Electric pedals!

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Smokin Joe

Legendary Member
How does an electric pedal transmit power to the cranks? Supposing you activated the pedals with the bike on a stand, would the back wheel go round? Never in a million years because thay are not pushing against anything.

And my engineering credentials include a five year apprenticeship, not enough to make me the worlds foremost expert but enough to know a poppycock idea that defies all logic and will be dead and buried in no time.
 

Arch

Married to Night Train
Location
Salford, UK
Smokin Joe said:
How does an electric pedal transmit power to the cranks? Supposing you activated the pedals with the bike on a stand, would the back wheel go round? Never in a million years because thay are not pushing against anything.

And my engineering credentials include a five year apprenticeship, not enough to make me the worlds foremost expert but enough to know a poppycock idea that defies all logic and will be dead and buried in no time.

But I HAVE activated the pedals on a bike in stand (as I've already said), and the back wheel DID go round. Because the pedals are pushing against the thing pushing down on them - in my case, my hand, or when riding, the foot.

So. Your theory vs reality....
 

bikepete

Guru
Location
York, UK
Smokin Joe said:
How does an electric pedal transmit power to the cranks? Supposing you activated the pedals with the bike on a stand, would the back wheel go round? Never in a million years because thay are not pushing against anything.

The whole point is that your foot on the pedal holds it flat, so the motor can spin the cranks.

How it behaves without a foot on it is simply irrelevant. The motor doesn't turn on without a foot pressing on the pedal.
 

Night Train

Maker of Things
I can certainly see how it would work by reacting against the foot and have no doubts that it would work, as Arch has experienced.
I'm just not yet convinced that the assistance would be very great given how short the reaction lever is.

I guess it can only really relieve the rider of the same anmount of effort that having a crank that is half a pedal width longer would achieve.

I guess it must suit someone's needs.
 

bikepete

Guru
Location
York, UK
Night Train said:
I'm just not yet convinced that the assistance would be very great given how short the reaction lever is.

I guess it can only really relieve the rider of the same amount of effort that having a crank that is half a pedal width longer would achieve.

I guess it must suit someone's needs.

Um, the calculations in my earlier post were trying to demonstrate that 100W per pedal (i.e. 200W total assistance) would be feasible at 60 rpm with a total pedal length of approx 6" (moment arm of 3" each side), as long as your foot was providing over 10 kg of force to keep the pedal flat. Most people's legs provide that just from their self weight.

Can anyone can see a flaw in my calculation? Or can anyone offer an alternative analysis?

Working out some numbers, even ever so approximately, tends to be more useful than guessing IMO.
 

Night Train

Maker of Things
bikepete said:
Um, the calculations in my earlier post were trying to demonstrate that 100W per pedal (i.e. 200W total assistance) would be feasible at 60 rpm with a total pedal length of approx 6" (moment arm of 3" each side), as long as your foot was providing over 10 kg of force to keep the pedal flat. Most people's legs provide that just from their self weight.

Can anyone can see a flaw in my calculation? Or can anyone offer an alternative analysis?

Working out some numbers, even ever so approximately, tends to be more useful than guessing IMO.
I see where you are going with that.
Are you calculating on the basis of the crank length being the effective lever or half the pedal width (front to back) being the effective lever? That would change it from 150mm to about 40mm.
You would need a greater static load, about four times as much, on the leading edge of the pedal then you would need on the crank.
 
Location
EDINBURGH
It is an interesting concept, however I can see a potential issue with the torque on the output shaft, I understand how it works. I also think it will be tiring on the ankles.
 

bikepete

Guru
Location
York, UK
Certainly not the crank length - that has no effect whatsoever. As the pedal motor is applying a pure torque to the crank via the pedal axle it could be doing so anywhere along the crank and the effect would be the same.

I was assuming that the pedal would be/is quite a bit more bulky than a normal pedal, with the battery packs each side. So the overall pedal length front to back could be about 6"/15 cm. That ties in with my memory of the prototype pedal from the show. It could be a bit less.

The lever arm I used was this entire length. My reasoning was that the limiting case is when the foot just lifts off the back edge of the pedal (and hence, assuming a flat rigid shoe sole, throttles back the motor via the pressure sensor close to the pedal axle).

At this point IF you neglect the weight of the leg (which you can't - on an unpowered bike there is always weight on the pedals - it balances out though because the two cranks are at 180 degrees) then there would be 10 kg of force between front edge and your shoe, and 0 at the back edge. As long as your leg weight and pedalling force provide over 10 kg the pedal stays flat.

Say your leg weight and a bit of pedal force provide 30 kg in total, applied only on the front and rear edges. At max assist there would be 20 kg on the front edge and 10 kg on the back edge of the pedal. So a difference of 10 kg over a distance of 6", i.e. enough to provide the necessary reaction torque.

I can see why using half of the pedal length is attractive, but it's the slightly counter-intuitive fact that you can apply a torque anywhere on a rigid body and it has the same effect which applies here I think.

If in our assumption the two contact points between pedal and shoe are the front and back edges only, then it must be this distance which is used for the torque calculation.

Even if I am wrong there and the forces involved are double my estimate then it's still a feasible proposition IMO. At least feasible enough not to be dismissed out of hand.
 

Smokin Joe

Legendary Member
Ok, I'm prepared to buy this electric pedal stuff. Now can someone get onto this bloke and get him to invent an electric football boot? :biggrin:
 

Armegatron

Active Member
Arch said:
But I HAVE activated the pedals on a bike in stand (as I've already said), and the back wheel DID go round. Because the pedals are pushing against the thing pushing down on them - in my case, my hand, or when riding, the foot.

So. Your theory vs reality....

Reminds me of Mythbusters: "I reject your reality and substitute my own"

Very good idea. Initially I was thinking how does that work, but Im starting to get it. Voted :biggrin:
 
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