Inside Floater Rear End Technology

It should come as no surprise that the axles on a drag racing vehicle undergo some incredibly demanding forces unlike many other types of racing – due both to the design of the typical rear end housing, and the dead-hook nature of the straight-line sport. The axles first carry the lateral load of the weight of the vehicle, but they are then exercised by the torsional load of transferring the horsepower and torque from the third member out to the wheel and tire at launch, putting a double-whammy of stress on them.

Despite the performance of racing vehicles growing exponentially over the years, axle manufacturers have kept up with the curve to deliver highly-engineered pieces that are up to task at hand, but the demands that are placed on the drivetrain as a whole when the transbrake button is released still prove too much in certain circumstances. We’ve all witnessed at one time or another a rear axle breaking at the flange, often right at the starting line but occasionally at speed in what quickly becomes a nightmare scenario for the driver.

Strange Engineering produces floater housings for everything from nitro cars down to Pro Mods and outlaw small-tire cars. This housing in particular is their all-aluminum version for the Pro Mods with carbon fiber brakes.

Fortunately, there’s a solution for this for the most extreme of drag racing machines that operate outside the realm of the typical flanged axle housing — one based largely upon a design envisioned by the major automakers decades ago for heavy duty vehicles.

Full floaters, as they’re known, separate the load-carrying and the torque delivery roles of an axle — the lateral and torsional forces described above — by placing the weight of the vehicle elsewhere on the housing, leaving the axle free to do the job that it’s intended to do: transmit the horsepower and torque to the wheels. In a full floater, the weight-bearing role is handed off to a spindle that’s welded onto the axle tube of the rear end housing, with the spindle itself and a set of bearings that ride on the spindle supporting the weight.

Such technology has been employed in drag racing for many years, particularly in the Top Fuel and Funny Car, Top Alcohol, and Pro Modified categories, where the forces far exceed the capabilities of any other design. And such technology has been made possible by renowned manufacturers like Mark Williams Enterprises and Strange Engineering, who we’ll hear from later in this text as we take a closer look at full floater rear end housing and components, discuss the benefits and advantages of a full floater, and show you how they work in practice.

This diagram shows a cross-section cutaway of a Mark Williams full floater assembly.

What Is A Full Floater, You Ask?

Full float housings and axles have long been utilized on higher GVW (gross vehicle weight) trucks and machinery — often 3/4-ton and larger — to hold up to the strenuous demands that such vehicles often undergo. They’re also used heavily in off-road vehicles, such as rock climbing trucks, racing trophy trucks, and those intended for going through mud and other extreme environments.

By their very nature, full float axles are not only far superior in strength to standard housings and axles, but they’re also much safer — which is a key selling point for the drag racing industry.

On our housings, the drive plate rests on the wheel studs which are pressed into the hub. The floater axle spline engages the drive plate which rotates the wheel and tire. – J.C. Cascio

A full floater housing begins with the axle tubes on the housing itself, where a set of spindles are welded onto the ends of the tubes as the support base for the floater hub assembly. Inside the hubs are a set of ball bearings — typically spaced apart with one at the inner and outer portion of the hub — with their inner race ‘riding’ on the outer portion of the spindle, allowing the hub to spin but also supporting the lateral weight of the vehicle.

The end of the spindle and the retaining method can vary from manufacturer to manufacturer, as Mark Williams explained to us. MW’s spindles feature a retaining ring groove and a retaining ring that rests up against the outside bearing that serves as the primary retaining method for holding the hub on the spindle. In the case of Strange Engineering, their spindles are threaded, and a spindle nut and retainer ring hold it in place on the spindle by delivering the proper amount of preload on the bearings.

Once the hub is in place, a drive plate that’s matched to the wheel studs on the hub is installed to hold the hub bearings in place. The drive plate is generally sealed at the inner half to keep the differential grease from working its way out of the axle housing. As far as the axles, a floater axle shaft is splined on both ends, with one engaging the spool and the other in the drive plate, which is splined to match.

A breakdown of the components in a Mark Williams full floater assembly. Shown at left is a fully assembled (minus the axle for photography purposes) hub assembly, while the individual components, including the spindle, hub/brake rotor, drive plate, axle cap, brake caliper mount, and hardware are displayed at right.

“On our housings, the drive plate rests on the wheel studs which are pressed into the hub,” explains Cascio. “The floater axle spline engages the drive plate which rotates the wheel and tire. There’s a snap ring on the floater axle that’s captured between the drive plate and the drive plate cap, and that prevents the axle from sliding out.”

Strange Engineering offers a complete line of floater components, including 40-spline gun drilled and solid axles, floater-specific carbon and steel brake rotors, and floater kits.

Mark Williams Enterprises, likewise, has a full line of components and even floater assemblies, on up to complete, assembled rear end housings in the nine and 11-inch varieties for such combinations as those in the Top Alcohol, Pro Mod, Top Dragster, Top Sportsman, and similar eliminators.

The Advantages Of A Full Floater

As you can likely ascertain based on the explanation above of how a full floater housing and axle setup work and what it’s designed to accomplish, there are two distinct advantages to their use. First, because the axle itself doesn’t support the lateral weight of the vehicle, and because it also being held into the housing with a cap, the axle is essentially unable to break free from the car. This means a floater is infinitely safer than a standard flange axle, because even in the event that a floater axle were to break, the wheels and tires would remain with the car, and the car would still be capable of rolling to a stop, because it’s the spindle, the hub, and the bearings within the hub that the vehicle is actually sitting on, and not the broken axle.

