Where do weight transfer/acceleration forces go? 1

[Fabrico]

Without consideration of aerodynamics, does a wheel driven vehicle become “heavier” against the ground while accelerating? Can a 3,000 pound car put more than 3,000 pounds of downward force against the pavement? How much downward force can be produced?

 

[BillyShope]

Hey, guys, I was referring to the 10 seconds of 1.2 friction. If the rear tires were doing anything at all, that means close to 200 miles per hour.

 

[GregLocock]

We've got one road that is straight for about 200 miles, and no speed limit.

You are right though, it must have been less than 10 seconds, it would have been from 70-80 mph.



Cheers

Greg Locock

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[mhejjas]

Two more thoughts on this:

1. Longitudinal weight transfer can be divided into two categories: let's call them "transient" and "consolidated" weight transfer.
The transient phase occurs when applying throttle or hitting brakes. In this phase it is possible that overall downforce on wheels exceeds the vehicle's weight, but this can occur only when CG is accelerating upwards or decelerating downwards.
"Consolidated" weight transfer means that the vehicle's longitudinal roll is stopped - accelerating/braking forces are in balance with suspension spring forces.

2. It is important that longitudinal weight transfer is mostly not a real transfer of the mass. The difference between gravity forces and the vectorial sum of gravity forces and opposites of accelerating/braking forces is responsiple for this effect.
This also means that mass is almost there where it was before, so the necessary lateral forces on the axles change much less than wheel downforces as a result of weight transfer. That's why a braking vehicle generally tends to oversteer, while an accelerating one usually tends to understeer.

 

[GregLocock]

"That's why a braking vehicle generally tends to oversteer, while an accelerating one usually tends to understeer."

Not in this neck of the woods!

Cheers

Greg Locock

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[mhejjas]

? I do not understand

 

[GregLocock]

Our cars are set up to understeer under braking, and demonstrably tend to oversteer when accelerating.

Cheers

Greg Locock

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[mhejjas]

Of course, there are a lot of other factors that affect (or even completely change) the car's balance, such as brake balance, type of transmission, engine power, etc.
But generally it is true, that a longitudinal weight transfer from front to rear causes major reduce in front traction, while only minor reduce in the necessary lateral force on the front axle.
And a longitudinal weight transfer from rear to front causes major reduce in rear traction, while only minor reduce in the necessary lateral force on the rear axle.

 

[patprimmer]

mhejjas

I suggest you find a section of road with safe run off areas and try driving a few cars around curves at near the limits of adhesion. Try changing from deceleration to acceleration mid turn and feel the effect. It will be very different to what you describe.

Regards

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[BillyShope]

Wait a minute, guys! Before we jump all over mhejjas, let's consider some possibilities. Yes, we're all familiar with "throttle induced oversteer" with a RWD car, but, can we say that, with smaller values of X acceleration, the capacity of the rear tires will always be diminished by the appearance of the traction vector MORE THAN the loss in capacity at the front?

 

[mhejjas]

Ridiculous. No one has driven a real racecar?
Then try thinking and learning at least the basic theoretics how a racecar behaves.
E.g. It is a well-known racing driving technique to increase front traction just by kicking the brakes before turning in. The car doesn't really loose speed, but the kick results in a weight transfer towards the front end and gives the car more steering.

"Competition car suspension design, construction, tuning" by Allan Staniforth would be a good starting point for You.
(Not for You, BillyShope

Cheers

 

[patprimmer]

mhejjas

I don't think it is as simple as you are describing, nor universal

Quick jab of brake certainly helps turn in, hard prolonged brake application decreases turn in to the extent a car can plough straight ahead.

Acceleration out of corner varies with front or rear wheel drive. some acceleration can promote under steer, but aggressive acceleration can spin wheels and cause very different effects depending on which end drives.


Also toe camber and caster changes due to roll, dive, squat or compliance can have a significant effect.


Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

mhejjas

If you are in a turn and apply the brakes gently, our corporate requirement is that the car should understeer, and it is part of my job to make sure they do.

If you are in a turn and apply the brakes heavily, our corporate requirement is that the car should understeer

I don't think I need to discuss power-oversteer, that is obvious from every pub car park in the country.

I should add that understeer is precisely defined by the SAE and bears little resemblance to your description. And yes, in this case SAE is the authority.





Cheers

Greg Locock

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[Fabrico]

OK, let's keep it simple. Picture a unicycle...

If you are going around a turn on a unicycle at a fairly high rate of speed (picture it, don't do it!) Would it not place more weight against the ground than when it just goes straight? Wouldn't the additional force last as long as the turn. For a given turn radius; shouldn't the force increase with speed until the tire rolls off the rim.

 

[GregLocock]

Weight? Wouldn't change

Vertical force? wouldn't change

Horizontal force? increases.



Cheers

Greg Locock

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[Fabrico]

I don't agree with that but, at least something was recognized as increasing

What you outline would apply well to a tethered unicycle.

Where does the increased horizontal force go?

 

[GregLocock]

it accelerates the mass of the rider and unicycle.

Cheers

Greg Locock

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[GregLocock]

and the other end of the force accelerates the Earth.

Cheers

Greg Locock

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[mhejjas]

Fabrico:
That's why the unicycle has to lean inside. The vectorial sum force of the unicycle's weight and lateral force has to be inline with the line connecting the ground contact point with CG. In other words, the horizontal force keeps the unicycle in the leaning angle - otherwise it would fall towards inside

 

[mhejjas]

GregLocock:
Are You working on passanger cars?
Racecars are designed to do fast laps not to keep alive panicked amateurs.

Serious: power-oversteer is one factor. weight transfer caused understeer is another one factor. and there are several other factors as well. The car's behaviour will be the sum of all factors.

 

[mhejjas]

Patprimmer:
This technique helps only in the turn-in phase, or when taking chicanes. Yes it's a transient only, but it is not rare that the car pushes on turn-in, while it has enough steering in mid-corner and out of turn phases.