike I use most often for that is a velodrome worthy bike except being pure road with brakes. (No one would even notice the drivetrain at a track. It's just good standard equipment that anyone could race.) But I also have done those rides on my namesake Mooney with its 110 BCD road crankset. Nowhere near as round so it is harder to set the slack correctly. It is a given that the max slack I will see on that bike will be considerably more than my pure fix gear.

In the early days of setting that bike up, I tried a KMC 1/8" chain. Beautiful gold chain. Nicely rounded sideplates, Dead silent. But - when the chain went slack it felt scary close to throwing off. One ride and an Izume went on and the KMC relegated to backup duty for the good bike. The one Surly cog I tried - same thing, Nice tooth shaping, quiet ride and scary. Another good bike only. (With the Sugino 75 crankset, full velodrome race quality, there is a little tight-slack to the chain but so little setting it up so it never gets loose enough to throw with the "slippriest" of cogs or chains is easy.)

Not really an answer to your issue but my experience as someone who's been riding fix gear a long time and has done his share of exciting locked wheel stops after throwing chains at speed. (And I love running those "square", rough running EurAsian Imports (EAI) cogs and likewise Izume chains for their rock solid no-throw reliability. Both also wear very well. Makes keeping on hand all the cogs from 12 to 24 teeth and using them on two bikes, changing often very feasible. EAI only makes to 22 teeth but as cogs get larger the throw issues get less.)

tangent70

Agreed