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What's the performance difference between getting speedplay zero stainless and titanium, 206g vs 164g. Only focus on weight and not quality of bearings etc.

I suspect the difference won't be noticeable, but I would like to see the math :-)

Lets say I pedal at 80 rpm, crank length 175mm, how many watt will I save getting the titanium edition? Feel free to define any other conditions, that I have left out.

Edit A lot of comments and answers, thanks :-) But they don't really address my question. As I wrote, I would like to see the calculations. Sure the whole thing is theoretical, but thats ok I guess?

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Just saying...... 40gm difference on a bike/rider combo of say 80kg. 0.05% of total weight. It is rotating mass though. Any measurable performance difference (due weight alone) is likely to be placebo effect. –  mattnz Feb 9 at 22:57
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The savings will be virtually nil -- essentially the same as shaving 42g off the weight of your clothing. –  Daniel R Hicks Feb 9 at 22:57
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@mattnz - And, of course, rotating mass only affects acceleration, not steady-state speed/energy. –  Daniel R Hicks Feb 9 at 22:58
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The point is that as part of a "make everything lighter" process each little saving adds up. So IMO a better question is "where does this fit in a cost per gram saving list". –  Mσᶎ Feb 10 at 0:27
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@Ӎσᶎ I cannot disagree, however starting with your argument ends up (for me anyway) with either a much lighter wallet or a bike that is not noticeably faster, or even worse - both. What I was trying to say is that for a vast majority of people (i.e. money IS an object and winning races is not the meaning of life) the benefits of 40g here and 100 there and another 35 on top are immeasurably small. If you are in that small group that measurably benefits you would not need to ask the question here. –  mattnz Feb 10 at 19:31
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2 Answers 2

I was answering the Physics.se version of this question but it got closed (probably rightly so), so I'm posting it here.

The lighter pedals will have very little effect on the performance of your bike, other than through the overall weight, and particularly if you're pedalling at a steady cadence instead of accelerating and decelerating repeatedly.

Making any part of the bike lighter means that accelerating will be easier as you have less inertia, and that you will spend less power at any given constant speed, through a decrease in rolling resistance (i.e. you spend less work flexing your tires). However, you could achieve a similar effect by simply using a lighter frame or handlebars or haircut. To optimize this, if you really are in a trim-all-the-weight-possible mood, you should be comparing this to all comparable weight savings across your bike.

The only real property of the pedals that is affected is their moment of inertia. This means that heavier pedals have more energy stored in their rotational motion than lighter ones. The thing, though, is that at a steady cadence this energy stays constant and you do not need to worry about it. It only comes into play when you are accelerating, and frankly I would be more worried about your translational energy than the rotational energy in the tires and pedals.

In a similar spirit to the above, if you want to minimize this effect, you shoukd compare this with the moment of inertia savings on your pedals, wheels and tires. Since the rims have a larger rotation radius, and rotate at a much faster angular velocity, they hold far more rotational energy than the pedals, and any savings in this area are likely to be easier by using lighter rims.

You asked for math, but I'm afraid it's a bit pointless to do it. The reason is that one can in principle calculate how much energy you save in a given situation, but that will give you a number that doesn't mean much. The only way to give it meaning is to compare it with similar weight losses in your frame and in your rims, at the very least, where 'similar' must be understood, in fairness, to mean a similar increase in price. But then you're in a completely different ball game, far broader than what is in your question.

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Spend the price difference on lighter tyres, tubes and rims, where the large radius has a significant contribution to inertia.

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Actually, no. The contribution to inertia of a tire is only twice that of weight on the frame, and it only affects acceleration, not top speed. How "significant" is 84 grams on the frame? –  Daniel R Hicks Feb 10 at 12:15
    
I see nothing in the question which specifies the kind of event the performance is demonstrated in. You can easily lose 100g per tyre by choosing a folding bead rather than wired and similar differences are possible in rim weights. ETRTO 622-13 rims range between ~400g and 700g+ –  Emyr Feb 10 at 14:30
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The point is that "significant contribution to inertia" is a major exaggeration. There are, eg, claims floating around that one gram on the rim is equivalent seven grams on the frame, but these are false. Basically, if you wouldn't consider the weight difference "significant" on the frame, it's not "significant" on the wheel. –  Daniel R Hicks Feb 10 at 17:16
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