Drivetrain

The drivetrain subsystem transfers mechanical energy from the engine or electric motor to the wheels. It includes the gears, chain or belt, differential, driveshafts, and CV joints.

Gearing

Main page: Gearing

Gears allow the engine to spin at different speeds than the wheels. Most combustion vehicles have multiple gears to achieve a high torque output at low speeds and high speed output when needed. The gearing determines crawl speed, top speed, and torque availability at any vehicle speed in between. For most motorcycle engines, this is a set of internal gears that can be selected by the driver. For electric vehicles, the transmission is often a single speed reduction gear.

The design engineer will need to also be mindful of how much tractive force the vehicle will generate in any given gear. This will influence tire selection and helps determine what happens when the driver firewalls the go pedal. Gearing will also factor into forces on the differential and driveline components.

Motorcycle Gearboxes

Motorcycle engines have internal sequential gearboxes. This is means that the gearing is located inside the crankcase. The lubrication is managed by the engine's oil system. Typically, these are 5 (YZ450) or 6 speeds (CBR600RR). Drivetrain efficiency for a geared system is often as high as 98%.

Gear Selection

Main page: Shifter

If your car has more than one gear, it's dependent on the driver to select which one the car should be in at any given time. This can be done by adapting the engine's shifting system to work in a vehicle. Many form factors to actuate the shifter can be used including but not limited to: manual shifting, pneumatic shifting, and electronic shifting.

Clutch

Most motorcycles use a wet plate clutch pack whereas most cars use a single dry plate clutch. Wet plate clutch packs are immersed in oil (wet) and use multiple friction plates (known as a pack).

The motorcycle clutch uses a set of linear springs or a diaphragm (less common). When engaged, the springs squeeze a series of clutch plates tight enough to restrict relative rotation. This mechanically links the engine output to the crankshaft. When disengaged, the plates are under no pressure and have the ability to rotate freely relative to one another. This allows the engine to spin without outputting a torque.

Continuously Variable Transmissions

These allow for an infinite combination of engine to wheel speeds. The benefits of this include allowing the engine to be run at peak power, peak efficiency, or peak torque at any vehicle speeds. The downsides are lower efficiency (~88% effficient) and a lower peak torque transfer.

Drive Mechanisms

There are two ways to get power from the engine to the tires: connecting the engine output shaft directly to the wheel(s) being driven (direct drive) or transmit the power from the engine to the wheels through an intermediate mechanism (indirect drive). For combustion vehicles, indirect drives are the prevalent design as it allows for a flexibility of engine placement relative to wheels, a greater degree of drive ratios, and it often easier to implement[citation or explanation needed].

Indirect Drive

Chain Drive

Main page: Chain Drive

Power transmission using a chain drive uses a drive sprocket on the engine connected by a chain to a driven sprocket on the rear axle(s). A chain drive is the most common system found in FS for a few reasons: most motorcycles already use this system, and it is easy to convert the existing system to drive a formula car, the system is robust, and the minimum design work to implement a functional system is limited. Factors that can limit design work include:

• The system has a low part count
• Many OEM or equivalent sprockets are available
• The system requires no pretension
• A chain has no slip
• There is a wide margin of error for design flaws except planar misalignment
  

Belt Drive

Power can also be transmitted with a belt and two pulleys. A belt and pulley system does not require lubrication, but still needs a tensioner to achieve the necessary pre-tension. Belt drive systems are more expensive than sprockets. These systems are frequently lighter and quieter than chain drive systems. The chordal action is much less apparent on belt drives because the pulley is more circular than a sprocket.

V-Belt (CVT)

CVT drive belts used in snowmobile engines are thicker and narrower than drive belts used with sprockets. The belt has a trapezoidal cross section to enable a large contact surface on the side for the CVT pulleys to contact. These are constructed of rubber laminated with steel cables.

Further reading: https://www.tec-science.com/mechanical-power-transmission/belt-drive/belt-types/

Pushbelt (CVT)

Some CVTs use a special type of belt to connect the drive and driven axes. This belt works by pushing instead of pulling. The belt is made of a series of stacked plates. These are unlikely to find relevant usage in FSAE as they are designed for much higher loads and lifetimes, and are much more expensive than rubber v-belts.

Gear Drive

A gear drive system transmits torque through a gear train to the driven axle, this can reach very high efficiency but is requires a custom design and implementation and is often heavier than an equivalent chain drive would be.

High-Octane Motorsports (out of Erlangen-Nurnberg) ran longitudinally mounted engine and bevel gear drive in 2020. Edith Cowan University built a custom engine block with an internal gear train to the driveshaft output used in 2014-2016.

