I’ve seen the WAG of 5W being roughly equivalent to 0.5s/K. So, for a half-iron bike leg, does an extra 10W really only improve your time by 90s? (Seems like it should be more than that.)
Thanks.
I’ve seen the WAG of 5W being roughly equivalent to 0.5s/K. So, for a half-iron bike leg, does an extra 10W really only improve your time by 90s? (Seems like it should be more than that.)
Thanks.
It’ll obviously depend on how much power you put ought to begin with. My very rough rule of thumb is that you can expect to go (X)^1/3 % faster where X is the percentage increase in watts. So let’s say you ride a half iron at 200w avg. Then 10w is a 5% increase in power. You’ll then get a (5)^1/3 % increase in speed. So you’ll go 1.24% faster. So if 200w gets you a 2:45 bike split, which seems reasonable, you’ll go about 2minutes faster on the extra 10w.
So yeah, your 90sec improvement is entirely reasonable of the 5w = 0.5s/km rule of thumb. I just like the cube root rule because it takes into account starting power. It’s a very crude simplification though, and becomes less accurate over a hilly course (of course).
Thank you, Jordan.
I’m trying to set my goal wattage for Kansas 70.3 (which is pretty hilly) using the Endurance Nation TSS tables, my estimated bike split and a FTP, and trying to determine what the “cost” of being a little extra conservative would be. (I’ve blown up on the run due to biking too hard in my two previous halfs - before I got a PM.) Seems like not much for someone who’s not a podium threat.
I just like the cube root rule because it takes into account starting power.
It’s also so easy to calculate in your head.
I’ve seen the WAG of 5W being roughly equivalent to 0.5s/K. So, for a half-iron bike leg, does an extra 10W really only improve your time by 90s? (Seems like it should be more than that.)
Thanks.
It’ll obviously depend on how much power you put ought to begin with. My very rough rule of thumb is that you can expect to go (X)^1/3 % faster where X is the percentage increase in watts. So let’s say you ride a half iron at 200w avg. Then 10w is a 5% increase in power. You’ll then get a (5)^1/3 % increase in speed. So you’ll go 1.24% faster. So if 200w gets you a 2:45 bike split, which seems reasonable, you’ll go about 2minutes faster on the extra 10w.
So yeah, your 90sec improvement is entirely reasonable of the 5w = 0.5s/km rule of thumb. I just like the cube root rule because it takes into account starting power. It’s a very crude simplification though, and becomes less accurate over a hilly course (of course).
Is the cube root of 5 not 1.7?
Extrapolating out from this are you saying that 200w would be in and around a 5:30 IM bike, and to go 5:00 you were need to go 9% faster. (750)^1/3 = 9 - so you’d need a 750% increase from 200 watts??? What have I missed?
Also would 200watts not yield a slightly faster IM bike split than 5:30? (for a reasonable weight person on a typical course with a reasonable cda)
Sorry, for some reason 3-x_root_y_5 on my calculator gave me 1.24. But it is 1.7.
And I’m actually a real dumbass.
It should be cube root of the percentage.
So to go 9% faster - 1.09^3 → 1.29 or a ~30% increase in watts.
And for the case of a 5% increase power, it’s 1.05^1/3 for 200w starting, so 1.016 or a 1.6% increase in speed.
Sorry. I just woke up…
@RChung, that’s why you always keep your smartphone with you. 
Sorry, for some reason 3-x_root_y_5 on my calculator gave me 1.24. But it is 1.7.
And I’m actually a real dumbass.
It should be cube root of the percentage.
So to go 9% faster - 1.09^3 → 1.29 or a ~30% increase in watts.
And for the case of a 5% increase power, it’s 1.05^1/3 for 200w starting, so 1.016 or a 1.6% increase in speed.
Sorry. I just woke up…
@RChung, that’s why you always keep your smartphone with you.
Cheers
So 260 watts avg for a 5:00 bike. Always wondered what it would take.
Sorry, for some reason 3-x_root_y_5 on my calculator gave me 1.24. But it is 1.7.
And I’m actually a real dumbass.
It should be cube root of the percentage.
So to go 9% faster - 1.09^3 → 1.29 or a ~30% increase in watts.
And for the case of a 5% increase power, it’s 1.05^1/3 for 200w starting, so 1.016 or a 1.6% increase in speed.
Sorry. I just woke up…
@RChung, that’s why you always keep your smartphone with you.
Cheers
So 260 watts avg for a 5:00 bike. Always wondered what it would take.
You’ll go faster than that on 260, mostly because 200w should get you a lot more than 5:30. W/kg also is a factor, and becomes more of a factor depending on terrain. If you are riding 200w and going 5:30, you have a lot of speed to gain just in “free” speed.
Sorry, for some reason 3-x_root_y_5 on my calculator gave me 1.24. But it is 1.7.
And I’m actually a real dumbass.
It should be cube root of the percentage.
So to go 9% faster - 1.09^3 → 1.29 or a ~30% increase in watts.
And for the case of a 5% increase power, it’s 1.05^1/3 for 200w starting, so 1.016 or a 1.6% increase in speed.
Sorry. I just woke up…
@RChung, that’s why you always keep your smartphone with you.
