FIRST PRINCIPLES THE CONSULTANT
According to Ortiz, the last place you would want an anti roll bar is on a Formula Vee, but in Europe they are a common feature, front and rear
transfer too, and there is a small lateral migration of the car’s c of g with roll, relative to the tyre contact patches. In addition to this, there can be an indirect increase in load transfer if roll forces us to run the car at a greater ride height to avoid the suspension bottoming out.
Transient manoeuvres So far, we have only discussed effects of
roll in steady-state cornering. In abrupt transient manoeuvres, greater roll displacements imply greater roll velocities and accelerations. Roll acceleration will add to lateral load transfer when
For road course competition, we want
ride stiffnesses and front / rear natural frequency relationships that will make the car ride well and take bumps well, and we need to balance these objectives against a need to keep roll within reasonable limits. For this, we will do well to get between 30 and 60 per cent of the overall elastic roll resistance from the bars, and the rest from the springs. For a pure racecar, we will generally be
more concerned about how low we can run the car without having the underside hit the track (much). We may also need to limit heave and pitch displacements to
“we will do well to get between 30 and 60% overall elastic roll resistence from the bars”
the direction of roll acceleration is into the turn. In taller vehicles, this added component of load transfer can sometimes be enough to make the difference between the vehicle staying upright or overturning, which explains why some SUVs, for example, will lift wheels or roll over in a lane change test, even though they will slide controllably in a skid pad test. Reducing roll displacements in such a vehicle will improve overturning resistance in abrupt manoeuvres.
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www.racecarengineering.com • August 2011
control under-car aerodynamics. Those constraints will dictate that we have the car very stiff in the heave and pitch modes, which means adequate roll stiffness will come almost automatically. Even then, however, we will generally run some anti-roll bar to provide fine (and quick) adjustment of roll resistance, including perhaps driver adjustability, and to allow steeply rising ride rate, or pitch and heave rate, via a third spring. Big bar / soft spring front-end set
ups have long been popular in American Stock Car racing, although this trend is fading. This approach makes sense here as soft springs let the front end compress due to banking and aero forces, while the bar maintains enough roll resistance to keep the car from being loose. The opposite exists at the rear of a Formula Vee car, where the rules require swing axle suspension, which has an extremely high roll centre and a tendency to jack up in cornering. Here, it works best to use springing that only works in ride, and a wheel rate of zero in roll. The last thing we’d want for this application is an anti- roll bar. Cars with beam axles at both ends
can have ample geometric anti-roll (high roll centres) at both ends, and can consequently have moderate roll gradients without anti-roll bars. Any beam axle suspension without an anti-roll bar has a smaller wheel rate in roll than in ride, because of the springs inevitably being inboard of the wheels. However, high roll centres carry some penalties of their own, even with beam axles, in the form of large lateral movement of the contact patches with respect to the sprung mass on one-wheel bumps. Beam axle suspensions generally perform better without anti-roll bars than independent suspensions do, but there is nothing wrong with using an anti- roll bar with a beam axle either.
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