Accuracy, Precision and Virtual Power...

I'm a big fan of using indoor trainers for building cycling power and strength.  I recommend the use of trainers for interval work all year round.  When the weather is nice, it is hard to beat riding outdoors for longer endurance rides.  But, utilizing a trainer allows you to target and execute on very specific intensity levels without the concern or interruption from traffic, intersections or weather.  How beneficial can quality trainer work be?  Check out this record setting 24-hour cycling performance, where the vast majority of this athlete's training was done on an indoor bike trainer.

One of the best ways to target and quantify workout intensity while cycling is to use power as a metric. Unlike heart rate, power is not impacted by fatigue, core temperature or hydration level.  Power also reacts very quickly to changes in intensity (which HR does not), allowing for quantification of short duration VO2 max sets, such as 30 second billats.  Although prices for power meters have dropped substantially in the past couple of years, the price of power meters is still a significant obstacle for many cyclists.  Likewise, electronic "smart" trainers than can actively control resistance (e.g. you dial in 200w and it provides 200w of resistance, like a Wahoo Kickr or CompuTrainer) tend to cost as much or even more than a power meter, putting them out of reach of many athletes.

During the past couple of years, a few software programs / apps have created a simulated power metric, called "virtual power" (e.g. TrainerRoad).  Using an Ant+ or Bluetooth speed sensor mounted to your rear wheel, the app reads your speed and uses a mathematical equation to estimate power based on a known or estimated resistance curve.  The user can then see their "virtual power" and change their intensity level to meet their specific workout targets.  Utilizing virtual power for indoor training purposes allows the user to get many of the benefits of training with power, without the need of the up front cost of an actual power meter.  

Each manufacturer designs their trainers with their own proprietary resistance curve, which is typically a function of the trainer design.  As such, some indoor trainers are much better than others for use with virtual power.  To understand why, it is important to understand the concepts of precision and accuracy.  Below is a great graphic providing an overview of both concepts, and how they relate to each other:

Ideally, your trainer would have both high accuracy and high precision.  In my experience (both personal and with athletes that I coach), I've found the Kurt Kinetic Road Machine to be a great trainer to provide both high accuracy and high precision.  They have been designed specifically for this purpose, and unlike other manufacturer's, Kurt Kinetic actually posts their resistance curve on their website.   

I've had a couple athletes that I coach use both virtual power and then actual power (from a power meter) using two different fluid-based indoor trainers.  The first used a Kurt Kinetic Road Machine, and the second used a CycleOps Fluid 2 trainer.  Both were using TrainerRoad to calculate virtual power.  I looked at a series of segments from multiple rides to see how the actual power compared to the virtual power calculation.  Here's how the Kurt Kinetic trainer performed:

Kurt kinetic, virtual power from website data.

Kurt kinetic, virtual power from website data.

This first graph (above) is a plot of the virtual power from the equation listed on their website.  The graph below is how TrainerRoad estimated virtual power versus the published data:

virtual power from trainerroad, plotted on kurt kinetic equation... a good fit of data!

virtual power from trainerroad, plotted on kurt kinetic equation... a good fit of data!

As you can see, the virtual power (blue diamonds) from TrainerRoad lined up very close to calculated data from the Kurt Kinetic website.  This shouldn't be a surprise, as they likely use the published resistance curve. Below is the actual power data (red squares) plotted on top of the previous graph:

Actual power plotted against virtual power for the kurt kinetic road machine.

Actual power plotted against virtual power for the kurt kinetic road machine.

As you can see, the actual power was very close to the virtual power calculation... well within the power meter's level of accuracy (within plus or minus 2%).  This is an example of a trainer that is both precise and accurate.  

Next we'll look at a CycleOps Fluid 2 trainer.  I find these to be very popular locally, with a lot of athletes using them.  Unfortunately CycleOps does not publish a resistance curve equation, just a picture of their resistance curve on their website.  

