Power from Tour of Georgia Bike race

[Nosmo]

power on the bike jumps all over the place, especially outdoors. but I think if you had a detailed (certainly if data were fractions of a second, but even stroke by stroke) plot of wattage from an erg, it would be significantly more variable than when you look at splits/500m.

btw- I bike and row, and I am curious what the relationship is between erg and bike power, but not so curious that I actually have done any erg testing! I do all out power efforts of various durations all the time, since I mostly bike race these days.

marc

Thanks for the Normalized power info. I did look for it but the web sites I found were rather vague. I wonder if the formula is emperical or if there is some theoretical reason that it is the 4th power. I assume they are taking 4th power of the power-- which would translate to the 7th power of the velocity on the erg!

I remember reading somewhere that someone measured the power into the erg with strain gauges on the chain, and determined that the true power applied is about 25W more then the monitor reading. This is in addition to the wasted energy moving up and down the slide, and the intermittent nature of the rowing stroke.

This link:
(http://www-atm.physics.ox.ac.uk/rowing/ ... #section13
estimates 37.5W moving the rower up and down the slide for a 75 Kg person rowing at 30 spm.

ErgMonitor software saves the power output from each stroke, so one can use it to monitor stoke by stroke variations. For a steady state effort, the variation in power between efforts is small compared to a bike.

I would guess that the normalized power formula would be more or less accurate for the erg also. It could provide an estimate to evaluate the
penalty of uneven splits. I'll run some calculations and post a follow up on this.

One of the tricky things about hilly bicycle time trials is the strategy of how hard to go when the terrain varies. These are the only bike races I've done in the last few years, so I have an interest in knowing in it.

On downhills a large increase in power makes a very small change in speed. On the flats the power is more or less proportional to the speed cubed, but on a steep uphill the power is proportional to the speed. One could use the formulas for normalize power, and for the formulas (found on the web) for power vs speed over varying conditions, to get a good idea for the optimal strategy for a given course.
I had an argument with someone about this after a time trial last year. He thought he should keep constant heart rate uphill and down. I'm sure he would have given my opinion more weight if I beat him but I can't.

[Orc]

Thanks for the Normalized power info. I did look for it but the web sites I found were rather vague. I wonder if the formula is emperical or if there is some theoretical reason that it is the 4th power. I assume they are taking 4th power of the power-- which would translate to the 7th power of the velocity on the erg!

I remember reading somewhere that someone measured the power into the erg with strain gauges on the chain, and determined that the true power applied is about 25W more then the monitor reading. This is in addition to the wasted energy moving up and down the slide, and the intermittent nature of the rowing stroke.

This link:
(http://www-atm.physics.ox.ac.uk/rowing/ ... #section13
estimates 37.5W moving the rower up and down the slide for a 75 Kg person rowing at 30 spm.

ErgMonitor software saves the power output from each stroke, so one can use it to monitor stoke by stroke variations. For a steady state effort, the variation in power between efforts is small compared to a bike.

I would guess that the normalized power formula would be more or less accurate for the erg also. It could provide an estimate to evaluate the
penalty of uneven splits. I'll run some calculations and post a follow up on this.

One of the tricky things about hilly bicycle time trials is the strategy of how hard to go when the terrain varies. These are the only bike races I've done in the last few years, so I have an interest in knowing in it.

On downhills a large increase in power makes a very small change in speed. On the flats the power is more or less proportional to the speed cubed, but on a steep uphill the power is proportional to the speed. One could use the formulas for normalize power, and for the formulas (found on the web) for power vs speed over varying conditions, to get a good idea for the optimal strategy for a given course.
I had an argument with someone about this after a time trial last year. He thought he should keep constant heart rate uphill and down. I'm sure he would have given my opinion more weight if I beat him but I can't.

The logic behind the NP formula is that various physiological responses to effort are curvilinear, but it might be partly empirical.
see, around page 9-10
http://www.midweekclub.ca/articles/coggan.pdf

My power meter (power tap) measures at the hub- used by many pros (like floyd landis in his infamous hilly ride). It loses about 10W due to chain friction compared to measurements at the cranks.(SRM, also popular but more expensive)

It would make more sense to compare rowing the erg to an indoor bike trainer. I am not disputing that outdoor cycling is highly variable- I see my plots all the time! My point is that aside from variation within the stroke (from zero power to some peak with each stoke) that stroke to stroke power variation is likely more variable than velocity. If you measured power in a scull you'd see even more variation due to wind, instability, steering, etc.

