Our goal for this 2013 Polaris RZR XP 900 EPS Orange Madness/Blue LE was ambitious: build a machine capable of racing the King of the Hammers UTV race without turning it into a single-purpose race car.
It still needed to work well in the dunes, handle high-speed desert terrain and remain narrow and maneuverable enough for trails such as the Rubicon. That requirement shaped every part of our Polaris RZR XP 900 suspension build.
Why We Retained the Stock RZR XP Suspension Geometry
The factory suspension geometry on the RZR XP 900 provided a solid foundation for a multi-purpose build. Its three-link trailing-arm independent rear suspension uses two long radius rods and a forward-mounted trailing arm to create a relatively strong and lightweight system.
The RZR XP 900 came with adjustable FOX Podium X 2.0 shocks, 14 inches of rear suspension travel, 13.5 inches of front travel and 13 inches of ground clearance. That was enough capability for most recreational riding, but desert and rock racing demanded better damping, additional ground clearance and stronger suspension components.
Long-travel suspension kits can improve high-speed desert performance, but the added width is not ideal for tight trails and technical rock crawling. Moving to longer control arms also normally requires custom axles and shocks.
By retaining the factory width and suspension geometry, we could select from a broader range of replacement arms, radius rods, axles and other components while preserving the RZR XP’s versatility.
Lessons From Our Previous King of the Hammers Race
We had raced our 2011 Polaris RZR XP at King of the Hammers after a head-gasket problem sidelined our Kawasaki Teryx. Despite the last-minute vehicle change, the RZR finished fourth overall and third in the Production 1000 class.
That machine used PURE Polaris trailing-arm guards and Factory UTV UHMW A-arm guards over DragonFire Racing replacement A-arms. Upgraded FOX 2.0 shocks helped it perform well in the desert, while the protective components helped us get through the rocks without major problems.
The result confirmed that the stock-width RZR XP platform could be competitive. With additional preparation and carefully selected suspension upgrades, we believed the 2013 RZR XP could perform even better.
FOX 2.5 Podium-X Shocks With Bottom-Out Control
The foundation of the suspension package was a set of FOX 2.5 Podium-X shocks. I had previously tested the shocks in Barstow, California, and they performed at a level beyond anything I had experienced on a RZR XP.
For more details on that initial test, see our dedicated FOX 2.5 Podium-X shock review.
The 2.5-inch aluminum shock body provides approximately 50 percent more piston area than a 2-inch shock. The larger piston allows the shock to produce the required damping force with lower internal pressures, reduced heat buildup and improved durability.
The shocks also include external dual-speed compression and rebound adjustments. That adjustability makes it easier to compensate for changing vehicle loads, terrain and driving styles.
One of their most important features is FOX Bottom-Out Control, or B.O.C. technology. During the final 25 percent of the shock stroke, a secondary piston enters a bottom-out cup and produces progressively greater damping resistance. FOX stated that the system could generate up to 3,000 pounds of additional damping force.
This design allows the primary valving to remain compliant over smaller bumps while preserving enough bottoming resistance for jumps, g-outs and other severe impacts.
Custom Shock Tuning by Magnum Offroad
After selecting the FOX 2.5 Podium-X shocks, we worked with Magnum Offroad to tune them specifically for our RZR XP 900 and its intended use.
Magnum Offroad had extensive experience revalving the factory FOX 2.0 shocks used on the RZR XP. The company also had the measurements and testing data needed to develop a baseline for the larger FOX 2.5 package.
The tuning process included shock-dyno testing throughout the shocks’ range of motion. Magnum Offroad then confirmed the settings through real-world desert testing in Barstow.
Lone Star Racing High-Clearance Trailing Arms
With the shocks selected, we turned our attention to the suspension arms and links. The factory trailing arms, radius rods and front A-arms work well for recreational riding, but striking rocks at race speeds can quickly expose their limitations.
Our objective was to increase strength and ground clearance without changing the factory suspension geometry or overall width.
The factory trailing arms hang lower than necessary near the leading edge of the rear tires. That area is particularly vulnerable when climbing ledges or crossing large rocks.
We selected Lone Star Racing STS high-clearance rear trailing arms. The arms are constructed from 4130 chromoly and save more than eight pounds compared with the factory trailing arms. Their shape also provides approximately 2.5 inches of additional ground clearance immediately ahead of the rear tires.
High Lifter Max Clearance Radius Rods
The two radius rods on each side connect the rear trailing arm to the chassis. The stock radius rods are relatively lightweight and can bend when they contact rocks.
