Gear equivalence

My friend is looking for a road bike, but can't decide if he wants a double or a triple. He's ridden my bike before, and with it being pretty hilly around here, he needs a good climbing gear. I took him to the steepest hill I know he would climb and had him get into a gear he felt comfortable with. He said it was on the smallest chainring on the front (30 teeth) and the third largest on the rear (21 teeth). So, to decide whether he wants a double or a triple, is there some sort of way to determine what the equivalent gear would be running a 39 tooth chainring in the front?

Do it the easy way ( english style ) result in inches .

........ 39 / 27 x 27 = 39 , 30 / 23 x 27 = 35·2 .............

it comes from the old pennyfarthing when the size of the wheel gave the gear so when gears came along (single speed at first of course ) it was convienient to continue with the same system , sort of , as everybody knows that 100 inches was long and 40 is short .

it´s gear ( chain ring ) devided by hub gear times wheel diameter( this is 26 or 27 normally ) result in inches : simple .

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' too old to rock 'n' roll : too young to die '

ps the answers a 27 , well almost .

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' too old to rock 'n' roll : too young to die '

Yeah, the exact same ratio would be given by a 27.3 tooth chainring (which obviously does not exist!) so a 27 would be the best bet. But I think you might struggle to find road bikes which come with 27-tooth sprockets as standard. Shimano do 12-27 cassettes but if you use one of these with a 53/39 chainset, that puts the total capacity required of the rear mech just outside the limit of Shimano's short cage mechs, and I haven't seen any double chainset road bikes that come with medium or long cage rear mechs. If your friend goes for Campag, they often do 53/39 chainsets with a 13-26 cassette, which gets him very close to the gear he needs (39x26 instead of 39x27 - there's not much in it) without the need to fiddle with the drivetrain. Of course, a 53x13 top gear is a little low if he's a serious racer - my road bike has a 53x13 top gear and although I'm only a recreational rider, I would like something a bit higher - I spin like a madman going downhill.

I've run 39/53 and 13-28 with short cage Shimano with no problems.

Yeah? I didn't know you could do that, the shimano website lists the short cage mechs' capacities as being 28 teeth, and the total capacity required for a 12-27 cassette with a 53/39 chainset is 29. But of course, maybe Shimano's figures are conservative. Can you use the extreme settings, i.e. small chainring, small sprocket and large chainring, large sprocket on your setup? (Of course this is kind of academic, those gears shouldn't really be used).

That is a barge load of hooey. A shimano SC works perfectly with a 53/39 x 27, and altho right at the limit for wrap and large cog spec, both specs are conservative and can be exceeded. The campy double mirage, veloce and centaur RDs are actually the medium cage that is recommended for a 29 cog.

The shimano SC wrap stated capacity is actually 29, and it's conservative.Do you think thay would actually sell a common combination of chainset/cogset that their dreailer could not handle? AFWIW you can fudge that by not being in the small /small where you shouldn't be anyway.

I stand corrected - Shimano's SC mechs are specced for a total capacity of 29 teeth, so a 53/39 chainset with a 12-27 cassette wouldn't exceed the spec of an SC mech.

FWIW, boudreaux, if someone makes a mistake it is possible to correct them politely - cut out the "barge load of hooey" talk.

Even if it was over 'spec' that does not mean it won't work. Spec is almost always conservative and some of them can be fudged. Blindly quoting spec without really knowing is the hooey part,and I always call a barge a barge.

If you look at my post you will see that I was simply observing that a 12-27 cassette plus a 53/39 chainset would put a mech specced for a capacity of 28 teeth out of spec, and this is accurate information. I never said it wouldn't work. My mistake was getting the spec of the Shimano SC rear derailleurs wrong, and I have acknowledged this. I just thought the original poster might find it a helpful observation - as it turns out, he would in fact be running the mech within it's specifications. Good for him.

As to your comment about blindly quoting spec; following manufacturers' specifications "blindly" is often very good practice because failing to follow them often results in voided warranties, regardless of whether or not you can "fudge" things. This is a statement of general practice - I have not read Shimano's rear mech warranties, so don't even bother looking them up and quoting them at me.

Why doesn't your friend just get a triple? He could use a racing cluster with 23-11 cogs and have lots of room in both directions, and have very fine grained shifting to boot!

Well, both he and I really only use the smallest cog in that very special case of climbing, so if he could eliminate it by just going to a double, it would save a lot of trouble. I'll still recommend him to try the double and the triple though, since he is still relatively new.

This is the best advice you've gotten so far. Why stretch the limits of what a double will give you and then waste it on a terrible stretched out far spaced cassette. Just get a triple and then you can go with a nice close spaced ratio cassette like an 11-21 or 11-23 and have all the gears you need.

If you want to look at gear calculations go to Sheldon Brown's website:

http://www.sheldonbrown.com/gears/

Just punch in your tire size, crank length and chainring configuration. Change the 'Gear units' to MPH @ 80 RPM and the calculations will give you your speed in each gear at 80 RPM cadence. Of coarse you can change this to what you want.

A triple set with 30 up front and 21 at the back runs 8.9 MPH at 80 RPM. To get this on a double with 39 up front you need a 27 which gives you 9.0 MPH at 80 RPM. If you went with an 11-23 and a triple you would have an extra gear at the low end to spare and beat the pants off of any cassette that included a 27.

Remember that 11 is realy nice for screaming down the hill after youv'e climbed it.

HFHooey.Don't play the warranty card on me!