Calling all physicists/Automotive Engineers
Discussion
Please explain what I am misunderstanding here. I don't get how applying the handbrake can cause the rear of the car become heavier.
http://www.pistonheads.co.uk/gassing/topic.asp?h=0...
http://www.pistonheads.co.uk/gassing/topic.asp?h=0...
I started a thread on this last year, and said something along the lines of my car rear goes up when I handbrake.
Lots of people said down, and that I didn't know what I was talking about, a few said up, and the most sensible posters after a few pages said it depended on the car.
It seemed strange to me that if the handbrake could increase the effectiveness of the traction when pulled on, we should all be using it as an adjunct to the main braking system, which appeared frankly daft.
I tend to stop reading threads about the first time a poster uses the word obviously
.
Lots of people said down, and that I didn't know what I was talking about, a few said up, and the most sensible posters after a few pages said it depended on the car.
It seemed strange to me that if the handbrake could increase the effectiveness of the traction when pulled on, we should all be using it as an adjunct to the main braking system, which appeared frankly daft.
I tend to stop reading threads about the first time a poster uses the word obviously
.NoelWatson said:
I don't get how applying the handbrake can cause the rear of the car become heavier.
I don't think it becomes heavier - but i have never seen the rear of a car rise when applying the handbrake when it's still moving.Looks like it's time for everyone to try it in their chosen car and report back.
Edited by Ozone on Friday 7th January 16:31
s2art said:
If the CoG is high (take it to extremes, imagine a car with a CoG 10 feet above the ground) then the braking force will act to rotate the car forwards, lifting the rear. If its low enough (unlikely) then it will increase the load on the rear.
That is my understanding - hence why I was confused with the suggestionIf you're moving, and apply handbrake, the rear wheels have more friction. As rotation is affected, and they're at the rear of the car, the natural reaction of the boingey back end bits (tech term - google it) is to compress, despite the cars movement forwards.
If you stamp on the footbrake, the front discs give more stopping power - and are set up that way to prevent instability under braking. Then the movement is more pronounced as the natural compression at the front is increased by the forwards movement.
Technically, hand braking might make the rear heavier, in terms of an increase in vertical force downwards, but by a tiny tiny amount as the c of g changes.
Without compressible suspension, there'd be no change in c of g end to end.
<awaits engineers answer>
If you stamp on the footbrake, the front discs give more stopping power - and are set up that way to prevent instability under braking. Then the movement is more pronounced as the natural compression at the front is increased by the forwards movement.
Technically, hand braking might make the rear heavier, in terms of an increase in vertical force downwards, but by a tiny tiny amount as the c of g changes.
Without compressible suspension, there'd be no change in c of g end to end.
<awaits engineers answer>
Ozone said:
Looks like it's time for everyone to try it in their chosen car and report back.
I believe car with a trailing arm will dip. Cars where the braking does not twist the rear suspension (such as wishbone, z arm) should rise slightly.Edited by Ozone on Friday 7th January 16:31
andy43 said:
If you're moving, and apply handbrake, the rear wheels have more friction. As rotation is affected, and they're at the rear of the car, the natural reaction of the boingey back end bits (tech term - google it) is to compress, despite the cars movement forwards.
If you stamp on the footbrake, the front discs give more stopping power - and are set up that way to prevent instability under braking. Then the movement is more pronounced as the natural compression at the front is increased by the forwards movement.
Technically, hand braking might make the rear heavier, in terms of an increase in vertical force downwards, but by a tiny tiny amount as the c of g changes.
Without compressible suspension, there'd be no change in c of g end to end.
<awaits engineers answer>
If you stamp on the footbrake, the front discs give more stopping power - and are set up that way to prevent instability under braking. Then the movement is more pronounced as the natural compression at the front is increased by the forwards movement.
Technically, hand braking might make the rear heavier, in terms of an increase in vertical force downwards, but by a tiny tiny amount as the c of g changes.
Without compressible suspension, there'd be no change in c of g end to end.
