Wheel upgrade - a question

ugo.santalucia

P_Tucker wrote:

unixnerd wrote:

On an uphill, you're effectively accelerating all the time, just to maintain speed (due to the battle against gravity which slows you down). Lighter rims are easier to accelerate and so it stands to reason that a lighter rim helps on the climbs (ignoring all other factors like lateral stiffness etc)

+1. As someone who climbs things like Cairngorm mountain's ski road on a regular basis I can assure you that light wheels and tyres have far more than a placebo effect.

On the flat if you put a bit more effort in it translates into extra speed far more easily with lighter wheels. This was very noticeable going from a 1900g to a 1500g set.

Why rely on anecdotes? Some nice people with A-levels in physics have done the sums - from Wikipedia

Advantages of light wheels

The advantage of light bikes, and particularly light wheels, from a KE standpoint is that KE only comes into play when speed changes, and there are certainly two cases where lighter wheels should have an advantage: sprints, and corner jumps in a criterium.[15]

In a 250 m sprint from 36 to 47 km/h to (22 to 29 mph), a 90 kg bike/rider with 1.75 kg of rims/tires/spokes increases KE by 6,360 joules (6.4 kilocalories burned). Shaving 500 g from the rims/tires/spokes reduces this KE by 35 joules (1 kilocalorie = 1.163 watt-hour). The impact of this weight saving on speed or distance is rather difficult to calculate, and requires assumptions about rider power output and sprint distance. The Analytic Cycling web site allows this calculation, and gives a time/distance advantage of 0.16 s/188 cm for a sprinter who shaves 500 g off their wheels. If that weight went to make an aero wheel that was worth 0.03 mph (0.05 km/h) at 25 mph (40 km/h), the weight savings would be canceled by the aerodynamic advantage. For reference, the best aero bicycle wheels are worth about 0.4 mph (0.6 km/h) at 25, and so in this sprint would handily beat a set of wheels weighing 500 g less.

In a criterium race, a rider is often jumping out of every corner. If the rider has to brake entering each corner (no coasting to slow down), then the KE that is added in each jump is wasted as heat in braking. For a flat crit at 40 km/h, 1 km circuit, 4 corners per lap, 10 km/h speed loss at each corner, one hour duration, 80 kg rider/6.5 kg bike/1.75 kg rims/tires/spokes, there would be 160 corner jumps. This effort adds 387 kilocalories to the 1100 kilocalories required for the same ride at steady speed. Removing 500 g from the wheels, reduces the total body energy requirement by 4.4 kilocalories. If the extra 500 g in the wheels had resulted in a 0.3% reduction in aerodynamic drag factor (worth a 0.02 mph (0.03 km/h) speed increase at 25 mph), the caloric cost of the added weight effect would be canceled by the reduced work to overcome the wind.

Another place where light wheels are claimed to have great advantage is in climbing. Though one may hear expressions such as "these wheels were worth 1–2 mph", etc. The formula for power suggests that 1 lb saved is worth 0.06 mph (0.1 km/h) on a 7% grade, and even a 4 lb saving is worth only 0.25 mph (0.4 km/h) for a light rider. So, where is the big savings in wheel weight reduction coming from? One argument is that there is no such improvement; that it is "placebo effect". But it has been proposed that the speed variation with each pedal stroke when riding up a hill explains such an advantage. However the energy of speed variation is conserved; during the power phase of pedaling the bike speeds up slightly, which stores KE, and in the "dead spot" at the top of the pedal stroke the bike slows down, which recovers that KE. Thus increased rotating mass may slightly reduce speed variations, but it does not add energy requirement beyond that of the same non-rotating mass.

Lighter bikes are easier to get up hills, but the cost of "rotating mass" is only an issue during a rapid acceleration, and it is small even then.

http://en.wikipedia.org/wiki/Bicycle_performance

Maths can be manipulated to get the result you want... all you need to do is to plot only the numbers you think are relevant... the criterium race example takes into account only a small portion of the events of a race, where the number of accelerations required can be far higher than those mentioned.

