Crankset question

[Catrin]

Wasn't sure where to put this... Next Friday my fitter and I are meeting to start the conversation about how to build my bike up. He is going to put together an initial parts list after we discussed it - the purpose of the first list is to both start the discussion and to give me a somewhat realistic idea what the cost will ballpark.

I think my biggest, and most difficult decision, will be whether to go with a triple or a compact double with a mountain cassette. I want the deepest range of gears I can have, and there are several ways we can approach this. I don't want to open that debate in this thread, there are plenty of other threads that I can read about that - both on this forum and on others.

What I am trying to understand right now is more basic. As an example, the crankset on my LHT is a 48-36-26t. What do those numbers actually mean? I know about Sheldon Brown's gear calculator and I will be putting in a lot of time on this - and I've a basic enough understanding of the rear cassette. It would be helpful to me to better understand what those numbers mean on the crankset description.

Are the numbers, basically, the travel distance for the chain between each of the three large rings? If this is correct, then how would smaller, or larger numbers on the crankset affect my overall gear choice? I've looked for links, but can't seem to find something basic enough :o

[OakLeaf]

Chainring numbers are the number of teeth, same as the numbers for cassette cogs.

So if you're in your 48x12 - a nice round number that's probably your top gear - for every revolution of your pedals/cranks, your cassette (and thus your wheel) will travel four revolutions.

Downshift your RD several clicks to your 48x24 (you're not crosschaining now, are you? ;)). Now, for every revolution of your cranks, your wheel will travel TWO revolutions.

If you're in your 26x32, then your wheel will travel 13/16 of a revolution for every turn of your cranks.

More teeth in front = more teeth/chainlinks of travel per revolution = higher gear
Fewer teeth in back = more wheel travel per tooth/chainlink = higher gear

[Catrin]

....So if you're in your 48x12 - a nice round number that's probably your top gear - for every revolution of your pedals/cranks, your cassette (and thus your wheel) will travel four revolutions.....


ooooooh, this helps, thank you. I think my brain has to chew on this a bit now but either the light is starting to dawn on me or a freight train is coming out of the tunnel :)

[laura*]

If this is correct, then how would smaller, or larger numbers on the crankset affect my overall gear choice?

Your choice of the biggest ring will mostly determine your top speed.

Top speed likely can't be tuned via the cassette in back. The smallest/fastest cog is 11 teeth. Most cassette options will have either an 11 or 12 tooth small cog. This leaves no option for further tuning via the cassette. Thus the crankset's big ring is the only way to choose your top speed.

Mountain bikes will have a big ring of 42 or 44. Touring bikes will be a bit faster with 46 or 48. Road bikes are faster yet with 50 to 52. Hard core racers might have 53 or 54 toothed rings.

Bigger rings have a downside of reduced ground clearance. Mountain bikes started out using touring cranks and had 48x12 as their top speed gear ratio. Around 1994, the industry changed to compact gearing with a top ratio of 44x11. Note that this is the exact same effective gearing!

Your choice of the smallest ring will affect your hill climbing. Here you have lots of options of chainring and cassette combinations.

If you choose a triple crankset, you might be able to tune your gearing such that you're in the middle ring most of the time. This simplifies your gear shifting decisions.

Double cranksets with huge chainring jumps won't shift well. If you rarely need to shift (say if you ride in a flat area with one big climb), this may be a worthwhile tradeoff. If you have to constantly shift, then a triple might be a better choice.

[Catrin]

Your choice of the biggest ring will mostly determine your top speed.

Top speed likely can't be tuned via the cassette in back. The smallest/fastest cog is 11 teeth. Most cassette options will have either an 11 or 12 tooth small cog. This leaves no option for further tuning via the cassette. Thus the crankset's big ring is the only way to choose your top speed.

Mountain bikes will have a big ring of 42 or 44. Touring bikes will be a bit faster with 46 or 48. Road bikes are faster yet with 50 to 52. Hard core racers might have 53 or 54 toothed rings.

Bigger rings have a downside of reduced ground clearance. Mountain bikes started out using touring cranks and had 48x12 as their top speed gear ratio. Around 1994, the industry changed to compact gearing with a top ratio of 44x11. Note that this is the exact same effective gearing!

Your choice of the smallest ring will affect your hill climbing. Here you have lots of options of chainring and cassette combinations.

If you choose a triple crankset, you might be able to tune your gearing such that you're in the middle ring most of the time. This simplifies your gear shifting decisions.

Double cranksets with huge chainring jumps won't shift well. If you rarely need to shift (say if you ride in a flat area with one big climb), this may be a worthwhile tradeoff. If you have to constantly shift, then a triple might be a better choice.

This is very helpful as well, thanks! This information helps me to put things in context and helps to provide food for thought.

[Becky]

Catrin, can you go test ride some bikes with compact doubles? Experience the shifting for yourself before you commit to it. Consider the force needed to shift it....

What are you doing for shifters? That can definitely influence the feel of a shift.

Personally, I prefer my compacts to my triple, both from a riding perspective and from a mechanic's perspective, but there are situations where the triple just makes sense.

[Catrin]

I will likely go with triples, and while unorthodox, my fitter and I are discussing the merits of a full mountain bike drive-train - at his suggestion. I will have flat-bars and trigger shifters. Road bars are not an option, period (physical issues). Of course with this approach I do run the risk of running out of gears at the fast end of things - but quite frankly - if I have to compromise on hill climbing options or speed I will keep my climbing options.

Have I made up my mind? Nope, but I already know that my gearing will wind up on the touring/mountain bike end of things - I just need the right combination that will give me the options my legs need...As I said earlier however, I am not making such decisions right now - just seeking a better understanding of how things work together so I can make an intelligent decision and be able to properly consider the advice I am given.

Ed: I am going to consider all of my options however - and I like the idea of test-riding a double compact to see how differently it might shift if I can find one small enough. If nothing else the LBS could pop it on one of their trainers for me to try...

[OakLeaf]

Top gear is pretty much the least of your worries.

Your top gear is for two things: sprints and fast descents. By definition, your top gear is NOT for speeds that you can sustain on the flats, even with a tailwind, because you always want some gears taller than that.

If you're not racing, then you're just sprinting for fun, so if you have to pedal 150 rpm to beat your friends to the city limit sign, that's good for you. When you're ready for a race bike, then you can worry about a taller top gear. :)

As an example (and I hope I've got my arithmetic right this time) - with a gear of 42x11 - a common mountain bike top gear - with 26" wheels, at 90 rpm you'll be traveling (very roughly) 26 mph.

I'm not saying that top gear isn't a consideration at all ... with that 42x11 you'll spin out very quickly on the descents, and that'll give you less momentum for the next climb.

But I do think it's a very small consideration.

[Catrin]

Oak, thanks for this. J. (my fitter) and I were discussing this yesterday. He made the same estimation you did, that I would top out about 25mph with the set-up he has suggested I consider but would have all of the deeper gearing I want/need.

This is all quite exciting :)

added: I do realize that it will be different climbing with my Gunnar - different geometry (based on cyclocross design), lighter frame, much lighter wheels (custom 26-inch) - and that it should be easier to climb with it. This bike will travel at least as much as my LHT however, so I have to think about much hiller terrain than what we have in central Indiana. I do wonder, at times, if my very upright riding position interferes a bit with climbing - but that isn't anything that can change.