Direct Drive

Direct drive configurations are uncommon in FSAE for combustion engines. Electric vehicles are commonly seen with direct drive, especially in the case on hub-motor designs. Direct drive systems are often more compact and can be more reliable but often have less opportunities for tuning.

An example of a combustion direct drive would be University of Virginia connecting the engine directly to a 2004 Miata differential in 2021.

Driveshaft

Not common on motorcycles, driveshafts are generally used for front engine, rear wheel drive cars. While some motorcycles do have them, mostly BMWs, they are more expensive and cannot as easily be customized to a new length. These are also the least efficient drive mechanism and can reach efficiencies as low as 80%. Becuase of their cost, lack of tunability, and low efficiency, these are not common in FSAE.

Hub Motors

Electric vehicles can use in-hub motors. These are frequently chosen for packaging efficiency, despite the drastic impact on yaw inertia and unsprung mass. Other benefits include increasing efficiency while reducing mass and cost, as driveshafts are not needed. Hub motors are the most common way to achieve driven front wheels, as packaging the motors inside the driver footwell can be prohibitively difficult. However, the constraint of fitting within the wheel may impose a size restriction on the motors causing them to be underpowered.

In order to execute the gear reduction needed by the electric motors, a hub motor design requires the use of a planetary gear set.

Drivetrain Shield

Drivetrain shields are covered under the FSAE rule T.5.2. Chain drive shields are subject to stricter rules, requiring a wider shield made of steel. A belt drive system is allowed an aluminum shield of a narrower proportional width, because belts are wider than chains, this may not yield a narrower shield. Frame tubes are allowed to act as part of the shield given they pass the material requirements. OEM chain guards on the engine itself such as on the CBR600RR are accepted as part of the chain guard as well. If any part of a brake line or cable carrying traction voltage passes across the chain, it also needs to be shielded by the chain guard or equivalent; this is an especially common issue in tech if the line crosses underneath the chain. This infraction is checked at tech by line of sight.

A drivetrain shield is easy to check before tech inspection and usually too time consuming to complete during.

Finger Guard (CVT)

Rule T.5.2.10 requires finger guards to enclose any part of the drivetrain that spins when the vehicle is stationary and the engine running. This primarily refers to the CVT mechanism: belt and both pulleys must be enclosed.

Final Drive Ratio

The final drive ratio (FDR) is the gearing reduction between the transmission output shaft and the differential. Because most motorcycle gearboxes come stock with the engine, and are usually similar in design, the final drive ratio is the easiest impact the team can have on the vehicle's gearing. Changing the FDR to a larger ratio will result in a lower top speed, close gearing, and higher acceleration. A smaller ratio will result in a higher top speed, wide gearing, and lower acceleration.

One often overlooked issue with especially tall FDRs is packaging the large rear sprocket. Usually the FDR is chosen to meet acceleration goals as the engines can rev high enough to achieve the top speed of the autocross style courses being driven. A shorter FDR can also reduce the number of gear shifts the driver will make over the course if driver competency is a driving factor in vehicle design.

Traction Model

Blank Tractive Force Diagram

A traction model or tractive force diagram is a useful tool in the design and analysis of a vehicle's drivetrain. Tractive force is plotted against vehicle speed to visualize how much torque, thrust or traction the vehicle will be able to generate at a given speed. Tractive force is derived from the engine torque, gear ratio and tire's dynamic radius. A curve showing the tractive force is drawn for each gear in the transmission. Some software will trim the traction curves at the optimum shift points for clarity, showing only the maximum tractive force available.

Curves denoting the limit of traction from the tire and drag are plotted on the same graph. Aerodynamic forces cause the limit of traction and drag forces to increase exponentially with vehicle speed.

A car that runs a high power engine like a CBR with a tall final drive ratio may find that the peak torque from the first gear is partially or entirely beyond the limit of traction. This would prevent the driver form being able to utilize the full torque the engine can produce until high in the first gear or even until shifting to second gear. The team could shorten the FDR to move more of the peak torque of the first gear beneath the limit of traction as shown in the second image. Additionally, the team could remove the first gear of the CBR, stretching the graph to the left as seen in the third image. Remember: this is not necessarily the route your team will want to go. A skilled driver can better handle the limits of the car without being spoonfed or strapped to an underpowered machine. Your Mileage May Vary.

