Camber, Caster & Toe

The three major alignment parameters  are toe, camber, and caster. Most enthusiasts have a good understanding of what these settings are and what they involve, but many may not know WHY a particular setting is called for, or HOW it affects performance. Here are the basic aspects of steering tuning.

TOE

When a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tyres or wheels. Toe settings affect three major areas of performance: tyre wear, straight-line stability and corner entry handling characteristics.

For minimum tyre wear and power loss, the wheels on a given axle of a car should point directly ahead when the car is running in a straight line. Excessive toe-in or toe-out causes the tyres to scrub, since they are always turned relative to the direction of travel. Too much toe-in causes accelerated wear at the outboard edges of the tyres, while too much toe-out causes wear at the inboard edges.

So if minimum tyre wear and power loss are achieved with zero toe, why have any toe angles at all? The answer is that toe settings have a major impact on directional stability. The illustrations at right show the mechanisms involved. With the steering wheel centered, toe-in causes the wheels to tend to roll along paths that intersect each other. Under this condition, the wheels are at odds with each other, and no turn results.

When the wheel on one side of the car encounters a disturbance, that wheel is pulled rearward about its steering axis. This action also pulls the other wheel in the same steering direction. If it's a minor disturbance, the disturbed wheel will steer only a small amount, perhaps so that it's rolling straight ahead instead of toed-in slightly. But note that with this slight steering input, the rolling paths of the wheels still don't describe a turn. The wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability.

If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So toe-out encourages the initiation of a turn, while toe-in discourages it.

The toe setting on a particular car becomes a tradeoff between the straight-line stability afforded by toe-in and the quick steering response promoted by toe-out. Nobody wants their street car to constantly wander over tar strips-the never-ending steering corrections required would drive anyone batty. But racers are willing to sacrifice a bit of stability on the straight-away for a sharper turn-in to the corners. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.










With toe-in (left) a deflection of the
suspension does not cause the
wheels to initiate a turn as with
toe-out (right).
 
 
 
 
 

The amount of toe-in or toe-out dialed into a given car is dependent on the compliance of the suspension and the desired handling characteristics. To improve ride quality, street cars are equipped with relatively soft rubber bushings at their suspension links, and thus the links move a fair amount when they are loaded. Race cars, in contrast, are fitted with steel spherical bearings or very hard urethane, metal or plastic bushings to provide optimum rigidity and control of suspension links. Thus, a street car requires a greater static toe-in than does a race car, so as to avoid the condition wherein bushing compliance allows the wheels to assume a toe-out condition. Trad Morgans have a single bush at each links No change is necessary for racers..

BTW, designers have been using bushing compliance to advantage. To maximize transient response, it is desirable to use a little toe-in at the rear to hasten the generation of slip angles and thus cornering forces in the rear tyres. By allowing a bit of compliance in the front lateral links of an A-arm type suspension, the rear axle will toe-in when the car enters a hard corner; on a straight-away where no cornering loads are present, the bushings remain undistorted and allow the toe to be set to an angle that enhances tyre wear and stability characteristics.

CASTOR

Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative.

Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. The mechanism that causes this tendency is clearly illustrated by the castoring front wheels of a shopping cart (above). The steering axis of a shopping cart wheel is set forward of where the wheel contacts the ground. As the cart is pushed forward, the steering axis pulls the wheel along, and since the wheel drags along the ground, it falls directly in line behind the steering axis. The force that causes the wheel to follow the steering axis is proportional to the distance between the steering axis and the wheel-to-ground contact patch-the greater the distance, the greater the force. This distance is referred to as "trail."

Due to many design considerations, it is desirable to have the steering axis of a car's wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tyre contact patch. The trail would be zero, and no castoring would be generated. The wheel would be essentially free to spin about the patch (actually, the tyre itself generates a bit of a castoring effect due to a phenomenon known as "pneumatic trail," but this effect is much smaller than that created by mechanical castoring, so we'll ignore it here). Fortunately, it is possible to create castoring by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tyre contact patch, and thus the same effect as seen in the shopping cart casters is achieved.

The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by imagining the unrealistically extreme case where the steering axis would be horizontal as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it.













