Bicycle Mechanics - Shimano 2303 FD - chainring compatibility
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01-11-13, 03:16 PM
I am currently setting up Shimano 2303 STI shifters (i.e. 3X8) on my old French Gitane touring bike. I have the rear shifting working using a Deore RD and an 8 speed cassette. However, the old Huret FD doesn’t work (no surprise) and I need an index FD that works with the STI. I was thinking of getting the Shimano 2303 front derailleur, simple and cheap. My chainrings are Stronglight 50/45/30T. But the 2303 FD specs state a minimum 10 tooth difference between the large and middle chainring. (I figure it has to do with the contours in the sides the derailleur cage that assist shifting) My Stronglight set up has a 5T difference (50-45). Will I need to replace my 45T chainring with a 40T or smaller chainring? Or will the 50/45/30T set work but maybe not as well as with the specified chainring sizes? The chainrings are 86 BCD, which are available, but not very common.
01-11-13, 03:54 PM
You may have to set the FDER higher than normal to clear the middle ring on its way to the small ring.
This would likely result in "less than pristine" shifting. Maybe worse?
Apples to pears, but I run a 22-32-36 on my Hybrid. The FDER is set WAY HIGH.
It shifts, but not wonderfully. I have to kind of 'ease" it up from middle to big.
Not a problem for me, since I spend about 98% on the middle.
Andrew R Stewart
01-11-13, 04:07 PM
With out actually testing a set up it's hard to say for sure how well the new ft der might shift. But here's a few comments any way.
The ft der likely has a inner plate height designed for the 10T difference. For the outer plate to clear the top of the big ring and still have the inner plate clear the middle ring. The 10t is not written in stone but only a 5t difference is a lot of variance from the spec.
The modern dirvetrains are designed around a shift gate/lift pin technology. So the amount of overshift the lever gives the der assumes that the chain will be grabbing a lift pin. Additional to this aspect of good ft shifting is that the der movement and cage shape is designed around specific ring to ring spacing. Some of the old cranks had much larger ring to ring spacing. So besides the possibility of the der not moving far enough to have the chain engage the next ring there may very well be the chance of chain rub after a shift. Remember that the cheap Shimano ft levers don't have much trim capicity.
You can modify the rings with filing down the tops of a couple teeth at a few vpoints around the ring. Shimano use to call this "W" cut back in the early 1980s. I have done this to some of my non pinned rings and have gotten better shifting response. Only about 1/4 of the tooth's height need be removed. I did 5 pairs roughly aligned with the crank's arms.
You can also change the cable pull/der movement ratio by attaching the cable on the underside of the der's anchor bolt. Shimano used to need this when friction frame mounted levers were coexisting with the early STI levers.
Two ultimate soultions are go to bar end levers which use friction front. And/or replace the crankset with a current one. Andy.
01-12-13, 02:27 PM
Thanks Andy. You've given me much advice and much to think about. The bike is on my trainer where I don't need to shift the FD. So I have a few months til spring when I will want to put it in the road. I will give an indexed 3 speed FD a try. I it doesn't work you've given me some good options.
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