Tips on Modifying Front Derailleurs to Handle Extreme Gear Range

Anatomy of a front derailleur

  • clamp -  attaches derailleur to the frame
  • arms - support and pivot the cage
  • cage - guides the chain
  • outer plate - pulls the chain in on a downshift
  • inner plate - pushes the chain up on an upshift
  • inner nose -  makes the chain climb onto the teeth of the outer chainring
  • outer nose - derails chain off of outer chainring
  • high stop - limits outward travel of the cage
  • low stop - limits inward travel of the cage
  • bolt - allows the derailleur to be removed without breaking the chain

It is difficult to make a 'one size fits all' derailleur.  Shimano's S.I.S. concept has let them develop drive trains that are optimized for one set of gears.  This has provided an optimal solution for the hardware while pushing the 'one size fits all' onto the rider in terms of gearing.  Since bikes are designed for the mass market and driven primarily by the racing demands, those who don't fit that predefined criteria are left with few options.  Because they are not part of that mass market, developers are reluctant to invest the time and money into developing specialized products for them.  Sometimes it just takes someone with a lot of passion and patience to preserver until an idea catches on.  Recumbents and hand cycles are a good example of that.  So until I have time to develop a set of super wide range derailleurs, let's step back to the early days of mountain biking and do what we did then; start with the closest part available and modify it to meet the need.  I'll give you the critical information you need to get started.  Your first attempt doesn't have to look like a show piece; you just want it to work.  Once you've accomplished that you can work on making it pretty.  If you get really good at it and are really ambitious you may develop it into a viable product that you can market or sell to someone else and get rich off of all the royalties.  (If only it was that easy)

The good thing is that almost all triple derailleurs have enough travel to handle four chainrings as long as the crankset is in the proper position.
The basic problem is that derailleur cages are not designed to handle very large and very small chainrings at the same time.
Raising the derailleur to clear a large outer chainring means the back of the cage is also raised and when you try to run a small triple or quad the chain may drag on the bottom of the cage.
Sometimes  you can cut the outer plate to match the outer chainring exactly, which will let you drop the derailleur some.  For example if the derailleur is designed to handle 52 teeth and you outer ring is only 46, you have quite a bit to play with.

The next option is to extend the bottom end of the cage.

In extreme cases, I've made the entire cage from scratch.

If you want to run 5 chainrings, you will probably have to modify the stops or make new, longer arms.

Key Points you would ideally like to achieve:
  • the outer plate should clear the teeth of the large chainring by ~ 1mm
  • the inner plate should just clear the middle (2nd largest) chainring
  • the cage should be stiff enough that it doesn't flex while shifting
  • the cage should lift the chain more than push it
  • the chain must not rub on the sides of the cage once the shift is completed
  • the chain must not rub on the tail of the cage
  • the top of the chain must not rub on the top of the cage
  • the tail of the cage should not touch the chainstay
  • the tail of the cage must not hit the tire (keep in mind the widest tire you want to use)
  • on suspended bikes, the cage cannot hit any part of the frame through the entire range of the suspension
Sound complicated?  It's really not that bad.  Most of this will already be taken care of - just points to consider.

Derailleurs come in several different flavors:
  • different mounting styles
  • different seat tube diameters
  • bottom pull/top pull
  • road/mountain  (wide range/close range)

wide range close range
Obviously the first  three have to match you bike.  It's the last one, we need to address here.
If the two outer chainrings are close sizes to each other (half step), the inner plate of the cage can not be wide, or it will hit the teeth of the second ring.  If you raise the derailleur high enough to clear it, the outer  plate will be to high.  Shifting will be poor and the chain could get stuck between the outer ring and the cage.

The Basic Process:      I'll add photos when I get a chance.
  • select a suitable wide range or close range derailleur to match your chainring selection
  • in some cases you can
  • cut the bottom of the cage
  • make a piece for the extension
  • solder an extension onto it
No way!  How are you going to solder to chrome?  That's impossible!!!
No problem.  In fact, it's really very easy, IF and
only if you have the right flux.  I use stay-clean flux from J.W. Harris Company with a regular rosin solder or a silver bearing solder.  It will do a beautiful job and won't damage the chrome.  It will also solder to stainless steel.

If you don't want to get into the soldering you can use an epoxy, such as JB Weld.  Don't use the cheap 5 minute stuff.  It's brittle and may not bond well.  Hysol makes some good ones.  If you go this route,  it would be advisable to secure it with a flat head screw as well.

Making the Extension
You'll need a small piece of metal sheet. 
I'd recommend .062" (1/16") stainless steel, though  you could use mild steel or brass.

If the derailleur cage is aluminum you will have to use aluminum. 
Use 6061 or 2024 and be very careful bending it or it will crack.
There are solder fluxes that work with aluminum also or you can use aluminum brazing rod which can be welded with a propane torch.  Practice before you try it on your derailleur. 

  • position the derailleur where you want it ignoring the bottom of the cage
  • make a template out  of thin (1/16") cardboard or plastic
  • the extension will go in the inside of the outer plate of the cage
  • trim it until you get the shape the way you want it
  • cut  a strip of metal to match the template
  • clean all edges with a file
  • drill a hole to match the inner plate bolt hole
  • bend the strip
  • pre-tin the surfaces to be soldered
  • keep the surfaces and soldering tip clean
  • use adequate flux
    • DO NOT breath the fumes
  • remove any flux after soldering is completed

An Alternative Method for Making the Extension.

You can use the bottom of the cage of one derailleur to modify another.

  • Choose a derailleur  to cannibalize that has a suitable section of the outer plate of the cage.  The rest of this derailleur will not be used so your junk box is a good place to look.
  • Using a hacksaw or dremel tool, cut the cage as shown here. 
  • Remove the screw.

  • On the derailleur you are modifying, cut the cage as shown here. 
  • Remove the screw and the little piece at the end.

  • It should look something like this.
  • Apply flux (described above) only where you want the solder to stick and pre-tin the surfaces. Make sure you have adequate heat.

  • Use and alligator clip or small clamp to hold the piece in place.
  • Solder the extention on the inside of the outer plate of the cage forming a lap joint.
  • Make sure you have a complete bond on all surfaces and edges.

Tip: Keep your soldering tip clean by quickly wiping it on a damp sponge.

  • Make a tie plate from a piece of aluminum, steel, or brass.
  • Tap on of the holes.
  • Use the original screws to hold it together.

Other Notes:
Be careful if you need to bend the derailleur cage.  Some use very hard steel which will crack if you bend it too far or too quickly.

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Half-step was very common place in the early days when freewheels only had 5 or 6 cogs.  With that set-up, if you wanted to shift through every gear (and you usually did) one would shift the front every time you shifted the rear.  This was the only way a wide gear range and close gear spacing could be achieved.
Today the ratios on the cassette are close enough and the chainrings are used as a range shift.  Many of the gears are duplicated.  There are ways to optimize the gears set for one's particular riding style.  We're back to that mass market - 'one size fits all' syndrome again.  An example would be to set your cassette spacing at 12% and using the half step to split it so you have 6% spacing when you want it.  On a mountain bike this usually isn't even an issue because the terrain changes constantly, but on a road bike and especially on tandems, it's really nice to have closely spaced gears when you want them. 
  If people are interested in this let me know and I'll do an article on it.