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  Example of a Tractive Force Diagram

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  The Same Tractive Force Diagram with a Reduced FDR

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  The Same Tractive Force Diagram with a Reduced FDR and Removal of the First Gear

This is why removal of the first gear is a common engine modification for the CBR

Differentials

Main page: Differential

A differential allows the drivetrain to split torque to two wheels (or two other driveshafts for AWD) while allowing each wheel to rotate at different speeds. This facilitates turning at higher speeds and can help reduce, control, or even magnify understeer caused by the rear wheels being driven. The two main varieties are open and limited slip. High performance cars and most FSAE teams opt for limited slip type differentials because of their locking functionality and tunability, but many successful FS/FSAE teams have competed using an open differential or a spool (functionally equivalent to a welded differential).

Driveshafts

Driveshafts, or half-shafts, are needed to transfer the torque from the differential or spool to the wheel. Because the wheels move with respect to the rest of the car, the driveshafts will have to be able to articulate while rotating. This is achieved by implementing a constant velocity (CV) joint. Vehicles that use a solid axle do not need diveshafts.

Driveshaft Specifications

Strength and Stiffness

The driveshafts should be designed or specified to ensure they will last throughout the competition races, but often off-the-shelf solutions like the steel case-hardened driveshafts seen on most FSAE cars are over built and will not fail in the lifetime of the vehicle.

Since powertrain designs are frequently asymmetric, the left and right driveshafts of many cars are not equal length. This can lead to a phenomenon known as torque steer, where the shorter driveshaft does not twist as much as the longer driveshaft and the wheel it is attached to spins more. While this is usually so minor as to not be noticed by the driver, design judges will want to see that the design engineer has looked into this: either showing the effect is too small to be an issue or what steps they have taken to minimize/mitigate it. Centering the differential or using driveshafts of stiffnesses that correspond to their lengths are the easy ways to fix or mitigate this. The calculation to show the torque steer is too minor to cause a driving issue is also quite simple. See Torque Steer page for example calculation and walkthrough.

Material

Steel is the most common driveshaft material in FSAE due to it's cost and availability in off-the-shelf products in FSAE specific or applicable sizes. For example, RCV sells FSAE specific steel driveshafts. Aluminum or carbon fiber may be used in instances where the weight reduction of the vehicle, or the reduction in rotational inertia is of high priority. Driveshafts from these materials are not sold off-the-shelf for FSAE specific applications and must be adapted or custom made. It is important to consider the hardness and impact resistance of steel versus aluminum or carbon fiber when looking into the usage of alternate materials for driveshafts. The splines of driveshafts should not be made from aluminum or carbon fiber, so the shaft must somehow be bonded to a spline or a spline-less connection to the CV joint must be utilized.

Length

Driveshaft Plunge

The length of the driveshafts shall be designed or chosen short enough to allow for the driveshafts to fit in the hubs when the wheel centers are in-line with the differential and long enough to keep the shaft side of the CV joint inside the socket when the wheel is at full jounce and full rebound. If the shafts are too long, the wheels will not be able to articulate fully and the vehicle will not pass technical inspection. RCV provides instructions on axle length calculation.

This change in distance between the CV joint at the differential and the CV joint at the wheel is called driveshaft plunge.

It is possible to change the length of a driveshaft after it has been purchased. This can be done by machining the splined ends to reduce the overall length or by cutting and rewelding the middle. Cutting and rewelding will destroy any confidence in the linearity, strength, temper, hardness, and reliability of the axle as purchased and is not recommended.

Retention

Most driveshafts are retained rotationally by a splined connection. These are bought of the shelf, or can be custom machined. If machined, the splines must lie outside of the outer diameter of the shaft. The shafts are retained to the CV joint tripod by a c-clip groove that must be machined in house even if axles are bought off-the-shelf.

CV Joints

The most common CV joint in FSAE/FS is the tripod design. Tripods are so ubiquitous they are considered as fundamental part of the drivetrain design in the 2022 FSAE design scoresheet. They can be found commercially off the shelf at most suppliers that sell steel drive-shafts such as RCV and Taylor. Tripod efficiency is well at low joint angles regardless of joint angle up to and beyond 10 degrees.

Alternative designs can and have been used before in FSAE. USC used rubber "guibos" from a BMW driveshaft in 2004. BYU in 2008 used aluminum flex plates (as well as CF driveshafts). UNSW used aramid flex plates in 2012

Validation

In vehicle simulations, including driveline affects become important the more detailed the vehicle model gets. The brake torque and horsepower are not what is seen by the tire's contact patch. A physical measurement of driveline forces should be undertaken at some point. It is not likely needed to do this every year however it should be done at least once to correlate model to reality.

This is typically a rotating measurement and typically involves the use of strain gauges. Wireless amplifiers are often needed. Several companies make them available such as Texense, Izze Racing and Lord Microstrain.