I found on my recent FF that I achieved my best power slightly outside the 7 second window. I am not alone in needing to wind up the power before the window started in order to achieve the best result.
A 10 second window to achieve the best 5second watts would be better
The point is that you’re not really supposed to wind up - it should be an explosion of power.
The two seconds extra, as I understand and use it, is to allow a shift of gears just before you spin up as fast and as hard as you can.
If you are needing to “wind up”, it’s not really measuring your NM output any more.
Perhaps you could explain exactly what you mean and do to wind up so further guidance can be given.
I just start 3 seconds early.
What type of trainer/ power meter are you using?
I have the same issue, but I just resolve it by starting a few seconds early. I use a Wahoo Kickr Core and Level 0.
About 5 seconds before the sprint, I shift to the correct cog and the big ring, but cadence is on the slow side. I need to get cadence up to maximize my NM output, but this takes a few seconds. Isn’t that normal? Is there a better way? I don’t have a problem with simply prepping a little early. I can see how it catches people off guard the first time though.
I’m using Wattbike. I have to start early. Started too early last time
I got on here to ask about this EXACT thing. I’ll ask in my own way with context - I’d love some expert insight. I’m using a manual kinetic trainer. My previous FF I hit 1421 NM and I was ecstatic. I did it as an explosion for 7 seconds from whatever recommended previous cadence/power I was at. I just completed “Cadence Builds & Holds”. I set a PR in the ramp up (192 RPM up from 164 the last time I did it. Here’s where it’s interesting. I was on my final hold - 115 RPM and 155 watts. About 20 seconds in I had an adrenaline rush and told myself that I have a S@#$ ton left in reserve. So I slammed into one of the hardest gears and went all out. I hit about 155 RPM and breached 2000 watts, but I held 1800+ for a solid 15-20 seconds. I was only able to do this because I was able to ramp up … so, what are my correct numbers?! BTW - I’m a sprinter - my FTP sucks (164)
I’d legit like the geeky scientific rationale as to what’s going on. TIA!
Power = torque x angular velocity (cadence)
Virtual power simply equates a wheel speed to the power required to sustain that wheel speed, AFAIK it doesn’t take into account the power required to accelerate up to that speed. This calculation has a big drawback because there is a hell of a lot of momentum in the flywheel and the bicycle wheel, once up to speed even it keeps turning and displaying big power number even when you have stopped pedaling.
It also depends on how accurate the power curve is for your trainer. I contacted the minions after I found a ceiling of 316w on my trainer at about 50kph wheel speed (which I can get to in the small ring), their answer was that they only had data for my trainer upto a certain speed so above that it just continues to display the same power. Their advice was to use a higher level.
In my next FF I moved the resistance level on the trainer and in the app up to lvl 9 and recorded a new 5sec best of 1765w (+1320 on the flagged and adjusted value from before). This changed my rider type from time trialist to sprinter 
I am under no illusion that I really have a sprint that huge, I would go further to say that I would only trust a sprint power figure that comes from a power meter that uses strain gauges and angular speed sensors or has a calibrated flywheel (Wattbike, ConceptII BikeErg) so the trainer’s computer can calculate the instantaneous power required to accelerate the flywheel.
I also go into the sprint a few seconds early so I’m at peak power for the whole duration. The first time I did FF I was in too low a gear/level and spun out in the sprints. I gained significant power by increasing the resistance level on my second attempt.
My Elite trainer has an optical power meter which I trust. I also have a single sided crank arm power meter to compare. Both give similar power results. I can’t quite break through the 1000W barrier, but I think that’s realistic when I see what power pro road race sprinters are putting down (1300-1500W peak) For reference I’m a reasonably strong 80 kg rider with a 283W FTP, but not a pure sprinter.
2000W is top professional track sprinter power, so I would seriously doubt those figures are real!
