First attempt with SPD's today in 30 mins

bike-a-swan

be nice, both of you

PBo

yeehaamcgee wrote:

I think a physics lesson is in order.
But, briefly... here's a thought experiment that may explain it:

Replace those scales with a bar pivoting around a central point. Now, attatch a forcemeter to the frontmost side of that arm.
Stand one that frontmost side of the arm with all your bodyweight, and see the force reading. Now, try lifting the rear of the bar with the other foot, and see the reading. Not much difference eh?

None of the arguments have fully convinced me yet - i think that the physics haven't been fully played out.


Now, I have to say, irrespective of whether yeehaamcgee is right or wrong, that is no use as a thought experiment. A thought experiment should help, by relating the idea to something familiar, and with a clear outcome that should be familiar or obvious.
There is nothing obvious about why the forcemeter won't change. If one believes rightly or wrongly that the up stroke adds, then the result of the thought experiment would be that the forcemeter increased!!

I don't know if you are right or wrong, but if you are going to dish out "physics lessons" then make them helpful.

I feel like, pushing down and pulling up, are 2 separate moments, acting the same direction around a pivot, so should be added together.....but I'm sure the "closed" nature of the system (i.e. the source of those 2 moments come from the same body, leveraging against the bike) complicates the analysis. (or maybe simplifies it - bodyweight being the maximum force possible, however it is distributed across the two pedals). Plus, bodyweight is a force due to gravity, but what about the force due to muscular contractions? Another red herring, cos the muscles only contract and move limbs with regards to the same body??

As i say, can't get a clear head around it yet....

Anonymous

Ok, consider it like this then. In the old scooby doo cartoons, shaggy and scooby would quite often try and run away from a guy pretending to be a ghost or somesuch, and in doing so would encounter a door.
Now, their pprefered method of opening the door was to jump at it, holding the doorknob in their hands, whilst simultaneously pushing against..... the door itself. Not the doorframe.
I hope it's obvious to anyone that in real life, this would only result in ripping the doorhandle FROM the door, and not actually opening it.
Had they instead pushed with their legs against the door FRAME, then they would actually be putting a lot of force into opening that door.


So, If you could hook your foot on something such as the bikeframe itself, to give you purchase whilst forcing down the frontmost pedal, great. However, the rearmost pedal is all a part of the same "system" and the net result is, no change. The rear will come up as the front goes down.
It is a little harder to grasp than the scooby doo door, because it is a dynamic system, not a static one. But I assure you that increasing the force with one foot will cancel out what the other is doing.
It might "feel" like the lifting foot is doing something, but all it is in fact doing, is unweighting the pedal, so the downwards stroke applies it's power unheeded.

Still not convinced? Here's another aspect to consider.
Ok, try doing a squat on one leg. Easy, there is a massive excess of power which makes this a simple undertaking.
Now, try hanging yourself upside down from ONE leg. And now try and pull your bodyweight up using just your knee - by that I mean, not by doing stomach crunches.

The amount of "power" that can be applied in this way is about the same as an, oooh, 5 year old girl pushing with her leg
Maybe not quite that low, but still, a pretty insignificant amount of strength.
So, when you're using ALL of that mighty lifting force, that might just be stronger than a 5 year old girl, it might seem like you're doing something, I mean, you can feel your shoe against the upper part of your foot, but no.
You're contributing nothing of any real use.

This is compounded by the fact that whilst putting maximum force through your leading leg, your weight will not be supported by the saddle - giving you nothing to brace yourself to apply the lifting force with. And we're back to square one.

PBo

yeah, i felt the closed system was probably key here...

bike-a-swan

yeehaamcgee wrote:

So, If you could hook your foot on something such as the bikeframe itself, to give you purchase whilst forcing down the frontmost pedal, great. However, the rearmost pedal is all a part of the same "system" and the net result is, no change. The rear will come up as the front goes down.
It is a little harder to grasp than the scooby doo door, because it is a dynamic system, not a static one. But I assure you that increasing the force with one foot will cancel out what the other is doing.

