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Long story why but I need to replace a front derailleur on my MTB.

Problem I have is I got the bike with the front derailleur missing so I am not sure what I need to replace exactly.

The path of the cable that goes down to the derailleur seems to go under the bike so I am guessing it needs to be a bottom pull.

Bike has 6 gears at the back that are working from a old Shimano SIS thumb trigger and 3 at the front.

Is this a simple fix or am I better of taking it to my local bike shop?

Thanks

  • Can you identify the shifters? This usually determines what derailers will work. Photos would help. – Will Vousden Sep 7 '16 at 12:30
  • Unless you have a box full of old derailers laying around, take it to a bike shop. – Daniel R Hicks Sep 7 '16 at 12:30
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    Look for a bike shop that deals in used bikes or see if there is a bike co-op in your area. They often have a box full of old derailleurs lying around. – Kibbee Sep 7 '16 at 12:44
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    A FD being a crude piece of work, any 3x(6-7) derailleur will be fine, meaning anything older than 15 years and still in shape. Look at a co-op. – Carel Sep 7 '16 at 17:11
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    The permissions on that file don't seem to allow the public to view it. – Adam Rice Sep 5 '17 at 14:11
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If you are not sure what you need, take it to the bike shop. Front derailleurs are complicated! You need the derailleur that fits your bike design and the gears that are installed on that bike.

There are five things to look at: attaching it to the bike, how many chainrings it needs to handle, the space it has to maneuver, whether the front shifter is indexed (sometimes it was only the rear) and how the FD is connected to the shifter.

The attachment for MTBs is just 2 ways, an E-type (which is the fairly rare bottom bracket mount) or a band clamp around the seat tube. You need the right band size (the clamp) for your seat tube - old steel frame MTBs usually had 28.6, or if it's aluminum, it's usually a 34.9. This number is the diameter of the seat tube, in millimeters - you need to measure.

Most of the older MTBs were built with triple chainrings, so most MTB front derailleurs will support that (you don't want a narrow cage double or one destined for the road, though it would probably fit the space).

If the shifter is not indexed you will have a much easier time, if it is indexed, then you will need to be more particular in picking a FD - one that matches your front shifter.

Now it gets complicated...there are top swing and bottom swing FDs. You need to be able to clamp the derailleur to the seat tube at the right height above the chainrings, but it can't be so low that it hits the chainstay when it swings to move the chain. This is the complicated part. Depending on the frame, and the size of your chainrings, either could fit. Older MTBs usually had bottom swing FDs. But there were interfering bottle bosses and weird high chainstays...who knows.

You have already said that the cable goes under the BB shell, so you will need a bottom pull FD. If it went under the top tube, then you would need a top pull FD. Shimano make a dual pull, so it could do both (very handy for cyclocross bikes!), but if you had one, you still need to choose how it swings.

Given the info you have provided, I would expect you to look for a Shimano 28.6, bottom swing, bottom pull...something like the FD-M650, which is a 90s era Deore DX. But I could easily be wrong :)

Good luck!

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  • Can you please explain more what do you mean at 'bottom and top swing'? – Alexander Sep 7 '16 at 19:18
  • Bottom Pull means the wire goes downwards toward the bottom bracket when it exits the FD mech as opposed to going upwards via the seat tube. – Criggie Sep 8 '16 at 1:38
  • Top Swing is also known as Low Clamp, meaning the construction of the mech and where it clamps to the seat post. By comparison, Bottom Swing is the same as High Clamp and hangs a bit higher on the seat tube. You can also get Direct Mount which uses a special braze-on mount common on aero/non-round seat tubes, or some whacky ones that go on the bottom bracket. – Criggie Sep 8 '16 at 1:44
  • Example of a bottom swing is Shimano FD-M611 Example of a top swing is Shimano FD-M615 Bottom bracket mount front mechs are E-Type and are used mostly for full suspension bikes. The Direct Mount FD is most often used for fat bikes, as the BB shell is so wide. I don't know of an MTB that has aero tubing :) – kellbell Sep 9 '16 at 13:27

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