The anatomy of a monster truck
It drives right over nearly every obstacle in its path and splits nearly every eardrum in its vicinity. It's supercharged, oversized, and extraordinary. It stands more than 10 feet tall, develops more than 1,500 horsepower, and can leap over a 727 jet airliner in a single bound.
It can be only one thing: a BIGFOOT® monster truck. And it contains an unbelievable amount of tube.
Just a Hobby
It all started innocently. Bob Chandler was a construction contractor by day, a 4x4 enthusiast by night, and a lead-foot all the time. His notoriously heavy foot led to a nickname-Bigfoot -and a continuous need for repairs on his truck. It needed a lot of repairs.
In the mid-1970s, four-wheel drive (4x4) vehicles weren't as common as they are today, and Chandler had trouble finding an experienced mechanic to work on the 4x4 that he used for work and recreation. Because parts for 4x4s were about as scarce as mechanics who knew how to repair them, Chandler realized he would have to rely on himself to find parts and do the repairs.
During a trip to the West Coast, he saw that the 4x4 trend was in full swing and realized it would just be a matter of time before it swept through the Midwest. As popularity of 4x4 trucks grew back home in Missouri and the demand for parts and repair services increased, he knew that he was at the forefront of something good. He took matters into his own hands and opened a 4x4 parts and service business. He then modified his work truck by installing many of the parts he stocked at his dealership. The truck he used daily in his contracting business thereby became a roving advertisement for his parts business.
One day while out doing a little off-road driving on a farm, Chandler and a friend, Jim Kramer, found a couple of junk cars. Eager to test the truck's climbing and traversing capabilities, they used the cars to make an obstacle, and Chandler drove right over the top of it. The rest is history.
Developing a Truck With Entertainment Value
"BIGFOOT has gone through several stages," said Chandler, creator and owner of BIGFOOT 4x4 Inc. "Stage One was a stock pickup truck. We beefed up the frame a little bit, put bigger tires on it, and put a bigger engine in it." BIGFOOT No. 1 was a 1974 Ford F-250® pickup truck, outfitted with a 460-cubic-inch engine (bored over to 640 cu. in.) and a C6 transmission.
The entertainment value was tremendous, and the truck soon was in demand for truck/tractor pulls, climbs, and mud runs. It wore 48-in. tires and weighed in at approximately 10,000 pounds.
The combination of the engine's torque and the tires' resistance revealed that the axle was the weakest link. Although it was a 2 1/4-in.-diameter hardened shaft, BIGFOOT No. 1 could snap an axle as though it were a matchstick. Chandler began to experiment with different combinations of tires and axles to increase the axle's service life.
He tried several larger tires, all the way up to 66 in. in diameter; heavier axle shafts; and a different method of transferring torque to the wheels: planetary gears.
"It was equipped with a 5-ton military center section with hardened axle shafts, and we used a Case® tractor planetary. So it had a good, solid rear end and a planetary that provided a 3-to-1 reduction," Chandler said. This reduction reduced the stress on the axles.
Chandler built several more trucks based on the Stage One concept. Although it was a state-of-the-art monster truck design that lasted for several years, competitors were catching up. Chandler abandoned this model and progressed to a heavier, stronger truck.
Stage Two was based on a 2-ton truck frame. The newer chassis, axles, springs, and shocks were much more massive than their predecessors, and the trucks weighed 12,000 to 15,000 lbs. To further exploit their entertainment value, promoters urged the drivers to race.
"How do you race a vehicle that weighs 15,000 lbs.?" asked Chandler rhetorically.
"It was brutal. The biggest flaw was the suspensions. We were still running leaf springs in those days, and you can't get a lot of travel out of a leaf spring, not with a heavy truck like that. Drivers were getting hurt," he said, as the trucks went airborne and came down hard. They simply weren't equipped with a suspension that could cushion the landing.
Something had to change.
Stage Three: A New Design
Chandler came up with Stage Three. For this design, he wanted something completely different-something that would really stand out when compared to the previous BIGFOOT models and other monster trucks, and something that would make the sport safer for the drivers.
