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Everyone says wheels are the key place to save weight but...

salsajakesalsajake Posts: 702
edited September 2009 in Commuting chat
What about shoes and pedals? Often overlooked I would have thought. My SPDs weigh 440g but I could get them down to 230g . My shoes weight who knows how much, but they are clonky Nike MTB shoes with big rubber soles and I bet a good road shoe would shave off 200g again. Total cost of the upgrade would be around £150 at a guess, for some top line pedals and good shoes. A wheel upgrade for £150 that would save around 400g over my existing ones - I don't think so!

Surely weight saved on every single pedal stroke is just as crucial as weight saved on the wheels too? With each stroke you are either pushing or pulling your other pedal and shoe up, so with weight saved there, you should have better acceleration, longer efforts, higher cadence and should climb quicker too as the whole ensemble will weigh less. Am I missing something?

Sorry if this is way too weeny for the commute forum but I know there are a few other Captain Upgrades on here...

Posts

  • Think I'll concentrate on th 5 kg I could save from my belly before I worry about 200g from my shoes....
  • But the heavy pedal and shoe on heading downwards on one side is getting pulled by gravity and balances the effort of lifting the shoe and pedal on the other side.

    The weight on your feet is no more significant than it is anywhere else on the bike.
  • choirboy wrote:
    Think I'll concentrate on th 5 kg I could save from my belly before I worry about 200g from my shoes....

    I lose a couple of hundred grammes from my belly before each ride in the morning, my body won't let me get in the saddle until it has (ahem) prepared itself. No more weight to come off me, has to come from the steed!
  • El Gordo wrote:
    But the heavy pedal and shoe on heading downwards on one side is getting pulled by gravity and balances the effort of lifting the shoe and pedal on the other side.

    The weight on your feet is no more significant than it is anywhere else on the bike.

    Doesn't seem right. Someone riding a 40lb full sus downhill bike with normal gear on would surely accelerate faster and keep going longer than someone on a 20lb machine with 10lb strapped to each foot? An extreme and ridiculous example granted, but if the principle applies, it should work to a lesser extent with lesser differences. Also, having seen cargo bikes, including those ludicrous images from China with all sorts being carried, clearly you can shift big weights fairly easily on a bike. BUT there comes a point where you simply couldn't pedal nice circles once you had a certain weight (say half a fridge) attached to each foot - granted the weight on one side would help a bit with the other, but it would still take a massive effort. Stick the fridge in a basket, and you are away, happy as larry.

    Where are Brainiacs when you need them?
  • The weight saving with wheels is to do with linear acceleration in the direction of your travel. If you can imagine parts of your wheel are speeding up and slowing down in the direction of travel at every rotation, the top going in one direction the bottom going in the other. Because the force generated is proportional to the wheel diameter, the speed squared and the radial weight distribution, weight saving on the rims and tyres is important.
    There is the same effect with your cranks, pedals and shoes etc. but the diameter is smaller and depending on your gear the rotational speed is a lot less. So weight saving in this area has less effect.
    Marin Highway One
    Trek 7.5FX
  • jetsstar1 wrote:
    The weight saving with wheels is to do with linear acceleration in the direction of your travel. If you can imagine parts of your wheel are speeding up and slowing down in the direction of travel at every rotation, the top going in one direction the bottom going in the other. Because the force generated is proportional to the wheel diameter, the speed squared and the radial weight distribution, weight saving on the rims and tyres is important.
    There is the same effect with your cranks, pedals and shoes etc. but the diameter is smaller and depending on your gear the rotational speed is a lot less. So weight saving in this area has less effect.

    Good man! So it has some effect, more marked than say a new fork which would save the same weight (which is what I was going to get before seeing a very good price on a very light clipless pedal!)
  • jetsstar1 wrote:
    The weight saving with wheels is to do with linear acceleration in the direction of your travel. If you can imagine parts of your wheel are speeding up and slowing down in the direction of travel at every rotation, the top going in one direction the bottom going in the other. Because the force generated is proportional to the wheel diameter, the speed squared and the radial weight distribution, weight saving on the rims and tyres is important.
    There is the same effect with your cranks, pedals and shoes etc. but the diameter is smaller and depending on your gear the rotational speed is a lot less. So weight saving in this area has less effect.

