Pursuit watts/cda?

whoawhoa said:

Does anyone have a model for the watts:cda ration it takes to ride various 3k pursuit times?

Of course. 😄 However, to accurately predict one's time you need some other information as well, e.g., peak power, mass, coefficient of rolling resistance, track length/path.

For a first approximation you could use the "power, given speed" calculator at www.analyticcycling.com and assume that 1) you'll lose 4-6 s getting up to speed, and 2) your overall power requirement will be ~10% higher (due to the start).

acoggan said:

For a first approximation you could use the "power, given speed" calculator at www.analyticcycling.com and assume that 1) you'll lose 4-6 s getting up to speed, and 2) your overall power requirement will be ~10% higher (due to the start).

Thanks...but why add 4-6 seconds and 10% to the average power?

whoawhoa said:

Thanks...but why add 4-6 seconds and 10% to the average power?

The calculator assumes that speed is constant over time. During the pursuit, however, you have to 'invest' additional kinetic energy in the system, which on average adds about 10% to the overall power requirement. In addition, however, you have to account for the time needed to get up to speed.

To give a numerical example: if you run the calculator using all of the default values except for slope (which needs to be set to zero), www.analyticcycling.com predicts that it takes only 336 W to ride a 4:00 3 km pursuit (i.e., 12.5 m/s). In reality, however, the power requirement would be closer to 391 W, which is the value you get by 1) calculating the power required to ride 3 km in 3:55, and then 2) multiplying this value by 1.1.

whoawhoa said:

Thanks...but why add 4-6 seconds and 10% to the average power?

Due to standing start / 1st lap. 1st lap will take 4 or so seconds longer than the remaining "steady state" laps. The power spike during the acceleration phase will lift overall average power for the full pursuit by approx 10% over your steady state power. Sort of depends on how you ride the pursuit.
Sample wko attached.
Avg for 3km = 384W
Steady State = 359W

OK ~ 7% difference but I had another a month later where difference between overall and steady state was 9.5% (last few laps were not pretty).

Alex Simmons said:

Due to standing start / 1st lap. 1st lap will take 4 or so seconds longer than the remaining "steady state" laps. The power spike during the acceleration phase will lift overall average power for the full pursuit by approx 10% over your steady state power. Sort of depends on how you ride the pursuit.
Sample wko attached.
Avg for 3km = 384W
Steady State = 359W

OK ~ 7% difference but I had another a month later where difference between overall and steady state was 9.5% (last few laps were not pretty).

Thats an interesting file. Your speed graph is sine graph (and to a degree so is the power graph). How does this correspond to riding around the track i.e. at what points on the track is the speed highest/lowest and does this correspond with the highest/lowest power?

mickthomas said:

Thats an interesting file. Your speed graph is sine graph (and to a degree so is the power graph). How does this correspond to riding around the track i.e. at what points on the track is the speed highest/lowest and does this correspond with the highest/lowest power?

Speed of the wheels (which is what is measured) is higher in the turns and lower on the straights. This is the result, in part, of trading potential energy for kinetic energy when you bank over at the entry of the turn, i.e., your center of gravity drops, which results in your speed increasing (since energy must be conserved). In addition, because you're now leaning over your center of resistance (which is primarily aerodynamic) travels inside/a shorter distance than your wheels, which enables you to apparently speed up even more. All of this is reversed when you exit the turn, which is why it feels a bit like you're briefly riding uphill at that point.

Since it is difficult to maintain power perfectly constant when cadence is increasing (e.g., cresting a hill and starting down the other side), but easier to do so when cadence is decreasing (e.g., as you hit at the base of a climb), power tends to flucuate in opposition to speed (i.e., lower in the turns and higher on the straights). Experienced riders are good at minimizing this flucuation, but it's always there, at least if you look closely enough.

acoggan said:

Speed of the wheels (which is what is measured) is higher in the turns and lower on the straights. This is the result, in part, of trading potential energy for kinetic energy when you bank over at the entry of the turn, i.e., your center of gravity drops, which results in your speed increasing (since energy must be conserved). In addition, because you're now leaning over your center of resistance (which is primarily aerodynamic) travels inside/a shorter distance than your wheels, which enables you to apparently speed up even more. All of this is reversed when you exit the turn, which is why it feels a bit like you're briefly riding uphill at that point.

