Traxxas TRX-4 with Deep-Terrain Traxx

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.

• Crawlers are built for slow, controlled climbing over rocks: very low gearing, flexible suspension and locked axles.

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

• The cost is more parts to maintain.
• A crawler needs every wheel pulling to climb.

A brushed motor carries current through carbon brushes that rub on the commutator.

• It is cheaper and smooth at low speed.
• The brushes wear and the motor loses power over time.
• That low-speed smoothness is why so many crawlers use one.

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 nominal size compared with a full-size vehicle. It works more as a class label than a measurement.

• A 1/10 monster truck is far bigger than a 1/10 touring car, so compare length and weight for real size.
• This one is 557 mm long.

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 9.1%.
• Crawlers run small pinions for slow, precise wheel speed on climbs; a bigger one is quicker on the trail but harder to control on rocks.

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, transfer case and axle gears.

• That chain is why a crawler's figure is much higher than other classes'.
• 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.

• Among 1/10 crawlers & trail trucks in the catalog, the middle half run between 27.1:1 and 44.0:1 on stock gearing; this one is 64.6:1, shorter than most.
• Crawlers run far higher ratios than other classes, so the truck can creep and hold on a climb.

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.
• Crawlers often lock both axles so every tire pulls on a climb.

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

• Crawler wheels are named by rim size, such as 1.9 or 2.2 inches; that is the wheel, not this tire diameter.

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

• 12 mm is the common size on 1/10 cars, so most 1/10 wheels fit; offset and width still vary.
• The wheel nut is a separate size, so check its thread too before buying nuts.

Brushed motors are rated by can size and turns rather than KV, so there is no KV to show.

The diameter of the motor's output shaft. A pinion's bore, the hole in its middle, must match it.

• 3 mm is slightly smaller than the 1/8 inch (3.175 mm) shaft on many motors, so choose pinions made for 3 mm.

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

• 3S is 11.1 V nominal and 12.6 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.

• Brushed motors are often named by can size, such as 540 or 550, and by turns: fewer turns is faster.

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.
• A brushed ESC cannot run a brushless motor.
• Crawler ESCs usually have a strong drag brake, which holds the truck still on a slope when you let off the throttle.
• Set its low-voltage cutoff for LiPo, so a pack is not run flat.

The plug on the battery lead. Your battery needs the same plug.

• An adapter works, but it adds resistance and another joint that can work loose.
• Traxxas's plug is its own. Packs from other makers need the same plug fitted or an adapter.

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.

What the maker says about water, in its own words.

• Waterproof electronics survive splashes, puddles and wet grass; they are not meant to be submerged.
• After a wet run, dry the car and check the bearings, which rust.

The stock servo that steers the car.

• Metal gears take hard knocks to the front wheels better than plastic ones.

A brushed motor has no KV rating, so no theoretical top speed is worked out. Compare gearing by rollout instead.

The vehicle's weight as the maker states it.

• The maker does not say whether it includes a battery.
• On a crawler, where the weight sits matters as much as how much: weight low and in the axles helps it climb.

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

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

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.

• On a crawler it also decides how well the truck clears a ledge without the chassis catching.