Hi all. Brand new to this forum. I ride a 2003 Trek 2300, with a standard crankset (53/39) and a 12-25t 9-speed cassette. Two weekends ago I did some mtn repeats with a couple others, for a total of 4600 feet of climbing. This is where I learned about compact cranks, since the other two guys had them, and were able to ride noticeably higher RPMs than me up that mtn.
So I ordered a 38t ring for the front (small improvement, but every tooth will help, and that's the smallest 130mm ring you can get), and a 12-27t cassette for the rear. They'll be here this week. We'll be doing a ride next weekend with around 6000 feet of ascent, and I'm looking forward to riding it with this new 38/27 ratio.
Well, all this new learning about gears and what hardware my bike has, and what's out there, got me thinking. On my 25-mile ride today I noticed that I was riding at my max comfortable cadence of 85 rpm (I am 6'3" and still weigh 270lbs, so my legs are beefier than usual cyclists) at around 20 mph, which is a very fast cruise for me (my typical 25-mile average speed is around 18mph), and I was in my 5th smallest gear! I changed to my 4th smallest gear and spun up to 85 rpm, and was going around 23 mph, which is far faster than I ever ride for a sustained period of time, unless I'm going downhill.
I'm thinking that my highest 4 gears are just wasted.
Bear with me here, I'm learning. I went to Sheldon Brown's gear calculator and started playing with different configurations.
I think a compact 50/34t crankset would probably be great for me, but I can't afford a whole new crankset right now. I already spent $56 on the new 38t ring and 12-27t cassette. I could probably afford another ring.
Can anyone explain why I shouldn't just buy a 48t ring for the front and compress the top gears down so that I'm riding in my 2nd or 3rd gear rather than my 5th smallest gear?
The thought hit me that although this stuff makes sense to me on paper, the fact that nobody sells bikes this way tells me there's probably a very good reason.
In fact, I was thinking a 48t/38t front and 13-27t cassette would probably be perfect for someone who rides in my speed range. There's gotta be a reason why I don't see this kind of setup anywhere.
Can anyone help clarify my thinking about this? What am I missing?