Opinions on wheel weight article in Cyclist Magazine?

imposter2.0

blinddrew wrote:

That would effectively test the heavy wheels vs light wheels. In order to test the heavy rims vs light rims you'd need the same set up but with one set of wheels with a nice heavy rim and another set with a light rim and weight added to the hub to make it up to the same weight.

Which bit of 'rim weight is not a significant concern when climbing' are you not getting?

blinddrew

I've read the other thread, all of it, and it's really not. And "Which bit of 'rim weight is not a significant concern when climbing' are you not getting?" this is the point that I'm suggesting could be conclusively settled one way or another. I'm actually struggling to see exactly what you're arguing some of the time. You agree that a lighter rim takes less energy to accelerate, but you don't think this has an affect when you're climbing?

imposter2.0

blinddrew wrote:

You agree that a lighter rim takes less energy to accelerate, but you don't think this has an affect when you're climbing?

Exactly - because you are obviously not constantly accelerating when climbing. And even if you are accelerating while on the hill, you still have to accelerate the entire mass of the bike and rider - not just the rims. All we are doing here is going over the same ground that was covered in the original thread, it's pointless.

blinddrew

Imposter wrote:

it's pointless.

Confusing is what it is, but at least we're doing this without resorting to calling each other names and actually reading each other's posts. I think that's a step in the right direction.

Imposter wrote:

because you are obviously not constantly accelerating when climbing.

Every crank of the pedals is, in effect an acceleration. The forces you're working against (gravity, & various linear resistanced) can be expressed as mass x acceleration, ergo to successfully move against that force you need to providing more acceleration in an opposite direction. My point was that climbing IS a series on constant accelerations. Slow it right down, for example, to some hideously steep ramp where you're stood up and cranking it out at less than 50 rpm, you can feel each acceleration before you hit the next dead spot.

Imposter wrote:

you still have to accelerate the entire mass of the bike and rider - not just the rims.

true, but because the radius of any other rotating part is very small the effect of that, compared to the linear inertia of the non-rotating parts, is neglible.

Pross

The thing I like with this thread and the old one is that those who have argued that wheel weight / rim weight makes a difference (as opposed to overall weight) back this up with anecdotal 'evidence' whilst those who think it doesn't make a difference where the weight is saved have put a lot of effort into explaining the science behind that belief on display. Yet the 'makes a difference' brigade are the ones crying troll. I am not qualified to say whether or not the science is right or not but considering there doesn't appear to be even an attempt at scientifically proving it does make a difference I'm inclined to agree with p_tucker on the original thread. This despite the fact I upgraded to lighter wheels about 18 months ago and felt they 'climbed better' as I appreciate that this is probably because they knocked 500g off the overall bike weight.

There also seem to be a lot of people missing the point (especially on that old thread) that the question is whether losing weight from the wheels is more beneficial than losing it elsewhere not 'is it better to have lighter wheels with everything else being the same'

Pross

blinddrew wrote:

Every crank of the pedals is, in effect an acceleration. The forces you're working against (gravity, & various linear resistanced) can be expressed as mass x acceleration, ergo to successfully move against that force you need to providing more acceleration in an opposite direction. My point was that climbing IS a series on constant accelerations. Slow it right down, for example, to some hideously steep ramp where you're stood up and cranking it out at less than 50 rpm, you can feel each acceleration before you hit the next dead spot.

But isn't the argument that p_tucker was making in the other thread that the heavier wheel will not decelerate so much during the dead spot and therefore it needs less acceleration to get it back to the same speed?

imposter2.0

blinddrew wrote:

Every crank of the pedals is, in effect an acceleration.

Followed by a deceleration when you hit the dead spot in the pedal stroke. Ironically, heavier rims cope with this transition better than light ones. Seriously, this is getting silly. This was also discussed in the other thread. You claim to have read the other thread, but you seem oblivious of its content.

blinddrew

Imposter: This comes back to my point about it not being a zero-sum game. Energy is lost throughout the process. And I did actually mention that in one of my previous posts. Please let's keep it polite.
Pross: I've talked about the relationship between rotational inertia and kinetic energy, I've talked about how not all of your kinetic energy is converted into potential energy and how a degree of this is lost throughout the process.

ugo.santalucia

Imposter wrote:

blinddrew wrote:

Every crank of the pedals is, in effect an acceleration.