If these match were right, there would be no benefit at all in reducing mass and/or rotating mass, but we all know this is not the case. Of course benefits are marginal, fractions of Km/h... but that's often all it takes to spot a difference or to win/lose a race against time

P_Tucker

Wirral_Paul wrote:

P_Tucker wrote:

Remember that reading thing we talked about? Well, try reading my CLEAR STATEMENT again and telling me how the mass of something solid affects how much air it moves out of the way.

Huh?? What on earth has this got to do with anything. As for proving YOUR physics - you dont have any of your own. You clearly just copy article and dont understand whats written - and so you cant translate that to the point!! You say yourself that lighter wheels are faster uphill, but then try and claim that heavier rims conserve energy better.

I really do give up. Believe what you want - i'm not rising to any more of your desperate attempts to cause yet another argument. You're famous for it.

Bye bye

Yes, I haven't invented my own physics, I just use the same physics that got mankind to the moon and back. No need to reinvent the wheel

As for your other "thought" - lighter wheels being faster uphill and heavier rims conserving more momentum are not contradictory in any way. Why do you think they are?

P_Tucker

ugo.santalucia wrote:

Maths can be manipulated to get the result you want... all you need to do is to plot only the numbers you think are relevant... the criterium race example takes into account only a small portion of the events of a race, where the number of accelerations required can be far higher than those mentioned.

If these match were right, there would be no benefit at all in reducing mass and/or rotating mass, but we all know this is not the case. Of course benefits are marginal, fractions of Km/h... but that's often all it takes to spot a difference or to win/lose a race against time

The problem with facts is that they can be used to prove anything that's remotely true

Davey C

Mikey23 wrote:

Only O level physics here and many years ago....

So are we to conclude then that for those of us who Pootle along at 15 mph average on our entry level road bikes there is no point whatsoever in spending our hard earned dosh on an expensive set of wheels?

Correct. Better off losing weight from your belly for improved performance and it costs nothing. If however you want a BLING set of Deeps to impress your mates that's fine too.

P_Tucker

Davey C wrote:

Mikey23 wrote:

Only O level physics here and many years ago....

So are we to conclude then that for those of us who Pootle along at 15 mph average on our entry level road bikes there is no point whatsoever in spending our hard earned dosh on an expensive set of wheels?

Correct. Better off losing weight from your belly for improved performance and it costs nothing. If however you want a BLING set of Deeps to impress your mates that's fine too.

Word. However I might suggest that wirral_paul buys a cheaper, heavier set of wheels so he can buy a physics textbook or 12.

For the record, I checked my understanding with a friend of mine who has a PhD from Oxford (no, not Brookes FFS) and works on some kind of particle accelerator I don't really understand. Also he's a national champion cyclist. Now whilst its not the same field, you'd expect him to have a fairly good grasp of the physics of this stuff and it turns out - wait for it - THAT I'M RIGHT!

Now, obv credentials don't make someone right - only being right does that - but it blows that A at A-level out of the water doesn't it?

gezebo

P_Tucker wrote:

Davey C wrote:

Mikey23 wrote:

For the record, I checked my understanding with a friend of mine who has a PhD from Oxford (no, not Brookes FFS) and works on some kind of particle accelerator I don't really understand. Also he's a national champion cyclist. Now whilst its not the same field, you'd expect him to have a fairly good grasp of the physics of this stuff and it turns out - wait for it - THAT I'M RIGHT!

Now, obv credentials don't make someone right - only being right does that - but it blows that A at A-level out of the water doesn't it?

I have a masters in physics and my dad has a PhD. He also knows Bradley Wiggins and he said that Thomas Voeckler said that light wheels and rims do make a difference to climbing hills on a bike.

In a nutshell the difference depends on how good a cyclist you are. The better/fitter a rider is, the bigger a difference to be gained.

