Ouch! Baptism by fire..... now I need a MTB >:)

[Cubist]

Hmmm... a bit of googling and I've found out that the crankset on the bike is an old shimano STX, with a very odd chainring set up - looks like the granny is bolted onto the middle chainring rather than to the spider. Guess this means I need an identical NOS middle chainring if I want to keep the granny ring (which I sure do). Can't find any online at the moment, will have to have a phone around my LBSs to see if they have any lurking in the parts bin.

A rather unexpected complication!

Unless you want to keep it original, then you'd be better off simply replacing the entire crankset.
http://www.ebay.co.uk/itm/Shimano-M...-Chainguard-/151755141256?hash=item23554fb088

 

[ChrisEyles]

I'd rather keep it original if possible, seems a shame to throw away the lot when it's only one ring that's shot, and I don't want to buy a new one and find I have issues with chainline or insufficient travel on the FD (plus the STX crankset looks a lot nicer than the one piece modern ones)... was also hoping a single new chainring might cost less than a replacement crankset.

Would a triple-izer middle chainring work do you think? From this diagram it looks like the STX middle ring is basically a triple-izer in the first place...

http://bernd.sluka.de/Fahrrad/Shimano/TM/FC-MC36-S_1997.gif

 

[ChrisEyles]

Haha, and I should have learnt my lesson by now that old bikes are always more complicated and expensive than you expect. Just when I've learnt most of the tricks on old ten speeds I've found a whole new can of worms!

 

[Cubist]

I'd rather keep it original if possible, seems a shame to throw away the lot when it's only one ring that's shot, and I don't want to buy a new one and find I have issues with chainline or insufficient travel on the FD (plus the STX crankset looks a lot nicer than the one piece modern ones)... was also hoping a single new chainring might cost less than a replacement crankset.

Would a triple-izer middle chainring work do you think? From this diagram it looks like the STX middle ring is basically a triple-izer in the first place...

http://bernd.sluka.de/Fahrrad/Shimano/TM/FC-MC36-S_1997.gif

That's the point. A new crankset can be cheaper than a new chainring or set of chainrings. Thing is, your current one is an obsolete standard, and a bizarre one at that. Yours dates back to 1995, and was replaced with a more current spider system in 1997. . There are plenty of chainsets out there that will look similar... I don't like the one piece cranks either.

You'll need to ensure you source the right BCD five hole chainring with chainring holes to match the spider and carry the granny. I imagine they aren't going to be easy to find. A quick search shows a useful thread on CTC, but be aware they were being described as scarce in 2009.... http://forum.ctc.org.uk/viewtopic.php?t=20168

In reality you would be better off fitting a different chainset.... you'll find loads of cheap square taper cranksets that will look the part, and be much easier to work with.

 

[ChrisEyles]

@Cubist looks like you might be right - I thought a single chainring would be a doddle to source and fit, but not this one apparently!

Still might be worth a check in the parts bin at the LBS... but I'll keep an eye out for a suitable replacement.

 

[ChrisEyles]

It's also faintly possible that the damage to the ring is in fact some very bizarre shaping to aid shifting rings (look at the squared off hole on the middle ring around 4 'o clock)... this is a pic of an allegedly brand new chainset and the middle chainring shows some very similar what-looks-like-damage to my one.

Tricky to tell, I guess! Think I'll see how it rides and only replace the crankset if the chain skips, or if it stretches really quickly (it's currently showing very little wear).

 

[Cubist]

Indeed. The middle ring teeth are shaped and ramped to aid shifting. Some of the teeth will look squared off and others appear sharper. The thing to look for in a worn ring are that all the teeth take on a shark fin appearance, and the scoop between the teeth looks asymmetrical as the chain wears the surface nearest the rear of the teeth..

 

[dan_bo]

*Coff* *have a look on retrobike* *coff coff*

 

[Hugh Manatee]

Those teeth don't look too bad. Shimano did all sorts of things with shaped teeth and little ramps to help get the chain across. If all is lost with that one I think I have a period LX chainset you can have. The rings on that are the standard pattern attachment and pretty easy to get hold of.

Whereabouts in Devon are you? I might be visiting home soon. Born and raised in the hippy stronghold of Totnes.

 

[ChrisEyles]

The teeth on mine are a little rougher looking than the crank in that pic. There isn't too much shark fin going on, which gives me hope, but there appear to be some nasty looking rips in the body of the middle chainring, in between the teeth. I'll see if I can get some pics to show what I mean.

@Hugh Manatee - thanks very much for the kind offer! If my current crank turns out to be toast, that sounds great. I'm based in a small village North of Exeter - it's a little bit of a detour from the M5 but not too bad (and it's a pretty drive down the Exe valley from Tiverton). If you bring a bike (or will fit one of mine) we could go for a potter along the lanes, if you like

 

[Cubist]

*Coff* *have a look on retrobike* *coff coff*

That's where I found the CTC link.... You flat earthers are all the same. 



 

[dan_bo]

That's where I found the CTC link.... You flat earthers are all the same. 



 

[ChrisEyles]

I had a look on retrobike, which has a fair few people lamenting the crazy desing of the chainrings. Apparently on some models the BCD of the large ring is 95mm instead of the usual 94mm, just to be extra difficult! I also came across a comment by a chap who worked in a bike shop, saying he had to deal with lots of complaints from people saying their chainrings appeared ripped or with damaged teeth but this was just how they were cast... so sounds promising.

Here's my chainring - note the "damage" on the middle ring around 6 'o clock and 9 'o clock. Other than that the profil of the teeth looks OK, no sharks fins going on.

And here's a NOS one - with very similar "damage". I guess it must just be a bizarre looking shifting aid.

So I guess my chainrings are probably fine after all. Bizarrely one of the chainring bolts is missing, so I need to sort this out before taking the bike out in anger.

Still need to sort the indexing of the gears out, but the suspension forks are working great after their cleaning/re-greasing. There are a couple of other small niggles (stiff jockey wheel, the bolt for the headset collar clamp sheared off, and the tension on the spokes seems a touch low), but it is basically ready to go. Here's where I am so far:

 

[ChrisEyles]

Took the bike into work today for a little test run and its all running quite nicely, if a bit slow compared to my not-very-fast-itself commuter! I was definitely glad I'd stuck the bar ends on, the position is no good for on-road riding otherwise. I was also informed I had mud all over my face when I got in, which reminded me just how awesome mudguards are (not for this bike though, I intend to get thoroughly plastered on the trails).

Stopped off at the LBS to pick up a chainring bolt, headset collar clamp bolt, and seatpost Q/R skewer (all for £3.50, cheaper than I could find online for definite - I love my LBS ) - just needs a little fettle over lunch and it'll be all ready for the woods tomorrow.

 

[ChrisEyles]

Went out to Woodbury common yesterday with the bike and did a couple of hours riding. Mostly cross country trails, but with the odd bit of single track and some technical-ish bits through the woods. Had a fantastic time and no spills this time

Seems to me that moutain biking takes a fair bit more skill than road riding though, I have a lot to learn! Planning on going back to the trail centre next weekend and seeing if I can get the knack of it a bit more. I'm particularly intrigued by all this "pumping" business, which is like nothing I've ever done on a bike before, and must confess a childish desire to learn how to bunny hop and wheelie

Only downside is I bent the rear mech a shade putting the bike back in the car boot, and am having difficulties getting it to work again after removing it to straighten the cage (have asked in the technical thread for help!)... may have to swallow my pride and ask the LBS for help so I can get the bike back on the road for the weekend.

But yes... this mountain biking business is really pretty good, isn't it?