Followed by a deceleration when you hit the dead spot in the pedal stroke. Ironically, heavier rims cope with this transition better than light ones. Seriously, this is getting silly. This was also discussed in the other thread. You claim to have read the other thread, but you seem oblivious of its content.

Imposter, I've had enough to hear your argument, please forward me your wheels and I will replace the rims with a pair of Gipiemme 40 mm alloy ones at my expense. I seem to recall they come just shy of 1 Kg each... after the first pedal stroke you should have enough momentum to read the newspaper for the rest of the climb

imposter2.0

blinddrew wrote:

Imposter: This comes back to my point about it not being a zero-sum game. Energy is lost throughout the process. And I did actually mention that in one of my previous posts. Please let's keep it polite.
Pross: I've talked about the relationship between rotational inertia and kinetic energy, I've talked about how not all of your kinetic energy is converted into potential energy and how a degree of this is lost throughout the process.

To be honest, I'm not really sure what you are talking about. If you are disputing the physics - or if you have a better way of describing the physics than has already been provided, then I'm sure we'd all be pleased to see it. Until then, I'll stick with the explanations we already have.

imposter2.0

ugo.santalucia wrote:

Imposter, I've had enough to hear your argument, please forward me your wheels and I will replace the rims with a pair of Gipiemme 40 mm alloy ones at my expense. I seem to recall they come just shy of 1 Kg each... after the first pedal stroke you should have enough momentum to read the newspaper for the rest of the climb

It's not my argument, ugo - I suggest you extend your kind offer to either 'scouselander' or 'p_tucker', both of whom were involved in explaining the physics involved in the original thread. I just happen to agree with it. Anyway, you'd have to supply hubs as well...

blinddrew

Imposter wrote:

To be honest, I'm not really sure what you are talking about. If you are disputing the physics - or if you have a better way of describing the physics than has already been provided, then I'm sure we'd all be pleased to see it. Until then, I'll stick with the explanations we already have.

ultimately my argument is pretty simple, entropy increases throughout the system (mostly that energy will be lost as a [EDIT]acceleration/deceleration of the air you're moving through but also as heat through bearings and tyres) ergo the less energy you have to put into the system you make it accelerate the less energy it will take to overcome a particular obstacle.

imposter2.0

blinddrew wrote:

ultimately my argument is pretty simple, entropy increases throughout the system (mostly that energy will be lost as a [EDIT]acceleration/deceleration of the air you're moving through but also as heat through bearings and tyres) ergo the less energy you have to put into the system you make it accelerate the less energy it will take to overcome a particular obstacle.

Not sure how many seconds 'entropy' will account for when riding up alpe d'huez.

thescouselander wrote:

Also on the acceleration point - although your speed will oscillate somewhat on a climb heaver wheels will smooth this out due to the flywheel effect. Any energy put into the spinning the wheels up in the power part of the stroke will be extracted again as power drops off.

thescouselander wrote:

the flywheel effects of the wheel rims are pretty negligible in theory anyway (although plenty will say they can feel the difference). I was just pointing out that the idea that heaver rims will require more power to keep spinning is false as any energy put in will always come out again at some point.

At the end of the day the amount of energy required to get up the hill is proportional to the all up weight of the bike and rider so the lightest setup should require the least energy - thats why lighter rims are better going up hill.

blinddrew - none of what you say changes the basic physics. And if you are convinced that lighter rims climb better than any other type, the onus is on you to explain - preferably using the laws of physics that are already established.

binkybike

Only looking at the physics of the "flywheel" isn't settling the debate, unless you include the friction of the entire system you aren't proving anything

imposter2.0

binkybike wrote:

Only looking at the physics of the "flywheel" isn't settling the debate, unless you include the friction of the entire system you aren't proving anything

I would guess friction between the two comparable systems is assumed to be the same, if we are comparing one bike/rider with lighter rims against another bike/rider with heavier rims. Can you explain where the friction issue is?

charlie_potatoes

So heavier wheel rims will make for a faster descent then?

blinddrew

Imposter wrote:

blinddrew - none of what you say changes the basic physics. And if you are convinced that lighter rims climb better than any other type, the onus is on you to explain - preferably using the laws of physics that are already established.