Maybe once you've left kindergarten, and allowed out on your own on a bike you can try the difference for yourself?!

P_Tucker

So you have a masters in physics and your argument revolves around what a professional cyclist, who's paid to promote cycling products, tells your dad?

Lets have some physics demonstrating it, or GTFO. I mean, it's not actually a difficult problem, is it? And if I'm wrong, which is perfectly possible, I'd be happy to have improved my understanding.

ShutUpLegs

gezebo wrote:

He also knows Bradley Wiggins and he said that Thomas Voeckler said that light wheels and rims do make a difference to climbing hills on a bike.

gezebo

P_Tucker wrote:

Lets have some physics demonstrating it, or GTFO. I mean, it's not actually a difficult problem, is it? And if I'm wrong, which is perfectly possible, I'd be happy to have improved my understanding.

Lol, It's a cycling forum. Also if you want to get academic then I'd suggest moving away from wikipedia!

Ringo 68

P_Tucker wrote:

Davey C wrote:

Mikey23 wrote:

Only O level physics here and many years ago....

So are we to conclude then that for those of us who Pootle along at 15 mph average on our entry level road bikes there is no point whatsoever in spending our hard earned dosh on an expensive set of wheels?

Correct. Better off losing weight from your belly for improved performance and it costs nothing. If however you want a BLING set of Deeps to impress your mates that's fine too.

Word. However I might suggest that wirral_paul buys a cheaper, heavier set of wheels so he can buy a physics textbook or 12.

For the record, I checked my understanding with a friend of mine who has a PhD from Oxford (no, not Brookes FFS) and works on some kind of particle accelerator I don't really understand. Also he's a national champion cyclist. Now whilst its not the same field, you'd expect him to have a fairly good grasp of the physics of this stuff and it turns out - wait for it - THAT I'M RIGHT!

Now, obv credentials don't make someone right - only being right does that - but it blows that A at A-level out of the water doesn't it?

And my mates uncle, Stephen Hawkins, has just told me that he always puts lighter wheels on his wheelchair if he has to go anywhere uphill.

Beat that.

P_Tucker

gezebo wrote:

P_Tucker wrote:

Lets have some physics demonstrating it, or GTFO. I mean, it's not actually a difficult problem, is it? And if I'm wrong, which is perfectly possible, I'd be happy to have improved my understanding.

Lol, It's a cycling forum. Also if you want to get academic then I'd suggest moving away from wikipedia!

gezebo

Great, as I'm not doing science is this an open invite for everyone to S#@g your girlfriend??? Nice!

P_Tucker

Good luck - she hates people who refuse to do science

gezebo

Great, guess she's off with your mate from Oxford then!

Right I'm off too.

thescouselander

Seriously, I am interested in what's going on here. The article linked earlier says:

Climbing wheels are built extra light because when the grade becomes steep, the bicycle slows and then accelerates slightly between each pedal stroke. The extra energy that this pulsing motion eats up can be considerable, so reducing the mass of the wheels—especially the rims and tires—can save a lot more energy than removing considerably more mass from

Now, I say this is not possible and the idea is in contravention of the law of conservation of energy. Clearly others here disagree so what is the supposed physics behind this? What are the variables and equations that describe how this energy is "eaten up" and where has the energy gone?

P_Tucker

No-one on the pro light wheels side has actually thought this through FFS. If they had, they'd have come to our way of thinking.

thescouselander

Yes, its strange how no-one is able to explain how this mysterious mechanism works.

You could compare the "pulsing motion" to that of a pendulum. The weight at the end of the pendulum is constantly accelerating from rest at the end of its swing to v max in the middle back to rest at the end and then back again. All because, as in the case of the bike wheel, kinetic energy is being traded for potential energy. And, if you were to conduct this experiment in a vacuum - the total energy "lost" in this constant acceleration = zip, zero, nothing, zilch, nada etc etc.

