The O/T Giant Anthem thread
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Grover
Likes Bikes and Dirt
wow, alchemist with gears. good choice - i run 2x9 and like my single speed. but for enduro and marathon events the 3x9 with a MTB cassette is the way to go. you might be going slower than walking pace spinning in the 22-34 but the legs are ticking over, the lactic acid is being dispersed and that is the most important thing - to keep moving with the least amount of fatigue.
Bodin
GMBC
Grover said:
<<insert maniacal laugh here>>alchemist said:
Yes, Grover, it was me. I love it how you continue a completely O/T discussion and then get sarcastic about someone inserting O/T in to the thread title. Nice one.
And I resent being likened to a genius. People might start expecting intelligence from me. I won't stand for that! I'd rather sit and spin in my 22x34 gear on a gentle incline and whistle a pretty little ditty.
DaGonz
Eats Squid
thecat
NSWMTB, Central Tableland MBC
DaGonz said:
A mate of mine just receive his new Dually. It's heavier than his old HT.
Last night he took 3 seconds of his lap record at our Short cource race... on the "Heavier Dually"... With little to no time to get use to the new bike... in the dark.
Your argument is flawed. .5kg isn't going to save you anything, it's going to cost you
No, he's not on his own. Yes, weight makes a difference. Yes, weight at the end of a lever (or the rim of a wheel) requires more effort to accelerate than weight in the middle. It's trivial.M@DM!KE said:
How about some maths?
A heavy tyre weighs 1kg, a light tyre weighs 600g. Yes, there's room to move at both ends, but this is *my* example, so shut up!
Moment of inertia is proportional to weight, angular velocity, and the square of the radius. Ground velocity is radius x angular velocity, so for a given speed we only need to worry about mass and radius. This is somewhat simplified, but sufficiently correct for this example. So the energy difference required to accelerate the 1kg tyre is about 1.7 times the energy to accelerate the 600g tyre.
How about case 2 - 100g hub and 1.5kg rim + tyre, vs 600g hub and 1kg rim and tyre. Same overall weight, different distribution. We'll ignore spoke weight, since it's not going to have a huge effect. Hub radius is 3cm, wheel radius is 33cm, and to make it simple everything is concentrated at that point. Let's say we want to accelerate at 5ms-1 to make life easy, which is a reasonable acceleration for a second or so. So with a 100g hub to get it to 5ms-1 we've got 5x(100gx0.03 + 1500gx0.3) =2.265N. With the 600g hub you've got (600gx0.03 + 1000x0.3) = 1.590N. Wow, that's a huge difference of 0.6N, or about 40% more to accelerate the heavy tyre!
Except we've accelerated at 5ms-1 for a second and to do that we'd need to push the bike + rider to that speed. Ignoring the rotation of the front wheel, and assuming a 70kg rider on a 10kg bike, we've put in 5x80 = 400N to accelerate. So that 0.6N actually equates to about 0.15%. You can see that's in the same ballpark as the figures JohnJohn quoted.
There are a lot of physics books out there that'll tell you angular momentum helps keep you upright on a bike as well, which is why it's easier to ride a moving bike than a stationary one. That's wrong as well, because the effect is trivial compared to keeping 80kg balanced on a 5cm tyre. While gyroscopic effects come into play, the main reason is that you're constantly making corrections to the direction of travel so that inertia keeps you upright - if you turn into the fall you'll be pushed back up. We don't notice this because we're so damn good at it that we correct almost before we start falling - just like we do when we're walking.
JohnJohn, can I borrow some more ammo? I seem to have used both barrels..
Edit: Sorry Bodin, the thread *did* say OT!
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