Cycleops fluid2 trainer plot and polynomial trend, based on trainerroad virtual powerA

Cycleops fluid2 trainer plot and polynomial trend, based on trainerroad virtual powerA

As you can see, the virtual power calculations from TrainerRoad follow a very consistent and predictable trend for the CycleOps Fluid 2 trainer.  Now, let's see how the actual power compares to the calculated virtual power:

cycleops fluid 2 virtual power plot versus actual power.  red squares are the actual power with blue diamonds being the virtual power.

cycleops fluid 2 virtual power plot versus actual power.  red squares are the actual power with blue diamonds being the virtual power.

In this case, you can see the TrainerRoad virtual power model for the CycleOps Fluid 2 is neither accurate nor precise. The average error is around 15%, but it ranges from less than 1% error on a few data points at the high power end to over 50% on the lower power values.  One could argue that the problem is simply that TrainerRoad's virtual power model is creating the accuracy problem.  That may be a contributing factor, but part of the accuracy problem could also be a result of the lack of precision or repeatability of the data... it's hard to create an accurate mathematical model with wide swings in data.  Without publishing an actual equation for the speed / power model, it's difficult to tell what the manufacturer had targeted for a resistance curve.  Likewise, although the plot above is the aggregate data, there was distinct ride-to-ride variation (one day to the next).  Even worse, there was variation within the ride... the longer you ride, the more the resistance increases for the Fluid 2.  Looking at one example, the average for three segments approximately three minute in length within a single longer interval set resulted in the following speed / power relationships: 14.6/126, 14.3/129, and 13.9/133.  As the segment continued, the speed dropped by 4.8%, while the power rose by 5.6%.  In this particular example, the virtual power would have shown power dropping from 154 to 143 as speed dropped, while power actually increased from 126 to 133.

So, what does this mean?  If you want to use virtual power, your best bet is to get an indoor trainer that is both accurate and precise. 

  • With a trainer that lacks precision and accuracy, your FTP (functional threshold power) would not be comparable to anyone else's, and you may not have confidence that you were actually executing on your targeted power zone.  I would be inclined to watch my heart rate closely under these circumstances, to ensure I was on target.  Or... if you are thinking about upgrading your trainer, rather than spending $300+ on a new trainer, consider the possibility of simply investing in one of the newer lower costs power meters instead (e.g. 4iiii Precision or Stages). Having actual power trumps the accuracy and precision issues of a trainer, and can be used outdoors for training too.  
  • If your trainer is precise (repeatable day to day and within the workout), but not accurate, then comparing your FTP with someone else's FTP is meaningless.  But, with a precise trainer and virtual power, you can establish a virtual FTP value and feel confident that when you are targeting zone 2 or zone 5 work, you are actually training within those zones.  
  • With a trainer that is both accurate and precise, you have much more confidence that you can compare your power output with others, and that if you are targeting Zone 2 or Zone 5 work... you are actually training within those zones.   

Train smarter... not harder.


  1. I have no vested interest in the Kurt Kinetic trainers, and get no income if anyone buys their product.  I have simply found that every time I have verified trainer performance with a power meter, the Kurt Kinetic Road Machines have followed the predicted model very closely.  Here is a great video giving more details about their fluid resistance unit.





The often unappreciated and sometimes misunderstood bike tire... :(

In cycling, aerodynamics is king and gets nearly all the attention... aero bikes, aero wheels, aero helmets, computer bike fitting... it's all high tech and "sexy" stuff for the geeky triathlete.   On the other hand, tires are often simply considered a "wear item" that needs to be replaced periodically and pumped up before you ride.  

Before you write off tires an unimportant, let me help you understand why tire choice matters.  For a typical age group triathlete who generates around 3 watts/kg on the bike, you have far more to gain or lose through tire choice than any gains you can make with a set of Zipp wheels.  

