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Thread: A question of physics>>

  1. #1
    Ext User(Jason James) Guest

    A question of physics>>


    If a vehicle is descending down a mountain, they used to say "dont ride
    the brakes". But is this so? If you brake hard you slow down quicker and
    dont need a re-application for some time. If you slow down slower [by
    riding the brakes] you wont generate as much heat [in the disc or drum],
    but you'll need more time to do the job of slowing down.

    On the side, you have peripheral issues like disc/drum-cooling,..but I
    suspect everything falls into line with the "hard application = less ON
    time,..while less application = longer ON time to achieve desired
    slowing down"

    If the brakes start to overheat, will this be the same for both braking
    examples ?Or is there another process at play ? In other words a fast
    and heavy cooling breeze will cool more per unit time compared to slow
    and long [time wise] cool per unit of time

    Just wondering,.. Jason

  2. #2
    Ext User(Bob Milutinovic) Guest

    Re: A question of physics>>

    "Jason James" <h6tgf22l@outlook.com> wrote in message
    news:l20e3o$nbd$1@dont-email.me...
    >
    > If a vehicle is descending down a mountain, they used to say "dont ride
    > the brakes". But is this so? If you brake hard you slow down quicker and
    > dont need a re-application for some time. If you slow down slower [by
    > riding the brakes] you wont generate as much heat [in the disc or drum],
    > but you'll need more time to do the job of slowing down.
    >
    > On the side, you have peripheral issues like disc/drum-cooling,..but I
    > suspect everything falls into line with the "hard application = less ON
    > time,..while less application = longer ON time to achieve desired slowing
    > down"
    >
    > If the brakes start to overheat, will this be the same for both braking
    > examples ?Or is there another process at play ? In other words a fast and
    > heavy cooling breeze will cool more per unit time compared to slow and
    > long [time wise] cool per unit of time
    >
    > Just wondering,.. Jason


    Use the rule of thumb my driving instructor told me all those decades ago -
    go down a hill in the same gear you went up it. Let the engine slow you down
    and only use your brakes when necessary.

    Don't use the argument that you drive an automatic; they too can be
    restricted to a maximum gear.

    --
    Bob Milutinovic
    Cognicom


  3. #3
    Ext User(D Walford) Guest

    Re: A question of physics>>

    On 26/09/2013 2:44 PM, Jason James wrote:
    >
    > If a vehicle is descending down a mountain, they used to say "dont ride
    > the brakes". But is this so? If you brake hard you slow down quicker and
    > dont need a re-application for some time. If you slow down slower [by
    > riding the brakes] you wont generate as much heat [in the disc or drum],
    > but you'll need more time to do the job of slowing down.
    >
    > On the side, you have peripheral issues like disc/drum-cooling,..but I
    > suspect everything falls into line with the "hard application = less ON
    > time,..while less application = longer ON time to achieve desired
    > slowing down"
    >
    > If the brakes start to overheat, will this be the same for both braking
    > examples ?Or is there another process at play ? In other words a fast
    > and heavy cooling breeze will cool more per unit time compared to slow
    > and long [time wise] cool per unit of time
    >

    If in both instances by braking you reduce your speed by the same amount
    you will generate exactly the same amount of heat.
    Maybe by using a single hard application they have more time to cool. In
    either case you should be driving slow enough so that you never have to
    worry about brake fade issues.

    --
    Daryl

  4. #4
    Ext User(Noddy) Guest

    Re: A question of physics>>

    On 26/09/13 2:44 PM, Jason James wrote:
    >
    > If a vehicle is descending down a mountain, they used to say "dont ride
    > the brakes". But is this so?


    I have no idea what "people" used to say :)

    > If you brake hard you slow down quicker and
    > dont need a re-application for some time. If you slow down slower [by
    > riding the brakes] you wont generate as much heat [in the disc or drum],
    > but you'll need more time to do the job of slowing down.


    Not really.

    If you "ride the brakes" down a hill you'll generate a *lot* of heat in
    a relatively short time, while at the same time reducing the
    effectiveness of the brakes to the point where you could have no working
    brakes at all.

    Just how long that takes to occur depends on a heap of things, but it
    can happen quite quickly in the right circumstances.

    > On the side, you have peripheral issues like disc/drum-cooling,..but I
    > suspect everything falls into line with the "hard application = less ON
    > time,..while less application = longer ON time to achieve desired
    > slowing down"


    Like I said it depends on a great many things.

