Rotating weight does it make any difference and why?

there is no gyroscopic effect its torque your should be refering to

Got a lightweight pair and through using normal tyres as tubeless I lost nearly a kilo and a half from my bike. It did feel quicker and it probably was because the moment i sliced the sidewall of my uber light 2.1 Exception Crossmark and replaced it with a 2.25 (was a mistake meant to get the 2.1 one) LUST version the bike simply drags itself.

On the few rides I managed on the Exception I remember sitting slightly back on the saddle and the lack of effort while pedalling was amazing. I am almost thinking of getting another and just putting a tube in.

Perceived or not it does not matter to me, what it does matter is that I did 27 miles on an average 10.2 miles per hour when I normally struggle at 9mph.

there is no gyroscopic effect its torque your should be refering to

Rewrite teh laws of physics!

What I've done is magnitize my spokes with the outer ends being the south pole. That way when I cycle north (which is uphil) the northern most spokes are attracted by the the northern earth pole and the southern most ones repelled by the southern earth pole. It's a small effect but combined with performance bearings gives me a definite edge.

I find that my wheels accelerate easier after I've curled one out before a ride, therefore it must be down to total weight rather than rotating weight 😀

yes it does, look at the distance the 'weight' travels

think there is some confusion from folks regarding effort.

i.e you only put effort in to accelerate, so this is when weight matters…

there is CONSTANT deceleration from air, rolling, gravitational and static contact resisistance.

therefore you are constantly putting in force against this combined effect.

the weight moves the same distance, roll your bike along the ground so the wheels revolve once, and you'll see that that the valve stems have moved the same distance as the rest of the bike.

But did they move in a straight line, like the rest of the bike?

Surely there is some german MTB mag that will have measured all this?

ballsofcottonwool – Member
the weight moves the same distance, roll your bike along the ground so the wheels revolve once, and you'll see that that the valve stems have moved the same distance as the rest of the bike.

There's more to it than that – google "moment of intertia" like I said.

to answer the OP that was

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?

It is better to save the weight off the rims.

This is due to the amount of effort required to move the weight a given distance. Mass that is farther out from a given axis must, for a given velocity, move more quickly than mass closer in. Therefore any effort put in will have a greater effect on acceleration where the mass is closer to the axis.

This is proven physics and not marketing hype.

If you would like me to I can extend these principles into why you are better saving weight off you wheels (as a whole) over other parts of the bike.

There is a clear point to lighter wheels and overcoming inertia – but only if the wheels are stiff enough for supporting rider weight and good tracking. That's where the cost comes in! I prefer lighter wheels but not stupidly light with 24 spokes, as weighing 95 kilos I will break them. On long endurance events like the kielder 100 or events with lots of climbing with all of the small accelerations adding up you will feel the difference. All personal choice though!

Tyres make a huge difference to how the bike feels on the trail as mentioned above, so note that you can save 200g on a tyre too!

think there is some confusion from folks regarding effort.

i.e you only put effort in to accelerate, so this is when weight matters…

there is CONSTANT deceleration from air, rolling, gravitational and static contact resisistance.

therefore you are constantly putting in force against this combined effect.

There's been a couple of different versions of this statement written on threads like these. While it's true that you're constantly putting in effort to combat air resistance, etc., this does not mean that you're accelerating anything.

If you're moving at a constant velocity, nothing is accelerating, rotating weight or otherwise.

As Al has said, rotating weight has more effect on acceleration, because you have to accelerate it in what could be thought of as 2 dimensions. The wheels have to move horizontally along with the rest of the bike, but they also have to be "spun up".

The sums are Force = mass x acceleration for the horizontal acceleration, plus torque = moment of inertia x angular acceleration for spinning up the wheels.

Whether or not the numbers you eventually get out of all that would be significant or not, I don't know (and can't be bothered working out), but for what it's worth, moment of inertia is quite complicated depending on the weight distribution in the rotating object, but it's generally based on mass x radius squared , so the usual stuff you see written about hubs, disc rotors, etc. not being too important compared to rims & tyres is also correct.

there is no gyroscopic effect its torque your should be refering to

I'm sorry, what? I don't think you've thought this through!

You can do the experiment yourselves:

Mount a wheel in a wheel jig on a bench. Wrap some rope around the rim, long enough so that it will reach the floor when it unwinds. Tie a mass to the free end of the rope and time how long it takes for the mass to reach the floor if it is allowed to free-fall whilst unwinding the rope.

Now, Torque = Inertia * angular accn. Knowing the distance to the floor the mass has fallen through and the time it takes to do that you can work out both the torque applied to the wheel and the angular acceleration – and therefore the interia of the wheel. But the maths doesn't really matter as the interia will be proportional to the time for the mass to fall if the mass and drop height is the same each time. Repeat for several wheels and see if the the difference in time is signficant.