To demonstrate this, I compared two very popular Continental tires: Gatorskins and GP4000s.  Gatorskins are a top seller for good reasons.  They are known for being puncture resistant, they last a lot of miles and are considered a top training or commuter tire.  The GP4000s are a top selling racing tire, known for it's low rolling resistance.  But how much does rolling resistance really matter?  I created a model in Best Bike Split and compared two sample riders (a hypothetical male and female) over Olympic and Ironman length courses to see just how much difference tires can make.  I ran a total of six scenarios per rider, per course (total 24 runs) and the short answer is for your average age grouper in the 16 to 20 mph range... choosing a racing tire over a training tire provided more speed gains than paying $250 for an aero helmet and $3000 for a deep carbon front tire and running a solid carbon disc on back.  For an Ironman length race, the GP4000s tires save around 16 minutes over the Gatorskins at the same power level and the full aero accessory package (helmet & wheels) saves only around 7 minutes.

I've listed the results below.  From a modeling standpoint, I've assumed 3w/kg FTP, IF's of 90% for Olympic and 70% for IM, CdA's of approximately 0.31 (typical of non-optimized body position), 6' 175 lb male, and 5'-4" 125 lb female.  The scenarios show below are 1) Gatorskin tires with standard road helmet and wheels, 2) GP4000s with standard road helmet and wheels, 3) GP4000s with Latex tubes (rather than standard butyl tubes), 4) Gatorskin tires with aero helmet, 5) Gatorskin tires with aero helmet, 808 front & disc rear, 6) GP4000s with latex tubes, aero helmet, 808 front & disk rear:

Because not everyone does long course racing, I also included an Olympic length race so you can see the impact there as well.

In terms of dollars spent for speed gained, tires are a great "bang for your buck".  Although Gatorskins and GP4000s tires are on opposite ends of the spectrum in terms of rolling resistance, most manufacturer's standard tires are probably somewhere in the middle of these two... they don't offer the puncture resistance of a Gatorskin, nor do they offer the speed of a GP4000s.  Depending on their actual performance, you may see half of the time savings above... which would still be comparable to the full aero accessory package.  But what about wear and puncture resistance of racing tires?  Everyone may have different results depending on the riding you do.  I personally had a lot of pinch flats when I rode Michelin Pro Race 3 tires, but have been riding GP4000s for over 3 years and haven't had one.  Your mileage may vary.

Want to compare some of the popular tires out there?  This site does a great job of testing tires and showing how much energy they consume.  The results are "per tire" so you need to double them for a set of tires.

Before running off any buying new tires, there are a few more important things to understand about tires:

