Relationship between Wattage/Cadence and Gearing?

Lonnie Utah said:

I hope somebody here can answer this question for me. (I know you can... 🙂 )

What's the relationship between output in wattage, cadence and gearing? If I'm going 20 mph on a flat road spinning at "X" rpm in one gear, do I expend the same amount of energy (in watts) spinning at a lower gear at a higher rpm? If so, then why is my perceived effort lower in the lower gear...

Just wondering....

Your power output is directly related to how much energy you are expending. As such, you can make a very accurate and easy conversion from kJs to calories - basically, they are equal. IOW, if you performed 2000 kJ of work (as measured by your power meter), you metabolized about 2000 calories to do it. Technically, most people are around 22% efficient, so in the above example, you would actually use slightly more calories than that (~2060).
I bring this up because your cadence has an effect on your efficiency. Generally, as cadence increases, efficiency decreases (meaning you would use a little bit more energy to complete the same 2000 kJ workout). So if you are riding at 20mph at 110rpm, you will be less efficient than you would be at 70rpm.
It gets more complicated than this though.... As intensity increases, higher cadences become more and more efficient. To not muddy the waters too much here, I would recommend training at your race specific cadences and not worry about the efficiency issue...it's a bit of a red herring.
Another factor is your self selected cadence. If you have been training at 70rpm for some time, that is what you are going to be used to and what you will find the easiest. This does not always mean that your self selected cadence is the best to train at however.

Smartt/RST said:

Your power output is directly related to how much energy you are expending. As such, you can make a very accurate and easy conversion from kJs to calories - basically, they are equal. IOW, if you performed 2000 kJ of work (as measured by your power meter), you metabolized about 2000 calories to do it. Technically, most people are around 22% efficient, so in the above example, you would actually use slightly more calories than that (~2060).
I bring this up because your cadence has an effect on your efficiency. Generally, as cadence increases, efficiency decreases (meaning you would use a little bit more energy to complete the same 2000 kJ workout). So if you are riding at 20mph at 110rpm, you will be less efficient than you would be at 70rpm.
It gets more complicated than this though.... As intensity increases, higher cadences become more and more efficient. To not muddy the waters too much here, I would recommend training at your race specific cadences and not worry about the efficiency issue...it's a bit of a red herring.
Another factor is your self selected cadence. If you have been training at 70rpm for some time, that is what you are going to be used to and what you will find the easiest. This does not always mean that your self selected cadence is the best to train at however.

Well said. Believe on the road, experienced riders tend to choose their most efficient cadence depending on the power required at the moment vs. their leg strength.

Going downwind, or drafting in a pack at 20 mph, 75-80 rpm may be enough to cruise along at the required power. However, when pulling at the front or into a headwind, 100-110 rpm is required to hold the same speed. Trying to produce the higher power in the same big gear at 80 rpm will cause the leg muscles load up pretty quickly.

The situation seems very analogous to driving a manual trans car.

I guess this is 'proved' by most of the hour record holders (the ultimate aerobic test ?) turning about 100rpm ? I heard O'Bree used a substantially bigger gear, though was unusual in that.

DavidM said:

I guess this is 'proved' by most of the hour record holders (the ultimate aerobic test ?) turning about 100rpm ? I heard O'Bree used a substantially bigger gear, though was unusual in that.

There is an interesting thread on the cadence of record holders, about 1 week back on page 2 of this Forum....."THIS is the optimum TT cadence".