I'm curious as to which of my explanations you think are breaking the laws of physics? my argument is simple, the less energy you put have to put into the system, the less energy it takes for that system to overcome an obstacle and you won't get all of that energy back.

In terms of the flywheel thing, that's a bit off, not all of the energy put into the system will be recovered until the descent as some of that kinetic energy is converted into potential energy, most of which will then be used combating wind resistance on the descent, but that's not strictly relevant.

We appear to fundamentally disagree on whether the total system effect of a lighter set of rims will be noticeable - hence I'd like to see a proper test. Hypothesis - experiment - results - review hypothesis.

mrfpb

Personal experience via theoretical understanding: an example.

A few years ago I watched professor Marcus Du Sotoy explian how air turbulence affects irregular spheres in flight, eg golf balls and footballs, and how this can be used to make balls curve on the horizontal plane, eg when taking a free kick. The equations belong to the chaos theory branch of mathemetics. Prof du Sotoy is a keen footballer, but can't actually apply the principles to make a decent free kick.

As far as I'm aware David Beckham does not understand the equations relating to air turbulence but has kicked footballs in excess of 100,000 times (possible exageration) and can bend a ball around a defensive wall to score a goal.

Now then, if the practice of pro-teams is to put lighter rims on their bikes before mountain stages and take a weight penalty elsewhere, then this is going to be borne out of realistic experience rather than a limited / incomplete understanding of the physics of bikes. Remember recent reports that said we don't fully understand why bikes stay upright!

Pro teams will also go to great lengths to make marginal gains, so their practices are not necessarily beneficial to a non competitive rider or commuter. However if bling wheels gives you bragging rights go ahead and buy them.

blinddrew

Well insomnia has struck again so I find myself with a bit of time for some maths.

Let's leave gravity out of it to start with and consider an example of the effect of moments of inertia on rotational speed. Take the figure skater who slides across the ice and then, with a deft flick of her blade, converts that linear kinetic energy to rotational kinetic energy. She can then control the speed of her spin (her angular velocity) by extending her arms out horizontal to slow her down (increasing her moment of inertia) or bringing them close to her body to speed up (decreasing her moment of inertia). From this we can see that with no change to input energy, or mass, we can change the rotational speed by solely changing the moment of inertia. The lower the moment of inertia, the faster the spin.*

Getting back to terms of relevance, consider two different bicycle wheels.
Wheel 1 has a hub of 0.5 kg and a rim of 0.3kg.
Wheel 2 has a hub of 0.3 kg and a rim of 0.5kg.
Both wheels have the same number and length of spokes weighing 100g, centred 150mm from the axis,
The centre of mass of both rims is 300mm from the rotational axis.
The centre of mass of both hubs is 15mm from the rotational axis.
In other words they are identical except for where the weight is carried.

If we assume the hub can be represented (2 dimensionally) as a disc then the formula for the moment of inertia is I = Mh * Rh * Rh/4
If we assume the rim can be represented (2 dimensionally) as a ring then the formula for the moment of inertia is I = Mr * Rr * Rr/2
We shall also assume the spokes can be represented (2 dimensionally) as a disc so the formula for the moment of inertia is I = Ms * Rs * Rs/4***
Wheel 1 has a total moment of inertia of 14091 kgm2
Wheel 2 has a total moment of inertia of 23079 kgm2
That's a 61% ratio. Quite significant I would say.

Now if we look at the kinetic energy of a rolling wheel the formula is E = 1/2 I WW + 1/2 MVV
So we can see that for the two wheels above, if the only variable is I then Wheel 1 will take only 61% of the energy to move at the same speed as wheel two.

Right, let's bring gravity back in and, because I'm thinking this through as I go along, we'll start with some basic force diagrams:

The first figure shows simply sliding a block along a surface, the force required to push it Fh has an equal and opposite force (thank you Mr Newton) resisting it Fr
The second figure shows that when you push that block up a slope, Fr is actually made up of two components, Fg (gravity) and Fl the linear component of the resistance.
The third figure shows a bicycle front wheel with the brake locked on, this is effectively the same as figure 2.

