Rotating weight does it make any difference and why?

As for acceleration, of course there is a difference. The question should just be re-written as "is 100g difference enough to notice?". If you made it 1000g difference it would obviously be noticable

Obviously? As I mentioned above, 1000g difference is ~2.5% (rotational and translational inertia), which I'm not convinced is that noticeable – I don't believe most people could put the same power into an acceleration consistently within 5%.

I rode my bike last night – it was ACE

the fella that claims F=MA thus at constant speed F = 0 is…

drumroll…

incorrect.

The momentum of the wheels vs that of the bike/rider is tiny – heavier wheels (within what is available) will have insignificant effect on keeping up momentum. "Aero drag" at mtb speeds – even racing – WTF?

So you're singing off our hymn sheet now, cynic-al? I agree, the difference will be insignificant, I was just pointing out that for the given scenario more inertia is better.

Yes aero-drag. I do other sports where cruising speed is ~12mph and top speeds on the flat (for me) ~15mph, and in that context aero drag is very significant at those speeds. Something you tend to neglect at those speeds on a bike because you do go faster and then get much more, but I can certainly cruise at 15-20mph on the flat off-road and you're expending a significant proportion of your energy on it then.

on a flat (relatively smooth) surface.. aero drag is BY FAR the dominant force in slowing the bike and rider.

I think that fella was Newton.

Third law.

the fella that claims F=MA thus at constant speed F = 0 is…

drumroll…

incorrect.

No, what I claimed was that at constant speed, acceleration = 0 which is

drumroll….

correct!

My point being that your post implied that because there were decelerating forces then the rider must be "accelerating" to resist them. Semantics, maybe, but the point of this discussion is whether or not light wheels make any difference to acceleration. Aero drag, etc. isn't relevant as it's the same whether you have light or heavy wheels (Ok, give or take 0.00001% for a wider rim/tyre!).

up there ^ somewhere, some numpty claimed

"….. i ride a normal 29r wheel……"

i just couldnt let it lie. 29r is not normal, it is freakish, it is disgusting, it is the root cause of evil in this world. i dont care what the guardian reading labour liberal democrat voting have to say on the subject, you and all your kind should be sterilized and outed from society along with anyone who doesnt believe in lighter tyres.

is this mic on?

Aero drag, etc. isn't relevant as it's the same whether you have light or heavy wheels

Well it is, because it's what you're expending energy on rather than acceleration. That and rolling resistance, which is where the differences in heavier tyres really are.

quite… so as i was saying – and aracer nicely puts it.

whether your net acceleration is 0 or not. you ARE expending energy maintaining your pace. F has not magically reached 0 as a result of your constant speed.

I'm having lunch in the Hilton, and reading all this with a smirk on my face.

You need to define F. you can't just have F as relating to Bike and person and then talk about the acceleration of one part of that system isolated from all other component parts. If you're talking about the whole system F= rate of change of momentum might be an easier way of thinking about it. The fact that lots of acceleration takes place going round corners as A has direction and thus the rotational mass effect has a diminished significance outside of straight line travel.

The fact that lots of acceleration takes place going round corners meand the F=MA argument as A has avector and thus the rotational mass effect has a diminished significance outside of straigh line travel.

Well actually zero significance if you're maintaining speed round the corner. I sometimes think on the "a little knowledge is a dangerous thing" basis that it's a mistake to teach people that cornering is acceleration!

You can do the experiment yourselves:

Mount a wheel in a wheel jig on a bench…

…Repeat for several wheels and see if the the difference in time is signficant.
Which is a good way of determining whether the difference in inertia of wheels is significant when they're mounted in a wheel jig on a bench, not such a good way of determining the significance when they're mounted on a bicycle which has a great big mass attached to the top which also has to be accelerated. The experiment for which can easily be done by using exactly the same experimental setup apart from having the wheel in a bike and a rider on the bike and then seeing how quickly the whole lot accelerates due to the weight falling!

quite… so as i was saying – and aracer nicely puts it.

whether your net acceleration is 0 or not. you ARE expending energy maintaining your pace. F has not magically reached 0 as a result of your constant speed.

I need a "knocking head against the wall" smily..

Yes, yes, yes, you are correct. You are indeed expending energy maintaining your pace at a constant speed. In what way is this relevant to the question of light vs heavy wheels at non-constant speed?

Exellent plane on a conveyer style troll Jon 😆

is this relevant to the question of light vs heavy wheels at non-constant speed

Friction from the ground and air (and resistance to deviation from the plane of rotation of the wheel)is what slows the bike down yeah? so surely heavier wheels would carry more momentum and thus would decelerate less (as they have greater mass) due to friction?