  • It is important to have the proper pressure in your tires.  Your tire pressure is determined by the weight you put on the tires (bike + body weight + water bottles + etc.).  This article does a fantastic job of explaining that ideally your tire will have a "15% drop" when optimally inflated, and has charts to demonstrate weight versus pressure with varying tire size.  If you want the "easy button", here is a calculator that will figure out how much pressure you need based on your weight.  For most triathlon bikes, you can use a 45/55 split of weight for front/rear (used the second calculator from the top... and don't forget to add your bike weight).  I suspect that many heavier riders fail to use a larger enough tire (25mm rear) and many light riders tend to add more pressure than necessary (particularly in the front tire).
  • Higher pressure isn't necessarily better.  On smooth surfaces, higher pressures result in lower rolling resistance... this has been demonstrated in multiple tests and can be seen in the rolling resistance testing site I referred to earlier.  But the real world isn't smooth like a stainless steel roller.  And there are diminishing returns... going above 100 psi provides very little improvement in rolling resistance.  In the real world, when you go over bumps there are "suspension losses" meaning some of the energy that is supposed to be pushing you forward instead pushes you upward as you go over bumps.  Excess inflation makes for very bumpy and jittery rides, particularly on a Tri bike where your elbows are resting firmly on elbow pads with no suspension from your wrists.  Way too many people put excess pressure in their tires on race day, assuming it will make them faster... when it could actually make them slower.  I noticed a significant improvement in both ride quality and control when I lowered my front tire to around 90 psi, based on the weight recommendations.  Here is a great article on suspension loss, where these people actually compared riding on the road versus rumble strips to see the impact on power loss.  Not surprising... there was a huge difference. 
  • Wider tires have less rolling resistance than narrower tires.  Again this can be seen on the testing on rolling resistance curves.  Does that mean you should get the biggest tires possible?  No, go with what fits in terms of pressure, under the first point above.  Wider tires can potentially have higher aerodynamic losses than narrow tires.  Most wheels made since 2014 are wider, making 23mm tires effectively 25mm wide which helps with rolling resistance and aerodynamics (the tires end up being flush with the wheels reducing turbulence and improving cross wind performance).  The important point is that if your weight dictates that your need wider tires, they will actually be faster than narrow tires for you.  There is also the option of mixing sizes, such as 23mm front and 25mm rear to optimize rolling resistance based on weight, while getting the benefit of having the rear tire tucked behind the frame on a Tri bike.  Note that you do need to verify that a wider tire will fit in the rear with your frame.  For some Tri bikes the frame is so close to the tire that you could potentially end up with tire rub with larger tires.
  • Not all tubes are created equal.  Because of additional elasticity, latex tubes have lower rolling resistance than standard butyl tubes.  The downside is they are more expensive, lose air faster and they need to be installed with some talc powder to avoid having them stick to the tire and pop (I've blown a couple when I didn't use talc powder on the first inflation).  But there are additional upsides as well.  Besides having lower rolling resistance, latex tubes are very light weight, and some people feel that they have better flat resistance and have a smoother ride.  My own personal experience is that bumps seem to be a little less "sharp" on latex.  The other option is to consider using a lighter weight butyl racing tube rather than latex.  They are between standard tubes and latex in terms of price and performance, but may not offer as good of flat protection.  Light butyl tubes are available in 650 tires, and I've yet to find latex in the smaller tire size.   Here is a test result showing the differences between standard butyl tubes, light butyl tubes (racing), latex and tubeless tires.  

So... something as simple as bike tires do matter.  And it's a relatively inexpensive and easy way to improve your bike speed.


  1. It's also important to understand that although bike tires trump aero in this example, that may not always be the case.  In optimized body positions, top age groupers are typically in the 0.24 to 0.27 CdA range and are traveling at higher speeds where aerodynamics starts being a much larger portion of total bike power. 


Cycling... what does that extra 5% in speed really cost you?

For most triathlons, you spend the majority of your time on the bike.  If you want to be a faster triathlete, it makes sense to optimize your bike speed.  So if you are doing long course races (70.3 to 140.6), this means that you can cut substantial time off your total race by maximizing your bike speed… right?

The short answer is yes & no.  Yes, you can have a good bike split, but it may result in a really poor run… more than negating the gains you made by riding fast.  Here's why:  The physiological cost of increasing your speed is not linear.  In the following example for a full IM, a 5% increase in bike speed will "cost" you more than 21% in additional total physiological stress on your body.

Let's play with some numbers:

  • If I average 18 mph on the bike for 112 miles, my total ride time would be 112 miles / 18 mph = 6.22 hours (6:13).
  • If I can squeeze out 5% more speed, my average speed would be 1.05 x 18 = 18.9 mph.  Getting an additional 0.9 mph can't be too hard, can it?  At 18.9 mph, my total ride time would be 112 miles / 18.9 mph = 5.93 hours (5:56).  Basically 5% more speed translates to a 5% time savings, or nearly 17 minutes on a 112 mile ride.  That is a big savings.

Of course… there's a cost of going faster.  And, as mentioned above, the relationships are not linear.

Power is not linear with speed, due to the increasing aerodynamic drag.

Power is not linear with speed, due to the increasing aerodynamic drag.

