When to replace eccentric BB shell and Rohloff sprocket

[pandanroll]

Hi,

Am heading off on an extended trip on our Raven Adventure Twin.

I took out both BB shells and there are, as expected, some pitch marks from the bolts to secure the them.

My question is, 1) at what point do people replace these? 2) What is the downside of not replacing?

Would prefer to avoid shelling (ahem) out on a pair, but would be happy to if required as we will be far from SJS for the entire trip.

[WorldTourer]

I replaced it once when I noticed some slippage from holes too close together, which it looks like you have allowed to happen, too. Then I ended up replacing the whole frame (a Nomad Mk2 with a Nomad Mk3) because that old-style EBB design is just daft, and I’d rather spend hundreds of euro more not to have to faff with it any more.

[PH]

The first needs replacing, the second looks fine.  The downside of not replacing is simply that it may not be hold secure, there is a chance of the set screw slipping from one indent to the next. The natural reaction to that is to overtighten the screw, too much of which can distort the frame.  Ideally you should avoid it getting like that, there is no need for that micro adjustment, both the drive and the timing chains can run a little slack.  What you're aiming to do when you adjust is create individual indents with reasonable space between them, once you've got three or four of these, they can be re-used.  It is a bit of a learning curve, I trashed my first in a couple of years, but lesson learnt the replacement would have lasted forever had I kept the bike.   I do prefer the design on the newer frames, but the set screw type has been in use on tandems for over 100 years, it's well proven, the error is users thinking the chain needs more frequent adjustment than it does.

[pandanroll]

Thank you WorldTourer and PH, very useful information -- I will replace the front one. I did have some trouble getting the front chain tensioned correctly, perhaps this was because of the slippage you mention, PH. My response was to adjust more frequently and tighten further, which probably exacerbated the problem.

I have a further question -- do you think the sprocket is worn to the extent that the used side will quicken the wear of the chain? If so, I may also replace it despite the fact that only one side is used. The bike did chew threw the rear chain significantly quicker than the front one (it was replaced several times, but the front one did two chains over 12,000KM) -- I wonder if that is because of sprocket wear?

[mickeg]

When I adjust the eccentric on my Nomad Mk II, I adjust it so that the pinch bolt holes (I call them divots) in the eccentric are not too closely spaced together.  That way, a groove does not develop.  I do this by removing one eccentric pinch bolt so that I can see where the nearest divot is.  Eccentric should last forever.  More detail at this post:
https://thorncyclesforum.co.uk/index.php?topic=14475.msg108340

I just added a photo to that post to make it more clear how I adjust the chain.

***

On the sprocket, I think the best time to flip it is when you wear out chains too fast.  Yours is worn less than mine was when I flipped mine last time, which was in 2020, but I do not ride my Nomad Mk II as much as several of my other bikes so it takes a long time to wear those parts out on mine.

Don't discard your sprocket if the teeth are only worn on one side, if you want to replace it you could carry your old one as a spare or keep it at home for next time.

My sprocket wear when I flipped mine is shown in the attached photo.  I have an even number of teeth on my sprocket (16), thus mine always has the chain links with inner plates on one set of teeth, links with outer plates on the other set of teeth.  And my chainline is slightly off.  Thus the tooth on the left in the photo shows some wear on the side of the tooth from the inner plates, where the other teeth with outer plate links have no noticeable wear on the sides.  From your photos, I suspect that your chainline is spot on to avoid wear in the sides of any teeth.

The way I look at a Rohloff sprocket to assess how worn it is is not to look at the shape of the tooth, but instead to look at the position of the tooth above the drilled hole in the sprocket.  The teeth start out directly above the hole, but if you look at my teeth in the attached photo, the right tooth has shifted somewhat from being directly above the hole, the left tooth has shifted even more because the left tooth has worn more than the tooth on the right.  The teeth did not move, but part of each tooth is now gone, so the teeth look like they moved, relative to the drilled holes.  Your teeth are much closer to being directly above the holes than mine are.