It is pretty clear that using constant effort (power, HR, or PE) on a hilly course is a bad strategy...

[Orc]

I remember reading somewhere that someone measured the power into the erg with strain gauges on the chain, and determined that the true power applied is about 25W more then the monitor reading. This is in addition to the wasted energy moving up and down the slide, and the intermittent nature of the rowing stroke.

This link:
(http://www-atm.physics.ox.ac.uk/rowing/ ... #section13
estimates 37.5W moving the rower up and down the slide for a 75 Kg person rowing at 30 spm.

PS half of 37.5W is similar to the 25W difference between strain at the wheel and that inferred from wheel accelerations. so, that might largely explain the discrepancy. on the recovery you don't get credit for the energy used to slide, but you do get to rest a little.

[Nosmo]

PS half of 37.5W is similar to the 25W difference between strain at the wheel and that inferred from wheel accelerations. so, that might largely explain the discrepancy. on the recovery you don't get credit for the energy used to slide, but you do get to rest a little.

These would be two separate effects. The power required to move the body up and down the slide is not measured by strain on the chain.
The 25W would have to be due to either losses in the chain and the shock cord, or due to an inaccuracy in the power calculation. (As long as the inaccuracy is repeatable it is not a problem)

From the paper you linked to, the formula is empirical.
He also notes on the top of page 10: "...exercise in which the power rapidly (~ 15 sec) alternates between high and low levels (400W & 0W), results in physiological, metabolic, and perceptual responses nearly identical to steady state exercise performed at the average intensity (200W)..."
This would indicate that the intermittent nature of the the rowing stoke does not decrease power output.

Thanks for that link. Now I understand some of the jargon better.

[Orc]

These would be two separate effects. The power required to move the body up and down the slide is not measured by strain on the chain.
The 25W would have to be due to either losses in the chain and the shock cord, or due to an inaccuracy in the power calculation. (As long as the inaccuracy is repeatable it is not a problem).

ok, never mind on the slide part- I wasn't thinking clearly. thanks

It seems reasonable that one could lose 25W (or something close to that) on chain friction plus overcoming the bungee force- people used to claim an old bungee made it easier to pull better scores. Assuming also the C2 power formula is perfectly accurate.


-marc

[Nosmo]

people used to claim an old bungee made it easier to pull better scores.
-marc

In theory, the energy put into the bungee is recovered by helping pull you up the slide. But "the difference between practice and theory is much bigger in practice then in theory". (not sure who said that originally.

[Orc]

people used to claim an old bungee made it easier to pull better scores.
-marc

In theory, the energy put into the bungee is recovered by helping pull you up the slide. But "the difference between practice and theory is much bigger in practice then in theory". (not sure who said that originally.

I don't think it would work that way. the exertion on your drive muscles is limiting, and I don't think making it easier to use a muscle group that is only marginally stressed would help much. (the only time my hamstrings have been an issue is stroking a boat with a crew that rushes the slide badly)

and anyway, those bungee jumpers never make it back up to the bridge, so there must be losses...

[Nosmo]

I don't think it would work that way. the exertion on your drive muscles is limiting...

As I said, "the difference between practice and theory is much bigger in practice then in theory".

This does bring up an interesting issue. The seat is on a slope and it does take some energy to lift your body up during the drive, which you get back on the recovery. If you are lifting say 50 Kg (your weight minus some of the weight of the legs) 30 times a minute up about 2.5 cm, that comes out to about 6W (mgh/time=50 * 9.8* 2.5 / 2). If you didn't get any of it back and the exertion is drive muscle limiting, then you would expect that your power would be about 6W greater with a level railing.

Boats and ergs are not designed with level tracks, and I would think the design would have changed if there really was a measurable penalty. (I know boats with sliding seats do not remain horizontal during the drive so the analogy to an erg is not exact)
I think (until I hear a good counter-argument), that the reasons for this is that there is an advantage to being really relaxed on the recovery which is enabled by the sloping rails, and for distances of 2K or more the limiting factor is not drive muscles but mostly oxygen availability which is averaged over the full stroke..

I think Paul or someone had a figure for the force of the bungee. Anyone know what it is? We could compare it to the force from the sloping rail. Lifting say 50Kg, 2.5 cm over a drive length of say 120 cm comes to a force of m * g *tan(slope) = 50 Kg * 9.8 m/s * 2.5/120 = 10.2N, = 2.3 lbs.

and anyway, those bungee jumpers never make it back up to the bridge, so there must be losses...

