Viewing 40 posts - 1 through 40 (of 134 total)
• Rotating weight does it make any difference and why?
• ballsofcottonwool
Free Member

Is it better to save 100g off of your rims or you hubs, its all going at the same speed so surely it makes no difference?

RaveyDavey
Free Member

Take your front wheel off hold it by skewer and get someone to spin it! No try moving it around. Rotational mass is a bitch man!! Keep it light.

crikey
Free Member

The only time it makes a difference is during acceleration. The average acceleration produced by the average STWer is very, very small, so in terms of bicycle riding, the idea that rotating weight is important is a myth.

Someone will be along in a minute to totally disagree, but that's STW for you.

(they'll be wrong by the way)

cynic-al
Full Member

Look up moment of inertia.

If acceleration didnt matter then you wouldn't know the difference between a light and heavy bike 🙄

do try harder!

crikey
Free Member

Look up the actual figures for how much 'acceleration' is actually occuring, then relate it to context, then to the average STWer, then come back and show me the maths that prove it's so important.

On a bicycle, in situations where accelerations occur, weight is weight is weight whether it rotates, spins, bounces, hovers or wobbles.

If you can prove differently, go ahead.

cynic-al
Full Member

🙄 you first

br
Free Member

Levers – hold your bike at arms length, now hold it close

Or

http://www.analyticcycling.com/WheelsConcept_Page.html

Also consider sprung vs unsprung weight.

westkipper
Free Member

I currently have a 6.5kg (road) bike with fairly light, slightly flexy wheels.
As I never do the kind of events that require frequent accelerations I'd happily swap for a 6.5kg bike with heavier stiffer more aero wheels.
I'd agree with Crikey.

brakes
Free Member

on a mountain bike you are constantly accelerating and deccelerating and changing the rotational velocity of the wheel.
you are constantly trying to maintain the rotational speed of the wheel as friction tries to slow it down.
also momentum is very important in mountain biking, whereby at slower speeds bumpy terrain has a greater impact on reducing your speed the slower you are going so to be able to get the bike up to speed quickly and efficiently is very important in terms of conserving your momentum, speed and energy.

buzz-lightyear
Free Member

"The average acceleration produced by the average STWer is very, very small"

Maths err no. But this is how I'm thinking about it… might be confused though.

Each pedal stroke results in a small amount of acceleration otherwise drag would bring you to a halt, no? All these little accelerations are where your effort goes (assuming you're cycling on the flat).

Imagine you are holding a weight on a 1" bit of string and think about how much effort you need to spin it around. Now make the string 26" long and resume spinning at same rotation rate as with the short string, again imagining how much effort you need. Which was greater, or was it the same?

1" is like the hub, 26" is like the rim.

brakes
Free Member

fair enough if you are bimbling along at 5mph, but if you are pushing it and screaming in and out of corners, then it becomes important

westkipper
Free Member

Surely most STWers aren't accelerating uphill much anyway? Just on downhills, where wheel mass is probably a plus 😉

andyl46
Free Member

Going around a corner is accelerating, so as you do that quite a lot on a mountainbike it becomes rather important.

Saying that I love the feeling of a heavy bomproof wheelset banging through a rock and root strewn route as much as I like the flickability of a lightweight xc wheelset.

buzz-lightyear
Free Member

Going uphill or downhill complicates the thought experiment because you are also converting between chemical->potential (going up) and then potential->kinetic (going down).

I'm guessing that heavier/bigger wheels assist downhill stability because of the increased flywheel and gyroscopic effects. Is this what 29er riders are experiencing when they say they "roll better"?

cynic-al
Full Member

buzz-lightyear
Free Member

It's a wind up
[as requested]

Free Member

My hack has super marathon plus 1.3's weighing in 950g (+-) ea.

I don't half feel the weight on the bike after 15 miles!

Great thing is jumping on my xc or road bike you fly from training with heavy roatational mass to riding a roadie wheelset weight, lighter than the two hack tyres alone!

I guess I would take heavy mass puncture resistance over a light puncture prone tyre in commutes or winter but with Summer here now-go for a lighter tyre and fly.

Some bike mags tested heavy tough tyres Vs. light racing and nearly 5 mins difference on finishing times over ten miles but how long would it take to mend a puncture? 5 mins or less with a tube swap? It's a personal choice.

westkipper
Free Member

Well, my views are coloured because I once actally swallowed the magazine bollox about rotating weight.
Swapped from my middleweight XT wheels and tyres to a very light Tune f/ XTR rear based set-up with the lightest tyres and real-world innertubes…
… and it made precisely **** all difference to how the bike felt, though, I worried more about crashing.

crikey
Free Member

It's a common misconception, based on years and years of cycling old wives tales…

Weight is weight, and the accelerations produced by cyclists are very small, plus the flywheel effect of heavier rotating bits allows 'conservation of momentum' to a certain degree. It's a bit like the 'I'll spend £200 to lose a kilo from my bike and will be at least 20 mph faster' rubbish.

