Question 1

.12 --

Accepted Answer

In general, this is the smallest nitro type engine available for sedan remote control/ radio control cars. Many car kits come equipped with a .12-size engine. Some .12-size engines are more powerful than many .15-size engines, which indicates they are more efficient, have more tuning capability and/or larger carburetor openings. The "".12"" refers to the displacement size of the engine in cubic inches, much like a full-size engine.

Question 2

.15 --

Accepted Answer

This is the second-largest generally available nitro type engine that is available for sedan remote control/ radio control cars. Most ready to run (RTR) car or truck kits come equipped with a .15-size engine. Although the larger displacement would lead you to believe the .15 engines are more powerful, there are some .12-size engines that are more powerful than many .15-size engines, which indicates they are more efficient, have more tuning capability and/or larger carburetor openings. Also worthy of noting is that .15 engines are not allowed in large sanctioned races in some countries, which could mean that a car or truck may not be allowed to participate in an organized race, because many race clubs or tracks follow these rules examples. The "".15"" refers to the displacement size of the engine in cubic inches, much like a full-size engine.
NOTE: Some .15-size engines are ""big block"" engines, while most are based on the .12-size engine case. ""Big block"" engines won't fit in most 1/10 scale cars without modifications.

Question 3

.21 --

Accepted Answer

This is the largest generally available nitro type engine that is available for remote control/ radio control cars. The "".15"" refers to the displacement size of the engine in cubic inches, much like a full-size engine.

Another larger nitro engine that is available for RC cars. It usually has a much larger case and head than the .12 or .15s. These are common in most monster trucks. The ¡§.21¡¨refers to the displacement size of the engine in cubic inches, much like a full-size engine.

Question 4

.28 --

Accepted Answer

This is the largest nitro engine that is available for RC cars. Its case is the same as a .21, but the piston and sleeve have a larger outer diameter, resulting in a larger bore. As the other engines, the ¡§.28¡¨ refers to the displacement size of the engine in cubic inches.

Question 5

2-Wheel-Drive vs 4-Wheel-Drive --

Accepted Answer

As in full-scale cars, there are two main drive types: two-wheel drive (2WD), where power is supplied to the two rear wheels, and four-wheel drive (4WD), where power is supplied to all four wheels. The 2WD vehicles are less expensive and require less overall assembly and maintenance than 4WD vehicles. Assembly and maintenance for 4WD vehicles tends to be more involved, though not necessarily more difficult.

Question 6

A Arm --

Accepted Answer

This generally refers to the lower suspension arm of the car, although it can refer to the upper arm also.   It is a suspension arm that resembles the letter 'A', being widest at the mounting location on the chassis and narrowest where the hub is mounted. An A arm is usually used on models that use pillow ball hub carriers (knuckles), requiring an upper and lower arm. This may also be referred to as a ""double wishbone suspension"". If your 'A' arm resembles a 'Z', it may be time to replace it.

Question 7

A MAIN --

Accepted Answer

The final race in any given class, consisting of the best of the best as determined by previous mains or qualifiers. Winning the A Main puts you on the podium.

Question 8

ABC --

Accepted Answer

Abbreviation - See ALUMINUM BRASS CHROME

Question 9

ABDC --

Accepted Answer

Abbreviation - See AFTER BOTTOM DEAD CENTER

Question 10

ABN --

Accepted Answer

Abbreviation - See ALUMINUM BRASS NICKEL

Question 11

ABS --

Accepted Answer

A form of plastic that is easy to form but is not crash-resistant. HPI Racing does not use this type of plastic for any kit parts, however some of our 24mm white wheels are made from ABS, which is slightly lighter than the high-impact nylon used in our other wheels.

Question 12

Acceleration --

Accepted Answer

A measure of how quickly a car can accelerate. Affected by items like the weight of the car and its rotating mass.