Like a shopping cart wheel (left) the trail
created by the castoring of the steering
axis pulls the wheels in line.
 
 
 
 

Most cars are not particularly sensitive to caster settings and they cannot be changed on trad Morgans without the use of muscle as there is no adjustment feature built in. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings.

CAMBER?

Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the car, it has positive camber (see next page). The cornering force that a tyre can develop is highly dependent on its angle relative to the road surface, and so wheel camber has a major effect on the road holding of a car. It's interesting to note that a tyre develops its maximum cornering force at a small negative camber angle, typically around neg. 1/2 degree. This fact is due to the contribution of camber thrust, which is an additional lateral force generated by elastic deformation as the tread rubber pulls through the tyre/road interface (the contact patch).

To optimize a tyre's performance in a corner, it's the job of the suspension designer to assume that the tyre is always operating at a slightly negative camber angle. This can be a very difficult task, since, as the chassis rolls in a corner, the suspension must deflect vertically some distance. Since the wheel is connected to the chassis by several links which must rotate to allow for the wheel deflection, the wheel can be subject to large camber changes as the suspension moves up and down. For this reason, the more the wheel must deflect from its static position, the more difficult it is to maintain an ideal camber angle. Thus, the relatively large wheel travel and soft roll stiffness needed to provide a smooth ride in passenger cars presents a difficult design challenge, while the small wheel travel and high roll stiffness inherent in racing cars reduces the engineer's headaches.

It's important to draw the distinction between camber relative to the road, and camber relative to the chassis. To maintain the ideal camber relative to the road, the suspension must be designed so that wheel camber relative to the chassis becomes increasingly negative as the suspension deflects upward. The illustration on the bottom of page 46 shows why this is so. If the suspension were designed so as to maintain no camber change relative to the chassis, then body roll would induce positive camber of the wheel relative to the road. Thus, to negate the effect of body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e., gains negative camber) as it is deflected upwards.

While maintaining the ideal camber angle throughout the suspension travel assures that the tyre is operating at peak efficiency, designers often configure the front suspensions of passenger cars so that the wheels gain positive camber as they are deflected upward. The purpose of such a design is to reduce the cornering power of the front end relative to the rear end, so that the car will understeer in steadily greater amounts up to the limit of adhesion. Understeer is inherently a much safer and more stable condition than oversteer, and thus is preferable for cars intended for the public.

Since most independent suspensions are designed so that the camber varies as the wheel moves up and down relative to the chassis, the camber angle that we set when we align the car is not typically what is seen when the car is in a corner. Nevertheless, it's really the only reference we have to make camber adjustments. For competition, it's necessary to set the camber under the static condition, test the car, then alter the static setting in the direction that is indicated by the test results.

The best way to determine the proper camber for competition is to measure the temperature profile across the tyre tread immediately after completing some hot laps. In general, it's desirable to have the inboard edge of the tyre slightly hotter than the outboard edge. However, it's far more important to ensure that the tyre is up to its proper operating temperature than it is to have an "ideal" temperature profile. Thus, it may be advantageous to run extra negative camber to work the tyres up to temperature.

(TOP RIGHT) Positive camber: The bottoms of the wheels are closer together than the tops. (LEFT) Negative camber: The tops of the wheels are closer together than the bottoms. (CENTER) When a suspension does not gain camber during deflection, this causes a severe positive camber condition when the car leans during cornering. This can cause funky handling. (RIGHT Fight the funk: A suspension that gains camber during deflection will compensate for body roll. Tuning dynamic camber angles is one of the black arts of suspension tuning.

Morgan Front Suspension Specifications (These are for earlier cars. to be sure please confirm with your Manual or contact the webmaster with your year and model)

If you take your car to an alignment shop, be sure to tell them you want to know the Camber angle, Castor angle and the kingpin inclination. (You can check by asking them what the values are, rather than telling them.

Toe in should be set to 2 degrees (or 1/8-3/16") of toe in..

Camber angle should be 2 degrees positive camber (the top of the tires slant outward like this "\ /") for all 4/4 Series II through Vs and +4s with the TR engines.

Castor angle should be 4 degrees for all 4/4 Series II through Vs and +4s with the TR engines.

