Where do weight transfer/acceleration forces go?
Where do weight transfer/acceleration forces go?
(OP)
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?





RE: Where do weight transfer/acceleration forces go?
Cheers
Greg Locock
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RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
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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RE: Where do weight transfer/acceleration forces go?
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
RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
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.
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RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
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RE: Where do weight transfer/acceleration forces go?
"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.
RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
Kevin
RE: Where do weight transfer/acceleration forces go?
Even accounting for shallow staging, the sixty foot times indicate that modern dragster tires achieve coefficients of "2" or more.
RE: Where do weight transfer/acceleration forces go?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
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RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
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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RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
Not in this neck of the woods!
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Greg Locock
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RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
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.
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RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
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
RE: Where do weight transfer/acceleration forces go?
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.
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RE: Where do weight transfer/acceleration forces go?
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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RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
Vertical force? wouldn't change
Horizontal force? increases.
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Greg Locock
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RE: Where do weight transfer/acceleration forces go?
What you outline would apply well to a tethered unicycle.
Where does the increased horizontal force go?
RE: Where do weight transfer/acceleration forces go?
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Greg Locock
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RE: Where do weight transfer/acceleration forces go?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Where do weight transfer/acceleration forces go?
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
RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
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.
RE: Where do weight transfer/acceleration forces go?
If the unicycle was doing the exact same thing on a banked purpendicular surface, there would be 100% of it's weight plus the lateral, which has now become centrifigul, force. But you tilt the ground back just a few degrees and whala! The forces have been completely separated again.
I think not.
RE: Where do weight transfer/acceleration forces go?
if you look at it that way, then in order for your unicyclist to NOT slide off the track into the trees, he will have to lower the total input force of the tire.
RE: Where do weight transfer/acceleration forces go?
So lateral force alone can increase traction?
I still think not.
RE: Where do weight transfer/acceleration forces go?
In corners you need to deal with centripetal and centrifugal acceleration . . . those are acceleration vectors normal (perpendicular) and tangential (in the same line as) to the direction of your travel. And yes, these are lateral forces. Remember the experiment of spinning with a bucket of water? These forces are also employed by figure skaters to spin faster or slower.
One other simple issue: Newton did not say that F = ma.
He said that the force was equal to the time rate of change of momentum (mv). Simplified, F = d(mv)/dt. When the chain rule is applied and mass is constant, then F = ma. For a car, mass can be considered constant, but in a curve, force vectors are no longer rectilinear, and acceleration becomes a complicated ODE (dv/dt).
Civil Engineers understand this in making either wide, sweeping turns or banking tighter turns.
Thus, the lateral or centrifugal acceleration is actually pulling you towards the center of the turn, so you don't go flying off the curve as long as you are under the acceleration that balances the vectors.
RE: Where do weight transfer/acceleration forces go?
An upward accelerating helicopter can have a pull on its rotor shaft that is much greater than the helicopter’s total static weight. It basically only pulls one direction. Why is a unicycle, car, or anything else, so different?
Virtually every force within the unicycle is on a single line with a single contact patch. How can it apply two distinct pressures going in two different directions against the ground?
I guess the question is: are lateral and vertical forces separated for teaching and analysis, or are they really separate?
RE: Where do weight transfer/acceleration forces go?
centripetal = seeking the center
centrifugal = fleeing the center
(meaning colinear, but opposite in sense)
RE: Where do weight transfer/acceleration forces go?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Where do weight transfer/acceleration forces go?
With regards to the combination of horizontal and vertical normal forces. I think that's why a negatively dynamically cambered tire, even more so than necessary to keep the contact area flat, provides more traction, since you're taking advantage of the horizontal component force to add to the compressing of the local points of contact. Thereby increasing the dynamic "coefficient of friction." True vertical load however will always sum to the weight, unless as previously discussed, c.g. is accel'ing up or deccel'ing down.
RE: Where do weight transfer/acceleration forces go?
Except when it isn't.
For instance, if you have forward brake bias, then braking in a turn will typically cause understeer.
"Take any RWD car, that's understeering already, an application of gas will worsen the situation."
Wrong. A gentle poke of the throttle on the production car I regularly drive will bring the back end out just a touch, very handy when cornering on gravel roads.
Thank you for your contribution.
Cheers
Greg Locock
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RE: Where do weight transfer/acceleration forces go?
You're talking about situations where the tires are way beyond optimum slip angles. For subtle, mid corner adjustments on the track, if you want to tighten up just a little, you let off or apply just a tad of braking, to lift the rear end induce slight rotation.
Pilun
RE: Where do weight transfer/acceleration forces go?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
I don't know what kind of cars they are working with, but those are far from real on-road racing.
RE: Where do weight transfer/acceleration forces go?
Play nice and objectively look through your arguments.
The original post makes no mention of race cars. This point is not mentioned until about the 30th post where you introduce it when confronted with some facts that contradict you statements.
Your statements by admission only apply to very specific circumstances introduced to the argument by you.
You only introduced these very special circumstances when your argument was contradicted, you then try to insist that the circumstances you introduced are the only parameters to consider and overrule the OP.
