## Rotating Mass...

Discuss light weight issues concerning road bikes & parts.

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asphaltdude
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ras11 wrote:Dudes... are you joking!!! I'm all about pragmatic equipment.. but come on! Light rims/tires/etc.. make a huge difference!

Listen, micro-accelerations (those 1-5cm movements your frame/body makes during the pedal stroke) account for a massive amount of energy. To slow down and speed up a rotating mass to adjust for these micro-accelerations is a lot. There are two things one can do... one is make your pedal stroke and body position better, or you can reduce mass.

It's true that with every pedalstroke your bike accelerates a bit (especially when you're climbing, at hight speed you won't notice it anymore), and between the pedal strokes it slows down a bit.
If you have light rims, your bike will be easier to accelerate with each pedal stroke. But it will also decelerate between the pedal stroke.

Heavier rims will result in a more constant speed uphill: smaller acceleration when you pedal, but also smaller deceleration when you don't pedal.

So when your riding uphill at constant speed, weight is the only thing that counts for your speed.
With light rims the micro-accelerations and decelerations are bigger, so the bike will feel lighter and more lively.

By the way, the rotational speed af the cranks is sooo small compared to the rims speed that the influence of the mass of the pedal is almost neglectible.

A small calculation:
You're riding 39-23 at 20 kilometer per hour.

The orbital speed of your tire is 5,555 meter/second

Your pedaling frequency is 92 strokes/min
That's 1.545 rotations / second.
So the orbital speed of your pedal is 2 * 3,14159 * 0,175 * 1,545 = 1.69 meter second.

Adding 100 grams to the tire weight will increase rotational energy with 0.5 * 0,1 * 5,5555555^2 = 1.543 Joule.

Adding 100 grams to the pedal weight will increase rotational energy with only
0.5 * 0.1 * 1.69^2 = 0.1428 Joule.

So: rotational mass of the cranks has much less influence than rotational mass of the rims / tires.
If you ride at a higher gear, the influence of the cranks compared to the influence of the rims will be even much less.
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asphaltdude
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LOL divve posted exactly the same thing as I while I was typing my post.
He's right: when going uphill at constant speed, weight is the only thing that counts.
(note: on the Alpe d'Huez TT Basso used 404's and not 202's!)
Whow! That's a pretty damn nice garage door!

ras11
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Thank you for your reply Divve. I think the forum needs more pious and close-minded posts like that. Let me break this down for your high-school-physics-limited-conservative thoughts on this subject. You can go kick your dog after you’re done reading this.

First, Dr. Ferrari is still a well-regarded bio-mechanistic professor. I think you should be more careful with the context in which you use his name, it came across as pompous and arrogant. I’m sure he knows more about the bike/rider then you and I will ever. I agree his blood-doping methods are ethically wrong, but in the strict scientific view he’s not too far off. I don’t agree with everything he says, but he does give the subject a lot of objective thought. When it comes to micro-accelerations, this notion has been around for decades, and it pains me to see it forgotten once again.

Yes, heavier wheels do have more potential energy. We all learned this in grade school. To further your point, back in the old days of the hour record, people used to use super-heavy disk wheels, for exactly the purpose you describe. Once up to speed, they are hard to slow down… much like riding with a flywheel attached to the bike. Now, what you have to remember is that the hour records were done on a perfectly flat oval with a perfect surface. The only significant forms of resistance are air and rolling. In this context, micro-accelerations are put into the background of the opposing forces on the rider.

Ok, so now you’re wondering what in the world is a “micro-acceleration”? Well, you could have asked for more details… Instead you make this sound like black magic that can only be explained through a Nobel effort. Well, I hate to burst your bubble, but cycling is not as complicated as think it is. Just because you don’t understand it, doesn’t mean that “micro-accelerations” don’t exist.

To visualize micro-accelerations, get onto a set of rollers and pedal a big gear… you will notice your bike surge forwards and backwards (looking at roller under your front wheel for a point of reference). Another would be to find a steep hill, and again, grind a big gear while standing. The cause of these forward and backward motions of your bike are from translating the imperfect motions of the legs into the circular motion of the crank. Remember the legs are designed for walking and running, not pedaling in circles.