In addition to wanting a safer truck, he also wanted a faster truck. To build safer, faster vehicles, he looked relentlessly for materials that had optimum properties, such as strength and rigidity. These materials also had to be low in weight. One of these materials was tube.
|BIGFOOT monster trucks use a mixture of steel and chromium-molybdenum tube. The frame and cage are 2-in.-OD, 0.120-in.-wall ASTM specification 1020 DOM tube, with tensile strength of 52 KSI (thousand pounds per square in.) and yield strength of 38 KSI. The four link bars are 2 1/2-in.-OD, 0.250-in.-wall 4130 chromium-molybdenum tube, with tensile strength of 162 to 194 KSI and yield strength of 123 to 159 KSI.|
Frame. Chandler collaborated with Dan Patrick, who had experience designing dragsters. Chandler spent nearly 800 hours using a CAD program to design and refine the tubular structure. The frame was built from welded drawn-over-mandrel (DOM) mild steel tubing and assembled with gas metal arc welding (GMAW) and heliarc welding.
"We use DOM tubing because it's stronger and more uniform in diameter than as-welded tube," said Jim Kramer, vice president of operations of the company. "That allows us to take a short length of a smaller-diameter tube and slide it into another and plug-weld it.
We do that usually where we need extra strength, such as where two tubes come together. This double wall allows us to make a stronger joint, both when building and repairing a frame.
"We do a little bit of heliarc welding too," Kramer said, "in areas that need a cooler weld, or where we have a difficult-to-reach spot that we can't get to with the other welding unit."
|The sudden acceleration of a monster truck reveals some of the tubular components, including nitrogen-charged shock absorbers that have 28 in. of suspension travel.|
Suspension. BIGFOOT trucks need about 28 in. of suspension travel, which is considerably more than the 4 to 6 in. of travel achieved by leaf springs. Research in shock absorbers led the BIGFOOT team to investigate gas shocks. Further research showed that they wouldn't be able to get the necessary gas shocks off the shelf, so the company began custom-manufacturing its own shock absorbers.
The shock absorbers are made from 3-in.-OD, 0.250-in. wall hydraulic tubing. "We made them out of aluminum years ago," Kramer said. "They weighed a lot less, but we found that the seals would wear the aluminum, so we switched to steel." The outside surface is ribbed, or finned, to save weight and help dissipate heat.
They are filled with nitrogen at 400 pounds per square in. (PSI) static. When compressed, the pressure jumps to approximately 2,100 PSI.
Axles, Wheels, Tires, and Brakes. The current axle housings, which are designed by Mittler Bros. Machine & Tool Co., Foristell, Mo., are lightweight replacements for the former cast steel housings. A manufacturer of several product lines, Mittler Bros. manufactures machinery such as tubing benders and notchers and components for high-performance automobiles.
"Mittler Bros. manufactured a front axle housing that weighed 200 pounds less than the previous housing, and a rear axle housing that weighed 350 pounds less than the one it replaced," Kramer said.
The axle housings hold ZF axles. The axle's total reduction is 16.5-to-1; the planetary reduction is 6.3-to-1; and the rear end (also known as the ring and pinion or third member) gear reduction is 2.6-to-1. The huge reduction at the planetaries means that the axles, ring and pinion, drive shafts, and transfer case are under much less stress than before. An example of the benefit of this reduction at the planetary shows up in the axles' size. They are the same size as standard axle shafts used on 1-ton pickup trucks.
"We use 1/4-in. chrome-moly tubes to make the four link bars, which are used to locate the axles," said Kramer. "We have them heat-treated in a salt brine. These tubes flex an awful lot. You'd never see it because it happens so fast, but they flex quite a bit. Heat treating raises its yield point, allowing the material to flex more without a permanent change to its shape. In other words, heat treating expands the tube's memory, so it flexes further and can still remember where it came from," Kramer explained.
The tires, which measure 66 by 43 by 25 in., are manufactured for heavy machines, such as fertilizer spreaders. Corresponding rims are commercially available. However, such wheels have very thick walls, and the weight is prohibitive for racing. So BIGFOOT's tires ride on rims cut from 25-in.-dia. tube.
Because the tires are so huge and contain such a large volume of air, they require just 8 to 10 lbs. of air pressure. When coming down from a jump, the tires flatten almost completely. Occasionally a tire's sidewall rolled right around the rim, and the rim's edge cut into the tire's sidewall.
"We got around that by putting a larger bead on the wheel-a bead made out of tube-and we haven't ruined a tire like that since," Kramer said. "The tire manufacturer's recommendations call for a 36-in.-wide wheel, but we use a 32-in.-wide wheel to get a crown effect. It also helps to hold the tire on the bead."
"We send the tires out to have them modified," Chandler said. They're mounted on a huge lathe, and 2 or 3 inches of lug, or tread, is shaved off of the tire. When the tires come back, the driver spends a day or two, per tire, shaving the rest of the lugs down with a hand tool." This removes about 450 lbs. of material from each tire. The tires weigh 880 lbs. uncut.
