Search
Close this search box.
Search
Close this search box.

Why Are Reverse Gears Straight Cut?

Manual transmission of a car with gears visible straight cut spur gears isolated against a blue background

There are several key advantages to using straight-cut gears for reverse gears in cars. They are more efficient, cheaper, and safer than the alternatives. In the article below, you’ll learn more about how reverse gears differ from the other types of gears found in cars.

Straight-cut gears are also called spur gears. This is because the teeth of these gears are cut straight so that they point directly outwards. You’ll also find helical and worm gears used in the automotive industry, but for different purposes other than the reverse gears.

It’s normal to hear a little bit of noise when you throw your car into reverse. However, persistent or worsening noise could be a sign that there is trouble afoot. Continue below to learn more about troubleshooting noisy gears.

Are All Reverse Gears Straight Cut?

All reverse gears are straight cut. When you look at a reverse gear, you’ll see teeth that are pointing straight out. These resemble the cogs seen in bicycles.

Reverse gears are straight cut because they reduce the amount of potential wear and tear on all components of the powertrain. They allow racing car manufacturers to maximize torque while also making efficient use of space.

What Are the Other Types of Reverse Gears?

Now that you know what reverse gears are straight cut, you may be wondering what the different types of reverse gears are.

What types of gears will you find in a vehicle:

  • Spur (Straight Cut) Gears: Transmit power through shafts that are parallel.
  • Helical Gears: The teeth are angled so that more than one tooth is in contact during operation. These are often used in automotive transmissions.
  • Worm Gears: Used to transmit power through right angles on non-intersecting shafts. Typically used in low horsepower applications.

There are many more types of gears, such as helical gears, that are not used in racing applications due to the type of load that they produce and their ability to contribute to safe and efficient operation.

Close up of gears visible inside a manual transmission car

Advantages of Straight Cut Reverse Gears

There are many advantages to using straight cut (spur) gears as the reverse gears in cars. Continue below to learn more.

No Restrictions On Torque- Great For Racing

Straight-cut reverse gears do not produce an axial load, which is an application of force that acts parallel to the direction of motion. As such, straight-cut gears do not restrict the amount of torque.

Torque is the power needed to get a vehicle in motion. It’s easy to see why torque is so vitally important for auto racing. A car with higher torque will always be able to accelerate easier.

Toyota Supra blue at the International Festival Auto exotics in Sevastopol, Ukraine
Toyota Supra – 4th Gen

Can Support a Large Powertrain

Straight-cut gears are the safest option for supporting a large powertrain. Once you start applying axial load onto output shafts and bearings with a large powertrain, you are going to run into trouble. Axial loads can cause these components to tear apart.

Heat is a concern with helical gears, which produce axial loads. Helical gears differ from straight-cut gears in that the teeth of the gear are angled.

Affordable & Easy to Assemble

The simplistic design of straight-cut gears makes them easy to assemble. Fewer parts are needed to construct a powertrain with straight-cut gears. Not only does this slim down the size of the car, but it also makes it less expensive to construct.

Disadvantages of Straight Cut Reverse Gears- Noise

The main disadvantage of straight-cut gears is the noise. All the parts of the gear come together at once, producing a loud whining noise. There are also limitations on the weight of the load that straight-cut gears can carry.

Straight-cut gears are typically only used as reverse gears for this reason.

Car mechanic inspecting and repair a manual transmission gearbox removed from a car at an auto body repair shop

Do Cars Ever Have Helical Reverse Gears?

Theoretically, couldn’t you construct a car with helical reverse gears, rather than straight-cut gears?

Well, the answer is no. The disadvantages mentioned above outweigh any potential benefits that could be had from switching out straight-cut reverse gears for helical ones.

Why Does My Car Whine In Reverse?

Now you understand why straight-cut gears are only used for the reverse gears in daily drivers. They are cheap and efficient, but they produce too much noise to be used in “forward” gears.

Can you stop your car from making so much noise in reverse? A little bit of noise in reverse is normal, but excessive whining could be a sign that trouble is afoot. Continue below to learn whether a vehicle repair may be in order.

Close up of a 6-speed manual transmission lever shift knob of a car

Clogged Fluid Transmission Line

If that whining noise is getting progressively worse, then this could be a sign that the fluid transmission line has become clogged. This usually is an indicator of a more serious problem with the transmission, so you are encouraged to visit a professional mechanic if you believe that this is the case.

If The Whining Noise Continues when You Are In Gear

Once you have shifted into reverse, the whining noise should stop. If you continue to hear this noise as you are backing up, then this could signify a problem with the torque converter.

A simple test would be to check if you can hear the noise persist while you are in “Park” or “Neutral.” If so, then you might have to take a look at your torque converter.

Illustration of gears inside a manual transmission gearbox of a car with shift knob lever and the corresponding gears
Manual transmission

Why Are Straight-Cut Gears So Loud?

You might consider cleaning the transmission out, particularly if you log a lot of miles in high-stress conditions.

High-stress conditions include:

  • Hauling heavy loads
  • Frequent stop-and-go driving
  • Trailer towing

During a transmission cleaning session, mechanics use equipment to pump fresh oil through the entire system. This process offers much greater potential to flush all of the gunk of the transmission versus a simple drain-and-replace.

With the manual transmission gearbox on the work table the mechanic is removing the casing getting ready to refurbish it

Conclusion

In cars, the reverse gears are always straight-cut. This is because they perform better with a large powertrain, are safer, and are also more efficient. Straight-cut gears require fewer parts than the helical gears used in the forward gears. Scroll above for advice on how to handle excessive noise when you throw your car in reverse.

Your subscription could not be saved. Please try again.
Thanks for subscribing, see your free e-book on your inbox!

References:

EDT Automotive

Rexnord

The Different Types Of Gears

Hubert Vester Auto Group

3 Transmission Sounds That Mean Trouble

Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

Leave a Comment