Race Car Stuff

In this article:

Ackerman? Or not? Does it matter?

there are some of the best explanations of the Ackerman effect in steering I have read anywhere. I have taught people about suspension tuning over the years and when the subject of Ackerman comes up, I can’t give any specifics. It is always difficult to explain all of the problems in applying simple mathematics to calculate what should be ideal. There is always are technical person that it is just impossible to explain all of the variables. Usually the main thing that is impossible to get across is the slip angle of the tires and how different they are between the inside and outside tire, because of the weight transfer. Here is an excellent example in Dave Hinde’s picture of my Shelby:

Notice the difference in the loading of the two front tires. Notice the vast difference in the deflection. This is explained in this article along with most of the other variables. Mathematics can be used to eliminate some of the variables; the best result is going to come from driver feel, the track configuration, all of the variables of the car, and the necessary testing results.

Give it a read, it is well worth it.

The Mark Ortiz Automotive
CHASSIS NEWSLETTER
PRESENTED FREE OF CHARGE
AS A SERVICE TO THE
MOTORSPORTS COMMUNITY
March 2006
Reproduction for free use permitted and encouraged.
Reproduction for sale subject to restrictions. Please inquire for details.

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: markortiz@vnet.net. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.

DIFF DIFFERENCES
Which differential would be best for road racing, in my 300hp Porsche? The question arises because a supplier suggests that a plate-type limited-slip is better suited to road racing than a worm gear style torque bias diff. I have used both and found Iliked the torque bias diff. I thought it was a better design, from what I read. The supplier states that the lsd will be better in corner entry and exit. What is your opinion?

Clearly, both worm gear and clutch pack differentials have their adherents, and both are used successfully in racing. You say you have experience with both types, and have already formed a preference. The most obvious answer would be that you’ve already answered your own question, and don’t need advice.

However, the situation is actually a bit murkier, because the behavior of both types of differential can vary according to design and tuning details. Both types are similar in that they generate a locking torque in response to the total torque being transmitted. In both types, the locking torque depends on pressure angles. In a ZF-style clutch pack design, the angles are those of the ramps on the spider shaft and the housing halves. In a worm gear design, it’s primarily the helix angle on the gear teeth, and secondarily the pressure angle of the tooth profile. Lubricant choice also influences behavior.

Consequently, all clutch pack diffs don’t act alike and neither do all worm gear diffs. A lot depends on how a specific example is tuned.

That said, the clutch pack design probably offers a greater range of tuning options, and probably greater wear resistance. With the worm gear designs, we are trying to make gear teeth act as a friction device. Clutch discs are designed to be a friction device; gear teeth can be made to act as a friction device, but they are less comfortable in that role.

This affects the ability of the differential to maintain consistent properties over time, and its longevity.

The pressure angles determine how rapidly locking torque builds as transmitted torque increases. The preload in the diff determines how much locking torque there is when no torque is being transmitted. A clutch pack is easily preloaded, and it maintains its preload relatively well, especially if the preload is applied by springs or some other compliant system such as dished clutch plates. Worm gears can also be preloaded, but because they are not very compliant, the preload rapidly goes away as the teeth wear.

One limitation in worm gears is that the pressure angle is generally the same for forward torque and rearward torque (as when engine braking, or when transmitting brake torque from a single rear brake, as seen in FSAE cars). In a clutch pack diff, we can use different ramp angles for power and decel.

Another peculiarity of worm gear designs is that because power and decel apply force to opposite sides of the gear teeth, preload doesn’t have identical effects in both directions. If we preload the gears in the direction they’re loaded under power, what happens under decel is that we have diminishing friction with increasing reverse torque, until the preload is overcome, at which point locking torque is zero. As reverse torque increases beyond that point, locking torque builds again. With a clutch pack, preload has similar effect in both drive and decel modes.

This does mean that we can make a worm gear diff act different in drive and decel, but not in a manner that’s independent of preload.

One interesting, though uncommon, trick we can use in a worm gear diff is to use plain thrust washers to absorb the thrust of the worm gears in one direction, and needle thrust bearings to absorb the forces in the other direction. This can afford us some limited measure of difference in friction depending on torque direction. Last year’s North Carolina State University FSAE car had a diff like this.

