The Mark Ortiz Automotive


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to the Motorsports Community

May/June 2005





Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by    e-mail to: Readers are invited to subscribe to this newsletter by e-mail. Just     e-mail me and request to be added to the list.





My question is regarding left side weight percentage on oval track cars, specifically dirt Late Models.  I have heard it generally stated that more left side is better in all situations, and I see a lot of paved track classes have limits on left side percentage.  I understand the concept of load transfer and equal tire loading in steady state cornering.


My question is about the point of diminishing returns.  My understanding would be that as grip decreases or banking increases, left side weight should be reduced to keep the left side tires from being more heavily loaded than the right sides.  Is this a correct assumption, or have I missed something?


In theory, yes it is possible to have too much left percentage, and to have the left tires more heavily loaded than the right tires even at the limit of adhesion in steady-state cornering.  In almost all cases, however, practical constraints or rules will stop us short of that point.


We can also have too much left percentage for the tire package short of that point, if the left side tires are smaller than the rights, or if the lefts are inflated to a much lower pressure than the rights.


Or, we might conceivably want more than 50% left dynamically, if the left tires are about as big as the rights, and we have a rule requiring a hard tread compound on one or both of the rights but not on the lefts.


Let’s consider a simple, if not very typical, case.  Suppose we have a car with a one foot c.g. height, a six foot track width, and  identical right and left tires.  Suppose that the overall coefficient of friction is 1.00.  That would be about what we’d get from sticky street-legal radials.  For this car to have 50% left dynamically at the 1.00g lateral acceleration that those tires will theoretically sustain, it would need 66.7% left statically.  That’s a wider, lower car than most, on tires with less grip than racing slicks.




If the same car is fitted with racing slicks that have a coefficient of friction of 1.30, the static left percentage needed to have 50% left dynamically increases to 71.7%.


If the car has a wing that acts equally on the right and left tires, lateral acceleration increases and the desired static left percentage goes up still more.


What happens if we put the car on a banking?  It’s a bit surprising.  If the coefficient of friction stayed the same, the ratio of car-horizontal (y-axis, per SAE conventions) force to car-vertical (z-axis) force would be unchanged, although all forces would increase.  This assumes the car is at the limit of adhesion both with and without the banking, not at an identical y-axis acceleration or an identical earth-horizontal acceleration.


However, due to the same tire load sensitivity that makes us want equal loading, on the banking the coefficient of friction will diminish, so the questioner’s intuition is correct after all, and the optimum static left percentage will decrease.


In an earlier newsletter dealing with this question, I noted that if we do get to the point where left percentage is excessive for conditions, wedge or diagonal percentage adjustments will work backwards, and so will roll resistance adjustments.  After that, a reader wrote in and said he had encountered this, with a go-kart on a very steeply banked dirt track.


Upon further discussion, it came to light that the kart had a much smaller tire on the left rear than on the right rear.  This not only affected the optimum load distribution for the rear wheel pair, it also meant the kart had a lot of tire stagger.  More load on the left rear increased the stagger-induced yaw moment on the kart, also causing more diagonal percentage to loosen the vehicle (add oversteer), contrary to what one might expect.  This effect can easily occur in any car with a locked or partially locking rear end.  This in turn affects our ability to infer whether left percentage is excessive, purely by noting how the car responds to adjustments.


I have also noted in earlier discussions that large left percentage makes a car tend to turn right under braking and turn left under power.  This tightens the car (adds understeer) during entry and loosens it (adds oversteer) during exit.  There are ways to counter this tendency with suspension design and tuning, but sometimes these are not legal, or the team doesn’t understand them.  In such cases, the car may well turn faster laps with less than optimal left percentage, even though it is slower in steady-state cornering.


These complexities can muddy the waters when tuning an actual car, but it is still fundamentally true that more left percentage is almost always better, provided we are able to work with the full package of consequences.