#4415 in its native habitat. (Image/Proving Grounds Racing)

With the annual King of the Hammers off-road race coming up, we thought it’d be a good time to talk about the vehicles in the race, specifically the 4400 Series Unlimited Class.

Not familiar with King of the Hammers? It’s a weeklong festival of off-roading, culminating in the grueling King of the Hammers race, which includes sections of both flat-out high speed desert running, and highly technical slow-go rock crawling. In short, it’s one of the most challenging off-road events on the planet.

Though the rocks are big, the trails tight, and the turns treacherous, building a rig to handle it all might be the toughest part. (Image/Proving Grounds Racing)

So, we got in touch with our pals over at Proving Grounds Racing and they gave us some details on the #4415 Ultra4 truck they’re campaigning in this year’s race.

The 4400 class is Ultra4’s top tier, where pretty much anything goes. The guidelines are loose: these trucks must use a transfer case with four-wheel drive and meet an array of safety standards. Other than that, there are no limits to chassis/frame, tire size, engine type, axle, or suspension design—which means both the builds and the racing get really, really wild.

We actually spoke with John Grounds, owner of Proving Grounds Racing and Co-Driver of #4415, in a recent podcast. Click here to listen to the interview.

While it wears a grille reminiscent of a Jeep Wrangler, the similarities end there—4415’s chassis is completely custom built. (Image/Proving Grounds Racing)

Anatomy of an Ultra4 Unlimited Class Off-Roader

Unlike some of the other classes of Ultra4 Racing, you don’t have to start with a production chassis. Instead, most competitors build their own. In the case of Proving Grounds Racing #4415, it’s a completely custom off-road chassis designed by Miller Motorsports. After countless hours spent welding together tubing sections, the Proving Grounds team turned the bare chassis into the scorching off-road racer you’ll see tearing up the desert in the King of the Hammers.

Some…errr….ALL assembly required. The Miller chassis arrived to the Proving Grounds team as little more than a stack of tubing. (Image/Proving Grounds Racing)

That means that, while they may use off the shelf parts from common manufacturers, everything still has to be custom installed. That said, here’s a quick rundown of some of the parts you’ll see on #4415.

Tucked inside the 4415’s custom chassis is our old friend, the GM LS. (Image/Proving Grounds Racing)


Transmission & Driveline

  • Reid Super Hydra 400 (Based on the GM TH400)
  • Advance Adapters Atlas Transfer Case

Chassis & Suspension

From the going…to the stopping, every part of the 4415 off-road racer was chosen to deliver the best performance in the most extreme conditions. (Image/Proving Grounds Racing)

Axles, Wheels & Brakes

  • Wilwood Disc Brake Rotors, Pads & 6-Piston Calipers
  • Spider 9 Fabricated Axle Housings
  • Motive Gear Ring & Pinion
  • Dirty Life Race Wheels
  • BFGoodrich 40 Inch Baja T/A Tires


Here’s a better look at 4415’s 40 inch BFG tires. (Image/Proving Grounds Racing)

…But That’s Far From Everything

Any time you undertake an automotive project, be it restoration, repair, or upgrade, the devil is in the details.

And when we’re talking about building a rock-hopping, desert-streaking off-road race buggy, there is no shortage of details either—the Proving Grounds folks gave us a jaw-dropping list of all the other, less-glamourous parts they used in putting 4415 together.

While they may not get the glory of the flashier bits, any build needs a smorgasbord of assorted odds and ends. The folks at Summit Racing kept the Proving Grounds team well supplied with these often-overlooked parts. (Image/Proving Grounds Racing)

We’re talking about the mundane stuff, like plumbing fittings, electrical connectors, and bracketry. While these parts are vitally important, they simply don’t have the mystique of, say, a fire-breathing LS7.

So, how crazy is the Proving Grounds parts list for these random odds and ends? The spreadsheet was close to 200 rows long and the parts subtotal easily tipped over $20,000.

….Suffice it to say, building an Ultra4 Unlimited Class racer is not for the timid.

Your typical Ultra4 race is a grueling combination of slow-go rock crawling and flat-out speed runs. (Image/Proving Grounds Racing)

Want to See More?

If your appetite for seeing high-powered off-roaders kick up some dirt is sufficiently whetted, you can watch the King of the Hammers and other Ultra4 off-road races streaming on the Ultra4 website and Ultra4’s YouTube Channel.

While the King of the Hammers is a marquee event on the calendar, there’s plenty of racing over a full Ultra4 season schedule. (Image/Proving Grounds Racing)

As for us, we’re rooting hard for John Grounds and his Proving Grounds Race team as they compete in the King of the Hammers and all of the other Ultra4 races this season!

(Image/Proving Grounds Racing)
Author: Paul Sakalas

Paul is the editor of OnAllCylinders. When he's not writing, you'll probably find him fixing oil leaks in a Jeep CJ-5 or roof leaks in an old Corvette ragtop. Thanks to a penchant for vintage Honda motorcycles, he spends the rest of his time fiddling with carburetors and cleaning chain lube off his left pant leg.