It's all in your heads, and it's probably quite lonely in there…

cynic-al
Full Member

Prove it

cheers_drive
Full Member

I've always been confused by the theory of the rotating mass argument, obviously a heavier rim will be slower to accelerate than a lighter one. I also understand the slight acceleration on each revolution argument by Buzz although you could also argue that a heavier wheel has more inertia and would hold it's speed better.
The wheels on my 29er are 2.5kg (and the mass is further from the centre of rotation) but on my full suss they are 1.7kg but if anything the 29er feels faster up hill (although that may be due to it being SS rigid). All in all I'm not convinced about the importance of wheel weight other than lighter wheels accelerate quicker and therefore feel nicer. Rolling resistance is much more important for me.

phutphutend
Free Member

The only time it makes a difference is during acceleration. The average acceleration produced by the average STWer is very, very small, so in terms of bicycle riding, the idea that rotating weight is important is a myth.

Someone will be along in a minute to totally disagree, but that's STW for you.

(they'll be wrong by the way)

Yup, you are accelerating the whole bike. But you are also accelerating the spinning wheels. The wheels spin faster than the bike moves forward so have more kinetic energy.

So a bike where the wheels are a higher proportion of the total weight will require a greater energy input to accelerate than a bike where the wheels are a lower propertion of the weight.

Similarly weight in the wheels more towards the outside will spin faster and therefore have more kinetic energy than weight at the centre of the wheel.

Energy balances can solve most question in this world!

crikey
Free Member

I can't prove it without digging out the actual physics arguments, which, as I'm off to that London tomorrow will have to wait.

It's about the context; are you really trying to say that in the total system of bike/rider/helmet/sunglasses/undershirt/shorts/socks/shoes/Camelbak/water/gel/multitool/spare tube/phone/wallet/keys that all need to be 'accelerated' each time acceleration occurs, that the change in weight of 100g or 200g at the wheel is a significant one?

If the total weight is 100kg, and all this, don't forget must be 'accelerated' otherwise you'll leave something behind, then the impact of rotating weight is as close to jeff all as makes no difference.

Weight is weight is weight.

cynic-al
Full Member

I can't prove it without digging out the actual physics arguments, which, as I'm off to that London tomorrow will have to wait.

LOL you are arguing this without even understanding the physics?

Let me help you. I'm not saying it's significant – just that gram for gram you'll feel weight loss more at the rim/tyre than static weight

you're probably trolling anyway.

buzz-lightyear
Free Member

"although you could also argue that a heavier wheel has more inertia and would hold it's speed better."

This is where I get especially confused. The positive flywheel effect would seem to cancel out the negative effects of getting it moving – at least in my lonely head!

But do you see my weight and string thought experiment back up there? Isn't that the clincher? Or can you explain some assumption I've missed?

crikey
Free Member

LOL you are arguing this without even understanding the physics?

I'm arguing it without presenting the physics to you, which would be casting pearls before swine.

I'm not saying it's significant…which would therefore imply that it doesn't matter, in this or any other context.

It's an old cyclists maxim and one which has no basis in reality; when you accelerate yourself on a bike, you accelerate everything you have on, everything you carry, everything that contributes to the total weight of you and your kit and your bike, whether that mass rotates or not is immaterial; it all has to be accelerated.

If you can prove differently, beeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee my guest, as the song goes.

cynic-al
Full Member

Is there a need for insults?

I started a reply but I really cba.

crikey
Free Member

Sorry, I was using a phrase rather than meaning to insult, apologies…

I will try to dig out the maths/physics bit when I've got more time, but I'm really not convinced by the whole rotating weight thing.

cynic-al
Full Member

Whatever.

mr_mills
Free Member

It's all to do with the quality of your bearings. If you don't believe me just ask Kaesae.

aracer
Free Member

Let me help you. I'm not saying it's significant – just that gram for gram you'll feel weight loss more at the rim/tyre than static weight

you're probably trolling anyway.
So your argument is that there is a difference, but it's totally insignificant? Well I think me and crikey actually agree with you then – are you sure it's not you who's trolling?

This thread is a good collection of all the usual myths trotted out by the rotating weight congregation. I'll just pull out a few points:

Each pedal stroke results in a small amount of acceleration otherwise drag would bring you to a halt, no? All these little accelerations are where your effort goes (assuming you're cycling on the flat).

Yes, but heavier wheels with more inertia will actually help to reduce these micro accelerations! Not that it really matters, because your effort doesn't go into these little accelerations, it mostly goes into rolling resistance and aero drag.

Going around a corner is accelerating, so as you do that quite a lot on a mountainbike it becomes rather important.