I'm not coming down on one side or the other here, but I'm not convinced by that- the forces in your xyz co-ords cancel out, you aren't lifting the bike or pushing away. In rotational terms, if you're already applying a moment from the downstroke, then the corresponding upstroke will add to the rotational moment, surely?

cgarossi

You know a thread has gone south with Scooby Doo analogies come into play. lol.

Anyway, yes I agree. Sort of.

The force of the upward stroke will compliment the downward stroke but not give it any extra power. Since the force is being applied to the axle through the bottom bracket, which transfers down the drive train. If there was somehow two axles which were independent and transfered their power to the drive train then, that would be a different matter entirely.

If you can train your legs to apply upward as well as downward forces then you would be a very effiecient cyclist. Extra power would come from muscle strength.

SPDs help cycling effeciency, not power.

Thewaylander

Now Yee,

Ever thought of being a teacher? i was thinking of how to explain the mechanics and forces envolved for people but hell that scooby one was genius!

John Stevenson

All this theory is, to be blunt, irrelevant. Every study of the way people actually pedal has measured a downforce on the pedal on the upstroke.

The flats vs SPDs decision is entirely down to what feels good and right to each individual. Trying to justify your choice with 'science' is like using equations to prove that Guinness is better than single malt.

Thewaylander

I just think spd are for the skilless noobs (sit's back and waits for a rise) muhahaha feed the troll

Anonymous

John Stevenson wrote:

All this theory is, to be blunt, irrelevant. Every study of the way people actually pedal has measured a downforce on the pedal on the upstroke.

Mainly because you have to support your bodyweight somewhere. Even the lightest XC race whippets don't actually float around.

John Stevenson wrote:

The flats vs SPDs decision is entirely down to what feels good and right to each individual. Trying to justify your choice with 'science' is like using equations to prove that Guinness is better than single malt.

Agreed, but I was trying to explain why commonly held theories are incorrect.

ride_whenever

You're all neglecting the other muscles in your body.

Usually when you're pedalling you don't pull up, your rear foot floats up and over as you get more efficient, which you can do adequately even with flats.

You can actively pull up, and this does add some, not insignificant extra torque, but this comes from the hip flexors, (I believe mainly the ilipsoas (sp)) which is small and not hugely strong, so fatigues quickly. This is good as an afterburner for clearing a section or really accelerating hard, but not for normal use.

The reason you can generate more is because pulling up with the rear foot enables you to keep your body weight in the same place, rather than expending energy lifting yourself up onto the leading pedal you stay more in the same place and turn the pedals round underneath you more, because, precisely as mentioned earlier, if you push any harder you stop accelerating the crank and start lifting yourself up.

Anonymous

weeksy59

Dunno about all this physics....

but i didn't like them

bails87

yeehaamcgee wrote:

Mainly because you have to support your bodyweight somewhere. Even the lightest XC race whippets don't actually float around.

Not that I want to get dragged into this....but surely your weight is supported on the leading pedal that's moving downwards. Good technique means that it isn't weighing down the trailing pedal and counteracting the effort you're putting into pushing the leading pedal down.

Whether that's different with SPDs or flats, I don't know, but if you overdo the lifting of the trailing foot with SPDs then you get a bit of a boost, if you overdo it with flats, your foot comes off the pedal.

Anonymous

But it is demonstratably true that there is always a force applied downwards to the rearwards pedal.
So, even professional riders aren't propelling themselves at full tilt most of the time (unsurprisingly)

bike-a-swan

I think the point is more 'can the rearwards pedal being pulled up help', 'not is it the case all the time'. At least, that's what I've got from this long and scary thread.

bails87

yeehaamcgee wrote:

But it is demonstratably true that there is always a force applied downwards to the rearwards pedal.
So, even professional riders aren't propelling themselves at full tilt most of the time (unsurprisingly)

Yes, I know, but it's down to the fact that they're imperfect human beings like the rest of us. Not because lifting the weight off one pedal would only be possible if they were floating in thin air. Weight stays on the back pedal because of imperfect technique, not because it's physically impossible to lift it off.

bails87

bike-a-swan wrote:

I think the point is more 'can the rearwards pedal being pulled up help', 'not is it the case all the time'. At least, that's what I've got from this long and scary thread.

