How important is it to save weight on your pedals?- Mtbr.com
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  1. #1
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    How important is it to save weight on your pedals?

    I want to upgrade my pedals from some Ritchey V2s (about 330gms a pair) to some crank brothers and was wondering how much performance gains there is to be had vs cost for the lighter weight pedals i.e. Ti, double Ti etc. The difference in weight appears minimal but an experienced hiking friend was telling me recently about the concept of weight savings off shoes when hiking i.e. "taking 200gms off your feet is like taking 1kg off your pack". So the numbers may not be right but the principle is there, kinda like the importance of reducing wheel weight/rotating mass on mountain bikes. So what do you think?

  2. #2
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    he's right, any weight that moves on your bike is called rotational weight, and the general rule of thumb is that it counts twice as much as static weight (ie your bar, seatpost) you won't feel as much of a difference as say take 200 grams off your tires, since pedals are a lot closer to their axle than tires are, but it will still make a much bigger difference than saving weight on static material.

  3. #3
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    In this case I would only upgrade if your current pedals are "caput".

    Or if you find that entry or exit is a problem with your current pedals.

    I upgraded to Eggbeater stainless steel because I was getting stuck in the pedals too many times and the old pedals were pretty banged up. Ease of entry and exit and weight of the Eggbeaters is awesome.

  4. #4
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    Quote Originally Posted by V-Tach
    I want to upgrade my pedals from some Ritchey V2s (about 330gms a pair) to some crank brothers and was wondering how much performance gains there is to be had vs cost for the lighter weight pedals i.e. Ti, double Ti etc. The difference in weight appears minimal but an experienced hiking friend was telling me recently about the concept of weight savings off shoes when hiking i.e. "taking 200gms off your feet is like taking 1kg off your pack". So the numbers may not be right but the principle is there, kinda like the importance of reducing wheel weight/rotating mass on mountain bikes. So what do you think?
    From an acceleration point of view, the "rotational weight" of the pedals counts like another 20% pedal weight, roughly and on average. So accelerating your 330 g pedals feels like accelerating 400 g of non-rotating weight.

    From a climbing point of view, there is no difference between "rotational" weight and "real" weight. Your 330 g pedals feel like 330 g at constant climbing.

    Here is the approximate "extra weight" that you feel when accelerating some other rotating parts (again, no "extra" effect on climbing):

    tires: 90%
    rims: 70%
    spokes: 20%
    hubs: negligible

  5. #5
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    Once again, rotational weight raised far to high up the flag pole

    Quote Originally Posted by ctracer01
    he's right, any weight that moves on your bike is called rotational weight, and the general rule of thumb is that it counts twice as much as static weight (ie your bar, seatpost) you won't feel as much of a difference as say take 200 grams off your tires, since pedals are a lot closer to their axle than tires are, but it will still make a much bigger difference than saving weight on static material.
    Oh will this amazing over valuation of "rotational weight" ever end?

    First the rule of thumb is completely wrong when it relates to pedals for the simple reason they are so much closer to the center compared to the tires etc.
    Then even tires don't hit the 2x mark.
    Lastly and most importantly is that the "effect" is ONLY taking place during acceleration which isn't happening all the time by any stretch of the imagination AND
    AND>.......happens even less at pedals because of gears. In other words, you can pedal in a much narrower range of revolutions than the wheels vary in their turns.

    But really, and I know people fail to understand acceleration, you are just not doing so much acceleration as you may think. The misconception is found in that the mere fact of being in the acceleration "mode" doesn ment much unless it is large acceleration.
    So when you hear folks saying that they are accelerating all the time or even "with every pedal stroke", they simply don't understand the concept of relative magnatude.

    The extra weight on a pedal and the "rotational" weight effect is so small as to be unimportant.
    Simply put, get the lightest pedal because its the lightest pedal, just like the lightest saddle is the lightest saddle.
    If I had to give some guess on the "average"weight of a 300g set of pedals over a typical cross country loop, I'd say it might be in the neighborhood of about 303 g to 306 g maximum when taken as a average over the whole, say, hour long loop.
    Certainly that 300g pedal would NOT weigh 310grams "average".

    The concept of rotational weight seems never to be digested in its true form, because the typical rider has a completely distorted view of acceleration. They tend to view any change in momentum as "acceleration", completely forgetting that acceleration comes in degrees. Small changes in acceleration account for small , even tiny, amounts of rotational weight, and despite what riders think, they are NOT acceleration all the time, either on flat or even up climbs.

    Get lighter tires because they are lighter....... and don't even worry about the "rotational" aspects of pedals.
    Better to concern yourself about clipping off those little nubs they leave on the tires when made in the factory....those little tags.

  6. #6
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    Quote Originally Posted by Chester
    Oh will this amazing over valuation of "rotational weight" ever end?

    First the rule of thumb is completely wrong when it relates to pedals for the simple reason they are so much closer to the center compared to the tires etc.
    Then even tires don't hit the 2x mark.
    Lastly and most importantly is that the "effect" is ONLY taking place during acceleration which isn't happening all the time by any stretch of the imagination AND
    AND>.......happens even less at pedals because of gears. In other words, you can pedal in a much narrower range of revolutions than the wheels vary in their turns.