Very well observed – cornering is acceleration. However the speed your wheels rotate at doesn't inherently change as you corner, hence there is no change in wheel inertia and no energy input required – rotating weight makes no more difference than static weight.

Some bike mags tested heavy tough tyres Vs. light racing and nearly 5 mins difference on finishing times over ten miles

Which is all down to the rolling resistance and nothing to do with the difference in inertia. 1kg more of tyres would make 2.5% difference to the total inertia of a typical rider and bike (and only 1.25% more difference than the same weight on your belly) – how does that account for 5 minutes difference over 10 miles unless you're taking over 3 hours to do those 10 miles and the only thing your energy is going into is acceleration?

But do you see my weight and string thought experiment back up there? Isn't that the clincher?

Not really – because the energy involved in spinning the rim is still insignificant compared to the energy involved in moving the rest of the system (see my 2.5%/1.25% figure above due to a massive difference in rotating weight).

westkipper
Free Member

Those of you that think it does make a big difference (other than road crits and Tour mountain stages) should do as I did- go out IMMEDIATELY! and purchase a light set of wheels, weight your bike with the difference in lead weight, then ride your usual route, then if you can show youve been noticably faster I'll believe ya.

Dasha
Free Member

A wheel of lower rotating mass will be easier to change the direction of travel of. This is also a contributing advantage of lighter wheels and rims.

Free Member

Rotating weight does it make any difference and why?

It does for a Land Speed Record car.

See here —> Bloodhound SSC

I doubt any of us could produce that kind of power on our bicycles though….

chiefgrooveguru
Free Member

If it makes such a big difference why would anyone consider using a 29er to race?

BigJohn
Full Member

A fragile rim will spoil your ride more than a sturdy one. Not only when it breaks, but when you think twice about riding stuff because of it.

coffeeking
Free Member

It matters on a number of levels that, while relatively minor, can make a large difference to the feel of the bike. The first is the gyroscopic effect when trying to turn while travelling at speed – heavier rims mean more gyroscopic action, meaning the wheel is harder to change direction. May only be small but so is the stiffness advantage gained by maxles etc, yet people don't question the logic in that.

Heavier rims/tyres make a huge difference to the feel of a bike when accelerating – sure the numbers involved may be relatively small, but they're noticable to a human and that has both physiological and psychological effects. I have two sets of tyres that are identical in size and very similar tread pattern but one set is about 300g heavier. Pump them up to the same high pressure (say 50psi) to reduce casing distortion effects and I challenge you not to notice a very large difference in the laggy spongey slow feel of the heavier tyres.

I don't think it's THAT significant to be worth losing sleep over, but I do think that's the main place I try to lose weight from as instinctually that's the place I feel it most regardless of the idealised physics models.

I think it obviously makes less difference if you're a more fit and steady rider, slower riders and those who struggle start-stop on climbs will notice it more as it's a larger percentage of their time. Those who can sit and spin comfortably are effectively negating its effect until they choose to stand up and blast away, so might as well just lose the weight from wherever.

Also remember that we're not machines, we're biological, we dont output forces with the same efficiency across the scale, at higher forces it may take more effort to apply (I'm speculating, I have no numbers) – i.e. applying 100Nm of torque might take 10 Kcals to maintain, but 200Nm might take 40, in which case the larger spikes of required energy for slightly heavier wheels may affect the efficiency of the human driving it more notably than the rather minimal increase in difficulty. Again, no numbers so that is somewhat questionable, but an interesting thought process – we treat humans as machines, X watts out = X difficulty, 2X watts out may not = 2X difficulty.

There's, of course, the positive side effect of heavy rims/tyres – the flywheel effect that helps you keep going over bumpy stuff. Increased work required by brakes when stopping though.

tron
Free Member

If you want to think about the maths, have a look at this:

http://www.pumaracing.co.uk/FLYWHEEL.htm

The figures quoted are for flywheels on car engines, which obviously get up to 8000 rpm or so, whilst bike wheels don't.

However, car engines also tend to produce north of 100bhp (~75kw). Whilst we tend to produce 2-300watts.

And a bit of a google produces this:
http://van.physics.illinois.edu/qa/listing.php?id=7559

Which says weight on the wheel is equivalent to double the weight on the frame.

cynic-al
Full Member

I should have been clearer – the weight difference feels significant and to that end it is significant for the rider, but I doubt it will make a significant time difference.

West kipper – I've ridden the same bike with different wheels – saving around 400gm per wheel at the rim. The difference is instantly noticeable.

Yes, but heavier wheels with more inertia will actually help to reduce these micro accelerations! Not that it really matters, because your effort doesn't go into these little accelerations, it mostly goes into rolling resistance and aero drag.

I disagree. 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?

Scienceofficer
Free Member

I think working against gyroscopic action during direction changes is understated.

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