# Power and Compacts



## Ghost234 (Jun 1, 2010)

1) My longer term power is fairly good for my weight, however my short term power is really lacking and shows very obviously on shorter hills. Outside of hill repeats and short high intensity intervals, is there any way of improving? 

2) This has to do with the application of compact cranksets. My riding buddies are pushing me to switch to a standard crank from my current 50/34 compact. I live in southern ontario, and so there are no mountains. My rationale for buying the compact was the fact that I am a spinner, usually averaging about 105-120 rpm on rides. My top end speed seems to rival my riding partners who are typically mashers. Is there any validity to me selling off my compact and going to standard? I use SRAM red so I can potentially go to 52/36 while remaining in compact rather than buying a new 53/39 crank.


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## Wookiebiker (Sep 5, 2005)

A gear inch is a gear inch...You can see the same ratio's regardless of crank type and in your instance the crank has no bearing on your problem. However, your shorter 1 min - 5 min power does have a lot of bearing on your problem.

If you have no long hills and need more base power for short climbs...work on that instead of purchasing a new crank. Or just try pushing a gear larger on the climbs and putting out more power.

The cranks however...not the problem


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## shomyoface (Nov 24, 2007)

Interestingly I was wondering the same. I have two cranksets, one 53/39 the other 50/36 both SRAM Red - I interchange based on riding conditions at destination. The "regular" crank is a newer purchase and I have found that power outputs are higher than the compact. Assuming my cadence will typically settle in my preferred range, the anecdotal observation makes me wonder.

I am using a PowerTap.


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## ukbloke (Sep 1, 2007)

shomyoface said:


> Assuming my cadence will typically settle in my preferred range, the anecdotal observation makes me wonder.


You can check your assumption by comparing the cadence data between two rides. For accuracy you will probably need a real cadence sensor though, not the virtual cadence of the PowerTap. For example, ride the same course (e.g. a serious hill individual time-trial) under the same conditions on different days with both cranks. You can then compare the data to see if you have the same cadence but significantly higher power output with the standard crank. If that was the case and the perceived effort was equal, then the question is where did that miraculous extra power come from.

It is undeniable that a gear inch is a gear inch. But it is likely that your available ratios with the two cranks are not exactly the same - they could be off by a half-gear change for example, and that taller half-gear happens to be optimal for you giving you the better performance. It might also be that use of the standard crank lures you into a slower cadence than you think, and that your body produces higher watts at that cadence.


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## STARNUT (Jun 19, 2005)

power is as simple as this; WATTS= FORCE X VELOCITY. Specifically, Force in Nm and Velocity in m/sec. Nowhere are gear inches used in the calculation. Nor is crank length for that matter. How hard you push time how fast you push...... that is power. Nothing more nothing less. 

I too am a 'spinner' and live in a hilly~ish area, use a 130 BCD, and I'm an ectomorph. I'll use a 52/38, 53/39, and a 54/44 depending on the terrain and race/ride. I pick the gear based on time at effort and terrain (which is time at effort).

Starnut


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## ukbloke (Sep 1, 2007)

Yes, I'm very familiar with the power equation. Even though gearing does not figure directly in the equation it is clear that any particular rider's attainable power output does vary considerably with gearing and cadence. To take it to an extreme, my power output is very different at 8rpm, 80rpm and 800rpm for example! You admit this yourself by immediately switching to a paragraph that talks about optimizing gear selection.

Ideally one might think that with appropriate choice of gearing that all grades can be made to be the same. For any grade choose a gearing that allows you to spin the cadence you select and the power output that you are capable of (for the duration of that climb). But experience tells me that there are other human factors involved (possibly second order), and my performance does diminish for hills that are not at my sweet-spot grade.


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## tom_h (May 6, 2008)

STARNUT said:


> power is as simple as this; WATTS= FORCE X VELOCITY. Specifically, Force in Nm and Velocity in m/sec. Nowhere are gear inches used in the calculation. Nor is crank length for that matter. How hard you push time how fast you push...... that is power. Nothing more nothing less.
> 
> I too am a 'spinner' and live in a hilly~ish area, use a 130 BCD, and I'm an ectomorph. I'll use a 52/38, 53/39, and a 54/44 depending on the terrain and race/ride. I pick the gear based on time at effort and terrain (which is time at effort). Starnut


Well, crank length does enter into the equation.
Most people, except maybe certain lab physiologists, use Cadence-RPM, not velocity of the pedals.

Equivalently, Watts=*Torque* x Cadence-RPM x proportionality constant.
And, Torque = *Crank Length* x Force (tangential to crank). In fact, you did reference the torque units: Newton-meter (N-m).

If calculating power using Tangential Force x Pedal tangential velocity, realize that short cranks will require _higher RPM_ with _same_ force, to produce same watts.


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