    With wheels the key difference is angular, rather than linear, acceleration, though your point about gearing is important.

    Put your bike on a stand and spin the wheels up to 20mph, then brake to standstill & do it again. All that energy is going into the system, then being wasted every time you accelerate and brake.
    Because the rims & tyres are the bits that spin fastest and furthest from the axis they are the most significant. Your feet will be turning much more slowly than your rims.

    A weight saving at the pedals will be more significant than on the frame or rider, but much less significant than at the rims.

    Cheers,
    W.
  • salsajake wrote:
    El Gordo wrote:
    But the heavy pedal and shoe on heading downwards on one side is getting pulled by gravity and balances the effort of lifting the shoe and pedal on the other side.

    The weight on your feet is no more significant than it is anywhere else on the bike.

    Doesn't seem right. Someone riding a 40lb full sus downhill bike with normal gear on would surely accelerate faster and keep going longer than someone on a 20lb machine with 10lb strapped to each foot? An extreme and ridiculous example granted, but if the principle applies, it should work to a lesser extent with lesser differences. Also, having seen cargo bikes, including those ludicrous images from China with all sorts being carried, clearly you can shift big weights fairly easily on a bike. BUT there comes a point where you simply couldn't pedal nice circles once you had a certain weight (say half a fridge) attached to each foot - granted the weight on one side would help a bit with the other, but it would still take a massive effort. Stick the fridge in a basket, and you are away, happy as larry.

    Where are Brainiacs when you need them?

    Ok, if we're going to get all precise about it then you should consider the rotational inertia of the crank / pedal / shoe / foot assembly. As Jetstar says the radius is small compared to the wheels but the effect is only relevent when accelerating / decelerating anyway. Since your cadence is more or less constant (unless you ride fixed) this is not important.

    In summary, yes it does make a difference but it is very small compared to everything else.

    ... and if you had half a fridge on each foot I think there may be other issues hindering your progress beyond rotational inertia. :D
  • anyone remember the radial relationship of force require to accelerate an object in a circle?

    Persumably it's got a significant influence...
  • markp2markp2 Posts: 162
    A roadie friend of mine says that 1kg taken off the wheels is roughly equivilent to 3kg taken of anywhere else.
    Can't see how that works up a 1:5 hill myself but that seems to be the perceived wisdom.
    Chers,
    Mark
    Genesis Croix de Fer - my new commuting mount
    Saracen Hytrail - the workhorse - now pensioned off
    Kinetic-One FK1 roadie - the fast one - hairy legs though!
    Cannondale Jekyll Lefty MTB - the muddy one which keeps tipping me into gorse bushes!
  • Yes, I didn't want to go into the rotational kinetic energy equations. I was trying to explain it understandable terms. :wink:
    But basically you have to calculate the sum of the translational kinetic energy and rotational kinetic energy. If you work through the equations then a wheel with all the mass at the outer rim will have twice the kinetic energy of an equivalent mass on a bike. For a disc wheel this works out at about 1.5. So a regular wheel mass saving on kinetic energy is between 1.5 and 2 of a fixed component.
    Marin Highway One
    Trek 7.5FX
  • markp2 wrote:
    A roadie friend of mine says that 1kg taken off the wheels is roughly equivilent to 3kg taken of anywhere else.
    Can't see how that works up a 1:5 hill myself but that seems to be the perceived wisdom.
    Chers,
    Mark

    That's the thing about perceived wisdom - it's not the same as actual wisdom (but it is much more common).
  • jetsstar1 wrote:
    Yes, I didn't want to go into the rotational kinetic energy equations. I was trying to explain it understandable terms. :wink:
    But basically you have to calculate the sum of the translational kinetic energy and rotational kinetic energy. If you work through the equations then a wheel with all the mass at the outer rim will have twice the kinetic energy of an equivalent mass on a bike. For a disc wheel this works out at about 1.5. So a regular wheel mass saving on kinetic energy is between 1.5 and 2 of a fixed component.

