Author Topic: Does a smaller outer chainring mean new front deralliuer?  (Read 1080 times)

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Offline Smudgy

Does a smaller outer chainring mean new front deralliuer?
« on: November 27, 2013, 10:01:29 pm »
I changed from a 28/47/52 to 28/40/47(stronglight 86 bcd). I simply put in a new middle ring and put the middle ring on the outside. The problem is that now it doesn't want to shift to that outer ring. Is it the FD? Is it the ring? Will lowering the FD on the down tube solve the problem?  Help please.

Offline DaveB

Re: Does a smaller outer chainring mean new front deralliuer?
« Reply #1 on: November 28, 2013, 09:34:39 am »
If you left the front derailleur high enough to clear the 52T big ring then, yes, that will make shifting to (and from) the replacement 47T ring worse.  The general rule is that the outer cage plate of the front derailleur should clear the big chainring's teeth by only about 2-3 mm.  You shouldn't need a new fd but should reposition your current one. 

The other problem is that newer chainrings have ramps, pins and shaped teeth to improve shifting and these enhancements are position-specific, that is the outer chainring will have different shifting aids than a middle chainring.  When you put a former middle ring on the outside, you lost the specific aids and they may, indeed, work against good shifting.  I don't know the vintage of your Stronglight crank but if it has enhanced chainrings, that could easily contribute to your problem.

Finally, chainrings made for outer position use often have a pin projecting to the outside to keep a spilled chain from jamming between it and the crank arm.  If you use a middle position ring, you won't have that protection unless your crank arm has the pin built into it.
« Last Edit: November 28, 2013, 09:37:12 am by DaveB »

Offline Smudgy

Re: Does a smaller outer chainring mean new front deralliuer?
« Reply #2 on: December 02, 2013, 09:28:09 pm »
DaveB,
Thanks for the reply. The crank and rings are vintage 1981, old school. I don't think there are any ramps or pins. Both surfaces of both rings, the 47t and the 52t, look the same, smooth and unremarkable. I will try lowering the FD, if that doesn't clear it up I will go back to he big outer ring. A 28/40/52 triple still gives me better choices in the mid range. That 52t is still good for long flat stretches with a stiff tailwind. As a matter of fact, it came in handy this past summer when out running a bad storm in Nevada. Thanks again.
Smugdy

Offline RussSeaton

Re: Does a smaller outer chainring mean new front deralliuer?
« Reply #3 on: December 03, 2013, 10:56:41 pm »
Lower the front derailleur so it just barely clears the outer chainring.  Newer chainrings for the outer and middle position will have ramps and pins designed to help move the chain between the rings.  These help and shift better than smooth plain rings.  Front derailleurs are generally curved so they follow the curvature of the outer chainring.  Your front derailleur has an outer cage that more or less follows the curve of the old 52 ring.  It will have a consistent couple millimeter gap between the cage on the front derailleur and the outer ring from the front to the back.  Same 2mm gap between front derailleur cage and ring all the way.  Now it will have a bigger gap at the tail end of the front derailleur.  I've not found this to matter much.  I have a Shimano Tiagra triple front derailleur on the touring bike.  It was designed to shift a 52-39-30 triple crankset.  I use it on a triple crank with 44-33-20 rings.  The bottom tail end of the front derailleur is about 1 inch from the chainring.  The top front of the derailleur is 1mm from the ring.  Shifts fine with STI levers.  If using bar end shifters on the front derailleur, then you have even more control over shifting.  Bar end shifters allow you to force the chain over at anytime.

Offline DaveB

Re: Does a smaller outer chainring mean new front deralliuer?
« Reply #4 on: December 04, 2013, 09:06:29 am »
As RussSeaton noted, a front derailleur intended for a 52T or 53T big chainring will work quite well with a smaller outer chainring if you adjust it so the front of the cage is close to the teeth of the chainring.  In fact, in the late '90's  bikes with Shimano's RSX group came standard with 46/36 or 46/36/26 chainrings and an RX-100 front derailleur designed for a 52/39 or 52/42/30 crank and it worked just fine.

Your old-school chainrings lack the newer shifting enhancements so they can be placed in any position on the crank.  So put the middle ring outside and lower the fd and you should be good. The only problem may be the lack of a chain catching pin on the outer chainring.   

That said, have you ever ridden a bike with modern chainrings?  The improvement is shifting, particularly under load, is amazing. 
« Last Edit: December 04, 2013, 11:52:17 am by DaveB »

Offline RussSeaton

Re: Does a smaller outer chainring mean new front deralliuer?
« Reply #5 on: December 04, 2013, 04:26:07 pm »

Offline DaveB

Re: Does a smaller outer chainring mean new front deralliuer?
« Reply #6 on: December 04, 2013, 04:52:50 pm »
Yes, I understood the age and limitations of that crank.  Frankly I'm also surprised new chainrings are still made for it, particularly by someone other than Stronglight.   

My question about having ridden a newer crank wasn't to recommend new-style replacement rings for his current crank but to ask if he was aware of the improvements since his Stronglight was made.  It implied a totally new crank might be worth considering.

Offline Smudgy

Re: Does a smaller outer chainring mean new front deralliuer?
« Reply #7 on: December 04, 2013, 08:17:52 pm »
I ride an older miyata 1000 which had a stronglight crank with the 86bcd. They are very hard to find. I upgraded to a sugino crank with 110bcd outer and middle, and 76bcd inner a couple of years ago. That one is easy to find replacement parts for. I recently acquired another old miyata 1000 with the old style crank. Just for grins I tried to find a new middle chainring rather than change the entire crank set. I found one on ebay from Poland. I love the old bikes, but the modern components are nice. On my original miyata I've upgraded where possible. On my new 'old' miyata I'm trying to keep it vintage. It's really a cool bike and a fun winter project.

Thanks for the help.