The Mark Ortiz Automotive

CHASSIS NEWSLETTER

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to the Motorsports Community

September 2008

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WELCOME

 

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions.  Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by    e-mail to: mortiz49@earthlink.net.  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.

 

 

MONOSHOCK SUSPENSIONS

 

I have seen photos of some of the alternate springing methods used on race cars these days including monoshocks and 3rd springs, but I don't understand how they work or what their advantages might be over coilovers in the usual methods of actuation.  Could you explain how those work?

 

 

Z-BARS

 

I have a question on the fundamental principle of the anti-roll bar.

 

Talking about any standard U shaped ARB, when the vehicle rolls the outside wheel is loaded and the inside wheel wants to be unloaded but faces the resistance of the spring or torsion bar (depending on the suspension construction), and therefore the inner wheel is unloaded by

an amount which is the difference of the weight transfer and the spring loading.

 

However, given the situation where you need a soft setup (ARB disconnected) would it be useful to use an S- shaped ARB instead? I have tried to find literature on this but found nothing. I would

assume that on a vehicle with a relatively low CoG like a Formula Ford, trying to load the inner wheel to an extent could yield some additional grip? Of course a new jacking force would be created, but do you have any comments on this?

 

 

STOCK CAR FRONT ENDS WITH BUMP RUBBERS

 

Now that the NASCAR COT is allowing bump rubbers (and will move away from spring coilbind setups), there is lots of interest in the use of bump rubbers in a number of stock car series.

 

Do you have any good advice to give on this subject?

 

I have grouped these three questions together because there is a common thread through all of them.  They all relate to ways we can get a wheel rate that rises as the suspension compresses.

 

To understand some important differences between ways of doing this, it is helpful to recognize that a four-wheel suspension system has four modes of movement roll, pitch, heave, and warp and the elastic resistance to wheel displacement, or rate of elastic loading change with respect to displacement, can be different in all four modes.

 

However, in most suspensions, the wheel rates at a given wheel are very nearly identical in pitch and heave, and in roll and warp.  Also, in road racing cars, the right and left wheel rates are identical, or very nearly so.  Consequently, for simple and conventional cases, we can break the suspension system down into front and rear systems, each having a wheel rate for ride, and another for roll.

 

In most suspensions, neither of these rates is truly a constant.  When the system has to contend with large variations in load, it becomes desirable to have the suspension get stiffer in ride as it compresses.  Before rising-rate suspension became popular in racing, it was widely used in road vehicles, particularly in the rear suspension on trucks and front-wheel-drive cars.

 

If we have a conventional suspension with two individual springs acting at the wheels, two dampers acting on the two wheels separately, and an anti-roll bar, and we arrange for the springs to have a rising rate, then the rate in both ride and roll rises as the suspension compresses in ride.  This also applies if we provide packers or bump rubbers on the shocks or elsewhere.

 

One effect that this will have on car behavior is that when we set up the aero balance to add more downforce at one end than the other, to make the end with more downforce stick better at high air speeds, the change in roll resistance distribution will change the handling balance oppositely to the change in downforce.  This makes the net effect somewhat more difficult to predict, and it may in some cases make the car unresponsive to the adjustment.

 

To get away from this, it is desirable to have a way to add wheel rate in ride that does not add wheel rate in roll.  Third springs, Z-bars, and monoshock systems are various ways of doing this.

 

For those not familiar with all of these, a third spring is most commonly seen in pushrod or pullrod suspensions, with rockers and a T-bar for anti-roll.  The T-bar is a torsion bar with the torsional part forming the leg of the T, and push-pull rods extending from the rockers to the ends of the cross of the T.  The T-bar can rock backward and forward freely about its lower end, so when the rods move synchronously, there is no resistance.   When the rods move oppositionally, the bar is forced to twist, and resistance results.

 

The "third spring" attaches to the middle of the top of the T, where the leg meets the cross.  It is most often a coilover, but it can be only a spring on a slider or dummy shock, or a shock or slider with a rubber snubber, or a coilover with a snubber.  Ordinarily, the third spring is designed to have a pronounced rising rate.  The third spring acts only when its attachment point to the T-bar moves,

 

which only happens in ride.  The total arrangement thus provides a steeply rising rate in ride, without a rising rate in roll.  Depending on the geometry used, it is theoretically possible to have rising or falling wheel rate in either mode.  The nice thing is that we control the two independently of each other.

 

A monoshock suspension generally has a single rocker, with a single coilover, actuated by pushrods from both right and left wheels.  The coilover acts only in ride, much like a third spring.  Roll is usually accommodated by having a laterally sliding shuttle in the rocker.  The pushrods anchor to the shuttle bar.  When the pushrods move synchronously, they rock the rocker.  When the pushrods move oppositionally, they slide the shuttle bar.  Roll resistance is most commonly provided by stacks of Belleville washers on the shuttle bar.

