geezerwheels

I have a cherry Motobecane Grand Touring, and I'm bidding on a MB triple crank...any guess whether this dbl FD can be stretched to handle a triple? Manipulating it by hand, it looks like it just might make it...

here's the crank:


and here's the FD:
VeloBase.com - Component: SunTour SL

if not, any suggestions for a bottom pull? (back end is a 6sp accushift FW)

dsbrantjr

You can try it and see but derailleur cages are generally shaped for a particular chain ring count and size range. If it doesn't work you can get one that does, derailleurs are relatively cheap.


You may also (quite likely) find that you need a different bottom bracket.


What kind of shifters are you going to be using?

fietsbob

It just has to swing 33 % further . need a different BB axle ,, Sekai is JIS like Shimano
52-44 is a Half step & so a Lot of Triple FD wont ,work , they require 10t difference ..

You could remove the 44 and just use a 52 36 as a double .. I used to .

JohnDThompson 

The SunTour FD should have sufficient swing to cover all three rings but as noted you'll likely need a longer BB spindle; another problem you may run into is the cage dragging on the inner ring. These vintage FDs work best with "half-step plus granny" type gearing, e.g. 50-45-28 than with evenly spaced jumps between rings.

Andrew R Stewart 

Yes, WRT the range of movement, sometimes. But the actual shifting can suffer. This is where the rider's technique can also contribute to positive or frustrating results. Andy

79pmooney

It is rated for a 16 tooth difference. 52-36 is 16 teeth. You may have to get a longer BB axle to move the chainrings out so the derailleur can reach the inner ring (which will also improve your chainlines). I used to do this with several SunTour mid-70s FDs though I never had an SL.

I'd try it. Worst is that it doesn't work. Nothing lost. My cranksets were all 52-42-28 or very similar. With the 28, the chain often did drag on the cage roller in small front, smallish rear combos but outside the noise, it never mattered and I never spent a lot of time in those combos anyway.

Expect to have to do several go-rounds of guessing a best derailleur height and rotational orientation to get the best shifting over the three rings, each time adjusting the inner and outer set screws and cable tension. Front derailleurs are crude devices. Think pipe wrench, not dentist drill. I would concentrate on getting the inner to middle shift working both up an down while making sure the middle to big is possible. Once that is done, you can often improve the upper shifts by bending the forward ends of the cage in toward the chain with a crescent wrench. (This was easy on pre CPSC derailleurs of the early to mid '70s. Those tabs disappeared once they became "dangerous" as protruding metal outlined by the new CPSC standards. (Deaths from front derailleur tabs were never recorded but there is speculation that a sick enough person could figure out a way to manage it. ) Prior to that, bending those tabs was common practice setting up new bikes.

Ben

Retro Grouch 

What are you using for a shifter?

Back in the olden days we didn't know there might be any difference between derailleurs used for double or triple cranksets. They pretty much all worked (except for that awful Simplex thingie). The key was friction front shifters. Friction shifting will cover up a lot of other issues.

fietsbob

I Looked at an enlargement Noted: swaged on arm to spider. So its rather overpriced

IMO, for what is being asked [Link: Next bid $62+], just to get the MB badge on it.

Kaze6

I was one of the first to put a 3rd chainwheel on, back in the mid 1970s (after reading an article in one of the magazines). I modified a standard Sugino crankset, and the Suntour FD adjusted with no problems.

bulldog1935

Older low-Q triples were intended to run with the double derailleurs of their day. What we call triple FD today came from mountain bikes.
Here's a Shimano 600 EX FD covering 46T down to 26T.
This particular crank requires a 122mm asymmetric BB spindle.

geezerwheels

thanks for the feedback...will be using shimano barcons...and a vintage accushift RD

geezerwheels

good point...then again, it's not like I have legs like Gino Bartali...

fietsbob

dont climb any hills either

Yan 

You'll need a new bottom bracket to change from a double to a triple. Your derailleur should work as long as you're using friction shifting.

Bike Gremlin

Longer BB spindle to have the correct chainline (and enable the FD to reach the smallest chainring). Just make sure it's not too long - so it can swing to the largest chainring as well.

Also, if the FD has cage shaped for say 50sh big chainring, and you're planning on using something smaller than 48-38-28 (like 44-34-24), shifting will suffer.

Detailed explanation of (cross)compatibility and what to look out for I put on this page:
5) Compatibility - front derailleurs - Cycle Gremlin