The micro-accelerations are directly proportional to wheel weight. Think of the micro-accelerations as a sinusoidal wave, with the y-axis being distance your bike moves forward/backwards and the x-axis being the radians of the crank. One can convert the y-axis from distance into force by taking into account the mass of the bike, e.g. Sum(F)=ma (note that this also makes the force absolute). But this is horribly simplified, as you might have guessed. The simplification comes in when accounting for the change in momentum of the wheels. Applying the same linear sinusoidal movement of the bike to wheels is easy, and you’d quickly realize that radial accelerations/decelerations are amplified by lighter wheels, meaning the PE as a function wheel displacement becomes intense, but for a much shorter period of time. Given that the displacements are fixed by the pedaling efficiency of the rider (I’m assuming the rider does not change positions during the climb), the integral force (work) applied to the light wheel is significantly less then that for a heavier wheel. Mostly because the time it takes to accelerate the heavier wheel is much longer, albeit not as much force. And obviously the displacement of the wheel multiplies the force (work) required. So this gets back to the basics… better pedal stroke and lighter rims/tires/spokes means a smaller integral, and thus less work done.

I think the guys at analyticcycling use too-static of models. It’s a great site and I hope they continue to grow into more stochastic calculations.

Well, not to call you a hypocrite, but it’s in fact you that have failed to produce the math. To your credit, I forgot to mention the lateral movement of the wheels, which while climbing at slow speeds, is not as important as you might think. But to be honest, I’m not sure where the speed cutoff versus angle away from perpendicular would be. But I do know that that while in a standing climb it’s best not to swing the bike beyond a couple of inches (measured laterally at the top tube). But that is from my own personal experience, and what several coaches have told me throughout the years. But I could be wrong.. obviously less is better.

But I’m not going on forever… this post is already too long… if you want to ride with heavy wheels, be my guest. And don’t take my word for it. Get out on the road and give it a try. I hate pontification.

See you on the hills... Cheers.
Toys-R-Us

Racing Aardvark
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One note on the "flywheel effect" people like to use as a reason for heavy wheels. Ever notice that in sports where there really IS a flywheel they go out of their way to LIGHTEN it?? Yeah, you can say that the heavier wheel helps push you along due to its inertia, but that's a lossy system. Takes more energy to spin it up then you will get returned.

OLver
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ras11, I think you should read again asphaltdude's post (or read it for the first time! your last post looked like you didn't read it).

it explains fairly why micro acceleration makes no sense.

yes, ther is micro acceleratin, but there's also a micro decelleration. and they are proportional...

divve
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Racing Aardvark wrote:One note on the "flywheel effect" people like to use as a reason for heavy wheels. Ever notice that in sports where there really IS a flywheel they go out of their way to LIGHTEN it?? Yeah, you can say that the heavier wheel helps push you along due to its inertia, but that's a lossy system. Takes more energy to spin it up then you will get returned.

It still returns more than a lighter wheel. Only the total system weight counts, unless you loose it through braking. For micro accelerations see above post.

Regarding Dr. Ferrari, his reputation and knowledge isn't relevant in providing proof. The facts presented should stand by their own merit regardless of who presents them.

Racing Aardvark
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divve wrote:It still returns more than a lighter wheel. Only the total system weight counts, unless you loose it through braking. For micro accelerations see above post.

But lighter wheels take less energy to wind up. Also, have you noticed that there is still an increasing urge to make track wheels lighter? Discs and the such? Even for say Kilo or pursuit riders.

ras11
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I'm sorry, I didn't explain myself better. Clearly you didn't understand what I wrote, and I should have used more careful words. How do I post an Excel sheet, maybe that would help? I totally understand your simple argument of conservation of energy. I'm not debating that. I totally agree with you... except what you think to be true about the actual physics of the situation. We are not talking about just a simple wheel here, we’re talking about a rider who’s bike is moving rapidly forward and backwards, and this bike has two wheels which turn with a certain amount of mass. The question is how does mass effect the rider… NOT how mass affects the wheel.

YES. greater mass at the rim has more total energy... but, because the body was not designed to pedal in circles, but it COSTS more energy to accelerate because it takes more time. I'm sorry if you don't understand that, and I encourage you to try this out on the road. I'm not just saying this for the heck of it. You have to think beyond just the wheel, and look at the whole system to see the obvious benefit.

This horse is dead... should we beat it some more!
Toys-R-Us

Racing Aardvark
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ras11 wrote:YES. greater mass at the rim has more total energy... but, because the body was not designed to pedal in circles, but it COSTS more energy to accelerate because it takes more time. I'm sorry if you don't understand that, and I encourage you to try this out on the road. I'm not just saying this for the heck of it. You have to think beyond just the wheel, and look at the whole system to see the obvious benefit.

This horse is dead... should we beat it some more!

Well, that's the way I look at it.

I just spoke to one of the M.E.s here at work. He says yes, in theory a slightly heavier wheel will help along the lines of a flywheel if you are dealing with a steady state effort. You do need enough time to recoup the initial investment of energy from the "windup". However, he also agreed that the total energy gained/lost is insignificant when you look at the whole system (versus spinning a wheel in your hand). For any race which would involve repeated efforts(or for a very short race like a sprint) the lighter wheel is the more efficient. Basically, for a few specific cases, yes, the heavier wheel might be better, but to any significant degree.

