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weight vs. friction

Posted by jaffar 
November 27, 2013 06:36AM
So I understand you want tech talks, here's one smiling smiley

Suppose we have a car.

First, we put the driver in, have it run 100km/h and break as strongly as possible. Let's assume car + driver = 1000kg.
Second, we load the car with crap to make it 2000kg. Accelerate again to 100km/h and brake again.

What is the second stopping distance compared to the first ? Please elaborate.

Thank you

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November 27, 2013 06:04PM
The energy involved is proportional to the square of the velocity so it will take about 4 times the stopping distance.

Bob in Everett
November 28, 2013 07:44AM
But the velocity is the same, only the weight changes.

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A physics truck just turned over outside. There's physics everywhere!
November 28, 2013 12:13PM
oops, you are correct. The mass being doubled will only double the energy that must be dissapated by the brakes. Since brake energy absorbsion is limited due to temperature increase capability, the larger mass may cause the brakes to overheat and fade. The stopping distance will likely be more than twice as much.

Bob in Everett
November 29, 2013 01:23AM
It largely depends on what the braking system was designed for. If ithas been made to handle 4000 kg, it will deal with 1000 and 2000 kg with little difference. Same for the tyres.
November 29, 2013 11:57AM
You are correct about the brake system design. Even making one maximum braking from 100 km/hr after another will have a noticeable degrading of brake capability. The friction coefficient is very much sensitive to temperature. With carbon brakes, it is better when the brakes are up to a certain temperature. Other materials it is the opposite.

Bob in Everett
November 29, 2013 04:58PM
The only different is the mass, and let's assume we have a flat leveled road, brakes are immune to fading, or tires are not wearing and all those minor variables.

The friction of the tires against the road depend on the rubber/road surface material (a constant) and the normal force pressing the tires. The surface being horizontal we may assume the normal force equals the weight.

The braking force is limited by tire adhesion, which is the very same as above, the weight times a constant.

So, heavier car will brake as an empty one...except:
- If the road is downhill or uphill, the weight will be different from the normal force and increase or decrease the braking distance.
- Weak brakes will not take advantage of the full friction available, and take longer to stop the heavier car
- worn tires will have less friction (the constant is lower), and take longer to stop, both loaded or unloaded.
-The heavier car will dissipate more energy (heat on the brakes) to stop than the lighter one, and fading brakes can come into play.

I am too tired to go look into the math...
If I am wrong, please show me.
November 29, 2013 10:16PM
If we just measure the energy dissipation required to stop the two different masses we can get an idea of what the difference will be. It is the opposite of increasing the velocity. It is just a change in velocity. with twice the mass, the energy change needed is twice as much. There are some limitations though, the energy will have to be converted to heat by the brakes. Since in your assumptions about the tires not wearing and brakes not fading do simplify the calculations, we might make some progress. In order to skid the tires. the brakes must squeeze the pads against the discs with more force than the tires press on the pavement. If we assume that the tires do not skid, and that all the heat from stopping the vehicle is absorbed by the brakes, the brake discs and pads will have to absorb twice energy of the heavier one. The temperature could get hot enough to melt the material they are made from.

Bob in Everett
December 04, 2013 09:38AM
Jose, I don't understand all you say, but I think you are on the correct track !
This question came from a test I did with some friends with motorcycles. I exagerated the weights for a car, but on 2 wheels it's more simple - the bike with a small rider is ~250-260kg, the bike fully loaded with passenger and luggage is ~425kg. To our surprise, the stopping distance is very similar in the given cases.

This brings another question that bothers me. Take two similar bikes, with similar weights, same wheel size, same tires. One bike has one brake disk, the other has two. Even if both bikes are perfectly capable of blocking the wheel (thus you can use the same braking force, without locking it), the bike with two discs stops way faster and more brutally than the one with one disc. My brain doesn't understand that.

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A physics truck just turned over outside. There's physics everywhere!
December 04, 2013 12:37PM
Quote
jaffar
Jose, I don't understand all you say, but I think you are on the correct track !
This question came from a test I did with some friends with motorcycles. I exagerated the weights for a car, but on 2 wheels it's more simple - the bike with a small rider is ~250-260kg, the bike fully loaded with passenger and luggage is ~425kg. To our surprise, the stopping distance is very similar in the given cases.

This brings another question that bothers me. Take two similar bikes, with similar weights, same wheel size, same tires. One bike has one brake disk, the other has two. Even if both bikes are perfectly capable of blocking the wheel (thus you can use the same braking force, without locking it), the bike with two discs stops way faster and more brutally than the one with one disc. My brain doesn't understand that.

It may be that the efficiency of the drum brake is a lot less than the diss brake. Even with the larger pad area on the drum brake it simply cannot apply the same force as the disc which clamps the rotor from both sides.