Several manufacturers offered reinforced or high-clearance replacements, but we chose High Lifter Max Clearance Series radius rods. These rods are manufactured from 1¼-inch by 3/16-inch DOM steel tubing.
The curved design moves more of the rod out of the path of rocks and other obstacles while the heavier tubing provides substantially more strength than the factory components.
UTV Inc. Front A-Arm Gusset Kit
The stock front suspension presented two concerns for desert and rock racing. First, the exposed lower A-arm could bend after a hard rock impact. A UHMW guard can provide some protection, but a stronger chromoly arm is preferable for extreme use.
The second concern was the factory upper A-arm mounting arrangement. The upper arm used a long pivot tube with a frame-mounting tab at each end. A hard impact at the tire could damage the front tab or elongate the mounting hole.
We began reinforcing this area with a bolt-on front A-arm gusset kit from UTV Inc. The kit was relatively easy to install and provided valuable reinforcement for the factory mounting points.
Cognito Motorsports Replacement A-Arms
We also wanted stronger control arms. Cognito Motorsports had RZR XP and XP 4 race vehicles competing in Best in the Desert and WORCS, giving the company useful experience with the platform’s suspension mounting points.
The Cognito kit includes replacement upper and lower A-arms with factory-installed heavy-duty spherical bearings. The upper arm uses a split-pivot design that changes the original two-shear-point frame mounting system into a four-shear-point arrangement.
Installing the system requires welding two additional mounting tabs on each side of the chassis. The extra work helps distribute suspension loads and reduces the possibility of premature wear or damage around the frame-mounting holes.
The upper load-bearing control arm is boxed to provide a favorable strength-to-weight ratio.
High Lifter Outlaw DHT Axles
Desert racing and rock crawling are both demanding on CV axles. We were also considering running 30-inch tires at King of the Hammers, which would add rotating mass and place more leverage on the drivetrain.
Because we retained the factory suspension width, we could use replacement axles designed for the stock-width RZR XP. We selected High Lifter Outlaw DHT axles.
The cages and housings receive heat treatments intended to improve their resistance to shock loads. The CV housings are larger than stock, while the housings, cages and axle bars are manufactured from 4340 chromoly steel.
The axles also use puncture-resistant thermoplastic polyester elastomer boots and low-profile boot clamps that are less likely to be damaged or dislodged on the trail. They arrive as complete assemblies, making them relatively easy to remove and replace.
UTV Inc. Heavy-Duty Tie Rods
The final steering upgrade was a heavy-duty tie-rod kit from UTV Inc. The replacement rods use thicker aluminum bodies and stronger rod ends than the factory components.
With larger tires adding leverage to the steering system, the upgraded tie rods provided additional protection against a bent rod or broken rod end in the rocks.
Testing the RZR XP Suspension at Prairie City
After installing the suspension and drivetrain components, we headed to Prairie City SVRA for the initial shakedown.
Prairie City does not have the large desert whoops found around Barstow, but it provides a useful combination of high-speed g-outs, rough terrain and jumps. We began testing with the stock tires and then installed 30×10R14 ITP Ultracross tires to see whether the larger tires exposed any suspension, steering or axle problems.
Paul Hart joined us for the test. His experience in short-course racing, rock crawling and desert racing provided another valuable perspective on the suspension setup.
Polaris RZR XP 900 Suspension Test Results
Both drivers were impressed with the ride quality delivered by the Magnum Offroad-tuned FOX 2.5 Podium-X shocks. The suspension remained compliant over smaller chop while retaining enough damping control for jumps, g-outs and harder impacts.
The best description was that the RZR now combined the plush ride of a Cadillac with the capability of a serious off-road race car. It floated across rough terrain and absorbed large hits without the harsh bottoming we wanted to avoid.
The replacement arms, radius rods, axles and steering components also felt solid throughout the initial test. We did not identify any immediate problems after switching to the 30-inch ITP tires.
The next step was desert testing in larger whoops, followed by a 30-inch tire comparison involving tires from STI, GBC, Maxxis and ITP. The winning tire would become a candidate for the King of the Hammers race setup.
Products Installed
- FOX 2.5 Podium-X shocks with Bottom-Out Control
- Magnum Offroad custom shock tuning
- High Lifter Max Clearance radius rods
- Cognito Motorsports replacement upper and lower A-arms
- Lone Star Racing STS high-clearance rear trailing arms
- High Lifter Outlaw DHT axles
- UTV Inc. front A-arm gusset kit
- UTV Inc. heavy-duty tie-rod kit