It will be clear, however, that using these tricks is not nearly as straightforward as varying the ramp angles in a clutch pack diff.

Finally, neither option is ideal, because neither is speed-sensitive. Both clutch pack and worm gear diffs rely on Coulomb friction, which is largely dependent on normal force and not speed. We would rather have the locking torque vary with the speed difference between the wheels, either entirely or at least in part. This argues for either a pure viscous limited-slip, or a design that uses a pump, driven by relative ouput shaft rotation, to load a clutch pack, or a design that combines viscous effects with a clutch pack.

From our friends at Autoblog.com:

Click on the picture for the full size image.

This weekend at the Proximus 24 Hours at Spa, the next chapter in Porsche’s storied racing history will be written. The Manthey Racing Team will campaign two of the new Type 997-based Porsche 911 GT3 RSRs in the event. Followers of ALMS (and FIA GT) know that teams have been running the previous-generation car this season as they await the introduction of the new racer.

Well, the wait is over.

Built on the latest 911 GT3 RS road car, the new RSR’s 3.8 liter six-cylinder is makes 485 horsepower at 8,400 RPM with a pair of mandatory 30mmm air restrictors in place. Torque peak is 435 Nm at 7,250 rpm.

The new body with its welded-in rollcage is 10% stiffer than the previous car and is also 7% more aerodynamically efficient. Relocating some components has resulted in better overall weight distribution as well. It has been constructed in accordance with FIA and A.C.O. regulations and you can expect to see it doing battle in the major racing series and at Le Mans next year.

Autoblog on the GT3 RSR

The Mark Ortiz Automotive
CHASSIS NEWSLETTER
PRESENTED FREE OF CHARGE
AS A SERVICE TO THE
MOTORSPORTS COMMUNITY
February 2006

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: [Email]markortiz@vnet.net.[/Email] Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.

ROLL AXIS INCLINATION
What is the influence of a roll axis inclination biased to the front suspension – meaning a front roll center always closer to the ground than the rear? At least in passenger cars, the roll axis is always inclined to the front except in some special cases, for example the BMW Series 1 which is reported by BMW to have the roll axis parallel to the ground.

I supposed I had an explanation, but after reading Race Car Vehicle Dynamics by Milliken my potential explanation has flown away. My explanation was based on the idea that the more the roll axis is inclined toward the front, the more load transfer there will be at the front axle, and the more understeer the vehicle will have.

But I have put into an Excel spreadsheet the formulation from Milliken and I find to my surprise that the higher the front roll center, the greater the load transfer at that end – which works against my intuition.

Can you explain this?

Short answer: higher roll center at the front implies more geometric roll resistance at the front, hence more load transfer at the front, other things being equal. So the typical nose-down roll axis inclination does not increase front load transfer.

There are cars that have a nose-up roll axis. They are all rear-engined. Probably the most extreme example is the Hillman Imp, which had a front roll center near hub height and a rear roll center near ground level.

Like many things, the subject of roll resistance and load transfer is fairly simple once you understand it, but will drive you crazy until you get to that point.

When discussing this subject, I am always quick to plug my video, Minding Your Anti, which covers the subject at length. It costs US$50.00, shipping included, payable by check or money order to me at 155 Wankel Dr., Kannapolis, NC 28083-8200, USA.

In steady-state cornering (constant speed, on a constant radius), on an unbanked road surface, the total load transfer from the inside wheels to the outside wheels depends entirely on the height of the whole vehicle’s center of mass (center of gravity, or c.g.) and the track width at the c.g.

Suspension design and tuning have almost no effect on the magnitude of the total load transfer. What we mainly do with suspension design and tuning is control the distribution of that total, between the front and rear wheel pairs.

We customarily consider the car to be a rigid object, supported by a single compliant structure at each end. The sprung structure is the rigid object; the front and rear suspension systems are the compliant structures.

As an analogy, imagine that you and a friend are carrying a sailboard, as used for windsurfing, along the beach. Each of you is carrying one end of the sailboard. The sail is up, and there is a breeze blowing. The force of the wind on the sail tries to overturn the sailboard.

