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Race Car Weight Transfer and Handling Examples for Suspension Set Up, Development, Tuning and Testing.

We will start with the static corner weights for the car, as measured on the scales.  Next we'll put the car into a corner, add braking and then add acceleration.  The weights are not calculated for any particular car, but are representative.  Note that all weight transfer during movements of the race car end up as vertical load on the tyres.  I hope to demonstrate how we can use the concept of wedge, and ask that you consider my conclusions at the bottom of the page.

        

   Static Corner Weights

Front weight is 60% of total weight. Therefore CG
is located at 60% of the wheel base distance from
the rear axle. In roll, total weight transferred
depends on height of CG, track width of the car,
and the total cornering force generated at the
tyres. The proportion of total weight transferred
at each end of the car is determined by relative
roll stiffness of front and rear wheel pairs.
wheeload.gif (1633 bytes) Front wheeload.gif (1633 bytes)
600 lbs         600lbs
          CG

400lbs        400lbs    

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   LH Turn - Neutral, No Wedge

Here, 300 lbs is transferred from left to right.                                 35%
Front and rear roll stiffnesses are so arranged that
weight is transferred in proportion to static
weight. Wedge is defined as greater inside
percentage at the rear than at the front. Inside
percentages are the same front and rear. So, as
expected, the car is not wedged.                                                35%
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420 lbs        780 lbs

280 lbs        520 lbs

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  LH Turn - New Stiffer Front Roll Bar

                                                                                               33.3%  

We are still transferring 300 lbs total weight.  But                               our new front roll bar stiffens the front roll
resistance.  Weight transfer is 200 lbs at the
front, and 100 lbs at the rear. Note the new inside
percentages. The car is now wedged (understeer).
Note that in every example, the sum of all weights
on the tyres is equal to the weight of the car - 2000 lbs.              37.5%

wheeload.gif (1633 bytes) Front wheeload.gif (1633 bytes)
400 lbs        800 lbs

300 lbs        500 lbs

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   LH Turn Plus Braking

With braking, we have added 300 lb of rear to
front weight transfer.  LH and RH wheel pairs have                     40.7%
the same pitch resistance, so 150 lbs of weight
transfers equally each side.  This weight transfer
is added to the LH turn with stiffer roll bar
example.  Note that we have de-wedged the car
(oversteer), even though the lateral load transfer
alone would wedge the car.  The car is now                                30%
unstable, and we can readily appreciate the driver
skill required for correct trail braking.
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550 lbs       800 lbs

150 lbs       350 lbs

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   LH Turn Plus Acceleration

Now we see 300 lbs of weight transfer going front
to rear.  Again the weight is added to the lateral                         27.7%
load transfer in the stiffer roll bar example.  The
car is now wedged like crazy (understeer). This
understeer is good because it offsets the
oversteer which rises sharply as the rear tyres are
used up in forward thrust, leaving less traction for                       40.9%
cornering.
wheeload.gif (1633 bytes) Front wheeload.gif (1633 bytes)
250 lbs       650 lbs

450 lbs        650 lbs

wheeload.gif (1633 bytes)       wheeload.gif (1633 bytes)

 

Conclusions

1. The vertical loads on the tyres change dramatically as we drive the race car.  So weight transfer
considerations should be our main weapon in balancing and setting up a race car.   However, if
the wheels lock, we hit oil, or the tyres leave the road over a bumpy section of track, all this goes
out the window.

2.  For steady state cornering it is relatively simple.   We would only have to be concerned with
relative roll stiffness between the front and the rear of the race car. (Add aerodynamic effects, if applicable.)

3.  But steady state cornering is only a very small fraction of lap time.  In some corners it is effectively nil.  Getting into and out of the corners is the main game.  Therefore my over simplified
but interesting combined roll and pitch examples apply.  There is very significant wedging and
de-wedging effects going on here.  (Contributions to understeer and oversteer.)

4.  We havn't done a warp (or twist) with cornering example because I think it can be considered
as a pure diagonal weight transfer between two wheels.  The wheels on the other diagonal remain
stationary - do not change weight.  (This is the assumption used by Neil Roberts in his shock tuning article.)  There is no other wheel pair to allocate a part of the weight transfer to.  So all
the weight just goes off one and on to another.  To work out what's happened to wedge, just
consider whether the weight movement increases or decreases inside rear weight percentage
compared to the front.  In fact, our "LH Turn With Acceleration" example has got nearly all the weight transfer going from the LH front to the RH rear.   Weights on the other diagonal are not
changed much from the static load.  So this example is really diagonal load transfer.   But we need
to break it down into roll and pitch so we can tune it.

5.  These examples are only snap shots in time.   What about the transients - accelerating and
de-celerating the various masses to cause the load transfers?  How important are they?  Transients are what happen most of the time, and are the reason why shock absorbers have been the focus of race car development in the 1990's.  See shock tuning for a discussion on this.

6. They say the driver feels intuitively the various net accelerations as they affect tyre loading, and
this sounds about right to me. (He also feels front tyre load through the steering wheel, and perhaps yaw angle to a lesser extent).  So we had better get the transients right so he can do his job.  Carroll Smith talks about making the race car responses linear in all his books..

7.  Consider what a large affect driver inputs have to the transients.  He's got to build the loads and
let them go constantly.  It explains why a good driver can sometimes drive around a handling problem and still be quick.  On the other hand, a driver and car set up may be so much in the groove that a lap record time may come in the course of a race, without balls out driving,no sweat at all.

                           

 

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