Whoa there chief – the OP implied he wanted to know if rotating weight was signficant with reference to wheels. The experiment descibed is a standard way of determining rotary inertia without the faf of working out the inertias of all the components parts and then playing with the effect of each analytically on the total inertia of the complete assembly – that is the context in which is should be viewed.

The discussion has moved on somewhat to consider the whole system and the gamut of inputs and variables that that entails.

Hitting squared off rocks and bloody big roots in the trail every 10 seconds and regular braking is what tends to slow my wheels down, and my main concern is then spinning the buggers back up to speed. Also, poindexters, you are reducing the unsprung mass in a suspension system, and no one's factored that in yet.

If they put this in physics lessons…

There is a nice empirical answer to this question, which is to look at the market. There you have millions of bike riders worldwide and an industry which is competitive and which is also active in and informed by sporting competition. It can very reasonably be assumed that mainstream products give you a good workable compromise between performance and reliability – if you want a more sprightly feel then use what the XC racers use, if you want a more solid planted feel and protection for terrain a bit tougher than an XC course then gravitate towards rims and tyres sold for the "trail" market. When you get up t the "all mountain" and freeride end of the scale I think most people will expect to feel a pretty significant decrease in acceleration.

So, basically, we all knew the answer to the OP before we started – none of us expected to put gert big 1000g 2.4s on and the bike to feel or be just as fast, although the increased confidence and surefootedness may well make it faster over the rough downhill bits.

Obviously true but there's the stuf manufacturers try to convince you make a difference and there's the down to the last milisecond attitude of world cup xc. I think the OP was more wanting know what difference it maks in the real world.

There are four reasons a heavy wheel (particularly the rim) is bad. Everybody's discussed three:

Linear acceleration/deceleration – the weight of the wheel contributing to the weight of the bike when you accelerate (flat or climbing) or brake.

Rotational acceleration – the flywheel effect that you can test yourself by pushing a spoke and seeing how much energy it takes to get the wheel rotating at a certain speed

Gyroscopic effect – making the wheel difficult to deflect with the steering.

The one I don't think anyone has mentioned is this:

Unsprung mass. Suspension works best when the ratio of sprung to unsprung mass is higher. A fat bloke on a heavy bike with light wheels will find it easier to set up suspension to track accurately accross the ground and keep the tyre in contact with the trail than a thin bloke will on a light bike with heavy wheels. Unsprung mass on a mountain bike is made up of tyres, rims, spokes, fork lowers, brake rotors and calipers and, on full-sussers, the rear triangle, cassette and rear derailleur.

So, lighten the wheels and gain in four ways.

Whoa there chief – the OP implied he wanted to know if rotating weight was signficant with reference to wheels. The experiment descibed is a standard way of determining rotary inertia without the faf of working out the inertias of all the components parts and then playing with the effect of each analytically on the total inertia of the complete assembly – that is the context in which is should be viewed.

The discussion has moved on somewhat to consider the whole system and the gamut of inputs and variables that that entails.
I'll take a wild guess here, but I think the OP was always more interested in the significance of the rotating mass as part of a complete system than when mounted in a jig on a bench!

I'll take a wild guess here, but I think the OP was always more interested in the significance of the rotating mass as part of a complete system than when mounted in a jig on a bench!

And how is he going to do that in any quantifiable way? How can the layman split out the effect of one part of a complex system? Where's the data to make any kind of judgement on the equipment he has? How's he even going to go about doing that?

Rather than the three pages of gassing about the maths and the interactions that I suspect the OP doesn't really care about he can go out into the shed and say – and I accept that this ignores the rest of the system/the interactions and whole-system variables – "I have two wheels – I've done some tests and the inertia of wheel A is lower than wheel B and it is therefore likely to accelerate faster." That's all – doesn't say anything about how the tyres grip, whether they are draggy, blah, blah, blah., but only further testign with wheels, tyres, trye pressures and everything else that is a variable on a bike will determine what is 'best' overall.

If you want to go ahead and generate a whole system model, be my guest – but this is really outside the realms of what the OP, and most people on here, can cope with or want.

No, what I claimed was that at constant speed, acceleration = 0 which is

drumroll….

correct!

Acceleration is defined by the rate of change of velocity, not the rate of change of speed. As velocity is a vector quantity it has both magnitude and direction. Speed is a scaler quantity and therefore only has magnitude. It is perfecty possible to accelerate without changing speed.

We could argue all day but it is pointless and we're at 4 pages nearly.

