[Closed] Armchair physicists

They're cretins. Why on earth even bother trying to explain it to them?

Most obvious way I can think of is that you're trying to undo a wheel nut with a big spider - you can either pull on one end only with your right hand, or use your left end to push on the other one too.

All this business about weights is neither here nor there.

if they are too stupid to understand that if you push in one direction and pull [b]in the same direction[/b] that you have increased the force of just one of these alone then thank God I dont frequent there?

When they have nearly understood it ask about a plane and a conveyor belt would you?

In the words of Ripley

[i]'Did IQ's drop while I was away?'[/i]

Are they talking about PowerCranks?

When they have nearly understood it ask about a plane and a conveyor belt would you?

Oh god please don't mention that I don't think some of them even understand the bullet being dropped vs fired from a gun thing.

if they are too stupid to understand that if you push in one direction and pull in the same direction that you have increased the force of just one of these alone then thank God

don't you mean [b]decreased[/b] ?? But of course if you pull up on one pedal you'll be pushing down harder on the other... and force isn't the same as torque etc

They still don't get it.

for sfb if you push on a lever to see how much force you can apply and then I pull in the same direction do you think we will get more force or less? Now imagine the lever is a crank and that you and I are your left leg and your right leg respectively we get more force

Assume a spherical cow...

Torque.

You're exerting forces in opposite directions - if they were inline with each other then they would cancel out.

But they're not, unless you've set your cranks up totally wrong!

Hence you increase the overall torque and go faster!

Do they not have legs that bend in the middle over there? Or is just their heads that are solid?

for sfb if you push on a lever to see how much force you can apply and then I pull in the same direction do you think we will get more force or less? Now imagine the lever is a crank and that you and I are your left leg and your right leg respectively we get more force

it may have escaped your notice that the legs are connected to each other

It's about frames of reference and what the people are pulling and pushing against, that's what's complicating the thought process I think.

It's about frames of reference

you mean hardtail or full suss, steel or alu ??

That's the one 😉

What about hand bikes?

When you pedal with one of those you push with one arm and pull with the other.

Why is it any different with a bicycle which uses your legs for power?

Ah, I was hoping this was a plane/ conveyor belt interface question. 🙁

You're exerting forces in opposite directions

Nuh-uh.

You're exerting forces along the circumference of a circle. Draw it on paper and the forces look opposed to each other, but you're only looking at the forces being exerted at that instant; they're actually in the same direction along the circumference of the circle and thus the total force is the sum of both.

Imagine you're sitting on the saddle, have really strong legs and are very light.

Without being clipped in, your pedalling force is directly related to your weight and nothing else. If you weigh nothing you won't go very far, you can be as strong as you like but the pedal's not going down, you're going up.

When clipped, you have a mechanical advantage in that you can operate both sides of a lever in opposition; now, pedalling force is directly related to your leg strength.

Question now is, in reality where is most of your power coming from, weight or leg muscles? (-:

they're actually in the same direction along the circumference of the circle and thus the total force is the sum of both.

is "around a circle" the same thing as a direction ?

as I said before, a force is a different thing to a torque

There can be little doubt that if you apply a downward force on one pedal and an upward force on the other, then the total force is the sum of the two forces...

The argument here can only be:-

Due to the limits of the system, is it possible to apply these two forces in a way that one does not interfere with the other.

For example, by pulling up on one pedal affect the amount of force you can apply downards to the other?

If when pedalling along there is any time that it is possible to lift with the back leg without affecting the downward force on the front leg, then it is simple to see that the total power would be increased.

However, whether this is actually a useful effect is not clear.

If you weigh nothing you won't go very far, you can be as strong as you like but the pedal's not going down, you're going up.

but it won't matter as you'll be really easy to accelerate 🙂

When clipped, you have a mechanical advantage in that you can operate both sides of a lever in opposition; now, pedalling force is directly related to your leg strength.

except for most people the push muscle will be about 10 times stronger than the pull muscle...

TBH its not even about planes of reference, it's just someone not understanding torque in this case.

We will say "the direction causing forward motion of the bike is positive"...

Magnitude of Torque from left pedal is 0.175 * Fl = TL
Magnitude of Torque from right pedal is 0.175 * F2 = TR

Now, assuming we are looking only at forces due to gravity (mass of rider * 9.81), and with pedals horizontal, the two torques cancel as one is opposing the other:

TL (left pedal forward) + TR (Right pedal backward) = 0

so no crank movement.

