Mams,Originally Posted by mams
My genuine, non-condescending, advice is, YOUR GOAL IS TO BUILD A CAR THAT CAN TRAVEL 20 MILES, AT AN AVERAGE SPEED OF 30 MPH, WITHOUT BREAKING DOWN.
No amount of "optimising" anti-dive will help you achieve this. Rather, time spent optimising will most likely harm your efforts. More below, but in short, any anti-dive between about -10% and +30% could be workable on a winning car.
~~~o0o~~~
Part of the reason for the above wideish range of acceptable numbers is that a secondary goal in FSAE (IMO) is to USE THE BRAKES AS LITTLE AS POSSIBLE. Instead of using the brakes, and then also having to stomp on the accelerator, just go fast around the corners, and thus be the fastest car WITH the least fuel burnt.
Dynatune covered most of the pros and cons of different levels of anti-dive back on page 1. On a smooth FSAE-typical track you can have quite high anti-dive without adverse consequences. On a bumpy road-racing circuit I would stay under ~20% (better under 10%?, else the front-wheels will start getting air over the bumps on corner entry).
Off-road racers typically have large amounts of pro-dive, mainly to give lots of "wheel recession" for better ride over the bumps (wheels move backward in bounce). But off-road racers also have front brakes the size of milk bottle tops (the 2WD cars are rear heavy, and racing is about going fast, not slow!).
There was a period in Lotus's history when they were trying to get as much pro-dive as possible. This was early in the aero era, and Chapman figured that lowering the front wing, via pro-dive, on corner entry would give the car better downforce through the rest of the corner. They also had pro-roll (RC below ground level) to jack the car DOWN in corners, because the straights were quite bumpy and the car had to have a highish static ride height.
~~~o0o~~~
For some understanding of antis (pitch or roll) have a look at the "Jacking Force" thread.... compared to roll center movement, how important is pitch center variation under braking and acceleration.
The key point to take from that thread is that you only have to know the longitudinal (for pitch) and lateral (for roll) n-line slopes (aka "force line slopes") to be able to calculate the anti-pitch, anti-roll, and jacking behaviour of the car. (Well, you should also know how to calculate the slopes correctly, realise that a single wheel can have different longitudinal n-line slopes for accelerating and braking (because, say, inboard drive and outboard brake), and you must know the magnitude of Fx and Fy road-to-wheelprint forces at each wheel.)
But, VERY IMPORTANTLY, the location of the intersection point of the pair of close to horizontal n-lines in side-view (= the "PC"), or in end-view (= the "RC"), DOES NOT MATTER AT ALL!!!!!
It should be apparent that n-lines that are always close to horizontal wrt the car floor-plane, have an intersection point ("PC" or "RC") that zooms off to infinity ("parallel lines meet at infinity"), or at least out past Pluto, with any small amount of body pitch or roll. But this make NO DIFFERENCE to the amount of anti-pitch, anti-roll, or jacking forces, which, again, depend only on the slopes of the n-lines.
Of course, Claude feels that the PCs/RCs have some sort of magical powers, the details of which he doesn't want to reveal, so you might get marked down in the Design tent if you give him an old-fashioned "Mechanical" explanation for how these things work.
Based on the longitudinal and lateral n-line slopes of all the cars I have ever driven, I reckon the closer to horizontal (wrt car floor) that the n-lines are (and remain), then the more benign the handling, and the better the ride and grip over bumps.
~~~o0o~~~
Bottom line, pick the antis that give you the simplest, quickest-to-build, most reliable car you can manage, providing the antis are roughly in the range given above (ie. n-lines closer to horizontal are better). If a particular concept has anti-dive = 13.7819...%, but it also gives you an exceptionally neat and tidy (and strong and stiff and light and easy-to-build+++) car, then go for it!
Z
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