THM launches 'world’s lightest' power meter crankset at just 320g

Roverman wrote:

Just wondering why a rider weight limit has been specified?

l am more than the limit but can't put out 1999 watts and some pro weighing 60kg can, surely the torque at 700 watts from a pro is greater than my torque at 300watts regardless of the weight of the ride?

Failure won't be caused by power, it will be caused by torque. Stand on the pedal in a high gear and the peak torque, and therefore peak bending moment of the crank, will be directly proportional to your weight.

Kind of, torque is just force x distance (crank length), the force is just the force on the pedal spindle.

The force is not directly proportional to your weight, otherwise you would have the largest/heaviest men in the world doing track cycling. You don't, Azizul is a good example. The force is simply how much you can push through the pedal, this is different for everyone, it could be said that in general heavier people could push through their leg more, but I would suggest that anyone who races in cycling would push more than someone weighing approx 10-15kg more than them who doesn't cycle. 

Forces equal and opposite etc, pushing force trying to make you stand up, but you have your rider weight and the fact you are clipped in on the other foot, holding the handlebars and engaging your core, which is what you 'push against', you are not just pushing against your own body mass.

Power is just Force x Distance covered in 1 second, this can basically be reduced to Force x Cadence x constant (where the constant accounts for gear ratio and wheel diameter etc). If we are realistic then cadence doesn't change that much, yes you have track cyclists goin at 130, but they are not the target market for this crank, I would bet it breaks if an elite track sprinter uses it. So basically cadence varies between about 75-100 rpm, but cadence is not in anyway related to rider weight. So in this case, when we disregard everything that varies rider to rider but is not effected by weight we have

Power = Force x random not weight based value

And we have shown that force is largely uneffected by rider weight.

So in short I have no idea why they have implemented a weight limit, it strikes me as odd, you can't base power meter design on typical human ergonomics as people who buy these devices aren't within the standard bell curve so it doesn't make sense in my mind.

Edit: just for completeness, you can have rider limits on things like wheels, handlebars and frames as these can be directly linked to rider weight for some of their maximum loads. However, cranks strikes me as strange.

Andystow is spot on. I think it is understood that Andystow is talking about the given crank length, at which point stationary torque is directly proportional to the weight standing on the pedal. Power at this point is zero since the crank is not moving - the weight simply needs to be standing on the pedal - so your comments about "largest/heaviest men" are moot.

Except its not, as I've literally just explained but you've decided to ignore or my explanation was inadequate.

There is more than just the rider weight pushing down on the crank, peak power for a track rider is on the standing start, the equivalent force value for this is approx 2500N (not pure tangential force, just force applied). I assure you an elite track rider does not weight 250kg (approx). Yes this is an extreme example, but it does highlight the point that cranks should not be limited by rider weight and should be done by max force.

Going by my old Stages meter, I only ever saw 2000W when I was standing on the pedals and free-wheeling down a bumpy road.  I suspect that generates far more force on the pedals that I can generate using my muscles.