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Trainers and Calculating Power

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Trainers and Calculating Power

Old 01-15-13, 09:54 PM
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Trainers and Calculating Power

I know that some manufacturers have published power curves for their trainers (Kurt Kinetics comes to mind). Does this only work for fluid trainers or can magnetic trainers also give you a somewhat reliable number for power depending on speed and resistance setting?

I just picked up a used Blackburn Trakstand Mag 3 from CL and was wondering if there would be anyway of getting a "guesstimation" of power output. I'm assuming it would be a function of gearing, speed (gearing and cadence), and the resistance setting (1, 2, or 3). Correct me if I'm wrong about that ... I'm no physicist. Would crank length come in to play as well? Maybe it would be just gear inches and resistance setting?

Anyone feel like a math problem?
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Old 01-15-13, 09:58 PM
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Not the most detailed, but from their website:

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Old 01-15-13, 10:13 PM
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Jeez - 30 mph for 300 watts?!
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Old 01-16-13, 12:42 AM
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Originally Posted by canam73 View Post
Not the most detailed, but from their website:
Thanks. Just out of curiosity, where on their website was that posted. I looked before posting and couldn't find anything even close to that.
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Old 01-16-13, 06:31 AM
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Originally Posted by hhnngg1 View Post
Jeez - 30 mph for 300 watts?!
People wonder why trainer miles don't count.
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Old 01-16-13, 07:04 AM
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I wouldn't be too worried about power on a trainer unless you have a power meter...Any number you calculate means squat!

What you can do, however, is use speed, cadence and HR as a measure of performance and to set structured intervals. I use a KK road machine and have a Powertap. With the PT I can view my actual power, but really when I am doing a longer interval 10min+ I don't concentrate on power, I watch cadence. From experience I know that to maintain X watts in Y gear I require Z cadence. The issue with power is that it's constantly jumping around, now your average power is consistent, but so is speed and cadence.
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Old 01-16-13, 07:21 AM
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Originally Posted by LowCel View Post
People wonder why trainer miles don't count.
(After one ride on it) I feel like miles on the KK road machine should count double . Resistance vs speed is closer to the road and damn it, I miss coasting.
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Old 01-16-13, 07:29 AM
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Originally Posted by WhyFi View Post
(After one ride on it) I feel like miles on the KK road machine should count double . Resistance vs speed is closer to the road and damn it, I miss coasting.
Some trainers are certainly like that, especially fluid. It's mechanical trainers that really screw up the distance, proof is in the graph posted above. At 300 watts (on flat ground) are you going 32 mph? Or at the lower setting 150 watts equals 40 mph, does that seem realistic?
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Old 01-16-13, 07:32 AM
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Originally Posted by hhnngg1 View Post
Jeez - 30 mph for 300 watts?!
Looks like it's actually about 32mph, and only 350 watts for 40 mph. If that's accurate, it wouldn't be very useful for shorter intervals.
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Old 01-16-13, 07:33 AM
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Originally Posted by merlinextraligh View Post
Looks like it's actually about 32mph, and only 350 watts for 40 mph. If that's accurate, it wouldn't be very useful for shorter intervals.
If that is accurate I would be one hell of a leadout man.
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Old 01-16-13, 08:28 AM
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I count trainer miles - on a KK or Cycleops2 where it's tuned to road acceleration on a flat road. It's been very close to my outdoor miles unless I take it to the mountains.

On trainers like these - yeah, miles don't count. Unless you double them.
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Old 01-16-13, 08:32 AM
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kind of.I still notice a huge difference with the speed needed to go x miles per hour. Just saying, it isn't even close to foul proof.
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Old 01-16-13, 08:34 AM
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The solution to your math problem is to just buy a Kurt Kinetic Road machine and you won't have to worry about it.

Then you can also count your "trainer miles" if that is important to you.
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Old 01-16-13, 08:44 AM
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I've posted a bit about this before at post #17: http://www.bikeforums.net/showthread...=#post13842306

If you understand how your trainer works from a very basic level (electronic control, magnetic resistance, fluid resistance, magnetic plus) and control for the very few variables that exist on a trainer, then you can get very, very, very precise and accurate power numbers from a trainer that can be verified with Powertaps, SRMs, etc. The charts published by the manufacturers probably won't be very accurate but maybe that doesn't matter anyway because if you use their chart it will result in precise numbers for you.

Accuracy: If you want accuracy, which I will use as meaning you can compare your watts to another person's watts, then you should borrow a powertap, put it on your bike, with your trainer, and calculate the power curve for yourself (if your trainer has resistance settings you will have a different curve for each setting). If you have a mag trainer it won't be a power curve it will be a power line. If you have a trainer that has been tested over and over and over, like the Kurt Kinetic, you can probably use their formula and be pretty accurate. But accuracy is only really important if you want to compare yourself to other people.

Precision: As long as you use the same speed to power formula for each trainer workout, clamp your tire down with the same pressure each time, and have the same psi in your tire every time, your numbers from one workout to the next will be precise. If you want to measure your own individual progress by comparing one workout to the next this is all you need. The fact that you can't compare those numbers to some other person may, or may not, matter to you. If it does matter then you need both precision and an accurate formula.

Crank length and gearing are irrelevant to watts on a trainer though it may impact which muscles you are using so your body's response may differ from one gear to the next. On my mag trainer, setting 4, if I am going 20mph I am producing 175 watts (I have verified this to be within 2-3% accurate with a powertap and checked it against an SRM too). It doesn't matter if I am going 20mph in a 50x12 at 61 rpm or going 20mph in a 39x18 at 118 rpm - in both scenarios I am producing 175 watts but am using different muscles in different ways to produce those watts.

