Rotational Inertia

[Mikey3]

Is there a simple way to explain the mentioned subject, without all the complicated formulas and calculations?
Trying to understand it in relation to suspension.
Thanks
Mike
____________________
'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'

[stinkwheel]

It's the resistance to a change in direction by applying an unbalanced force to a mass currently subjected to a balanced force.

Or even more simply. The tendancy of moving objects to keep moving in the direction they are already going and of stationary objects to remain stationary.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Mikey3]

thank you
I understand that now, but how does that link into suspension and types of suspension increasing and reducing rotational inertia?
Mike
____________________
'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'

[Teflon-Mike]

OK, if you lean against a mini, you can probably push it up the road with the hand brake on, becouse it ent very heavy, yeah? Try that with a Midland Red bus... well you ent going to shift it are you, even if the hand brake was off and you had a few mates to help.

Heavier something is, more effort it takes to get it moving, or make it change direction.

Moments of inertia are about making things move AND change direction.

Instead of 'forces' we are looking at 'torques' a force multiplied by an amount of leverage.

So, heavy flywheel, takes more energy to spin it up, becouse it is heavy, than a lighter one. BUT most of the resistance is coming from the mass at the rim of the wheel, which has to be accelerated further and faster becouse its further away from the axle.

So, if you take metal away from the midle, but keep the weight at the rim, you get nearly as much flywheel effect...

The moment of inertia being MASS x its distance from the axle.... so a small flywheel with a LOT of mass at its rim can have the same moment of inertia as a big flywheel with most of its mass packed around the axle.....

On to suspension.....

Suspension moves up and down, BUT, on a swing arm system, its not moving in a straight line, but an arc... if you let the swing arm rotate all the way around you could transcribe a full circle with it...

So same deal... moment of inertia is how much 'flywheel effect' the suspension links all have, and its quite important, becouse you tend to have a long swing arm, fixed at the pivot on the frame, supporting the pritty big mass of the wheel at it's far end.

Wheel is heavy and the swing arm long, so it will have a LOT of flywheel effect and NOT want to accelerate very much when you give it a shove.... like a bump does....

Worse, having got it moving, very soon after you are going to want to slow it down and shove it back again.... yeah?

So the lighter the assembly, and the less 'flywheel effect' or moment of inertia it has, the more easily it will go up or down... hence the more 'responsive' the suspension will be...

Responsive suspension is useful, because if you have the wheel still wanting to carry on going 'up' after hitting a big bump, the wheel can 'ramp' over the top of the bump and loose contact with the surface, and if you have a succession of bumps, rather than following the surface and keeping the tyre in close contact with the road, you can sort of ride the crests, skipping from one to the next.

SO; we have 'unsprung mass' which is the total amount of weight thats hung on the suspension a bump has to shift, and we want that as low as possible, BUT we also have the moment of inertia, and if we have a heavy braced swing arm, where a lot of the mass is packed around the pivot, we can tolerate that more, as it adds less to the moment of inertia, than if its hanging on the end of teh swing arm, where it has a lot further to move and has much more effect on the suspension responsiveness.

Make sense?
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[G]

[Mikey3]

I won't quote but yes, Thank you teflon, a long explained explanation is what i needed, Much much clearer now

and G - Yeah i understand it doesn't, just the wording of the Haynes book confused the s**t outta me! And generally assumes you know stuff like this, providing no explanation even in the 'technical terms explained' section. Which doesn't help.

so a clear up, its easy to shove a penny miles along the floor (suface providing) but a large tyre or something with much larger diameter, is a lot harder to get moving, and so, stop moving, due to it's rotational inertia or 'moment of inertia'?

Thanks guys
Mike
____________________
'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'

[pepperami]

If you want to experiance rotational inertia to help you to understand it a tiny bit better do the following.

Get a bycycle wheel and spindle, put the spindle in the wheel and hold both ends of the spindle, then get a friend to spin the wheel, once it is spinning try moving the axis of the spindle.

It will give a little idea about what is going on with your wheel when you try to change direction
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I am the sum total of my own existence, what went before makes me who I am now!

[Mikey3]

[Babba]

It's simple to explain the basic principle of rotational inertia if you happen to have a toy gyroscope in your hand at the time.

Explaining it's effect in relation to suspension might prove a bit more involved & won't translate into something my missus would understand (as an example).

I once went to a 'talk' given by Tony Foale, who stands out as a God amongst thinking motorcyclists, he knows his stuff so well that he can either baffle you with total bullshit or give you that 'Ahhh' moment where everything falls into place & you suddenly understand life, the universe, everything.

WHY?

Are you trying to design your own chassis?

[Mikey3]

[Mikey3]

[G]

[Frost]

Surely rotational inertia does play a small part for bike suspension as a swing arm is a pivot. So the heavier the rear wheel, tire and swing arm are the more rotational inertia there is to overcome to get the wheel moving or stop the wheel moving with the spring.
It's likely to be quite a large effect, but fairly consistent between all bikes and will have been taken into consideration at the design phase.

As said above the rotational inertia is responsible for the gyroscopic effects of the wheels which must be taken into account when designing suspension systems, setting steering head angles etc But this is all done in computer simulation before a bike is built and should never need to be worried about by a normal user. Even someone building their own bike would get by perfectly well by copying an existing bike or by simple trial and error.

[Teflon-Mike]

[Mikey3]

[Frost]

A rim disc requires less sheer strength in the wheel spokes allowing them to be thinner and lighter.

[Babba]

[Pete.]

[Babba]

Here's one to ponder & keep you awake the rest of the night.

(A) A motorcycle rear wheel hits a bump at 50mph.

(B) It hits the same bump at 100mph.

Qu' 1. Are A & B equal in the work that the rear shock has to do?

Qu' 2. What is the differance between the ratio of speed effect + rotational inertia against the rear shock at 50mph & 100mph?

If you understand the question, I might give you a guided tour of a BSB teams workshop.

If you answer correctly, then I'll blag you a ride of the bike !

[Mikey3]

i understand the question......
thats it
are you part of that racing world then?
____________________
'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'

[Frost]

[Mikey3]

idiot question?

all makes sense to me, except regarding the tyre rigidity at high speed, wouldnt that be more to do with centrefugal force instead of rotational inertia?
the tyre being pushed outwards by the centrefugal from high speed and therefore holding its shape more effectively by 'wanting' to continue in a straight line?
Its more a query than a correction
____________________
'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'

[Mikey3]

going back to the original question, it seems to make relative sense.
so the closer the mass is to the centre of a pivot, the easier it is for a given torque to rotate the mass round the pivot compared to the same torque but with weight at a greater distance from the pivot?
The further out the mass from the pivot - the higher its rotational inertia?
____________________
'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'