Drivetrain Layout

Introduction

When I open the front hood of my Mazda Protege sedan, I find its engine and many other goodies. Same for my S2000. I also saw many people opening their front hood to inspect the engine. This gave me an impression that all cars have engine stored under their front hood. I used to think this is the truth. But after going to my first car show, I realized that I was like a toad that was living in the bottom of a well.

I am always an admirer of Porsche. They made a lot of uncompromising performance car while maintain the exquisite styling. [Lately they disappointed me with the introduction of automatic 911 Turbo and Porsche SUV(!)] So naturally the first car I wanted to check out at the car show is the legendary Porsche 911 Turbo. After watching it inside out, I lifted the front hood to look for the engine that powers this Godly machine. Heck! There was nothing there, under the hood it is like a front trunk for people to store things. I found the rear trunk too but still no engine. I asked people around and finally someone answered me.

That person says the 911 Turbo is a rear-engine car. The engine is mounted after the rear driving axle. So it is located under the rear trunk. I finally find the engine by unlatching a lock. It is under the smallish rear trunk. Well, why do Porsche put their engines at the back of a car?

There was another thing about cars that bugged me. I am an avid player of the Gran Turismo racing game. Initially, I was baffled by the many designations like FF, FR, MR, RR and so on. I later learned that the first letter of the designation is used to designate the engine location and the second letter is to designate the which pair of wheels the engine drives.

Drivetrain Layout

The car body, the front panel, the rear trunk, the seats and other amenities that come with the car are just distractions to a performance enthusiast. What makes a car move are its engine, its transmission and its wheels. Collectively speaking, these three components comprise the drivetrain.

Engine Positioning

There are two major characteristics of a drivetrain that impacts the performance of a car: the engine placement and the driving wheels location. The engine placement is a big factor to determine the moment of inertia and the weight distribution of car because many other mechanical/electrical components of a car are usually located close to the engine. The driving wheels location determines which wheels the transmission to send the engine-generated torque to. Due to weight transfer, you will soon find out that driving wheels location is a very important factor in car handling.

For simplicity purposes, car nuts classified different engine placements into three types: front-engined, mid-engined and rear-engined. Front-engined cars have their engine placed in front of the passenger seats. Mid-engined cars have their engine placed behind the passenger seats but in front of the rear driving axle. Rear-engined cars have their engine placed behind the rear driving axle. There is also a subclass of front-engined cars called front-mid-engined. In this subclass, the engine is behind the front drving axle but in front of the passenger seats. An example is the Honda S2000.

Driving Wheels

As most people know, the engine isn't necessarily driving all the four wheels. There are actually four types of driving wheels. The first is All Wheel Drive (AWD) in which all four wheels are driven by the engine all the time. Then there is Front Wheel Drive (FWD) in which only the front wheels are driven by the engine. It then follows by Rear Wheel Drive (RWD) in which only the rear wheels are driven. The final one is an odd one. It is called Four Wheel Drive (4WD). It is usually employed by Sports Utility Vehicles (SUV). There is a switch in the car that allows you switch between AWD and FWD mode. Since 4WD doesn't really introduce anything new, so by reading materials related to AWD and FWD should help you completely understand 4WD.

Some common Drivetrain Layouts

In this section, I will give you examples of cars that represents the seven common drivetrain layouts. There will also be some simple description of their handling and acceleration characteristics. Acceleration characteristic will be explained in details at the bottom of this page. However, to understand the handling characteristics, you need to understanding the concepts described in the Cornering Chapter.

  1. Front Engine, Front Wheel Drive (FF)
  2. Front Egnine, Rear Wheel Drive (FR)
  3. Mid Engine, Rear Wheel Drive (MR)
  4. Rear Engine, Rear Wheel Drive (RR)
  5. Front Engine, All Wheel Drive (FA)
  6. Mid Engine, All Wheel Drive (MA)
  7. Rear Engine, All Wheel Drive (RA)

Quantitative Effects of Drivetrain Layout during acceleration

Here we would like to use a simple example to quantify the effects of different driving wheels. We will still use our Skyline GT-R as an example. This time in addition to the default all wheel drive (AWD) setting, we will also study the cases when it is front wheel drive (FWD) and rear wheel drive (RWD). To compare, we will check to see what is the maximum force that can be applied to the ground in each case.