This internet link describes why the teeth with the chain inner plates wear faster than the other teeth, that is because the links on a chain do not wear evenly.  Only half of the chain links elongate with wear.
https://www.sheldonbrown.com/chain-life.html

Thorn fits sprockets with 17 teeth to their Rohloff hubs, thus your sprocket teeth will wear equally, unlike mine.

[PH]

I have a further question -- do you think the sprocket is worn to the extent that the used side will quicken the wear of the chain?

Of a new chain? Yes. 
There are two approaches to chain replacement:
Change the chain when it reaches 0.5 - 0.75% elongation, this will prolong the life of the sprocket and chainring.
OR
Let them all wear together until the sprocket looks too worn, then replace (Or flip) the sprocket (And maybe chainring) and add a new chain.  The hard part of that is defining how worn is too worn, I've always lost my nerve before I've reached that point.
I prefer to do the second, but I'd argue it suits me, rather than any other measure of better. What wouldn't make sense is a middle way of putting new chains on part worn sprockets. 
As mickeg says there's no reason not to use the other side, the only circumstance I wouldn't is if you'd worn the teeth to points.

Disclaimer - I have no tandem experience, the wear rates are going to be different, but I have no idea by how much, or if that would influence me.

 

[mickeg]

I am still using the older threaded sprockets, they cost more than the new splined sprockets.  Thus, I pushed the sprockets pretty far.

The splined sprockets are cheap enough that, you could certainly run a sprocket until the chain wears out, then flip the sprocket and buy a new chain.  Thus, two chains per sprocket.

I run my Rohloff bike chains to 1.0 percent before changing them.  But my derailleur bikes, I change those at 0.75 percent elongation.

[Andyb1]

Going back to the OP - is there any way those ‘dimples’ could be filled?  JB Weld is very good for that sort of thing.  File it round and off you go again.
Or strip it down and silver solder.

[Danneaux]

Or strip it down and silver solder.

The eccentric is milled from aluminum, so sadly not compatible with silver solder or brass.

Best, Dan.

[Andyb1]

You are right Danneaux, not standard silver solder, but I think there are special fluxes and non lead solders for use with aluminium although the heat might be an issue - giving distortion and damaging seals.

If it was mine I would clean the divets with acetone and fill them with JB Weld.  Despite the name it is a 2 part epoxy.  It can be used to reform damaged metal and be drilled and tapped.  Engine crankcases have been repaired with it!  It is an American product.

[PH]

Going back to the OP - is there any way those ‘dimples’ could be filled?  JB Weld is very good for that sort of thing.  File it round and off you go again.

I don't know what JB Weld is, I read of someone who tried it with Araldite, it didn't work for long, I think it didn't get a good enough bond.
It's a £30 part, which might lead to considerable inconvenience if it failed on tour, I'd just replace it.
OTOH, I met someone with a set screw type Thorn Raven Sport, who was running the EBB upside down, thin side at the bottom, against the advice and apparently likely to damage the BB itself.  He'd been deliberately doing so for years, to lower the BB height, without problem, though he said he was careful not to overtighten.
Which reminds me, there are some old tandem EBB's which use dog point machine screws and the insert pre drilled to take them.  I was considering converting mine, but never got round to it.

[Andyb1]

Araldite comes in different ‘flavours’ and the more commonly available quick set stuff can to go rubbery and peel off fairly quickly and personally I don’t use it.   The 24 hour slow set (old style araldite) is much better as a long term general purpose adhesive but is getting more difficult to source.

J B Weld (cold weld epoxy) is advertised to give ‘the world’s strongest bond’ and seems to work very well at bonding metals.   In the case of the dinted BB it would only be acting as a filler.  If the OP decides to use it then after cleaning, apply a small quantity into each void and wrap sellotape around the BB to give a smooth surface.

But do whatever you feel comfortable with.

[pandanroll]

Hi all thank you for the useful responses. I bought another shell as it’s not too expensive and if I tighten the chain correctly should last a long time, as per advice on this thread. Cheers.