Weight? What is the big deal?

PB Forum :: Downhill
Weight? What is the big deal?
Author Message
flag letsgetit
Posted: Jan 9, 2011 at 8:21 Quote
NICE!!! Excellent post! Un-bias, fact based, post! If anyone took the time to read after, the heavier stone falls faster post, they would have seen in was a "dumb down" argument, and really just a funny comeback. Not meant to be taken too serious. Some people only see what they want too though. If you read all of it, you will probably just get blasted with a bunch of in-coherent, opinion based ranting, just a heads up. Hope not though.

Thanks again, awesome post! Salute
flag firedudecndn
Posted: Jan 9, 2011 at 8:41 Quote
MooMoo2323 wrote:
z-man wrote:
Reading this quote here, makes me have some serious doubt on your knowledge on the laws of physics. This and various other things that you have stated have totally invalidated you're argument of weight not being a big deal

bigquotesheavier stone fall faster?

Weight doesn't have an effect on the speed that anything falls. Just resistance and the force of gravity. Its all about resistance to weight ratio. If you crumple up a piece of paper into a tight ball, it will fall at the same speed as a 14lb bowling ball. Weight does however have an effect on momentum, which on a bike, is a hindrance around corners and against the forces on your suspension.

Let me discuss the fallacies in both statements here. This is a matter of mechanical physics and it's clear that BOTH of you do not understand. First, it must be known that there is a difference between static objects and objects in motion. In reference to the "heavier stone falling faster", it just depends on stone's state when the question was asked: was the stone in motion or sitting in place? Your correction of his statement makes no sense. I think what you were trying to say is: objects, regardless of size or mass, fall at the same RATE (this is different than speed), in the absence of air resistance, and assuming the objects start their fall from zero velocity. Then you went on to mention momentum, which implies the "stone" (or our bikes) are in motion. This changes everything!

Everything in DH relates to momentum. It's all about "carrying your speed" (I'm sure you all use that phrase when looking at different lines/corners). When you include momentum into our physics discussion, things behave differently as our "stone" (our bikes) are not falling from zero, but have already had FORCE applied to them. Let us consider these scenarios:

For this example, let's assume 2 bikes of different weights enter a rock garden at the same speed. The equation for momentum is p=mv (p is momentum, m is mass, and v is velocity). Since "v" is the same for both bikes, the bike with the greater mass will have the greater momentum. Now, each bike will hit the same rock. This rock pushes the bikes in the opposite direction they want to travel. The bike with the greater inertia and momentum (the heavier bike) will be LESS AFFECTED than the lighter bike and continue down the hill at a faster speed. This means, the heavier bike can go through the rock garden while keeping a higher speed due to it's greater momentum. So here, the heavy bike wins.

Now, these same 2 bikes are exiting a corner and have to pedal to get back up to speed. We'll need the basic equation: F=ma (f is force, m is mass, and a is acceleration). Based on this relationship, in order for the heavier bike to accelerate at the same rate as the lighter bike, MORE force is required (assuming they exited the corner at the same speed). Even though the heavy bike still has more momentum when leaving the corner, the rider still has to work harder to get the bike to go faster.

Also, please note that my examples above assume the weight differences are in the same spot on the bike. It seems everyone is aware of the benefits of rotational weight savings over static weight savings. And this also assumes the riders are the same weight. The question of the thread was referring to whether or not one would notice a 1-2lbs. weight difference on his bike. So, whether or not the rider is fat has no affect on the answer to the question. Finally, my examples also assume the bikes have the same tires and tire pressure as well as suspension set up.

So, who wins? I'd say it depends on what terrain you're riding and what type of rider you are. Take a look at some of the WC riders out there and see what their bikes weigh. They range from under 35lbs to 42lbs+ (and there's no correlation between lighter bikes and podiums). Each type of bike has it's own advantages and disadvantages, and it's up to the rider to capitalize on which of those he chooses. For the type of rider I am and the trails down here in Southern California, a light bike seems do the trick for me. And yes, I can notice a 1 or 2 pound difference on my bike.

Hope this helped!

Well said. +1

 


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