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TECHNOLOGY – CONSULTANT


they’d be the best, but I don’t have the visibility to confirm or dispute that. I do know that there is


lots of incorrect information published about anti-dive and related effects. I’ve seen one graphic recently that apparently has circulated quite a bit that purportedly shows geometry for 100 per cent anti-dive. It shows the control arm pivot axes in side view – the axes defined by the control arm attachment points to the tub – meeting at the sprung mass c of g in side view. This replicates illustrations that appear in a number of old chassis books, but it is incorrect in at least two ways. First, the side view geometric


properties depend on the actual side view projected control arms. These are the lines where the control arm planes intercept the wheel plane, not the control arm pivot axes as seen in side view. Second, we do not have 100


per cent anti-dive when the side-view projected control arms intersect at the c of g. We have 100 per cent anti-dive when the side view force line intercepts the side view resolution line at sprung mass c of g height. The side view force line is the


line from the contact patch centre through the side view instant centre. The side view resolution line is a vertical line located rearward from the front axle line by a percentage of the wheelbase equal to the percentage of ground plane retardation force exerted by the front wheel pair when braking. This will generally be a greater percentage than the static front weight percentage, so the resolution line will generally be aft of the c of g.


that sprung mass pitch, and front wing height, also depend on what the rear suspension does. 100 per cent anti-dive only results in zero pitch if the rear suspension has 100 per cent anti-lift. It is quite possible to provide that. There are many production cars that have more than 100 per cent anti-lift. Almost any car with trailing arm or semi-trailing arm rear suspension jacks the rear suspension down in braking. This potentially results in wheel hop but, in practice, as long as either


“It is possible that a designer


could deliberately make the front end lift in braking”


If the front suspension


meets this criterion, the front suspension will neither compress nor extend in braking, as the questioner correctly understands. If the front end is lifting in braking, that implies that the anti-dive is more than 100 per cent and that the force line slope and jacking coefficient are greater than described earlier. But it is important to note


ABS or the brake bias keeps the rear wheels from approaching lock up, such cars brake just fine. With SLA or double wishbone


rear suspension, 100 per cent anti-lift requires a lot of inclination of the side view projected control arms – more than is needed for 100 per cent anti-dive at the front. The need for extreme-looking geometry results from the fact that the


ground plane force we have to work with is smaller at the rear than at the front. So it is possible that a


designer could deliberately make the front end lift in braking because the rear lifts, or it is possible that even an F1 designer might have read the wrong literature… Is there a penalty in the car’s ability to absorb bumps while braking when there is that much anti-dive? Yes. However, when the track is very smooth, that may not matter so much. And when the alternative is to use stiffer springing instead, some anti-dive may be deemed preferable to that. My own default


recommendation regarding anti- dive lies somewhere between the recent former practice of using little or none, and the current fashion of using a huge amount. I generally suggest that the side view force line should have a slope at static of around four degrees, and no more than eight degrees in any condition. Depending on brake bias, wheelbase, and sprung mass c of g height, this will generally result in somewhere between 25 and 60 per cent anti-dive.


Some Formula 1 cars raise the nose under braking, but 100 per cent anti dive only results in zero pitch if there is also 100 per cent anti lift at the rear 44 www.racecar-engineering.com • July 2012


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