Carrera Subway Zero Chainline woes...

You're not one for the easy life! :)

As mentioned already 126mm OLD is odd for a purpose built SS bike: that's the standard for 5-7 speed bikes.

However, having recently had my hands on a friend's Halfords POS, that's much more recent and he's had from new, nothing would surprise me: a random selection of dangerously mismatched parts.

Entirely likely that when they jumped on the bandwagon, they just bought what was cheap, ie outdated 126mm frames, and cobbled the lot together with no regard to proper function.

That or they just adapted leftover MTB: look at the bus-sized gap twixt tyre and fork of the bike on eBay!

What is the gap between the back of the crank and the chainstay like? Should only be a couple of mm or so.

If the chainring is on the inside of the spider, then it isn't a SS crankset but a double (or even a triple, any "spare" holes below those used to mount the existing ring).

In order to mount a bashguard (and hide their bodgery), the ring has been mounted on the inside.

In order for the ring (which is larger than what would normally be mounted in that position) to clear the chainstays (which are flared to accommodate the gears you don't have), a longer BB has been used (113.5mm is typical of a triple set-up).

"Chainline? What's that?"

You could ditch the bashguard and mount the chainring on the outside of the spider (so it clears the chainstays) and use a shorter BB to bring everything inboard, but my guess is that the cranks will bottom out on the BB shell before you get the desired chainline.

In fact, having written all the above, here's what I think happened:

• Halfords bought a load of old 3x7 speed MTB (hence the huge gaps above the tyres, V-brakes and 126mm OLD);
  
• they removed the two smallest chainrings, moved the large ring inboard and slapped on a bashguard;
  
• ditched the 7 speed freewheel and slapped a SS one on.
  
  

Result: a POS that will never work as intended and cash in the bank from unsuspecting punters.

If I'm right, then I'm afraid it really is more trouble than it's worth to get this running a SS set-up. Either turn it back into a geared MTB or write it off to experience.