    I remember when my mum's Getz wasn't very old I took it for a drive
    through the Dandenongs and in the space of 100mtrs down a long hill it
    went from having very good brakes to both feet on the pedal giving
    nothing at all and I mean absolutely nothing. It's soft "blocks of tasty
    cheese" factory fitted brake pads were great for around town in traffic,
    but in applications more involved than that they went off *very* quickly
    indeed as they had no heat ability at all. You could actually feel them
    go away under you foot, where the car would be perfectly fine one
    second, and then accelerate as if you'd released the brake and stood on
    the gas the next all while you were holding the same pressure on the
    brake pedal.

    In the case in question, a car in front (which was a mid 90's Camry as I
    recall) was being driven on the brake down one of the grades and I had
    to brake to avoid hitting it and adjust to it's speed but the little
    Hyundai's chocolate chip biscuit brake pads went off *way* before the
    Camry's seemed to and I ended up having to drive around it and then
    stopping with the trans and hand brake or I would have run right up it's
    clacker.


    > If the brakes start to overheat, will this be the same for both braking
    > examples ?Or is there another process at play ? In other words a fast
    > and heavy cooling breeze will cool more per unit time compared to slow
    > and long [time wise] cool per unit of time


    Having air move over the brakes is the key, and they'll always cool
    faster while moving rather than standing still unless you park the car
    in a river. The problem is that if you reach the point where you're
    desperate for cooling air you're likely to have little or no working
    brakes at all, and moving generally isn't the best option.

    Unless you actually *want* to park in a river :P


    --
    --
    Regards,
    Noddy.

  5. #5
    Ext User(Xeno Lith) Guest

    Re: A question of physics>>

    On 26/09/13 4:24 PM, Noddy wrote:
    > On 26/09/13 2:44 PM, Jason James wrote:
    >>
    >> If a vehicle is descending down a mountain, they used to say "dont ride
    >> the brakes". But is this so?

    >
    > I have no idea what "people" used to say :)


    That doesn't surprise me at all.
    >
    >> If you brake hard you slow down quicker and
    >> dont need a re-application for some time. If you slow down slower [by
    >> riding the brakes] you wont generate as much heat [in the disc or drum],
    >> but you'll need more time to do the job of slowing down.

    >
    > Not really.


    Highly detailed explanation - NOT!


    See Bob's explanation for an example of conciseness.
    >
    > If you "ride the brakes" down a hill you'll generate a *lot* of heat in
    > a relatively short time, while at the same time reducing the
    > effectiveness of the brakes to the point where you could have no working
    > brakes at all.


    Really? Didn't you get taught not to ride the brakes down a long hill?
    Didn't it seem patently obvious to use the gears instead of the brakes?
    >
    > Just how long that takes to occur depends on a heap of things, but it
    > can happen quite quickly in the right circumstances.
    >
    >> On the side, you have peripheral issues like disc/drum-cooling,..but I
    >> suspect everything falls into line with the "hard application = less ON
    >> time,..while less application = longer ON time to achieve desired
    >> slowing down"

    >
    > Like I said it depends on a great many things.


    You have no idea of how to drive down a hill by what you wrote below.
    Did you get your licence in a cornflakes packet?
    >
    > I remember when my mum's Getz wasn't very old I took it for a drive
    > through the Dandenongs and in the space of 100mtrs down a long hill it
    > went from having very good brakes to both feet on the pedal giving
    > nothing at all and I mean absolutely nothing. It's soft "blocks of tasty
    > cheese" factory fitted brake pads were great for around town in traffic,
    > but in applications more involved than that they went off *very* quickly
    > indeed as they had no heat ability at all. You could actually feel them
    > go away under you foot, where the car would be perfectly fine one
    > second, and then accelerate as if you'd released the brake and stood on
    > the gas the next all while you were holding the same pressure on the
    > brake pedal.
    >
    > In the case in question, a car in front (which was a mid 90's Camry as I
    > recall) was being driven on the brake down one of the grades and I had
    > to brake to avoid hitting it and adjust to it's speed but the little
    > Hyundai's chocolate chip biscuit brake pads went off *way* before the
    > Camry's seemed to and I ended up having to drive around it and then
    > stopping with the trans and hand brake or I would have run right up it's
    > clacker.
    >