Now let's release the brake:

What happens here of course is that the force Fr (equal and opposite to the force being applied at the hub) then acts to rotate the wheel round (Fa) acting at the furthest point of the radius.
Opposing this acceleration (force is mass * acceleration remember) is the resistance of the bearings (Fb) and the Inertia of wheel I multiplied by the angular velocity of the wheel (alpha)***

For every millimetre that you climb you are putting these forces in constant opposition.
Energy is force * distance, in working against gravity (Fg = mass * 9.8m/s/s) you are applying a constant acceleration away from the centre of the earth. Otherwise you'd come to a halt and roll back down again.

So if we bring back our two wheels above, the only variable in here is the moment of inertia.
Wheel 1 therefore takes 61% of the energy to be pushed up hill as Wheel 2
This is actually slightly optimistic as a lighter wheel also gets bounced around more by surface irregularities but, on road, this is not in the same order of magnitude.

Hence where you carry the weight in the wheel makes a difference.

* see an old self-regulating pressure release valve for a great example of change in moment of inertia, angular velocity and energy all at once.
** technically we should be using cylinders, rods and Toruses but that gets unnecessarily complicated for the point being made (and the differences are negligible compared to the key ratio.
*** god I wish I could do superscript and greek characters.

charlie_potatoes

We need this P_ Tucker chap back.
Imposter is.... just an imposter

Barbarossa

blinddrew wrote:

......

So if we bring back our two wheels above, the only variable in here is the moment of inertia.
Wheel 1 therefore takes 61% of the energy to be pushed up hill as Wheel 2
This is actually slightly optimistic as a lighter wheel also gets bounced around more by surface irregularities but, on road, this is not in the same order of magnitude.

Hence where you carry the weight in the wheel makes a difference.

That was an awful lot of effort to show some irrelevant physics and arrive at the wrong answer.

If the start and end conditions are the same (at rest, or at the same speed), the only only change in energy is the change in gravitational potential energy which is a function of total mass. Any energy used to accelerate your heavier wheels is returned to the system as they slow down.

blinddrew

blinddrew wrote:

Wheel 1 has a total moment of inertia of 14091 kgm2
Wheel 2 has a total moment of inertia of 23079 kgm2
That's a 61% ratio. Quite significant I would say.

It's also, of course, bollox. For one thing the unit will be kgmm2 and for another I've forgotten to factor in the weight of the tyre. Assume 300g at a distance of 320mm for both sets of wheels. This gives us a revised set of figures of:
Wheel 1 has a total moment of inertia of 29451 kgmm2
Wheel 2 has a total moment of inertia of 38439 kgmm2
That's a 77% ratio. I thought 61% sounded too good, this is what happens when you're trying to think at stupid o'clock in the morning.

blinddrew wrote:

So if we bring back our two wheels above, the only variable in here is the moment of inertia.
Wheel 1 therefore takes 61% of the energy to be pushed up hill as Wheel 2

This is also bollox. There are two parts to the kinetic energy term, the rotational part and the linear part, only the rotational part is affected by the moment of inertia.
If we assume 2Pi radians per second and an outer radius of 330mm:
total KE for Wheel 1 = (1/2 *0.029451 * 4) + (1/2 * 1.2 * 2.074) = 1.833 Kj
total KE for Wheel 2 = (1/2 *0.038439 * 4) + (1/2 * 1.2 * 2.074) = 2.013 Kj
So Wheel 1 takes 91% of the energy to move as Wheel 2

imposter2.0

Barbarossa wrote:

blinddrew wrote:

......

So if we bring back our two wheels above, the only variable in here is the moment of inertia.
Wheel 1 therefore takes 61% of the energy to be pushed up hill as Wheel 2
This is actually slightly optimistic as a lighter wheel also gets bounced around more by surface irregularities but, on road, this is not in the same order of magnitude.

Hence where you carry the weight in the wheel makes a difference.

That was an awful lot of effort to show some irrelevant physics and arrive at the wrong answer.

If the start and end conditions are the same (at rest, or at the same speed), the only only change in energy is the change in gravitational potential energy which is a function of total mass. Any energy used to accelerate your heavier wheels is returned to the system as they slow down.

