What do we do most times when buying aftermarket springs for our race or road performance car? We depend on advice from our supplier or our car builder. Or we might talk to our competitors, maybe check out the forums, to see what others with our brand of car are using.
There are so many different views. Some may feel it doesn’t matter that much, what springs you use. Others may think softer springs are better; “I’ll settle for the 8kg/mm spring rather than the 10kg/mm for XYZ reasons”. Maybe somebody has fitted up the 14kg/mm spring and they're happy.
Then, when the springs have been in the car for a while, we tend to forget about it. We don't question the spring rates anymore.
In our experience at Racing Car Technology, it's super important what spring rates, what suspension stiffness you have.
It can be worth seconds per lap for a race car.
Cornering grip improves remarkably as we increase spring rates from standard, right through to an optimum, where the gains have started to level off.
The thing is, if we could put a test driver in the car, and he/she could say if we needed softer or harder springs, this would be great. But we can’t. It’s almost impossible to tell whether softer or stiffer springs would improve grip, improve performance, just by driving the car in isolation.
If we could test different springs back to back at the race track, and see which is faster, that would work. But track testing is expensive. Also, fraught with difficulties, if we don't understand what we're looking for.
What we need is a scale of suspension stiffness, so that we can compare the suspension stiffness of various race cars.
The measure we use is Suspension Frequency - the “undamped natural frequency” – ie the oscillation of the body up and down, assuming we have no damping and no friction.
We measure suspension frequency Suspension frequency is often referred to as the “ride frequency” or “ride stiffness”. It is measured in Hertz (cycles per second), or CPM (cycles per minute.)
The scale we use for non-aero cars starts around 80 CPM for road cars, through to 170CPM at the top for racing (non-aero cars).
It’s almost 20 years ago, we started out in our business, Racing Car Technology. We wanted a way to do the suspension set-up in the workshop, so we could give clients a fast race car, straight out of the box. We didn’t want to try a lot of changes at the track. That gets expensive.
But we did get to check if the cars were as fast as we hoped. A lot of times, we got a good comparison of the speed of the car before and after the workshop set-up. Sometimes with the same driver, same engine, same tyres.
The results were startling.
Of course, you have to have good balance…. but if you do……
The standout improvement comes from optimizing suspension stiffness – sometimes seconds per lap.
What happens is, the cornering grip improves remarkably as we increase spring rates from standard, right through to an optimum, where the gains have started to level off.
Further increase in suspension stiffness can degrade braking and acceleration grip. Compliance in suspension and chassis now becomes a significant factor. The transitions, corner entry and corner exit, can be affected.
The idea that we can select springs rates for racing and road performance with a good degree of certainty and only positive results is pretty much our thing at Racing Car Technology. Any recommendations you see in print are too broad, and generally a bit softer at the higher frequencies than we have found best in our experience.
Of course, high downforce race cars are a completely different matter. The frequencies that apply are much stiffer. This is the domain of the race engineers in professional racing. When the professionals talk about softer springs making more mechanical grip, they are talking about spring frequencies way above what we use in non-aero cars. (ie Does not apply to you and me working with spring stiffness much lower on the scale.)
In one of his books, “Engineer in Your Pocket”, Carrol Smith said “My own opinion is that on most road courses, a rear anti-roll bar is a bad thing. Anti-roll bars transfer load from the unladen tyre to the laden tyre – exactly what we don’t want at the rear.”
I used to think there was something in this. I was inclined to agree that we should limit anti-roll bar stiffness at the rear.
See our blog article, Anti-Roll Bars? A Good or a Bad Thing?
Now I’ve come full circle. I believe we can say unequivocally – the anti-roll bar is only a good thing. I’ve used the VW beetle front suspension as a practical demonstration to show that the anti-roll bar works the same as the spring.
The anti-roll bar works like a secondary spring, providing stiffness at the tyre contact patch. But it works in roll mode only. It does not add any stiffness in two-wheel bump.
The suspension “natural frequency” is our first consideration when it comes to improving our race car’s suspension set-up. To improve grip, we want the stiffest springs we can have for our use of the race car. The scale we use to measure suspension stiffness starts at around 80 CPM for road cars, through to 170 CPM, the stiffest recommended for non-aero race cars.
Anti-roll bars are the next piece in the puzzle, after we have selected our springs. We do want to run anti-roll bars front and rear, for all our cars, except where the rules dictate we can’t.
In selecting our anti-roll bars, we use the Racing Car Technology Weight Transfer Worksheet™. And just as for springs, we need to have our thinking right – our view of how things work. This way, we can take maximum advantage of all the options available to us.
If you're interested to learn more about how you can choose the springs and anti-roll bars you need for best grip and balance, then check out our training course here....