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MORE ON SUPERMODIFIED REAR ENDS

I have a minor correction for your September newsletter. Supermodified rear suspensions are generally an open tube axle (like a sprint car). Some folks run splined wide 5 or six pin adapters, but since splined wheels have become much more commonplace, a lot of guys have switched over to them on supers. There were a few experiments with closed tube axles back in the late 80's, but the added weight of a closed tube axle means they are generally not run.

There is always a right birdcage with an open tube axle, but in a lot of cases the left birdcage is not run and the torque arm does double duty, both locating the axle longitudinally and reacting drive/brake torque. If there is no left birdcage, the left brake caliper gets bolted directly to the side bell of the rear end using lugs cast in place by the manufacturer. There are a ton of variations between cars in this area, which is one of the things that makes supermodifieds cool.

This comes from Graham Gott at Stackpole Engineering. He’s correct. This response led me to update myself a bit on recent developments in the Supermodified class. My comments in the September newsletter were based on my recollections from helping with a Super in California in the ‘80’s, plus some pictures I’d seen much more recently. Supers don’t run in North Carolina, where I’ve lived since 1999.

I did find pictures online now of Supers with wide-5 hubs (not just adaptors – hubs like you’d find on a dirt Late Model), but the pictures – and those I was viewing earlier but fairly recently – were all at least a couple of years old, and all were of cars from the western US. It appears that east of the Rockies, open tube axles have been the norm for many years.

It further appears that Supermodified racing on the west coast has been in a state of collapse since 2011. On the other hand, it appears to be newly popular in the Midwest, and still popular in Ohio, Pennsylvania, and New York.

I definitely agree that having lots of variation in the cars makes the class interesting. Back in the ‘80’s on the west coast, it was really wild. A “best rules is no rules” philosophy prevailed. I don’t remember open tube axles, but there were rear-engined cars. One of them, fielded by the Triguiero brothers, had four-wheel drive. Another had the engine mounted backwards, in front of the left rear wheel, with the driveshaft running back along the engine’s right side to a conventional (for a Super) radically offset quick-change beam axle with a torque arm.

Beam axle rears with torque arms were almost universal, but quite a few cars had independent front ends.

Engine choice varied too. Big-inch aluminum small block Chevys were the most common, but the car with the backwards rear engine had an aluminum big block. One car had an injected, unblown Keith Black hemi. Sometimes Keith Black himself showed up to help tune it. There was one car with forced induction: the Gerhardt Offy. It had an Indianapolis-style turbocharged four-cylinder Offenhauser. Nobody else used a turbo. This was short track racing, and turbo lag was a disadvantage. Roots blowers would have been legal, but I never saw one.

Nowadays, where Supers still run, they all have aluminum big block Chevys, and for the most part they are required to have beam axle front ends. However, there are some interesting possible variations in the design of the rear suspensions.

Other than weight, in terms of suspension dynamics an open tube rear is not much different from a closed tube with a spool, with one or more birdcages. In either case, there is no law of nature that requires us to have birdcages at both ends. With the open tube, we do have to have one at the right end.

It would theoretically be possible not to have a brake at that end of the axle, but the rules require four brakes, one for each wheel. The two rear brakes both act on both wheels, as with any locked axle, but each caliper reacts its torque through its own linkage. Therefore, we cannot get yaw moments from staggered caliper or rotor sizes, or from side-to-side hydraulic proportioning, as we might at the front, but we can have various effects on dynamic wedge in braking depending on side-to-side rear brake proportioning combined with right and left anti-lift or pro-lift properties.

I am not personally aware of anybody actually using left/right brake bias as a tuning variable on a Super, but in a car where the left wheels statically carry about twice as much weight as the rights, it would seem to be something to seriously consider.

Lateral brake bias probably offers possibilities that have not yet been explored, and not just for Supermodifieds. It might be possible to use four master cylinders, and three balance beams in series-parallel, but that sounds pretty scary. I have seen the right front brake eliminated entirely on sprint cars – in fact, that’s the norm. I have seen shutoff valves and proportioning valves used to limit right front brake apply pressure. I have seen the use of front tire stagger, whose main effect is to provide front lateral brake bias.