P_Tucker

Whoa, slow down there. You're applying rational thinking to a bicycle upgrade question. Everyone knows that the only difference between us and Contador is that he has lighter wheels.

dubcat

Last time I rode up a steep hill I noticed my bike decelerate slightly after every peddle stroke. In order to maintain speed I had to accelerate with each peddle stroke. This is called 'the law of speeding up a bit when you slow down a bit to maintain constant speed sort of'.This acceleration would have been easier with lighter wheels.

P_tucker I disagree with you about contador. I think in addition to his lighter wheels his ability to get a super close shave on legs is also a major contributing factor. In addition to this all his gear is super nicely coordinated from a colour scheme perspective.

P_Tucker

Dubcat wrote:

Last time I rode up a steep hill I noticed my bike decelerate slightly after every peddle stroke. In order to maintain speed I had to accelerate with each peddle stroke. This is called 'the law of speeding up a bit when you slow down a bit to maintain constant speed sort of'.This acceleration would have been easier with lighter wheels.

With lighter wheels you'll decelerate more than you would with heavier wheels. Thus although heavier wheels are harder to accelerate, you won't need to accelerate them as much. In fact they'll exactly cancel each other out (air resistance and weight of the system as a whole excluded). This is called the "conservation of momentum" and, unlike yours, is an actual law of physics in the universe in which we currently reside.

slowbike

Where does all the energy go?

A bike isn't perfect - no matter how much you spend on it ..

Heat & Sound is generated as you pedal - both take energy to create - this is where some of it is going ...

If anyone doesn't want their lighter wheels anymore I'll be happy to swap them with mine ...

thescouselander

Slowbike wrote:

Where does all the energy go?

A bike isn't perfect - no matter how much you spend on it ..

Heat & Sound is generated as you pedal - both take energy to create - this is where some of it is going ...

If anyone doesn't want their lighter wheels anymore I'll be happy to swap them with mine ...

Yes, all valid factors contributing to rolling resistance and aerodynamic drag but nothing to do with the mass of your wheels or the smoothness of pedalling up hill.

slowbike

thescouselander wrote:

nothing to do with the mass of your wheels or the smoothness of pedalling up hill.

I wasn't going to enter that debate!

P_Tucker

Slowbike wrote:

Where does all the energy go?

A bike isn't perfect - no matter how much you spend on it ..

Heat & Sound is generated as you pedal - both take energy to create - this is where some of it is going ...

If anyone doesn't want their lighter wheels anymore I'll be happy to swap them with mine ...

Well no f**king sh!t. The question was "where does the extra energy that heavy rims supposedly require go" - the implication being over and above light wheels. FFS

lochindaal

I can't remember what I got in my Physics exam but here goes

Lets say you’re travelling along a nice 5% section at 15kmh and happen upon a 10% section. Now, because the bike weight is the same, it takes no more energy to climb said wall due to total weight being equal – however the heavier rims have more energy stored so – wait for it - the heavier rims will release this energy as you decelerate and will actually get you to the top of the 10% bit ever so slightly sooner. I KNOW, RIGHT? However, once you get to the top of the 10% section it’ll take more energy to spin your rims back up to 15kmh. These effects will largely cancel each other out.

I agree with the constant weight theory in regards to energy required but if we are overcoming gravity on a slope it is not constant it is based upon a force calculated using the angle of the slope. If the angle gets steeper then more energy will be required to get up the 10% incline than the 5% incline if you want to maintain a constant speed. If you maintain a constant energy output you will go slower but could climb the steeper hill for the same effort.

Any energy being stored as you climb is potential energy. This will continue to increase as you continual to climb. This PE is what then allows you to freewheel back down a hill. As you move from the 5% hill to the 10% hill you aren't releasing any PE from your heavy rims when you decelerate it is just the increased force of gravity acting upon you. Your energy level has remained constant.

I'm not sure with your statement that the heavier rims will release energy helping you get there quicker?