What we need is a controlled experiement repeated multiple times and then statistically analysed.

Anybody actually want to do this? (It has been done countless times)

Maybe get some professionals with equipment and sponsership?

Volunteers?

glenh – Member
Exellent plane on a conveyer style troll Jon

Rumbled

I've got these empty honey jars, if could just somehow attach them to the hubs and rims of 2 identical sets of wheels I'm sure we could carry out some kind of meaningful experiment.

Hi Guys,

Interesting article and interesting to hear the arguments. I run a blog as part of my Research group and thought this would be a good subject for a topic (I'm a sports Engineer) so have done some quick calcs with some simple assumptions. I'll write it up this week and get it on ASAP (there's a good article waiting to go up so can't push in front) but to get a sneak peek on the numbers.

Assuming a rider + gear weight of 80kg, total bike weight of 12.7 kg and wheels weight (in tyres etc. of 3.12 kg) by decreasing the rim mass by 100 g (and putting the mass elsewhere on the bike) you get a 4% increase in acceleration for a zero change in the bike mass.

Interesting stuff, I'll write it up properly and put in some real examples (i.e. how does it relate to time, given an equal torque at the wheel)

Also, realise this is a very simple assumed system, accleration is exactly that, accln of a point mass with two rotating bodies depending on a torque applied at on a rotating mass.

Hope that's of some help to the OP. I can't answer whether that 4% is noticeable though!

Si C

Some numbers! At last
Good stuff weirdchops

The OP was something to do with the hub going at the same speed as the rim. Which part of the rim?

The hub goes at the same speed as the bike.
However the rim (and tyre) at the ground is going at the same speed as the ground (its either stationary, or it is skiding).

The top of the wheel is travelling at twice the speed of the bike.

If you try to trace out the path off a single point on the rim as the bike moves forward. Something that you can try to do with your finger it is a curious pattern.

For a point on the rim to accelerate from zero to twice the speed of the bike and back to zero each revolution is fairly rapid acceleration.

Incidentaly one of the limiting factors for a 100m sprinter (Usain Bolts for example) is the ability to take the foot from zero (mph) on the track surface to twice the speed of the rest of the runner (approx 60mph).

😳
Apologies guys, let me correct that, everything's the same other than the answer. A classic case of not carrying the 10. Let me make that .4% ! Error in the typing, my mistake.

Further analysis:

100 g from bike (non-rotating) = 1.5 % increase in accln

100 g from bike (rotating) = 1.9 % increase in accln

These assume weight of bike reduces.

I stand by original claim. I can't say whether the .4 % is noticable!

Si C

Wiredchops.
I only get a 0.1% change by slicing off 100g from the rim based on your data.
If you manage to shave off 1kg then I calculate you'll accelerate 1% quicker.
Look forward to your calcs.
I think all this is overrated. We don't get the massive advantage seen by engines due to the fact that the tip of our cycle wheels are travelling at the same speed as the bike itself whereas a flywheel will rotate much faster than the transverse speed of a vehicle. In low gears the lightening of a flywheel offers a much better advantage due to the relative speed difference between the transverse speed and the flywheel rotational speed.
Perhaps we should investigate trimming down the teeth on our outer driving cog!!!!

The above calcs assume that weight is put back on the bike, I suppose this is unreasonable so double the % figure to get some idea of the advantage of losing the weight entirely.

asol,

😳 Again, rushing through some calculations to try and get them on a forum is proving costly to my kudos. Didn't include the mass of the rider in my spreadsheet which explains the discrepancy. I did think the improvement was quite large.

We're talking a .06% difference by removing 100 g from both wheels.

I think we're getting in to 'it's neglgible' territory 🙂 Especially if I've got the chuffin' calculations right this time!

It might not be the massive gain some expected, but a gain is a gain, and its a gain that feels better to ride (unlike say loosing 100g from the bars/stem).

It might not be the massive gain some expected, but a gain is a gain, and its a gain that feels better to ride

On the contrary, it's impossible to feel a 0.06% difference – we're into placebo effect here.

I only agree with half of that statement. I think to claim it feels better to ride is pretty spurious and opens up a big can of worms. However, this place feeds off of worms, so dig in.

This is like another Kaesae thread, only not as funny.

Bring back Kaesae!!

I think we worry too much about weight. All the bike companies and magazines go on about it all the time because it's another way for them to sell their stuff. I recently put heavier forks on my bike and was worried about the weight penalty because everyone seems to think it's so important to shave off grams here and there. This fork upgrade has made my bike much faster uphill and down. A heavier bike may affect handling a bit but I rate strength and functionality more highly.