    The Camry driver probably had his car in a lower gear and only had the
    foot on the brake pedal very lightly, just enough to light up the brake
    lights but not enough, in reality, to apply the brakes. Driver
    re-education course for you, lad!
    >
    >> If the brakes start to overheat, will this be the same for both braking
    >> examples ?Or is there another process at play ? In other words a fast
    >> and heavy cooling breeze will cool more per unit time compared to slow
    >> and long [time wise] cool per unit of time

    >
    > Having air move over the brakes is the key, and they'll always cool
    > faster while moving rather than standing still unless you park the car
    > in a river. The problem is that if you reach the point where you're
    > desperate for cooling air you're likely to have little or no working
    > brakes at all, and moving generally isn't the best option.
    >
    > Unless you actually *want* to park in a river :P
    >

    A particularly trite comment!


    --

    Xeno

  6. #6
    Ext User(Noddy) Guest

    Re: A question of physics>>

    On 26/09/13 5:59 PM, Xeno Lith wrote:

    > A particularly trite comment!


    Why are you still talking to me? Aren't you dead yet?



    --
    --
    Regards,
    Noddy.

  7. #7
    Ext User(Xeno Lith) Guest

    Re: A question of physics>>

    On 26/09/13 8:06 PM, Noddy wrote:
    > On 26/09/13 5:59 PM, Xeno Lith wrote:
    >
    >> A particularly trite comment!

    >
    > Why are you still talking to me? Aren't you dead yet?
    >
    >
    >

    You first!

    --

    Xeno

  8. #8
    Ext User() Guest

    Re: A question of physics>>

    Actually, you should coast downhill as fast as you can without losing control or exceeding the speed limit. Then you are using more aerodynamic drag, so you need less brakes!!
    READ THE PDS
    No liability accepted if you kill yourself following this advice.

  9. #9
    Ext User(Noddy) Guest

    Re: A question of physics>>

    On 26/09/13 10:54 PM, pedro1492@lycos.com wrote:
    > Actually, you should coast downhill as fast as you can without losing control or exceeding the speed limit. Then you are using more aerodynamic drag, so you need less brakes!!
    > READ THE PDS
    > No liability accepted if you kill yourself following this advice.


    I hope you don't live in my part of the world and share roads with me.


    --
    --
    Regards,
    Noddy.

  10. #10
    Ext User(Xeno Lith) Guest

    Re: A question of physics>>

    On 26/09/13 11:07 PM, Noddy wrote:
    > On 26/09/13 10:54 PM, pedro1492@lycos.com wrote:
    >> Actually, you should coast downhill as fast as you can without losing
    >> control or exceeding the speed limit. Then you are using more
    >> aerodynamic drag, so you need less brakes!!
    >> READ THE PDS
    >> No liability accepted if you kill yourself following this advice.

    >
    > I hope you don't live in my part of the world and share roads with me.
    >
    >

    With your self confessed inability to avoid brake fade, I was thinking
    precisely the same thing about you!

    --

    Xeno

  11. #11
    Ext User(John_H) Guest

    Re: A question of physics>>

    Jason James wrote:
    >
    >If a vehicle is descending down a mountain, they used to say "dont ride
    >the brakes". But is this so? If you brake hard you slow down quicker and
    >dont need a re-application for some time. If you slow down slower [by
    >riding the brakes] you wont generate as much heat [in the disc or drum],
    >but you'll need more time to do the job of slowing down.


    As an exercise in physics, two factors to consider. The velocity
    (speed) you want to maintain and the increase in velocity due to
    acceleration down an incline. It's the kinetic energy relating to the
    velocity squared that will be converted to heat under braking (law of
    conservation of energy), since KE equals mv^2.

    IOW if the speed is doubled the KE increases fourfold!

    So by delaying the braking, as opposed to 'riding the brakes',
    there'll be a lot more energy involved, corresponding to the increased
    speed (due to gravity) between brake applications.

