Classic & Vintage - Front Chainring ratios, 6sp rear
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08-15-12, 11:00 AM
I am working on a city bike that can also do trails occasionally as I have quite a few dirt/loose gravel paths that are great scenery for casual cycling..
My rear wheel is a 6sp shimano 105 hub so my rear ratios are sorta stuck between 12-21.. and as much as I'd like to make a climber with 12-28 or something like that, i figured changing the chainrings is easier/more economical (unless I stumble upon a rear wheel with a better config)
I picked up a 34t front chainring and went 42/34t on the front and this felt ok with a bit more to climb with, but I thought the spread would be more dramatic.. but instead there is a lot of overlap ratios - not sure if this is really a bad thing, but I'll ride with it as it for a few days and see how it goes.. I want to go easy on my knees and I don't mind being speed limited on the flats as this is my upright-ish, fendered/luggage bike..
If I find some downhill, I will prob throw on the 52t and run the 34t inside and see how that goes, but I see a lot of mtb cranks do a 42/34/28... which i find a bit odd since my experience is that the 42t doesn't gain much..
is a 46-48t going to gain me anything?
08-15-12, 05:28 PM
Not enough info; clarification needed. Is your front a double or triple? If double, you'll get the most bang for your buck by upgrading to a triple. You can get low end MTB take-offs pretty cheap (<$30, with chainrings) on Ebay all day long. If you want something nicer, well, you're going to have to pay a little more.
Judicious selection of chainring tooth counts will yield you a wide range with close ratios and minimal duplication.
08-15-12, 07:41 PM
I've seen a bunch of 'new' SR SunTour Superbe triple cranksets on eBay of late. 52/42/30. Going rate is $25 or so...
I upgraded my old Fuji from the original Sugino Maxy 51/39 to a Sakae triple 20 years ago, but I just bought one of these Superbes for a spare. My rear freewheel is an old Suntour 13-21 six-speed (I think - it might be a 13-24) Around here, I find myself primarily leaving it on the 42T front chainring and using mostly the middle gears on the rear. (Then again, I'm old and slow ;) .) I guess I average 16-17mph or so. If I were to do more riding in hilly terrain, things might be different.
A road triple with 52/42/30 will get you a 2.88 gain ratio low, which is pretty low. Of course I'm suffering with a 3.53 gain ratio (42-24) so I'd kill for a 2.88 sometimes. Still, its not super low. Not mountain bike low. And road triple cranks cost more than MTB triples.
A MTB triple 42/36/28 (let's say) will give you a 2.31 gain ratio low, which I'd consider low enough for anything short of heavily loaded touring up a serious climb and what the heck would anyone do that for . . .
You might need a new front dérailleur but used MTB FDs are cheap. Hopefully your shifter will pull enough cable. You might ideally want a long cage rear dérailleur too, but I'd simply remember to stay out of the little-little combo. The MTB triple may put your pedals further apart but who cares.
I don't think there is too much overlap. Half of the small chain ring range is to be avoided anyway (little-little). You'll probably spend all your time in the big ring on the flat, the middle ring in the hills, and bail to the little ring when the going gets tough and the tough look for granny.
08-15-12, 08:52 PM
Check out this classic article by the Great Guru of Gearing, Frank Berto:
Assuming your tooth progression is 12-13-15-17-19-21, you probably want something like 41-34 in front to obtain a 1.5-step progression. (I run 50-42/14-16-18-20-23-26 on the Bianchi and 46-38/13-15-17-19-21-23 on Capo #2.) This would give you 92 gear-inches on top, about the same as an English 3-speed, and 44 on the bottom, which is about what I run on all of my road bikes.
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