The overturning force depends entirely on the design of the sailboard and the amount of wind. The total counterforce that you and your friend together need to exert to balance this does not depend on you and your friend. However, the amount of counterforce that you individually need to exert depends on the amount exerted by your friend, and the amount of counterforce he has to exert depends on you.

You and your friend are like the front and rear suspension systems. The sailboard is like the sprung mass.

There are portions of the load transfer that come from the unsprung components, and there are portions that come from the dampers if the car is rolling upon corner entry or de-rolling on exit. However, for simplicity in answering the present question let’s look just at the components of the load transfer that come from the inertia force (centrifugal force) of the sprung mass acting through the suspension, in steady-state cornering. There are only two such components: elastic load transfer and geometric load transfer. Elastic load transfer comes from elastic roll resistance: the roll resistance supplied by the springs and anti-roll bars. Geometric load transfer comes from the properties of the structural components attaching the wheels to the sprung mass, which can be arranged to generate forces opposing roll, or geometric roll resistance.

With independent suspension, these two components influence each other more than is commonly recognized. The load distribution on an independently suspended wheel pair affects how much geometric roll resistance the wheel pair has, for any given suspension geometry. To illustrate with an extreme case, if the inside wheel is off the ground, the geometry of its suspension linkage is irrelevant and only the geometry of the outside wheel has any effect on the car. My video deals with these effects in detail. For simplicity, I will ignore them here, but I do want note in passing that they exist.

When we speak of roll center height, we are speaking of an imaginary point whose height represents the amount of geometric roll resistance for the front or rear wheel pair. If this point is assigned properly, we can closely approximate the geometric load transfer at one end of the car as: roll center height times sprung mass centrifugal force at that end of the car, divided by track width at that end of the car.

When the suspension is symmetrical, the point you generally see in the chassis books – the force line intersection – is a good approximation. When the suspension is not symmetrical, using the force line intersection as the roll center can lead to major mis-predictions of car behavior. Sometimes the force lines may be parallel, in which case there is no intersection.

We may define a line connecting the front and rear roll centers, called the roll axis. The car doesn’t really roll about this line, but as a crude approximation we can reasonably think of it as doing so.

If we raise the roll axis at both ends, the geometric roll resistance is greater at both ends. If we raise one end of the roll axis and lower the other, leaving its height at the c.g. unchanged, the total geometric roll resistance is unchanged, but we increase the geometric roll resistance at one end and lower it at the other. The elastic elements – the springs and anti-roll bars – are not affected by this.

So the end where we lowered the roll center has less geometric load transfer and the same elastic load transfer as before – hence less load transfer overall. This will make that tire pair grip better, because they will be sharing the work more equally. At the opposite end, the elastic component will likewise be unchanged, but the geometric component will be increased – hence more load transfer overall.

Okay, so if we want understeer for most drivers, why have a nose-down roll axis? There are a number of explanations.

The most obvious explanation is that when the car has independent suspension in front and a beam axle in back, we don’t have much choice. Independent suspensions with roll centers much above four inches generally jack excessively. Front suspensions with high roll centers generate lateral contact patch motion over bumps, which creates kick at the steering wheel. It is possible to build a beam axle suspension with a roll center below any component of the suspension, but the linkage required is somewhat complex. Consequently, beam axles on cars with enough ground clearance to be practical on the street generally have roll centers at least six inches high, and usually at least ten inches. Of course, with independent rear suspension, the roll center is usually much lower, but most often still a bit above the front one.

The next most obvious reason is that passenger cars are generally too nose-heavy to have balanced handling, and the front suspension doesn’t control camber when cornering nearly as well as the rear suspension. Consequently, we need to kill understeer, not increase it.

A somewhat less obvious reason has to do with driver-perceived car behavior in abrupt transient maneuvers, such as the lane-change test commonly used in passenger car testing. With a nose-down roll axis, there is a small yaw component with roll. The nose points out of the turn slightly, relative to the four contact patches. This makes the car feel steady to the driver, rather than twitchy.