^^ this. We are either witnessing the emergence of some new physical conventions, or the embarrasing ramblings of someone whose basic science education is being over-stretched.

blinddrew

Imposter wrote:

^^ this. We are either witnessing the emergence of some new physical conventions, or the embarrasing ramblings of someone whose basic science education is being over-stretched.

You asked for my physics, I have provided my working, and corrected it. Please feel free to make further corrections.

blinddrew

Barbarossa wrote:

That was an awful lot of effort to show some irrelevant physics and arrive at the wrong answer.

If the start and end conditions are the same (at rest, or at the same speed), the only only change in energy is the change in gravitational potential energy which is a function of total mass. Any energy used to accelerate your heavier wheels is returned to the system as they slow down.

So by that statement there would be no disadvantage in riding heavier wheels at all?

imposter2.0

blinddrew wrote:

So by that statement there would be no disadvantage in riding heavier wheels at all?

Yawn. For the 58th time, this isn't (and never has been) an argument about heavy v light wheels. The original discussion (the one you said you had definitely read) was only ever about rim weight in the context of overall bike/rider weight.

blinddrew

Imposter wrote:

blinddrew wrote:

So by that statement there would be no disadvantage in riding heavier wheels at all?

Yawn. For the 58th time, this isn't (and never has been) an argument about heavy v light wheels. The original discussion (the one you said you had definitely read) was only ever about rim weight in the context of overall bike/rider weight.

And that is exactly what the maths I've set out above shows: a difference in the energy required to accelerate a light rim.

imposter2.0

blinddrew wrote:

Imposter wrote:

blinddrew wrote:

So by that statement there would be no disadvantage in riding heavier wheels at all?

Yawn. For the 58th time, this isn't (and never has been) an argument about heavy v light wheels. The original discussion (the one you said you had definitely read) was only ever about rim weight in the context of overall bike/rider weight.

And that is exactly what the maths I've set out above shows: a difference in the energy required to accelerate a light rim.

Well, you needn't have bothered, because nobody has ever disputed that accelerating a light rim requires less energy!! It will also deccelerate quicker - we've been through all this before. We seem to be going round in circles here (pun intended).

bompington

blinddrew wrote:

Imposter wrote:

blinddrew wrote:

So by that statement there would be no disadvantage in riding heavier wheels at all?

Yawn. For the 58th time, this isn't (and never has been) an argument about heavy v light wheels. The original discussion (the one you said you had definitely read) was only ever about rim weight in the context of overall bike/rider weight.

And that is exactly what the maths I've set out above shows: a difference in the energy required to accelerate a light rim.

Aaaaaaaaargh! Its's not a wheel rolling merrily along on its own, it's two wheels attached to a bike and rider whose weight in total something like 50 times greater.
My simple calculations (based on kinetic energy rather than moment of inertia, 'cos it's much simpler) came out with a similar answer for the wheels. But 61% more energy needed to make a heavier wheel spin is only 1% of the energy needed to make a bike and rider move! What on earth is so hard to understand about that?

TBH I find the wilful misunderstanding of physics a bit strange. I'm still sticking with Confirmation Bias as the source of both perceived improvement in performance, and the desperate need to invent dodgy physics shown in this thread.

mrfpb

Barbarossa wrote:

That was an awful lot of effort to show some irrelevant physics and arrive at the wrong answer.

If the start and end conditions are the same (at rest, or at the same speed), the only only change in energy is the change in gravitational potential energy which is a function of total mass. Any energy used to accelerate your heavier wheels is returned to the system as they slow down.

But that's a gross oversimplification. Energy is always consumed due to friction, wind resistance etc. I start and end my bikes rides at the same altitude ie on my drive, yes the energy "in the man/bike system" should be the same, but a lot of energy has been consumed along the way. If i only did a huge hill climb the energy consumed by my body would be a lot greater than the potential energy added to the system. The point is does the distrbution of weight on a wheel make a difference to the rate at which energy is consumed (over and above the adding of potential energy to the man and bike). Their is a set of theories one way or t'other, but i return to my earlier point that physics can't currently explain why a bike stays upright, so the practice of cycling pros (which I assume is based on properly calibrated power measurements) is more relevant than the physics and maths on this thread,which however rigorous is likely to still miss factors that add to the marginal gains that the pros seek.