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

A BIT MORE ON SCRUB RADIUS

Last month I responded to a question on scrub radius, in relation to upright design. In my answer, I said that scrub radius per SAE terminology is not really a radial measurement but a front-view offset. A reader has pointed out to me that the ISO has more recently created a standard of its own for this terminology that is more semantically rational.

Per ISO standard, scrub radius is not synonymous with front-view steering offset. ISO steering offset is what the older SAE standard calls scrub radius, and ISO scrub radius is a quantity not included in the SAE terminology. It is the radial distance from the contact patch center to the steering axis, taken normal to the steering axis. That is, it is the length of the moment arm for ground plane forces about the steering axis.

That moment arm can then have any angular orientation, but its length cannot have a negative value. So if we’re using this definition of scrub radius, there really cannot be such a thing as a negative scrub radius.

However, there can be such a thing as a negative ISO steering axis offset or SAE scrub radius, and the sign convention is the same per ISO or SAE: negative when the steering axis intersects the ground plane outboard of the contact patch center and intersects the wheel plane above ground; positive when the steering axis intersects the ground plane inboard of the contact patch center and intersects the wheel plane below ground.

ISO steering axis offset or SAE scrub radius is largely independent of trail and is close to being a strict function of upright and wheel design. It is not greatly affected by caster. ISO scrub radius is also influenced by wheel and upright design, but it changes a great deal when we adjust caster.

LEFT-OFFSET LOWER CONTROL ARM MOUNTS WITH LEFT-OFFSET ENGINE?

We are building our first ever straight rail super late. In looking at suspension mounting choices, I have noticed most manufacturers, when building front clips, are utilizing offset inner pivot points. We have built front clips here in the past as well and have also utilized offset. The difference is that our offset was to the right (moving equal length lowers to the right of the mass). The manufacturers I have seen are moving their inner pivot location to the left, which would require shorter left side lower control arms to enhance left side weight. My question is: what are the benefits of this design? I realize that a shorter left LCA would affect camber gain, as well as moment center – what else could be going on here?

First of all, the length of the lower control arms has no effect on left side weight, and the theory that one can or should take moments about the force line intersection is incorrect, although modeling based on this theory ends up close to correct when the suspension is close to symmetrical and both right and left suspensions have some anti-roll or both have some pro-roll.

That said, does it make sense to use a longer lower control arm on the right side than on the left, for an oval track car with the engine offset to the left? Probably so. In particular, there is a case for this when the track includes dissimilar turns, as for example at Pocono. When the turns are alike, we can get away with almost any front end geometry by adjusting the static settings to suit. When the turns are dissimilar, and the suspension displacements are therefore different in the different turns, or when there are right turns, suspension geometry becomes much more important.

In a stock car, there is generally a limit to engine setback which dictates that the engine will be between the control arms, not behind them. Generally, the block, cylinder heads, and exhaust headers will limit where we can put the inner pivot axes of the upper arms. If the engine is offset to the left, then the inner pivots of the upper arms will likewise be offset to the left, and the left upper arm will be shorter than the right upper. If we want similar upper to lower length ratios right and left, the inner pivots for the lower arms should be offset to the left as well.

The length of the upper arm, relative to the lower, doesn’t really control camber gain (camber rate of change with respect to ride displacement). Instantaneous front view swing arm length controls that, and that depends on the relative angles of the arms rather than their lengths. The length and length relationship of the control arms affect front view swing arm length rate of change. We might say this is not camber gain but camber gain gain. Camber gain is the first derivative of camber with respect to suspension displacement. Camber gain gain would be the second derivative of camber with respect to displacement, or the first derivative of camber gain with respect to displacement.

By keeping the control arm length ratio similar on both sides of the car, we keep the rates of change of camber gain and geometric anti-roll similar, or at least in a somewhat similar relationship to each other, on both sides. If the track is banked, right suspension displacement will be greater than left, so having both upper and lower arms longer on the right probably will be desirable for most applications.

