Restrictions Breed Innovation in the Tilden Motorsports Ultra4 Car

• June 22, 2017
• Story By
  Ali Mansour

When it comes to Ultra4 Racing, the premier 4400 cars typically get most of the notoriety. While there’s no question that rigs like Erik Miller's solid-axle buggy and three-time KOH champion Shannon Campbell's Dragon Slayer are exciting to watch, the restrictions in some of the lower classes have bred some incredible innovation. With a wide variety of rules, the 4500 class has become one of the most creative from a builder's standpoint in the Ultra4 series.

Some of the 4500 Class restrictions include:

• 37-inch DOT-compliant tire
• Mechanical steering
• OEM-style chassis

Since there are a limitations in the 4500 series, it has evolved into an excellent place for drivers to showcase their skillset and for builders to roll out some slick, out-of-the-box ideas. One rig that’s a great example of this is Kevin Stearns’ #4502 racer. The BDS Suspension-sponsored Tilden Motorsports car went from a CAD drawing to the track in just 90 days. Since Stearns stays booked with his Pacific Fabrication shop in Gilroy, California, the car could only be wrenched on at nights and on the weekends. Thankfully, his shop foreman Jason Howerton, along with Rick Baker, Christian Gardner and Rob Llanes, all chipped in to make this race car become a reality.

Its first time out would be the 2017 King of the Hammers race where, despite battling a few new-car blues, Stearns locked in an 18th-place finish. Currently, he has locked in two podium finishes (third place at the Vaquero 212 and second at the MetalCloak Stampede).

While the video above offers insight into the 4502 car direct from Stearns, we're diving more into the details in the feature breakdown below.

Tilden Motorsports is known for building incredibly reliable and powerful LS-series GM engines. The 4502’s 367ci LS2 is belting out 650 horsepower and 585 lb-ft of torque, partly thanks to AFR CNC ported heads, stainless steel equal length headers with 1 7/8-inch primaries and a host of Tilden Motorsports components and machining magic. Bolted behind the hot V8 is a 4L80e built by Maximum Offroad Transmissions. Going with a four-speed transmission allows Stearns to take advantage of the NP205 transfer case’s 2:1 low-range ratio. Where a three-speed transmission would typically require an exit out of low range in faster sections of the course, the fourth gear allows him to stay moving with the extra torque of the lower ratio. 

When it’s time to tackle more technical rockcrawling sections, Stearns can increase his crawl ratio thanks to a Magnum Box from Offroad Design. The underdrive box bolts to the gear-driven NP205. Fit with a 2.72:1 low range, the box can be engaged when more gear reduction is needed. When multiplied with the 205’s 2:1 (technically 1.96:1) low range, you end up with a 5.33:1 ratio, which is perfect for ‘crawling.

Sitting behind the T-case is a Torq Super 14 third member, which is bolted to the RuffStuff Specialties housing. Inside, the 10.5-inch ring gear is locked to a spool feeding 300m RCV Performance 40-spline axleshafts. For stopping power, the full-float axle also uses a ¾-ton GM disc-brake conversion.

Up front, another Torq Super 14 third member was used; this time it was locked in a Torq housing. The 5.13:1 gear set works with an ARB Air Locker, which sends power to the RCV Performance 300M axleshafts and drive flanges. Hiding behind the gates of what used to be of a GMC grille is a Warn 9.0 competition winch using Warn’s Spydura synthetic rope.

While you can run two shocks per corner in the 4500 class, the bodies of the shock cannot exceed 2.65 inches, nor can they have more than 14 inches of stroke. Since the shocks also have to be connected to the axle assembly and chassis, Stearns had to get creative when looking to squeeze out more travel from his Fox 2.5 race series coilovers and four-tube bypass shocks. He accomplished this by sweeping the shock mounting position back, which nets him a total of 14 inches of suspension travel (6 up and 8 down).

One of the most calculated and unique parts of the 4502 car is the steering system. Since mechanical linkage is required for the 4500 class, simply placing on a fully hydraulic steering ram isn’t an option. Needing strength and bump-free steering, Stearns' team engineered a custom steering system that starts off with a side-mounted manual steering gear box from a Jeep Wrangler YJ and placed at Stearns' feet in the cab. The YJ box then feeds the custom steering direction changer (shown below).

From the direction changer box, a PSC Motorsports steering servo and steering column gets its marching orders. The pitman arm actually pivots at the upper control-arm mounting point, which sends a link to the axle.

At the axle, a bell crank was added to the WFO high-steer arm. Given the PSC steering pump runs off of an XR series pump, it’s able to direct fluid to the PSC Motorsports double-ended hydraulic steering ram. The end result? A steering system that retains the mechanical requirement of the class, but has the added strength of the double-ended hydraulic ram. Even more important, since the steering system was paired with a four-link suspension (triangulated lowers, straight uppers), there’s no bump steer during travel.

The chassis is primarily a mix of 2-inch, 0.120-wall DOM tubing, with a few spots of 1.75-inch, 0.120-wall mixed in. Under the modular belly pan is a substructure that’s a mix of 1/8-inch and ¼-inch plate and 3x2-inch framerails. Weighing in around 5,200 pounds, the car has a 118-inch wheelbase with a 96-inch track width. The belly pan rest 18 inches off the ground.

To secure the rear axle, a triangulated four-link suspension was comprised using aluminum lower links and chromoly uppers. Each adjustable link is fit with 1.25-inch FK rod ends. Like the front, the rear of the car uses Fox 2.5 coilovers and bypasses to net 16 inches of vertical wheel travel (8 up, 8 down). Helping to keep the car stable is a PAC Racing Springs rear sway bar, while Fox 2.0 bumpstops prevent the shocks from bottoming out.

Fueling the rig is a 32-gallon Jazz fuel cell using Tilden Motorsports fuel filters and regulator. Since the drivelines are so close in matching length, Stearns only hauls one extra of the Southbay Driveline 1350 double-Cardan ‘shaft.

Special care was taken to keep the weight and overall height of the vehicle very low. Since cooling in the desert is critical, Stearns fabricated a mount for the Ron Davis radiator that would keep it low, while still retaining plenty of air flow. When they were building the mount, they crafted the custom tool box to keep not only essential tools onboard, but recovery gear as well.

Inside you’ll find a quick-release Grant steering wheel along with a Samsung tablet used to display a running diagnostics app. Stewart-Warner mechanical gauges sit on the passenger side along with a Lowrance GPS and Magellan TRX7 navigation system. Keeping occupants riding in comfort are suspension seats from Twisted Stitch.

As one of the most dominate tires in Ultra4 Racing, the Nitto Trail Grappler has proven time and time again that it has what it takes to survive the brutality that is off-road racing. Stearns is running a 37x12.50R17 Trail Grappler, which is mounted on a 17-inch Trail-Gear creeper beadlock wheel. While this rig could definitely handle the more common 40-inch Trail Grappler of the 4400 class, the rules limit the tires to 37 inches for all 4500 vehicles.

BDS is known for building suspension systems for Jeeps and fullsize trucks alike. To let this rig stand out from the pack, it was dressed using a stock GMC hood and fenders. The doors and side panels are all composite and formed in a way to help it resemble the look of a heavily modified S-10 pickup.

HARD FACTS

2016 Tilden Motorsports Chassis

(Photos courtesy of BDS Suspension)

See more photos in the photo gallery below — and if you like this one-off GM buggy, be sure to check out this Killer K30.