sci_femme

I am high-speed long distance commuter - 15 mi one way, and I ride Tricross Comp with 48-34 and 11-28. I live in Florida and the only place flatter than Florida is probably Indiana. I plant my chain in 48 ring and just sweep up and down the cassette depending on speed, traffic, wind, etc. My average speed is 15-17 mph, and I can cruise pushing 20 for coupla miles.
FSA is a decent crankset.

Good Luck

SF

8Ring

The advantage of a standard compact crank (50 and 34 teeth) is that you only have to do one double shift (shift both the rear and front derailleurs) at the same time in order to get a smooth progression through all the available gear ratios. If you have a ten speed casette, as a practical matter, you can only use eight speeds on either the small or big chainrings due to cross-chain problems.

You didn't say what rear casette you have, but this example will work with a 10 speed 12-23, 12-25, or 12-27. The "lowest" you can go on the big (50 tooth chainring) is 50 x 21 or a ration of 2.38. The "highest" you can go on the small (34 tooth) chainring is 34 x 14 or 2.42. 34 x 15 = 2.266. Therefore, if you are shifting up through the gears, you should go to the big chainring (50 x 21 =2.38) after 34 x 15 (2.266)

If you change to a 48 tooth front chainring, the 48 x 21 ratio will be 2.285 or almost identical to the 34 x 15. However, the next ratio (48 x 19) is 2.526 which is a big jump from 2.266. So essentially, you lose the 48 x 21 gear. On the top end, you will lose some top speed because a 50 x 12 = 4.166 while a 50x 48 = 4.

So, if you have a 10 speed cassette, a 48 x 34 will lose you a gear in the middle and will lose speed at the top end with a standard 10 speed casette. If you have a different cassette you'lll just have to do the math.

Dad

rm -rf

I don't think it'll be much of a problem. See the chart on post #9 above. If the rider is planning to stay below 16-17 mph, they would use the 34 chainring. If the road looks favorable to ride at above 15 mph, then the 48 or 50 is good. The 48 actually keeps the cog combinations closer together than a 50 chainring. I don't always run my small chainring right up to the second to smallest cog before shifting to the big chainring. And if I'm going to be slowing back down soon, I'll cross chain the small cog and the 34 chainring to get a temporary speed boost without having to shift the front.

For a top end speed, the 48-12 is faster than my 50-13 high gear, and I can spin my 50-13 up to about 33 mph (on the downhills, of course).

Phil85207

Very interesting read. I have been thinking of making some gearing changes as I am getting a little long in the tooth and looking for someway to compensate for dragging teeth on the ground.

dizzy101

Try 50-36, it's a good combo (better than 50-34 IMO).

clink83

It must be winter if people are debating over which compact crank is better for flat land riding. Sheesh. It's really simple: Get a 50/34 or a 53/39 and stop overthinking it. Don't go with some weird combo that any LBS doesn't carry replacement chainrings for.

datlas 

Lookit, the difference between 48 and 50 up front is like 4% so the difference is minimal.

Based on what the OP said, if he is a recreational rider who is cruising around at 14MPH he would probably be better served by the 48 than the 50, but it probably matters little....get whichever one is cheaper or more readily available.

Nogyro

I went through this same scenario about a year ago. I also had the 50/34 with a 12/25 and most of my rides are on gently rolling to flat roads with an avg. speed of 16 to 17 mph. I was always in the middle between cross chaining on either of the front sprockets. I did my homework on Sheldon's site with his gear charts and also came up with dropping to a 48 would only be a 4% drop. I took a chance and dropped down to a 46. That put me in the 4th sprocket down when running 16 mph or so at 80-85 cadence, right were I wanted to be, very little cross chaining. I've never spun out even going down hill, and the shifts from small to big in the front are very smooth.

icyclist 

My two cents worth, after looking at the Sheldon Brown chart, and owning a variety of similar set-ups:

It doesn't make any difference, except for losing a gear off the top end, which chainring you use, 50 or 48.

To ensure a smooth progression of gear changes with a 12-27 cassette, the jump from the small chain ring (34) to a 48 involves at three shifts up the range of cogs on the back. The same is true in reverse.

The transition is a little closer with the 50, but takes four shifts on the back.

It gets worse with a tighter cassette, because it takes about five shifts on the back to make a smooth transition.

But making smooth transitions, while efficient, and important to racers, don't matter that much for most riders, who will shift from one chainring to the other and maybe shift a couple of gears in the back.

So, again, it doesn't matter which chainring, 50 or 48, is used.

fogrider

I've ridden a 48/34 with 12x25, had to go with a 11x25 but I really liked the 48. with a 48, I can keep it in the big chainring on the flats and rollers and just shift the rear. I spunout the 12 but with an 11 cog, I was able to get up t 42 mph. I also have bikes with 50/34 and 53/38 cranks...the right bike for the right ride...