My bike has a braze on front mech and is currently running a double chainset.
Its a lot overgeared for me so how can I work out what is the smallest chainring I can run on it (I suspect a triple is out of the question) but I maybe able to change the chainrings to a 36 or something
thanks
braze on mech and crank sizes
Re: braze on mech and crank sizes
Whilst not exactly answering your question this thread contains some useful information: http://forum.ctc.org.uk/viewtopic.php?f=5&t=96888
Typically the limiting factors for the outer ring are the minimum height to which a mech can be set in the braze-on or the tail of the mech clashing with the chainstay.
The inner ring will be limited by the BCD of the existing chainset, typically 38T for 130mm and 34T for 110mm. Another consideration is whether you want an indexed front shift which will raise other issues.
On my own tourer I have a band on front mech and friction shifting, the chainstay is the limiting factor. I run a 38T outer but it's a huge gap between the mech and the chainring.
Works for me.
Typically the limiting factors for the outer ring are the minimum height to which a mech can be set in the braze-on or the tail of the mech clashing with the chainstay.
The inner ring will be limited by the BCD of the existing chainset, typically 38T for 130mm and 34T for 110mm. Another consideration is whether you want an indexed front shift which will raise other issues.
On my own tourer I have a band on front mech and friction shifting, the chainstay is the limiting factor. I run a 38T outer but it's a huge gap between the mech and the chainring.
Works for me.
Last edited by gaz on 2 Jun 2023, 9:21am, edited 1 time in total.
High on a cocktail of flossy teacakes and marmalade
Re: braze on mech and crank sizes
there are such things as triple mechs with a braze-on fitting. They come in different fitting depths so that they will reach down to smaller chainrings in some cases.
You can work out how much leeway you have by allowing about 2mm per tooth , then 2-3mm clearance above the chainring for the front mech.
A few front mechs have come with two mounting holes in them, so that you can vary the height more than usual. Others eg this one;
look to have enough meat in them to allow a second hole to be drilled and tapped if necessary. Another option is to make an adaptor bracket (eg from a length of tube, bisected) to lower the mech on the original bracket. A further option is to use a top swing (low band) mech that sits lower down on the seat tube and to just ignore the braze-on bracket.
BTW maybe I've misunderstood you, but I'm not sure why (or indeed how) you would set up a bike with a 36T outer and a triple chainset... also, it isn't compulsory to use an 11T sprocket, either. If you use a 13, 14 or 15T smallest sprocket, you will probably find that you will get the gears you need with (say) a 44/28 double, or a 26,36,46T triple. This will usually allow a standard front mech to be used , on a standard bracket, with no worries about the mech tail hitting the chainstay, either. The gears will shift better, the chain and sprockets will last longer... what is not to like?
Using a sub-40T outer chainring can create knock-on problems of one kind or another; these are surely best avoided. Going from a ~50T outer ring (I'm guessing) to a 36T outer just smacks of overreaction to me...?
cheers
You can work out how much leeway you have by allowing about 2mm per tooth , then 2-3mm clearance above the chainring for the front mech.
A few front mechs have come with two mounting holes in them, so that you can vary the height more than usual. Others eg this one;
look to have enough meat in them to allow a second hole to be drilled and tapped if necessary. Another option is to make an adaptor bracket (eg from a length of tube, bisected) to lower the mech on the original bracket. A further option is to use a top swing (low band) mech that sits lower down on the seat tube and to just ignore the braze-on bracket.
BTW maybe I've misunderstood you, but I'm not sure why (or indeed how) you would set up a bike with a 36T outer and a triple chainset... also, it isn't compulsory to use an 11T sprocket, either. If you use a 13, 14 or 15T smallest sprocket, you will probably find that you will get the gears you need with (say) a 44/28 double, or a 26,36,46T triple. This will usually allow a standard front mech to be used , on a standard bracket, with no worries about the mech tail hitting the chainstay, either. The gears will shift better, the chain and sprockets will last longer... what is not to like?
Using a sub-40T outer chainring can create knock-on problems of one kind or another; these are surely best avoided. Going from a ~50T outer ring (I'm guessing) to a 36T outer just smacks of overreaction to me...?
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: braze on mech and crank sizes
Bruce sorry I was unclear. The bike currently has a double 52/39 and it's a crit frame so 74 head and seat tube angles.
It's fun to ride but I do struggle on hills so would like to lower the gearing.
The simplest would be to have a triple with the spa chainset running something like 48/36/26 but on reflection I am not sure I can get the mech low enough, but then again I am not sure how low I need to go, hence the question.
I will follow later with whether I should just ditch the frame as its high maintenance with all that chrome which is pitted and I need to find a way of preventing the rust getting any worse. Maybe frame saver will just do it.
It's fun to ride but I do struggle on hills so would like to lower the gearing.
The simplest would be to have a triple with the spa chainset running something like 48/36/26 but on reflection I am not sure I can get the mech low enough, but then again I am not sure how low I need to go, hence the question.
I will follow later with whether I should just ditch the frame as its high maintenance with all that chrome which is pitted and I need to find a way of preventing the rust getting any worse. Maybe frame saver will just do it.
Re: braze on mech and crank sizes
you could try a clear wrap to protect the frame on the outside. Frame saver is really for the insides of the frame.
To fit a 48T chainring you need to be able to lower the front mech by 8mm. If you manage to do six mm that will be enough. I'd suggest that you buy a front mech and offer it up; if it is wrong you can always send it back to CRC or whatever. Shimano mechs have published specifications; I don't expect you to have very great difficulty in finding a mech to suit your application.
cheers
To fit a 48T chainring you need to be able to lower the front mech by 8mm. If you manage to do six mm that will be enough. I'd suggest that you buy a front mech and offer it up; if it is wrong you can always send it back to CRC or whatever. Shimano mechs have published specifications; I don't expect you to have very great difficulty in finding a mech to suit your application.
cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: braze on mech and crank sizes
I am going to use the Krust this week and then see what it looks like. I am guessing the brown stains will go blue/ black and then either a lacquer paint or helicopter tape.
On the gearing, I am going to have to find some compact cranks with 170mm arms that are campag, so I can use the existing BB. I am not sure I have all the tools for a total change.
I am going to measure tonight to see if I can get away with a 50/34 (not my favorite gearing as I quite like a 36-39 chainring for 95%) of my riding
On the gearing, I am going to have to find some compact cranks with 170mm arms that are campag, so I can use the existing BB. I am not sure I have all the tools for a total change.
I am going to measure tonight to see if I can get away with a 50/34 (not my favorite gearing as I quite like a 36-39 chainring for 95%) of my riding