Question 13

Ackerman --

Accepted Answer

Rudolf Ackerman is a man who worked out a steering system for horse-drawn carts, and we use his name today to describe the angle of the inside tire in relation to the outside tire when the wheels are turned to full ""lock""--the farthest the wheels go to the left or right. Normally, when the front wheels are turned all the way left or right, the inside wheel is at a sharper angle than the outside wheel. If you extend the center line of each front tire to a point where the intersect and measure that angle, that is the Ackerman angle. Ideally, for perfect steering, the Ackerman angle will cross at the center line of the rear axle.
In a wide turn, the front tires are not turned very far to the right or left, the inside wheel is not steering at a sharper angle than the outside wheel, and the Ackerman angle is not very wide. In a tight turn, the inside wheel is steering at a steeper angle than the outside wheel, and this is what is called the ""Ackerman effect"". A bellcrank steering system approximates a way to copy the Ackerman effect, and is adequate for RC cars because of tire slip, tire sidewall folding and other factors.
Adjusting the Ackerman angle can be done by changing the length of the center link, also called the Ackerman link that connects the bellcrank steering arms, or changing the mounting location on the steering arms without changing the link length. Most racers won't need to change the Ackerman angle, and actually it is best left to experienced racers who wish to try something new.
If you wish to learn what changing the Ackerman angle will do, see below:
The Nitro Racer 2, Pro 2 and Pro 3 use bellcrank steering arms that have two sets of mounting holes (inner and outer) for the Ackerman link. The Pro 2 and Nitro Racer 2 kits use the outer holes, while the Pro 3 uses the inner holes on its new bellcranks. Mounting the Ackerman link to the outer holes will result in a small Ackerman angle. Using the inner holes will increase the Ackerman angle.
A smaller Ackerman angle (done by lengthening the Ackerman link or using the outer link holes) will give you more aggressive steering into a corner with a possibility of oversteer at the middle of turn, when the most weight is on the outside tires. The RS4 Pro 2 and Nitro RS4 Racer 2 kits use the outer Ackerman link holes in stock form, so they has more aggressive steering than other cars, everything else (damping, tires, suspension) being equal.
A larger Ackerman angle (achieved by shortening the Ackerman link or using the inner link holes) will give the car more predictable and smoother steering. The new RS4 Pro 3 uses the inner Ackerman link holes, so its steering will be more predictable than the Pro 2.

Question 14

Ackerman Link --

Accepted Answer

The center link of the bellcrank steering system that connects the two steering arms.

Question 15

After Bottom Dead Center --

Accepted Answer

(ABDC) An engine timing term commonly used to pinpoint the timing marks where a port opens or closes, expressed in degrees of crankshaft rotation from the point where the piston is at bottom dead center.

Question 16

Air Dam --

Accepted Answer

An extension of the front bumper that blocks, or dams, too much air from getting under the car and producing lift. Most RC bodies for Touring Cars have air dams built in.

Question 17

Air Leak --

Accepted Answer

This is a major enemy of nitro engines. Air leaks can cause a very inconsistent tune and excessive leaning of the air/fuel mixture (especially at high RPM). If you suddenly discover that the rc engine isn't holding a tune, an air leak is most likely the problem. The usual causes of an air leak are:

( 1 ) A poorly fitting fuel tank lid - The seal on most tanks can be adjusted by tightening a screw that applies pressure to the O ring seal. Make sure your lid snaps shut and creates a good seal.
( 2 ) Holes or cracks in the fuel tank - This can be tested by emptying and removing the tank, plugging the fuel line, then submerging the tank in water and blowing into the pressure line. If you see bubbles, you have found a leak. Leaks usually develop in fuel tanks at the mounting points (where stress is created) and at the seam where the two halves were joined. Fuel tank leaks may be quickly patched at the track with silicone, JB Weld, or even duct tape, but this is only temporary. In some cases, you may be able to carefully melt the crack closed with a hot soldering iron (make sure you empty and air out the tank first - nitro fuel is flammable). Still, pony up the $10 for a new tank ASAP, just to be safe.
( 3 ) Pinholes or tears in the fuel tubing - Fuel tubing is cheap, so if you even suspect that this is the culprit for an air leak, install some new stuff and eliminate any doubt. Also make sure the tubing is securely attached to the pressure fittings.
( 4 ) Rotted or cracked O rings in the fuel filter - Inspect the fuel filter O rings and replace them if there are any signs of cracks or damage. While you're there, don't forget to clean the filter, since this is often overlooked.
( 5 ) The carb or backplate isn't properly sealed against the block - Check to make sure the carb and backplate are properly sealed, and replace any cracked or damaged O rings and gaskets. Many people use sensor safe RTV silicone instead of gaskets, and it does not hurt to use it with O rings, either.