I have yet to see anybody use different size brakes on the two front wheels, but that would definitely be possible. Just one possibility, using popular calipers, would be to use a caliper with four 1.75” pistons on the left, and one with four 1.375” pistons on the right. With equal-size rotors and tires, that would give about a 62/38 lateral brake bias. On a Super, that would probably give better straight-line braking than symmetrical front brakes. The car would have less tendency to turn right, and less tendency to prematurely lock the right front.

With enough built-in left brake bias, we might even contemplate using an adjustable proportioning valve for the left front rather than the right front.

Proportioning valves generally output a fixed proportion of the input pressure, once they reach their actuation threshold; short of activation threshold they output the input pressure unchanged. If we plot output pressure as a function of input pressure, we get a straight line with a slope of 1 up to actuation threshold. There, the plot has a “knee” and is linear again from there on, with a slope of less than 1. What we change with the adjuster is the actuation threshold, or the location of the knee. The slope of the plot after the knee is determined by the rate of the spring in the valve. The adjuster generally changes the spring’s preload.

When we use a proportioning valve to the right front, we get equal line pressure to the right and left up to actuation threshold, and then an increasing left bias from there. We have greatest left brake bias in hardest braking. If instead we had asymmetrical front brakes and a proportioning valve for the left, we’d have greatest left bias in gentle braking and decreasing left bias in harder braking. This could be highly desirable, if the objective is to use lateral brake bias to free the car up when trailbraking on entry, yet not have the car try to spin when braking hard in a straight line.

In any case, it should be noted that in braking, as with lateral force, ground plane force distribution influences the action of any “anti” or “pro” effects we have in our suspension geometry. We might also note that in the case of a locked rear axle, caliper force distribution affects the linkage forces, without necessarily changing ground plane force distribution, and that with any beam axle the right side linkage acts partly on the left tire and vice versa.

Open tube axles are always locked, as a matter of structural necessity. With a closed tube axle, we can run a locker or a limited slip. In that case, rear lateral brake bias generates yaw moments the same as it does at the front, in addition to affecting wheel loads by influencing “anti” effects.

It is quite possible to eliminate the drop link at the front of the torque arm and just have a single pivot attaching it to the sprung structure, and then eliminate the left birdcage and its usual two trailing links. This is possible with a closed tube axle as well. This option saves parts, and therefore weight and complexity. If a structure similar to a torque arm has no drop link, and locates the axle longitudinally, I would call that a trailing arm or radius rod assembly rather than a torque arm.

To behave similarly under power to a torque arm plus left birdcage on horizontal trailing links, the trailing arm needs to have the front pivot at axle height, at the same longitudinal location as where the drop link would otherwise be. The system then appears to be equivalent, and we’ve simplified it.

To mount the caliper on the left bell of an open tube center section, we just use off-the-shelf sprint car parts.

However, the penalty is that we give up having separate control of left rear anti-squat (under power) and anti-lift (under braking), because we are using the same linkage to react both the torque from the left rear brake and the torque from the pinion. Is that a problem, actually?

The main reason for having so much anti-squat, so far to the left, is to counteract the tendency of a left-heavy car to turn left under power and consequently be loose on exit. The anti-squat makes the car gain a lot of wedge under power, reducing the oversteer – sometimes to the point of creating a power push.

A left-heavy car also has a tendency to turn right under braking, making it tight on entry. In addition to lateral brake bias, we can use wedge change to control this, as we do on exit. Having a lot of anti-lift on the left rear and relatively little on the right rear will make the car de-wedge on entry, reducing the understeer – but again, perhaps excessively.

There is no guarantee that the same left side geometry will give us the balance we seek for both entry and exit. However, assuming that we are setting up for minimal roll steer, the right side linkage will not generate any significant pro-squat or anti-squat under power, but it can be made to generate just about any amount of anti-lift we want under braking. That means we can optimize the left side linkage to balance exit behavior, and balance our entry behavior with the right side linkage. We merely have to be able to vary the geometry on both sides.

That would involve having two separate trailing links on the left, rather than a single trailing arm, and two separate links on the right, per usual practice, with sliders or multiple holes to mount the links.

One other possibility, when using a trailing arm or two trailing links on the left in place of a torque arm, would be to put the left caliper on a brake floater. That’s like a birdcage, but with only one link. It reacts the torque from the brake, generating whatever pro-lift or anti-lift we wish, while letting something else provide longitudinal location.