Cars, Engineering and Other Ramblings: RWD - Not Necessarily the Ultimate Driving Machine

Not Necessarily

The masses have been trained through careful advertising campaigns to believe that rear-wheel-drive is the end-all be-all of performance, but is it really? What about front-wheel-drive and all-wheel-drive? Surely there must be some benefit to these other wanna-be 'Ultimate Driving Machines', right?

Up a creek, or in a creek?

In 2010 BMW CEO Norbert Reithofer let it slip (link) that an internal study revealed "80% of BMW 1-Series owners thought their ultimate driving machine was front-wheel-drive." Well, that's embarrassing. It would seem that even BMW owners aren't aware of what makes their 'Ultimate Machines' so ultimate. Do you?

A pendulum has never been so interesting

In Mechanical Engineering, complex problems are often related to any of several basic principles, one of which is the pendulum. You are probably thinking right now "wow, a pendulum, so what?" Pay attention, your mind is about to be blown. A pendulum is inherently stable because gravity pulls the mass downward and aligns the pendulum to a vertical position. If the pendulum is pushed such that it rocks side-to-side, it will always return back to the stable, vertical position.

An inherently unstable inverted pendulum

The opposite of a pendulum? You guessed it, an inverted pendulum. This model is inherently unstable, but vertical, for now. If the pendulum is pushed, sneezed at or even looked at wrong, gravity rapidly swings the mass 180 degrees around. The clever ones in the group have likely caught onto the analogy by now, but the author will proceed for the rest of the class.

Why your rear-drive car is an inverted pendulum

Getting back to cars, a rear-wheel-drive car is a classic inverted pendulum. In a perfectly straight line, untouched by turns, that rear-wheel-drive car is going to stay straight, for now. But what happens if you enter a turn while under heavy throttle? The inverted pendulum, or car, swings around 180 degrees and your BMW 1-Series owner ends up in a creek without a paddle. Probably wishing you had an inherently stable front-wheel-drive car right now, huh?

The Honda Civic, losing races since 1973

Now hold on a moment, before everyone rushes out to buy a Civic Si or Neon SRT-4, you may want to keep reading. Front-wheel-drive has it's drawbacks too. Imagine you are launching from a standstill in your Honda Civic, what happens? The vehicle rotates about it's center of gravity due to the forward force, this rotation, or weight transfer, shifts weight off of the front wheels and onto the rear wheels. This transfer of weight is bad for a front-wheel-drive vehicle as it reduces traction and results in a big smoky burnout without any forward motion. A rear-wheel-drive vehicle benefits from weight transfer, but if launching on a street - traction is still a limiting factor.

Having one wheel spin like crazy isn't ideal, neither is a hole in your transmission.

To make matters worse, your front-wheel-drive econobox likely has an 'open differential' which acts as a power fuse. Essentially, once a wheel loses traction, all power is transferred to that wheel which results in even more spinning. Ever notice during a front-wheel-drive burnout that one wheel spins while the other simply rolls? Open differential. (Note, the Neon SRT-4 came stock with a limited-slip style differential which did not have this issue, thanks Getrag!)

Can your rear-wheel-drive vehicle do this?

By now you are likely asking, "Alright, so what vehicle drivetrain is not an inverted pendulum and is able to grip while rapidly accelerating from a standstill?" All-wheel-drive is the answer. Again, before you run out and buy a brand new Subaru Impreza WRX STI, keep reading as there are more tradeoffs. The addition of both front and rear differentials with axles and supporting driveshaft, an all-wheel-drive setup is heavier than both front and rear-wheel drive setups. Beyond additional mass, all-wheel-drive vehicles such as the Mitsubishi 3000GT VR-4 experience significantly more power loss through the drivetrain than their front-wheel and rear-wheel drive contemporaries. Given 300 horsepower at the crankshaft, a stock '93 VR-4 (link) sees roughly 240 horsepower at the wheels - a whopping 20% drivetrain loss! Ouch! Subarus see similar results (link).

It turns out that the answer to the question of "What drivetrain setup makes for the 'Ultimate' Driving Machine?" is quite complicated and subjective. A quick recap of everything presented so far is in the table below.

Muddying the drivetrain selection waters

First, a front-wheel-drive platform is inherently stable with low drivetrain loss and weight, but does not have traction from a standstill - not to mention the understeer introduced by this setup when turning (a topic for another post). Next, the rear-wheel-drive platform is inherently unstable with low drivetrain loss and weight, but can only obtain decent tracking when launching on a well prepped track surface. Finally, all-wheel-drive has the bonus of stability depending on front/rear bias and can launch in the rain, but it has marks against it for being heavy with high drivetrain loss.

Which is best? It depends on what you are looking for. Do you want to go really fast in a straight line, and enjoy having the rear end kick out when turning under throttle? Maybe go rear-wheel-drive. Are you looking for a daily-driver that can only take on adversaries from a roll? Perhaps front-wheel is your choice.