Question 18

Alligator Clips --

Accepted Answer

You will most likely find these narrow toothed 'pincers' on the end of the power wires on your charger (used to hook the charger up to a power supply or battery), but some chargers may have alligator clips on the battery side as well (great for charging custom built packs, but if you are still sticking these clips into your battery plug, it may be time to invest in some connectors). You may also find alligator clips on your 3rd hand tool, commonly used to securely hold wires while soldering.

Question 19

Aluminum --

Accepted Answer

In general terms, aluminum is a metal that is lighter than steel, but not as strong. It can be machined (cut on a machine) to replace many plastic parts of an RC car, but is not recommended for replacing suspension arms.

Question 20

Aluminum Brass Chrome (ABC) --

Accepted Answer

This is the current standard for 2 stroke RC nitro engines. ABC refers to the materials used in the construction of the piston and sleeve - Aluminum piston and a Brass sleeve plated in Chrome. The aluminum and brass allow the parts to expand and contract as the engine heats up and cools down, while the chrome plating provides a hard, smooth surface on the inner walls of the sleeve, which prevents wear and tear from the piston.

Question 21

Aluminum Brass Nickel (ABN) --

Accepted Answer

Refers to the construction method used for some high end 2 stroke nitro engines. ABN refers to the materials used in the construction of the piston and sleeve - Aluminum piston and a Brass sleeve plated in Nickel. The aluminum and brass allow the parts to expand and contract as the engine heats up and cools down, while the nickel plating provides a hard, smooth surface on the inner walls of the sleeve, which prevents wear and tear from the piston. Nickel tends to be harder than the more traditional chrome plating, which results in parts lasting longer and running smoother.

Question 22

Amplitude Modulation (AM) --

Accepted Answer

Short for, an AM radio in RC is considered a ""budget"" radio, what most newcomers to RC will start out with. Most hobbyists will be fine with an AM radio, but some racers can benefit from FM or PCM radios. An AM radio will have more glitching than these other, more expensive, radios.

Question 23

Angle of Attack (AOA) --

Accepted Answer

Refers to the angle that a surface contacts the air, usually mentioned when talking about spoilers and wings. A higher AOA helps solve oversteer but increases drag and decreases top speed. A lower AOA is used to alleviate understeer and increase top speed.

Question 24

Antenna ¡V (1) Receiver - RX  --

Accepted Answer

The flexible wire on the receiver that 'captures' signals broadcast by the transmitter. The length of AM and FM antennas is calibrated to the frequency and channel of the radio system, so don't cut or shorten it. Always route the RX antenna as far away from other electronics or batteries as possible to prevent glitching (also try to avoid direct contact with graphite or carbon fiber parts), avoid moving parts that can damage the antenna (such as linkages and gears), and mount the end as high as possible to increase reception. Also, try not to bundle or cross the antenna over itself, as this can also reduce reception. The RX antenna on 2.4 GHz systems is usually stiffer than other systems - this is also known as a 'coaxial antenna'. Always follow the manufacturer's instructions, but coaxial antenna can usually be shortened to within 3 or 4 inches of the receiver.

Question 25

Antenna - (2) Transmitter - TX --

Accepted Answer

This is what sends (transmits) the signal from the radio to the receiver in the model. If a transmitter uses any other modulation than 2.4 GHz, the antenna will be the collapsible metal type, which is replaceable and screws into the radio case. 2.4 GHz antennas are encased (similar to cell phone antennas). The transmitter antenna radiates signals from the side, so pointing the top of the antenna at the model actually reduces reception. Always extend or raise your radio antenna before powering up your model to ensure reception.

Question 26

Anti-squat --

Accepted Answer

Refers to the angle of caster on the rear wheels. However, that angle prevents the squatting of the rear suspension, so 'anti-squat' it is. Anti-squat is most effective on acceleration from a stop, when much of the car's weight is forced onto the rear suspension. Lifting the front of the hingepin of the rear arms gives a caster (anti-squat) angle, and helps to transfer the power that makes the car want to do a wheelie into forward motion.

Question 27

Arm Brace --

Accepted Answer

A suspension mount (pivot block) that utilizes captured hingepins.

