What's a good single chain ring size for a 7s 12-28 cassette for better hill climbing?

Question

I have a Decathlon Triban RC100 road bike with a 7 speed 12-28 cassette and a 44t single chain ring crankset. Since I live in a city with a lot of hills and use this bike to commute and carry weight around, I'm looking into changing the chain ring for a better climbing experience. I'm considering a 28t to get a ratio of 1 on the lowest gear, but since it's my first time fiddling with a drivetrain and don't have a lot to spend on this, I want to be sure I'm heading on the right way.

Thanks in advance for any help

Answer 1

You're on a 1x7 bike - so there's not a lot of range available unless you're happy to take enormous jumps between gears.

You need to figure out if 28:12 is a high-enough gear for your normal riding, and if 28:28 is low enough for the grades you want to climb.

Best way to do this is ride comfortably in a still wind, then count your crank RPM over 60 seconds. This gives you an approximate RPM value to aim for.

https://www.bikecalc.com/cadence_at_speed tells me you'll be doing 102 RPM to ride at 30 km/h in 28:12 and 85RPM at 25 km/h. That's quite high for most people, but you'll have to decide if its right for you.

https://www.sheldonbrown.com/gear-calc.html can also be useful once you figure out what your ideal crank RPMs is on the flat.

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Your bike probably has a 7-speed Shimano A050 rear derailleur, which are rated for a high of 11T and a low of 28T. It should be capable of 30T easily, and probably 32T with some luck, but 34 and above will be pushing the limits I think.

Bolting another chainring on your existing bike is relatively easy, though chainrings themselves tend to be relatively pricy. You will need to match the Bolt Hole Circle of your existing chainring.

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Another option is to swap out the cranks, crankset, spider, and chainring for a double, and fit a left-hand shifter, front derailleur, and inner/outer cable. The costs will not be small, and your frame probably doesn't have the cable stops for a second gear wire.

Personally I'd keep eyes open for another used bike which has better gearing for climbs. Nothing wrong with owning two or more bikes if you have the room.

Answer 2

12-28 is a small range for a 7 speed cassette, with only a ratio of 2.3:1 between the highest and lowest gears. You can get an 11-34 cassette compatible with Shimano (S-Ride CS-M200) which gives you 3.1:1, though 11-28 (2.5:1), 12-32 (2.7:1), and 12-34 (2.8:1_ are more common. This will help even with the chainring you have - if you can find a 34 tooth biggest sprocket you will have a 22% easier gear. Cassettes are properly consumable, so you'll be replacing it soon enough anyway. Note that a bigger max sprocket may not work with your current rear derailleur. 32T probably will, but 34T may not. 7 speed mountain bike derailleurs will have enough range, and could almost certainly be swapped on. The Decathlon website doesn't specify the model of the RD, unfortunately. Changing the cassette does make for big gaps between gears, but on 1×7 I reckon that's preferable to a narrow range in most terrain.

As for changing the chainring, 44T is likely to mean a 104mm BCD (bolt circle diameter) or possibly bigger - smaller BCDs are available for 44T but they're more expensive. You must match the BCD and hole count. At 104 BCD you're unlikely to get less than about a 32T chainring, and less than 29T wouldn't have room for the bolts so would be physically impossible: 104 BCD means a circumference of 12.8" at the bolt centres; the bolts are 10mm diameter so at the outer edges of the bolts we have a circumference of 14.1" or 28.2T, which we have to round up to 29. you might be able to find a 30T though I couldn't.

Answer 3

There's a lot to recommend looking at the back end first. It's not clear what rear derailleur you have, but it can very likely support a 34t large cog. An 11-34 cassette and a new chain to give the extra length needed is a very simple change that will give you a much lower gearing. You're widening out your jumps in between gears, but for utilitarian riding that is not a big concern.

Your chainring is almost certainly one piece with your cranks. If that's true, it can't be replaced on its own. For a bike like this, at the point where you're replacing the crank, you should expect to be replacing the bottom bracket as well, because different cranks need different spindle lengths. Since 1x bikes really need the chainline to be exactly right, you shouldn't fudge this. Hypothetically you could only look at cranks that use the spindle length you already have while giving the chainline and gearing you need, but that is impractical even for a pro. Replacing the crank and BB and either getting the tools to do it yourself or having a shop do it is a much greater expense than a cassette and chain.

If you do your crank plan and the goal is to spend an amount that is within reason for this bike, and if you're actually okay with having the high end of your gearing be so low, you're talking about getting a mountain 30t 1x JIS crankset and then figuring out what BB you need to match whatever the rear end on your bike is (not clear if 130 or 135).