The result will obviously be influenced by bearing smoothness, but it's the best that can be done in a simple experiment. Used to do a similar experiment teaching undergad Mech Eng

"You can do the experiment yourselves" they won't though, they'll keep droning on and on and on and on and on and plane on a conveyor belt and on and on…

6 pages by 5pm tomorrow, you watch.

Yes, I was always in the camp of 'it makes a huge difference', but having just done some fag packet calcs (it's a slow day….), as I=mr^2, I'm probably better off fitting BMX wheels to my road bike, to reduce the radius, and seeing if Kaesae can sort me out with some magically smooth hub bearings than agonising over 100g at the rim. Bang goes the new road wheels – the wife/credit card will be pleased 😉

Heavier rim or tyre will produce a stronger gyroscopic effect, which has two consequences. First you get stronger stability because the wheels "want" to go straight, secondly when you do turn the torque reaction fed through front the front wheel (gyroscope) into the frame will be larger and make a more forceful difference to the attitude of the bike, helping the turn once you have overcome the threshold. This is why, I believe, chunkier bikes feel better with shorter stems – the lighter steering feel of a shorter stem goes with the heavier initial feel of a heavier tyre/rim.

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, so 100 does make a difference, but you might not be able to feel it, or care.

secondly when you do turn the torque reaction fed through front the front wheel (gyroscope) into the frame will be larger and make a more forceful difference to the attitude of the bike, helping the turn once you have overcome the threshold.

Not sure I understand what you mean here.

OK, let's do the maths. Hope I don't cock any of this up…

Imagine the effect of adding an additional weight 'm', either to the rim at a radius 'r' from the hub, or to somewhere on the frame, and how much energy you need to put in to get your bike moving to a certain speed 'v' relative to the ground.

Rotational energy = 1/2 * I * w^2

I -> moment of inertia, ~= m*r^2, (actually the formula for a point mass, but a close enough approximation to a rim or tyre)

w -> rotational velocity = 2*pi*v/r

Substitute those two in, you get:

Rotational energy = 1/2 * m*r^2 * 4 * pi^2 / r^2 * v^2

Here, something interesting happens: the r^2 and the 1/r^2 cancel out, meaning that the energy required is not affected by the radius of the wheel (whether bmx or 29er): the bigger wheel will be spinning more slowly at the same ground speed.

So, we end up with rotational energy = 2*pi^2*m*v^2

Compare this with the translational kinetic energy from the weight moving forward with velocity v, the well known E = 1/2*m*v^2

So, a weight on the wheel needs 4*pi^2 times more energy to get moving to the same speed than if it were on the frame. That's 40 times, which actually is pretty shockingly much!

One thing I'm not sure about: the weight on the wheel is also moving forward and so I suspect you need to put in an aditional E = 1/2*m*v^2 of energy to get it moving forwards….

Dr Mike

Glad you went through the maths, I couldn't be bothered!

Not sure I understand what you mean here.

You know when you hold a wheel and spin it and then turn it? Ever done that? If you do it sitting on a swivel chair the torque will turn the entire chair with you on it. Well that's what happens every time you turn your front wheel – the twisting force feeds into the fork and then the frame.

One thing I'm not sure about: the weight on the wheel is also moving forward and so I suspect you need to put in an aditional E = 1/2*m*v^2 of energy to get it moving forwards….

Yeah, you do! I was just about to pick you up on that, good save! 😉

The gyroscopic effect is (I think) a bit more subjective than for simple acceleration in a striaght line. The gyroscopic force is proportional to the inertia and rotation speed, but also the input torque and turning rate at the bars. This is acting in combination with the moment due the lateral force at the contact patch on the tyre as you turn….. but the gyroscopic torque builds as you put steering input in and drops as you hold the line around the curve (your turning rate on the bars has dropped to zero if you hold a constant line), but the torque due to the lateral force on the tyre in increasing as the banking angle increases through the turn.

There's a lot more variables here than just the wheel interia, so saying situation X is better than situation Y is more subjective depending on what you want to achieve.

You know when you hold a wheel and spin it and then turn it? Ever done that? If you do it sitting on a swivel chair the torque will turn the entire chair with you on it. Well that's what happens every time you turn your front wheel – the twisting force feeds into the fork and then the frame.

Yup, I know, just not sure how that translates to "feeling" on the bike.

Right my Physics and Math’s are getting a bit old now. (Back when 'A' levels where hard and all that)

Bike wheels spin to move forward so to move a wheel forward, more of the force is required to move the outside of the wheel than it is at the centre (rotational mass) and more than a saddle or frame or rider (ish) (static mass)

More force is required the further out the mass is.
(ie You'd be better saving weight 5g on tyres than 10g on hubs)

As this is all to do with inertia (not torque) it only matters when changing speed. At a constant speed it makes no difference.