I know for a long time I always compared wheels spinning them in my hands, but that is one very tiny piece of the whole system. A 400 gram difference at a rim isn't very signifcant to a system that comprises 80000grams total.

asphaltdude
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ras11 wrote:The micro-accelerations are directly proportional to wheel weight. Think of the micro-accelerations as a sinusoidal wave, with the y-axis being distance your bike moves forward/backwards and the x-axis being the radians of the crank. One can convert the y-axis from distance into force by taking into account the mass of the bike, e.g. Sum(F)=ma (note that this also makes the force absolute). But this is horribly simplified, as you might have guessed. The simplification comes in when accounting for the change in momentum of the wheels. Applying the same linear sinusoidal movement of the bike to wheels is easy, and you’d quickly realize that radial accelerations/decelerations are amplified by lighter wheels, meaning the PE as a function wheel displacement becomes intense, but for a much shorter period of time. Given that the displacements are fixed by the pedaling efficiency of the rider (I’m assuming the rider does not change positions during the climb), the integral force (work) applied to the light wheel is significantly less then that for a heavier wheel. Mostly because the time it takes to accelerate the heavier wheel is much longer, albeit not as much force. And obviously the displacement of the wheel multiplies the force (work) required. So this gets back to the basics… better pedal stroke and lighter rims/tires/spokes means a smaller integral, and thus less work done.
.

I read this over and over again, and it's just not true. In calculating power / work you mix up distance and time, work and impulse etc.
Last edited by asphaltdude on Fri Apr 22, 2005 9:09 pm, edited 1 time in total.
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yourdaguy
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Basically you are all mostly correct. Any extra momentum put into the wheel will be more or less recovered later if you don't brake and energy is always conserved, but as a person with a backround in physics I would like to look at the entire system and in order to accelerate a heavier wheel there are greater frictional losses for example since you have to push harder there is more heat generated in the shoe/pedal boundry area, in the bearings, more flexing of the pedals, crank, stretching of the chain, etc. These are small differences, but they are measurable. Many other small things come into play, but generally; all things else equal, lighter is better. Generally stiffer is also better and that is where the argument really starts. Is it better for a certain part to be lighter or stiffer? Hence my tagline.

The stuff the pro's use is not just because it is lighter. They are definately taking stiffness and aero into account also. The teams probably have graphs of lightness vs efficiency and stiffness vs efficiency overlaid and try to get the largest area under both curves and that is the part they use. Of course aerodynamics is also a huge part of the equation with the pro's. In fact, aero is probably the number one influencer with the pro's because at the speeds they ride those forces are probably way more important than for a 20 miles per hour guy like me.
For certain parts stiffer is more important than lighter.

Racing Aardvark
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yourdaguy wrote:The teams probably have graphs of lightness vs efficiency and stiffness vs efficiency overlaid and try to get the largest area under both curves and that is the part they use.

Most people I know who work or ride on pro teams would be lucky to even know how to make a graph. Seriously.

divve
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To clear things up. I didn't contend that heavier wheels are actually better compared to lighter wheels for climbing. Only that it doesn't make a whole lot of difference either way when you compare it to the total system weight.

ras11, it doesn't matter whether you take the body into account. Your initial premise doesn't agree with the basic laws of physics. Therefore anything beyond that will then be erroneous by default.

brianwchan
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Racing Aardvark wrote:
yourdaguy wrote:The teams probably have graphs of lightness vs efficiency and stiffness vs efficiency overlaid and try to get the largest area under both curves and that is the part they use.

Most people I know who work or ride on pro teams would be lucky to even know how to make a graph. Seriously.

Hehe, details?

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Racing Aardvark
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brianwchan wrote:
Racing Aardvark wrote:
yourdaguy wrote:The teams probably have graphs of lightness vs efficiency and stiffness vs efficiency overlaid and try to get the largest area under both curves and that is the part they use.

Most people I know who work or ride on pro teams would be lucky to even know how to make a graph. Seriously.

Hehe, details?

All I'll say is, what kind of education do you think most people working (or riding) for cycling teams have? Heck only a SMALL percentage of American pros have any college education. In Europe it's even worse(there are a few exceptions!

Now, remember that MOST people working for pro teams either were pros or have always been involved in cycling. How many business people or people with careers give it up to work for a team? Very damn few. So, given the limited pool of candidates, what do you think the average IQ is?

(I do know a few, for instance a mechanic with a Masters in Sports Psychology, and a team director with a degree in Physiology, and there's actually quite a few MTBers with college degrees in advanced fields, but they are the EXCEPTION).