    But really, and I know people fail to understand acceleration, you are just not doing so much acceleration as you may think. The misconception is found in that the mere fact of being in the acceleration "mode" doesn ment much unless it is large acceleration.
    So when you hear folks saying that they are accelerating all the time or even "with every pedal stroke", they simply don't understand the concept of relative magnatude.

    The extra weight on a pedal and the "rotational" weight effect is so small as to be unimportant.
    Simply put, get the lightest pedal because its the lightest pedal, just like the lightest saddle is the lightest saddle.
    If I had to give some guess on the "average"weight of a 300g set of pedals over a typical cross country loop, I'd say it might be in the neighborhood of about 303 g to 306 g maximum when taken as a average over the whole, say, hour long loop.
    Certainly that 300g pedal would NOT weigh 310grams "average".

    The concept of rotational weight seems never to be digested in its true form, because the typical rider has a completely distorted view of acceleration. They tend to view any change in momentum as "acceleration", completely forgetting that acceleration comes in degrees. Small changes in acceleration account for small , even tiny, amounts of rotational weight, and despite what riders think, they are NOT acceleration all the time, either on flat or even up climbs.

    Get lighter tires because they are lighter....... and don't even worry about the "rotational" aspects of pedals.
    Better to concern yourself about clipping off those little nubs they leave on the tires when made in the factory....those little tags.
    I am not sure if you are referring to what I called "average" extra effect. My "average" refers to the effect on acceleration for the different gears, nothing else. That effect goes from around 50% down to practically zero.

    I too think that a few extra grams is a corresponding additional "loop time effect weight". But it may be tens of grams for a pair of tires. And this is the weight weenie board, where details are important :-)

    Of the energy that goes to accelerating the bike, roughly 10-20% may go to reving up rotating parts. But how much of your weight-related loop riding output energy goes to acceleration? My guess is around 10%, but I admit it's just a guess. Do you have any kind of basis for your claim for acceleration's share of consumed energy? It's little more than a claim now.

  7. #7
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    Quote Originally Posted by Chester
    they are NOT acceleration all the time, either on flat or even up climbs.
    If you climb then you need to accelerate period. You might be going at constant velocity, but there is this thing called gravity that pulls you down. If you want to go at constant velocity then you need to have some kind of force that equals the force that gravity pulls you down according to the vector mgsin(theta).

  8. #8
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    OK.........not a rocket scientist but ...........

    Quote Originally Posted by anden
    I am not sure if you are referring to what I called "average" extra effect. My "average" refers to the effect on acceleration for the different gears, nothing else. That effect goes from around 50% down to practically zero.

    I too think that a few extra grams is a corresponding additional "loop time effect weight". But it may be tens of grams for a pair of tires. And this is the weight weenie board, where details are important :-)

    Of the energy that goes to accelerating the bike, roughly 10-20% may go to reving up rotating parts. But how much of your weight-related loop riding output energy goes to acceleration? My guess is around 10%, but I admit it's just a guess. Do you have any kind of basis for your claim for acceleration's share of consumed energy? It's little more than a claim now.

    Well, I think we would need some very complexed models and data imput to get exact numbers. Having said that I think the misunderstanding most riders have with regards to rotational weight are
    1. That it only is in effect, when you are accelerating. You'd be surprised how many don't understand this.
    2. That it is only in "significant" effect when under great acceleration. Meaning that if you are climbing a hill and every 10 to 15 seconds, you go from 4.6 mph to 6.2 mph or from 8.7 to 10.1 mph and this is done over 2 to 5 seconds, then that is not "significant" acceleration as it relates to "rotational weight" either in the tires/wheels or especially in the pedals where the rotational effects are more moderated due to the gearing adjustments, should they become too pronounced.
    3. Much, very much of the time, you are dealing with relatively slow rotational effects as it relates to increasing the weight of a object during acceleration.

    One often used example....Take the largest rotational effect, your tires and rims.
    Turn the bike upside down. Using only your fairly weak arm, spin that wheel up to speed.
    Notice you can do it easily in about 3-4 seconds from zero to 20+ miles per hour.
    You never, ever encounter this kind of rotational effect in the real world, and yet you can easily do this with one arm (and I am rather a wimpy upper body guy)
    This kind of acceleration is many times what you will ever get in the real world.
    Now look at the typical acceleration you will get in 90% of the acceleration you encounter in a typical 1 hour loop with uphill climbs, downhill, and flat.
    Almost all of it involves only micro acceleration compared to the experiment I spoke of and remember we were only using 1 wimpy arm to do that.

    Acceleration is highly over rated as it relates to rotational specific weight.
    Besides, the major reason you'll want a lighter pedal is because its just lighter.
    Rotational aspects are something that much thinking is wasted on unless you are on some weird course made up almost entirely of hairpin turns.

    "Of the energy that goes to accelerating the bike, roughly 10-20% may go to reving up rotating parts."

