Big vs Small Chainring - Same Power

power-meter

#1

I recently did the Antelope workout which has 10-min intervals at around 90% FTP. Since I use a smart trainer in ERG mode, I normally do not shift gears and just leave it in my small chainring and a cog that makes the chain straight.

Five minutes into the first interval, and holding a cadence of 85-95 (Antelope recommendation), I found myself really struggling. I shifted to the big chainring and one cog easier (one cog easier just to offset the big chainring a bit), and finished the second and third intervals easily.

My questions are: (1) Is shifting to the big chainring “cheating”? The same power is achieved. (2) What does this say about my physiology, if any?

Additional info: My normal cadence is 90 and I am on a Tacx Vortex.


#2

Yes, gearing in ERG matters and affects the feel.

Higher gearing = Higher Flywheel Inertia
That generally means “easier” and the flywheel works to keep the drivetrain movement easier.

This is covered in several threads. Usually you want to consider applying the gearing in a way to mimic your event needs.

The rule of thumb is to try to match the flywheel inertia to the majority of your events.

  • Flat and Fast Road Riding = Use Higher Bearing and Faster Flywheel Inertia

  • Hilly and Off-Road = Use Lower Gearing and Slower Flywheel Inertia


#3

Thanks @mcneese.chad this makes a lot of sense now. I did feel like I was climbing when in the small chainring setup mentioned.

One quick follow up, does this mean I’m currently a better cyclist on the flats and could probably benefit more by training with the small gear? Assuming I’d like to improve on my weak area that is.


#4

Not sure about “better or worse”. I’d say it’s a good deal to recognize for one. I think people can choose one as their primary option, but it’s worth mixing it up on occasion to get more variety and keep a bit more capable.

Mix it up in a particular workout or from day to day, depending on the workouts.


#5

And as an added benefit of changing chainrings and cogs you get to share the wear on your driveline.


#6

There was a post on the AACC Facebook page by someone indicating that in the big chain ring, his trainer under-reports power compared to his power meter.

I wonder if for some trainers, it’s not just an inertia difference, and the power being generated is actually less??


#7

Yeah, I think it was about a Kickr Snap and stages PM.

Inertia could be part of it. Not to mention the fact of a single-sided PM (and potential variability) along with the way the Snap measures power.

All those could contribute to the power delta. So it’s hard to pin that down to one factor.


#8

Can I get advice on the FTP test regarding the typical rules above. I’m an MTBer, I always train in the small front ring and approx middle cassette ring, however, this presents wattage/floor ceiling issues for big watt stomps and in order to do so I have to jump up a front ring (you know the score). So, doing an FTP test, how does this affect me and what should I do?

Example, last FTP test I got to approx FTP step on the ladder and switched up front cog in order to ensure the Wattage could still rise accordingly. Was this the best way? Or should I have used the rest of the cassette in the small ring but causing chain alignment penalties? OR something else?


#9

Do what you have to to work within the trainer limits. I’d try to use the cassette range first, then move to the big ring as needed.

What matters more for an FTP test is consistency from test to test. So, whatever you do, make it consistent and repeatable. Keep track of when you swap during the test and try to do that the same when you test again.

Or just use the big ring when you start. I like testing in the same gear I train, but I don’t think that’s really all that important in the big picture.


#10

@Dan_Dima-ala this subject is close to my heart as I made a big mistake in my training last winter as a direct result of precisely this flywheel inertia effect.

So based on the results of a friend who made unbelievable progress in his racing after two winters of smart turbo training, I decided to do an experiment and do only turbo training, no road riding at all, for an entire winter. My target events are a whole season of 10 and 25 mile TTs from flat through rolling to hilly.

I live in a hilly area and thought I could get the same strength gains on the turbo as doing real hill repeats for example. The mistake I made was that I did my entire Trainerroad plan in the big ring, with the exception of the low wattage valleys. All on a Kickr Snap on ERG.

Come the season I was confident and feeling really strong. I’d hit all the workouts full bore and even discovered a new max HR in a VO2 session. My ftp was up from 242 to 275 at 70kg. I thought i was going to kill every course.

I was in for a shock. My PBs came rolling in on the flat stuff, sure enough, but my climbing performance was abysmal. I lost minutes over my PBs on the hillier courses. I worked out straight away what the reason was.

Lesson learnt and next steps: this winter I’m doing plenty of big ring high cadence workouts but I’ll be supplementing these with lower gear and lower cadence work too, via appropriate Trainerroad workouts and real hill repeats. Even on the big ring workouts I’m using a bigger rear cog than last winter to reduce the helping hand of flywheel inertia even if it means my stated ftp will be lower.

Good luck with your training!


#11

How does big vs small chainring play out on dumb trainers? I use a Feedback Sports Omnium Over-drive for almost all TR workouts (Rollers get the rest). For my recent base block I’m in I’ve done all the workouts almost exclusively in the little ring (39t) upfront and somewhere in the middle of the cassette in back (73-54" gear inch range mostly). I started doing this as it’s quieter on the drums (I do my workouts between 5-6am in a NYC apartment with a sleeping wife). I only will shift into the big ring (53t) if I need to maintain power (beyond a few seconds) over 380 watts.

I started using the small ring due to lower noise…but then justified it that it’s closer to the feel of the bikes I primarily ride (fixed gear - 47-49t front generally).