Then again, it may be something else since in hard brakng, as much as 90% of the stopping power has to come from the fron brake...
December 05, 2013 07:14AM
one bike has one DISC, the other bike has two discs. talking about front brake only.
the rest is the same - weight is similar, wheel size and tires are identical.

one disc is perfectly able to block the wheel, so there's plenty of braking power.
however, two discs stop the vehicle much faster (and throw you over the front end much harder smiling smiley )

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A physics truck just turned over outside. There's physics everywhere!
December 05, 2013 12:07PM
Quote
jaffar
one bike has one DISC, the other bike has two discs. talking about front brake only.
the rest is the same - weight is similar, wheel size and tires are identical.

one disc is perfectly able to block the wheel, so there's plenty of braking power.
however, two discs stop the vehicle much faster (and throw you over the front end much harder smiling smiley )

This is an hard one, cause any vehicle can only brake as much as it's tire adhesion on the pavement.
Adding more brakes, will only allow to lock the wheel faster, and you said both bikes had plenty of braking.
However, having 2 disks instead of just 1 allows the braking energy to be dissipated faster, but I don't know how that comes into play here because the distance depends only on the forces.
You said the tire was the same, are you sure about that? Different rubber can have different grip, and affect the stopping distance. Other than that, I'm puzzled.
And, of course, both riders must apply brakes to the limit of adhesion.
December 06, 2013 03:33AM
Yes, the tire is exactly the same, brand, size and age.
Of course, there are some differences in vehicles' suspension, I think those may have a big influence. I should find a bike with two discs and disconnect one, to make a proper test smiling smiley

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A physics truck just turned over outside. There's physics everywhere!
December 06, 2013 12:06PM
Quote
jaffar
one bike has one DISC, the other bike has two discs. talking about front brake only.
the rest is the same - weight is similar, wheel size and tires are identical.

one disc is perfectly able to block the wheel, so there's plenty of braking power.
however, two discs stop the vehicle much faster (and throw you over the front end much harder smiling smiley )

There is a brake equalizer valave that proportions braking power between the front and rear.
My guess is that the sharing is much more equal on the dual discs sothe front isn't working quite as hard as opposed to the disc/drum where the disc has to work much harder so the drum doesn't lock up. Drums are far easier to lock and loose control of.
December 06, 2013 01:11PM
Quote
Archeo-peteriX
Quote
jaffar
one bike has one DISC, the other bike has two discs. talking about front brake only.
the rest is the same - weight is similar, wheel size and tires are identical.

one disc is perfectly able to block the wheel, so there's plenty of braking power.
however, two discs stop the vehicle much faster (and throw you over the front end much harder smiling smiley )

There is a brake equalizer valave that proportions braking power between the front and rear.
My guess is that the sharing is much more equal on the dual discs sothe front isn't working quite as hard as opposed to the disc/drum where the disc has to work much harder so the drum doesn't lock up. Drums are far easier to lock and loose control of.

I understood the 2 disk was at the front only, but the rear braking and suspension may have a role here.
The way the weight shifts to the front when braking or splits front and rear may have huge impact on the adhesion, and therefore the braking force that may be applied to each wheel.
I Imagine the bike more loaded to have the cargo more towards the rear, and the other one with only the rider leaning over the tank and to the front.
The contact area of a bike is about the size of the palm of a hand at each wheel, if the rear wheel is not loaded, no braking can be expected there and the front must take all the force.
I'm talking without any experience, cause i have no motorbike licence. tongue sticking out smiley
In my car experience, the braking distance suffers more from adhesion conditions (wet pavement, worn tires, faded brakes) than the cargo or the lack of it. But then again, cargo is more critical at trucks or vans, which i have little experience in driving...Most passenger cars have more brakes than needed anyway.
December 07, 2013 08:14AM
The trucks are not 100% comparable, but still interesting to see.
December 10, 2013 07:03AM
Quote
Michiel 318iS
The trucks are not 100% comparable, but still interesting to see.

This test is not made under scientific methods, yet some data can be used to our discussion.
-The loaded truck stop distance is in the same order of magnitude of the unloaded, and the mass is 3 times bigger.
-The loaded truck is about 40 times heavier than the skoda car, and stops in twice the distance.
-The bike is much lighter than the car, and takes longer distance to stop

So, I think we may rule out the mass from the braking distance, as the theory was suggesting from the beginning.
Now I think of it, the weight changes in a car upset the acceleration, balance and the suspension much more than braking distance.
Also a loaded car demands more energy to be dissipated, therefore more brake fading will occur.

:popcorn:
December 11, 2013 07:44AM
On the bike, the rear brake is mostly useless in case of hard braking - the weight shifts to front THAT much. If you brake too hard, you can completely lift up the rear long before locking the front.
There's a lot of physics packed in a small thing, very easy to notice lots of things that remain hidden on a car. Too bad I cannot understand all of them sad smiley

I have another one - apparently (people say), a driveshaft transmission burries the wheel, but a chain tranny unburries it (talking about soft ground, mud, sand etc.). Beats me why, the wheel is still turning, why the hell would that make any difference ?

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A physics truck just turned over outside. There's physics everywhere!
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