Weight distribution when freewheeling on quick/downhill corners

[PaulSB]

I'm more of a "b" but not pushed back anything like as much.

 

[CXRAndy]

Coming down a steep descent I tend to employ 'B' to some extent. Your are automatically loading up the front wheel and transferring weight back as much as possible keeps a more even balance. This stops rear wheel going light/lifting allowing harder braking of both brakes

 

[PaulSB]

I hope no one feels I'm trying to stir this thread. On my ride today I took particular note of how I was sitting. It was quite clear to me the bulk of my weight was supported by my sitting bones and then through the saddle.

The pedals are probably 10-12cm forward of the saddle. This makes it difficult to understand how the legs are taking anything but a marginal amount of weight other than when standing.

There was no noticeable pushing back or up from the saddle created by the force of pedalling.

 

[Twilkes]

It's a function of how hard the legs are pushing the pedals, not really of their position. I test rode a few roads and a hill that I know, to compare two bikes, and when pedalling hard it was quite clear to me that my bum was barely staying on the saddle, and rose off a few times on the hill when I had to push harder. When sat stationary most if not all of my weight was on the saddle. If you're not getting the physics of it I'm not really sure how else to describe it. It's probably less noticeable at lower power levels, or higher cadences where the power is in the 'speed of rotation' rather than the 'pushing force', but it will still be there as long as there is pedal resistance.

 

[Ajax Bay]

I hope no one feels I'm trying to stir this thread.
The pedals are probably 10-12cm forward of the saddle. This makes it difficult to understand how the legs are taking anything but a marginal amount of weight other than when standing.
There was no noticeable pushing back or up from the saddle created by the force of pedalling.

Stirring you are not. Difficult to understand = frustrating.
The distance the 'front' pedal is forward of the saddle (at 9 o'clock) is irrelevant. If the rider is pushing down on it, the effective weight on the saddle will be reduced - very broadly an equal and opposite reaction (see Newton, with the assumptions and limitations noted in my earlier post). (Btw, the front pedal is much more likely 40cm in front of the saddle and the rear one about 5cm in front - dimensions depend on length of crank and seat tube and exposed seatpost, and seat tube angle.)
Your assessment that "the bulk of my weight was supported by my sitting bones" is right (well it's actually the saddle that's supporting your weight: the sit bones describe the point(s) of contact), but the reaction to the force on the pedal reduces the "noticeable pushing back or up from the saddle (on the rider's backside).

 

[swansonj]

We can resolve the forces acting on the cyclist separately into vertical and horizontal components. The diagram shows just the vertical components: the weight W, and three forces exerted by different parts of the bicycle on the cyclist: Fp from the pedals, Fs from the saddle, and Fh from the handlebars. Each of these are the equal-and-opposite reaction force to the forces exerted by the cyclist on the bicycle.

The cyclist is not accelerating vertically so there can be no net vertical force on the cyclist: W=Fp + Fs + Fh.

Let's assume Fh, the force exerted on the cyclist by the handlebars, is constant (it will be small anyway, so it won't matter much even if it's not strictly constant). And W is clearly constant.

Fs = [W-Fh] - Fp, where the bit in square brackets is constant.

Then, if Fp increases, Fs decreases. Or, as many people have already said, the saddle takes whatever part of the weight is not taken through the pedals.

For a cyclist working at 100 W, delivering that power output at 80 rpm through 170 mm cranks, Fp, averaged over a revolution, is about 10 kgf. It presumably actually varies from closer to zero at 12/6'o'clock to probably nearer double, say 20 kgf, at 3/9'o'clock. For an 80 kg rider, that means that the amount of the weight taken by the pedals is in the region of 10-20% body weight, and the amount of weight transmitted through the saddle reduced accordingly - significant but not dramatic.

But the force on the pedals increases as the power output increases and as the cadence drops. Outputting 400 W at 60 rpm gets you to the whole of the body weight taken through the pedals and none through the saddle - which makes sense because that's what we do when we climb hills out of the saddle. (Well, not the 400 W bit for some of us, obvs.)

 

[Ajax Bay]

Thank you for finding the diagram.
Worth noting that the force FH can be negative (in the sense of the diagram) ie pulling on the bars so a downward force on the body, normally when uphill power is required.

 

[freiston]

I don't push hard enough in low enough cadences to take much of my weight on my feet - I prefer to be in a gear that will allow me to spin fast enough to avoid lifting my backside out of the saddle - though saying that, there are extremes where I will take more weight on my feet, like when I'm carrying a lot of weight uphill or I need to put a sprint on to clear what I consider to be a danger zone (like a junction with several lanes to cross). Freewheeling round a bend/corner and I don't think I transfer more weight to the pedals - I tend to have the cranks anywhere between horizontal and inner crank at 12 o'clock. When the ground is a bit rough (or there are speed humps/potholes), I will sometimes push down on the pedals and lift my backside off the saddle (or at least take most of the weight off) and use my bent knees for "suspension" pretty much like a skier.

With regards to the riding position diagrams posted by cyberknight, if I want to reduce my wind resistance, I tend to use "b" when going downhill. Other possibly relevant detail - my saddle is about the same height as my handlebar tops but I have a deep drop on them and I suffer with arthritis in the knees.

 

[Ajax Bay]

On sketchy (ie variable surface) roads downhill, most of my weight is on my legs with some through my hands and firm contact but not much weight on the saddle - like @freiston . This is designed to mitigate the risk of a kick in the b*****ks. On smoother downhills I'm pedaling so weight distribution is 'normal' or once the aero drag / cadence balance threshold is crossed, I'll be a 'Position b' too.