Big vs Small Chainring - Same Power (ERG Mode Gearing)

It can be very individual, with my Kickr 2017 direct-drive I’ve found big ring to give the most realistic feel and allows using the same FTP inside and outside. You need to experiment and find out for yourself. If you decide to switch from little to big ring, then I would recommend a new ramp test in big ring just in case you have different FTPs (little vs big).

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That was my decision. I would rather train in Small Gear for MTB rather than have a larger FTP with the Big Gear for more road riding. Basically I did my first SSB block in Big Gear until first MTB race come up for the season. I was a beast on the flats but had no punch or sustained power in climbs. Going to Small Gear corrected this for me. As far as feel goes, yes IMO - Big Gear feel more roadie realistic, but Small Gear feels more MTB relaistic to me since there is less inertia. Really boils down to what you are training for IMO.

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I was training for Etape and other big events COVID-19 stopped that. But I only ride not he road, my DH days are well behind me. I love putting down watts on the flats, ( I am more of a sprinter / breakaway rider) But climbing is one of my biggest weakness.

Training in the small ring I believe has helped this, but realistically unless I am on a climb im always in the big ring. I have this week worth of MV SSP and then a ramp test on Wednesday next week…

So here’s a question… I had been training with my road bike in the big ring for the last year or so, but recently put my cross bike on the turbo (36t 1x at the front, mid cassette). Since then, I’ve had to drop workout intensity by about 6% to feel like the workouts are achievable, although my heart rate is much higher.

My question is, will I lose fitness if I was to carry this on? Would I be better served training at higher power on the road bike, or will my muscles adapt to the change in flywheel momentum followed by a bump back up in FTP?

I always train on the small ring too, and my outdoors FTP follows my indoors FTP. If I increase by 5% indoor, it increases outdoors as well. No worry as far as I am concerned.

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Most of the advice in this thread (and others) say it’s best to train for the kinds of riding you want to do. So if flat time trials, big ring on the trainer. Small ring for climbing.

One thing I’d also say is that your power may not actually be 6% lower. Some of the difference could be measurement error of your power meter/trainer between high vs low inertia riding. This has also been talked about in this and other posts - although there doesn’t seem to be conclusive evidence, so this is a bit more speculative.

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Cheers Dave! I tried to skim the thread, but there’s a lot here!

FWIW - I think that difference is too big to be just due to you not used to riding small ring. I’d suspect something is also up with your power measurement. An interesting experiment would be to ride with your cross bike in small ring for a training block, then switch back to road bike In big ring and see how that feels. That might give some insight into the root cause of the difference.

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Some quick math that probably doesn’t mean much directly, but interesting at least:

50t x 17t = 2.94 input to output ratio
2.94 ior x 90 rpm pedal cadence = 264.7 rpm of rear axle (wheel)

36t x 17t = 2.12 input to output ratio
2.12 ior x 90 rpm pedal cadence = 190.6 rpm of rear axle (wheel)

190.6 rpm / 264.7 rpm = 0.72 = 72% of the higher gearing axle rpm

That’s a 28% decrease in trainer rpm that directly drives the flywheel. No idea what it really means, but as they say " it’s not nothin’ ".

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I think you are on to something here! We need a math nerd to take this info and tie into the weight of the flywheel in our trainers. Need an equation that calculates how long a flywhee/rear wheel will spin on its own at each gearing once you stop pedaling. Call it an inertia assistance time, I suppose? The longer the time the more help you have in peddaling? This could be good in making it clear why gearing (and maybe trainer choice with multiple flywheel sizes) does make a difference in our training. It would also prove why almost all trainers struggle with correct/consistent power at higher gearing vs lower gearing. With great inertia/higher gearing you dont have to be as consistent/smooth with your pedalling technique so it makes it harder for the trainer to measure your power. With less inertia/lower gearing you have to be more consistent/smooth with your pedaling which makes it easier on the trainer to measure your power.

I would love to see the inertia assistance time in camparison for the various Kickrs (past/present/core, non-core, etc.) and the Hammer since they all use different sized flywheels. Of course the elephant in the room is the Tacx Neo Series virtual flywheels. I wonder if I could use the inertia assistance time to mimic the same of Kickr/Hammer by entering a different body weight in for the Neo? I might try this when I get my Neo 2T in today. Has anyone tried this with a Neo? I assume the greater the body weight the greater the weight of the virtual flywheel? I find it strange that gearing does matter for the Neo from a linearity standpoint since it uses a virutal flywheel. I know Tacx shared their own documentation proving this between the Neo OG, Neo 2, and Neo 2T. But then again, it makes sense based upon my previous argument from above that trainers find it easier to measure power at lower gearing vs high gearing. Anyway, be interesting if you can customize the Neo flywheel size to what you type of riding you are training for.

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I’m a math nerd but honestly think it comes down to feel and individual preference. Setup your inside trainer to replicate outside efforts as best you can, and if inclined, do some inside/outside FTP testing (not ramp testing).

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The inertia of the flywheel (or bike + body if riding outside) does not add to your power output. There are two things that can happen at different inertias:

  1. Different muscle groups are used
  2. Power meters or trainers can calculate different power numbers even if the power input is the same.

Both of these are present and contribute to different RPE in high vs low inertia, but to varying, usually unknown degrees.

Just pick which setup you prefer (high vs low inertia) - either based on feel or to mimic the kind of riding you are training for.

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Hello good people. Finally setup my new Kickr Core and love it. For me, the small ring was tougher than the larger ring, and the larger ring felt more like the Kurt Kinetic (which I’m impressed how good it is now that I’ve also tried the Kickr Core) and also more like the riding here in the Midwest, mostly flat.

I do have two technical question though.

  1. I assume the calibration feature in TrainerRoad app is the same as the Spindown feature in the Wahoo app, and that I can simply do the TR one every two weeks. (?)

  2. If I choose to do 80% of my rides in big chain ring, and 20% in small, do I need to recalibrate when I change (or even change mode, from Erg to Standard or Resistance?)

Thanks, ride on

  1. Yup, calibration in TR is fine. Most important is to be consistent. So, calibrate when the trainer is a bit warm, usually pause right after the warm up steps, and in the recovery before your first interval.

But with a wheel-off and assuming it stays in place in the same basics temp range, you can calibrate one time (once warmed) per 2-3 week period. No need to do it every ride, like is recommended for wheel-on smart trainers.

  1. Calibration is not gearing dependent. You need to get it to around 23 mph (can’t remember the exact speed) and hold it there for a bit. This usually requires a big ring effort and then you coast. Once done, the calibration is good for any gear.

Always a pleasure reading your replies. Thanks! Have a great weekend.

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This could be the reason why my workouts on my new smart trainer have seemed so much harder. For some reason. Realized on my “dumb” Fluid trainer I was running in the big ring and on my new smart trainer I’ve been in the small ring.

Is there a decent middle ground between the two?

Sure. Try on the smaller end of the rear cogs. It that’s not enough flywheel speed, swap to the big ring and larger cogs. You can experiment and find something you should like.

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Ironically, I bike is a 9-speed (Fuji Tread 1.5). My Elite Suito comes with an 11-speed 105 cassette. I’m stuck with the bigger gears on the cassette for a bit (as the chain jumps around on the smaller gears) at least until the 9-speed cassette for my Suito comes in.

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Did this one today, first and third interval in small chainring and second and fourth in big one, just to see (and feel) the difference. Definitely noticeable for me after having done a year in the big chainring on a dumb trainer, but not undoable in the small chainring. Just different muscles being engaged and fun to seek them out.

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its hard to see, was there much difference in HR between the small vs large chainring intervals?