Reducing front wheel weight

[400bhp]

I'm hoping @Yellow Saddle returns to the debate. I understand his frustration with us but with patience I think his knowledge gently explained and respectfully debated is valuable.

From my own POV, as a scientist (but not physics or engineering) I get much of what he says. Just have some difficulty squaring it with my own 'experience'.

So let's try and break the problem into smaller pieces and see where we agree or disagree.

For example, turn a bicycle upside down and crank the rear wheel. A heavier wheel WILL take more effort to spin up than a lighter one. F=MA and all that.

It also makes 'sense' to me that for a given weight of wheel having the same mass distributed around the rim as opposed to at the hub makes the wheel also harder to spin BUT doesn't that contradict the simple F=MA equation? Or is my 'sense' plain wrong.
Inertia plays a part here I'm certain! Flywheels and all that.

The Brompton Question:
Again it's widely written that Brommies accelerate so quickly (and my experience says that's true) because their little wheels 'spin up so quickly'. Brommies are not light bikes and neither are their wheels.
So again, I'm conflicted. If our upturned bicycle has a small diameter 1.5 kg wheel or a large diameter wheel of the same weight, which will spin up quickest. F=MA states that they'll be the same.

The thing with riding a bicycle however is that we don't spin wheels in free space. We spin them in contact with the road and in my case with 100kg connected to them.
So to take @Yellow Saddle point regardless of the wheel weight, diameter and mass distribution, as long as the total package weight (wheel plus bike plus rider) remains the same ( add the weight loss in the wheel to the bike) then the amount of force required to move or accelerate it remains the same.
Thus with Yellow Saddles analysis the only benefit to a wheel that is 0.5 kg lighter is that you have removed half a kg or 0.5% of the total weight from the system to be accelerated. Like removing a full water bottle.
I can understand that too.

BUT: It just doesn't seem to chime with people's experience and reportage .... But myth busting is line that and perception not scientific. Alternatively the model might be too simple?

So I'm stuck in my mind why 2 world record attempts built special heavy wheels when a heavier bike would have done .... And why Brommies are the fastest accelerating bikes on the planet :-)

You've kind of answered your own question with the world record attempts.

Overall, a lighter wheel will not make a "material" difference for the reasons you cite.

However, "material" difference to the average Jo is completely different to what "marterial" means to (in your example) a world record attempt, where an infinitesimally small difference may mean the difference between winning and losing. With my maths head on, think of the "materiality" here for the world record attempt as being in continuous space, whereas whether the attempt is succesful or not is discrete (win or lose).

What everyone has missed here is the difference to how the bike handles. Having such a reduction to the front wheel weight and doing nothing to the rear wheel is going to shift the centre of mass backwards and should make the front end very twitchy.

 

[Profpointy]

You've kind of answered your own question with the world record attempts.

Overall, a lighter wheel will not make a "material" difference for the reasons you cite.

However, "material" difference to the average Jo is completely different to what "marterial" means to (in your example) a world record attempt, where an infinitesimally small difference may mean the difference between winning and losing. With my maths head on, think of the "materiality" here for the world record attempt as being in continuous space, whereas whether the attempt is succesful or not is discrete (win or lose).

What everyone has missed here is the difference to how the bike handles. Having such a reduction to the front wheel weight and doing nothing to the rear wheel is going to shift the centre of mass backwards and should make the front end very twitchy.

I suspect the weight shift would be comparable with moving your arse back about 2mm on the saddle

 

[Profpointy]

I'm hoping @Yellow Saddle returns to the debate. I understand his frustration with us but with patience I think his knowledge gently explained and respectfully debated is valuable.

From my own POV, as a scientist (but not physics or engineering) I get much of what he says. Just have some difficulty squaring it with my own 'experience'.

So let's try and break the problem into smaller pieces and see where we agree or disagree.

For example, turn a bicycle upside down and crank the rear wheel. A heavier wheel WILL take more effort to spin up than a lighter one. F=MA and all that.

It also makes 'sense' to me that for a given weight of wheel having the same mass distributed around the rim as opposed to at the hub makes the wheel also harder to spin BUT doesn't that contradict the simple F=MA equation? Or is my 'sense' plain wrong.
Inertia plays a part here I'm certain! Flywheels and all that.

The Brompton Question:
Again it's widely written that Brommies accelerate so quickly (and my experience says that's true) because their little wheels 'spin up so quickly'. Brommies are not light bikes and neither are their wheels.
So again, I'm conflicted. If our upturned bicycle has a small diameter 1.5 kg wheel or a large diameter wheel of the same weight, which will spin up quickest. F=MA states that they'll be the same.