<awaits engineers answer>
andy43 said:
rotation is affected, and they're at the rear of the car, the natural reaction of the boingey back end bits (tech term - google it) is to compress
Agreed if a trailing arm, otherwise, I don't know why this would be the case.andy43 said:
Technically, hand braking might make the rear heavier, in terms of an increase in vertical force downwards, but by a tiny tiny amount as the c of g changes.
Where would this weight come from?NoelWatson said:
Ozone said:
Looks like it's time for everyone to try it in their chosen car and report back.
I believe car with a trailing arm will dip. Cars where the braking does not twist the rear suspension (such as wishbone, z arm) should rise slightly.Edited by Ozone on Friday 7th January 16:31
NoelWatson said:
andy43 said:
andy43 said:
Technically, hand braking might make the rear heavier, in terms of an increase in vertical force downwards, but by a tiny tiny amount as the c of g changes.
Where would this weight come from?Gah, buggered up the quotes too.
Edited by andy43 on Friday 7th January 17:04
Ozone said:
NoelWatson said:
Ozone said:
Looks like it's time for everyone to try it in their chosen car and report back.
I believe car with a trailing arm will dip. Cars where the braking does not twist the rear suspension (such as wishbone, z arm) should rise slightly.Edited by Ozone on Friday 7th January 16:31
I didn't read the four pages of the link.
But..... if the car is moving in a straight line then the energy of the car would force down the front. The front suspension would absorb all the energy.
However, once the car was in a turn the direction of the moment changes (dramitically). The rear outside wheel start to absorb the energy and compress.
I'll rephrase that....the direction of the energy doesn't actually change.It can't, but it does change relative to the car.
ETA the application of the handbrake is a bit of a red herring with regarding the energy and where it will go.
Of course apply the handbrake in a turn will influence the direction of the car
(and thus the direction of energy relative to cars new direction)
Watch video of cars in silly mud racing.
But..... if the car is moving in a straight line then the energy of the car would force down the front. The front suspension would absorb all the energy.
However, once the car was in a turn the direction of the moment changes (dramitically). The rear outside wheel start to absorb the energy and compress.
I'll rephrase that....the direction of the energy doesn't actually change.It can't, but it does change relative to the car.
ETA the application of the handbrake is a bit of a red herring with regarding the energy and where it will go.
Of course apply the handbrake in a turn will influence the direction of the car
(and thus the direction of energy relative to cars new direction)Watch video of cars in silly mud racing.
Edited by jeff m on Saturday 8th January 08:19
If you use the D'Alembert theorem to convert this dynamics problem into a statics one the one thing you can count on is that any decelleration, however achieved, will always result in a weight transfer towards the front of the vehicle, because the centre of gravity on a car is always above the ground.
How the suspension responds is another matter but there will always be a weight transfer to the front.
If a vehicle has a poor braking setup I could understand using a handbrake to get some decelleration if the use of footbrakes always resulted in locking up the front wheels, but given that this would imply a mu level of almost zero you'd barely get any decelleration anyway and the likelihood of this must be unbelieveably slim tending to nil.
How the suspension responds is another matter but there will always be a weight transfer to the front.
If a vehicle has a poor braking setup I could understand using a handbrake to get some decelleration if the use of footbrakes always resulted in locking up the front wheels, but given that this would imply a mu level of almost zero you'd barely get any decelleration anyway and the likelihood of this must be unbelieveably slim tending to nil.
First class degree in aerospace engineering, ex-braking systems engineer for Airbus
Applying the hand brake will always produce a nose down pitching moment (assuming you are going forward). This increases the vertical load on the front wheels and decrease the vertical load on the rear wheels.
As Max_braking_force = coef_friction x vertical_load (a simplification, but it will do), this makes the rear brakes less effective, and the front brakes more effective in terms of total braking force. This is the reason all brakes are forward biassed.
Applying the hand brake will always produce a nose down pitching moment (assuming you are going forward). This increases the vertical load on the front wheels and decrease the vertical load on the rear wheels.
As Max_braking_force = coef_friction x vertical_load (a simplification, but it will do), this makes the rear brakes less effective, and the front brakes more effective in terms of total braking force. This is the reason all brakes are forward biassed.
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