Can someone Mythbust this please

LuigiSham

Hi,

I have been perplexed about this for a good while, and i don't know what the answer is:

Two cars (Car A & Car are traveling from the same point (X) at the same speed (S). They both stop at the exact same point (Y).
Car A applies the brakes earlier, at a constant rate of deceleration, and slows down to a stop at Point Y.
Car B slams on the brakes (at a constant rate) at a later point and comes to a stop at point Y.
So, Car A brakes gently and Car B brakes more aggressively.

X

---

Y
.................Car A brakes

X

---

Y
................................................Car B brakes

Which car wears the brakes down more (or is the wear the exact same)? Are the results the same in Theory as Reality?

PS: Anyone have the number for Myth Busters?

bnt

Here's the way I see it: both cars have the same amount of kinetic energy before the braking, and the same amount afterwards, so the brake pads in both cars have dissipated the same amount of energy in stopping each car. So far so good.

However the second car did it in less time, meaning that the brakes got hotter, and the effect of temperature on brake pads is not good. According to this FAQ, uneven wear on pads also reduces their life, for the same reason: excessive temperatures. (It's not that simple, of course, e.g. some types of racing brake pad need to be hot to work properly, but we're talking about general pads.)

LuigiSham

bnt wrote: »

However the second car did it in less time, meaning that the brakes got hotter, and the effect of temperature on brake pads is not good.

Sorry about the headache. I fixed it up and thanks for spotting it. I was discussing this on my lunch (brake!!!!) break and wanted to post it, before getting back to work.

Thanks for the FAQ. Very interesting.

Car B might produce a higher max temperature, but would the sum of the heat produced over the braking period not be the same for both cars?

Fringe

The heat in A has more time to dissipate so it would actually be lower in temperature.

bnt

LuigiSham wrote: »

Car B might produce a higher max temperature, but would the sum of the heat produced over the braking period not be the same for both cars?

Since heat = energy, that's what I meant with the first paragraph; but then I thought about the max. temperature at any time as the problem, not the total heat dissipated. The brake system dissipates the heat, but that takes time, and if it's not allowed the time (aggressive braking), the temperature rises.

If you want to model this mathematically, you get in to differential equations e.g. here. More Math than Myth. :cool:

grateface

The greater braking speed though means that the brakes will have a higher convection coefficient.

Theres too many variables here to actually say. It'd need to be done experimentally or with a CFD program.

mickdw

Car A is getting the benefit of all the friction losses whether it be air resistance or resistance in the mechanics over a longer time than car B thereby requiring less effort from the actual brakes to bring it to a stop.
Car B is using it power source for longer also to overpower the friction forces.
Car A will complete the journey using less energy & therefore less brake force required

Tora Bora

LuigiSham wrote: »

Hi,

I have been perplexed about this for a good while, and i don't know what the answer is:

Two cars (Car A & Car are traveling from the same point (X) at the same speed (S). They both stop at the exact same point (Y).
Car A applies the brakes earlier, at a constant rate of deceleration, and slows down to a stop at Point Y.
Car B slams on the brakes (at a constant rate) at a later point and comes to a stop at point Y.
So, Car A brakes gently and Car B brakes more aggressively.

X

---

Y
.................Car A brakes

X

---

Y
................................................Car B brakes

Which car wears the brakes down more (or is the wear the exact same)? Are the results the same in Theory as Reality?

PS: Anyone have the number for Myth Busters?

Now. lets assume car A is a Toyota Yaris and car B is say a Mercedes CLK. Different set of maths required here eh

Gurgle

LuigiSham wrote: »

Which car wears the brakes down more (or is the wear the exact same)? Are the results the same in Theory as Reality?

Assuming car B doesn't have ABS, then car B wears the brakes less and the tyres more.

If car B does have ABS, then rather than applying dynamic friction (as per car A) the brakes repeatedly cross the static-friction threshold as the ABS pulses them. This would be akin to chipping rather than scraping, so I would guess that car B wears the pads down more.

Then again, if car B doesn't slam on, just applies the brakes smoothly but harder than A (i.e. ABS doesn't kick in) then I guess the same kinetic energy is transferred into the brakes so the wear would be equal.

jomo99

I'm don't have a clue how to do the math on this, but I know for a fact that car B's brakes will last longer. We run a few race cars, and have the experience. Car A brakes for a longer time, and thus builds up more heat, while car B brakes harder, but for a shorter distance. We have examined brakes from both type's of driver, and compared the the brakes, as we can see the difference.

John

FarmerGreen

Car A is using engine braking and aerodynamic drag during the long decelleration.
Car B keeps the power on 'till the last moment and cooks the brakes.
Extreme comparison ie old bloke/boy racer.

LuigiSham

jomo99 wrote: »

I'm don't have a clue how to do the math on this, but I know for a fact that car B's brakes will last longer. We run a few race cars, and have the experience. Car A brakes for a longer time, and thus builds up more heat, while car B brakes harder, but for a shorter distance. We have examined brakes from both type's of driver, and compared the the brakes, as we can see the difference.

John

Thanks for the real life experience Jomo.