But does the ring matter that much on my dumb trainer as long as I hit the watts needed in the cadence range I’m comfortable at (90-110rpm for me)?


#12

Since a dumb trainer/roller is using the overall roller/drum speed to create the resistance, they are different.

Effectively, the roller/drum speed is what dictates the resistance level. The faster it spins, the higher the resistance level. As such, there is only one roller/drum speed for a specific resistance point.

For ERG and smart trainers, they can achieve a specific resistance level at different axle speeds (the basic equivalent to roller/drum speed in dumb trainers.

So, for dumb trainers, there is really only “one setting” compared to the options present with smart trainers in ERG mode. The inertia you get is effectively set by the weight and sized of the flywheel or roller drums. Generally speaking, rollers have much lower inertia when compared to even the most basic trainer.

Perform a simple spin-down test for comparison.

  1. Get on a dumb trainer, spin the bike up to 20 mph.
  2. Stop pedaling to coast.
  3. Record the time it takes to go from 20 mph to a stop.

You will find that rollers are usually very short coast-down time. Where a good fluid trainer like the Kinetic Road Machine, will be much longer. That is what people usually associate with good “road feel”, when you have a reasonably long coast/down time. That means there is reasonable inertia in the system to mimic the coasting action we get when riding outside.


#13

Shane Miller did a video on this. In addition to inertia, muscle activation and where force maxes out in the pedal stroke are different.


#14

Yup, and that’s why it’s in the link I posted above, when we discussed this the 3 other times this week :stuck_out_tongue: Popular topic for some reason. Must be a sign of the shift indoors.


#15

Do to the nature of SSB (building base fitness) I tend to do all of it in the big ring with a straight chain line. Build and speciality phases are done in the gearing that matches the effort. i.e. sprints in big ring/small cog(s), climbing small ring/large cog(s), everything else big ring/middle cogs all of which would simulate my outdoor riding style.


#16

I did SS base 1 using the big ring and still think it was one of the factors for my drop in FTP. Probably not the main factor but just contributed to the decline, especially since I do the test in the small ring. So far, I’m using the small ring for SS base 2 workouts and going to stay away from the big ring on the trainer.

Usual disclaimer here: n=1


#17

Thanks @Darkgerbil, I appreciate you sharing your experience. Really helpful stuff. Good to know though you hit a PB on the flats. Cheers!


#18

I have a Kickr for a year, and have only used big ring. I’ve had multiple setbacks this year, but have completed SSB1 multiple times. Each time I’ve improved climbing performance including best HC climb earlier this year (despite lower ftp), first 20mph metric century (flat terrain), and recently was only 3 seconds off a PR on 35 minute climb despite a lower W/kg.

Tonight I did Colosseum, a 90 minute aerobic endurance ride between 50-80%. First 35 minutes in little ring, the rest in big ring. Always in middle of cassette. The little ring felt like riding down a shallow hill, even during 80% interval. It felt like my pedaling could never “catch up” just like when soft pedaling down a small false flat. And the power chart after the ride shows it was unnaturally smooth (a little jaggy, but not much). Honestly felt like I was getting nothing but junk miles in little ring, again like I was going down a gradual decline for 35 minutes. The big ring felt like I was on the road, either on the flats or a small incline (going up a false flat). Here it is:

Same experience tonight as when I tried little ring last month: Form Sprints in ERG mode

Honestly don’t understand why my experience is so different. Ok, so you have a wheel on trainer while I have direct drive. But others with a Kickr swear by the little ring. I’m around 200lbs / 95kg but that can’t explain the “too easy like riding down a false flat” feeling of little ring as my weight has no influence on a trainer.


Easier in the small ring?
#19

To summarize:

  • Colosseum 90 minute endurance workout on a Kickr 2017 in Erg mode
  • 35 minutes in little chainring (middle of cassette)
  • 55 minutes in big chainring
  • latest apps, firmware, zero offset, recent advanced spindown

How come when I’m in the little chainring feels like I’m riding DOWN a false flat? Note how much smoother the power is during first 35 minutes in little chainring.

And how come the big chainring feels like I’m riding on the flats, or even UP a false flat?

A month ago I tried little chainring during 15 minute interval in Gibbs workout and it felt the same. All the recommendations on this forum to ride in small chainring “to simulate climbing” so why is my experience seemingly so different? The little chainring feels like the opposite of climbing, it feels like I’m descending and soft pedaling.

@GPLama your YouTube video ended with the hypothesis that the differences led to different muscle recruitment. Fair enough. From the video I believe you were using Rotor Q rings. I’ve got stock Ultegra 6800. EDIT: you were not using Q rings.

Captured a couple of screenshots of the video when power and rpm was as close to identical as I could get:

Little chainring:

Big chainring:

Spent the last hour giving myself a refresher course on torque and power (power is integral of torque over one complete pedal revolution). The ovals appear to be different sizes to accommodate the small differences in max torque (about 5 Nm) in the two screenshots above, Which makes sense to this engineer/math major if I’m not too tired.

I dunno, need more data to run the math and do real science. That and more hours in the day. Was thinking of using Stages app to do a high-speed data capture of torque and force, but looked at app tonight and that feature is gone (but referenced in May 2018 release notes?!).

:man_shrugging:


#20

I’ve never used Q-Rings and don’t plan to unless there is conclusive evidence they’re beneficial.