If you focus more on building knowledge rather than defending face, we might all learn and progress.
As far as automotive industry goes, I am only a hot rodder with I believe some flare for mechanical design. I have been involved in building and tuning a number of very successful race cars and boats.
Greg is a professional automotive engineer, currently employed by a very large automotive company that builds some very good handling large everyday passenger sedans but has a history in suspension design with some of the industries icons re excellent handling road going sports cars, sporting large and luxury sedans and race cars. I would not dismiss his view without considerable thought.
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RE: Where do weight transfer/acceleration forces go?
They are not in extreme conditions, they are in the linear handling range.
However, to be fair " For subtle, mid corner adjustments on the track, if you want to tighten up just a little, you let off or apply just a tad of braking, to lift the rear end induce slight rotation"
Is fine, and typical behaviour.
But I have driven rear wheel cars with throttle-on understeer, and some with throttle off-understeer, and some with throttle on oversteer and some with throttle off oversteer (albeit probably nearer the limit). Some cars do both. I have driven front wheel drive cars with throttle on understeer (most Audis), and some with throttle off understeer (VW Golf Mk I), some cars do both. I have driven front wheel drive cars with throttle off oversteer (Mini).
So, I'd be very wary of generalizing.
With brakes I have certainly had brake induced spins, but most modern cars are set up to give brake induced understeer.
But on the car I drive most often, a slight addition of throttle, even when not driving hard, will move the tail out when cornering on a gravel road.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Where do weight transfer/acceleration forces go?
Since you woke things up...
Questions still remain on the simple act of going straigt ahead.
On lateral and vertical forces, the “boss” says: “the forces are one.”
This means there is only one dynamic force acting upon the ground at whatever angle. If the front wheel(s) of the vehicle is/are off the ground while traveling forward, then there is only one force acting on one wheel of the 2 wheeled vehicle, or pair of wheels of the 4 wheeled vehicle.
The "boss" also said: "moments of inertia will be irrelevant in steady state cornering"
This of course would apply to steady state acceleration in any direction on a vehicle with any number of wheels.
Again, during steady state acceleration, the two above conditions are constant.
For clarity; some 2 or 4 wheeled race vehicles can initiate and maintain a wheelie without involving inertia. With pliable suspension, the same type of vehicle can wheelie without raising the height of CG above the ground. Obviously this represents a wheelie induced by torque alone. Proof of these conditions are shown in low level wheelies maintained for several hundred feet, accompanied with no letup of the throttle or rate of acceleration. If CG becomes too high or unstable, for whatever reason, acceleration will cease. A wheelie from inertia or balance cannot represent steady state acceleration. How do all the wheelies relate to you and me? All vehicle acceleration falls somewhere between sitting still and maintaining a low level wheelie.
All vehicle acceleration involves weight transfer. In a straight line, all acceleration is steady state or more. Tire contact patch is no longer perpendicular to the chassis centerline. CG is not relavant.
Questions remain:
What is the angle of the one non-vertical force?
What is the magnitude of the force?
RE: Where do weight transfer/acceleration forces go?
----> Ok on the topic of the unicycle. No, the lateral and longitudinal are not seperated and applied at 2 different magical tyre contact patches. For analysis it is split into lateral and longitudinal co-ordinate system relative to the ground so to make calculation of the actual force witnessed by the centre of mass of the item easier.
As you said earlier the neck witnesses a higher compressive force. This is perfectly true. But this is because the neck axis isnt in the vertical plane anymore, as its on the angle the neck also sees a fraction of the lateral force. So the ground itself does see more load from the tyre, BUT its exactly the same force in the vertical direction.
RE: Where do weight transfer/acceleration forces go?
How can the neck witnesses a higher compressive force, but not other components such as the butt, axle, wheel, or tire?
A fraction of the lateral force? There is only one force and it's fully experienced by the neck and everything else.
Perhaps I missed it if someone mentioned it, but this seems to point to something that may make both sides look a little more sensible.
As far as a unicycle, it seems that downward vertical force from weight must decrease as acceleration increases. In other words, an acceleration that requires the unicycle to hold a 45* tilt may apply less, perhaps half, the vertical pressure from weight it had at rest. No?
While vertical pressure from weight goes down, vertical pressure from G force goes up. It's balanced and is neither fully vertical or horizontal. If you accelerate in a big bowel, the entire force would eventually become "lateral".
Of course this leads to questions about accelerating past that 45 degree mark, and adding long weighted arms to the mix. Obviously the G-force can become considerable, multiples of the original weight.
RE: Where do weight transfer/acceleration forces go?
RE: Where do weight transfer/acceleration forces go?
If we set the dragrace 4-linkage systm up in such way that it separate the rear axle from the chassis and thereby creating a lifting force at the chassis, we should for a period of time exceed the total car weight at the rear wheel. Front wheel is of ground slightley. The duration may be adjusted by shock absorber setting.
As the moost important for et times is the first feet of acceleration, we can pay the price of less rear wheel weight as the car retain to its "normal" height longer up the strip.
Seen over the total strip the average weight on ground is the same, as I understand the situation.
Goran Malmberg