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

Offline DoctorBeaver

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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 »


 

Offline daveshorts

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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
 

Offline ukmicky

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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. :-X
« Last Edit: 25/04/2007 18:09:27 by ukmicky »
 

Offline DoctorBeaver

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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.
 

Offline daveshorts

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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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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.

I had not answered this because I thought Dave had given as good an answer as anybody could to this question.

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).
 

Offline DoctorBeaver

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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?
 

Offline daveshorts

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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
 

Offline ukmicky

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:)
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. ;D  ;D
« Last Edit: 02/05/2007 00:44:39 by ukmicky »
 

Offline DoctorBeaver

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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.
 

Offline ukmicky

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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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