TT power

powermad

What kind of power (in watts) is needed for get under 1hour for a 40km TT? What kind of power level does everyone on the list achieve at lactate threshold (ie. what's your 1hour power). It would be interesting to know... mine is 260W.

Copenhagen

Well, the figures aren't that interesting without the weight of the riders. A heavy rider needs to produce more Watt than a lighter rider to maintain the same speed. Anyway, my threshold is at 320 W and I weigh 73 Kg

ric_stern/RST

The answer will be dependent upon many factors, including, but not limited to

topgraphy of the course
environmental conditions
aerodynamic equipment used
bike position
size, frontal area and CdA of the rider
mass of the rider

Furthermore, power that can be sustained for ~1-hr is generally far in excess of power at lactate threshold (which is generally defined as a 1mmol increase in lactate over resting levels). There was a discussion on this last year.

It's also, not of great use to just state an absolute power output without specifying any anthropometric data (e.g., if you can 260 W and are 1.52m tall, and weigh 50 kg, your velocity on a given course would be far greater than if you're 1.80m and weigh 85 kg!)

Ric

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ric_stern/RST

CdA and/or frontal area are far more important than body mass, with body mass making little or no difference on flat or generally flat circuits.

Ric

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ric_stern/RST

It's also imperative when talking about power meters, to state, which meter is used as there can be errors and differences associated with certain ones. For instance, Power Tap hubs read about 2% less than (correctly calibrated) SRM cranks, due to power being 'lost' within the drive train.

Ric

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2LAP

That is an interesting point Ric, I would assume then that SRM cranks give an indication of what is happening at the pedals (i.e. the power produced at the pedals) and PowerTap what is happening at the hub.

I assume that powertap is better when predicting performance (particularly when different bikes are used) and SRM's are better when training (i.e. the riders power output is the same no matter what the bike is).

Is this correct? (I appreciate that the difference is very small!)

ric_stern/RST

Obviously, the difference is small, say 5 W at TT power for good riders. You might possibly, think of the SRM as being the power that gets to the pedals and is therefore, what the rider produces, whilst with the PT this is what's available to the rear wheel and thus affects the velocity of the wheel. But, because the difference is so small especially at typical training power it doesn't really make much difference, but is worth bearing in mind!

Both systems are very accurate and both have their advantages and disadvantages.

Ric

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2LAP

Thanks Ric, you might have realised by now I am very interested in small differences particularly when they are intereting themselves.

Shibumi

Yes, but to go back to the first post, what power output do I need to hit 25mph?!!! I know it depends on a variety of factors, so I typed them into the analytical cycling web site. Apparently its 593W for the local 25 course I use a lot, for my weight of 78kg. Now Chris Boardman broke the hour with, I think, around 430W. So where do AC get their figures. Am I missing something?

Shibumi

Yes I am - just found out what. The AC model has a slope factor in. Once you sort that out things start to make sense. Thus to answer my own question, it's 287W for 25mph/40km/h. on a good day, on my local course.