Front Derailleur Problem

[Karakoram]

Hoping to tap into the combined wisdom of all the Hubbers out there.

 

I've recently bought a Momsen STR 29 steel 29er frame and transferred most of the components from my outgoing Silverback Segma 29er onto the new frame. The original Shimano front mech (model: fd-m770-10) worked perfectly on the old bike, but the cage seems too long for the new frame.

 

It is not possible to position the mech at the recommended minimum spacing over the big ring, without the bottom of the cage interfering on the chain stay. See image:

 

At the moment shifting performance is the best at this high position, but not very good at all. The chain often hunts when moving from the middle to the big ring. Adjusting the chain tension helps a little.

 

I run a 24/32/42 chain wheel and 10 speed 11-34 casette. The seat tube is 28.6mm and the angle between the seat tube and chain stay is 73 degrees and requires a bottom pull setup.

 

What replacement Shimano mech will work here?

Thanks for your advice in advance

[fanievb]

My bb has a spacer on the drive side (between the bb cup and the frame)

 

Maybe your new frame needs one

[Karakoram]

My bb has a spacer on the drive side (between the bb cup and the frame)

 

Maybe your new frame needs one

 

Thanks fanievb, but I need to get the derailleur in a lower position down the seat tube, towards the bb, not further away from the bb, if I understand the function of the spacer you describe correctly.

[Tankman]

 

The seat tube is 28.6mm and the angle between the seat tube and chain stay is 73 degrees and requires a bottom pull setup.

 

 

Is the that a top clamp or bottom clamp deraileur you have on there currently?

[Karakoram]

Is the that a top clamp or bottom clamp deraileur you have on there currently?

 

Hi Tankman. It looks like a bottom clamp to me. (Not being an expert, I could be wrong). Image here: http://www.jensonusa.com/Shimano-XT-FD-M770-10-Front-Derailleur

[Tankman]

Yup, that is bottom clamp.

 

A top clamp deraileur like this might solve your problem.

Most of them these days are dual pull (the cable can be pulled from either the top or the bottom)

 

But you should be able to mount it much lower with a top clamp.

 

Try and borrow one somewhere to test it ... before buying.

(I have one somewhere you are welcome to borrow ... but I cant find the bloody thing now)

 

[Karakoram]

 

 

Try and borrow one somewhere to test it ... before buying.

(I have one somewhere you are welcome to borrow ... but I cant find the bloody thing now)

 

 

Thanks a lot for the offer. I could also remove mine and bring it along to compare. (Stellenbosch?) The interference of the derailleur is at the bottom part of its cage against the chain stay, when selecting the smallest ring. The seat tube has plenty of mounting space for the derailleur clamp and there is lots of room to move it up or down.

 

I've written to Momsen with the same query and the response I got was that most of their customers build up their steel frames as single speed rigs, or with dual front ring set-ups. They recommended that I should also look at SRAM front derailleurs, which apparently would work with my XT shifters. Not exactly much help. I was hoping to get a derailleur model number that would fit the geometry and my drive setup.

 

The selection options on the web is overwhelming and the jargon not so clear for a novice at component sourcing. I've heard of short, long and medium cage derailleurs, but it is not obvious from the online spec-sheets which is which. Then there is a 'total teeth number' and seat tube angle just to confuse you even more.

 

I think a derailleur with a shorter cage (perhaps a medium) will work on my frame, but I am not sure if it is compatible with a triple chain-ring.

[Steven Knoetze (sk27)]

STR should only ever be built up as a SS.

Just saying........

 

But agree with tankman, top clamp and proper setup should solve the problems

[Tankman]

I think a derailleur with a shorter cage (perhaps a medium) will work on my frame, but I am not sure if it is compatible with a triple chain-ring.

 

A shorter cage would be for 2x10.

That won't work with a 3x10 setup.

The chain will be rubbing against the bottom of the cage while in the smallest chainring.

 

You can use a 3x10 (long) cage on a 2x10 setup but not the other way round.

 

No need to worry about the number of teeth on the chainring for now either. That only comes into play when using an E-type or direct mount derailleur.

 

Let me see if I can find that derailleur for you, work in Stellenbosch, live in Somerset.

[Karakoram]

A shorter cage would be for 2x10.

That won't work with a 3x10 setup.

The chain will be rubbing against the bottom of the cage while in the smallest chainring.

 

You can use a 3x10 (long) cage on a 2x10 setup but not the other way round.

 

No need to worry about the number of teeth on the chainring for now either. That only comes into play when using an E-type or direct mount derailleur.

 

Let me see if I can find that derailleur for you, work in Stellenbosch, live in Somerset.

 

Thanks a stack. I also live in Somerset West.

[Karakoram]

Problem Solved (I think)

 

I had a spare Shimano Acera M390 9 speed compatible front derailleur floating about in the garage. I thought, what the heck, and fitted it. The shifting seemed to be really sweet during the setup on the bike stand. I should get an opportunity to try out the mech's performance on the trail tomorrow.

 

The clearance between the replacement derailleur's clamp mechanism and the rear tyre is a bit smaller than the outgoing XT mech, so it is going to be a real crud catcher in winter. It was also possible to fit the Acera derailleur a little closer to the big chain wheel than the outgoing derailleur.

[johanpre44]

Just curious if your problem have been solved?

 

I'm busy getting all the parts together for my Momsen STR29 build, and will be running it Geared (not nearly strong enough for SS yet). I will be running 2x10 (which may be solving some of the problem you had with the bottom-clamp/top-swing front der). The LBS where I bought the frame suggested that I use a top-clamp/bottom-swing model because they last longer (much less play on the pivots). I'm slightly worried about if the clamp will be far enough form the bottle cage bolt so that I will still be able to use a bottle cage.