Question 28

Armature - ARM --

Accepted Answer

The backbone of a brushed electric motor - the coils (wire) are wrapped around the armature, and it spins in the center of the motor can. Armatures consist of the stack (laminations), motor shaft, commutator, and coils (wires). Armatures are commonly created by laminating thin slices of iron together as they are slid onto the motor shaft. Some armatures may be skewed (the 'slices' are rotated slightly as each one is stacked on the shaft) to help reduce cogging, or some armatures may have gaps (missing 'slices') to alter a motor's magnetic field or RPMs. Armatures are commonly balanced for smooth operation, usually with epoxy or by drilling.

Question 29

Axle --

Accepted Answer

Refers to the shaft that transfers power from the diff to the tires, or to the shaft that the tires rotate on (such as a stub axle). A driveshaft (which transfers power from the tranny to the diff) is also commonly referred to as an axle. In many cases, the axle may only refer to the portion of a universal or CV  shaft that is supported by the hub (the end that the rim bolts to). The term ""drive axle"" is used to denote an axle that actually transfers power to the tires - such as the rear axle on a pan car. Solid axle models (such as rock crawlers) use an integrated axle and diff housing.

Question 30

Axle Pin - Drive Pin - HEX PIN --

Accepted Answer

A small metal pin that is inserted into a hole in a drive axle just outside of the hub carrier. This holds the axle in the hub and provides a mounting key for the rims. Depending on the model, the rim may be keyed directly to the axle pin, or a wheel hex may be keyed to the pin with the rim keyed to the hex. Either way, the pin allows the rim to be secured to the axle and rotate with it. Keep an eye on your axle pins, they tend to wander off once your rims are removed.

Question 31

Axle Stall --

Accepted Answer

This is a concern for electric, dual motor rock crawlers. Axle stall occurs when the model is attempting a steep climb and the weight of the model exceeds the power of the motor on an axle - causing the axle to 'stall'. Increasing battery voltage or wiring the motors in a series can help eliminate axle stall, as well as installing a smaller pinion or larger spur gear (gearing down).

Question 32

Axle Wobble --

Accepted Answer

Refers to a wobble or shaking of a solid axle (such as those on pan cars). This may be caused by an axle that is mounted at an angle or has worn bushings, bearings, or cam style inserts. However, the most common cause is a bent axle or axles that aren't ground true (such as graphite or carbon fiber axles).

Question 33

Back Plate --

Accepted Answer

The 'cap' that bolts to the back of a nitro engine, sealing it and preventing all of the fuel from pouring out. For engines that use an onboard starting system (pull, shaft, spin, or EZ start), the backplate also houses the one way bearing. Always use a gasket or sensor safe RTV silicone (such as PERMATEX GOLD) to seal the backplate and prevent air leaks.

Question 34

Back Pressure ¡V Plugging Pulse ¡V Super charging --

Accepted Answer

The percussion (pressure) a pipe or muffler sends back to the combustion chamber of a nitro engine just before the exhaust valve closes. Also known as a ""plugging pulse"" or ""super charging"", back pressure helps to push the fresh air / fuel mixture back inside the combustion chamber instead of allowing it to follow the exhaust gases out. Mufflers provide back pressure, but tuned pipes are optimized to provide the right amount of pressure at the right time (this can be tuned by pipe length or size) - this means more fresh air and fuel in the chamber, better efficiency, and more torque and speed. With tuned pipes, the speed of the returning back pressure can almost reach the speed of sound. The pressure line routes some of this back pressure to the fuel tank to help push the fuel towards the carb.

Question 35

Backlash --

Accepted Answer

Refers to the amount of free movement between two gears, specifically, the amount one gear will continue to travel once the other gear has stopped. Also refers to the amount of excess movement caused by play (slop) in an axle assembly.

Question 36

Backmarker --

Accepted Answer

Refers to someone that has been lapped (passed by the leader) in a race. Unlike NASCAR, there are no yellow flags or 'lucky dogs' in RC, so once you're backmarked you need to get on the ball or deal with it.

Question 37

Backslash - \Slash --

Accepted Answer

This is a popular mod where a TRAXXAS SLASH is converted into a buggy. Basically, this is a SLASH SC truck with a buggy suspension, tires, body, and wing.

Question 38

Baffle (Fuel Tank) --

Accepted Answer

A molded-in barrier (or series of barriers) in a fuel tank that reduces the tendency of the fuel to slosh around. Baffles reduce fuel foaming, which can cause an inconsistent engine tune.

