Smithees Race Car Technologies                                                                              racetech@ozebiz.com.au                    

 

Springs and Anti-Roll Bars

Deciding on spring rates is one of the most important decisions you have to make about your race car set up.

Unless your team has done the necessary analysis and testing, do not accept the springs as they are from the previous owner of your car, or from a Supplier ("these are right on such and such car").

We can do the calculations to get your springs (including leaf springs) and anti-roll bars in the ball park, and help with testing if you require.   If we have access to the car, there is a lot we can do to check that they will work as expected, or we can send you some forms for the data we need.  You could send us the data and the springs off your car.

The main job of the spring is to allow the tyre to follow the bumps in the road without unloading the tyre. (The shock works in concert with the spring to achieve this.)

Starting with the front, we would consider what range of wheel frequencies could work with your car.  Wheel frequency is the number of times the chassis will bounce up and down if unrestrained (in cycles per minute.)  A given wheel frequency will relate directly to spring rate, if we know the suspension leverage, and the sprung weight the spring supports.

Part of the job of the spring is to restrict body roll, and we can work out what spring rate will be required to provide the  roll resistance we want to restrict body roll to say 2 degrees at 1.0g cornering force.   This spring
would be stiffer than ideal, so we generally allocate 25% to 50% of roll resistance to the anti-roll bar.  Bars have the added advantage that they are easier to adjust than springs, for purposes of balancing the car.

It all comes down to optimising your spring/anti-roll bar/ shock combination.  Some people may use less spring and more anti-roll bar, others less spring and more shock.  In general, most drivers who have the opportunity to test,
should end up with similar roll and pitch resistance overall, and with adjustable bars and shocks, be able to balance the car to individual requirements.

Many cars used in racing will not be able to make good use of premium racing shocks - for any number of reasons - poor motion ratios, suspension geometry, bushings etc.  In these cases, you optimise the springs and anti-roll bar
package and then choose shocks to suit.  In historic racing, we recommend Pedders Sports Ryder or Koni or adjust the valving on the lever shocks. (In HQ racing the specified shocks are Pedders Sports Ryder.) 

In fact, prior to the mid 1980's, shock technology in racing was not well understood, so few outside the very top professional teams were tuning with shocks.  Now, even club level racers can work with racing shocks, if they
are permitted in the regulations.

Real World Example of Suspension Tuning Problems Experienced....

Andrew Moyle was trying to cure the terminal understeer
on his just purchased Nissan Pulsar GTi-R. 
His experience helps us.  Click on:

Type GTi-R into their site search engine box (at www.autospeed.com).

1.  He bought springs without knowing what he is getting.
2.  He used Koni inserts.  Probably no insert will be up to the performance level of this car.  Monotube struts with adjustable coil overs would be much better...but maybe cost was a factor.
3.  Shocks on full hard.  He is outside the performance range of the shocks.  Either the springs are too soft or he needs to re-valve the shocks.  If they were double adjustable, he could have experimented with less rebound, so the rebound didn't overcome the soft springs.  Probably, it would never have worked on the soft springs - the plough understeer was caused by excessive front roll giving him positive dynamic camber, even with the 3 deg static camber setting.
4.   The tyre pressure differential of 14 psi would only work while the tyres were cold.  If he put it on the track, the heat build up in the front tyres would probaly take the pressure up over 50 psi, and the understeer would get progressively worse.
5.   The strut camber pin movement.  This is very common.  As soon as the camber moves, it puts the toe setting out dramatically.  Re-check camber bolt tension and toe setting often.  Even the Kmac pins, described as OK in the article, can move.

Real World Example 2

Go to and type "No Float"
into their site search engine. You do need to be a member - very worthwhile for all the great info on this
site.

They took a BMW with worn out suspension, and tested it
before and after the fitment of Koni red shocks and Kings
lowered springs.  As expected, they got improved times
on a swerve test through the witches hats, and far
greater stability on full throttle runs around the Calder
oval.

Interestingly, they got excessive understeer out of the new package.  I reckon the front springs were probably too stiff in relation to the rear.  The problem is they used progressive rate springs in the front.  In most strut cars,
there is not enough room to fit in the tightly wound progressive part of the spring.   If you looked under the front guard of this car, at normal ride height, the progressive coils would be already closed - this solid block taking up about 1/3 the available space.  So the spring designer is likely to go too stiff, because he hasn't got enough spring length left to work with.

The reason they do it is so the springs don't rattle loose on full droop.  Kings also claim an advantage in that the inside wheel loads and unloads in a more desirable way.  Pedders don't use a progressive rate spring in these applications.  Their better solution is a single rate spring with a shorter travel shock to capture the spring.  While on the subject of aftermarket springs, it has always been of some concern to me as to how the spring designers have affected the balance of the car.  In general, I guess they keep the percentage increase in spring rate the same for the front and rear.  So all things
being equal, the balance of the car will be unchanged.  Yet we fitted some springs supplied from Koni Europe recently, where the front spring rate increase was double the rear percentage wise.  The car didn't need it.  It comes standard with a monster front anti-roll bar.

I believe the after market spring makers would do us all a favour if they would perform skid pad tests to determine the desirable steady state balance of the car. (this is the 200ft diameter circle test they use in the US.
We discuss it here.)  It is very safe and easy on the car -  low speed, low revs - yet you get a good feel for the steady state balance of the car at maximum cornering force.

Phone Dale Thompson  (02) 4472 8225
racetech@ozebiz.com.au

Pricing for our Services

 

Smithees Race Car Technologies                         ABN 42 911 718 836                                      racetech@ozebiz.com.au
Get a good suspension set up with the "Smithees Weight Transfer Worksheet"