[Closed] 'Turning weight'

Interesting.

Figure I got quoted last week in a Not-So-LBS was that rotating weight was worth 4 times non rotating weight, ie 1g saved on a wheel was worth 4g on the frame.

Wasn't sure if it was bollocks or not - he was trying to sell me a wheel upgrade at the time.

Now feel happier that I fell for it...

This is only for accelerating. Doesn't really apply to touring bikes where you sit for long periods of time at the same speed.

Basically like flywheels, doesn't affect how quick an engine can spin or the power it puts out, but a heavier flywheel takes longer and more energy to spin up.

Similarly a heavier wheel will want to keep rolling more over bumps than a lighter one.

Rotating weight/rotating mass. You'll see it discussed in various threads about wheel size and whether or not disc brakes are a good idea because you can have lighter rims.

Just how much it matters will depend on the fitness/ability of the rider and the types of trails being ridden.

Also see: unsprung weight.

Thanks; its all bringing back hazy memories of physics / mechanics lessons.

And should of course be in the knowledge armoury of anyone involved in using machinery.

So as with most of these things...
How it actually affects overall performance?

And action to take / purchases to make?

" it depends "

Certainly depends upon who, is trying to sell you what ...

This thread might be worth a read. TiRed seemed to nail it.

http://singletrackworld.com/forum/topic/wheel-weight-physics

He didn't. If a wheel requires more energy to accelerate then it stores more energy and resists deceleration better. Light wheels feel faster but don't actually go faster in situations where near constant velocity is required.

ie 1g saved on a wheel was worth 4g on the frame.

Its a question of what you're applying the 'worth' to. Outside of sprint disciplines on the track out and out sprint performance isn't 'worth' much in competitive or efficiency terms because its such a small aspect of your journey. But in terms of the sense of money well spent - which might be what an [i]LBS[/i] is referring to - you'll probably [i]feel [/i]the money you've spent more on wheels more than anywhere else the bike, not least because you'll feel the difference on the first few pedal strokes after you've fitted them, even if they make practically no measurable difference for the rest of your ride.

I thought it was only twice, but either way, my fat bike wheels weigh a ton, yet I've set personal records on that bike all over the place. Go figure.

Pretty sure it is only a factor of two. There is a wikipedia article on moments of inertia somewhere which has the relevant maths.

Sorry, but the OP is bullshit, CGG has it and it's been well proven.

Al and CGG +1

Outside of sprint disciplines on the track out and out sprint performance isn't 'worth' much in competitive or efficiency terms because its such a small aspect of your journey.

You mean those disciplines where they use deep section aero wheels which may loads more than equivalent shallow ones?

Im with cgg too, however I would add that as the energy available to accelerate (from the legs) is finite, then surely, all other things being equal, a lighter wheelset should accelerate more quickly. And brake more easily.
And lose speed more easily when the rider backs off the power.
But as noted already, its a small percentage whichever way you look at it, also the heavier the rider/bike combo the smaller that percentage would be.

@ cynic-al

Excellent news - thanks 😀

You just saved me a wad of cash.

Nigh on 37 years ago when I time-trialed some of the disc wheels had cavities to add weights near to the rim. On a flat fast course once you were up to speed they felt brilliant, and set fast times.
Needless to say for hilly races you took out the weights as the speed was much more varied.

The matter becomes clouded when you realise that very few amateur cyclists seem to be able to manage a constant speed and are constantly accelerating and decelerating subconsciously

You really notice wheel weight on tt bikes. I went from super light wheels to 100mms and the flywheel effect was ace until you hit a prolonged hill Like wise I borrowed a disk on the track once and compared to the relatively light alloy spoked wheels I had it just wanted to go faster and faster was ace.

Well that settles it then.