    Hmmm........I'd have to think about that but suppose we accept 15%. But pedals might, on weight percentage alone, only equal 10% of that 15% and then we come to position.
    You still have to remember that pedals make up a very very tiny portion of that because of their close-in position. So even at 10 percent of that 15% or 1.5% you have to make a huge adjustment for where they are. Perhaps you; end up with 10% of that 1.5% or now at .15%....and then you begin to factor in how much time is spent on "significant" acceleration.
    And again this all hinges on how much of the time and to what degree acceleration is really taking placel. Riders despite what they think are more "steady state" movers.
    It is through the natural inclination to conserve energy that one rides as close to a steady state as possible. All movements are done to retain as much energy as possible in doing a loop.
    In some people;'s minds, all acceleration is acceleration with all the rotational aspects in full fury. But think a about a rider starting out and accelerating from zero to 25 mph over 5 minutes. The entire time is spent accelerating and yet I don't think anyone would think all the rotational weight effect amounts to anything.
    Turn your bike upside down.......wish we could adapt a speedo to the wheel....
    Accelerate from 5.9mph to 8.9 mph over 10 seconds using one arm. It takes an amazingly little amt of energy......almost imperceptable and remember that is looking at the entire, wheel and tire etc......not the pedals.

    I remain convinced.....that when averaged over a loop, that a 330 grams pedal set is not even 340 effect grams. I'd guess closer to 333 grams.
    Not saying it isn't anything at all, but I would not look at pedals with rotational apects in mind.

    If you are getting down to performance and not looking at weight for asthetic reasons, then shift your attention to aerodynamics where far greater real gains can be made without counting how many fairys can dance on the head of a pin.
    In performance, aero outweighs rotational weight on pedals 10 to 1 or 100 to 1

    OK......and you said or guessed that...
    " But how much of your weight-related loop riding output energy goes to acceleration?"
    "My guess is around 10%, but I admit it's just a guess. Do you have any kind of basis for your claim for acceleration's share of consumed energy?"

    IF we admit your guess, and I'm don't have a exact answer because it would vary.
    But if we take your 10% and we give it the 15% for the rotational aspect equaling 1.5% and then further we reduce it because of the percent of that that is for pedals at about 10% giving us .15% and then further reducing it because pedals are so close to the center of their rotation.......and then we end up at perhaps.....under .05% maximum.......
    Leaving us with a maximum of 1 part per 2000.............( or am I lost?)

    So if you take a aspect of doing that loop and you find that the factor for acceleration the extra "rotational" weight of the pedals amounts to 1/ 2000, then that is something.
    Could equal 30 feet at the end of a one hour 12 mile loop.

    But then again, look at the following
    http://www.cptips.com/energy.htm

    "Your frontal surface area affects your air resistance. Wind tunnel results show that eliminating the drag created by projecting 4.5 inches of a pencil into the airstream will provide a 158 foot finish line advantage to a cyclist in a 25 mile time trial. That baggy jersey or upright position may be costing you minutes."

    So compare the pedal related rotational aspects with what a few extra folds in your jersey may cost you. I admit that example is a time trial, but looking at most mountain bike riders, they are a aero disaster.
    Thus I think dropping weight is a good idea, but focusing on the rotational aspects of pedals is getting a bit absurd, when there are many other more important aspects to be looked into for maximum performance.

    Now if you just like lower weight for the sake of lower weight, then by all means get the lightest pedals, but not because of their rotational weight nature.

  9. #9
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    There is no other way to describe your post

    Quote Originally Posted by b12yan88
    If you climb then you need to accelerate period. You might be going at constant velocity, but there is this thing called gravity that pulls you down. If you want to go at constant velocity then you need to have some kind of force that equals the force that gravity pulls you down according to the vector mgsin(theta).
    From what you appear to be saying, is that you when climbing you are accelerating all the time or something of equal effect.

    How can I say this..... You are WRONG

    Acceleration is acceleration and has nothing to do with gravity which is a separate matter.
    If you climb at a steady rate of speed up a long climb, you have ZERO acceleration and there are no "rotational" weight effects, .......understand?

  10. #10
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    Let me ask a question. I admittingly don't udnerstand all the ins and outs of accellerations and rotational mass, and climbing etc... But, doesn't mass matter the most when climbing? Take this expample, roll your bike back and forth on the ground. Stand next to it and push it forward for a bit. Now find a steep road, and roll it up the hill. Aren't you, in effect, lifting the weight of your bike against gravity? Isn't it the whole inclined plane thing we all learned about back in high school? When you are lifitng a mass against gravity, whether verticle or in a slant, you are doing the same amount of work, but doing it in a slant you have mechanical advantage so you are exerting less force but for a longer amount of time. I'm not considering rotational weight, just static weight.

    BM
    "I've come to believe that common sense is not that common" - Matt Timmerman

  11. #11
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    Yes, you are correct

    Quote Originally Posted by bmadau
    Let me ask a question. I admittingly don't udnerstand all the ins and outs of accellerations and rotational mass, and climbing etc... But, doesn't mass matter the most when climbing? Take this expample, roll your bike back and forth on the ground. Stand next to it and push it forward for a bit. Now find a steep road, and roll it up the hill. Aren't you, in effect, lifting the weight of your bike against gravity? Isn't it the whole inclined plane thing we all learned about back in high school? When you are lifitng a mass against gravity, whether verticle or in a slant, you are doing the same amount of work, but doing it in a slant you have mechanical advantage so you are exerting less force but for a longer amount of time. I'm not considering rotational weight, just static weight.
    BM
    Yes, you have it correctly in mind.
    And to get idea of how that change in weight affects your climbing, you can take a look at the following site

    http://www.analyticcycling.com/Force...ight_Page.html

    Now, some will instantly tell you its all different because every trail and every stick and stone make it all much more complicated. Sure some things afffect climbing, but to narrow it down, so you can study one weight and how much lowering that weight will affect the "relative" climbing speed, then you can make good use of that site and its calculator.

