Traxxas XRT Ultimate

The kind of vehicle the maker sells it as. It sets what the car is built to do and which cars it is fair to compare it with.

• Monster trucks have tall tires and long suspension travel, built for jumping and rough ground rather than a race track.

All four wheels are driven. That gives more grip under power, especially on loose ground.

• The cost is more parts to maintain.

A brushless motor has no brushes to wear, so it makes more power and runs more efficiently than a brushed one.

• It needs a brushless ESC.

Ready to run: built at the factory with the electronics installed.

• Some come without a battery or charger; the maker's box contents say which.

The maker's product number for this exact version.

• A V2, a new color, or a kit instead of an RTR usually has a different number, and parts can differ between them.
• Check it against your box or receipt before ordering parts.

The small gear on the motor shaft. Fewer teeth gear the car shorter: quicker acceleration, lower top speed and a cooler motor. More teeth do the opposite.

• On this car one tooth changes the gearing by about 5.0%.
• A bigger pinion loads the motor and ESC harder, so check the maker's gearing chart and feel the motor temperature after a run.

The large gear the pinion drives. A bigger spur with the same pinion gears the car shorter.

• Pinion and spur must have the same pitch, or tooth size, to mesh.
• The Gearing parts table below shows this car's pitch and part numbers.

Every reduction after the spur, multiplied together: transmission, differentials and axle gears.

• It is fixed by the design; the gearing you change is the pinion and spur.

How many times the motor turns for one turn of the wheels. A higher number is shorter gearing: more acceleration and pulling power, less top speed.

The type of each differential, as the maker states it.

• A gear or ball diff lets the wheels on one axle turn at different speeds in a corner; a spool or locked diff turns them together for straight-line drive and climbing, at the cost of steering.

The outside diameter of the mounted tire, tread to tread. A taller tire covers more ground each turn, which gears the car taller.

• At high speed tires can balloon and grow, so the real gearing gets taller as the car goes faster.

The hex on the axle that the wheel locks onto and is driven by. A wheel must match the hex size to fit.

• 24 mm is used on the largest trucks.
• The wheel nut is a separate size, so check its thread too before buying nuts.

The oil in the shocks slows how fast the suspension moves. Thicker oil settles the car more slowly and resists bottoming out; thinner oil lets the wheels follow bumps.

• At the front it mostly shapes the steering: thicker oil makes the car turn in more calmly and dive less under braking; thinner oil turns in sharper and keeps the front tires on the ground over bumps.
• Bashers and monster trucks often go thicker to take big landings without bottoming out.
• It is graded in wt here. A wt number is the oil maker's own grade and differs between brands, so match the brand, or use the oil maker's chart to convert.

The rear shocks mostly shape grip under power and how the car lands.

• Thicker oil at the rear holds the back up on landings and keeps it steadier on the throttle; thinner oil lets it squat and follow rough or loose ground for more grip.
• This car runs thicker oil at the rear (40 wt) than the front (30 wt). The rear stays steadier on landings and under power, while the front follows bumps and turns in more readily.

RPM per volt with no load on the motor. Higher KV spins faster on the same pack; lower KV on more cells can reach the same RPM with less current.

• On 8S (29.6 V nominal) this motor spins about 37,740 rpm with no load.
• Under load the real RPM is lower.

The highest LiPo cell count the maker rates the electronics for. Each cell is 3.7 V nominal and 4.2 V fully charged.

• 8S is 29.6 V nominal and 33.6 V off the charger.
• Running more cells than this risks the ESC and motor.
• Many cars also run on fewer cells, only slower.

The motor fitted at the factory, as the maker names it.

The electronic speed control. It feeds power from the battery to the motor as you work the throttle, and usually powers the receiver and servo too.

• An amp figure in its name is normally the maker's continuous rating.
• Set its low-voltage cutoff for LiPo, so a pack is not run flat.

The biggest pack that fits the tray, as the maker states it: length and width, and height where given.

• Compare it with the pack's case size, not its capacity.
• Allow room for the leads.

The stock servo that steers the car.

The stock transmitter, the radio you hold.

It needs a motor KV, which this vehicle does not have on record, so no theoretical top speed is worked out.

The vehicle's weight as the maker states it.

• The maker does not say whether it includes a battery.
• More weight takes more power to accelerate and lands harder on the parts; it changes top speed very little.

Overall length as the maker states it, usually with the body fitted.

• Useful for comparing real size, since scale labels are loose.

Overall width as the maker states it, usually with the body fitted.

• A wider car is more stable in corners and harder to roll over.

Overall height as the maker states it, usually with the body fitted.

• Useful when checking it fits a shelf, a bag or a car trunk.

The distance between the front and rear axle centers. A longer wheelbase is steadier at speed and over bumps; a shorter one turns more tightly.