Another reason sometimes cited is that when a car is abruptly steered into a turn, the geometric component of the load transfer is the first to act on the car. If this component is greater at the rear, we will momentarily have less understeer and the car will turn in more responsively. Note that this explanation is somewhat at odds with the one immediately preceding it.

There are somewhat logical variations on both of these two explanations. We could say that if the main mass of the car is yawing out of the turn relative to the four contact patches, that steers the contact patches into the turn, or steers the rear wheels out of the turn, momentarily adding oversteer!

Some people also believe that tire load sensitivity momentarily works backwards until the tires start heating. I personally don’t believe this, but if so it means that if there is initially more rear load transfer, that adds understeer rather than oversteer, and makes the car feel stable.

Isn’t this fun? If it weren’t for vehicle dynamics, I’d have to do something sane for a living.

The Mark Ortiz Automotive
CHASSIS NEWSLETTER
PRESENTED FREE OF CHARGE
AS A SERVICE TO THE
MOTORSPORTS COMMUNITY
January 2006

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: markortiz@vnet.net. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.

BIG TIRES ON THE FRONT

Last issue, I mentioned that there is a performance gain to be had in a front-wheel-drive car by making the car markedly nose-heavy and using larger tires in front than in back. This was in a rather lengthy response to a reader’s question partly relating to the Pontiac Grand Prix GXP. I have recently noticed in magazine reports about this car that GM is in fact using larger tires on the front of the V8 versions of this car: 255 section on the front, 225 on the rear.

TIRES IN THE SNOW
I have heard two schools of thought on tire pressure for winter driving. This applies to driving on snow-covered roads. The first is that tires should be kept at the upper end of the manufacturer’s specifications to help in cutting through the snow. The thinking is that the contact patch is smaller, hence more weight per square inch, as well as less sidewall deflection – which may decrease the potential for hydroplaning on the snow. The other is that running the pressure at the lower end allows for better bite for the tread in the snow, and more stability. What do you think?

I’m with the low-pressure school.

It’s said that one measurement is worth a thousand expert opinions. Really, you’d think this might be measurable. Surely somebody has tried measuring, say, how steep a hill a vehicle can climb, at various tire pressures. I would be willing to defer to any actual measurement that contradicts my expert opinion.

That said, I offer my expert opinion, and that expert opinion is based on a lot of time spent in Wisconsin, where there are long, snowy winters.

First, tire pressure effects in snow are surprisingly subtle compared to other variables, and effects due to other variables are surprisingly large. That explains why there is controversy about inflation pressures, even though people have been driving cars through more than a hundred winters now.

One reason tire testing was moved indoors, with rollers or belts substituting for pavement, is that even on hard, dry pavement, weather, surface contamination, pavement temperature, pavement age, and other factors make enough difference that small variations in tire performance are hard to measure repeatably.

When we’re dealing with snow, we have similar variability, exaggerated at least tenfold. Snow and ice come in dozens of different varieties and depths, and all of these have properties that are highly temperature-sensitive. Compared to snow, pavement is simple and consistent.

The explanation I have heard from sources that advocate high tire pressures is that the tire needs to penetrate the snow and get to the pavement, where it can find traction. But clearly, in most snow conditions that never happens, at any inflation pressure. If it did, we’d see bare pavement where the tire passed.

Instead, we see compacted snow, with an imprint of the tire tread. Or at least we see that if the tire is rolling, and not spinning or sliding – and if we are dealing with snow that has not already been compacted. We also usually see a small area alongside the track where the snow appears slightly raised, apparently having been pushed out of the way, perhaps just by the sidewall. This is not a lot of snow, however. Most of the snow stays put horizontally, and gets compacted vertically.

The tire evidently gets traction by packing the snow into a relatively solid form, and simultaneously interlocking with it. To break traction, the compacted snow projections residing in the tread grooves must be sheared off, and the layer of snow lying under the tread blocks must also fail in some manner.

The failure of the snow in the grooves is easily visualized as simple breakage. The failure of the snow under the tread blocks is a bit harder to visualize. It appears that the snow under the tread blocks contributes more to traction than one would imagine, because the tire’s grip is greatly improved by siping the tread blocks. It also helps to roughen the surface of the tread blocks.