Cheers
So 260 watts avg for a 5:00 bike. Always wondered what it would take.
You’ll go faster than that on 260, mostly because 200w should get you a lot more than 5:30. W/kg also is a factor, and becomes more of a factor depending on terrain. If you are riding 200w and going 5:30, you have a lot of speed to gain just in “free” speed.
Ahhhh. Suppose I was just trying to work out what 230w avg on a course like Austria would get me around in. (70kg)
Thanks!
Do you know what X watts gets you - speedwise - for a reasonably flat 20km TT? Then you can back calculate a rough CDA.
I would think that with reasonably good equipment, on a fast course like Austria 230w @ 70kg should get you 4:45 or so.
I’ve seen the WAG of 5W being roughly equivalent to 0.5s/K. So, for a half-iron bike leg, does an extra 10W really only improve your time by 90s? (Seems like it should be more than that.)
Thanks.
It’ll obviously depend on how much power you put ought to begin with. My very rough rule of thumb is that you can expect to go (X)^1/3 % faster where X is the percentage increase in watts. So let’s say you ride a half iron at 200w avg. Then 10w is a 5% increase in power. You’ll then get a (5)^1/3 % increase in speed. So you’ll go 1.24% faster. So if 200w gets you a 2:45 bike split, which seems reasonable, you’ll go about 2minutes faster on the extra 10w.
So yeah, your 90sec improvement is entirely reasonable of the 5w = 0.5s/km rule of thumb. I just like the cube root rule because it takes into account starting power. It’s a very crude simplification though, and becomes less accurate over a hilly course (of course).
Is the cube root of 5 not 1.7?
Extrapolating out from this are you saying that 200w would be in and around a 5:30 IM bike, and to go 5:00 you were need to go 9% faster. (750)^1/3 = 9 - so you’d need a 750% increase from 200 watts??? What have I missed?
Also would 200watts not yield a slightly faster IM bike split than 5:30? (for a reasonable weight person on a typical course with a reasonable cda)
Hi - 200 watts sounds about right for 5:30 bike split I did Canada last year with a Norm Power of 192watts and rode a 5:36hrs. My watts per Kilo 3.5watts 178lbs.
Sorry, for some reason 3-x_root_y_5 on my calculator gave me 1.24. But it is 1.7.
3^(1/5) = 1.24. 5^(1/3) = 1.7.
Your smartphone experience shows why everyone should do cube roots in their head.
I’ve seen this site mentioned before - I’ll leave it to others to discuss how accurate it is/isn’t:
Do you know what X watts gets you - speedwise - for a reasonably flat 20km TT? Then you can back calculate a rough CDA.
I would think that with reasonably good equipment, on a fast course like Austria 230w @ 70kg should get you 4:45 or so.
I don’t have data from a 20km TT with my present set up unfortunately.
Is it possible to calculate Cda any other way?
Position is reasonable I think. Could be lower.
Equipment - P2c, with 404 front, 900 rear, sapre tub behind seat, bottle on seat tube, bottle on a hed lollipop.
Do you know what X watts gets you - speedwise - for a reasonably flat 20km TT? Then you can back calculate a rough CDA.
I would think that with reasonably good equipment, on a fast course like Austria 230w @ 70kg should get you 4:45 or so.
I don’t have data from a 20km TT with my present set up unfortunately.
Is it possible to calculate Cda any other way?
Position is reasonable I think. Could be lower.
Equipment - P2c, with 404 front, 900 rear, sapre tub behind seat, bottle on seat tube, bottle on a hed lollipop.
Go do a 20km TT.
I don’t have data from a 20km TT with my present set up unfortunately.
Is it possible to calculate Cda any other way?
It’d be neat if there were.
I don’t have data from a 20km TT with my present set up unfortunately.
Is it possible to calculate Cda any other way?
It’d be neat if there were.
Okay so I do one, then what formula do I plug the data into?
Does this forumla allow me to factor in sh!te Irish roads? 
Sorry, for some reason 3-x_root_y_5 on my calculator gave me 1.24. But it is 1.7.
3^(1/5) = 1.24. 5^(1/3) = 1.7.
Your smartphone experience shows why everyone should do cube roots in their head.
It was too early in the morning…
Okay so I do one, then what formula do I plug the data into?
Does this forumla allow me to factor in sh!te Irish roads?
Depends on what kind of test you do.
There are tests that allow you to factor in crappy roads. There are tests that allow you to do relative comparisons but don’t give you an absolute estimate of CdA. There are tests that have high precision but require additional equipment and care. And then there’s the wind tunnel.
Choosing from among the alternatives depends on what you have available to you in time, in equipment, in venue, and in money.
Do you have a power meter, or just a wrist watch and speedometer?
Okay so I do one, then what formula do I plug the data into?
Does this forumla allow me to factor in sh!te Irish roads?
Depends on what kind of test you do.
There are tests that allow you to factor in crappy roads. There are tests that allow you to do relative comparisons but don’t give you an absolute estimate of CdA. There are tests that have high precision but require additional equipment and care. And then there’s the wind tunnel.
Choosing from among the alternatives depends on what you have available to you in time, in equipment, in venue, and in money.
Do you have a power meter, or just a wrist watch and speedometer?
I have SRM pros. Recently calibrated.