    For example on one hill I regularly climb......approx 1.4 miles with about 580 ft elevation, I have found that for every 2.5 pounds I drop off the bike, I will go about 8.4 seconds faster.
    Not as great as some would hope given that the time in question is about 10 minutes or 600 seconds.
    That is why I focused more on items such as rolling resistance where the gains in time can be much greater and easier. Of course ideally you would have both the lower weight and the best tire possible.......and don't forget your aerodynamics which begin to play a greater and greater roll as low as 9-10 mph and get much larger as speed increases.
    So if the average speed up a climb such as this one is 8.4 mph but certain faster sections are as much as 12 to 14, then aero come into play.
    On this particular climb, being not very technical or with great variations in speed over short times, the rotational weight effect is almost nil.

  12. #12
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    Interesting discussion

    and instructional. moment of inertia is the key here. (distance from the rotating center times the mass). Accelleration is the key too. if not accellerating, then the weight is just weight. Good discussion.
    Kindacreeky,
    Tennessee Singletrac Sculpter

  13. #13
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    my riding experience...

    Quote Originally Posted by kindacreeky
    and instructional. moment of inertia is the key here. (distance from the rotating center times the mass). Accelleration is the key too. if not accellerating, then the weight is just weight. Good discussion.
    may i just talk about my own riding experience with wheels and pedals as well...

    wheels:
    whatever math you do...a lighter wheelset will accelerate faster - period.
    usually it's also the rims that are lighter, not just the hubs. when i compare my 1220g Amclassic magnesium road wheelset to Ksyrium SLs that weigh about 1500g the difference is just HUGE. even comparing them with the already light CR420 (1370g) they are much faster. to have a valuable comparison we would need to add the weight saved on the wheels to the bike so that it's not the lesser weight making you faster...but it definitely is a BIG difference. not just imagination.

    pedals:
    i have some of the lightest pedals on my bikes (tuned 130g Speedplays on the roadbike and Triple-ti on my MTB). paired to SIDI shoes which are also the lightest mass production shoes this combo is really light.
    now during the cold season i use some neoprene booties that weigh about 350g. i can definitely feel that my legs spin slower. that is a big difference! now imagine you have regular Shimano shoes (+300g over the SIDIs) and regular pedals (+150g)...that's over 1 lbs and more than just the weight of my booties! no thanks - i wouldn't want to ride that way knowing how easy my legs spin with the lighter parts on.

    same with saddles that were mentioned above:
    i can definitely feel if i have my 45-60g saddles on or a SLR with regular post when sprinting out of the saddle. when hammering you move the bike from one side to the other and the lighter saddle makes this just much easier. it is placed high on the bike therefore it really affects how the bike handles when standing in the pedals.

  14. #14
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    I agree with this post.....

    Quote Originally Posted by Chester
    Yes, you have it correctly in mind.
    And to get idea of how that change in weight affects your climbing, you can take a look at the following site

    http://www.analyticcycling.com/Force...ight_Page.html

    Now, some will instantly tell you its all different because every trail and every stick and stone make it all much more complicated. Sure some things afffect climbing, but to narrow it down, so you can study one weight and how much lowering that weight will affect the "relative" climbing speed, then you can make good use of that site and its calculator.

    For example on one hill I regularly climb......approx 1.4 miles with about 580 ft elevation, I have found that for every 2.5 pounds I drop off the bike, I will go about 8.4 seconds faster.
    Not as great as some would hope given that the time in question is about 10 minutes or 600 seconds.
    That is why I focused more on items such as rolling resistance where the gains in time can be much greater and easier. Of course ideally you would have both the lower weight and the best tire possible.......and don't forget your aerodynamics which begin to play a greater and greater roll as low as 9-10 mph and get much larger as speed increases.
    So if the average speed up a climb such as this one is 8.4 mph but certain faster sections are as much as 12 to 14, then aero come into play.
    On this particular climb, being not very technical or with great variations in speed over short times, the rotational weight effect is almost nil.

    People throw a a hole bunch of figures out there about being X% more efficient, etc. The best way to calculate just how much that 400 grams is going to cost you is to do a back to back comparison. Believe me it is going to be a lot less than everyone thinks.

    I used to be the ultimate weight weenie until I started breaking parts and became more concerned about the weight of my bike than the ride itself. You guys out there know exactly what I mean.... How many of you out there cannot ride without their computers?? Since I am not concerned with how fast I am, I have changed my riding group (for the better) and the challenge now is going big and just having fun.

    Later Mon.

  15. #15
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    Quote Originally Posted by nino
    same with saddles that were mentioned above:
    i can definitely feel if i have my 45-60g saddles on or a SLR with regular post when sprinting out of the saddle. when hammering you move the bike from one side to the other and the lighter saddle makes this just much easier. it is placed high on the bike therefore it really affects how the bike handles when standing in the pedals.

