FIRST PRINCIPLES THE CONSULTANT
Why use anti-roll bars?
Question In the past you have talked a lot about anti-roll bars and how they affect
mechanical traction. My question though is why do we use anti-roll bars (sway bars) at all? What effect will removing front and rear bars have on the behaviour of the car? Sure, the car will roll on the corners, but what about traction on the bends and overall performance?
Answer I would definitely not advise
Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. Here Mark answers your chassis set-up and handling queries. If you have a question to put to him Email: markortizauto@
windstream.net Tel: +1 704-933-8876 Write: Mark Otiz 155 Wankel Drive, Kannapolis NC 28083-8200, USA
taking the anti-roll bars, or sway bars in US parlance, off an existing car, and then taking it for a lap at speed, as on most modern cars the front / rear roll stiffness distribution is tuned with the bars, and the car will not have the same oversteer / understeer balance if the bars are removed. It is quite true that for vehicles
Dive planes added a useful increment of front downforce, albeit not very efficiently, and knocked a little downforce off the rear
with the sprung mass c of g heights commonly encountered in cars, roll has only a small effect on overall load transfer. It is also true that we can achieve any roll gradient (how much the car rolls per unit of lateral acceleration) using only the ride springs. And finally, it is true that we can get any front / rear distribution
of load transfer using only the ride springs. So why not just do that? After all, many cars have been built with no anti-roll bars, but they basically fall into three categories: ones that roll a lot, ones that ride really hard and don’t absorb bumps well and ones with beam axles at both ends. Anti-roll bars and other
interconnective springing devices offer the following advantages; they let us independently control wheel rates in the four modes of suspension motion – roll, pitch, heave and warp. They let us achieve better control of roll for a given ride quality and they afford us a way to readily adjust front / rear load transfer distribution, with a minimum of effect on other things. So what’s wrong with
just letting the car roll? With independent suspension, the wheels inevitably lean with the sprung mass to some degree anyway. We can compensate for this by using geometry that makes camber go toward negative in compression in ride, but we can’t get 100 per cent camber recovery in roll without excessive camber change in ride. Generally, we have to accept 50 per cent camber recovery or less. So roll hurts camber, and consequently reduces grip with any independent suspension. Roll also uses up suspension
travel and ground clearance and can disrupt the under-car aerodynamics enough to cause problems (these disadvantages occur even with beam axles). Roll does increase lateral load
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www.racecarengineering.com • August 2011 57
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