There is some loss in any spring, especially bungees, but in your example most of the energy loss is from wind resistance, so they would not make it all the way back even with a lossless bungee.

[Bob S.]

There is some loss in any spring, especially bungees, but in your example most of the energy loss is from wind resistance, so they would not make it all the way back even with a lossless bungee.

It has been over 60 years since my physics classes, but I keep thinking of the term hysteresis. As I remember it was applied mostly the energy loss in an electromagnet with an alternating current, although the definition in my dictionary implies that it might be appropriate to use it for mechanics as well. From the class in analytical mechanics I remember that, in an oscillating mechanical system like a spring (or a bungee), we used the term damping factor. Air resistance is one part of the factor, but there are others, like the chemical bonding changes in the steel of the spring or the rubber of the bungee. If you stretch a rubber band, heat is produced that is just lost to the surroundings.

I am sure that you have more up-to-date terminology to describe these effects.

Bob S.

[Nosmo]

Bob,
Most people don't remember that much 60 days after their last physics class let alone 60 years! But you got it right. The terminology hasn't changed.
Wikipedia had an article on hysteresis in a variety of systems including "elastic" hysteresis which applies to rubber bands: http://en.wikipedia.org/wiki/Hysteresis

In the example of the bungee jumping off a bridge, it would certainly play a roll, but my guess is that the bungees are of pretty high quality so the losses would not be huge. However when jumping off a bridge the jumpers speed gets pretty high very fast, so I would think the energy lost to air friction is much greater then anything else, at least for the first couple of bounces. I could do the calculation and get an idea of what it was, but I'm a rather rusty and am sure it would take me a few hours to get it right

[AtlantaCyclist]

[quote="Chris Brett"]I can't remember the specifics but I do know that when I was in sub 6 shape I was always able to generate over 1KW on a 'bike' with significantly less effort than pulling a sub 1:10 split on an erg. quote]

How long could you hold 1000 watts? A 1:10 split rowing is much more impressive in my mind (as an experienced cyclist)...

[gkucera]

I can't remember the specifics but I do know that when I was in sub 6 shape I was always able to generate over 1KW on a 'bike' with significantly less effort than pulling a sub 1:10 split on an erg. quote]

How long could you hold 1000 watts? A 1:10 split rowing is much more impressive in my mind (as an experienced cyclist)...

I have to chime in since I love both sports. I agree with the general convergence of this thread and want to add some soft data to highlight some of the points:
1. Comparing different sports is challenging, and perhaps pointless, but interesting, both from a training standpoint and a self-ranking standpoint. It is hard not be vested/biased by the latter. Here are my comments on that:
a. Cyclists must train many more hours per day to be competitive; this is more about the nature of the sport than a comparison. On average they burn fewer calories per hour, but must be able to maintain it longer.
b. By contrast, competitive distance rowers spend more energy per hour than cyclists. I can tell myself that I expend more energy rowing than cycling per hour (except for the few 10%+ grade hills), but the fact that a rower can't pull a marathon every day while a cyclist can ride 200km with 8km+ of climbing every day for a week or more (not me personally, but a competitive cyclist).
c. Competitive cyclists can not carry extra weight around, period; for rowers, the fat penalty is smaller. This only suggests that competitive cyclists, on average, probably have a lower body fat %. However, like with rowing, I do know some overweight (10kg) cyclists who do very well because they train a heck of a lot; they would simply be better if they lost the weight.
d. My intuition/gut feel tells me that my best cross-training for cycling is the longer rows where the slower pace feels more like cycling than the "fast" expenditure of AT capacity in the 5k and shorter rows.
e. I am curious/interested about the fact that cyclists must only replenish energy to their leg muscles (albeit, almost all of them) whereas rowers must replenish to back/shoulders and arms as well as over half of the leg muscles (by mass). My hypothesis is that rowers must have better capacities for this oxygen exchange, but that cyclists have better endurance/recovery with a relatively smaller capacity. I am again influenced by the fact that cyclists must train 2x (or more) as much time as rowers do to be competitive.

2. Cycling has been very popular and widely available for a long time. It has had far more press as well. As a result, there are perhaps 1000x as many people trying to be good at cycling than there is with rowing. I think the $ is a side effect of the above, but of course also contributes to a harder push by more people towards competitive excellence.

Thank You for the wonderful thread everyone!

[AtlantaCyclist]

As a cyclist who ergs occasionally, I believe that my cycling fitness helps more on the longer rows than the shorter efforts. My 2k times stink, but my 10k times are much more competitive. Just my observation.