I know what you mean. I don’t generate peak torque going from low to normal cadence but from a normal cadence to high. It’s also too short a time prior to change gear multiple times and the difference in power is so huge. I think it depends on what sort of rider you are. It’s like being a car without a clutch. You don’t calculate the peak torque of an engine from a low rpm but from the best part of the curve.
I don’t think they actually calculate the peak 5s power only from inside the on screen interval though. I peak and hold for 5s even if it means continuing longer than the video. I approach the interval in the gear I’m going to sprint in at a power of about 80% of FTP and a low normal cadence. Then when the interval starts I try to spin out as quickly as possible. Power inevitably tails off as you spin out I reckon it takes about 10s.
Power = Torque multiplied by cadence.
As with a car engine you will tend to hit your peak power at a higher cadence than you hit your peak torque. In other words as you approach peak power your torque is actually starting to reduce but the higher cadence multiplier gives a higher power figure until the torque drops away too far.
So if you want to maximise your power over 5 seconds you need to hit your peak power cadence at the start of the interval otherwise you will be still ramping up power with cadence throughout the interval.
There is a tendency for people to assume they hit their peak power at maximum torque because that’s when your leg muscles are most loaded, but peak power actually comes as the cadence rises faster than the torque is reducing. It is the multiple of cadence and torque that gives power. So you need a relatively high cadence and torque to hit your peak power. For me that means 100+ rpm with as much torque as I can dig out at that cadence.
“So if you want to maximise your power over 5 seconds you need to hit your peak power cadence at the start of the interval otherwise you will be still ramping up power with cadence throughout the interval.”
What is “peak power cadence”?
The cadence at which you hit your reported peak power, I assume
It’s at whatever cadence you usually hit your peak power output in a sprint. It’s not going to be exactly the same for everyone. For me peak power cadence is at around 100-110 rpm. Beyond that leg speed I simply can’t generate enough torque to continue pushing my power any higher. Likewise at low cadence, even though I can generate more torque, power output is not maximal because of the low cadence (Power = torque x cadence).
The hardest part is getting exactly the right resistance to hit that peak power at your optimum combination of both torque and cadence. If the resistance is too high then you get bogged down at low cadence and if the resistance is too low you spin out with relatively low torque. Both of those scenarios lead to sub-optimal power generation. So it involves a bit of trial and error to get it spot on.
But the point I’m making is that you need to be up to speed just before the start of the interval to get the highest 5 sec power, otherwise power will still be rising throughout the interval. Obviously if you go too early then you will hit peak power too soon and burn out during the interval. So timing has to be spot on too! So it’s not that surprising that I find it much easier to hit a higher 5 sec power figure in an open sprint rather than in a specific timed interval like FF.
Edit: I just had a look at my last FF and I hit peak power at 101 rpm and pretty much held it through to 106 rpm over the 5 sec interval. This was on my second sprint interval. The first interval I was one gear lower and hit peak power at 110 rpm, but at a slightly lower power level.
That’s it and it’s extremely unlikely to be at a low or medium cadence. It’s going to be somewhere north of 100 rpm for most people.
Something else I’ve learnt from the physics of this is that it’s better to slowly ramp up your speed approaching the sprint interval rather than giving it your absolute max torque right from a low cadence. Otherwise your legs will have given out before you reach a high enough cadence to really nail your max power. It’s slightly counter-intuitive but it does work like that.
My power record for 5 seconds is from the standing starts video - not full frontal. Often when I do full frontal I select a level where I am pedaling at FTP and as soon as the test starts I quickly ramp up my cadence to 130.
“So if you want to maximise your power over 5 seconds you need to hit your peak power cadence at the start of the interval otherwise you will be still ramping up power with cadence throughout the interval.”
So why do you need to be at that optimal cadence at the “start of the interval”? I’m pretty sure the software just finds the highest 5 seconds so does it really matter when?
If the software actually does work like that then of course the timing is not critical at all. I hadn’t really given that any thought but you are probably right. I just make sure I’m at full gas when the interval starts.
That would also contradict the OP’s original post about hitting peak power outside the 7 second window. Be interesting to know how the software actually does work.