If that's the right name for it. I've heard this phrase bandied about.....as I understand it, the more weight you have around the rim the harder it'll be to accelerate or maintain speed?
Is the effect compounded by the distance from the hub / diameter of the rim?
Harder to accelerate, easier to maintain speed, greater gyroscopic effect, and yes the effect is greater the further the mass is from the axle.
larger mass (rim/tyres) makes it harder to knock the wheel off-line.
It's only a factor during acceleration right?
Yes that is correct Gnar.
Any time you are changing the velocity of a mass you are accelerating it.
Sooo.
Speeding up = acceleration
braking for a corner = acceleration
changing direction (cornering) = acceleration
If you are going up a hill then you have to accelerate the wheel each pedal stroke as well...
Much easier to pedal a bike with lighter wheels. Slows down quicker for corners and the wheels are easier to make go around a corner.
There's also the issue that mass at the wheels is unsprung mass, and the lower this is the better.
What's this unsprung mass then?
Unsprung mass is the mass of the bike that's not supported by the suspension. Essentially the heavier it is, the more energy is used to move it out the way when it hits a bump on the trail, therefore the more speed that is lost.
I'm not sure why pedalling uphill envolves accelerating the wheels each pedal stroke ?
Because when you pedal up hill your not just working against friction like on the flat. Your also working against gravity, this mean if you stop pedalling you decelarate. You therefore have to increase the wheel speed with every pedal stroke to keep moving.
but by increasing that speed, you're adding momentum which helps keep you going so it's not that simple.
The fact is that when riding, we're always speeding up and slowing down. Sometimes a heavier bike may actually help your riding but since as soon as you brake, you lose energy, having heavier bike/wheels means more work to get it back up to speed.
To make the point, the current hour record holder used a really heavy back wheel on his bike to help keep his speed more even - slow to accelerate up to speed but helped maintain speed through the bends once going but then he didn't have to speed up/slow down through the hour.
http://en.wikipedia.org/wiki/Angular_momentum
http://en.wikipedia.org/wiki/Moment_of_inertia
duntmatter - a swingarm is unsprung mass, fork lowers are unsprung mass - it's the bit that moves.
Wheels are unsprung mass.
if the moving parts / unsprung parts are lighter, they will move quicker, so the suspension will be more responsive.
MrGnar, The earth has loads of rotating mass, it's not slowing down very much. i'm happier riding Dh stuff on a bike with heavier wheels because the bike tends to hold a line better, it feels less nervous.
so for me, there's a dilema: lighter wheels = better suspension action, heavier wheels = more confidence through tricky bits.
If you are going up a hill then you have to accelerate the wheel each pedal stroke as well...
You therefore have to increase the wheel speed with every pedal stroke to keep moving.
No, this is entirely wrong.
The force you're acting against when climbing is purely a downward force which starts to pull the bike backwards as the bike points upwards. Draw a vertical line through the hub (regardless of the bike's inclination) and that force pulls on each half of the wheel to the exact same extent (let's ignore the valve, eh?) - which means it's opposing and augmenting rotation in exact equal measure. A net effect of zero.
To put it another way, riding a 30lb bike which has 10lb of weight in its rims and tyres takes exactly the same power output to ride up any given hill as a 30lb bike with 5lb of weight in its rims and tyres.
Bez - only on a perfectly smooth surface.
Bez - only with a constant input of power as well 
Good point, I was finding hard to believe that a set of heavy wheels would be as easy to ride up a hill as light ones.
A pedal stroke is definately not a constant input of power, well mine aren't anyway 
Borrow a BMX, wheels weigh about the same, but the 150mm less diamete means they wobble about like no ones business!
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