    I never thunk of that, but it makes sense. but then again, I don't sprint out of the saddle a whole lot......

    BM
    "I've come to believe that common sense is not that common" - Matt Timmerman

  16. #16
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    Quote Originally Posted by nino
    SIDI shoes which are also the lightest mass production shoes
    I would just like to note here, that my Specialized Comp shoes are only 15g heavier than
    my SIDI Dragon SRS.

    Quote Originally Posted by nino
    when hammering you move the bike from one side to the other and the lighter saddle makes this just much easier. it is placed high on the bike therefore it really affects how the bike handles when standing in the pedals.
    yep, a light saddle is definitely more important than a ligt pedal but you also move
    many other parts of the bike all the time and that's why everything on your bike
    (maybe except the Bottom Bracket) should be as light as possible.

  17. #17
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    Where do you ride???

    Only reason to take such extreme measures is if you race. If you do not, have you ever broken anything 20 miles into the wilderness and had to walk out?? I have broken 2 headsets, 1 carbon crankset, tacoed superlight wheelsets, ripped knobs off of light tires and locked up some super light disc brakes on a downhill due to fluid expansion.

    I moved back to the states recently and now have to ride inside these idiotic bike parks that feel unatural and inside a 1 sq/mile area. Guess if you ride under these conditions you can go light, cause all you have to do is walk out into the parking lot. For the rest that ride in wilderness areas or nowhere civilization, keep it strong.

  18. #18
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    Ride barefooted....

    Just have the cleat surgically inserted in your foot. You can lose over a lb of weight.

    Jaybo

  19. #19
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    For those that fail or refuse to understand why climbing is not accelerating against gravity :
    Get a wheel, hold it by the axle ends. Lift it. Lower it. Again.
    Same wheel, give it a nice 20mph spin. Lift it. Lower it. Again.

    Now tell me how much heavier the 20mph spinning wheels was, 5x heavier surely? Did you suffer an injury attempting it?

    Same with cranks/pedals.
    Remove chain.
    Take out the pinky finger, and find out how much effort it costs to accelerate the cranks from 0 to 100rpm. Ouch, that much have hurt? Just don't get your pinky stuck between crank and frame.

    And a wheels? One swing with the arm accelerates it from zero to 20-25mph in a wink. Hardly any effort at all.

  20. #20
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    Don't cranks without chains spin on their own.......perpetual motion or something?

    Quote Originally Posted by Cloxxki
    For those that fail or refuse to understand why climbing is not accelerating against gravity :
    Get a wheel, hold it by the axle ends. Lift it. Lower it. Again.
    Same wheel, give it a nice 20mph spin. Lift it. Lower it. Again.

    Now tell me how much heavier the 20mph spinning wheels was, 5x heavier surely? Did you suffer an injury attempting it?

    Same with cranks/pedals.
    Remove chain.
    Take out the pinky finger, and find out how much effort it costs to accelerate the cranks from 0 to 100rpm. Ouch, that much have hurt? Just don't get your pinky stuck between crank and frame.

    And a wheels? One swing with the arm accelerates it from zero to 20-25mph in a wink. Hardly any effort at all.
    I agree with all three examples.
    Especially like the cranks without the chain. My gosh, it hardly takes a fly landing on one side to push them down.
    I aways tell folks on even the "hugest" part of rotational weight to just flip that bike upside down and go from zero to 25........with on hand.....BAM....up to speed in a couple seconds....perhaps 3.........
    This whole area of "rotational" weight has been given so much "weight" in the minds of mountain bikers and even road bikers, that it is simply amazing.
    Aerodynamics is far more important to even mountain bikers than "rotational" weight and yet you don't see endless discussions and concern about making sure parts and clothes are more aero. In fact they look at you at being some kind of nut for discussing aerodynamics in mtn biking.

    Part of the reason for this obsession with rotational weight is because it is always a good selling point for upgrading parts. Stores and manufacturers are only too happy to keep passing on the hugeness of rotational weight. Turning it into some kind of virus or something.
    Buy light wheels and pedals and tires, because they are light, and forget about whether they are rotating.
    Spend your time more wisely on rolling resistance and being more aero.....and of course being lighter in general, body and bike.

  21. #21
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    Maybe you are just unlucky. In the past two years I've only had to walk out for a chain that broke, and I wasnt' carrying a powerlink or a chain breaker, and for flat tires. I'm only prepared to fix one flat per ride, so if I get two, I'm done. One ride a sidewall ripped and I found some roadside trash to cover the hole, but the tube still popped and I had to walk, so now I carry a few lengths of duct tape in my saddle bag too. I carry a good multi tool and two powerlinks so I'm ok for any minor mechanical issue.
    "I've come to believe that common sense is not that common" - Matt Timmerman

  22. #22
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    Formula from MOTORCYCLE DYNAMICS

    To find out rigorously how much force is required to accelerate a bike the concept of "reduced mass" is used. This is the actual mass of all the components and the rider plus the inertial effects of all the rotating parts.

