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?

 

[GregLocock]

Instantaneously, yes, the total vertical force exerted by a non-aero car can exceed its weight. On average, on a flat road, it will not.



Cheers

Greg Locock

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

 

[BillyShope]

As for how much:

With a RWD car, the front wheels will eventually unload as the front of the car rises. This, with the increased loading at the rear wheels, will provide the couple necessary to balance that couple generated by the tractive force at the rear tire patches and the inertial force acting at the CG. But, the only way the fronts can unload is with an extension of the suspension springs. So, as Greg said, the instantaneous effect is an increase in total loading as the suspension springs begin to extend. With high traction tires (drag slicks), this can easily be an increase of half again the total car weight. But, again, it happens over so short a time interval that it has little effect on performance.

 

[Warpspeed]

A bit like standing on the bathroom scales and stamping your foot.

The scale pointer may fluctuate a bit higher, but only for a moment.

Where do the forces go ? Into the springs, then out of the springs back into the sprung mass, with some slight heating of the oil in the dampers.

 

[Fabrico]

So, the forces go away even while the car is still accelerating at a similar rate? In a drag racing scenario, if a front engined car did a low altitude wheelie for 200 feet, wouldn’t the full effect last for the entire 200 feet? An F1 car can experience over 5g’s while stopping from almost any speed. What percentage of this 5g's becomes downward force? Is this also instantaneous, or does it last until the car is stopped?

 

[GregLocock]

Ignoring aero renders any discussion of F1 cars irrleveant

Ignoring aero, if the car is exerting a downward force in excess of its weight then it will accelerate upwards.

Since cars do not finish 200 ft runs several feet in the air (usually) we can deduce that on average they exert their own weight on the ground, on a flat road.

Cheers

Greg Locock

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[Fabrico]

Ouch!

BillyShope: Thank you for some intellegent thought on this. IMHO there is a lot more to it and it is much more prevalent than most think.

Cheers

 

[Warpspeed]

There is a great deal of nonsense prevalent about weight transfer in drag racing, and what actually happens.

If the car does the whole quarter mile with the front wheels completely off the ground, all that means is 100% of the vehicle weight is then on the rear wheels. Simple.

It also makes no difference if the springs are soft or hard, or how much it squats. If all of the total vehicle weight is on the rear wheels that is all there is.

 

[BillyShope]

I would only add, Warpspeed, that, if the fronts lift while the rear wheels are unequally loaded, rear wheel loadings will continue to oscillate until the limited damping in the tires settles things down. I liken it to a barefoot boy on a hot sidewalk. This is why you'll see cars dart to the left or right after the fronts lift (at the dragstrip).

 

[patprimmer]

You get greater than the weight of the car as the centre of gravity lifts due to wheel stands or due to very aggressive anti squat actually causing the car to rise. This can only happen while the car is still rising. It must occur for only a very short time as already explained by Greg.

It is very difficult to imagine how one might continue to increase traction from downward force while continuing to raise the centre of gravity beyond the length of the car.

Regards

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[Warpspeed]

Suspension tuning and geometry can sure effect how the tyres react with the track, and vehicle stability, but that is not the same as making the tyres push down with more weight than the vehicle has.

 

[patprimmer]

With a live rear axle, the rear suspension geometry can only affect bump steer and weight on the wheels. If the suspension allows the CG to raise, it increases weight due to the equal and opposite reaction to the lifting of the weight of the car. The limitations are how high you can go before becoming unstable. Once your suspension tops out or the drive thrust drops off and therefore the anti squat falls off or the front wheels stop climbing, you lose at least everything you gained, but with some luck, aero is helping at that stage.

Regards

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[BillyShope]

Since the front of a RWD car will ALWAYS rise, those following this thread might be interested in further posts in the "simulation info" thread. Since suspension travel at the rear is either limited or non-existent (anti-squat), "tricks" that can affect rear wheel loading can sometimes be best accomplished at the front of the car.

"Extra" loading can only appear as the front of the car is lifting. And, this occurs so quickly that Greg is justified in calling it "instantaneous." So long as the car continues to accelerate, however, there will continue to be additional loading on the rear wheels, but this will be at the expense of front wheel loading; the sum remains constant.

 

[jabonet]

You are asking for anti-squat rear suspension, which can actually lift the rear of the car for a few yard, and put quite some extra load on the back wheels at the cost of lifting the CG. in F1 cars, there is practically no suspension whatsoever, and most of the force is aerodynamic.

 

[c2s]

Hmmmm ... I think the item not addressed are the fundamentals of tire mechanics. A tire can generate a longitudinal force greater than it's vertical force (Newton is frowning) irrespective of weight transfer and aero. As I remember, one of the members of this group is a tire guy and could probably elaborate on it.
Kevin

 

[BillyShope]

There is no upper limit to the coefficient of friction. Go to your Mark's Handbook and you'll find coefficients greater than unity for certain material combinations. When I was "on the board," a little parlor trick was to take a Pink Pearl eraser, place it on a plastic triangle, and slowly tilt the triangle until the angle exceeded 45 degrees. An angle of 45 degrees corresponds to a coefficient of unity, but angles far beyond 45 degrees were often achieved.

Even accounting for shallow staging, the sixty foot times indicate that modern dragster tires achieve coefficients of "2" or more.

 

[GregLocock]

I measured the friction coefficient (Fx/Fz) for a standard production tire on smooth asphalt, during an ABS stop. The front tires were at 1.2 for the entire 10 seconds of the stop.

Cheers

Greg Locock

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

 

[BillyShope]

10 SECONDS??!!! I didn't think they had enough paved roadway in Australia to go that fast

 

[patprimmer]

We have a few Top Fuel cars running mid 4s, Our door car runs high 7s

Regards

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[Fabrico]

I did'nt know Greg ever went that fast! J/K!

As far as this discussion goes, testing G's with anti-lock brakes is like drag racing with snow chains.

The only thing instantaneous is the assumption that additional downward force is limited to weight transfer or the CG suddenly trying to circle around the rear axle or tire contact patch. This is not correct. The suppositions would apply well to acceleration of a front wheel drive car or stopping with rear brakes only.