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        riddles >> medium >> Weight on stopping distance
        
(Message started by: aicoped on Mar 4^th, 2012, 9:32am)

Title: Weight on stopping distance
Post by aicoped on Mar 4^th, 2012, 9:32am

OK, I just got qualified to drive a bus for the military and several times throughout the course the instructor made the statement that buses stop quicker when fully loaded with passengers and take longer to stop without them.

I have searched on the internet for why this might be the case and it seems like everyone disagrees on whether its true or if in fact the exact opposite might be true.

So a couple of assumptions, so we aren't bickering about semantics later.

2 identical model buses that have equal braking ability moving at the same speed and one is loaded with passengers( I am qualified to drive up to 44 passengers so let's start there) weighing 175 each. bus weight empty is 20,000 pounds and passengers weigh 7700.

My thought process is that the empty bus has less momentum at a given speed so the brakes force will stop it quicker, but a lighter bus will have less traction by the same amount so it will take longer to stop based on friction and thus the two factors cancel out.

If anyone would be willing to convince me that this isn't the case, I would love to see why.

Title: Re: Weight on stopping distance
Post by towr on Mar 4^th, 2012, 10:48am

I don't think increase friction/traction should be a factor. Unless you let the wheels block, which is generally a bad idea, the traction of the wheels on the ground shouldn't make much of a difference, what's relevant is the friction of the break on the wheel, which should not depend on load. So therefore I'd expect only momentum to make a real difference.

Maybe you should send this in to mythbusters, or try to convince your instructor to put it to the test :P

Title: Re: Weight on stopping distance
Post by rmsgrey on Mar 5^th, 2012, 6:03am

What determines the maximum safe braking force? That is, the force at which the wheels don't quite lock, and the bus doesn't quite start skidding.

Intuitively, it's going to be related to the maximum friction of the ground on the wheels, which does depend on load...

Title: Re: Weight on stopping distance
Post by SMQ on Mar 6^th, 2012, 5:16am

Another consideration: the moment around the front axle.

Consider an extreme example: a bicycle. With a light rider, if you brake too hard it's relatively easy to lift the rear wheel off the ground, resulting in reduced traction at best, and an "endo" at worst. With a heavy rider, the additional mass over the rear wheel makes this less likely--it takes a lot more braking force to lift the rear wheel and upset the bike.

The same forces, though less pronounced, are at work in the bus. More passengers means proportionally more friction at the rear wheels and so more stopping power.

Edit: another consideration that hasn't been mentioned; with inflated rubber tires, more weight = larger contact patches (at the same tire pressure) = increased stopping power.

--SMQ

Title: Re: Weight on stopping distance
Post by Noke Lieu on Mar 7^th, 2012, 6:59pm

I'd agree with your points SMQ.
Riding to work, I consider both these things- dropping the tyre pressure when it's wet to get more rubber on the road.
The other is start braking whislt standing on the pedals, and throwing the bike forward and sitting down to try and reduce my stopping distance. (I don't know if that actually works, but it seems to).

Besides, the increase in mass is linear, whereas the increase in rubber is squared, no?

I, too, would like to see this tested in reality.

Title: Re: Weight on stopping distance
Post by SWF on Mar 7^th, 2012, 7:48pm

If you believe the Coulomb friction model applies, maximum friction force that can be supported across an interface depends on the the normal force pushing the objects together and coefficient of friction. Contact area does not enter into it.

Title: Re: Weight on stopping distance
Post by Noke Lieu on Mar 7^th, 2012, 9:02pm

waah! So much more to friction than I realised!!
http://nersp.nerdc.ufl.edu/~wsawyer/AIM2005.PDF

Title: Re: Weight on stopping distance
Post by SMQ on Mar 8^th, 2012, 5:14am

on 03/07/12 at 19:48:32, SWF wrote:

And yet more powerful vehicles--from sport cars through custom racing cars and dragsters--consistently use wider, larger tires with less tread in order to increase the amount of rubber in contact with the pavement. ;)

--SMQ