I do not claim to perfectly understand the mechanics of structural failure of snow in a tire contact patch, but I do know that it is normally a combination of breakage and melting. Ice (and snow is ice crystals) can be melted by mechanical pressure – or, stating it a bit differently, the melting point of ice is lowered by mechanical stress, either compressive stress or shear stress. Anyplace that the snow or ice liquefies, its mechanical strength disappears, and it turns into a lubricant. The closer the ice or snow is to its melting point, the less mechanical stress is required to turn it to liquid.

So, when we compact snow, we make it stronger, but only up to the point where we start to get localized melting. The unit loading required to reach this point depends on how cold the snow or ice is. Moreover, short of the point where we encounter melting by compression alone, we see an increased likelihood of melting by the combination of compression and shear. In other words, as unit loading increases, we gain hardness but lose melt resistance. The hardness gain is fairly independent of temperature. The melt resistance loss is heavily affected by temperature, or at least its importance is.

From this, we might logically expect that the ideal contact patch size would be smaller in really cold weather than when we’re near thaw temperature.

I suspect that this is academic, however. I think the optimum contact patch size is far bigger than we can ever get with a tire. Consider the transportation devices that people have devised specifically for snow: snow cats, snowmobiles, snowshoes, cross-country skis. All of these operate by compacting the snow minimally, over a large area, and then trying to get maximum purchase on that large interface. For best performance on snow, or any other soft surface, we really want a belt or track, not a tire.

It would seem to follow that the more we can get a tire to act like a track, the better it should work. That would suggest a radial tire, at low pressure.

Note that it does not necessarily follow that we want a wide tire. It is generally agreed that for most winter conditions, a tire should be narrow. I think, based on the reasoning above, there will be winter conditions where a wide tire may be preferable. These may include bare ice and hard-packed snow, probably even shallow soft snow. But in snow of significant depth, narrow tires are better.

The reason for this doesn’t have to do with an increase in traction when the tire is narrow, as such. Rather, it has to do with the force required to move the tires, which is less when the tires are narrow.

As the tire rolls forward, it is resisted by the snow in front of it. To advance, the tire must, in effect, climb a ramp of snow. The ramp of snow is not strong enough to support the tire, and it is continually collapsing under the weight of the car. The amount of collapse is fairly similar regardless of the width of the tire; for any practical tire size, we will compact the snow to a pretty solid state, no matter what. Yet the snow has substantial resistance to this compaction, and this translates to a resistance to the wheel’s forward motion. The taller and wider the mass of snow we must compact, the greater the resistance to motion. The height of the snow we must compact depends on the snow’s depth. The width we must compact depends on the width of the tire.

It would also seem that a narrow tire should provide more directional control, since it is better shaped to act like a blade or rudder.

From this reasoning, we might expect that the ideal tire for deep snow would resemble a bicycle tire. Such a tire would be easy to push along, and should have good directional stability.

However, it doesn’t quite work that way with really narrow tires, as anybody who has tried riding a bicycle in deep snow will attest. The problem is that the ramp of compacted snow that the tire rides on is so narrow that the tire is forever sliding off the side of it into the soft snow alongside. As soon as the tire moves forward again, another narrow compacted ramp is formed beneath it, and again it slides off one side or the other – no predicting which side. The result is that the tire absolutely will not run straight.

So there is such a thing as too narrow. The tire needs to be wide enough to sit on top of the compacted ramp it is making for itself. A square-shouldered profile, or one with concave shoulders that compact a sort of retaining berm along the side of the main compacted ramp, also can be expected to help.

Returning to the question of inflation pressure, this also affects resistance to forward motion. And even this relationship is not as straightforward as one might think. Based on our experience with tires on pavement, on a smooth, hard surface, the higher the inflation pressure, the easier the tire rolls, at least within practical limits.

But on a rough surface, a softer pressure can actually roll more easily. For this to be so, the surface must have roughness as opposed to waviness: the ups and downs must come fairly close together. The tire rolls easier because it can yield to the bumps rather than having to climb over them. This was realized very early in the history of the pneumatic tire. John Dunlop immediately noticed that his new pneumatic tire would roll further across his bumpy back yard than a solid tire.