Sat on the seat but not pedalling we have:

TL+TR = 0

Stood up and pedalling (the point he's confused about I think) we have:

TL (left pedal forwards) - TR (right pedal back) = +VE, i.e. we have weight on the left foot and we "unweight" the right foot without pulling, meaning our resultant torque is positive. In this case the best we can do by unweighting the pedal is achieve the torque provided by the weight of the rider on one pedal.

But we're not that simple, and we can pull up while pushing against the other arm, to the limit of our muscles - we now have the situation where TL is no longer limited by weight but by the force on the other arm, the harder you pull up on one arm the harder you can press down on the other without rotating around your crank yourself.

Is the point that the force of one leg pushing can't be greater than the pulling force of the other, but that force (and hence torque/power) is still at least twice as much as one leg can do on it's own.

[img] [/img]

http://www.ncbi.nlm.nih.gov/pubmed/18418807

I read that as saying you can produce more power but it's harder work.

Thats counter to my experience but I'd have to read the full paper. My experience is I'll last ~25% longer clipped in and pulling up, suggesting my efficiency is improved due to load balancing.

is "around a circle" the same thing as a direction ?

"around a circle" is the same thing as a direction, yes. I don't know what your point is though.

as I said before, a force is a different thing to a torque

Yes, but where does the torque come from? From the forces exerted around the circumference of the circle. If it was a force exerted upwards [i]only[/i] and a force exerted downwards [i]only[/i] rather than around the circumference, you'd end up with the "up" pedal/crank arm pulling at TDC of the circle and the "down" pedal/crank arm pulling at BDC of the circle. In that situation they would cancel each other out.

Is the point that the force of one leg pushing can't be greater than the pulling force of the other, but that force (and hence torque/power) is still at least twice as much as one leg can do on it's own.

only if you have no weight, which will not be the case as your legs require a considerable support system involving digestion, respiration and control (AKA [b]body[/b])

"around a circle" is the same thing as a direction, yes. I don't know what your point is though.

to my mind a direction can be expressed as either x,y & z or angle and azimuth, which a circumference cannot.

Well I'm with the counter-intuitives on this one. Somebody posted on here not so long ago clip ins automatically give you 30% more pedaling efficiency! So that means my 3 hour loop will take 2 if I swap pedals?

The only times I've found them useful is in long climbs to give your quads a break. You can't use both the same time in a meaningful fashion, and the 'studies' confirm it.

Somebody posted on here not so long ago clip ins automatically give you 30% more pedaling efficiency!

twaddle :o)

Efficiency measured in what ? (Watts ?)

sfb... you are being obtuse, and you know it!

tinribz... just because someone exagerated somewhat, doesn't mean it can't generally be true! Now, I would imagine that in some circumstances that SPD's do help with pedaling efficiancy otherwise we'd see Tour riders and track riders all using flats!

Now, whether it has anything to do with pulling in the upstroke is moot, as on the whole riders don't tend to do it, so I would imagine it has more to do with ensuring foot position and getting good power transfer than anything else.

Back to the main topic, it would be interesting to know if people like Chris Hoy pull up on the back stroke when starting a sprint. As has been said above, I have a feeling that even though it might produce more power, it is less efficient than not pulling up, so for short sharp sprints it might be useful.

sfb... you are being obtuse, and you know it!

I know I'm trying to separate inchoate handwaving from measureable, relevant physical quantities. People keep talking about force when they mean torque, or suggesting meaningless concepts like having no weight, or unspecified efficiency

that SPD's do help with pedaling efficiancy

I don't think it has anything to do with efficiency. They allow the rider to apply more torque as the rising, otherwise idle, leg can also contribute, provided the cardiovascular system can deliver the extra demand.

or suggesting meaningless concepts like having no weight

It's not meaningless, it's perfectly sensible assumption if you think of someone sat on a saddle - they have no weight WRT the crank arms, so all torque generated is due to muscle input.

I don't think it has anything to do with efficiency. They allow the rider to apply more torque as the rising, otherwise idle, leg can also contribute, provided the cardiovascular system can deliver the extra demand.

Or reduce demand on the downward-pushing leg and share it with the upward pulling one, meaning each muscle can be used within a reasonable % of max force assuming the cardio side can keep up.

they have no weight WRT the crank arms, so all torque generated is due to muscle input.

but is that a realistic scenario ? Don't we instinctively apply enough body weight to allow us to push the pedals ? I certainly must do as I use flat pedals, and as soon as it gets bumpy I transfer most or all my weight to the pedals

[i]Don't we instinctively apply enough body weight to allow us to push the pedals[/i]

So recumbent bicycles are propelled by magic?

but is that a realistic scenario ?

Yes. And it was there to set up the example/calcs, not as a difinitive solution to the question, surely you can see that.

So recumbent bicycles are propelled by magic?

