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A "Mindset" for Analysing Suspension Set Up Problems

Here we try and bring together the two areas of weight transfer theory and suspension geometry, and consider their relevance in achieving our single goal in suspension set up - optimising the use of the tyres.

Tyres

Nothing is more important than the tyres.  We must optimise the temperature and pressure range, and adjust dynamic camber for reasonable tyre temperature gradient.  We could get baseline numbers for these using a "skid pad".   But because such a facility is not generally available in Australia, we collect tyre temperature and pressure data during testing and practise at a race circuit.  This data base will be the best window into tyre performance available to us, short of high end data logging.

We will only be able to consistently use all the grip potential of the tyres if we have a stable chassis platform and good tyre compliance (tyres stay in contact with the road.)  To this end, overall roll and pitch resistance must be such that we keep suspension geometry within workable limits, avoid suspension bottoming out, and hold the chassis platform to maximise the desired effects of aero devices, if used.

Grip and Balance

"Wheel pair stiffness" and "wedge" are our key concepts in weight transfer theory.  Weight transfer is distributed across the  front and rear of the car in accordance with the proportional stiffness of the wheel pairs that come into play for the particular movement of the car we are looking at.  The spring/anti-roll bar/shock combination are our main tuning devices.  We can also change roll stiffness by changing the proportional height of front and rear roll centres.  When we change the set up to balance the car, we have also changed overall grip.  If the car is now better balanced in the various corner phases, then overall grip will have improved, and the car will be faster.

The set up combination needed to achieve the amount of roll and pitch resistance the car needs to be balanced is unique to each car set up (ie the identical car set up identically.)  A multitude of factors can have an influence, the short list including weight distribution, front and rear track, tyre sizes, relative downforce, and dynamic settings for just about everything to do with the spring/anti-roll bar/shock and the suspension geometry - including suspension jacking forces and tyre scrub.*(see below)

Maintaining tyre compliance with the road is a major consideration.  In theory, we want to run the lowest rate springs, consistent with a stable chassis platform, and reasonable response for the driver.  How much of the roll resistance will be allocated to anti-roll bars?  How much to shock absorbers?   What are the downsides of too much anti-roll bar and/or too much shock?

Transient Response

The driver constantly builds and lets go the weight transfer in the car as he or she traverses the race track.  As well as roll, we have pitch under braking and acceleration.  Diagonal weight transfer (or warp) can be considered as weight transfer in a combination of roll and pitch, as we discussed for corner entry and corner exit. 

To allow the driver to use all the potential of the tyres, we considered the importance of developing good response to all driver inputs - good transient response.  The driver should be able to develop weight transfers smoothly.  Transitions for understeer to oversteer should be rogressive.  Steering and suspension geometry is a factor, as are basic considerations of track, wheel base and CG height.  But tuning wise, we again look to our spring/roll bar/shock package.  Stiffer wheel pairs transfer weight faster.  So for faster response in either roll or pitch we stiffen the relevant wheel pairs.  For instance, we may choose to stiffen the front the front and rear anti roll bars equally, for faster response in roll, without changing the balance of the car.

It is clear that most suspension upgrades for road cars are aimed at improved transient response - lower CG, stiffer springs bars, and shocks can all dramatically improve driver feel and the reduction in roll and pitch will reduce the affect of any suspension geometry deficiences.  The car may well be a lot quicker on the road, even though no attempt has been made to optimise the balance of the car at the limit, as we have described in the weight transfer section.

For racing, we do need a baseline set up for steady state balance of the car.  But even this apparently simple aim has many twists and turns.  Can it be achieved straight off the drawing board?  Not always.  A new sports car in the UK needed a spring change on the rear in it's very first test session.  In developing our own car, we may find ways of improving grip, and then need furthur adjustments to re-balance the car.  For most amateur racers, this is about as good as it gets.  We look for wheel stiffness that gives the driver the response he likes and then adjust the steady state balance of the car by whatever means we have available - springs, bars,  aero adjustments etc.

Transient Behaviour

We emphasised the importance of transients because that is the majority of our lap time outside of straight line acceleration.  So we really need a greater insight than steady state understeer or oversteer.  We looked at combination pitch and roll during the corner phases.  Certain movements of the race car add greatly to understeer or oversteer.   Transient behaviour is more than just the response, or speed of weight transfer, that the driver feels is OK.

There was a story sometime back about the Swift people putting their Formula Ford on one of those "shaker" suspension testing machines and finding a completely new set up for springs and shocks.   The car was a lot quicker.   They said this set up would not have been found in the normal course of track testing the car.  You can bet the improvement was in the transient behaviour.  The steady state balance would have been much the same, before and after the changes.