This has relevance to driving in snow because often the situation that gets us stuck is one where one or more wheels are in a fairly modest-sized depression, and we have to move the tire over the lip of the depression with the meager traction available. In at least some such situations, soft inflation will make getting over that lip easier.

It will be apparent that I am writing here from a mixture of practical experience and inference. I invite readers with further experience, or contradictory experience, to comment.

If you have ever wanted to build a budget race car, Keith Tanner is someone you should be aware of. He wrote a book about building a Lotus/Catherham Super Seven replica on a budget. The book is called How To Build a Cheap Sports Car. Be sure and check out the web site. He built the car and developed it into an autocross and time trials car.

Well Keith had undertaken another budget project. This one is called the Targa Miata. the intent is a budget competition Miata. He has just started the build, but will keep an online build log. If you want to know what it might take you to build a Spec Miata or similiar type car, it is worth following along.

I have been watching the current NASCAR Nextel Cup and Busch Grand National cars run for the past few years. Admittedly, I am currently not in contact with anyone in the garages anymore (guess I should get in touch with Elton Sawyer again sometime). But watching the cars I have become really annoyed with their current setups. The current aerodynamic rules have really changed the driving style necessary to drive the cars. They have also told me about a new dimension of the drivers that are winning as well.

The current are rules have pretty much eliminated front springs from the steady state handling equation. A soft rate spring is made by using a smaller wire diameter and a larger number of coils in the spring. The current spring rates, coupled with some well developed shock absorber design, have been set up so that the front suspension completely bottoms out (either with bump stops or coil binding) on the banked turns. The matching rear springs are set up very stiff giving the car a significant rake as the cars are in the turns. You might ask why? It all comes down to aerodynamics being developed so that it is more important than the true handling of the car.

The problem with this is that the bottomed out front suspension leaves very little feel to the front of the car. Some drivers have taken to this well, and others have gone from front-runners a couple of years ago, to backmarkers that often are blaming other drivers. As a fan, admittedly with more than average technical knowledge of the workings of the sport, I think it has severely hurt the wheel to wheel racing. I guess I like the driver to driver challenge, and I feel that this has removed many good drivers from being competitive at most of the tracks. I also feel it has eliminated much of the side-by-side racing.

But as long as you look at the full grand stands and increased television ratings, maybe NASCAR knows more about what is best than me. I just wish we could see the finish of the 1976 Daytona 500 every race, just maybe sometimes without the crash…..

Mark Ortiz Automotive News Letter 06-06

June 2006
Reproduction for free use permitted and encouraged.
Reproduction for sale subject to restrictions. Please inquire for details.

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: markortiz@vnet.net. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.

EFFECTS OF SPRING RATES AND DAMPER SETTINGS DURING CORNERING

I would be interested to hear your comments about the effects of relative spring rates (front/rear) and damper settings on steady-state cornering characteristics. I do not race, but I am interested in the effects of suspension settings on the relative under/over-steer characteristics.

I presume that (all other things being equal) any increase in spring rate at the front will increase the apparent roll stiffness and therefore make the car trend towards an increase in understeer (which I presume is the same as an increase in static directional stability) and an increase in the rear will cause the reverse. And an increase in damper rate at one end would seem to have a similar dynamic effect during turn-in, while the load transfer is actually taking place.

I own a Morgan which as you are aware does not have any anti-roll bars and I am about to change the front springs to a higher rate (140 lb/in vs. 105 lb/in) and I presume that this is going to cause some change in behavior. It would be interesting to know what order of magnitude of change I could expect, though, and whether it would be worth experimenting with the (adjustable) damper rates to try to modify turn-in behavior.

If I had more extensive experience with Morgans, perhaps I could predict the change with more confidence. As things stand, I can say that you have correctly understood the effect that spring rates have on steady-state handling balance in most cases: stiffen one end, and you get more load transfer at that end and less at the other, so you reduce grip at the end you stiffened and add grip at the other end.