Question 39

Ball Cup --

Accepted Answer

A hollow plastic piece that snaps onto a ball end that provides a pivot point, usually for suspension movement. Using a ball cup/ball end assembly instead of a rigid system allows the car to absorb crash damage, so the ball cup will ""pop off"" instead of break. Ball cups are usually paired with turnbuckles for the ultimate in ease of use and assembly.

Question 40

Balance - (1) Armature & Crankshaft --

Accepted Answer

To reduce vibration and improve efficiency, many nitro engine crankshafts and motor armatures are balanced at the factory. This is usually done by adding weight with epoxy resins, or by removing weight by drilling or milling. With crankshafts, drilling or milling is usually preferred since it removes weight, but this alters the magnetic field of armatures, so epoxy is preferred for electrics.

Question 41

Balance - (2) Rim & Tire --

Accepted Answer

A tire that is out of balance causes erratic handling and vibration. A tire balancer is commonly used to balance tires, but the front tires on most 2WD models can be balanced on the ride. Either way, just spin the tire and mark the top position where it stops. Spin it again - if the mark stops elsewhere, you're good to go. But if it stops in the same spot, add a little weight inside the rim at the mark and repeat until there is no longer a heavy side. Self adhesive tire weights are available for balancing, but in a pinch, poster putty, chewing gum, or even a folded up dollar bill glued to the rim will work. For tires that are really out of balance, try gluing $20 or even $50 bills to the rim (or better yet, send the bills to rcMart.com and we'll send you a balanced rim).

Question 42

Balance Port --

Accepted Answer

A port on lithium based batteries that allows a battery balancer to be plugged into it. This helps to monitor the difference in individual cell voltages. Depending on the intended use or manufacturer, the types of balance ports may vary, which means adapters may be required for the specific balancer to work with various batteries.

Question 43

Ball Bearing --

Accepted Answer

A row of small balls or cylinders sandwiched between two rotating rings. Bearings are used to support rotating parts, such as wheels, axles, or driveshafts. They are available in a variety of sizes and styles to suit nearly any application or driving condition. Bearings are infinitely smoother and more efficient than bushings. Keeping your bearings oiled increases their lifespan and efficiency - use specific bearing oil or any light oil (such as 3 in 1).

Question 44

Ball Differential --

Accepted Answer

A differential that uses small balls and pressure rings instead of grease or diff fluid to tune diff action. A basic ball diff consists of; the diff gear, small balls, 2 pressure rings, a tension spring, and an adjustment nut. The balls are inset in the gear and then sandwiched between the pressure rings. As the adjustment nut is tightened against the tension spring, it applies more force to the pressure rings and against the balls, tightening the diff action. Likewise, loosening the adjustment nut loosens the diff, allowing the opposing tires to spin more freely. Ball diffs are known for precise adjustments and smooth operation, as well as being free of backlash or play. Always use light grease or silicone spray to lubricate ball diffs.

Question 45

Ball End --

Accepted Answer

A metal ball that has a hex (6-sided) and screw threads on one end. A ball cup will attach over the ball end to provide a pivot point.

Question 46

Ball Stud --

Accepted Answer

This is a metal ball with a threaded shaft, used with a ball cup. The ball stud is threaded into a part (or held in place with a nut) and a ball cup is snapped onto it. Most commonly found on suspension and steering linkages.

Question 47

Balloon - (1) Battery --

Accepted Answer

Refers to LiPo cells that have swollen or expanded due to over charging or over loading. These batts are done for - dispose of them properly.

Question 48

Balloon - (2) TIRES --

Accepted Answer

At extreme speeds or during diff unloading, the rubber tires of a model will expand considerably - this is known as ballooning. Ballooning causes erratic handling and reduced traction.

Question 49

Banjo Fitting --

Accepted Answer

A slotted circular clamp used to hold the carb body in place on some nitro engines. A screw or bolt is used in the slot to compress the clamp, which applies the retaining pressure to the carb body.

Question 50

Barrel (Shock) Spring --

Accepted Answer

A type of progressive rate shock spring that uses a barrel shape. The transition from the smaller coils on the ends to the larger coils at the center as the spring compresses creates the progressive effect.

Question 51

Bashier --

Accepted Answer

No track. No rules. Just drop your ride and peg the throttle. That piece of plywood leaning against the doghouse should make a nice ramp. . .

