# Question about power interval range on a trainer vs. the road



## Nevermiss (Jun 7, 2011)

I was wondering if anyone adjusts the power range for doing power intervals on a trainer vs. on the road.

I'm doing various sets of 3x3 min PIs on my trainer. My range of power for PIs is 95-100% of my CTS Field test.

Because they are longer power intervals, I'm trying to stay in the 95% range. It seems that it require more effort (Increased RPE) when doing PIs on the trainer vs. the road for maintaining the same power.

Just wondering what more experienced riders have noticed or suggest.

Thanks!


Update: It looks like some people suggest adjusting power goals down about 10% when performed on a trainer, if your FTP power was calculated using a test on the road. Thanks!


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## Peter P. (Dec 30, 2006)

Yes; you need to have different RPE's for trainer work vs. on the road.

Depending on the type of trainer you're using i.e., fluid vs. mag vs. rollers, etc., the resistance curve can vary. Some resistance curves are based on a specific weight of rider such as 160lbs. So, if you weigh less or more, the trainer's resistance level at speed X will not quite be the same for you vs. on the road.

That's all OK; it doesn't need to be that precise. Establish a separate set of RPE's for your trainer work and you'll be fine. The way to establish your "trainer correct" RPE is to perform a Carmichael Field Test ON THE TRAINER. You'll have to do this at least at the beginning of each trainer season.


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## Alex_Simmons/RST (Jan 12, 2008)

Nevermiss said:


> I was wondering if anyone adjusts the power range for doing power intervals on a trainer vs. on the road.
> 
> I'm doing various sets of 3x3 min PIs on my trainer. My range of power for PIs is 95-100% of my CTS Field test.
> 
> ...


Intervals are self correcting. If you can't complete the minimum number in the set at the target power level, then adjust the power level down next time such that you can.

Whether you have and how much the gap is between your indoor and outdoor power is variable and depends on a range of things. Some people actually put out more power indoors than out, although that's relatively rare.


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## bikerector (Oct 31, 2012)

I've found the more fans I use the closer my indoor power comes to outdoor power. I've got to believe the air, at least in my home, is less oxygen rich than training outdoors and the lack of moving air had me overheating more. With more air movement I've been able to have better workouts.

I'm not truly sure on the actual reason, but 2 full-size fans was a large improvement over just the one especially when doing longer intervals, 5 minutes or more, where the sweat really starts flowing.

My power is usually less impressive indoors though. I'll find out this spring I guess how much difference there will be over the improved indoor setup.


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## Clyde250 (Feb 24, 2007)

I notice that my indoor power is about 10% less than outdoors. And without the mental distractions it feels even lower. That being said my CTS field tests (always done on a trainer) seem to put my FTP a little high. High enough to scare me into never going that fast for the full hour.


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## deviousalex (Aug 18, 2010)

Peter P. said:


> Depending on the type of trainer you're using i.e., fluid vs. mag vs. rollers, etc., the resistance curve can vary. Some resistance curves are based on a specific weight of rider such as 160lbs. So, if you weigh less or more, the trainer's resistance level at speed X will not quite be the same for you vs. on the road.


Huh? If the OP is doing power intervals on the road it means he's using a power meter on his bike, i.e. not something like Trainer Road which does calculated power. So different types of trainers wouldn't matter.

On another note I had a similar effect with my PowerTap. I could keep 300 watts for 20 minutes on the road but could barely hold that for 3 on my trainer. My theory (I may be completely way off base here) is that the bike being fixed in the trainer was loosing a lot of energy in other movements. When I would look down at my bottom bracket while doing a hard interval I could see it swaying back on forth quite a bit, as in over ~1cm.


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## Peter P. (Dec 30, 2006)

deviousalex said:


> Huh? If the OP is doing power intervals on the road it means he's using a power meter on his bike...


I don't think so. He mentions nowhere in his post that he's using a powermeter. Since he DOES mention he's using RPE, I can safely assume that's how he's basing his effort levels.

It's quite possible the OP is using speed as his reference. Not all trainers have the same resistance curves and they're not all calibrated for the same rider weight.


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## Alex_Simmons/RST (Jan 12, 2008)

deviousalex said:


> My theory (I may be completely way off base here) is that the bike being fixed in the trainer was loosing a lot of energy in other movements. When I would look down at my bottom bracket while doing a hard interval I could see it swaying back on forth quite a bit, as in over ~1cm.