Kingpin inclination should be 2 degrees for all 4/4 Series II through Vs and +4s with the TR engines.

These last 3 items are fixed by the design and assembly of the front suspension. While it is conceivable that they can be adjusted it is done with brute force. So stop and think about it before changing any of these settings unless the car has just been in an accident. (I would be remiss if I did not point out at this time that there is a "modification to the front suspension call a "negative camber" mod. This will change the tire stance from " \ /" to "/ \" ). However, this modification will take a little work and there is some discussion as to its applicability for street use. Fred Sisson as a nice write up about it in his book.

If none of the above has helped solve the front end vibrations, you will have to come face to face with the mystique of the Morgan front end.

ALL Plus 8 Steering Dynamics (1968 to 2004)  (MMC manuals)

Toe in set to 0-1/8" (0 to 3.2mm) of toe in..

Camber angle set at 2 degrees non-adjustable.

Castor angle set at 4 degrees non-adjustable.

Kingpin inclination set at 2 degrees.
 

Tuning Up Front (EARLY CARS THAT SWITCH TO RADIAL TYRES ONLY)

by Gerry Willburn

DECAMBERING A MORGAN (for a 0 camber): I will not attempt to go into the pros and cons of decambering your Morgan. Suffice it to say that all of the racers do it, most people recommend it if you use radial ply tires, and our SNOBMOG is not decambered.

To decamber your Morgan, you must move the bottom of the kingpin outboard with respect to the top. This can be accomplished in several ways. On the Le Mans cars, Chris Lawrence had longer bottom tubes made for the cross-axle. The local slalom crowd has been using "Decamber Plates". This is a replacement base plate with the hole for the kingpin moved 3/4" outboard. This is a very simple modification to make and the only drawback seems to be that a greater strain is put on the two mounting bolts for the base plate (because of the greater offset). Grade 5 or better bolts should be used here and torque them to 20 lbs. feet.
 
Webmaster Note: I would suggest that you manufacture the new base plate (see drawing) before starting the following procedure.

 
Webmaster Update: (July 2012) Considering the LATEST. one can now purchase adjustable camber plates at from the MMC.


THE PROCEDURE

You should have 1/8 to 3/16 inch of toe-in. ie. shorter in the front.

Toe-in Adjustment

  1. Loosen the stop-nut(s) at the end(s) of the track rods between the front wheels and also the clamp on the rod from the steering drop-arm to the track rod. Note the position of the clamp on the track rod.
  2. Rotate the track rod to lengthen it or to shorten it as required.

  3. Note: If the front has just been decambered, the rod should be too short and will have to be lengthened (about one inch).
  4. When the toe-in is correct (1/8 to 3/16 inch), tighten the jam-nut(s) on the ends of the track rod.
  5. Tighten the clamp on the tie rod in the same position that it was in before.
Note: If you forgot to mark the position of the clamp or if you just wanted to check the centering of the steering, do it now before you tighten the clamp on the tie rod.
 

SPECIAL NOTE: CAMBER ON WIDEBODIES ( +8s from 1998 on and all cars from 2004)
July 2012

There are "rumors" that the crossheads made for the widebody (used on some Plus 8s from 1998 and later on classic models from 2004) are imprecise, resulting in non-matching and/or non-ideal camber set-ups for the cars affected. A tyre shop or mechanic who is fully equipped with a precision steering alignment device can quickly confirm whether your Morgan is afflicted. The cure will require a Morgan knowledgeable machinist to remachine the camber as required. However, though there is no news that the crosshead has been remachined as yet, the Morgan Motor Company started using adjustable camber plates on the 2011 Plus 4 Super Sports and reported in the Morgan Wire at that time) . These are now being fit to the wider Roadster 3.7. The factory is now using them to adjust out the errant camber before delivery. Owners who confirm the issue on their cars can then inquire whether it can cure the problem. We suggest you have the camber checked before approaching your Morgan dealer and expert (bring the written results to them). Car manufacturers will always have recommended toe and caster settings. They are supposed to arrive at these numbers through exhaustive testing and experience. The only owners who need normally change the stock camber are racers.