 

This arrangement offers us independent control of roll and ride properties, much like a third spring, with reduced weight and cost.  Generally, there is no damping in the roll mode, which is a disadvantage.  Monoshock systems are generally used only at the front, in smaller-displacement formula and sports-racing cars.

 

A Z-bar is what the second questioner describes: a transverse torsion bar, similar to an anti-roll bar, but with one arm pointing back and one forward, so that it has the shape of a Z rather than a U.  It affords a way to spring only the ride mode, in a suspension that doesn't have pushrods and rockers.

 

We should note that there is no such thing as a negative-rate spring.  In certain unusual cases, it is possible to mount a spring so that the force generated by the assembly diminishes with increasing displacement, but there is no way to make a spring do that by itself.  A Z-bar, at least as we normally encounter it, cannot, in itself, reduce roll resistance or promote roll.  It just adds rate in ride without affecting roll.  This means that we can use it to get reduced roll resistance with a given ride rate, if we use it in conjunction with soft individual wheel springs, or with no individual wheel springs at all.

 

We most commonly see this with swing axle rear suspensions.  By having a high wheel rate in ride and little or no wheel rate in roll, we can minimize the limit oversteer and upward jacking that a swing axle rear end tends to generate.  Even when the Z-bar is the only spring in the system, the

suspension still resists roll and creates load transfer.  It just does this entirely through its geometry, rather than elastically.

 

If we were to use Z-bars instead of anti-roll bars at both ends of a modern formula car with low roll centers, we would increase roll, but we would not increase the loading of both inside wheels.  Increasing loading of the inside wheels i.e. equalizing the loading of inside and outside wheels, reducing load transfer is beneficial in all conditions, wet or dry, but there is no way we can do that for both wheel pairs by making changes to the suspension.  The suspension only controls how much of the load transfer occurs at the front, compared to the back.  The only way to reduce the total load transfer at both ends combined is to lower the c.g. or widen the track.

 

 

With stock car front ends, we have a somewhat similar situation, but with some differences.  Generally, in oval track racing a given track's turns are all taken at similar speed, but the car has to pass a ride height check.  With the stock car, we are trying to get the splitter or valance as close to the track as possible, and keep it there, without destroying it against the track surface, yet still have the car pass ground clearance check statically.  So we run soft springs and a big anti-roll bar, and arrange for something to bottom when the car reaches the desired ride displacement.  Until recently, at least in the upper NASCAR divisions, bump rubbers were prohibited, so people arranged for the springs to reach coil bind.  Now bump rubbers are permitted, and teams are learning to work with them.

 

The bump rubbers are on the shock shafts.  They unavoidably act in both ride and roll.  They can be about as soft as we like, but we still face a tradeoff between controlling the splitter and having highly non-linear roll resistance at the front of the car.  If we try to soften the rubbers to get some roll compliance, we give up some aerodynamic control.

 

On high-speed ovals, aero forces are so significant that reducing front roll resistance can actually cause a push, by costing us front downforce as more air gets under the nose.  On short tracks, this is less of a concern, but still a consideration.

 

The problem with having a steeply increasing wheel rate in roll at just the front of the car is that it becomes difficult to get good handling balance through the entire turn, and as grip level varies.  We can increase rear roll resistance easily enough, but giving it a non-linearity that harmonizes with the front is not possible.

 

Rules permitting, there would be a strong case for having a stiff Z-bar at the front of a stock car, in addition to the anti-roll bar, and having sliding drop links with bump rubbers for the Z-bar.  Another possibility would be to have a third arm in the middle of the anti-roll bar, and a bump rubber acting on that.  I'm sure NASCAR would not allow this, but if you are running with a different sanctioning body, or running to track rules, maybe you could get your officials to allow it.

 

I have heard of one instance of a Z-bar being allowed as an anti-roll bar.  This was on the swing-axle rear end of a Triumph Spitfire, in SCCA.  Aftermarket anti-roll bars were legal, and the officials ruled that the bar still qualified, even if one arm pointed the wrong way.  To me, it's a bit of a stretch to call a device an anti-roll bar when it doesn't resist roll, but that was their ruling.

 

In many cases, the ruling would probably come down to whether the device is reasonably affordable, can be retrofitted to existing cars, and improves the show.  Having a car that is more consistent is definitely desirable from the competitor's standpoint, but from the promoter's standpoint there is a strong case for deliberately making it hard to achieve consistency.  When all the cars are inconsistent, some cars are better in some conditions and in some parts of a run, and others are better at other times.  This makes for more lead changes and less predictable race outcomes.