The thing with riding a bicycle however is that we don't spin wheels in free space. We spin them in contact with the road and in my case with 100kg connected to them.
So to take @Yellow Saddle point regardless of the wheel weight, diameter and mass distribution, as long as the total package weight (wheel plus bike plus rider) remains the same ( add the weight loss in the wheel to the bike) then the amount of force required to move or accelerate it remains the same.
Thus with Yellow Saddles analysis the only benefit to a wheel that is 0.5 kg lighter is that you have removed half a kg or 0.5% of the total weight from the system to be accelerated. Like removing a full water bottle.
I can understand that too.

BUT: It just doesn't seem to chime with people's experience and reportage .... But myth busting is line that and perception not scientific. Alternatively the model might be too simple?

So I'm stuck in my mind why 2 world record attempts built special heavy wheels when a heavier bike would have done .... And why Brommies are the fastest accelerating bikes on the planet :-)

Your experience and upside down bike experiment is right. Your F=ma thing whilst per se true is half the story. The energy needed to accelarate a wheel can be consired in two chunks : 1- the energy needed to accelerate the weight (stricly the mass) of the wheel to a given velocity - this is same as that needed to accelerate same mass of frame to that velocity.2 - In addition there's the energy needed to spin the wheel. Obviously this is on top - a (so to speak) stationary spinning wheel (eg a gyroscope) contains a fair bit of energy despite not moving along at all. How much energy to spin it up depends both on the speed of spin and the distribution of mass in the wheel. This is something termed "moment of inertia" - which I'd need a pencil and paper and a bit of rusty calculus to work out but a rim heavy wheel has higher moment of inertia and thus contains more energy when spinning than a hub-heavy wheel of same mass - the latter can approximate a point mass if rim if very light and hub very heavy.

Small wheels contain less energy for a given rpm but I'd have to think a bit to be certain they contain less energy for a given forward speed for the same mass - but they are also lighter of course. I strongly suspect they would get up to speed quicker but I'd need to do some (hazily remembered) sums to be sure.

 

[Fab Foodie]

It would help me understand @Yellow Saddles way of thinking. I know a coupe of guys who talk bikes all the time but they ride 10 miles a week on a £200 mtb.

Well from what I've read of his other posts, he seems very knowledgeable and experienced. But, argue the points made, not assumptions about the poster.

 

[Yellow Saddle]

For example, turn a bicycle upside down and crank the rear wheel. A heavier wheel WILL take more effort to spin up than a lighter one. F=MA and all that.

Yes, a heavier flywheel does take more effort to spin up than a lighter one. It is laid out nicely in these posts. The information is perhaps a bit dense, but it is kinda important to understand. It is nicely peer-reviewed, if we can use that lofty term here.
I am not disputing that fact, I've published data on that and quantified it here:

https://www.cyclechat.net/threads/road-racing-aero-wheels.173743/post-3523626
https://www.cyclechat.net/threads/29ers.180730/page-2

The summary is that a heavier wheel only requires a decimal point of a percentage of extra energy to spin up to a given speed. The figure in the one example is 0.4% but as you'll see from my answer to your next question, it is a worst-case scenario. I think it is probably about half of that for two real wheels.

The key issue to consider is that the ratio of the "burden" of accelerating a light-weight wheel (even if it is rotating) to the burden of accelerating a large mass in a linear fashion is very small. We are accelerating two objects. The one has to spin up and move forward (hence twice the energy required to move a object of equal mass forward only) and the other has to move forward. But one is very small and the other very large. Hence the logic of focusing on cheap large savings rather on expensive small ones.

An analogy would be an NHS that invests billions in a very rare disease that kills 1 person per decade but only one thousand pounds on cancer. It is a silly analogy but I want to drive the point home. Newbies (and veterans I see) usually claim or believe that wheels make a big difference. They don't.

It also makes 'sense' to me that for a given weight of wheel having the same mass distributed around the rim as opposed to at the hub makes the wheel also harder to spin BUT doesn't that contradict the simple F=MA equation? Or is my 'sense' plain wrong.
Inertia plays a part here I'm certain! Flywheels and all that.

A uniform flywheel like a millstone or cast iron car flywheel is a relatively simple structure to theorise with because it is uniform. A bicycle wheel "that's X grams lighter than another bicycle wheel" is a different story. It is absolutely crucial we know where the mass is located. In the examples above I've tried to overlook that for simplicity sake and stated that all the wheel's mass is at the circumference. This is thus a worse-case scenario. The fact that the mass is more evenly spread makes the difference even smaller between light and heavy wheel.

But again, see all of this in the context above -- one of proportions.

It is nicely laid out in a logical argument in the first link above.

The Brompton Question:
Again it's widely written that Brommies accelerate so quickly (and my experience says that's true) because their little wheels 'spin up so quickly'. Brommies are not light bikes and neither are their wheels.
So again, I'm conflicted. If our upturned bicycle has a small diameter 1.5 kg wheel or a large diameter wheel of the same weight, which will spin up quickest. F=MA states that they'll be the same.

