# The Naked Scientists Forum

### Author Topic: Why does the front of a vehicle dip when braking and rise when accelerating?  (Read 12819 times)

#### DoctorBeaver

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##### Why does the front of a vehicle dip when braking and rise when accelerating?
« on: 25/04/2007 08:56:51 »
Why do the fronts of cars dip when you brake and rise when you accelerate?
« Last Edit: 03/05/2007 08:21:33 by chris »

#### daveshorts

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #1 on: 25/04/2007 09:57:37 »
it is because the force applied is on the tyres but the centre of mass of the car is somewhere nearer the centre

So if you are breaking you are moving forwards and have a force backwards on the wheels twisting the car forwards

And if you are accelerating the wheels are being pushed forward by friction so twisting the car backwards

#### ukmicky

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #2 on: 25/04/2007 14:28:34 »
Also isn't it due to the breaking and accelerating force's being  applied to the wheels/drivetrain of the car and not the body. And due to the body being connected to the drive train via the springs and shocks the body of the car feels a delay to the force due to play built into the suspension system and like when accelerating gets momentarily left behind.

Or is that the same as you said.
« Last Edit: 25/04/2007 18:09:27 by ukmicky »

#### DoctorBeaver

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #3 on: 30/04/2007 13:34:05 »
Micky - I'm sure the suspension adds to the effect but I don't see that it's the cause. If the suspension at the front & rear were exactly the same then what you've said should cause the whole body to sink. And what if you built a car without suspension?

I've got a sneaky suspicion it's to do with momentum and the centre of gravity but I'm not sure quite how.

#### daveshorts

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #4 on: 30/04/2007 13:53:13 »
yes, when you brake the force is applied at the bottom of the tyres backwards, your centre of mass is higher than this and the car wants to rotate about it's centre of mass. this means (as in the diagram above) it will rotate forwards.

When accelerating the force is forwards so the car is rotated so the back goes down.

#### another_someone

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #5 on: 30/04/2007 13:54:43 »
I've got a sneaky suspicion it's to do with momentum and the centre of gravity but I'm not sure quite how.

It is.

It is about the transmission of force (both in acceleration and braking) being at that point where the wheels touch the ground, whereas this force has to overcome the inertia of the vehicle that is focused at the centre of gravity.

Since the transmission of force is below the centre of gravity, you then develop a rotational force caused by forces at one poiint acting opposite to forces at another point (effectively, it is as if you had a lever running from the ground upward, and you were pushing one end of the lever in the opposite direction to the other end of the lever, so the lever rotates).

#### DoctorBeaver

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #6 on: 01/05/2007 08:32:40 »
So what if the vehicle had large diameter wheels and the body was slung under the axles. Would the front lift when braking or still nosedive?

#### daveshorts

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #7 on: 01/05/2007 16:10:21 »
The car as a whole would try and rotate forwards, but you are right the body would now although still rotating forwards would rotate upwards

red arrows are forces on the body green and yellow the car as a whole

#### ukmicky

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##### Re: Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #8 on: 01/05/2007 23:56:43 »
:)
Micky - I'm sure the suspension adds to the effect but I don't see that it's the cause. If the suspension at the front & rear were exactly the same then what you've said should cause the whole body to sink. And what if you built a car without suspension?

I've got a sneaky suspicion it's to do with momentum and the centre of gravity but I'm not sure quite how.
Yes Doc but the question was "Why do the fronts of cars dip when you brake and rise when you accelerate?"

And the reason a car dips is due to the suspension and play built into it as i said, stiffen the suspension and remove the play and the car wouldn't dip and the vehicles kinetic energy would be tranfered to the brakes rather than the suspension leaving your question with no where to go . Change the suspension to a different version and instead of dipping it could be made to rise rise as you said.

OK that may not be the scientific answer and didn't mention the car body wanting to continue on its journey due to its unwillingness to stop with the drivetrain so it tends to try and rotate due to its kinetic energy but my answer is still a valid answer and one which any motor mechanic would be proud of so there.
« Last Edit: 02/05/2007 00:44:39 by ukmicky »

#### DoctorBeaver

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##### Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #9 on: 04/05/2007 10:32:34 »
Micky - if the vehicle was rigid, wouldn't the back lift off the ground if you braked hard? Surely, that indicates that the dipping is not suspension related although suspension may augment the effect.

#### ukmicky

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##### Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #10 on: 05/05/2007 15:47:59 »
Quote
Micky - if the vehicle was rigid, wouldn't the back lift off the ground if you braked hard? Surely, that indicates that the dipping is not suspension related although suspension may augment the effect.

Hi DOC

The brakes would overheat or lock up first. A Formula one car rapidly slowing down from 200mph to 10 doesn't tip over and its not due to downforce overwise when they break a rear wing they would have to limit there speeds on the straights on there way back to the pits.

Its a bit of both i suppose  there are really sort of 2 answers to the question because forward roll of a car can be eliminated with the right tyre shock absorber ,spring and brake setup. All the forces involved can be directed and transfered into heat at the shock absorbers, brakes or roadwheels preventing all forward roll or front lift/rear squat.
You will always have a weight transfer to either end of the car on acceleration or breaking but the effect of the car dipping at the front or squatting at the rear is caused by suspension travel.

And just to add something in case your wondering about dragsters and why do they have weelie bars its to prevent the front of the car changing places with the rear its called blowover and is due to air getting under the front wing forcing the front of the car up due to the speed that they accelerate through it.
« Last Edit: 05/05/2007 19:19:12 by ukmicky »

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##### Why does the front of a vehicle dip when braking and rise when accelerating?
« Reply #10 on: 05/05/2007 15:47:59 »