I think it's beards...

it may have escaped your notice that the legs are connected to each other

Force means to apply energy to accelerate a body - eg kinetic via gravity downhill.Torque means to apply a force around a fulcrum or pivot say like pedalling a bike. Pray exaplain why it matters here ? An increase in force equates to an increase in torque I am fairly certain that when I pedal with more force it increases the torque and I tend to go faster and get up hills rather than when I apply less force and get less torque when I have a tendency to stop. Perhaps there is a relationship at work here between force and torque?
I assume you can create a scenario whereby I increase the force applied to a grank but the torque does not increase? Trackstand?????

Pulling up with one foot whilst pushing down with the other creates exerts a torque on the cranks, but also an equal and opposite one on your body. You need to balance this out to stop you rotating backwards off the bike. You can do this by pulling on the handlebars (or by pushing less).

There's no inherent reason why you can't exert a torque greater than that created by putting all of your weight on one pedal. Whether it's worthwhile is another matter.

I assume you can create a scenario whereby I increase the force applied to a grank but the torque does not increase?

if you read what I said, I didn't comment on the amount of torque in the case you mentioned. You said force so I remarked on that. Inevitably, if you pull up with one foot you'll have to press harder with the other unless your centre of gravity moves with respect to the crank axle.

[i]30% more pedaling efficiency! So that means my 3 hour loop will take 2 if I swap pedals?[/i]

My new van's apparently about 30% more fuel efficient than my mates car. Why won't it go 180 mph?

if you read what I said

"or suggesting meaningless concepts like having no weight"
It's not meaningless, it's perfectly sensible assumption if you think of someone sat on a saddle - they have no weight WRT the crank arms, so all torque generated is due to muscle input.

They allow the rider to apply more torque as the rising, otherwise idle, leg can also contribute, provided the cardiovascular system can deliver the extra demand.

That's what I mean. I'm a lowly biological chemist though, so my brain doesn't compute in forces and moments and torques. As I have so obviously demonstrated.

Wrong. As sfb suggests, when pushing hard you have far less than your whole bodyweight resting on your saddle.

I'm aware of that, but that's not the situation I used it in, maybe I didn't explain what I meant properly. It is possible to sit on the seat and pedal hard without reducing your force on the saddle, you cause do so by pulling up on the upstroke to match your downstroke. The point was to remove gravity effects from the build-up so it was a little more clear, unfortunately it made it a little less clear.

Any net force in any direction from the two cranks gives you an increase or decrease in apparent saddle reaction force. As long as you can match the downstroke force with your upstroke force you can hold your body still wrt the cranks. If your net force goes positive (pushing harder than pulling) you'll start to unweight your body off the saddle, therefore the maximum net force you can apply is whatever your upstroke can apply+force from your body weight. After that you have nothing holding you down anymore, unless you're also able to stop your rotation about the handlebars, but I'd expect that your grip isnt that strong.

I suspect the reason they say (in that link provided) that net efficiency goes down is that the upstroke is less mechanically efficient (due to the position, size etc of the muscle and limb) and so drags down the overall efficiency even if it shares the loads. But that's in sub-maximal case, in the maximal case where your push muscles are working at full tilt, bringing in a little help from the pull muscles may well extend your range.

is "around a circle" the same thing as a direction ?

to my mind a direction can be expressed as either x,y & z or angle and azimuth, which a circumference cannot.

SFB
It all depends on your co-ordinate system, you essentially have mentioned two possible coordinate systems
1) cartesian (x, y,z)
2) Spherical (or polar) (Radial , polar, tangential) well you said asimuth /angle but its part of the same thing.

But there are other coordinate systems such as cylindrical.

You can express a vector using cylindrical co-ordinates - (radius, tangential and axial)

It is possible to sit on the seat and pedal hard without reducing your force on the saddle, you cause do so by pulling up on the upstroke to match your downstroke.

yes it is, but not a technique I could recommend...

yes it is, but not a technique I could recommend...

Something I do quite a lot if I'm struggling up long hills, it transfers some of the load from my push to my pull and lets them "rest" so I can continue climbing while "easing off" the painful muscles.

but not a technique I could recommend...

particularly to those of us using flat pedals 🙂

Something I do quite a lot if I'm struggling up long hills

Pulling up with one foot whilst pushing down with the other creates exerts a torque on the cranks, but also an equal and opposite one on your body. You need to balance this out to stop you rotating backwards off the bike. You can do this by pulling on the handlebars (or by pushing

My legs bend in the middle. And also where they meet the rest of me. And at the ankle come to think of it.

I think the underlying mechanism is fully encompassed here -
[url]