Since it is difficult to maintain power perfectly constant when cadence is increasing (e.g., cresting a hill and starting down the other side), but easier to do so when cadence is decreasing (e.g., as you hit at the base of a climb), power tends to flucuate in opposition to speed (i.e., lower in the turns and higher on the straights). Experienced riders are good at minimizing this flucuation, but it's always there, at least if you look closely enough.

Yes, and the power meter shows that even though I'm an experienced trackie, I'm still learning the subtleties of pursuit riding....

There are other factors at play in the way PT records data but certainly smoothness is something for me to consider as I practice.

Only a power meter would tell you such info BTW, there is no way a stopwatch could identify this.

Alex Simmons said:

Yes, and the power meter shows that even though I'm an experienced trackie, I'm still learning the subtleties of pursuit riding....

The fact that you race on a 250 m track also plays a role.

Alex Simmons said:

There are other factors at play in the way PT records data but certainly smoothness is something for me to consider as I practice.

An interesting question to contemplate*: precisely how steady should your power really be? If it's perfectly constant, then you'll speed up even more in the turns, which means that during that time you're battling greater aerodynamic drag than you would be if you achieved the same lap time by riding the turns and straights at a more constant speed. OTOH, to keep speed perfectly constant requires the largest flucuations in power output, which is likely to hurt your performance when you find yourself unable to generate the requisite surge in power at the exit to the turn late in the race.

*That is, if you find this sort of navel-gazing to be interesting! 😄

Alex Simmons said:

Only a power meter would tell you such info BTW, there is no way a stopwatch could identify this.

True 'dat!

acoggan said:

An interesting question to contemplate*: precisely how steady should your power really be?

It's an interesting point. I almost universally get complimented by how "smooth" I appear on the bike and also by those that follow my wheel, yet I will still see these fluctuations in power data. I certainly don't expect it to become completely smooth as that would be counter productive. The wko file pic I posted before does show the power smoothing out the further into the pursuit I got.

Then there are those that suggest taking a wide line in the straights. I'd need some convincing of that one. I'd say that's a track specific issue.

acoggan said:

*That is, if you find this sort of navel-gazing to be interesting! 😄

Well I don't know how much power data there is in belly button fluff but I'm always willing to try and chart it😄😄

acoggan said:

An interesting question to contemplate*...

I remember reading about Sosenka (SP?), the big Czech who broke the hour last year using like 4KG wheels… and I thought “what good will that do”... doh, forgot about the turns.

wilmar13 said:

I remember reading about Sosenka (SP?), the big Czech who broke the hour last year using like 4KG wheels… and I thought “what good will that do”... doh, forgot about the turns.

4KG wheels ! I thought that concept of a flywheel type effect had been rejected back in the 1980s after Moser first record - the first disc wheels weighted a ton and Moser apparently had his weighted towards the rim.

Hi guys

I know this thread is forever old but the information still interesting for me to try to interpret

I have seen a few different watts vs time for pursuit and they seem to be a pretty big difference in watts from around 380 to 411 for around 03:45.

I plugged in .44 * .5 for my CdA which I think would be high for a fairly aggressive position.

82 kg, 0.03 rolling resistance (concrete track), 0.00 slope and 380 watts. I got 13.7 m/s. I tacked on 10% and got about 420 watts. So in theory if I can pace myself and do 420watts with the above CdA I should be able to do a 3:45?

InPursuit said:

Hi guys

I know this thread is forever old but the information still interesting for me to try to interpret

I have seen a few different watts vs time for pursuit and they seem to be a pretty big difference in watts from around 380 to 411 for around 03:45.

I plugged in .44 * .5 for my CdA which I think would be high for a fairly aggressive position.