I come from a place where we didn't have Brommies so I have never heard this claim, hence me questioning your statement of hearsay. But I think it is a valid one to address. My answer is basically the same as for the answer in Link 1 above: A lighter wheels spins up to a given speed with less energy than a heavier one. But the competition is not between spinning up two wheels. It is between a Brommie rider and a Cervelo rider taking a 1/4 mile dice. Two 80kg lumps of metal and flesh. Not two wheels rolling down the road.
If the two lumps weigh the same and the one has smaller wheels, the smaller wheel one will win if all things are equal. However, this notion of spinning up a wheel is false. People imagine a car engine revving up quickly or as you said, inverting a bicycle and spinning up the wheel using just one hand. In an instant you can accelerate the wheel to 60kph on your bike computer. Almost effortlessly. However, spinning up the wheel when it accelerates the lump is a different story. Here you require two strong legs and just reaching 30kph will take you a minute. Getting to 60kph is not within reach for most of us. Again, within that context, the light wheels help zero (my terms for little enough to not matter).

The thing with riding a bicycle however is that we don't spin wheels in free space. We spin them in contact with the road and in my case with 100kg connected to them.

You've said it. We think alike.

So to take @Yellow Saddle point regardless of the wheel weight, diameter and mass distribution, as long as the total package weight (wheel plus bike plus rider) remains the same ( add the weight loss in the wheel to the bike) then the amount of force required to move or accelerate it remains the same.
Thus with Yellow Saddles analysis the only benefit to a wheel that is 0.5 kg lighter is that you have removed half a kg or 0.5% of the total weight from the system to be accelerated. Like removing a full water bottle.
I can understand that too.

BUT: It just doesn't seem to chime with people's experience and reportage .... But myth busting is line that and perception not scientific. Alternatively the model might be too simple?

People are very poor judges of time and speed and acceleration and when they've spent money, they will always err on the positive side. Test this for yourself. If you give someone ten time tests, you'll see how silly it is. Here's the test. Take a stopwatch and base your scattergram on say 10 seconds. Ask them to say whether time one or time two is the shortest. You've tick off 10.5 seconds (time 1) on your stopwatch and 11 seconds (time 2). Do this ten times with slightly varying times in the pairs to average out chance and you'll see they don't "time" very well in their minds. They will fail the test.

Devising an acceleration test is perhaps a bit more difficult but they will fail such a test spectacularly. Seeing that the difference in acceleration is in the order of half a percent, I can't see how anyone can "feel" it. It is of course my word against theirs.

So I'm stuck in my mind why 2 world record attempts built special heavy wheels when a heavier bike would have done .... And why Brommies are the fastest accelerating bikes on the planet :-)

I'm not aware of the details or facts of these record attempts. It is the first time I've encountered this claim and would like to read up more about it. Who were the individuals? However, considering that they ride fixed-gear bikes in these attempts, there are benefits to be had from a heavy flywheel. If you've ever pedaled a cheap exercise bike with a light flywheel and an expensive one with a heavy flywheel you will know that the latter evens out your stroke and make things smoother. A heavier wheel vs a heavier frame will require less mass to get the same effect. It's all I can think of off the cuff.

 

[derrick]

Well from what I've read of his other posts, he seems very knowledgeable and experienced. But, argue the points made, not assumptions about the poster.

It'a a question not an assumption.

 

[fatblokish]

Whilst I approve of the pedantry, your conclusion doesn't sound right. I don't think the kinetic energy changes after these changes of direction - just like the kinetic energy of a spinning wheel doesn't change despite each component of the rim continuously changing velocity as it goes round in a circle. No work is done by the (centripetal) force which keeps the rim going round in a circle - work is force x distance and the spokes ar fixed length. The wobbling about changes in direction you mention are akin to this.

I dare say we're losing the non-pedants here !

To further expand on my pedaling pedantry, by way of explanation, if the front wheel changes direction then its velocity must also have changed, which in turn requires the application of a force. Given that the only force propelling the bike on the flat is the rider then I'm guessing that this force, or at least the corrective element of it, must come via the pedals, robbing the rider of a tiny element of forward velocity. Or maybe that's claptrap.

By way of an analogy, a rider going around a velodrome must apply extra force to maintain velocity when going round the bends as opposed to the straights since round the bends the velocity is constantly changing. Or may that's gibberish too.

 

[Yellow Saddle]

To further expand on my pedaling pedantry, by way of explanation, if the front wheel changes direction then its velocity must also have changed, which in turn requires the application of a force. Given that the only force propelling the bike on the flat is the rider then I'm guessing that this force, or at least the corrective element of it, must come via the pedals, robbing the rider of a tiny element of forward velocity. Or maybe that's claptrap.