The threaded-style spindle that Strange Engineering utilizes on their housings. this spindle is welded onto the ends of the axle tubes of the housing, and the threaded end of the spindle becomes the primary lateral weight-bearing mechanism for the vehicle. The two ball bearings inside the hub are press-fit onto this spindle and handle the load of the vehicle.

“With a regular flange axle, you can develop cracks near the flange, and when the axle breaks the flange is gone and therefore the wheel is gone. So a floater prevents that wheel loss,” Williams explains.

You may be wondering, as well, what would happen if the retaining rings themselves were to come loose. According to Williams, the snap ring used in their floater housings is rated at around 31,000 pounds, making them virtually impervious to failure. In the rare circumstance where a snap ring were to fail however, he explains, “When you have the brake calipers over the rotors, it would have to shear the rotor off in order for the hub and the wheel to come completely off. The caliper is essentially holding the hub on in that case. Of course that’s only as good as the strength of the brake rotor itself — on a car with steel brakes, it would be pretty substantial, but it wouldn’t take much to break carbon fiber rotors in that instance.”

Full Floater Requirements

Per the current NHRA rules, full floating or live axle rear ends are required on all Top Fuel Dragsters, Funny Cars, Top Alcohol Dragsters, Top Alcohol Funny Cars, and Pro Modifieds running either a supercharged or turbocharged engine combination, along with all Nostalgia Top Fuel and Funny Cars in the NHRA Hot Rod Heritage Series. Nitrous oxide-injected Pro Modifieds, although not at present required to run floaters, are to be evaluated prior to the 2015 racing season, according to NHRA Tech Department personnel. The PDRA requires full floaters in their Pro Extreme, Pro Nitrous, and Pro Boost classes.

The other advantage of the floater, which was also hit on earlier, is the ability to handle more of a total vehicle weight load or higher degree of torque, which is why, in addition to the safety aspect, they’re required on 8,000 horsepower nitro cars, 4,000 horsepower Pro Mods, and often advised for use in heavier racing vehicles, such as heads-up small-tire doorslammers. This is accomplished not only by the weight riding on the welded spindle, but by the presence of the two ball bearings inside the hub.

In the case of a nitro car, the axles undergo perhaps the highest torsional load of any piece of machinery on the planet, but they also carry an extreme amount of weight at speed, brought on by the estimated 4,000 to 5,000 pounds of downforce the rear wings and rear spoilers are delivering to the cars at over 300 miles per hour. This being the case, you can understand why nothing less than a full floater is acceptable.

Says Williams, The floater carries a heavier load because it has the two bearings out there supporting the load, instead of a single axle bearing on a standard flange axle. So it’s got more overall bearing strength.”

Although it was likely coming at the time, Williams cites the devastating crash and fire experienced by Top Fuel racer Herm Peterson at Orange County in July of 1973 as the turning point to the NHRA requiring full floaters on their quickest and fastest race cars. In that accident, a broken axle allowed the right rear wheel to break free, turning the car upside down, where the leaking fuel tank and sparks from the roll cage ignited, causing life-threatening burns to Peterson’s body.

In general, as Cascio points out, it’s very rare for a floater axle to fail, given the reduction in load placed on them, adding that Strange’s 40-spline Hy-Tuf axles are capable of handling “massive” amounts of horsepower and torque.

The floater carries a heavier load because it has the two bearings out there supporting the load, instead of a single axle bearing on a standard flange axle. So it’s got more bearing strength overall. – Mark Williams

“The full floater is just an all-around safer setup. The housing absorbs the bending load created by the large amounts of horsepower and in the case of tire shake, he explains”

Are There Disadvantages?

There are two main advantages to a full floater, and likewise, there are two primary disadvantages. One, of course, is the added cost — although it’s hard to put a price on your own safety. But in addition to that, it also comes down to weight for many race teams. Pro Stock racers, for example, are still holding out on the use of full floaters for this very reason, instead running two-piece axles likes those from Strange Engineering that sport a high mis-alignment bearing that captures on the axle, or run-of-the-mill flange axles, in order to keep the weight down.

Obviously the manufacturers look at it more from a safety standpoint, with Cascio stating, “The safety and longevity of the floater assembly far outweighs a part failure on the track in the name of weight.”

Mark Williams’ beefy 11-inch Modular housings, designed with Top Alcohol and Pro Modified racing in mind and sporting carbon fiber brakes, are almost exclusively equipped with full floaters, although they can be optioned with flanged axles.

According to Williams, the spindles used on their modular housings weigh in at about five pounds each, with the floater components on each side around nine pounds each, making for a complete, installed weight of about 28 pounds. This can certainly vary between manufacturers, but it provides a ballpark figure to help you understand what these components weigh.

More complex? Sure. And there’s no doubt that full floaters come at both a cost and weight penalty, but there’s no denying that full floaters are a far superior design for a rear end and axle system than the alternatives. This is particularly true if you’re competing in a heavy or high horsepower vehicle where the demands either stretch the boundaries of a flanged axle or blow them right out of the water, in the case of today’s modern fuel cars. Chances are, if you fall into this category, you’re already well aware of what a full floater is and how they work, but for those aren’t, we hope that you’ll be even more informed the next time the conversation comes up or you’re at the track and wonder, “How the heck do they keep the axles together in those things?”

About the author

Andrew Wolf

Andrew has been involved in motorsports from a very young age. Over the years, he has photographed several major auto racing events, sports, news journalism, portraiture, and everything in between. After working with the Power Automedia staff for some time on a freelance basis, Andrew joined the team in 2010.
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