Aha, yes, this!

meesterbond

bails87 wrote:

yeehaamcgee wrote:

But it is demonstratably true that there is always a force applied downwards to the rearwards pedal.
So, even professional riders aren't propelling themselves at full tilt most of the time (unsurprisingly)

Yes, I know, but it's down to the fact that they're imperfect human beings like the rest of us. Not because lifting the weight off one pedal would only be possible if they were floating in thin air. Weight stays on the back pedal because of imperfect technique, not because it's physically impossible to lift it off.

So, the best that anyone can realistically hope to achieve is to reduce the amount of pressure on the back pedal during the upstroke (which is surely a positive in itself as you're reducing the amount of effort working against the downstroke due to your own weight), rather than actually increasing the effectiveness by 'pulling up'.

Obviously this 'unweighting' can be achieved with flats as well as SPDs thus the hypothesis is disproved and we can all continue using whichever system we like in the knowledge that if we get beaten to the top of the next hill it's because the other guy's quicker, not that he's get better pedals.

Now, can someone explain why red bikes are quicker than other colours?

llamafarmer

yeehaamcgee wrote:

But it is demonstratably true that there is always a force applied downwards to the rearwards pedal.

What were the conditions for these tests? Are we talking riders on turbo trainers in a lab or out on the road or on the trail? How were the forces measured?

Like bails, I don't really want to be drawn into this argument because ultimately I don't care - I use clipless because I prefer them, not because I think it gives me any kind of performance advantage and I don't feel any less capable a rider because of that choice. I just think there's a lot of pseudoscience coming from both sides of the argument here.

Anonymous

bails87 wrote:

yeehaamcgee wrote:

But it is demonstratably true that there is always a force applied downwards to the rearwards pedal.
So, even professional riders aren't propelling themselves at full tilt most of the time (unsurprisingly)

Yes, I know, but it's down to the fact that they're imperfect human beings like the rest of us. Not because lifting the weight off one pedal would only be possible if they were floating in thin air. Weight stays on the back pedal because of imperfect technique, not because it's physically impossible to lift it off.

The downwards pressure on the rear pedal is down to flaws in technique - however small.
But the fact remains that it would not be doing any good anyway.

I'm out of this thread. It's just infuriating talking physics and mechanics with people who have no fundamental understanding (bullet drop thread anyone? Front wheel braking anyone?) :roll:
Sit down, do some maths, analyze it critically rather than just rushing in because you've spent money on something for the wrong reason.

Oxygen Thief

Stick some striped on your bike, you know, the go faster ones! That'll help you up that hill.

I reckon if you have perfect technique on how to use an SPD and perfectly perfect technique on how to use a flat pedal then the same person will be quicker using an SPD pedal. But there'll be nobody with near perfect techniques out there.

bike-a-swan

yeehaamcgee wrote:

people who have no fundamental understanding

oi!

legin

i use the platform spd pedals as some of my local terrain is quite technical.i used to hate them but perservered and have noticed more eficiency on the climbs and rocky bumpy decents that used to knock my feet off the pedals.
the benefit of the platforms is you can unclip and still use them like regular flats.as im still not totally committed to them on the downhills i find the platform spd the best of both worlds.
strangely i prefer being clipped in on drop offs and jumps its adds more security to your landing.
i read a study that stated they are 30% more eficient.i would like to see the source but wouldnt argue as ive noticed the benefit on long 5 hrs rides.

RealMan

hamstrich wrote:

I think that RealMan is talking sense.

Dear god. I never meant to do that. :shock:

Will Snow wrote:

First of all, ha! So who are these studies funded by, big companies such as shimano (who make mainly... err clipped in pedals) or boutique flat pedal companies, such as burgtech (who are so small they couldnt afford the huge expense of a scientific study). According to you, the latter.

The company RealMen LTD just did a study and found that RealMan is a 4000% better rider then anyone else in the world, and that toe clips give more power and control then any other type of pedal system in the world.

That was hard.

Will Snow wrote:

than it is to pull up (using muscles that cyclists use less) against gravity. Course a good way of testing this would be to get a clipped in rider with only one pedal:- can they propel themselves bearing in mind they can only push down half the time?