Question 52

Batteries --

Accepted Answer

This is what powers all of the electronics in an RC model, and they are usually grouped into packs designed for specific duties. For RC, alkaline, nickel (NiCad and NiMH), and lithium (LiPo, LiMN, and LIon) batteries are most commonly used. With the exception of alkaline, RC batteries are rechargeable.

Question 53

Battery Bar --

Accepted Answer

A short piece of metal (usually a copper compound) that is soldered between individual cells (batteries) to form a battery pack. Battery bars are waaay more efficient that the 'foil plated cardboard' commonly used for factory assembled packs, as well as offering rigidity to the finished assembly.

Question 54

Battery Box --

Accepted Answer

A case on nitro models designed to hold the receiver pack. On most models, this is found near the center line of the chassis, but on 2WD stadium trucks, it usually 'hangs' off the rear.

Question 55

Battery Brace --

Accepted Answer

A narrow plate that is placed on top of a battery pack to hold it in place. The battery brace is usually held down with body clips or thumbscrews.

Question 56

Battery Jig --

Accepted Answer

A tool used to hold individual batteries (cells) in place while building battery packs. Most battery jigs are designed for 4 to 6 cells.

Question 57

Battery Pack --

Accepted Answer

A collection of batteries that are joined together to combine the voltage of the batteries to power an electrical device. This term is used for electric RC cars (normally a pack to power the motor) and also for Nitro cars (to power the servos controlling steering and throttle/brake).

Question 58

Battery Slots --

Accepted Answer

A series of thin rectangles cut into the chassis of an electric model. The batteries sit inside these slots so that the batteries are flush with the underside of the chassis. This lowers the CG of the model. Along with a battery brace, battery slots also keep the batteries from shifting forward, backward, or side to side. It is a common practice to file (bevel) the edges of the battery slots to lower the batteries even further and to reduce rubbing on the battery casings. Battery slots are designed to be used with hand built battery packs (not stick or shotgun packs).

Question 59

Battery Strap --

Accepted Answer

A belt used in place of a battery brace to hold the batteries in place on electric powered models. This belt secures the batteries with a velcro or buckle closure. Strapping tape also acts as a battery strap.

Question 60

Battery Tray --

Accepted Answer

A flat 'platform' used as a base for mounting battery packs. The battery packs are usually secured with zip ties, velcro, or servo tape. Battery trays are commonly found on TVP (Twin Vertical Plate) style models, and may be a stressed part of the chassis.

Question 61

Bearing --

Accepted Answer

A hollow metal ""donut"" that uses balls inside to increase the efficiency of a rotating shaft (like an axle). Highly recommended as the first option part for any kit equipped with bushings, bearings have become a standard accessory in most RC kits today.

Question 62

Bearing Puller --

Accepted Answer

A "screw and plug" type of tool used to efficiently remove bearings from electric motors without damaging the bearing or mounting surface.

Question 63

Bearing Seal -- Bearing Shield -

Accepted Answer

A 'washer' that rides between the bearing races, designed to pretect the bearing balls from contamination, or to prevent oils or pressure from passing through the bearing. Not all bearings are sealed (or shielded). Bearing seals come in 3 basic styles:

(1) METAL - Metal bearing shields offer the least seal protection, but spin more freely compared to other seals. These are commonly found in drivetrain components, such as hub carriers, where protection from dirt and debris is needed but reduced friction is desired.
(2) RUBBER - This offers the best seal of all bearing shields, but increases friction and drag in the bearing. Rubber sealed bearings are commonly found supporting the crank journal in nitro engines (a.k.a. front bearings). Here, they help prevent pressure and fuel from exiting the crankcase through the bearing.
(3) TEFLON - Teflon is the all around bearing shield - striking a compromise between the sealing ability of rubber and the free spinning ability of metal shields. Teflon shielded bearings may be found anywhere that a shielded bearing is needed.

Question 64

Bead Rings - Bead Lock Rings --

Accepted Answer

The flat, circular rings attached to the outside of bead lock rims to hold the tire in place. These clamp (lock) the tire bead to the rim. Some people use only every other screw in a bead lock ring to reduce rotating mass.

KO Propo SE-1 Type-T Servo Set

Description:

Feature:

Specification:

Include:

Write a review