However, in certain cases we can add roll resistance at the front and reduce understeer! This is most often seen in cars with beam axles in back, and independent suspension with poor camber recovery in roll in front most commonly small rear-drive sedans that roll a lot and have lowered MacPherson strut suspension in front. What’s going on in these cases is that although the front tires are less equally loaded, the reduction in roll improves their camber so much that the camber improvement more than makes up for the more unequal loading. At the rear, the beam axle gives 100% camber recovery at any roll angle, so rear camber is unaffected.

The Morgan is a similar case in some respects. It rolls a lot less than a tall, narrow sedan, but it has no camber recovery in roll at all with that sliding-pillar front suspension. The front wheels lean the same amount as the body. So when you add roll resistance at the front, you are hurting the load distribution at the front but helping the camber. At the rear, you are helping the load distribution and leaving the camber largely unchanged.

Also complicating prediction in the case of the Morgan is that the frame is unusually flexible in torsion. That mutes the effect of relative roll stiffness changes.

Actually, all cars with independent suspension in front and beam axles in back have poor camber recovery in front compared to the rear, so they all are subject to the same conflicting effects when we add front roll stiffness. Interestingly, when we change rear roll resistance the effects on front load distribution and front camber are additive rather than subtractive, and we can predict the effect on car behavior with much better certainty. Reducing rear roll stiffness will hurt both camber and load distribution at the front, while helping load distribution and not affecting camber at the rear. We know that will add understeer. Conversely, adding rear roll stiffness will help both camber and load distribution at the front, while hurting load distribution and not affecting camber at the rear. We know that will add oversteer.

As to the effect of shocks, yes stiffening the fronts will add understeer during entry and stiffening the rears will add oversteer, provided that the road surface is smooth. This effect requires that the car have a roll velocity outward, and that this be the main source of suspension movement. When the car is cornering steady-state on a smooth surface, the roll velocity should be zero, the suspension should have displacement from static but not velocity, and shocks shouldn’t matter. During exit, the car has a roll velocity inward (it’s de-rolling). In this situation, the effect of the shocks reverses. Stiffening the fronts adds oversteer; stiffening the rears adds understeer.

So to add understeer or oversteer overall, we use the relative stiffness of the front and rear springs (and/or bars, if present). To change entry and exit properties in opposite directions, we use the relative stiffness of the front and rear shocks (remember, only on smooth surfaces).

I sometimes refer to damper forces as creating frictional anti-roll or pro-roll (anti-de-roll). Even forces generated purely by a liquid may be termed a form of friction if they act in opposition to motion. Speaking of friction, I believe I have observed a phenomenon watching Morgans run that may be of interest here. I think that these cars can easily experience excessive friction in the sliding pillar mechanism when subjected to the forces modern racing tires can generate. This causes understeer until the driver finally gets the car rotating, gets on the power, and starts unwinding the steering. Then the car snaps into oversteer as the front end suddenly frees up and can roll. I therefore always tell people running these cars to keep the pillars in good condition and well lubed.

Readers may be a bit baffled by the questioner’s reference to “static directional stability”. After all, aren’t a car’s static properties the ones it has when it’s motionless? And any car is directionally stable when it’s sitting still, right?

The questioner is in fact using terminology that is familiar to engineers. It has been traditional to analogize a car’s directional stability to a statically stable stationary object, i.e. one which rights itself when disturbed a moderate amount by an outside influence or force, rather than tipping over. A directionally stable car tends to “right itself” similarly in yaw. If the car is disturbed in yaw while running straight, say by one wheel hitting a piece of debris, it will then travel down the road in a yawed condition, with all tires running at a slip angle. The car’s inertia then has a car-lateral component, as in cornering. If the car understeers in gentle cornering, it is said to have greater cornering stiffness at the rear than at the front. If that is the case, it will also tend to straighten itself out when disturbed; it will tend to rotate in the direction of its own inertia rather than the other way, absent any steering input from the driver.

Usually, discussion related to this ignores aerodynamic factors in directional stability, but actually the analogy applies, and its relation to under/over-steer applies, when aerodynamic yaw moments and downforce/lift are present.