I am definitely an 'amateur cyclist' , in the truest sense of the word.

i love cycling, just for its own sake, and no one is ever likely to pay me to go for a bike ride ... 🙂

In what world do people ride where they are not decelerating or accelerating frequently?
What about junctions, round abouts, traffic lights, traffic, headwind (when the wind and road change direction), road surface change, gradient change etc?
True a couple of hundred grammes here or there aren't going to make much difference but when I went from a cheap heavy wheelset, steel band tyres and tubes to a light tubeless setup on my road bike my average speed went up a good bit plus they felt great too.
I know it's not all about the weight but I'd rather ride around where I live on my new wheels and tyres than my old setup and I think that's what's important.

There is a lot to be said for not having your wheels decelerate rapidly when the terrain changes.

chiefgrooveguru - Member

Light wheels feel faster but don't actually go faster in situations where near constant velocity is required.

Which really only happens when grinding along a consistent gradient road, everything else we do is frequent changes of speed. (and when speeds drop and deceleration is stronger- riding up a steep hill, or into a headwind, or similiar- you can reach a point where you're essentially accelerating with every pedal strong to counter the deceleration.

Course, for mountain biking you're doing a lot of different changes of velocity, it's not just pedalling.

[quote=Northwind ]and when speeds drop and deceleration is stronger- riding up a steep hill, or into a headwind, or similiar- you can reach a point where you're essentially accelerating with every pedal strong to counter the deceleration.

In which case heavier wheels would be beneficial as your speed would vary less. In reality it makes very little difference - 200g difference at the rim makes 0.5% difference to the rate of acceleration of a typical 80kg rider/bike combo, which is well below the threshold of human perception. Those who claim they can feel the difference light wheels make to acceleration are actually feeling something else.

I also ride in the real world, where like most cyclists I don't actually accelerate and decelerate much in real terms. Not that much time spent at junctions, traffic lights or roundabouts. Maybe if I was riding in central London where you get a traffic light every minute, but that's not where I ride. Of course as mentioned above acceleration due to road surface and gradient changes don't provide an advantage for lighter wheels.

aracer - Member

I also ride in the real world, where like most cyclists I don't actually accelerate and decelerate much in real terms. Not that much time spent at junctions, traffic lights or roundabouts

This is a mountain biking forum.

Strange, it calls it "bike forum" at the top. Though plenty of discussion about road riding on this thread and one a couple above yours talking about junctions and traffic lights. If anything differences due to acceleration are likely to be more lost in the noise when mountain biking anyway.

I'll accept arguments on how lighter wheels make a difference to handling and unsprung weight, but that doesn't appear to be what this discussion is about.

riding up a steep hill, or into a headwind, or similiar- you can reach a point where you're essentially accelerating with every pedal strong to counter the deceleration.

But heavier wheels mean you're decelerating less too, so it makes no difference overall. You're not losing any more energy to drag through having heavier wheels, or having to shift more weight up a hill than an equivalent amount of weight on the frame.

Speaking from experience swapping to lighter wheels on my road bike certainly made the bike accelerate quicker.

As for carrying speed? I really couldn't say..

[quote=mrlebowski ]Speaking from experience swapping to lighter wheels on my road bike certainly made the bike accelerate quicker.

What was your testing protocol?

What was your testing protocol?

My testing protocol would be how it "feels". But I think your arguing black is white if you don't think lighter wheels don't help with faster acceleration.

Of course they do, just not enough for it to be detectable by a normal human, or enough for it to make any real difference to performance if you're competing in an event where the winning margin is likely to be more than hundredths of a second.

How it "feels" is a pretty rubbish testing protocol.

Feeling is of utmost importance if you are doing it for fun, of course it's also utterly subjective

Edit. Lighter wheels also make a massive diffference in some race scenarios. It's not just the speed of acceleration, it's the reduction in fatigue over multiple accelerations. In something like cx where you are going from slow coast round a hairpin to full sprint a dozen times a lap that is noticable

Most road biking is aiming for near constant velocity - minimal changes of speed and direction. The main benefit of low wheel weight is that it reduces the weight you have to haul uphill. The fact that velocity isn't constant is irrelevant, as a wheel with greater flywheel effect will reduce your deceleration between moments of peak torque when pedalling up climbs.