Nice theory, but no, that's not the cause of your power gap. Such losses from frame flex are tiny.

The BB moves when you're out on the road too, although the wheels take some of that movement. You just can't see it when the bike isn't fixed in position to give you a steady frame of reference.

Here are some more plausible reasons:
Alex's Cycle Blog: Turbocharged Training


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## deviousalex (Aug 18, 2010)

Alex_Simmons/RST said:


> Nice theory, but no, that's not the cause of your power gap. Such losses from frame flex are tiny.
> 
> The BB moves when you're out on the road too, although the wheels take some of that movement. You just can't see it when the bike isn't fixed in position to give you a steady frame of reference.
> 
> ...


Do you have anything to backup that the loss of power from flame flex is quite tiny? I can see this being true when the bike is out the road, but when the rear axel is fixed down like in a trainer I can see it being different.

I read up on that article and I don't disagree that those points could also contribute to this. As for the inertial load I was using the Kurt Kinect Road Machine which your article points to as a good trainer to use because of it's large flywheel. You also state you have little evidence to support this.

In reality I'd like to have an SRM/Quarq + PowerTap and see what the measured difference between them is.


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## Alex_Simmons/RST (Jan 12, 2008)

deviousalex said:


> Do you have anything to backup that the loss of power from flame flex is quite tiny? I can see this being true when the bike is out the road, but when the rear axel is fixed down like in a trainer I can see it being different.
> 
> I read up on that article and I don't disagree that those points could also contribute to this. As for the inertial load I was using the Kurt Kinect Road Machine which your article points to as a good trainer to use because of it's large flywheel. You also state you have little evidence to support this.
> 
> In reality I'd like to have an SRM/Quarq + PowerTap and see what the measured difference between them is.


Well many people (including myself) have measured power with a Powertap and an SRM, so it's not something new. When the devices' respective calibrations have been verified, then this is the level of drivetrain loss most report.

There have of course been actual studies of such things by Spicer and others, e.g. as reported in this paper:
http://www.ihpva.org/HParchive/PDF/hp50-2000.pdf

As for frame flex, well the frame is a spring, the energy to distort the spring in one direction is returned to the system when the force causing that distortion is released, and most of that energy goes into the drivetrain, not back into the rider's muscles, but here's an explanation of that:
Bicycle Frame Efficiency | Bike Think

else think of it this way - if substantial energy really were being lost in the frame, then it would heat up - the energy has to go somewhere (basic thermodynamics - conservation of energy) and usually that means heat. Does your frame get hot?


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## deviousalex (Aug 18, 2010)

Alex_Simmons/RST said:


> Well many people (including myself) have measured power with a Powertap and an SRM, so it's not something new. When the devices' respective calibrations have been verified, then this is the level of drivetrain loss most report.


Was this on a trainer? My bike was shooting flames out by the end of the trainer session


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## Alex_Simmons/RST (Jan 12, 2008)

deviousalex said:


> Was this on a trainer? My bike was shooting flames out by the end of the trainer session


Both on a trainer and on the road. No flames I'm sorry to report.


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## elbee (Jun 10, 2007)

My theory (I may be completely way off base here) is that the bike being fixed in the trainer was loosing a lot of energy in other movements. When I would look down at my bottom bracket while doing a hard interval I could see it swaying back on forth quite a bit said:


> .. or your head is moving slightly side to side as you pedal so that you are viewing the BB from different angles on each pedal stoke - giving the impression that the BB is moving.


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## jspharmd (May 24, 2006)

For me, the mental part is the biggest key. As Alex states in his blog, motivation is a factor that has an effect on indoor vs outdoor power. I find that it sometimes feels extremely hard to maintain a certain power output (that is normally easy outside). However, occasionally I find myself not paying as close attention to my power when indoors and when I look down, the power that was previously more difficult feels much easier. Being too focused on the power when indoors hurts my efforts. I don't look down at the power readings that much when outdoors. 

For me the temp is not as big a factor as I live in the South and rides in 90+ degrees with 90%+ humidity can be worse than an indoor trainer ride in the winter, although a fan makes it feel more like I'm riding outside.


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