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Power output vs RPMs

Old 05-11-15, 08:43 AM
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bikenh
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Power output vs RPMs

Does higher power output limit the amount of RPMs you can pedal?

I realize the reason I' asking the question is putting me at lower power output, downhill riding but since I'm still applying power to the pedals going downhill would being in a lower gear benefit me more by giving me a higher possible cadence or would it limit me more by making me pedal faster? So far I'm still doing fine riding a 53x17 even when riding 42-43 mph. I know the cadence would raise with smaller gearing but I would think the powe output neccesarry to acheive the same speed would also reduce but would that give me a higher overall possible cadence before I started seeing any wobbling?
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Old 05-11-15, 08:53 AM
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Power = Torque x RPM
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Old 05-11-15, 09:01 AM
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I think you are using "power" to mean "gain ratio" or something like that. If you are looking to go faster downhill, then you should have a higher gear rather than a lower gear. Are you walking your bike up the hill and riding down or something?
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Old 05-11-15, 09:34 AM
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Originally Posted by TejanoTrackie View Post
Power = Torque x RPM
basically this, and on a bike torque really only matters for accelerating from a stop

the faster you can spin the pedals the better. it doesn't matter how hard you're pushing on them. in fact, the harder you push the quicker you'll spin out
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Old 05-11-15, 09:46 AM
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Originally Posted by neamatoad View Post
on a bike torque really only matters for accelerating from a stop
Not quite. Wind resistance increases as the square of the speed, so your maximum speed will depend on both the applied torque and RPM.
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Old 05-11-15, 11:13 AM
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I've seen people using power meters on rollers, so how does that work? Is it just the resistance of the the wheels?
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Old 05-11-15, 11:40 AM
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Originally Posted by prooftheory View Post
I've seen people using power meters on rollers, so how does that work? Is it just the resistance of the the wheels?
Well, there's some fan effect from the wheels and small rollers can offer significant rolling resistance with the tires, but it's a bit silly to use a powermeter on rollers unless they are connected to some sort of resistance device. A powermeter only makes sense on a stationary trainer.
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Old 05-11-15, 02:03 PM
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all of the watts
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Old 05-11-15, 04:08 PM
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The push from your legs does not equal power. The speed(cadence) at which you apply force equals power. So, if you pedal with less force you will need to increase cadence to achieve your current power. The other way around, if your pedaling slows down then you need to push harder to achieve the same power.

downhill riding but since I'm still applying power to the pedals going downhill would being in a lower gear benefit me more by giving me a higher possible cadence or would it limit me more by making me pedal faster
This is up to your physical abilities and preference. Do you want to ride downhill pedaling harder or faster? Can you pedal faster as it is? Maybe you're topping out with your gear and a higher gear, going downhill, will get you the same cadence while allowing you to push harder.

Last edited by hairnet; 05-11-15 at 04:13 PM.
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Old 05-12-15, 12:50 PM
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Originally Posted by hairnet View Post
This is up to your physical abilities and preference. Do you want to ride downhill pedaling harder or faster? Can you pedal faster as it is? Maybe you're topping out with your gear and a higher gear, going downhill, will get you the same cadence while allowing you to push harder.
This is actually the scenario I'm dealing with. I know I climb slowly, some by being lazy, aka not a racer just a long distance rider so I'm not trying to push the speed. I know climbing with a 53x17 is also part of the reason as well. Thus far I haven't topped out the 53x17, even at 43.7 mph on a downhill. I'm not sure what my max rpm is, on pavement...the only place I ride. I'm thinking about downshifting and seeing if I can find the max but I'm not sure how much I want to downshift. Yeah, I generally a tough boy...I don't reach for the brakes on the descents around here unless I run into horrible pavement conditions like earlier this morning.

I'm just trying to figure out what I might like to even try when downshifting. I know around here the fastest drops, in the past, have been nothing over 48.5 mph, coasting. This year on the same drop I haven't even hit 42 mph pedaling all the way down.

I know this sounds strange/stupid does pedaling down a hill actually add a little resistance to the chain and help to slow you down. One of guys I ride with actually brought up the idea the other day and I'm still surprised I haven't hit 45 yet this year. The drop I rode yesterday at 43.7 I had only seen 40 on it once before this year and that was coasting down. Yet in the past week I've seen over 40 four times. It's not the 48.5 mph drop. I've been down it a couple of times and can't seem to get any decent speed going down it anymore.

So I'm really trying to increase the uphill speed while not going over max on the downhills. Yeah, being a bit greedy.
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Old 05-13-15, 03:09 AM
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Basically, if you would "cruise" downhill, the pedals would be going a certain RPM at a certain speed. The speed is determined by the grade of the hill, your weight, resistance, etc. The RPM is determined by the gear in combination with that speed. To go faster, you have to increase the RPM beyond the "cruising" RPM. The amount of force required to do so, downhill, isn't high. So, in a very big gear, you will be able to go a lot faster, since your max RPM will result in a high top speed. In a lower gear, you would probably reach your max RPM before running out of strength.

Going uphill is the opposite: You are more likely to reach the end of your strength than the max RPM you could achieve, so you use a low gear to use your RPM's to the greatest advantage.

If you do both in the same gear, it will always be a compromise. Of course, you can decide which is more important to you and focus on that (big gear for higher downhill speed, small gear for higher uphill speed). But there is no "magic gear" that is the best for both.
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Old 05-13-15, 07:00 AM
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OP what do you mean by "downshift"? Sounds like you are on a geared bike and not a fixed geared or single speed. If so you need to find a more appropriate forum for your question. If the downhill is steep and sufficiently long you will spin out on a geared bike or single speed and will go faster by freewheeling and getting super aero on the bike. I've approached 60 mph a couple of times and only weighed 135-137 at the time. But you had better be sure you do this on a good road surface and traffic conditions allow playing this game as safely as possible. You mentioned "bad road surface"-a sure trip to the ER if you are "lucky".
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Old 05-13-15, 07:03 AM
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Huh?
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Old 05-13-15, 09:13 AM
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Originally Posted by bowzette View Post
OP what do you mean by "downshift"? Sounds like you are on a geared bike and not a fixed geared or single speed. If so you need to find a more appropriate forum for your question. If the downhill is steep and sufficiently long you will spin out on a geared bike or single speed and will go faster by freewheeling and getting super aero on the bike. I've approached 60 mph a couple of times and only weighed 135-137 at the time. But you had better be sure you do this on a good road surface and traffic conditions allow playing this game as safely as possible. You mentioned "bad road surface"-a sure trip to the ER if you are "lucky".
I did kinda say that misleading like. I meant changing the entire gearing around on the bike I am riding fixed gear on a flip flop hub.

Funny thing is I find I get more speed by power pedaling down the hills rather than coasting in a nice tuck position. I'm measuring that from numerous descnets down the same hill coasting over the past several years versus what I now see power pedaling down the same hill. A hill in particular I had only hit 40 on once, with a nice tailwind, last fall. Last week I hit 40+(43.7 the fastest thus far since going fixie) four times going down the same hill.

Actually something you posted just gave me an idea I'm going to have to think on and do some data(speed and cadence) checks with. As well as see how efficient I can make the switch. Thanks.
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