For all those people that make sarcastic comments about "speed" and "miles" on a trainer being stupid, it reminds me of those guys in social settings where if you say something their response is always "technically that's true but . . . blah, blah, blah." When someone says I went X miles on my trainer, or X mph on my trainer, do you really think they are suggesting they rode around the perimeter of their room for 10 miles or at 20 mph? In case you do think that, just keep in mind most people when their bike is on a trainer know they aren't actually going anywhere.

Speed on a trainer IS power so long as you know the conversion. Distance is a product of speed/power and time. If you know your distance and your time, you know your speed and therefore know your power as long as you know the conversion. So when someone asks whether going 30 miles on their trainer in an hour is good or not, a helpful response is it depends on the resistance of your trainer; a not so helpful response is - boy you are dumb if you think your room is 30 miles long.
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Old 01-16-13, 08:58 AM
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^ All that boils down to the important measures when riding a trainer are time and intensity.

Speed and miles on a trainer are only relevant to the extent that they allow you to extrapolate intensity.
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Old 01-16-13, 09:00 AM
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Originally Posted by merlinextraligh View Post
^ All that boils down to the important measures when riding a trainer are time and intensity.

Speed and miles on a trainer are only relevant to the extent that they allow you to extrapolate intensity.
There we go.
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Old 01-16-13, 09:11 AM
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Originally Posted by merlinextraligh View Post
^ All that boils down to the important measures when riding a trainer are time and intensity.

Speed and miles on a trainer are only relevant to the extent that they allow you to extrapolate intensity.
And if you want to accurately and precisely measure intensity you can do so best by knowing your watts which you can determine most, and very, accurately from your speed.
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Old 01-16-13, 09:33 AM
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Just wanted to pop in and mention trainerroad.com, it's basically a website that uses ANT+ speed/cadence sensors and tested power curves for a bunch of common trainers to give you a live estimate of your power on your laptop. Assuming you're using it as instructed (proper tire, proper psi, proper pressure from the drum on the tire, ect.) it can apparently give you a power estimate that's no more than a few percent off from real power data. I don't have any ANT+ devices, but if I did, I'd consider giving it a try although I'd probably end up using my HR exclusively as a measure of effort on the trainer as I do now.
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Old 01-16-13, 12:03 PM
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Anyone feel like a math problem?
Sometimes. It's usually either my digits getting too cold, or a bout with negativity.
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Old 01-16-13, 12:07 PM
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Originally Posted by vandalarchitect View Post
Thanks. Just out of curiosity, where on their website was that posted. I looked before posting and couldn't find anything even close to that.
It's the alternate picture: http://www.blackburndesign.com/train...l#.UPbsYCfAdCg
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Old 01-16-13, 05:41 PM
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Originally Posted by merlinextraligh View Post
^ All that boils down to the important measures when riding a trainer are time and intensity.

Speed and miles on a trainer are only relevant to the extent that they allow you to extrapolate intensity.
But without speed or miles on a trainer, how do you get any objective measure of intensity if you don't have a HRM or powermeter? RPE is notoriously variable for long ride efforts.
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Old 01-16-13, 05:43 PM
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Originally Posted by Dan333SP View Post
Just wanted to pop in and mention trainerroad.com, it's basically a website that uses ANT+ speed/cadence sensors and tested power curves for a bunch of common trainers to give you a live estimate of your power on your laptop. Assuming you're using it as instructed (proper tire, proper psi, proper pressure from the drum on the tire, ect.) it can apparently give you a power estimate that's no more than a few percent off from real power data. I don't have any ANT+ devices, but if I did, I'd consider giving it a try although I'd probably end up using my HR exclusively as a measure of effort on the trainer as I do now.
Just finished a workout using trainer road. Best $10 a month a person can spend training related when stuck inside. I also like that it's easy to change the ftp depending on the workout you are looking for.

http://www.trainerroad.com/cycling/rides/162249
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Old 01-16-13, 07:35 PM
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Originally Posted by hhnngg1 View Post
But without speed or miles on a trainer, how do you get any objective measure of intensity if you don't have a HRM or powermeter? RPE is notoriously variable for long ride efforts.
I think that is his point. If you don't have a PM or HRM, then speed/distance can at least give you something quantitative to work with.
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Old 01-16-13, 08:13 PM
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Completely agree with most of what has been said, I was really just looking for a way to basically say, "I just rode the trainer for X minutes at Y mph which is apparently close to Z watts ... hmm, interesting." Maybe at some level to understand watts vs. intensity (perceived exertion or HR). Not really looking to "count" trainer miles, whatever that means. Do people really get all giddy because they rode a bunch of miles, while sitting in their living room? Maybe I underestimate the competitiveness of the 41.

More than that I had seen graphs for the KK road machine and wondered if there were graphs for the trainer I just purchased, or if it would be feasible or realistic to try to calculate it myself without a power meter. Turns out there is a graph, so the rest is just extra. Of course, if anyone wants to send me a power meter they have just lying around so I can verify the graph and its accuracy ... I'll pay shipping! [Disclaimer: you may not get your power meter back, I hear the mail can be awfully unreliable.]

Originally Posted by canam73 View Post
Thanks again. It was hiding in plain sight.
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