82 kg, 0.03 rolling resistance (concrete track), 0.00 slope and 380 watts. I got 13.7 m/s. I tacked on 10% and got about 420 watts. So in theory if I can pace myself and do 420watts with the above CdA I should be able to do a 3:45?

You need to add in the 4-7 s you'll lose getting up to speed in the first place. Other than that, you're in the right ballpark, e.g., my PR is 3:47 on a power of 397 W whereas my wife's is 3:49 on an average power of 411 W.

Alex, Andy....fair to say that to do a sub four min 3k pursuit requires a w/cda greater than 1700 at sea level?

DancenMacabre said:

Alex, Andy....fair to say that to do a sub four min 3k pursuit requires a w/cda greater than 1700 at sea level?

Depends on the track surface but no, I'd say less than that, more like >1400W/m^2

I have a long overdue blog post on this to do, with chart and table comparing pursuit times with W/CdA.

Let us know when that blog post goes up!

I have few times and power to compare. Alex, Andy and AaronT have given up power for their 3k and 5k times.

CdA seems to be the biggest unknown to me. I am estimating it at .22 but really hard to say with the widening of the arms compared to TTing adding to the drag, but also by the more aggressive bike and long sleeve skin suit reducing it... shorter duration allows for a more aggressive position to be held if I can find it.

Going a little off topic but it applies to the energy used in the start and it's affect on your ability to go faster during the steady state laps.

My understanding is that here in the USA, the masters races are TTs not pursuits. If you are doing a "TT" versus a pursuit (where you have to worry about an opponent) would it be suggested to start slower, and preserve that energy for your time when you can go much faster in an aerodynamic position?

Finally is there a generally accepted point where it is "smart" to get into your aero position... On the road I have read anything above 15mph-17mph which would be pretty quick on the track I imagine.😕

Alex Simmons said:

Depends on the track surface but no, I'd say less than that, more like >1400W/m^2

I have a long overdue blog post on this to do, with chart and table comparing pursuit times with W/CdA.

I was perusing your blog but must have missed the fabulous new pursuit watts/cda entry *Nudge, nudge* :-) :-)

Right now on a road bike, I'm not at 1400 but getting closer with more and more TT specific riding...

Anyhoo, I'm not asking for coaching advice here Alex ( i have respect for the profession & those that do it seriously) and putting aside the physiological requirements....but do you think someone (re: that would be me) can do an excellent, say < 4m pursuit without living near a track? IOW, training overwhelmingly on a road bike with very specific sessions that would mimic track riding (aka, high cadence L5/L4, standing starts, etc) ? Andy if you happen to read this, your opinion would be most welcome too.

DancenMacabre said:

do you think someone (re: that would be me) can do an excellent, say < 4m pursuit without living near a track?

Depends on what you mean by excellent.

If you'd asked whether you could do the best you are capable of without access to training on the track, then I would say no. There are a number of things that require practice and experience. Of course if you have prior substantial track pursuit experience, then you don't need all that much track time leading up to race day.

Can you get into excellent physical shape to ride a good pursuit without access to a track? Most definitely.

Alex Simmons said:

Depends on what you mean by excellent.

If you'd asked whether you could do the best you are capable of without access to training on the track, then I would say no. There are a number of things that require practice and experience. Of course if you have prior substantial track pursuit experience, then you don't need all that much track time leading up to race day.

Can you get into excellent physical shape to ride a good pursuit without access to a track? Most definitely.

Thanks Alex.

When I say "excellent", I would envision that as being a sub 4 minute 3k at say, 1800m of altitude. Now I know for some that might be a rubbish pursuit, but I'd be quite happy and maybe that's good enough for a respectable placing in "masters" track here in America:-)

Guess I figured as you said, that you can get all the fitness necessary without being on the track per se. Two big things seem kinda daunting from my point of view....a) Being able to make 300+ watts........at 120+ rpm and b) holding a precise line around the track while riding at aforementioned cadence & power.

I vaguely remember reading an archived post somewhere from A. Coggan (not Andy 😄 ) about how difficult it was to do very high cadence L4/L5 work and how much lower power was as compared to "self selected" cadence. Sounds painful just thinking about it...