Gates sprocket on Nexus Hub - what's wrong

plet

Can anyone give me any ideas as to what's wrong here?

I've got myself a brand new bike (a Civia Kingfield since you ask) with a nexus 8 hub and a Gates belt. The bike is designed for a belt so there should be no problems there but the drivechain (is that the right term here?) has never really felt right. Lots of clicking in all gears and it just doesn't feel smooth, certainly not the silent running as advertised!

So after a lot of tinkering with belt tension, cable tension, sacrificial offerings, etc. I think I've finally tracked the problem down to how the rear sprocket attaches to the hub. There just seems to be way, way to much play and movement in it. Unfortunately never having owned either a Nexus hub or a belt drive I don't know if this is normal or what should be correct.

I've uploaded a couple of youtube videos so you can see what I mean:

This one: http://www.youtube.com/watch?v=56IGX84hlXU shows the sprocket on the hub without the lock ring. It that amount of movement normal? It almost seems as if there's something missing since if the sprocket were to sit a little higher it feels "right".

This one: http://www.youtube.com/watch?v=tX6my9Y1Z2Q is a very poor attempt to show that with the snap ring in place there's still a bit too much movement. When the belt is on and tensioned, it's obviously going to cause problems. And is that snap ring sitting correctly?

Many thanks for any advice.

majormantra

Hard to tell about the position of the circlip from the video, but there shouldn't be that much movement. With the cog all the way on, it shouldn't move at all relative to the axle.

Many bikes supplied with Shimano hub gears and Gates belt systems come with a crappy aluminium cog as stock. These are totally unfit for purpose and the tabs that engage with the hub can wear down or snap in a few weeks or months of riding. The solution is to get a steel cog, which does exist. You should be able to get one under warranty if your cog is already damaged.

plet

Thanks for the reply.

It's a steel sprocket I have. Certainly agree that there shouldn't be that much movement! When the clip is on it stops most of it but there's just a little bit left.

When I look closely a the back of the sprocket I notice that there there are some nicks out of the pins which come into contact with the hub: http://twitpic.com/d6x8il

Are these normal? They look like they are there by design and it's impossible to wear steel on steel like that after a few 10s of miles, right? But its almost like the sprocket would fit properly without them. It's not something stupid like the wrong sprocket has been fitted? A Sturmey Archer one maybe (I'm clutching at straws!)

Thanks again.

majormantra

That looks like damage, the tips of the tabs should be symmetrical.

Are you sure it's steel? Have you checked with a magnet? I'm wondering because it doesn't look like the steel Gates cog I've handled.

plet

Yep, checked with a magnet.

I know what you mean it looks like damage but I just don't understand how it's possible to do it. There were no metal shavings when I removed the sprocket and apparently no damage on the hub.

Not overly impressed with either the Nexus or the belt drive so far!

majormantra

Hmmm, funny one. Wasn't an ex-demo bike or something?

Honestly I don't think they're worth the trouble either. And if you ever have to replace the drivetrain the components are bloody expensive.

plet

Brand new bike, assembled out of the box myself. Unfortunately bought online so returning it is going to be more trouble than it's worth.

Looking again really closely at the nicks on the sprocket I'm almost certain they're not damage. They're too similar to each other and there's even paint on the surface. I can't find anything similar on the Gates website so I'm really confused.

Might just have to try a genuine Nexus sprocket just to try and get to the bottom of it. You're right though, bloody expensive and a pain to source!

MichaelW

I have had some experience fitting Sturmey Archer 1/8 style rear sprockets onto Shimano Alfine, which is designed for Shimano 3/32 sprockets.
The circlip fits into a inverted cone-like recess on the hub so the radial compression is translated into axial compression.
If the sprocket is too thick, the circlip wont fully engage into the recess. If the sprocket is too thin, no matter how tight the circlip grips the recess, it will never translate axially to compress the sprocket in place.
Spounds like your sprocket is too thin at the critical point. You may be able to add a thin washer to the inside of the sprocket.

Post a small pic of the damaged tabs.

plet

It's something like that I'm sure. Here are the "damaged" tabs http://twitpic.com/d6x8il comparing these to pictures of a genuine alfine/nexus sprocket on the Gates website the difference is certainly going to cause problems.

Anyway, a new sprocket is on order. I'll update as to whether it fixes the problem.

plet

Got there finally. After fitting a proper Nexus sprocket, but perhaps even more importantly the correct, much thinner, circlip locking ring the difference is obvious. It now looks and feels right with no movement of the sprocket and therefore a smoother ride. Goodness only knows how I was sold a new bike with a wrong sprocket!

I had some trouble getting the belt tension right. It feels a lot less tight than I was expecting but it's obvious when it's too tight as the hub makes it's objections known with a horrible knocking sound.