    To get the inertial effects you first need the mass moment of inertia of each rotating part. This is not an easy thing to calculate, but it's going to be quite small, as Chester has said. You can rotate the cranks and pedals without a chain using one finger. And you can rotate the wheels, even relatively heavy ones, with not much more effort.

    But the formula reduces this already low figure down much further. You have to divide the moment of inertia of each wheel by the square of its radius. And you have to divide the moment of inertia of the cranks by the square of the rear wheel's radius and by the square of the gear ratio.

    Practically, for almost all puposes, you can just use the dead weight mass of the bike and rider and not worry about the rotating inertia.
    Last edited by Steve from JH; 03-01-2006 at 06:43 AM.
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  23. #23
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    Some approximate calculations

    An engineering textbook I have gives a formula for the mass moment of inertia of a thin, uniformly dense disk, rotating around its center. That's not exactly the same as a wheel of course, but it should be fairly close. The formula is 1/4mr^2, where m is the mass of the disk and r is its radius. Using the formula discussed in my previous post we divide by r^2, leaving 1/4m. So in practical terms if you had a bike and rider weighing 90 kilograms and added one kilogram at the wheels, it would produce a bike as hard to accelerate as a 91.25 kilogram bike with the original wheels. It would require l.4% more force to achieve the same acceleration as the original bike.

    Changing the weight of the pedals would have considerably less effect. Let's use the same assumption of the crank and pedal set as being like a disk. Since you again have to divide by the square of the radius of the rear wheel and since that radius is more than twice as big as the crank length, you're going to reduce the mass down by another 1/4, leaving 1/16 of the mass of the crank and pedal set to be added. Then you have to factor in gear ratio.

    To cut to the chase, in the highest gear found on conventionally geared mountain bikes, the increase in apparent pedal or crank weight from rotation would be equivalent to .1.004 times the dead weight increase. In the lowest gear it would be 1.1 times.
    "Don't criticize what you can't understand."

  24. #24
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    IMHO - field "test"

    Quote Originally Posted by nino
    may i just talk about my own riding experience with wheels and pedals as well...

    pedals: ...
    now during the cold season i use some neoprene booties that weigh about 350g. i can definitely feel that my legs spin slower. that is a big difference! .

    OK, here's my "seat of the pants" test. I recently upgraded from heavy Lake M80 shoes and HEAVY old skool Sh!mano 737s to Shimano M181s and Time Atac XS. I forget the weight savings, but on the order of 200gr for the pedals and "a lot" for the shoes.

    I am VERY accustomed to the Lake/737s. Years accustomed.

    In two test (ie change to new, back for a month on old - don't ask!, then back to new) I can without a doubt say that my bike felt lighter, faster, and more responsive with the lighter combo.

    Lighter may not always be a large difference, and there's little question that I've "upgraded" in the quality and technology of my parts, but I am sold on the improvement.

    YMMV, and thx for yer time.

    back to the grindstone (gawddamn busted water main!$%^$%#$^#@!!!)

    -capt pearl
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    We are what we repeatedly do. Excellence, then, is not an act but a habit.
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  25. #25
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    Revising the calculations

    It seems I shouldn't have used a uniform disk as a model. Since the weight added to wheels is usually added entirely at the rim, the formula mr^2 would be better. Likewise for weight increase at the pedals at the end of the crank.

    This means a kilogram of weight added at the wheel (heavier tires and/or rims with same spokes and hub) would have to be counted twice. A 90 kilogram rider/bike with one kilo added at the wheel would accelerate like a 92 kilo rider/bike with the original wheel.

    The maxim "a pound added to the wheel equals two pounds on your back" would seem to be true.

    For pedals the weight increase would amount to 25% extra weight added for a 1:1 gear ratio. 100 grams added to the pedals would make the bike accelerate like you added 125 grams of nonrotating weight. For higher gears you would further reduce the effect by dividing by the gear ratio squared. For lower gears it would obviously increase the effect. Thus 100 grams extra at the pedals and a 4:1 gear ratio would feel like 101.5 grams of dead weight added. For a 2/3 gear ratio 100 grams would feel like 156. Remember you're adding this on to the overall bike and rider weight so it's a tiny percentage increase.
    "Don't criticize what you can't understand."

  26. #26
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    Explaining my original numbers and adding numbers to the acceleration share:

    Kl = linear kinetic energy of the object
    Kr = rotating kinetic energy of the object
    I = moment of inertia for the object
    m = mass of the object
    v = linear speed
    w = angular speed of the wheel
    r = radius of the object
    R = radius of the wheel
    n = gearing ratio between object and wheel
    the object = hollow cylinder mimicing the object

    Kl = m*v^2/2
    Kr = I*w^2/2
    I = m*r^2
    R = v/(w*n)

    This gives:

    Kr/Kl = (r/(R*n))^2

    My bike (a 29er) has:
    R=36 [cm]
    r for tires and tubes = 34
    r for rims = 30
    r for spokes = 16
    r for hubs = 2
    r for pedals = 17.5
    n for pedals = from 22/34 to 44/11
    n for everything else = 1

    Which gives roughly:

    tires: 90%
    rims: 70%
    spokes: 20%
    hubs: 0.3%
    pedals: 20% for 1:1 gearing (typical singlespeed)

    And that is only for acceleration. It is difficult to estimate the share of weight-related energy that goes to acceleration, but let's say it's 10% (the rest includes climbing and rolling). So, the final "rotational weight effect" for rotating parts is 10% of the above, giving:

    tires: 9%
    rims: 7%
    spokes: 2%
    hubs: forget it
    pedals: 2% for 1:1 gearing

    So decreasing the weight of the original question's 330 g pedals with, let's say 100 g, would be like taking 102 g off the frame, from a total riding effort point of view. That "rotational effect" indeed doesn't seem worth considering.