First, a brief background on bike power.  Whether you train with a power meter or not, it is important to understand the basics of bike power.  The total amount of power it takes to achieve a given speed is a function of aerodynamic drag, rolling resistance, transmission losses (chain/gear friction) and the effects of gravity if you are on a hill.  What is important to understand is that while the impact of gravity and rolling resistance is a linear relationship, changes in speed impacts the aerodynamic power component as a cubed function.  Unless you are climbing or cruising at less than 10 mph, this means that changes in speed require a much greater corresponding change in power.  For example, using the speeds above and an online speed / power calculator, the resulting power is:

  • At 18 mph (see note 1 below for model / input), the required watts on a flat road is 116.51.
  • At 18.9 mph (same input conditions), the required watts on a flat road is 131.93.
  • The increase in power required is 131.93/116.51 - 1 = 13.23%.  So to go 5% faster… you need to produce 13.23% more power (physical work).

But, that isn't really the end of the story, as you also need to consider the impact of this increase in power on your body.  A common factor used to quantify the total physiological impact of a workout on the body is Training Stress Score (TSS).  Essentially it combines the intensity of the workout (Intensity Factory - IF) and duration, to come up with a single factor of how hard you worked your body in the workout or race (see note 2 below).  TSS is calculated by:

TSS = IF x IF x Duration (hrs) x 100

The Intensity Factor is simply how hard your average output is versus what your maximum output is for a 1 hour period.  So, if you could produce 185 watts for 1 hour at maximum capacity, your IF for a ride at 116.51 watts would be:  116.51/185 =0.63.  At 131.93 watts (second scenario), your IF would be:  131.93/185 = 0.71.  Using these to calculate Total Stress Scores:

  • TSS for 18 mph = IF x IF x Duration x 100 = 0.63 x 0.63 x 6.22 hrs x 100 = 246.9
  • TSS for 18.9 mph = 0.71 x 0.71 x 5.93 hrs x 100 = 298.9
  • The resulting increase in physiological stress:  298.9 / 246.9 - 1 = 21.1% increase.

So the question is:  Is the 5% bike speed increase is worth the 21.1% increase stress on your body?  As a rule of thumb, the upper limit of TSS scores during an IM is around 280 for a strong IM athlete and an upper limit of 260 for weaker runner or novice IM athletes.  Few pro's push to 299 TSS values, so likely the 5% increase above would likely result in a poor IM run overall (see note 3 below for more information).

Ideally, you balance your pacing to get close to the target TSS values without going over and resulting in a significant negative impacting your run… meaning your run turns into a long and leisurely stroll.  I'll discuss strategies for this in a future article.

In the interim, train smarter not harder.


  1. Calculator example.  I used 165 lbs for the rider, 20 lbs for the bike, CdA of 0.271, CRR of 0.004, Rho 0.076537, 3% drive loss, with no incline (these are similar to my numbers).  There are several similar calculators online, converting speed to power or power to speed.  Note these models assume constant speed, and don't factor in acceleration.  With hills, turns, stop/starts, your average speed versus average watts will be quite a bit different than you will see in these models (higher average wattages for lower speeds).  They are still useful tools at comparing scenarios, if used properly.  Other bike power/speed calculators I use are here and here.
  2. Note that universally a 5% increase does not exactly represent a 21.1% increase in physical "cost" across the board, as it is a function of your specific IF factors as well as the starting point (speed) you are referencing.  If you were comparing 20 to 21 mph, your "cost" would actually be higher than this (due to the increased aerodynamic drag) and it would be lower if you were comparing 15 to 15.75 mph.  The math stays the same, as does the general trend… increasing your speed 5% takes a much larger toll on you body.
  3. Definition of NP, IF, TSS (TrainingPeaks).
  4. Joe Friel IM Bike Pacing blog (TrainingPeaks).
  5. One caveat to Joe's site above and my comments on Ironman TSS values, is that if you look at the pro women, they seem to run higher TSS values than the guys (overall) and do push into the 300 range.  Here is a list of files for 2013 and 2012 for reference.  Two top age group female athletes have shared IM files with me, and I have noticed that they also seem to be able to push higher TSS values and still run well, making me wonder if women are able to hold higher biking IF values for longer duration than men.