By way of an analogy, a rider going around a velodrome must apply extra force to maintain velocity when going round the bends as opposed to the straights since round the bends the velocity is constantly changing. Or may that's gibberish too.

That force that changes the directional vector of the velocity comes from the body, not the pedals. It comes form the rider throwing her weight left and right. In other words, it doesn't rob pedal input and that force for a larger or smaller wheel are equal, bar a very small portion of it that is used in steering corrective torque which will differ slightly depending wheel size.

 

[bonsaibilly]

It would help me understand @Yellow Saddles way of thinking. I know a coupe of guys who talk. bikes all the time but they ride 10 miles a week on a £200 mtb.

I'm not sure but I think he does have and ride bikes a bike; wrote a book on bikes; and used to own his own bike shop. More than that I couldn't say. BB

 

[Spoked Wheels]

Sorry, and I really mean that, don't let one individual ruin your usage of this forum, please.
Like @derrick says let it go over your head.
Think of poor yellow, it can't be nice to be so unloved, got to mess with your head that has.

Yellow saddle reminds me of my uncle, my dad and his 3 years older cousin studied law and while my dad become a barrister his cousin ended up as a law professor at university. They used to have huge disagreements over Sunday post lunch debates my uncle knew the law well but he could not apply it well in court, in real life.

While I agree with him in some areas I have very different views on others. For instance, he would still put a 60 kg rider on 36 spokes wheels. That is Jobst Brandt's thinking to the T, although a master, at the time, on bicycle wheels behaviour, but that was a long time ago and some views have changed considerably.

Theory or models based on physics are great to understand concepts but those models not always represent real life experiences. I can never rider a bike at a constant speed, constant power output or a constant cadence, the wind is never constant. Where do I find a hill with a constant gradient? People tend to ride their bikes very differently, with many accelerations, stops, slow downs, etc. even when I do hills my speed changes as I get tire or I ride off the saddle.
I do understand that people some times have excessive expectations from saving a couple of hundreds grams for example, but there is no need to slam the physics book on their heads. A small improvement is better than nothing.... where did I hear about marginal gains? . Besides, some people just want to have new wheels full stop.

There are still many areas, in wheel building, where his views appear antiquated to me but my biggest doubts are whether he has actually built wheels as he says or he is just simply quoting from The Art of Wheelbuilding

justinslow, you are right, I can't let trolls upset me

 

[PhilDawson8270]

The issue I have with upgrades and light weight wheels. Is that I honestly do not see the point. I don't understand the logic in spending hundreds or even thousands on wheels, to save less weight than cutting out beer and pies for 2 weeks for free.

This of course doesn't mean people shouldn't buy them, I buy plenty of stuff I really don't need. Their money, their enjoyment, but I will still voice my opinion that lightweight components isn't the most cost effective performance increase. The gains are small compared to the money, and the gains that can be made by improving yourself.

Of course, cycling is for enjoyment. If you will enjoy it more by having new wheels, then buy new wheels.

 

[winjim]

Do you people seriously not know how to use Google?

 

[PhilDawson8270]

Do you people seriously not know how to use Google?

I use http://www.lmgtfy.com when people ask me for stuff that can be googled. Though it appears sending to to customers (accidentally) isn't appropriate

 

[Yellow Saddle]

Yellow saddle reminds me of my uncle, my dad and his 3 years older cousin studied law and while my dad become a barrister his cousin ended up as a law professor at university. They used to have huge disagreements over Sunday post lunch debates my uncle knew the law well but he could not apply it well in court, in real life.

While I agree with him in some areas I have very different views on others. For instance, he would still put a 60 kg rider on 36 spokes wheels. That is Jobst Brandt's thinking to the T, although a master, at the time, on bicycle wheels behaviour, but that was a long time ago and some views have changed considerably.

Theory or models based on physics are great to understand concepts but those models not always represent real life experiences. I can never rider a bike at a constant speed, constant power output or a constant cadence, the wind is never constant. Where do I find a hill with a constant gradient? People tend to ride their bikes very differently, with many accelerations, stops, slow downs, etc. even when I do hills my speed changes as I get tire or I ride off the saddle.
I do understand that people some times have excessive expectations from saving a couple of hundreds grams for example, but there is no need to slam the physics book on their heads. A small improvement is better than nothing.... where did I hear about marginal gains? . Besides, some people just want to have new wheels full stop.

There are still many areas, in wheel building, where his views appear antiquated to me but my biggest doubts are whether he has actually built wheels as he says or he is just simply quoting from The Art of Wheelbuilding

justinslow, you are right, I can't let trolls upset me

So, am I on your mind all the time or just sometimes? It is kinda creepy, innit? There's a name for that type of behaviour.

 

[nickyboy]

I'm almost sorry I've been out riding my bike and missed all the fun on here