    Finally, that is not considering that only a small part of the bike weight is the part (say 1/50), only a small part of the total weight is the bike (say 1/10), and only a part of the whole effort is weight-related (say 1/2). So in terms or race effort, divide the effect again with 1,000
    Last edited by anden; 03-01-2006 at 02:08 PM.

  27. #27
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    People look through the wrong end of the telescope at "rotational" weight

    Quote Originally Posted by Steve from JH
    The maxim "a pound added to the wheel equals two pounds on your back" would seem to be true.
    OK.....I'm gonna have to read all three posts again, when I have time to "study"....
    Still, I am comfortable with the outer part of the wheel, tire, tube and rim being "as much as" requiring 2 times greater effort during acceleration. Folks must remembet though that the hub is far less magnified as well as the inner portion of the spokes and the skewer should be zero (just case people are weighing their wheels with everything).
    I must say though, that the case for lighter tires, (and tubes) normally has far more to do the the lower rolling resistance that usually comes along with less rubber and the degree to which performance is gonna be affected will be more due to that, than the extra "rotational" weight effect. You can of course kill two birds with one stone by getting lighter tires with lower rolling resistance.

    Now, comes the part of the question I've never been able to clearly impress some of the "rotational weight" magnifiers..... Those who insist that "rotational" weight is some kind of monster.
    1. They insist they are doing tons of acceleration. Since they insist that their speed is constantly changing they insist they are accelerating on a nearly continous basis.
    Now, for some of the more reasonable ones, you can at least make the case that they have to spend about half the time decelerating just to get to a place where you can once again accelerate.
    I, on the other hand try to make the case that on a relative basis, there is very little time spent accelerating. Or to put it another way, though you increase and or vary your speed, you are not doing it in a huge way, such as a car heading up a on ramp to a freeway.
    We are talking about small rates of acceleration during short times, gettting up to the "general speed zone" and then mini-micro accelerations to stay in that general area.
    Of course, those doing very technical riding will still ague, but their actual speed changes still in a lower range.

    The question is.........given that we will accept the ball park theory that weight out near the tire-rim area can be as much as 2x "during" acceleration, just how much acceleration is actually going on?
    For example, if someone began at 1 mph on a gentle slope and increased his speed 1 mph per minute, then at the end of 15 mintues he would be going 16 mph and would have been accelerating the entire 15 minutes. Those who argue that "rotational" weight is a huge factor, would suggest that that 1 pound increase would be acting like 2 pounds the entire 15 minutes would suggest that the influence is great.
    But as anyone can, with common sense, see is that to accelerate from 1 to 16 mph over 15 minutes is to almost NOT be accelerating at all.

    Also, when climbing up a long climb, that may be varied, but not hugely technical, you wil have hundreds of small accelerations, but only from 5 to 6 mph or from 7 to 9 mph, and they won't happen in 1 second. Though they are indeed "accelerations, they are mini accelerations and as such the magnitude is minimal. That is a concept seemingly lost on those focused on "rotational" weight.

    Even worse are those who insist there are micro accelerations on every pedal stroke.

    So the magnitude of the acceleration is the key factor that is normally left out of riders assumptions when giving a value to rotational weight.
    Personally I think when you factor in the magnitude, duration, and even total time spent accelerating, the effect shrinks to very low impact made up by normal variances in rotational weight.

    Rotational weight is a concept that most riders should ignore. Buy lighter tires and wheels because they are lighter, but not because of rotational weight.
    Normally you would do best to spend the dollars per gram lost....on any part, rather than trying to focus it on "rotational" parts.

    For performance on a cross country loop
    1. Aerodynamics
    2. Rolling resistance
    3. Overall weight
    4. Rotational weight ( a very distant 4th)



    Quote Originally Posted by Steve from JH
    For pedals the weight increase would amount to 25% extra weight added for a 1:1 gear ratio. 100 grams added to the pedals would make the bike accelerate like you added 125 grams of nonrotating weight. For higher gears you would further reduce the effect by dividing by the gear ratio squared. For lower gears it would obviously increase the effect. Thus 100 grams extra at the pedals and a 4:1 gear ratio would feel like 101.5 grams of dead weight added. For a 2/3 gear ratio 100 grams would feel like 156. Remember you're adding this on to the overall bike and rider weight so it's a tiny percentage increase.

