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front mech questions

Posted: 5 Jul 2007, 11:25am
by specialist
Ok, I've got 2 front mechs, one triple, one double. I'm currently using the double to shift a 48-40-28 triple with an rsx brifter. I want to change to a 54-40-28, do I have a chance of getting this to shift or will I need a 44 middle ring. The mechs are just sora, chainrings are not ramped or pinned and it is an 8 speed chain on a 7 speed freewheel. The other issue is that the triple mech is braze on, the double band on, can I dissasemble the mechs and change the clamp over? It looks to be just allen key bolts there?
Thanks

Posted: 5 Jul 2007, 11:34am
by PW
With a double mech on a triple chainset they tend to work better if the middle ring size is closer to the outer than it is to the inner. I don't say it won't work, just that it may not work well. Whether the mechs can be converted between braze-on & band-on I can't tell without looking - I've never tried it. You could use a band-on converter for the triple, they're fairly cheap from most bike shops. Finally an increase of 6 teeth on the outer chainwheel may have implications elsewhere. It's possible the rear mech may not be able to cope with a total 34 tooth jump across the chainset. That's a lot of chain slack to take up.

Posted: 5 Jul 2007, 5:28pm
by dkmwt
I have a similar question. Can I get a front mech to cope with a 29 tooth difference in the chain rings?

As you see by my profile, I ride a trike with 20 inch wheels. To get a decent top gear I have a 30/42/55. If I can I would like to change the small one to 26 for a lower bottom gear. I don't think chain tension would be a problem because if I'm constantly above 4th gear on the back while on the small ring I'd go to the middle ring.

Posted: 5 Jul 2007, 5:38pm
by specialist
well I've changed the mounting over on the band for the braze on fitting, so that isn't a problem. getting it to shift right is though, and I think I can see when on the inner plate of the mech. So I'm thinking I'll have to go to that 44. The lack of ramps and stuff probably doesn't help along with having to use an 8 speed chain on 6 speed rings.
To the other guy, I believe that the size of the inner ring is much less critical than the middle for getting a triple to work, as long at the bcd is such that you can fit a smaller ring.

Posted: 5 Jul 2007, 11:17pm
by dkmwt
The spider is deffinatly ok for fitting a 26 toorh ring as ICE the makers of my trike offer it but don't advise using both the 26 and the 55. I was wondering if they say this because the mech they put on it won't handle the 29 tooth difference.

Posted: 6 Jul 2007, 11:52am
by PW
dkmwt.
It would have to be a mountain bike mech for a difference that large. Beware that the difference between outer & middle for Shimano mtb front mechs has to be at least 12 teeth or the inner cage plate hits the middle ring. You're ok at the moment but don't be tempted to increase the middle ring size. I don't know if it will work but I suspect the answer is "Yes - badly!"

Posted: 6 Jul 2007, 12:29pm
by CJ
If all you want to do is fit a smaller inner ring, it's easy to see how much smaller your existing front mech will let you go.

1. Engage your existing inner ring with the smallest rear cog that you really need to use with it (probably not much beyond the middle of the cassette).

2. Measure how much gap there is under the chain where it passes through the front mech cage, i.e. how much lower, in millimetres, the chain could be before it scrapes on the bottom of the cage. Tip: an easy way to measure this is to poke allen keys under the chain, the biggest size you can poke into the cage, flat sides horizontal, without quite touching the chain, tells you how many millimetres. Another tip: lightly press down the right pedal to tension and lift the chain.)

3. Divide the number of millimetres gap by two. That is how many teeth smaller your inner ring can be before the chain will bottom on the cage