TT power budget

AshesGlory said:

I have entered a short ~30min TT which the first half is flat and the second half is uphill.
It seems that conventional wisdom for a flat TT is to maintain an even power output throughout. However I will be going faster for the first half than the second half, and the power required to overcome wind resistance is proportional to the cube of my speed so it would seem from a physics perspective I should lower my power for the first half.
From a metabolic perspective is it reasonable to assume that it is possible to ride at a lower power than my 30 min MMP for 15 mins then ride above my 30 min MAP for 15 mins so that my overall NP is equal to my 30 min MMP ?
And how can I evaluate by how much to lower my power for the first half of the TT to maximise my average speed?

yep you're on the right track. Speed on the flats is roughly proportional to the cube root of power while speed climbing is roughly proportional to power (no root). So it's clear that you can modestly drop you power on the flats and modestly increase it on the climb to maximize your average speed over the course. I say modesly as for a duration of 15-minutes - even while fresh you probably can't exceed your FT by much more than 10%.

I think RDO has a model whipped up that can easily handle a 2-segment course like yours. Important variables are rider/bike mass and accurate road gradient. That's leaving out wind of course ...

rmur

rmur17 said:

yep you're on the right track. Speed on the flats is roughly proportional to the cube root of power while speed climbing is roughly proportional to power (no root). So it's clear that you can modestly drop you power on the flats and modestly increase it on the climb to maximize your average speed over the course. I say modesly as for a duration of 15-minutes - even while fresh you probably can't exceed your FT by much more than 10%.

I think RDO has a model whipped up that can easily handle a 2-segment course like yours. Important variables are rider/bike mass and accurate road gradient. That's leaving out wind of course ...

rmur

I do indeed have a simplistic 2 segment optimization model for road gradient only (with a little manual manipulation for wind). There is another version well along that moves the ball forward on such problems by a few light years, but it's still in the development stage. In the meantime, if you have the data, post the distance and road grade for each segment, the magnetic heading (approx) of each segment, the prevailing winds (direction and velocity), your total weight in kilos (you + bike), your frontal area as per AnalyticCycling and your MPs (max powers) for 30, 20 and 10 mins (or better yet, your AWC and CP from the Critical Power Model). BTW, the wind direction should be as given by the meteorological service (i.e., direction is the direction from which the wind is blowing, speed can be in mph, kph or knots, as long as I know). Depending on the distance and grade of each segment, the push segment could well be >10% of your FT and that is probably the critical constraint (e.g., your max 20 min power).

RapDaddyo said:

I do indeed have a simplistic 2 segment optimization model for road gradient only (with a little manual manipulation for wind). There is another version well along that moves the ball forward on such problems by a few light years, but it's still in the development stage. In the meantime, if you have the data, post the distance and road grade for each segment, the magnetic heading (approx) of each segment, the prevailing winds (direction and velocity), your total weight in kilos (you + bike), your frontal area as per AnalyticCycling and your MPs (max powers) for 30, 20 and 10 mins (or better yet, your AWC and CP from the Critical Power Model). BTW, the wind direction should be as given by the meteorological service (i.e., direction is the direction from which the wind is blowing, speed can be in mph, kph or knots, as long as I know). Depending on the distance and grade of each segment, the push segment could well be >10% of your FT and that is probably the critical constraint (e.g., your max 20 min power).

Wow, I'll give it a try. The wind direction would be a pure guess until a couple of days prior. I dont even know my start time yet. So can we assume a windless day?
The course is 5km flat and straight going due West, then 5km @ 3% going due North, then 500m @ 13% due North.
My bike, clothing, shoes and I weigh about 81kg.
From the monod spreadsheet: CP=4.1, AWC=184 based on 3 points (1.5, 5 and 20 mins) with R squared=1.0000. This is based on my weight of 73kg.

AshesGlory said:

The wind direction would be a pure guess until a couple of days prior. I dont even know my start time yet. So can we assume a windless day?

The prevailing winds by month can be found here ncdc.noaa.govavgwind.htmlOpen ↗. You'd be better off assuming it will be an average day per this chart than to assume it will be a windless day. There are almost no windless days.

RapDaddyo said:

The prevailing winds by month can be found here ncdc.noaa.govavgwind.htmlOpen ↗. You'd be better off assuming it will be an average day per this chart than to assume it will be a windless day. There are almost no windless days.

The problem is that typically it will be windless at 7:30 and the wind will pick up as the morning progresses. I have no idea of my start time. According to the average wind page it is 9.3 mph. The prevailing wind is SW.
I forgot to mention I usually use the default frontal area in analytic cyclist. I am 5'8'' and dont have a great TT position, but working on it.

RapDaddyo said:

The prevailing winds by month can be found here ncdc.noaa.govavgwind.htmlOpen ↗. You'd be better off assuming it will be an average day per this chart than to assume it will be a windless day. There are almost no windless days.

okay let me play and you can correct me later on 🙂

My back of the napkin estimate:

[font=Courier New]5k flats 275W ~7.5min[/font]
[font=Courier New]5k 3% 325W ~10.0min[/font]
[font=Courier New]500m pitch 425W ~2.0min[/font]

Total time ~19.5min, AP=315W, NP=325W

I assumed density 1.2 kg/m3, Crr=0.005, Cda=0.275 (mid-range). The OP mentioned 30min duration so unless it's quite windy -that's sounds way too long to me. Maybe I screwed up :o

rmur

rmur17 said:

okay let me play and you can correct me later on 🙂

My back of the napkin estimate:

[font=Courier New]5k flats 275W ~7.5min[/font]
[font=Courier New]5k 3% 325W ~10.0min[/font]
[font=Courier New]500m pitch 425W ~2.0min[/font]

Total time ~19.5min, AP=315W, NP=325W

I assumed density 1.2 kg/m3, Crr=0.005, Cda=0.275 (mid-range). The OP mentioned 30min duration so unless it's quite windy -that's sounds way too long to me. Maybe I screwed up :o

rmur

I haven't done any numbers yet, but you're probably in the ballpark with a 50w differential between the flat and (long) upgrade segment. Trying to finish a 30 page writeup of a major project that may affect you (at least on the bike) as well as me. But, I knew in advance without looking at any actual numbers that the gating constraint was going to be the upgrade segment(s). The solution is probably going to be to go at his MP from the CP curve for the two upgrade segments and do the flat segment at whatever it takes to not blow his NP for the full duration. But, chances are the optimal (but impossible) solution is to push the upgrade segments even harder and take it off the flat section. Of course, that will be mitigated a bit because the prevailing winds in the US are from the west and his flat section is likely to be upwind. This is the classic case of the importance of anaerobic capacity for TTs even though many think it's all about FT. I'll work out some #s when I finish this draft.