  28. #28
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    Quote Originally Posted by Chester
    OK.....I'm gonna have to read all three posts again, when I have time to "study"....
    Still, I am comfortable with the outer part of the wheel, tire, tube and rim being "as much as" requiring 2 times greater effort during acceleration. Folks must remembet though that the hub is far less magnified as well as the inner portion of the spokes and the skewer should be zero (just case people are weighing their wheels with everything).
    I must say though, that the case for lighter tires, (and tubes) normally has far more to do the the lower rolling resistance that usually comes along with less rubber and the degree to which performance is gonna be affected will be more due to that, than the extra "rotational" weight effect. You can of course kill two birds with one stone by getting lighter tires with lower rolling resistance.

    Now, comes the part of the question I've never been able to clearly impress some of the "rotational weight" magnifiers..... Those who insist that "rotational" weight is some kind of monster.
    1. They insist they are doing tons of acceleration. Since they insist that their speed is constantly changing they insist they are accelerating on a nearly continous basis.
    Now, for some of the more reasonable ones, you can at least make the case that they have to spend about half the time decelerating just to get to a place where you can once again accelerate.
    I, on the other hand try to make the case that on a relative basis, there is very little time spent accelerating. Or to put it another way, though you increase and or vary your speed, you are not doing it in a huge way, such as a car heading up a on ramp to a freeway.
    We are talking about small rates of acceleration during short times, gettting up to the "general speed zone" and then mini-micro accelerations to stay in that general area.
    Of course, those doing very technical riding will still ague, but their actual speed changes still in a lower range.

    The question is.........given that we will accept the ball park theory that weight out near the tire-rim area can be as much as 2x "during" acceleration, just how much acceleration is actually going on?
    For example, if someone began at 1 mph on a gentle slope and increased his speed 1 mph per minute, then at the end of 15 mintues he would be going 16 mph and would have been accelerating the entire 15 minutes. Those who argue that "rotational" weight is a huge factor, would suggest that that 1 pound increase would be acting like 2 pounds the entire 15 minutes would suggest that the influence is great.
    But as anyone can, with common sense, see is that to accelerate from 1 to 16 mph over 15 minutes is to almost NOT be accelerating at all.

    Also, when climbing up a long climb, that may be varied, but not hugely technical, you wil have hundreds of small accelerations, but only from 5 to 6 mph or from 7 to 9 mph, and they won't happen in 1 second. Though they are indeed "accelerations, they are mini accelerations and as such the magnitude is minimal. That is a concept seemingly lost on those focused on "rotational" weight.

    Even worse are those who insist there are micro accelerations on every pedal stroke.

    So the magnitude of the acceleration is the key factor that is normally left out of riders assumptions when giving a value to rotational weight.
    Personally I think when you factor in the magnitude, duration, and even total time spent accelerating, the effect shrinks to very low impact made up by normal variances in rotational weight.

    Rotational weight is a concept that most riders should ignore. Buy lighter tires and wheels because they are lighter, but not because of rotational weight.
    Normally you would do best to spend the dollars per gram lost....on any part, rather than trying to focus it on "rotational" parts.

    For performance on a cross country loop
    1. Aerodynamics
    2. Rolling resistance
    3. Overall weight
    4. Rotational weight ( a very distant 4th)
    i have to say i disn't read all that theries about rotational mass...BUT you say "don't care about rotational mass"...

    well - that's just wrong! a heavy wheel at speed won't let you change your line. lighter wheels make a bike much more flickable. changing lines and directions is much easier than with heavy wheels. on the other hand at high speeds heavy wheels help you stabilize the bike. i have found light weights are a BIG plus in technical terrain. on the other hand on my roadbike if i go down at over 40 miles the thing gets really nervous and a hair of wind makes you grip the handlebar real thight

  29. #29
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    People ain't flicking all that much in most cc races.

    Quote Originally Posted by nino
    i have to say i disn't read all that theries about rotational mass...BUT you say "don't care about rotational mass"...

    well - that's just wrong! a heavy wheel at speed won't let you change your line. lighter wheels make a bike much more flickable. changing lines and directions is much easier than with heavy wheels.
    yes, in technical riding, what you say is true. However in most cross country racing, the nature of the course is such, that it isn't considered to be very technical, requiring much, if any, flicking around.
    There are exceptions as there always are.
    I'm all for light wheels and especially for light "low RR" tires, but it gets to the point that the average guy thinks spending $100 to save 100 grams on his wheels is gonna make him faster than the guy who spends $100 to save 150 grams else where.
    The 150 gram loss is always having a impact.......and the extra 50 gram loss is always working for you, while the "rotational" extra 100 grams is only happening when you are accelerating.
    Now without getting some formula invoved, and just guessing to support my position, I say if the 50 grams is involved in 35% of your riding then the "rotational" 100 grams (100+100) involved in well less than 10% or your riding.
    People do a lot more relatively steady-state riding then they are willing to believe.
    For the same dollars spent, I'll take 150 grams of lower weight compared to 100 grams of "rotational" weight lost, even though many suggest that the outer rim/tire/tube is 2x normal weight. (but only during real significant acceleration)

  30. #30
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    Yeah, but those pedal weights do not include the weight of the cleats. Your lightest Speedplay pedals also have the heaviest cleats on the market. It is about the combo of those two items, not the manufacture's weight for the pedal which does not include the cleat. I ended up in this thread as a misclick and entertained it. I am not nor do I get weight geekery. Ti axle pedals are dangerous unless you weigh sub 150 lbs.

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