Lastly I'm not clever enough to do the math’s on this but I suspect that for your every day cyclist. Any weight saving that is being noticed is because of the general weight saving and not that of rotational mass.

If it's a few grams you would only notice it under sprinting up to a speed or heavy braking.

Like I say not clever enough to do the proof but half a brain and a bit of common sense says.

Rotational Mass = Of course it makes a difference but not enough to worry about.

Get some lighter tyres and tubes.

And if you still think it makes a big difference take off your dust caps 😉

That's 40 times, which actually is pretty shockingly much!

Yes, in isolation, but the mass of the rotating parts (the wheels – a couple of kg's) compared to the mass fo the rest (80kg?!? – including rider) diminishes the effect in the overall system in this particular example?

I put lighter wheels on my bike, and they made the bike feel great…
Until I came to rolling downhill on road.. All my friends are there freewheeling, and I'm having to now pedal to keep up with them. Whats that about!

It took me a few minutes, but here's where you went wrong, majk:

w -> rotational velocity = 2*pi*v/r

Actually w=v/r

You're adding an extraneous 2 * pi term on the mistaken assumption that w is measured in degrees, where it's actually measured in radians. The end result being that the actual rotational inertia is exactly the same as the translational inertia, hence nowhere near as big an effect as you (and everybody else) seems to think.

To those who can feel a difference in acceleration due to 300g difference in tyres, can you also feel the difference in acceleration due to your water bottle being full or empty?

Yes, in isolation, but the mass of the rotating parts (the wheels – a couple of kg's) compared to the mass fo the rest (80kg?!? – including rider) diminishes the effect in the overall system in this particular example?

Well, what I describe is relevant to when it comes to getting the bike up to speed – so how lively the bike feels in response to applying power. But mountain biking we tend to do a lot of slowing down and speeding up (well, speaking for myself) so I guess it's pretty important.

If you're grinding up a hill at constant speed, then the total mass is of interest. If you're cruising on the flat at constant speed then your aerodynamic profile, rolling resistance etc. is what is relevant.

The force which is twisting the person in this vid is channelled into your bike when you turn the wheel.

Actually w=v/r

You're adding an extraneous 2 * pi term on the mistaken assumption that w is measured in degrees, where it's actually measured in radians. The end result being that the actual rotational inertia is exactly the same as the translational inertia, hence nowhere near as big an effect as you (and everybody else) seems to think.

Thanks for the catch (d'oh!) Thought that 40 was too big a number.

But still, it means a factor 2 (due to both the rotational and translational kinetic energy).

glenp – I understand the concept of gyrposcopic torque…. I was pointing out that it isn't the only torque acting on the wheel in cornering and whether the gyroscopic effect is significant will depend on alot of variables. You may be right in your situation.

Are you boys talking about weight or mass?

bristolbiker – that was actually for the benefit of coffeeking. My point is that the effect is there, and if you add rim and/or tyre weight the effect gets bigger. If you think about riding along and someone just nudging your shoulder, it doesn't take much force to lean a bike, so that torque from turning the gyro must be significant, and does get bigger with a bigger tyre.

I run my bike with a) a normal 29er wheel and tyre and b) Surly Endomorph tyre which weighs in at 1150g plus a 450g DH innertube – I can tell you – the difference in turning feel is (gyroscope effect?) is significant, to the point where it feels like I'm riding two totally different bikes. I did try the Endo with a light 100g innertube but I can't say I noticed the difference in performance, it punctured first time out and proved to be unreparable – so I've stayed with DH innertubes – not had a puncture yet.

Some people probably don't disagree with the maths, they just think if it doesn't seem to affect them much personally then clearly it's all just "marketing bollox", innit?

Not "bollox" at all, even if you are unaware of it. A lot of people think they don't counter-steer, but they do and are simply not consciously aware of what they are doing. You learn to ride a bike very young, so all the inputs don't have a name or understanding – you just do them. But that doesn't mean they don't exist, or are "bollox".

I lost nearly a kilo and a half from my bike. It did feel quicker and it probably was…

Perceived or not it does not matter to me, what it does matter is that I did 27 miles on an average 10.2 miles per hour when I normally struggle at 9mph.
Perceived I'm afraid – in combination with a genuine reduction in rolling resistance (from lighter tyres which roll better, as does going tubeless). Even if that 1.5kg was all at the rim it would only make 3-4% difference in total inertia, which I'd suggest is below the margin you can genuinely tell the difference, and certainly not sufficient to account for a ~13% improvement in performance!