Having seen a couple of threads where some confusing information has been given about load transfer under braking and cornering, I thought I might have a go at clarifying the subject.
Firstly, I want to restrict the post to steady state behaviour - by which, I mean once the car is settled into the corner, or has been braking for some time, and the car body is no longer continuing to roll or pitch any further. Considering the full response including transients is much too dull for a leisure time activity!
Tyre load transfer under braking is *solely* due to the fact that the tyre force occurs at ground level and the vehicles centre of gravity is above ground level. (This is the vital idea people tend to get wrong)
A pitching moment or torque is created by the braking force multiplied by the height of the centre of gravity - this is entirely balanced by the load transfer multiplied by the wheelbase. Whether the braking is done by the front or rear wheels does *not* affect the amount of load transfer.
Note here, I'm talking about how much extra vertical force is pushing the front wheels down into the road, not how far the vehicle's body pitches - that's a different matter.
The response of the vehicle body to this weight transfer, i.e. how far the car pitches is proportional to the weight transfer, the pitch angle, while governed by the stiffness of the suspension, may be modified by anti-dive and anti-squat suspension geometry, so that the pitch angle under braking can be related to how much braking occurs on the front compared to the rear.
Exactly the same situation occurs in cornering - with one extra significant complication.
The *total* amount load transfer across the car is governed by the height of the centre of gravity, and the track width of the car, together with the car's mass and lateral acceleration, that's all there is to it.
The amount of roll angle is governed by the roll stiffness of the car. But, this may be modified, both by suspension geometry, and by devices like anti-roll bars.
Here's the complication - in roll, the weight doesn't need to be transferred equally front and rear. As an aid to picturing this, consider chopping a hypothetical car into two functioning halves, left and right, only joining them with a ball joint type bearing at the centre of gravity. If you were to drive this car in a straight line and then brake, the two halves of the car would still move as one (you have mirror symmetry about the cut). If you took another car, and replaced the cut along the length of the car with a cut across the car with the same type of ball joint at the centre of gravity and try to go round a corner, you would find that either the front of the car, or the rear would roll further - depending which end had the stiffer suspension in roll.
An anti roll bar attracts more vertical load. For example, consider a car with a 50:50 weight distribution and a stiff front anti roll bar (or a suspension geometry naturally stiff in roll). During a corner, the anti roll bar will make the front outer wheel carry more than 50% of the load transferred from side to side.
For a given lateral or longitudinal acceleration, all you can do to reduce load transfer is to reduce the height of the centre of gravity, reduce the total mass, or increase the track or wheelbase respectively.
Number_Cruncher
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Whether the braking is done by the front or rear wheels does *not* affect the amount of load transfer.
Whether the braking is done by the front or rear wheels does however effect the amount of braking force that can be applied therefore it has an effect on the actual amount of load transfer as opposed to the theoretical amount of load transfer for a given braking force.
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Im not sure I agree with that. Why should it not be possible to apply the same brake force to the rear as the front ?
Isnt bias braking more about stability and control?
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Sorry - You must mean that because of load xfr, less weight on the rear hence less friction available ?
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To amplify a little on Cheddar's reply;
For a given amount of deceleration (or a given amount of total braking force), the amount of weight transfer is not at all dependent upon whether it is produced at the front or rear - it happens purely because the centre of gravity is some distance above the road surface.
Cheddar is quite right to point out that the front wheels become more heavily loaded and can produce more braking force than the rear before the wheels loose traction.
The equivalent in cornering is that the outside wheels are more heavily loaded than the inside. For passenger cars, an onerous load case for carrying out stress analysis on the front suspension was a combined braking and cornering load - if I had a better memory, I would be able to remember the values in 'g' for each direction.
Number_Cruncher
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'For a given lateral or longitudinal acceleration, all you can do to reduce load transfer is to reduce the height of the centre of gravity, reduce the total mass, or increase the track or wheelbase respectively.'
Makes me wonder if manufacturers have forgotten this,there seems to be an incresing number of tall,narrow wheelbased cars about.One,cannot think of the name,would be Postman Pat's van if it was red.
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