    Not that either is the correct way to go down a mountain... but you
    already knew that! (The alternative to 'riding the brakes' is to
    select the correct gear.) :)

    >On the side, you have peripheral issues like disc/drum-cooling,..but I
    >suspect everything falls into line with the "hard application = less ON
    >time,..while less application = longer ON time to achieve desired
    >slowing down"
    >
    >If the brakes start to overheat, will this be the same for both braking
    >examples ?Or is there another process at play ? In other words a fast
    >and heavy cooling breeze will cool more per unit time compared to slow
    >and long [time wise] cool per unit of time


    You've already answered your own question! Obviously the more time
    the brakes have to dissipate the heat the lower their ultimate
    temperature. Wear associated with abuse is something else again. :)

    --
    John H

  12. #12
    Ext User(Jason James) Guest

    Re: A question of physics>>

    John_H wrote:
    > Jason James wrote:
    >>
    >> If a vehicle is descending down a mountain, they used to say "dont ride
    >> the brakes". But is this so? If you brake hard you slow down quicker and
    >> dont need a re-application for some time. If you slow down slower [by
    >> riding the brakes] you wont generate as much heat [in the disc or drum],
    >> but you'll need more time to do the job of slowing down.

    >
    > As an exercise in physics, two factors to consider. The velocity
    > (speed) you want to maintain and the increase in velocity due to
    > acceleration down an incline. It's the kinetic energy relating to the
    > velocity squared that will be converted to heat under braking (law of
    > conservation of energy), since KE equals mv^2.
    >
    > IOW if the speed is doubled the KE increases fourfold!


    Ooouch,..should have got that energy formulae.


    >
    > So by delaying the braking, as opposed to 'riding the brakes',
    > there'll be a lot more energy involved, corresponding to the increased
    > speed (due to gravity) between brake applications.
    >
    > Not that either is the correct way to go down a mountain... but you
    > already knew that! (The alternative to 'riding the brakes' is to
    > select the correct gear.) :)



    On that occasion I tried that. Engine too small to help !

    >
    >> On the side, you have peripheral issues like disc/drum-cooling,..but I
    >> suspect everything falls into line with the "hard application = less ON
    >> time,..while less application = longer ON time to achieve desired
    >> slowing down"
    >>
    >> If the brakes start to overheat, will this be the same for both braking
    >> examples ?Or is there another process at play ? In other words a fast
    >> and heavy cooling breeze will cool more per unit time compared to slow
    >> and long [time wise] cool per unit of time

    >
    > You've already answered your own question! Obviously the more time
    > the brakes have to dissipate the heat the lower their ultimate
    > temperature. Wear associated with abuse is something else again. :)



    Thanx John


  13. #13
    Ext User(John_H) Guest

    Re: A question of physics>>

    Jason James wrote:
    >John_H wrote:
    >>
    >> Not that either is the correct way to go down a mountain... but you
    >> already knew that! (The alternative to 'riding the brakes' is to
    >> select the correct gear.) :)

    >
    >
    >On that occasion I tried that. Engine too small to help !


    Then you either tried to descend too fast, or the mountain would've
    been too steep to climb as well! ;-)

    There's a good reason for the 'trucks use low gear' signs at the
    beginning of steep descents.... *If the driver of a truck or bus is
    driving on a length of road to which a trucks and buses low gear sign
    applies, the driver must drive the truck or bus in a gear that is low
    enough to limit the speed of the truck or bus without the use of a
    primary brake*.

    --
    John H

  14. #14
    Ext User(Noddy) Guest

    Re: A question of physics>>

    On 27/09/13 12:14 AM, Xeno Lith wrote:

    > With your self confessed inability to avoid brake fade, I was thinking
    > precisely the same thing about you!


    It's a shame you don't live next door to me, as I'd back my GT up
    against your house, drill a pair of holes through your wall and ****ing
    gas you.



    --
    --
    Regards,
    Noddy.

  15. #15
    Ext User(Jeus) Guest

    Re: A question of physics>>

    On Fri, 27 Sep 2013 10:28:11 +1000, Noddy <me@wardengineering.com.au>
    wrote:

    >On 27/09/13 12:14 AM, Xeno Lith wrote:
    >
    >> With your self confessed inability to avoid brake fade, I was thinking
    >> precisely the same thing about you!

    >
    >It's a shame you don't live next door to me, as I'd back my GT up
    >against your house, drill a pair of holes through your wall and ****ing
    >gas you.


    Very creative :)

  16. #16
    Ext User(Xeno Lith) Guest

    Re: A question of physics>>

    On 27/09/13 10:28 AM, Noddy wrote:
    > On 27/09/13 12:14 AM, Xeno Lith wrote:
    >
    >> With your self confessed inability to avoid brake fade, I was thinking
    >> precisely the same thing about you!