    Indeed, but this is only important when accelerating. If you are moving along the road at a constant 20mph the weight of the bike, rotating parts or not, is irrelevent. It's rotating weight is only relevent when accelerating and all weight is relevent when climbing.
  • El Gordo wrote:
    jetsstar1 wrote:
    Yes, I didn't want to go into the rotational kinetic energy equations. I was trying to explain it understandable terms. :wink:
    But basically you have to calculate the sum of the translational kinetic energy and rotational kinetic energy. If you work through the equations then a wheel with all the mass at the outer rim will have twice the kinetic energy of an equivalent mass on a bike. For a disc wheel this works out at about 1.5. So a regular wheel mass saving on kinetic energy is between 1.5 and 2 of a fixed component.

    Indeed, but this is only important when accelerating. If you are moving along the road at a constant 20mph the weight of the bike, rotating parts or not, is irrelevent. It's rotating weight is only relevent when accelerating and all weight is relevent when climbing.

    Yep you are right. Doesn't matter if you are travelling along at constant speed.
    Marin Highway One
    Trek 7.5FX
  • I knew this would bring the scientists out! I do actually have to do a fair bit of stop start riding on my commute, so accleration does come into it. I have a couple of big(ish) hills too, so weight is a concern. I figure if I can make the bike lighter, I can ride it faster and/or be less tired.

    The problem is, my wheelset weighs about 1940g I think, which is far from flyweight (Alex 450 rims, Deore front hub, Tiagra rear), for 200g saving I would need to spend about £150 and for 400g (1500g ish appears to be pretty light) I'd be looking at about £300 at least. But if £300 buys me the equivalent of 1.2kg weight saving, then it seems much better! Not sure about weights of road shoes vs weight of clonky Nike mtb shoes, but it seems I have some decisions to make about my next spends:

    Kinesis Evo Crosslight Carbon fork and Chris King headset: £280 (saving 400g over a ridiculously heavy steel Kona Project 2 fork)
    Look Keo Carbon pedals and some decent-ish road shoes £150-170 (saving about 400g at a guess, possibly more, with a slight advantage when accelerating)
    Some 1550g wheels (at least £300) (saving 400g or 1.2kg depending if accelerating!)

    I'll need to pedal some more miles and save me some more commuting money for any of these options
  • New shoes will be all shiny and nice.

    Surely that's a good enough reason?
  • New shoes will be all shiny and nice.

    Surely that's a good enough reason?

    As would a new fork and wheels... If I get all of them, everything will be shiny and nice, apart from my bank balance - I only save £6.02 a day by bike...

    Then again, if I put a new light front wheel and fork on, bearing in mind I have panniers, I might be wheelying everywhere!
  • deswellerdesweller Posts: 5,271
    There are corollaries to the 'save weight on your wheels' argument.

    The inertia of a spinning object is a function not just of its mass, but of the radial distribution of said mass. So a 1kg wheel with all the mass at the hub has less inertia (i.e. less resistance to acceleration/deceleration) than a 1kg wheel with all its mass at the rim.

    Unfortunately wheel manufacturers do not quote mass moment of inertia figures. Cowards ;-).

    This means that, as your cranks, pedals etc. have a smaller diameter, chances are they have quite a low rotating inertia anyway, so there are less savings to be had.

    Another point to make is that the energy stored in a spinning wheel has a square relationship with the angular velocity of the wheel. So, to double your wheel speed requires four times as much energy. Because the angular velocity of your cranks, pedals etc. is so much lower than your wheels, the amount of rotating energy stored in them is nowhere near as high.
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  • markp2 wrote:
    A roadie friend of mine says that 1kg taken off the wheels is roughly equivilent to 3kg taken of anywhere else.
    Can't see how that works up a 1:5 hill myself but that seems to be the perceived wisdom.
    Chers,
    Mark

    Cool. What I'm going to do is put some really heavy wheels which weigh 7kg between them on my bike, so taking it's total weight to around 15kg. Then I'll put my current wheels back on, saving around 5kg. With a 3:1 ratio, this means the 5kg saving will be worth 15, and my bike will effectively weigh nothing!

    Back in the real world, it will take twice as much energy to accelerate a wheel of a given weight to a given speed than it would to accelerate a wheel of half the weight to the same speed, but only if all the weight was at the very edge of the rim. This is a long way from saying that '1kg of the wheels = 2kg off anywhere else'. It's usually going to be more worthwhile worrying about the aerodynamics and stiffness of a wheel, but those properties are more difficult to measure and so don't get as much consideration.
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