Maybe muscles that you use less. When I first started pedalling properly in a circular motion rather then just downwards, it puts muscles that aren't usually used into play, and they get stronger over time.

Can a clipped in rider pedal with one foot? Yes. Of course. Its a good way to train yourself to pedal properly. There's even a crank system out there that immobilizes one crank at a time when selected so you only have to move one leg. Very good training, even if it does look weird.

Will Snow wrote:

Also in another thread you said braking with two fingers was wrong, so Im annoyed with you. Try having hands that are 20cm long, it is literally impossible to pull the levers with one finger before it hits your other knuckle before the biting point. Anyway that was a bit off topic.

Yes, it was a bit off topic, but its because your brake levers are too close to the grips. Try moving them away, into the bars a bit. This will help your technique.

yeehaamcgee wrote:

The amount of "power" that can be applied in this way is about the same as an, oooh, 5 year old girl pushing with her leg
Maybe not quite that low, but still, a pretty insignificant amount of strength.

Again, maybe true for someone who hasn't trained those muscles.

yeehaamcgee wrote:

But it is demonstratably true that there is always a force applied downwards to the rearwards pedal.
So, even professional riders aren't propelling themselves at full tilt most of the time (unsurprisingly)

Sorry but that's just wrong. When you pedal properly there will be an upwards force on the rearwards pedal. Like supersonic (I think) said, you should be applying force at a tangent to the circular motion of the pedal at all times. But while you can sort of do this on flats for maybe 66% of the circle, you can't do it as much as SPDs.


Any other questions?

cgarossi

Sorry but that's just wrong. When you pedal properly there will be an upwards force on the rearwards pedal

Which is derrived from the acting downwards force. Unless you are pulling up with equal force to your downward leg. In which case both actions have a dividing effect.

RealMan

You are getting confused with force and velocity I think.


A perfect example, the other day I was sprinting on my road bike from the lights, and on the upstroke (due to my IMMENSE power), my foot came unclipped, and I almost fell off. If there had been a downward force on my rearward foot, it would not have come away from the pedal.

cgarossi

Thats down to the resistance of the bike.

You cannot expect 100% of the energy you put in to be applied to the motion of turning your cranks. Its divided between all resistances.

RealMan

Here's those cranks I mentioned.

http://www.youtube.com/v/nRErXdmZVXw&hl=en_GB&fs=1&

cgarossi wrote:

Thats down to the resistance of the bike.

You cannot expect 100% of the energy you put in to be applied to the motion of turning your cranks. Its divided between all resistances.

You're being very vague there. Resistances? Between the energy you output and the energy going into the cranks the only wasted energy is heat energy in your legs, any resistance the pedal has to spinning, and pedal and shoe flex - that I can think of anyway. But to be honest, I'm not quite sure why you are talking about energy.

cgarossi

RealMan wrote:

Here's those cranks I mentioned.

http://www.youtube.com/v/nRErXdmZVXw&hl=en_GB&fs=1&

cgarossi wrote:

Thats down to the resistance of the bike.

You cannot expect 100% of the energy you put in to be applied to the motion of turning your cranks. Its divided between all resistances.

You're being very vague there. Resistances? Between the energy you output and the energy going into the cranks the only wasted energy is heat energy in your legs, any resistance the pedal has to spinning, and pedal and shoe flex - that I can think of anyway. But to be honest, I'm not quite sure why you are talking about energy.

Where do you think the power in your legs comes from?

Its simple:

You have have two cranks joined with an axle through the bottom bracket. Its the same system and in turn it drives the chain which drives the wheels.

If you push down on one crank and pull up on the other, the action of one will cancel out the other. So, if you pull up, the opposite crank will pull away. If you push down on one crank the other is forced up. If you try to simultaneously pull up and push down with exactly the same force (which is impossible) then the total power exerted will be divided between forcing down and pulling up, combining in the axle to propel the bike forward via the drive train.

The reason why your foot popped out of the cleat is because you cannot apply 100% power into just turning your cranks. The power you are applying is going toward propelling the bike forward.

When I say resistances, there are too many to list. In order to get the bike going forward there are many resistances at work.