I will say I run a stiffer rear spring than in the front, and my setup started at 400 lbs/in front and 500 lbs/in rear. Motion ratio is one part of the reason I selected those, but even at those rates (if my math is right) I still ended up with a faster natural frequency in the rear. The short answer is you get excessive chassis pitch or roll with equal or higher front rates; by making the rear slightly faster, when you go over a bump the front wheel will compress and rebound first followed by the rear, and the slightly faster rate allows the rear to catch up and finish its rebound motion the same time the front is. You can run a higher NF in the front and still overcome this with good shock tuning, but the higher rear NF will naturally make the car better over bumps.
I will admit I followed the rule of thumb as fiat without totally understanding it, but I've seen it in more than one place and I personally like the car better this way (I have tried a square setup).
Here is a guy from an FSAE forum.
http://www.fsae.com/forums/archive/i...p/t-5177.html?
Quote:
The out of phase motion between front and rear vertical motion, caused by the time delay
between when the front wheel and rear wheel hit the bump, is accentuated by the frequency
difference. A result of the phase difference is pitching of the body. To reduce the pitch
induced by hitting a bump, the rear needs to have a higher natural frequency to “catch up”
with the front. This notion is called producing a “flat ride”, meaning that the induced body pitch from road bumps is minimized.....
For a given wheelbase and speed, a frequency split front to rear can be calculated to minimize
pitching of the body due to road bumps. A common split is 10 – 20% front to rear.
The above theory was originally developed for passenger cars, where comfort takes priority
over performance, which leads to low damping ratios, and minimum pitching over bumps.
Race cars in general run higher damping ratios, and have a much smaller concern for comfort,
leading to some race cars using higher front ride frequencies. The higher damping ratios will
reduce the amount of oscillation resultant from road bumps, in return reducing the need for a
flat ride. Damping ratios will be explained in the next tech tip in detail. A higher front ride
frequency in a race car allows faster transient response at corner entry, less ride height
variation on the front (the aerodynamics are usually more pitch sensitive on the front of the
car) and allows for better rear wheel traction (for rear wheel drive cars) on corner exit. The
ride frequency split should be chosen based on which is more important on the car you are
racing, the track surface, the speed, pitch sensitivity, etc.......
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Additionally, Dennis Grant at Autocross to Win suggested the same thing. You're not necessarily wrong if you don't follow this, and certainly plenty of people are doing extremely well on 400 lbs/in square, but it can be help your shock tuning to understand the dynamic your spring selection creates.