Bicycle Mechanics - road derailleur/MTB bottom braket compatibility

Hi;

I've been running a single speed MTB rear hub, with a single chain ring on a road crank on a 113 width ISIS bottom braket. So far so good.

I have decided to replace the chain ring guard with a second chain ring. I'd hoped that a Shimano LX front derailleur (for MTB triple) I had in the parts bin would do for shifting between the two rings. Of course it doesn't.

Now my question is: If I buy the road front derailleur designed for shifting between the two rings up front, will it have enough travel to work with the 113 width BB? Or am I going to get another surprise?

Thanks in advance. DanO

Hi;

I've been running a single speed MTB rear hub, with a single chain ring on a road crank on a 113 width ISIS bottom braket. So far so good.

I have decided to replace the chain ring guard with a second chain ring. I'd hoped that a Shimano LX front derailleur (for MTB triple) I had in the parts bin would do for shifting between the two rings. Of course it doesn't.

Now my question is: If I buy the road front derailleur designed for shifting between the two rings up front, will it have enough travel to work with the 113 width BB? Or am I going to get another surprise?

Thanks in advance. DanO
Ok so this is a single speed at present correct? What are you using for a chain tensioner? Or are you simply using horizontal dropouts? Without a tensioner to take up the slack you won't be able to run a double up front as the chain won't allow for it.

Ok so this is a single speed at present correct? What are you using for a chain tensioner? Or are you simply using horizontal dropouts? Without a tensioner to take up the slack you won't be able to run a double up front as the chain won't allow for it.Besides that , what's the issue with the derailer that doesn't work?

I am using a Melvin tensioner by Paul's components. It routes the chain the same way a rear derailleur does, but is constructed much lighter since it doesn't have to actually shift gears. It has enough take-up to handle a twenty tooth spread between front chainrings. It worked great on a MTB 3x1 Drive train which featured a 16 rear and a 28/38/48 front.

The problem with the MTB triple front derailleur I first tried is that the cage is too wide. By the time the inner side of the cage is pushing the chain onto the big ring, the outer side of the cage is making contact with the drive side crank arm.

I am supposing that a derailleur engineered for only two chainrings (either MTB or Road) would be narrower, and thus would not grate up against my expensive carbon crank - OUCH!

I just don't know if a 2 ring road derailleur might also be designed to work with a narrower chain line - ie, a BB width of say 108, as opposed to the MTB bb I'm now using which is a 113.

So my question is, before I order up a road derailleur, is if it will push the chain out far enough to shift onto the big ring.

Hope that helps you... help me. Thanks again. DanO

I am using a Melvin tensioner by Paul's components. It routes the chain the same way a rear derailleur does, but is constructed much lighter since it doesn't have to actually shift gears. It has enough take-up to handle a twenty tooth spread between front chainrings. It worked great on a MTB 3x1 Drive train which featured a 16 rear and a 28/38/48 front.

The problem with the MTB triple front derailleur I first tried is that the cage is too wide. By the time the inner side of the cage is pushing the chain onto the big ring, the outer side of the cage is making contact with the drive side crank arm.

I am supposing that a derailleur engineered for only two chainrings (either MTB or Road) would be narrower, and thus would not grate up against my expensive carbon crank - OUCH!

I just don't know if a 2 ring road derailleur might also be designed to work with a narrower chain line - ie, a BB width of say 108, as opposed to the MTB bb I'm now using which is a 113.

So my question is, before I order up a road derailleur, is if it will push the chain out far enough to shift onto the big ring.

Hope that helps you... help me. Thanks again. DanOThink about this. Pretend there is no granny ring on a triple crank. Does that triple derailer now know it's shifting only a double? Your problem is somewhere else.

I'm thinking that the arc on the cage of a road derailleur will match your road chain rings better and solve your problem, but I've never tried it so I can't say if you will run into a different problem.

I'm thinking that the arc on the cage of a road derailleur will match your road chain rings better and solve your problem, but I've never tried it so I can't say if you will run into a different problem.Good catch on the road ring, and true enough, especially if he's trying to use a FD designed for a mtb crank with 44 and smaller rings.