In the video, you can see shaker rig tests for road cars compared to race cars....
Road Cars: Minimise disturbance for the driver
Racing Cars: Minimise disturbance at the tyre contact patch
Over 15 years ago, when we started doing set-ups for race cars, we noticed that the cars with stiff racing springs were seconds faster per lap than cars with factory or aftermarket sports springs. The evidence was clear and continues with every set-up today – you need stiff racing springs for grip at the tyres.
Our #1 Rule for All of Suspension Set-up….
For circuit racing, hill climb and autocross, stiff racing springs will create the most grip at the tyres. At Racing Car Technology, we have identified the stiffness required by measuring the “spring frequency”, a measure we use to compare suspension stiffness between all race cars. When you test the stiffness we recommend with good results, you can be assured there is no softer setting you could try that could somehow give you more grip at the tyres.
Racing spring rates are generally three to four times stiffer than standard. Sports and aftermarket springs are generally only 30% to 50% stiffer. Note: With coilovers, many suppliers do offer springs that are stiffer, but most times, they’ll still be a road option - won’t be as stiff as needed for racing. Or, they could be too stiff. What’s important is to optimise the stiffness for your race car.
At this stage we should consider the counter argument – that soft racing springs will create grip. The idea is stated in many books on racing and high performance. One example, “Soft settings guarantee minimum tyre load fluctuations as the tyre follows the road with smaller changes in suspension forces.”
That this statement and others like it are wrong, can be clearly be demonstrated by looking at 7 Post Rig tests.
. Go and tell the guy what you want.