osiris419

I'm building up an old road bike and I have a 6 speed 13-21t freewheel that I'm going to use. I was just wondering what you folks would think a good chainring set-up would be up front? I have a couple of cranksets that I could use a 130 BCD and a 110 BCD both doubles. Any input would help I'm kind of a noob with multi-speeds.
Thanks

hamster

What's the intended use of the bike? 13-21t is very narrow and, no matter what, you will end up with a bike that is seriously deficient at one or the other end of gear range. In fact, with 21t low, it will be a very poor hill climber regardless of your choice of front. If you designate it as a flatland bike, 130 BCD 39-53 in front should work for you. If you want to climb hills, you may need to rethink your cassette choice. (A low of at least 34/25, lower than that if possible, is highly recommended for hills.)

osiris419

Thanks. It's going to be a commuter, beater, something to ride when I don't want to take out my Allez, mainly flats I don't have huge climbs around me. I know its a pretty narrow freewheel but its what I had on the wheel I got. I could put a wider freewheel on I have a few kicking around but they are all 5 speed.

hamster

For a commuter, I'd say lower is better. Get a 110 BCD 34-50, it will be a little easier to spin out but more accommodating at the low end.

LesterOfPuppets

I rolled 42/52 x 13-21 when I was young and dumb. Would do again only if I lived in Florida.

Drew Eckhardt 

For flat ground 53/52/50-39 will be fine. 50-34 x 13-21 won't have any overlap between rings, the ends are in a comfortable cruising range, and you're not going to be happy. I rarely want anything bigger than 39x14 or smaller than 39x21 commuting.

For hills you want 53/52/50-42/40/39-30 unless you need lower gears in which case a 28 or 26 would be better; although 30x21 is a bit smaller than the 42x28 we all use to ride in the hills and like a contemporary 39x27 or 34x24 which are ample for fit riders on most paved roads like those in the Rocky Mountains (apart from Magnolia Road and the other Mike Horgan route where they work but you'd do better with lower gears).

Drew Eckhardt 

I think that's a horrible idea. Wide rings do not mate well to narrow cogs. I tried 50-34 x 13-23 9 cogs because it provided the same range and spacing as the 50-40-30 x 13-21 8 cogs I rode in that era and 2 rings are better than 3.... front shifts went from infrequent to constant with small bumps or changes in wind.

50x21 is the same as 34x14
34x13 is the same as 50x19

Eschewing the fully cross-chained combinations there's no overlap between rings and even if you don't do that there's still going to be a _lot_ of front shifting.

NMBuff

How strong of a rider are you and how much do you weigh? I'd assume Massachusetts is flat, but I could be wrong.

hamster

Not that 53-39 is a huge improvement. 50-34 x 13-21 has an overlap of 1 gear (this being a 6sp, it's probably 13-14-15-17-19-21), and 53-39 x 13-21 has an overlap of 2 gears.

In terms of speed, at 90 rpm and likewise excluding cross-chaining, 50-34 would result in the low ring ending at 17.1 mph and the high ring beginning at 18.6 mph, 53-39 would be 19.7 and 19.7. We are looking at a lot of front shifting in either case. If 53-39 is better at all, it's mainly because it has gears shifted to the point where the commuter has no real need for 53 anyway.

A good compromise could be 46-36, with the small ring ending at 18.1 mph and the big ring starting at 17.1 mph @ 90 rpm (or 14.2 mph @ 75 rpm.) That way flatland riding could be done while staying mostly in the big ring.

hueyhoolihan

i don't know what kind of rider you are, so i'll say what i would do.

i'd put a 4o-45t ring on there. BCD doesn't really matter to me. then i'd ditch the front derailleur and center the chain with the better of the inside or outside chainring positions on the crank spyder.

JimPz

You *could* run into one other problem, you are talking 5 & 6 speed freewheels, at least with the compact crank, I take it your talking 9/110 speed cranks.. You need a wider chain for the rear & a narrower one for the fron\t. You may not get good shifting with this. The suggestion above may be your best bet - a single ring.