The Math

First, let's assume for a while that our Skyline is FWD. For this kind of car, the maximum force that can be applied to the ground is limited by the limiting friction on the front tires. From the Weight Transfer section, we know that

Wf* = (mgLr - hFf - hFr) / (Lf + Lr)

Since this is FWD, Fr is 0. Note that the maximum of Ff is μWf*. Beyond that, the front tires will slip and hence no force can be transmitted to the ground. By substituting Wf* with Ff/μ, we can solve for maximum Ff:

Ff=(mgLr - hFf) / (Lf + Lr)
Ff=μmgLr / (Lf + Lr + μh)
=0.94×1550×9.8×1.519 / (1.146 + 1.519 + 0.94×0.34)
=7267.04N

Next we consider the case when it is RWD. In this case, the maximum force is limited by the limited friction on the rear tires. Again, from the Weight Transfer section, we know that

Wr* = (mgLf + hFf + hFr) / (Lf + Lr)

This time, Ff is 0. The maximum force will be limited by μFr. Beyond that, no force can be transmitted to the ground. Again, by subsituting Wr* with μFr, we can solve for maximum Fr:

Fr=(mgLf + hFr) / (Lf + Lr)
Fr=μmgLf / (Lf + Lr - μh)
=0.94×1550×9.8×1.146 / (1.146 + 1.519 - 0.94×0.34)
=6976.75N

For the AWD case, thanks to all the all powerful technology called Limited Slip Differential (LSD), the math is way simpler. LSD can distribute the torque generated by the engine to not yet slipped tires. So during acceleration, when the rear tires start to slip, it can transfer the torque to the front tires that still has traction. Therefore, the car basically won't slip until all four tires slip. To make all four tires slip, you have to apply a force greater than μWf* + μWr* = μmg = 0.94×1550×9.8 = 14278.6N.

Conclusion

As we can see AWD allows the most torque to be transferred to the ground. Then what about RWD and FWD? In our example, FWD allows for great acceleration. But if you look at the formulas more carefully, you can discover that there is a condition for this to happen. Suppose we want to know what is the condition such that RWD works better than FWD, we look at the condition Fr >= Ff:

μmgLfμmgLr
-------------------->=-------------------
Lf + Lr - μhLf + Lr + μh
μh>=Lr - Lf

Note that Lr and Lf is determined by the weight distribution of the car. The higher the weight is at the front, the longer Lr is and the shorter Lf is (Note that Lf+Lr is a constant). The converse holds true for rear-heavy cars. For a perfectly balanced (ie 50:50) car, Lr equals Lf and hence this condition holds. But as we can see if the front is heavy enough such that Lr is way longer than Lf, then FWD will prevail. Now we can check whether AWD is the correct configuration for our Skyline GT-R. Recall that our car gives out 392.27Nm of torque at 4400rpm. When it is transmitted to the rear tires at the first gear, it is Γ×g1×G/r = 392.27×3.827×3.545/0.3266 = 16,294.6N. In reality, there should be about 20% loss, so only 13,035.68N can be transmitted to the tires. Note that this is beyond what an RWD GT-R can handle but below what an AWD GT-R can handle. Therefore we can conclude that the engineers at Nissan did the right thing to make GT-R an AWD car.

After all these analysis, a question naturally arises: so if AWD is better than FWD and RWD for powerful engines, why the race cars still use MR configuration? The reason is that installing an AWD system put more weight on the car. For an example, compare the curb weight of BMW 325i and 325xi sedans: 1,463kg vs 1,573kg. There is about 110kg difference. For BMW 325 sedan, there is an increase of 7.5% in weight and hence 7% reduction in acceleration. However, for a 500kg race car, the weight increases by 22% and the acceleartion decreases by 18%!

Discussions related to understeer and oversteer in relation to the drivetrain layout will be discussed in the Understeer and Oversteer section.


First Draft: January 18th, 2004
1.0 Published: February 29th, 2004
Yee Man Chan