For each wheel you would again need to consider the angular velocity and forces acting upon the wheel and what is required to rotate them. If you imagine a single point on the top of the wheel it will require more energy to move the heavier section than the light one. This is of course something constantly ongoing through out your pedal stroke. Once you have the section moving you will get an inertia effect with the heavier section giving you more but as the slope angle increases this will be balanced out against overcoming gravity getting up the slope causing a deceleration faster. Hence on the flat a heavy rim will maintain high speed easier and lighter wheels will go up a slope easier.

Discuss

slowbike

P_Tucker wrote:

Slowbike wrote:

Where does all the energy go?

A bike isn't perfect - no matter how much you spend on it ..

Heat & Sound is generated as you pedal - both take energy to create - this is where some of it is going ...

If anyone doesn't want their lighter wheels anymore I'll be happy to swap them with mine ...

Well no f**king sh!t. The question was "where does the extra energy that heavy rims supposedly require go" - the implication being over and above light wheels. FFS

Do you thrash out at everyone?

Perhaps you need some anger management classes ...

neeb

One factor that might explain any discrepancies between the physics and subjective experience is that the physics is excluding the biological components in the system, or rather simplifying them by assuming that they behave consistently in different circumstances.

If the angular momentum of wheels of different weights means that their acceleration/deceleration profiles are different over the course of the climb or the course of a crank rotation, this could interact differently with the muscular/physiological system. You might need to put the same amount of energy in to get the bike up the hill, but if the energy input is distributed differently this could interact with muscle fatigue in different and potentially extremely complicated ways.

thescouselander

neeb wrote:

One factor that might explain any discrepancies between the physics and subjective experience is that the physics is excluding the biological components in the system, or rather simplifying them by assuming that they behave consistently in different circumstances.

If the angular momentum of wheels of different weights means that their acceleration/deceleration profiles are different over the course of the climb or the course of a crank rotation, this could interact differently with the muscular/physiological system. You might need to put the same amount of energy in to get the bike up the hill, but if the energy input is distributed differently this could interact with muscle fatigue in different and potentially extremely complicated ways.

At last - a half sensible answer! There could be something in that I suppose although the difference in acceleration profiles would be minute so I'm not sure how much difference this would actually make. I have no idea about biomechanics though so I might have to give up there.

P_Tucker

neeb wrote:

One factor that might explain any discrepancies between the physics and subjective experience is that the physics is excluding the biological components in the system, or rather simplifying them by assuming that they behave consistently in different circumstances.

If the angular momentum of wheels of different weights means that their acceleration/deceleration profiles are different over the course of the climb or the course of a crank rotation, this could interact differently with the muscular/physiological system. You might need to put the same amount of energy in to get the bike up the hill, but if the energy input is distributed differently this could interact with muscle fatigue in different and potentially extremely complicated ways.

Could do. Any evidence? And do you think it would have more or less impact than the weight of the valve on yer inner tubes?

I'd suggest a far more likely explanation is that people are gullible idiots and after they've been bummed by a wheel manufacturer's marketing department to the tune of four figures they engage in purchase justification.

neeb

P_Tucker wrote:

Could do. Any evidence? And do you think it would have more or less impact than the weight of the valve on yer inner tubes?

I'd suggest a far more likely explanation is that people are gullible idiots and after they've been bummed by a wheel manufacturer's marketing department to the tune of four figures they engage in purchase justification.

No solid evidence, but given the complexity of bio-mechanical systems it would almost be surprising if there wasn't some difference. I've often thought something similar about that other great marketing favourite, frame stiffness. Irrespective of how much energy is lost or not lost through a flexy frame, different frames feel different to ride and must interact differently with the biological components in the system. How much differently is anyone's guess, there are far too many variables to meaningfully speculate.

We often assume that our off-the-wall speculations on these sorts of things must have been tested ad infinitum by people who really know what they are doing, but there doesn't seem to have been much real science done on bicycles and especially the overall bio-mechanics of the human-bike cyborg, which is what a cyclist is.