A car conveyor / plane question

[rhinochopig]

A car is driving down a very steep, fairly long hill. At the bottom of the hill is a 90 degree bend that must be taken at a maximum of 30mph.

Which approach would introduce the least heat into the braking system:

1. Stay off the brakes until the last minute and then drop from high speed to 30mph?
2. Stay on the brakes all the way down maintaining 30mph?
3. Adopt a cadence braking approach?

Or would the three approaches generate the same heat?

[Glosphil]

I ssume that as all 3 methods remove the same amount of energy from the car, they must generate the same amount of heat. However, different methods will allow the brakes to disipate the heat at different rates.

[Krikkit]

Assuming we're doing this all in neutral, the total energy you need to dissipate is near-enough the same in all scenarios (in the last-minute brake stamping situation you'll lose a bit more to wind resistance, but not significant for reasonable hill lengths). To keep the brakes as cool as possible, braking continuously down the hill gently and letting them cool would be best, but in all cases if you .

[vrooom]

drive normally down the hill, go around at the bend at 60mph anyway.

[xRIEx]

Assuming that the car is only bring accelerated by gravity in all situations, then theoretically (spherical chickens in a vacuum etc.) the same amount of heat will be generated - you are converting the same amount of kinetic energy and potential energy into heat.

If you change the question to "what will the brakes be best at coping with?" then it depends on the brakes, speeds and distances involved I think.

[tigger1]

Do you need to brake again for another corner shortly afterwards?

Assuming you're on a road, select a lower gear, and let engine braking keep the descending speed a little slower?

[rhinochopig]

OK lets assume that you will need the brakes again and it's a long road - think Stelvio - and not free-wheeling.

Same gear for each scenario.

[Hugo a Gogo]

go as slow as possible, using the brakes
dragging the brakes at very low speed won't create much heat

release the brakes completely at the point near the bottom where you will only accelerate to 30mph before the corner comes up

[walm]

^Hugo is right.

Remember that the cooling of the brakes is a simple matter of time (some wind/speed effect but not much).

The point that the energy required in each scenario is the same BUT the time available for cooling is different.

If you slam on at the end they will not have much time to cool down.

If you brake from the start - they will have the whole descent for that heat to dissipate.

So, if you crawl down the hill taking a few hours - you probably won't ever raise the temperature of the brakes much above ambient.

Could be pretty boring though.

[nixon1]

I'd go for (3) as it has the most time allowed, airflow, and the less friction at the lowest speed.

[Dr Jekyll]

But presumably it's possible to apply a large amount of heat over such a long period that the temperature rise is insignificant. Is it sustained heat or peak temperature that makes brakes less effective.

[nixon1]

I suppose its the system's ability to shed the heat that matters. Introducing heat <> amount of friction?

I'm intrigued to know if Rhinochopig has had an argument with physics student of late

[rhinochopig]

nixon1 said:

I suppose its the system's ability to shed the heat that matters. Introducing heat <> amount of friction?

I'm intrigued to know if Rhinochopig has had an argument with physics student of late

No, I was driving down a very steep hill the other day in a long procession of traffic and was idly wondering whether there was any practical - rather than theoretical - difference between the various ways in which you could drive it.

There are quite a few factors involved after-all: Higher speeds offer greater airflow and thus cooling; constant braking offers a more slower thermal transient vs hard-braking from high speed; etc.

Just wondered whether there was an optimum strategy :geek:

[Chicane-UK]

I change down to a lower gear and use the engine braking effect to reduce my speed and then gently brake as I finally approached the corner.

[walm]

rhinochopig said:

Higher speeds offer greater airflow...

Not really.

[AJI]

The potential energy of the car from high point to low point is the same, therefore the poential engery conversion to heat (or other) is the same for all cases.

But, a smooth use of the brake controlling car during its decent would see the least heat build up in the braking system. This will be due to the rate of heat dissipation. Give the brakes longer time to dissipate the heat to the surrounding medium and the cooler the pads/discs will be.

Ask them to dissipate the rapid build up of heat from a short and heavy braking phase would see them reach much higher temps.

The peak temperatures in the brakes would be different in each case as will the amount of potential energy conveted to heat by the brakes.

Edited by AJI on Monday 18th February 14:02

[Disco You]

The brakes are doing the least overall work if you apply the brakes only at the end of the hill.

I'll make this simple:

The amount of kinetic energy to dissipate is fixed (initial kinetic energy + gravitational potential energy).

This energy can be dissipated by drag and braking. - Maximising drag minimises the amount of work to be done by the brakes of the car.

Drag is proportional to the speed squared.

Therefore, it is advantageous to have your speed as high as possible in order to maximise drag and minimise the amount of braking that you need to do.

[Krikkit]

Disco You said:

Therefore, it is advantageous to have your speed as high as possible in order to maximise drag and minimise the amount of braking that you need to do.

But, given the large rate of work required at the end of the hill, will the brakes have cooled sufficiently, and maintain their performance, immediately after the bend?

Technically you are correct - using engine braking and drag down the hill will minimise the brake work needed, but you are then doing the work in a very short time, possibly overloading them.

[xRIEx]

Disco You said:

Therefore, it is advantageous to have your speed as high as possible in order to maximise drag and minimise the amount of braking that you need to do.

I know what you're trying to say, but that is quite badly worded: 'speed as high as possible' necessitates a greater amount of braking.

Krikkit said:

Disco You said:

Therefore, it is advantageous to have your speed as high as possible in order to maximise drag and minimise the amount of braking that you need to do.

But, given the large rate of work required at the end of the hill, will the brakes have cooled sufficiently, and maintain their performance, immediately after the bend?

Technically you are correct - using engine braking and drag down the hill will minimise the brake work needed, but you are then doing the work in a very short time, possibly overloading them.

This. Yes, to answer the question: "which scenario will put the least heat in to the braking system?" Disco You is correct; however, "which scenario will the brakes be able to best cope with?" is as Krikkit and others have said - extra time to dissipate the heat will allow the brakes to operate well within their capability (assuming the possibility that a given system cannot deal with hard braking at the end of the hill).

walm said:

rhinochopig said:

Higher speeds offer greater airflow...

Not really.

AIUI, greater airflow in itself does not help cooling - if the air is moving too fast over the surface there is less time for the heat to be transferred to the air (and therefore inefficient). In terms of radiators, it can be beneficial to slow the airflow to increase efficiency.

[walm]

xRIEx said:

walm said:

rhinochopig said:

Higher speeds offer greater airflow...

Not really.

AIUI, greater airflow in itself does not help cooling - if the air is moving too fast over the surface there is less time for the heat to be transferred to the air (and therefore inefficient). In terms of radiators, it can be beneficial to slow the airflow to increase efficiency.

But there isn't more airflow in the first place!
He travels the same distance in every scenario therefore the flow of air past the discs is exactly the same.

Obviously for a set period of TIME if you drive faster you have more airflow (because you travel further).
But this is a set DISTANCE.