"Racing guys have a phrase about this-they say that taking 10 pounds off of a tire or wheel is like taking 100 pounds off of the chassis. This is because removing any weight from the tire or wheel makes it that much easier for all those parts-the engine, transmission, differential, axle shafts, wheels, and tires—to get up to speed," said Chandler, explaining his company's relentless efforts to reduce weight at the wheels and tires.
"Something else we have that is unique in the monster truck industry is an internal planetary disk brake, also called a wet brake," Chandler said. "BIGFOOT trucks have one at each wheel. Ninety-nine percent of other monster trucks out there have disk brakes on the drive shafts. If they break an axle, differential, drive shaft, or a planetary, they have no brakes on two of the wheels." Although the truck still would have brakes on the other two wheels, it presents a hazard nonetheless, Chandler explained. After a jump, monster trucks often land on two wheels. If those wheels don't have the ability to slow or stop, the driver doesn't really have any control over the vehicle until the other two wheels touch down.
Bringing It All Together. One of the main design considerations is the sprung/unsprung weight. This ratio refers to the amount of weight that rides on the suspension (the sprung portion) versus the weight that doesn't ride on the suspension (the unsprung portion). In layman's terms, the sprung portion is the chassis, and the unsprung portion consists of the axles, wheels, and tires.
Having 95 percent of the weight sprung is ideal, which is typical of passenger cars and trucks. However, monster trucks aren't passenger cars, and BIGFOOT trucks typically have an astonishing 65 percent of their weight tied up in the unsprung portion. The truck with the new Mittler Bros. axle housing is closer to 50-50.
"Having more of the weight upstairs is the key to keeping the wheels on the ground, and keeping the wheels on the ground is the key to winning races," Chandler said.
|BIGFOOT trucks performed two jumps over a 727 jet airliner. Videos of these jumps can be viewed at the company's Web site, www.bigfoot4x4.com.|
Can a Monster Truck Fly?
Even though the current design uses tubular frames and many other weight-saving components and strategies, trucks are trucks, and they spend most of their time where trucks should: on the ground. However, BIGFOOT drivers occasionally demonstrate that their trucks sail through the air pretty well too.
One such occasion occurred on Sept. 11, 1999. Starring at the Tennessee Aviation Days Air Show at Smyrna Airport near Nashville, drivers Dan Runte and Eric Tack each took a short flight over a parked 727 airliner. Tack, behind the wheel of BIGFOOT No. 15, went airborne for more than 148 ft. Runte, the pilot of BIGFOOT No. 14, set a record at a stunning 202 ft.
Approximately two months later, Runte was preparing to jump seven cars and a semi tractor-trailer lengthwise for a network television broadcast. The BIGFOOT team worked out all of the details and was confident that the jump would be successful. The landing, however, was questionable.
"We knew that the jump would be OK, but we weren't quite sure what would happen when the truck came down. When a monster truck is in the air for that long, anything can happen," Chandler said.
It was a formidable jump. And because it was to be taped for a television show, a Hollywood stunt coordinator was brought in to help. "It will never work," he said.
Based on the height, angle, and location of the ramp, he was sure that the truck would smash right into the semi. "It won't clear it," he warned.
He showed Chandler and Kramer several figures and calculations that proved the jump was impossible. After BIGFOOT successfully completed the jump, Chandler and Kramer realized that the BIGFOOT team and the stunt coordinator were looking at two different sets of factors.
Attempting the jump with a Hollywood stunt car would have been a disaster, but a BIGFOOT truck isn't a Hollywood stunt car. When the truck hits the ramp, the eight nitrogen-charged gas shock absorbers compress and build up a huge amount of pressure. Then they spring back with tremendous force and give the truck a huge boost. The tires also compress when they hit the ramp, and they spring back and give BIGFOOT more than enough lift to help clear a semi tractor-trailer and seven automobiles.
Or a 727.
Bigfoot 4x4 Inc., 6311 N. Lindbergh Blvd., Hazelwood, MO 63042-2876, 314-731-8112, fax 314-731-8114, firstname.lastname@example.org, www.bigfoot4x4.com. BIGFOOT 4x4 Inc. builds and races BIGFOOT monster trucks and markets associated merchandise.
ZF Group North America, 7310 Turfway Road, Suite 450, Florence, KY 41042, 859-282-4300, fax 859-282-4311, www.zf-group.com.
BIGFOOT is a registered trademark of Bigfoot 4x4 Inc.
Case is a registered trademark of Case IH.
F-250 is a registered trademark of Ford Motor Co.
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