For mountain biking low wheel mass is clearly a good thing due to frequent braking and turning - but a bad thing if it compromises strength or stiffness.

What was your testing protocol?

Feel.

Feel isn't rubbish.

E.g Ned Overend trains on how he feels & he knows a thing or two about riding a bike.

Hydration? Established protocol used to be 1l/hr, now medical advice is drink when you FEEL thirsty.

Going on feel isn't rubbish.

when I went from a cheap heavy wheelset, steel band tyres and tubes to a light tubeless setup on my road bike my average speed went up a good bit

I'll wager that had a lot more to do with the change in tyres than the weight of the wheels...

2:1 is the ratio usually quoted, but that's for road bikes really - with suspension MTBs you've also got the factor of unsprung mass, so I'd guess a slightly higher ratio would make sense, depending on the terrain.

Over my ~4.5 mile commute each way, my fat wheels/tyres are ~75 seconds slower on the way in (more downhill) and ~210 seconds slower on the way home (more uphill) compared to my skinny wheels/tyres.

Difference in weight is ~7.2Kg vs ~4.2Kg off the top of my head (4.0" JJ vs 38c Marathon Cross).

However, my (3) fat times were from a month ago when I was less fit and I had only just began to use Strava, I've been on my skinny setup since the start of October. Not to mention, traffic light sets massively influence my time, especially on the way in.

Thereby inadvertently saving 250g x 5 - 1.25 kg of extra weight, not carried up and down Scottish hills. , if the physics is sound.

It's not.

The wheel weight is just weight that you are taking UP the Scottish hills - if you are riding at a constant pace.

Rotating weight has to be accelerated twice as you increase speed, even on the flat - forward momentum and angular momentum. But they are not the same actual value of energy, so it's not simply double the energy you have to accelerate. I forget the formula, but it depends on wheel diameter - the further the mas from the hub, the more angular momentum it has and the more rotational energy, and you have to supply that. So 29er wheels with the same mass as 26er wheels will take more energy to accelerate. But then they won't need rotating as fast so that might cancel out, not sure off the top of my head.

Whether or not this makes you faster or slower is a much more complex issue. On rocky trails I found having much lighter 26" wheels made it far more difficult to climb than with my heavier 29" wheels. And once you've got up to speed the heavier wheels will probably be quicker. This might account for people's good times when riding fatbikes.

On road it's a different story.

The question is how much difference the 3kg weight difference would make were it on your frame. And of course aero and rolling resistance will also make a difference.

2:1 is the ratio usually quoted

Weight placed at the rim will take twice as much energy to accelerate to a given speed than the same weight placed on the frame, so I guess in that sense 2:1 is correct.

Weight placed on the hub is equivalent to weight on the frame.

But even at 2:1, for most riding, the effect on overall speed is negated by the fact that'll you'll decelerate more slowly.

Here's the critical bit: The wheel is geared one to one with the road!

So, doing the maths. Taking Linear accel force (f = ma) and Rotational Accel Torque (T = Ia) and resolve those forces into a total linear force, parallel to the road.

Taking the following assumptions:

Rider Mass: 80 Kg
Bike Mass (inc wheels): 10Kg

Wheel/tyre Mass: 3kg
Wheel/tyre OD: 0.725m (about 28.5", i'd roughly measured my 650B wheels and tyres)
Effective radius of Wheel/tyre rotational mass: 90% of wheel OD

Start speed: 0mph
Finish speed: 20mph
Accel Time: 5sec

(ie, we start accel, and 5 sec later, are doing 20mph, pretty typical of a full bore sprint off the line on a pedal bike i suggest)

And the Answers:

Linear force to accel bike & Rider: 152N
Linear force to accel wheel/tyres: 5.37N
Linear force to rotationally accel wheel/tyres: 4.35N

As you can see, because the absolute rate of accel is pretty slow in the grand scheme of things, the force to rotationally accelerate the wheels is also pretty low, especially compared to that requried to linearly accel the 80kg sack of spuds doing the pedalling!

