Rlaarlo JK07Pro Brushless RTR

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 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.
• Crawler brushless systems are usually sensored, so the motor turns smoothly from a standstill.

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 805 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 6.7%.
• 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.

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.
• This one is worked out from the tooth counts the maker publishes.
• This is the high range, which sets top speed and is what the calculators load first.

The same chain in low range: the transmission's second, lower set of gears.

• The pinion and spur are shared, so a pinion swap changes both ranges by the same share.
• This one is worked out from the low-range final drive the maker publishes.

Spur teeth divided by pinion teeth: the part of the gearing that a pinion or spur swap changes.

• Here it is 2.333:1.

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.

• Crawlers run far higher ratios than other classes, so the truck can creep and hold on a climb.
• This is the high range, which sets top speed and is what the calculators load first.

The final drive in low range, where a crawler spends most of its time: slower and stronger, for climbs and fine control.

• It is 1.9 times the high range.
• Switch the calculators to Low to work in this range.

This vehicle has a two-speed transmission: a high range for speed and a low range for crawling.

• The maker does not say how the range is changed.
• Both ranges share the pinion and spur.

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.

How far the car moves for one turn of the motor: tire circumference divided by final drive.

• It lets you compare gearing across different tire sizes; more rollout is taller gearing.
• Here it is 15.9 mm.
• This is the high range, which sets top speed and is what the calculators load first.

How far the truck moves for one turn of the motor in low range.

• Here it is 8.6 mm.

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 4S (14.8 V nominal) this motor spins about 38,480 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.

• 4S is 14.8 V nominal and 16.8 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.

• In a name like 4282, the digits usually give the can size: 42 mm across and 82 mm long. A bigger can makes more torque and sheds heat better.

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.
• 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 same no-load ceiling in low range: how fast the truck crawls at full throttle.

• Low range trades that speed for pulling power and control.

Motor RPM times rollout, worked out on 4S at nominal voltage with stock gearing.

• It assumes no load, no battery sag, no wheel slip and no drag, so read it as a ceiling for comparing setups.
• On a crawler it matters little: the gearing is chosen for control on climbs.
• This is the high range, which sets top speed and is what the calculators load first.

The motor's RPM with no load: KV times pack voltage.

• Under load it drops, which is one reason real speed is lower than the theoretical figure.

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 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.

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