Additionally, offsetting the steering rack along with the lower arm inner pivots will probably simplify steering shaft routing.

HOW BIG ARE WHEELS GOING TO GET?

When does the current trend to increasing wheel diameter reach a limit, or sharply diminishing returns? Even on street cars 20 and 21 inch diameter wheels are common. As wheel diameter increases the weight increases, but also the rotational inertia increases. It takes power to compensate both characteristics, and both increase more than linearly with diameter. Yes, there is more room for bigger brakes, but 18 inch wheels would seem to provide more than adequate brake space for most applications. There are other factors, including gyroscopic forces, increased car polar moment of inertia (more stability but lower maneuverability) and unsprung weight. I can appreciate this may be a difficult question to answer.

One thing that makes this tricky is that this is not purely an engineering decision. For road cars, it is heavily influenced by fashion, which in turn is influenced by the desire to cultivate planned obsolescence. In racing, generally the rules dictate wheel size.

From an engineering standpoint, there isn’t any “knee in the curve”: there isn’t any point of sharply diminishing returns on increased wheel diameter. There are gradually increasing penalties and gradually diminishing returns.

To some extent, we can look at tires and brakes similarly, and also clutches. They are all friction devices. With all of them, there is a lower size or swept area threshold below which there is simply no way to get even marginally adequate performance. Above that minimum, there exists a tradeoff matrix that involves force capability, size, weight, cost, temperature sensitivity, and longevity. We can improve any of these at the expense of some or all of the others.

The size, weight and cost aspects are pretty straightforward. The other aspects include some subtleties that may not be immediately apparent.

When we make a tire larger in diameter, with similar section dimensions and construction, for a given inflation pressure the contact patch theoretically should stay the same width and length. Its area should equal load divided by inflation pressure, or a fairly constant percentage of that. The static deflection should therefore decrease, because with a larger diameter that contact patch length subtends a smaller arc. Any given portion of the tread should spend less time in contact with the road. This should reduce operating temperature and tread wear. Rolling resistance should also decrease. We can then trade some of these gains away for better traction if we wish, by using a softer tread compound with more hysteresis. We may opt to make the sidewalls more vertically compliant and get similar static deflection but a longer contact patch.

Larger diameter tires do a better job of bridging small surface irregularities. They have an easier time climbing over and/or mashing down deep snow or mud in front of them.

I mentioned that larger diameter tires theoretically should last longer. In a street use context, it is logical to question whether it is worthwhile to cart around an extra six months’ or year’s supply of rubber, when it would cost no more and maybe even cost less to simply replace cheaper tires more frequently. Also, tires sometimes do not last the life of the tread. Sometimes they get damaged. Sometimes they get blisters or belt separations. Sometimes they just get too old and hard. Most of these factors favor cheaper tires, replaced more often – ergo, smaller diameter.

For roadgoing performance cars, there is also a limit to how soft we can make tread compounds and still have reasonable puncture resistance.

This may not matter where tire size is governed by the rules, but there is in theory some advantage to having larger diameter for racing in that the tire will heat less in a long-duration turn and will therefore not go off as rapidly due to heat cycling. This could advantage a car with large diameter wheels in a production car class where the rules require use of the same size tires as original equipment. On the other hand, if the tires that are needed to be competitive like to run hot and cars have difficulty getting them up to temperature, large diameter could be a disadvantage. It could even be good or bad according to the weather.

With brakes, bigger size lets us use pad compounds having more friction at low temperatures without encountering fade in hard use. Alternatively, more size will let us use a pad with less low-temperature friction but better high-temperature properties and still get adequate panic stop torque. Overall, the task of finding a suitable mix of torque, temperature tolerance, and longevity gets much easier as the brake gets bigger.

Larger diameter wheels, then, are something of a fad at the moment but they do have some functional advantages. They are not an unmixed blessing but they probably make more functional sense than upholstering the outside of the roof.