Sizing 1x chainring based on ftp

I am wondering is there is some calculation to relate reletive effort vs power for different size rings.

I ran a 30x9-46 cassette last year and the granny was just right for high Alpine +10,000ft rides. Maybe a bit excessively deep down low.

If my FTP bumps up 10 %, does that = a 2 tooth bigger ring to keep the reletive effort on the climbs the same? Is there a way to guess that without swapping rings back and forth in the spring?

I ask because I want to take a stab at sorting out the drivetrain over the winter and not waste a few hundred dollars on rings and chains jumping up 2 teeth at a time.

Maybe I am just a nut job😋

I seem to remember TR had a podcast on this sort of thing. Maybe it was the Leadville prep podcast from last year. I run a 34 tooth crank and I’ve thought about upgrading to a bigger crank… Good question, I’m interested to hear what everyone else says.

Certainly, the stronger you get, the bigger the chain ring you can run. But you also need to match that to the terrain you are riding.

I have a 32 tooth chain ring with eagle cassette. That’s fine for me for most rides in the mountains, but on long rides, I think I might benefit from a 30 tooth ring. I’m going to buy one and put it on this spring to check it out.

FWIW, I’d not be too keen on making any bigger jump that 2 teeth at a time, unless something significant had changed.

If a 30 tooth was too easy for you last year, and you increase FTP by 10%, and you plan on riding the same terrain, it may make sense to bump up to a 34.

The other factor is your cadence and how fast you like to spin on the climbs.

If you don’t need more top end, I’d be inclined to leave it as is. If you are spinning out on the roads and descents, then perhaps it makes sense to go bigger.

I agree here, no reason to change anything unless you are running out of gears on either end, or if a specific race requires special gearing (ie: Leadville). Better to have a gear and not need it than need a gear and not have it, expecially if your races/rides go beyond 2 hours.

Pro XCO racers end up with bigger rings up front to get a better chain line, torque, preference, etc. (marginal gains), as they’re normally in the middle of the cassette anyways.

Fair enough. I would like to get off the 30 so I have more options for 104bcd. I may just grab a 32 and call it good. I switched from a Shimano 11 to try 9 so fireroads are not the pits spin out fest they had been. 32 9 should get the job done on the roads going from my house to trailheads.

BTW, I still wonder if there is a way to calculate the change someone would feel on a given climb with xy gearing on a climb bases on ftp change, or maybe better to guess based on watt/kg. Just something to dream about while on the trainer I guess.

On a steep climb where speed is slow, aero drag is negligible, rolling resistance is low, and gravity is the primary force to overcome, the ratio increase in chain ring size would be the same as ratio increase in FTP - assuming you keep your cadence the same.

Basically, 10% more power = 10% faster up the climb = 10% increase in chain ring size to keep cadence the same.


Time for someone to make odd number chainrings… :innocent:

Wickwerks makes them for CX. Don’t know if it is a narrow wide issue for mountain.

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If that is true it leads me to wonder about the fact that I would then be riding longer distances and may need to retain light gearing for the return trip… I was doing a shuttle on Monarch Crest this year and found myself considering riding back up the pass to grab my car rather than going to the brewery and waiting for a ride back up (bigger chainring would have shut that idea down for sure). Too much time to think about bikes while riding on a trainer.

Yep. Got to also consider ride length, and how tired you’ll be when you are riding. On a 1 hr ride, you may be able to clean a section, but if it’s 4 hrs in, maybe not.

I pre-rode Leadville last year over 2 days, and was able to clean sections that on race day I was walking. So the gearing you choose has to be matched to the riding you’ll do. That’s why I’m planning to try a 30 - to see if it might help me ride for a little longer vs hike-a-bike.

You can plug the numbers in to a calculator and see your speed at certain rpm. I use the sheldon brown gear calculator myself. I have an 11 to 46, switching from 32 to 34 makes a .5 mph difference in thr 46 at the same rpm but makes a 2 mph difference in the 11.

I use this as a gear calculator. It let’s you compare alternatives side by side:

As others have mentioned here, cadence is also a factor. The calculator let’s you look at this too for different speeds.

:thinking: I did not know that, intriguing.