How to Choose Your Racing Springs - Racing Car Technology's Bounce Test™

Uncategorized Feb 06, 2020
 

Racers hate losing money buying the wrong stuff for their race car and then having to fork out more money for the right part.

A case in point is buying suspension springs. How do you know what spring rates to order?

Parts suppliers, shock absorber and suspension specialists, and car preparation shops are likely to make a the choice for you, unless you say what you want.

Yet, even with very experienced people making the decision, most racing car spring selection is little more than informed guess work. It doesn't need to be that way.

In racing, it's the stiffness of the spring

What we’re showing you in the video is a unique way of measuring suspension stiffness.  We’ve called this simple test the Racing Car Technology  Bounce Test™.

The recognized measure of suspension stiffness is the "suspension frequency". Or as I call it in the video, "spring frequency".

What we are after is the spring rates we need to achieve our desired suspension frequency.

With the Bounce Test™ above, for this Triumph TR3 race car, we have 122 cycles per minute (CPM) spring frequency, with 550 lb/in springs in the car.  If we want to go stiffer, say 130 CPM, we can work out the spring rates we need to do that in a simple Excel spreadsheet. 

Getting the right spring rates in the car is a huge factor in optimizing grip at the tyres. For us at Racing Car Technology, it’s the single biggest improvement we make to non-aero race cars. The single biggest contributor to improvement in lap time.

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On the road, it's the springs that give the suspension compliance necessary to isolate the passengers from bumps in the road surface. i.e. Soft springs are needed for good ride.

But in racing, we don't care about a comfortable ride for the driver. We're only concerned about getting the suspension stiffness higher (by fitting stiff racing springs) so we can get more grip at the tyres. To give you a feel for how stiff we are talking about here, we’ve found for production race cars, the springs needed for best grip will be around 2 - 4 times stiffer than the standard springs. 

Our procedure is to remove the shocks, leaving the springs in place. Then to bounce the car as shown in the video.

It's decidedly low tech.  But way more important than I thought nearly twenty years ago, when I first came up with the idea. It's worked out to be consistent and reliable. More accurate and way easier than traditional methods.

The stiff springs needed in racing provide resistance to the high frequency vibration at the tyre/road interface. As the tyre cycles up and down in response to the road surface, there is a degree of stiffness that helps the tyre key into the road surface and give least "contact patch load" (CPL) variation.

The beauty of the suspension frequency measurement is that it is directly comparable between all race cars, so we can predict what spring frequency is likely to work best for any particular car. 

Finding the suspension suspension stiffness we want, by optimizing the spring frequency, is where the speed comes from in all our race car setups. 

Our Bounce Test™ is one of two basic procedures we use in determining the suspension frequency. The other requires measuring the Motion Ratio for the spring. We need this for coil-over suspensions where we cannot remove the shocks to do the Bounce Test™.

Why is the Bounce Test™ so important?  Because it gives us a direct read of the suspension frequency, front and rear, our most important number, and the starting point in setting up the car.  And nothing could be simpler, as you see in the video. Right?

Well, there are fine nuances and things you find out when you try this.  It's our experience with the Bounce Test™ over nearly twenty years that gives us confidence in the recommendations we make.

By working with the Bounce Test™, we have added a new dimension to spring selection for production race cars.

Without the bounce test, racers trying to work out suspension stiffness have to calculate a spring motion ratio. Sometimes the methods used to do this are just too rough to be useful. So, you end up with little understanding of the spring rates you need.

Choosing Spring Rates for a Historic (Vintage) Muscle Car/Touring/Sports Car, (Group N and Group S in Australia.)

Most of these race cars ran with too little spring rate in the day. How do we know this? Only by accurately measuring suspension frequency with the Bounce Test Method™. That way, we can really get a handle on how much performance these cars are giving away by being too soft.

Recently, one of our clients with his Group N race car made a huge discovery about the suspension stiffness of his marque. I can't give you his numbers (that's his own proprietary information) but I can give you ballpark numbers on what happens with the Bounce Test™, particularly the effect of spring bending in increasing the suspension stiffness on historic cars. 

For this and many other examples of different cars and the suspension stiffness issues, do get our free report below.  

See How Suspension Frequency Measurement and especially the Racing Car Technology Bounce Test™ Can Help You Get the Suspension Stiffness You Need for Racing....

We cover all types of race car suspension (excluding cars with aero downforce).

Examples of race cars from the 60's and 70's why were racing cars so softly sprung? You'll be surprised at the answer to this and how racers found out about the extra speed you get with stiffer springs.

What racers are currently doing with spring frequency measure