    >
    > It's a shame you don't live next door to me, as I'd back my GT up
    > against your house, drill a pair of holes through your wall and ****ing
    > gas you.
    >
    >
    >

    You would have done well in Germany 1939-45

    --

    Xeno

  17. #17
    Ext User(atec77) Guest

    Re: A question of physics>>

    On 27/09/2013 10:38 AM, Xeno Lith wrote:
    > On 27/09/13 10:28 AM, Noddy wrote:
    >> On 27/09/13 12:14 AM, Xeno Lith wrote:
    >>
    >>> With your self confessed inability to avoid brake fade, I was thinking
    >>> precisely the same thing about you!

    >>
    >> It's a shame you don't live next door to me, as I'd back my GT up
    >> against your house, drill a pair of holes through your wall and ****ing
    >> gas you.
    >>
    >>
    >>

    > You would have done well in Germany 1939-45
    >

    I think that big pink patch on his chest would be an impediment ?

    --









    X-No-Archive: Yes


  18. #18
    Ext User(Noddy) Guest

    Re: A question of physics>>

    On 27/09/13 11:56 AM, Buzz^| wrote:

    > 2 things.
    >
    > 1) Bikes are nasty things when they get brake fade.


    Bikes are *way* easier a prospect without brakes than any car. Ask a
    speedway bike rider :)

    > 2) When confronted with snipers shooting from the tunnels under Lae the
    > "solution" was to seal the entrances and introduce exhaust gasses to the
    > miles of tunnels under what is now Top Town. The tunnels being a huge
    > war hospital, when the Japs evacuated they could only take those that
    > could march leaving plenty of wounded to act as snipers. The gassing was
    > done by the Aussies and the Yanks.


    What the **** does any of that shit have to do with brake fade?



    --
    --
    Regards,
    Noddy.

  19. #19
    Ext User(Buzz^|) Guest

    Re: A question of physics>>

    On 27/09/2013 8:58 AM, John_H wrote:
    > Jason James wrote:
    >> John_H wrote:
    >>>
    >>> Not that either is the correct way to go down a mountain... but you
    >>> already knew that! (The alternative to 'riding the brakes' is to
    >>> select the correct gear.) :)

    >>
    >>
    >> On that occasion I tried that. Engine too small to help !

    >
    > Then you either tried to descend too fast, or the mountain would've
    > been too steep to climb as well! ;-)
    >
    > There's a good reason for the 'trucks use low gear' signs at the
    > beginning of steep descents.... *If the driver of a truck or bus is
    > driving on a length of road to which a trucks and buses low gear sign
    > applies, the driver must drive the truck or bus in a gear that is low
    > enough to limit the speed of the truck or bus without the use of a
    > primary brake*.
    >


    2 things.

    1) Bikes are nasty things when they get brake fade. The RMX250 I have,
    being a 2 stroke needs special care when descending the mountains around
    here. Frequent stops are the best precaution. From sea level up to 2000m
    being one of the rides I like doing fairly frequently. The XR400 it's
    just a matter of gearing but the 2 smoke doesn't have the compression to
    slow it down too much. Boiling the brake fluid is usually caused by
    water build up and can contribute a lot to brake fade. Replacing the
    fluid yearly is a good idea.

    2) When confronted with snipers shooting from the tunnels under Lae the
    "solution" was to seal the entrances and introduce exhaust gasses to the
    miles of tunnels under what is now Top Town. The tunnels being a huge
    war hospital, when the Japs evacuated they could only take those that
    could march leaving plenty of wounded to act as snipers. The gassing was
    done by the Aussies and the Yanks.


    --
    Brad Leyden
    6 43.5816' S 146 59.3097' E WGS84
    To mail spam is really hot but please
    reply to thread so all may benefit
    (or laugh at my mistakes)

  20. #20
    Ext User(Jason James) Guest

    Re: A question of physics>>

    Noddy wrote:
    > On 27/09/13 10:38 AM, Xeno Lith wrote:
    >
    >> You would have done well in Germany 1939-45

    >
    > I think so. I would have joined the SS I think. I never cared much for
    > their politics but their uniforms were awesome :)


    Simon Weisthanthale [sp]


    Called them "Nazi princes"..

    Jason

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