And you can also see that with our assumption that the wheel rotational mass acts at 90% of the wheel/tyre radius, then because the wheel/tyre is geared 1:1 with the road, the rotational accel loads are 81% of the linear loads (because of the squared term in the moment of inertia calc)

It's also worth noting that wheel diameter cancels! the forces (linear/rotational) are always simply proportional to the effective radius of the mass on the wheel/tyre!

So, a 29er, has a larger total force to accelerate it's wheels, but only because it has a higher percentage of its mass at a greater percentage of it's wheel diameter!

BTW, to make things clearer, if we ignore the rider (which of course is never a sensible thing to do when talking about push bikes!) our 10kg bike, with it's 3kg wheels, if we assume the wheel weight is ALL at the wheel/tyre OD, then the rotational accel forces match the linear force for those wheels, because each wheel has to be accelerated linearly AND rotationally at the same moment in time.

But as said all that applies to a sprint off the line. When winching up a long climb at a steady pace it's all just mass.

Probably of more relevance to us though is the act of a few stiff pedal strokes to clear a technical section on a climb.

pdw - Member

2:1 is the ratio usually quoted
Weight placed at the rim will take twice as much energy to accelerate to a given speed than the same weight placed on the frame, so I guess in that sense 2:1 is correct.

Are you able to back that up with, you know, some, like, science, or something?

It's only 2:1 if the mass is absolutely and 100% placed completely at the tyre OD as my maths earlier showed, and due to the square term in the inertia calc, it drops off drastically as the mass is placed further towards the wheel centre. Ie at 90% of the wheel/tyre OD, it's 1.8:1.

However, in all cases, it's small beer compared to accelerating the riders mass!

Are you able to back that up with, you know, some, like, science, or something?

Yes, although I think maxtorque has done it to death.

KE (linear) = 1/2 m v^2

KE (rotational) = 1/2 I w^2

where I = moment of inertia and w = angular velocity.

Speed of wheel at tyre is v = w * r (r is wheel radius here)

Moment of inertia = m * r^2 (r is distance of mass from axle)

Assuming we're adding mass right where the rubber touches the road, r is the same in both cases and cancels to give:

KE (rotational) = 1/2 m v^2

So for mass on the edge of the wheel the rotational energy is the same as the linear energy, so a ratio of 2:1 compared to something with no rotational energy. This intuitively makes sense: a mass on the rim of the bike is moving at the same speed as the bike as a whole, it's just going round in circles.

At any other point it's scales with the square of the ratio i.e. (r(mass)/r(wheel))^2, so weight at the axle itself has zero rotational energy.

[quote=ferrals ]Edit. Lighter wheels also make a massive diffference in some race scenarios. It's not just the speed of acceleration, it's the reduction in fatigue over multiple accelerations. In something like cx where you are going from slow coast round a hairpin to full sprint a dozen times a lap that is noticable

Massive difference? See the calc I did a bit earlier. 200g less on the rims/tyres makes 0.5% difference, 400g less 1% (I'm assuming a factor of 2, which is good enough for this discussion). So over 8 laps that's like doing one extra hairpin in terms of fatigue even on your rather unusual CX course with 12 hairpins. I very much doubt you could notice the difference.

[quote=mrlebowski ]Feel isn't rubbish.

It is as a means of determining whether they "certainly made the bike accelerate quicker".

Sure training based on feel is a valid thing to do, as is drinking when you feel thirsty, but those involve physiology where feel is important. Trying to determine a 1% or less difference is a totally different matter - here physiology is a problem, because it's quite easy to have 1% difference on any given day without being at all aware of it. The fundamental issue though is that's a limit to how small differences humans can detect - IIRC it requires ~3% to 5% difference in something like this for a normal human to be able to tell the difference.

It's possible you're feeling something other than the difference in acceleration, but most likely your "feel" is just placebo effect.

I need to get me set of them placebo wheels!

What tyres for placebo wheels?

Seriously though, why upgrade the stock wheels on a bike then if it's all in the mind? A new set of decals should in theory be enough.