Whyrear wheel drives have better gradeability ???

HI
can anyone tell me why rear wheel drive cars have better gradeability than fornt wheel drive cars ....... gradeability means the max. angle of slope they can climb...........i read in one of the books ........ but didn;t find the reason.......

The key is in where the car's center of gravity (CG) is relative to the powered wheels. It's harder to explain here than just showing a figure, but I'll try.

For a front wheel drive (FWD) vehicle going uphill, the CG is located behind the powered wheels. Thus, the wheels have to "pull" the rest of the car uphill. There is only a fraction of the car's weight pushing down on the driven wheels, thus the amount of grip provided by the tires is much less.

For a rear wheel drive (RWD) vehicle, the CG is located above and in front of the powered wheels. As a result, the car's higher CG helps to push down on the rear wheels, increasing the available grip, thus providing more traction.

Hope the helps!

Good answer.
Also, the RWD traction advantage applies only on dry pavement. Often, on wet pavement or loose surfaces, like gravel, FWD is better. And in snow, FWD is always better.

As you go up a grade, the weight on the front wheels is reduced and transferred to the rear wheels. This adds traction to RWD cars, but reduces traction for a FWD car.

I also have to disagree with magicrat. FWD is not always better in the snow. Its better as long as you maintain traction. Once you lose traction in a FWD car you also lose steering. FWD is a great thing and has its applications, but the automotive manufacturers did an insanely excellent job of convincing us that FWD was for safety and traction, but the truth is, it was entirely a financial move to start with.

The truth is, the best wheels to drive are the ones that get traction. In 90% of the situations, that means RWD. In low-power econobox situations where the drivers are not looking for performance as a priority, FWD is excellent.

My winter car growing up in PA and ONT was a Caprice RWD with 255mm summer-only rubber. If things got really hairy I would put on 235mm snow treads. I also had a FWD Beretta. The Beretta with 205mm all season rubber was not as trustworthy in the snow as the RWD Caprice. The tire footprint of the Caprice was bigger (theoretically reducing traction in snow), but it weighed considerably more. There was much more weight over the drive wheels. Plus, if I did lose traction with the drive wheels, I could still steer.

It also has a lot to do with driver experience.

I almost challenged MagicRat on the FWD and snow comment as well, but realized the OP was referring to a straight grade, not trying to perform any maneuvers.

For the past several years my wife drove a RWD Lincoln LS year round here in Iowa (she just bought herself a 2010 Lincoln MKZ which is FWD). When the first snow fell that amounted to something I installed the Blizzaks. She had total confidence in that car in the winter, mostly because even if she lost traction to accelerate she was easily able to steer. The LS has a pretty good weight balance from the factory, and it was probably even better with the stroller, jump start pack, and other stuff that she kept in the trunk. Of course traction control and ABS help out a lot these days as well. I have my doubts that her MKZ will perform as well in the snow as her LS did with Blizzaks.

-Rod

I almost challenged MagicRat on the FWD and snow comment as well, but realized the OP was referring to a straight grade, not trying to perform any maneuvers.

True, I was just discussing forward traction here. Not only can a RWD car steer when motive traction is lost, but, of course, the ability to steer with the throttle (aka slide) adds a new dimension for handling in slippery conditions.... at least for experienced drivers, anyways.



I also have to disagree with magicrat. FWD is not always better in the snow. Its better as long as you maintain traction. Once you lose traction in a FWD car you also lose steering. FWD is a great thing and has its applications, but the automotive manufacturers did an insanely excellent job of convincing us that FWD was for safety and traction, but the truth is, it was entirely a financial move to start with.

The truth is, the best wheels to drive are the ones that get traction. In 90% of the situations, that means RWD. In low-power econobox situations where the drivers are not looking for performance as a priority, FWD is excellent.

My winter car growing up in PA and ONT was a Caprice RWD with 255mm summer-only rubber. If things got really hairy I would put on 235mm snow treads. I also had a FWD Beretta. The Beretta with 205mm all season rubber was not as trustworthy in the snow as the RWD Caprice. The tire footprint of the Caprice was bigger (theoretically reducing traction in snow), but it weighed considerably more. There was much more weight over the drive wheels. Plus, if I did lose traction with the drive wheels, I could still steer.

I knew my use of the word "always" would start a fight!!:)

Yes, a skilled driver can use RWD in snow to their advantage. But, IMO for the vast majority of drivers out there, the superior traction and greater predictability of FWD makes them a better choice. Here's why:

1. Usually, with similar tires, having the greater proportion of mass over the driving wheels means FWD gets better motive traction.

2. For the average, modestly-skilled driver, FWD handling is often better in the snow.
Yes, when the front wheels spin, you can't steer, but you tend to go in a straight line. It's predictable, and you usually regain steering when you lift off the gas and the wheels stop spinning.
IMO this means you still have considerable control of the car.

3. When you lose motive traction in a RWD, the rear end often starts to slide sideways (especially with an LSD). Even though you can still steer the front wheels, the rear end is steering itself, and you are still losing control of the car. Furthermore, the pendulum-effect means that even if you let-off the gas, the rear end can still continue its sideways movement..... and you are still losing control. For the majority of drivers, this sideways movement is unpredictable for them (but not necessarily for a skilled driver)


As for your example, comparing an all-season-equipped Beretta and a snow tire Caprice is apples and oranges. Tires make a huge difference in such a comparison. :) Even FWD cannot overcome the disadvatage of poor winter tires.

RWD cars tend to be pretty variable in snow conditions. Some, like the GM B-bodies (Caprice etc) were not bad, because of substantial rear-end overhangs means decent weight in the back. My old Fleetwood is fairly decent in the snow. But RWD coupes, such as Mustangs, Camaros, older Supras etc are dreadful in the snow. FWD coupes of the same size are much, much better.

The best two-wheel drive winter car I ever drove was a Dodge Omni, with tall, skinny snow tires. It got great traction, yet was light enough to easily corner and handle very well on snow, much than my heavier FWD cars or my RWD cars.

Finally, yes the switch to FWD cars was financial...... but only because the consumer was embracing FWD imports and the domestic manufacturers had to remain cometitive. Frankly, they would have preferred to stick with RWD cars because they were cheaper to engineer and manufacture. This is why the first subcompacts, (Pinto, Vega, Chevette) were all RWD... for cost reasons.)
The switch to FWD at the time had other advantages, such as more efficient packaging, more interior room and, for the time, better handling. 30 years ago, domestic FWD cars all handled much better than their equivalent RWD cars.