We extend the concept of transient behaviour to include "handling balance variation".  There is the potential to influence and tune understeer and oversteer characteristics during transient movements of the car.   We look for improvements that will help the corner phase we are looking at, without causing too much of a problem somewhere else.  Say you want the car to turn in better.  Increasing the rate of the rear roll bar would probably be the worst decision - you would reduce corner exit traction.  Increasing rear spring rates may be a better way, or raising the rear roll centre better again. If you had an adjustable 4-link rear suspension, the best way might be to set it for slight roll oversteer in forward pitch to help turn in, and roll understeer in rearward pitch (acceleration out of the corner). You improve two areas of handling balance with the one change! 

Never let your thinking revolve around the idea that springs and anti-roll bars only control steady state balance (and in some way that shocks look after the transient stuff by themselves).  I hope I have been able to show this is definitely not the case.

Before shock tuning for balance came into the picture, possibilities for influencing handling balance in one area without affecting another were limited for road racers.  For instance, if you increased front roll stiffness (say with the anti-roll bar) to improve power down on corner exit, the driver may have to deal with increased understeer in mid corner. Speedway has never been so restricted because of the multiplicity of available asymetrical set ups. But shock absorbers give us the potential to influence wheel pair stiffness differently in bump as opposed to rebound, just as the 4 -link can give us different axle steer in bump as opposed to rebound, opening the way for asymetrical like effects, even though the car is still square!   Speedway tuners can go one better again - they might only have to change the rebound or bump on one shock to get the change they want.  All is revealed in the shock section of this web site.

Transients and Shocks 

Shocks can add a lot of stiffness in a wheel movement.  They have to, because of the loading they must absorb to fullfill their basic task of controlling oscillations in the springs.  As soon as wheel movement stops, the shock loading goes away.  So they have no affect on steady state handling.

Could we use these quite considerable loadings in the shocks to tune for understeer/oversteer balance during transients?  You bet we can.  And in our shock section, we show you how, with a generalised approach which will work with any movement of the race car.   Every book I have read on suspension has missed this point.  Only Mark Oritz, in his excellent articles in Race Car Engineering Magazine showed the way.  The concept is fascinating, because for the first time road racing cars can use a set up that performs like an asymetrical one, such as a speedway car might use, yet the car is still square (performs the same in RH and LH corners).

So it turns out that there is no mystery at all, about what we are trying to do with shock tuning, only in the execution.  We have to be absolutely clear about what wheel movements are happening and we need a very accurate, highly consistent and adjustable device for our shock (read also expensive.)  Paul Haney says that when monotube shocks were first used in Indy car racing in the early 80's, these cars ran away by 1/2 a second a lap towards the end of the race.  Using the old twin tube shocks, the teams thought their loss of speed was the tyres going off.  But it was the shocks overheating and loosing efficiency.  The monotubes stayed consistent longer.

Evidently, we do not need expensive racing shocks on the road.    A high quality precisely valved non adjustable unit will do just fine.  But what about your road registered performance car, used in club motor sport?  We hope to do some track tests soon with Pedders Extreme shocks on a road car, to see if we can tune for transient behaviour.   We'll report what happens on this web site.

Transients and Dynamic Wedge

To assist in our understanding of transients we defined the term dynamic wedge.  Speedway teams have used the wedge concept for decades because of their need to run asymetrical set ups.  Road racers need it too, to gain a better insight into transient behaviour.  See the weight transfer section of this web site.

*A note on suspension geometry...
You should know and understand the pros and cons of the suspension geometry you have. We can help with this by analysing your suspension in the Susprog 3D program.   If we can't get to see your car, email us, and we can send you the sheets to measure up your car your self.  You should know what tuneable devices you've got e.g. 4 link rear suspension, and what potential you have for tuning with roll centre heights and static wheel alignment angles.

The situation will be different in every class of racing.  For instance, they say the new Pilbeam Sports Prototype has got very good pitch control so they can run softer springs - anti dive geometry in the front, and a combination of anti squat and anti lift in the rear.  But if your running a 50's or 60's historic production sports or sedan, considerations like these go out the window.   You might have one of those front suspensions with the pick up points all over the place. (Toyota were still doing this up to the late seventies.)  The result is very strange happenings with wheel bump or droop - track change (scrub),  wheel base change, caster variation, and bad bump steer.  If everybody is in the same boat, no worries.  You can still tune the set up and get a nice handling car that's quick and fun to drive.  Only problem is,  if one of the makes in the class has more straight up and down geometry (as per later model production cars), they are potentially faster.

The reason for wanting to hold consistent dynamic geometry in a race car is firstly not to scrub the tyre over the road surface or build jacking forces in the suspension, and secondly to give the driver reliable linear response to his inputs of steering, braking and acceleration.  For instance, roll centres should move up amd down in the same direction and by similar amounts.  Linear response is also the reason why excessive rising rate suspension and progressive springs are now well out of  favour on race cars.   You'd think that the same would hold for anti dive and anti squat geometry - they also cause rising rate and/or wheel base variation.  The reason the're still used (in moderation) must be that it is the least hurtfull way of achieving a stable chassis platform in pitch.  There is no effect in roll, only in pitch.

 

  We welcome your comments or queries on any of the ideas presented here.

Smithees Race Car Technologies          racetech@ozebiz.com.au