Heavier flywheel

What is the main difference I will see and feel coming from a 6.5lb flywheel to a12.5lb fly wheel? Currently on a KKRM and hoping the wheel on resistance itself feels better with a heavier flywheel? What’s anyone else’s thoughts about it?

I have a KKRM as well and originally used the heavier flywheel for two years. I sold it last year after seeing that I liked it without.

People feel (reviews) that the heavier flywheel has better road feel. I don’t think this is true. It does seem the heavier one is better for TT-like efforts that are more steady. I like that it’s easier to accelerate and have power fluctuations akin to road cycling on the lighter flywheel. I remember it being so hard to do those quick bursts that some workouts require, in terms of matching the right power levels, with the heavier flywheel.

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Mechanically, the weight of the flywheel is (almost) irrelevant. What matters is the moment of interia. So two flywheels of the same weight, geared the same way, one a solid disc of metal, the other one a thin disc with most of the weight on the circumference, will have very different moments of inertia and give a very different equivalent inertia and therefore different “road feel”. The exact same flywheel, geared with two different ratios, will give a very different road feel.

You just look at the difference in feel between the same power on the large vs small chainring to get an idea.

I have the same trainer for my wife and got the heaver flywheel attachment. It takes longer to spin it up but once you get going it has a feel more akin to riding around 20 mph on a flat road. The smaller flywheel is more like you’re riding along at 15mph up a 1-3 percent grade.

If my wife is doing sprint efforts we take off the heavier flywheel so it spins up faster and slows down faster. Intervals like 20 on 10 off or quick sprints are better with the smaller flywheel. If you’re doing SST, LT, Tempo, or Base I find the heaver flywheel is better for those efforts.

Yea I do my steady state stuff on my trainer. What I am finding is indoors on a trainer it feels like my knees are being punished until I get cadence and speed up. Hoping the more road like feel is better. Although maybe a direct drive is the best way to?

Direct drive advantage is the elimination of wheel slip. That applies more to jump/sprint efforts as the point where slip occurs most for wheel-on trainers, especially with a setup like the Road Machine and the heavier flywheel addition. That mass resists accelerations and leads to slips in some cases.

For steady state, there is less of a difference between wheel-on/off.