HPI Racing Sport 3 BMW 2002 Turbo

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.

• Touring cars are 4WD on-road cars for asphalt and carpet tracks, often raced in classes with fixed motor and battery rules.

All four wheels are driven, which puts power down more evenly and keeps the car steadier accelerating out of corners.

• The cost is more parts to maintain.

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.

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 431 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 3.7%.
• 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.
• This one is worked out from the tooth counts the maker publishes.

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

• Here it is 2.778:1.

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.

• Among 1/10 touring and pan cars in the catalog, the middle half run between 6.3:1 and 8.5:1 on stock gearing; this one is 8.5:1.
• On-road racers often compare gearing by rollout instead, because tire sizes differ between cars.

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.

Hub bearing size: The documented drivetrain connection or size for this exact model. Match the mating component before buying a replacement.

The nominal size of the hole that mates with the motor shaft.

• A nominal shaft fit comes from the exact replacement pinion listing. It does not specify a measured finished hole or manufacturing tolerance.
• Match the pinion mounting method as well as its size. A press-fit hole cannot be inferred from motor shaft diameter.

The tooth spacing of the fitted gear, stated as metric module or diametral pitch.

• The meshing pinion and spur need matching pitch. A missing pitch can be inferred from the other fitted gear; module and diametral pitch are not silently converted.

The tooth spacing of the fitted gear, stated as metric module or diametral pitch.

• The meshing pinion and spur need matching pitch. A missing pitch can be inferred from the other fitted gear; module and diametral pitch are not silently converted.

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

• This approximate diameter comes from a replacement advertised for the identified OEM tire. Calculated speed and rollout use this working size; measure your fitted tires for a closer comparison.
• Race tires wear down over a day, and a smaller tire gears the car shorter.

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 23.2 mm.
• Touring and pan car racers often gear to a target rollout, because it stays comparable when tire diameters differ.

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.

Axle thread: A documented stock wheel, tire, or axle measurement. Stock dimensions alone do not establish the largest size that will clear the body and suspension.

• This working size comes from a replacement advertised for the identified OEM part. Confirm the fitted part before ordering; a replacement's size may differ.

Stock wheel width: A documented stock wheel, tire, or axle measurement. Stock dimensions alone do not establish the largest size that will clear the body and suspension.

Stock wheel offset: A documented stock wheel, tire, or axle measurement. Stock dimensions alone do not establish the largest size that will clear the body and suspension.

Stock tire width: A documented stock wheel, tire, or axle measurement. Stock dimensions alone do not establish the largest size that will clear the body and suspension.

What the main chassis is made of and how it is built, as stated for this car. Braces, stiffeners and skid plates are not counted.

• Composite is the makers' word for molded plastic, usually nylon with glass or other fibers: it flexes and shrugs off hits, and it is light and cheap to replace.

What the shocks are and how they are built, as the maker describes them.

• Oil-filled shocks damp with silicone oil; friction shocks, found on some entry-level cars, damp with a sprung plunger and cannot be tuned with oil.
• Threaded bodies set ride height with a collar you turn, rather than clips.
• The shaft size matters when buying seals, ends and rebuild kits.

The stated length of the complete shock assembly. Sources may use mounting-eye centers or a nominal assembly length.

• Compare fully extended mounting-eye spacing, compressed length and mounting hardware before ordering. A longer shock changes ride height and travel, and can bind at full droop or bottom out.
• This approximate length comes from a replacement listed for the identified OEM shock set. Check the fitted shocks; their original dimensions may differ.

Front shock lengths: A documented shock mounting interface at the named end and eye. A ball stud or standoff is distinct from a through-bolt. Match the mounting type, thread and eye dimensions when choosing a replacement shock.

• This working size comes from a replacement advertised for the identified OEM part. Confirm the fitted part before ordering; a replacement's size may differ.

Rear shock lengths: A documented shock mounting interface at the named end and eye. A ball stud or standoff is distinct from a through-bolt. Match the mounting type, thread and eye dimensions when choosing a replacement shock.

• This working size comes from a replacement advertised for the identified OEM part. Confirm the fitted part before ordering; a replacement's size may differ.

Brushed motors are commonly described by can size and turns. No motor KV is published for this vehicle; turns alone cannot supply an exact value.

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

• 3.175 mm is 1/8 inch, the usual size on 1/10 motors.

The highest LiPo cell count the maker rates the electronics for.

• The maker does not publish it.

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.
• Set its low-voltage cutoff for LiPo, so a pack is not run flat.

The size of the stock motor's can, as the maker names it.

• 540 is a standard can size; a larger number is a longer can with more torque.

The current the stock speed control is rated to carry, as the maker states it.

• The continuous figure is what it can carry all run; the burst figure only for moments, such as a launch.
• A motor or pack that pulls more than this risks the ESC.

ESC mounting dimensions: A documented battery, motor, or ESC installation detail. Electrical ratings and physical fit are separate checks.

The stock servo that steers the car.

• Plastic gears are light and cheap but strip more easily from hard knocks to the front wheels.

How hard the servo can turn, at the voltage the maker rates it.

• More torque steers heavier cars and bigger tires more precisely and holds the wheels straight over rough ground.
• Kilogram-centimetres and ounce-inches measure the same thing; the other unit is converted for comparison.

How long the servo takes to turn 60 degrees, at the voltage the maker rates it.

• A smaller number is a quicker servo, which feels sharper when changing direction.

Stock servo spline: A documented servo or steering interface for this model. Matching torque alone does not establish mounting or horn fit.

• This working size comes from a replacement advertised for the identified OEM part. Confirm the fitted part before ordering; a replacement's size may differ.

Stock servo dimensions: A documented servo or steering interface for this model. Matching torque alone does not establish mounting or horn fit.

• This working size comes from a replacement advertised for the identified OEM part. Confirm the fitted part before ordering; a replacement's size may differ.

Theoretical top speed is unavailable because motor KV and rated battery cell count are missing.

• Enter the missing inputs in the calculator to work out a theoretical speed.

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.