This book was published before the days of traction control.

 

Well, that's good because we don't want to consider stability control in this context.

The point is whether braking the front wheels would cause a car to yaw not whether it could be prevented by a computer. I'd rather not get into a dangerous situation and hope technology will save me.

 

Imagine getting a BSOD (or spinning beach ball) when you go into a skid

 

The book's recommendations are in agreement with those of most modern tire manufacturers — certainly Michelin. Unless you're an expert driver, and know how to control your car when its rear starts to break away, the tires with the better grip should be put on the rear wheels, irrespective of whether the car is front- or rear-wheel drive. I believe that it follows from this that front-wheel-only braking is more likely to result in rear breakaway than 4-wheel braking or rear-wheel-only braking.

Stan

P.S. I retract the last sentence! See post #48.

 

And you would be 100% wrong.

PLEASE try my toy car experiment. It is very counterintuitive until you see what happens. The experiment is very easy to do. Just put a piece of tape from the top of one tire over the trunk or hood to the top of the opposite side tire to lock up the rear or front wheels.

With regards to weight distribution, any car that has more weight on the rear wheels will be unstable. PERIOD. The more weight (CG) up front, the harder it is to turn a car, and the more weight behind the center of the wheelbase, the harder it is to keep a car going straight. It is a fact that all stock vehicles (even rear engine sports cars) have more weight on the front wheels, and those very high performance sports cars that are close to 50/50 have warnings not to overload the trunk.

With regards to front versus rear braking, when the rear wheels are locked up, they can fishtail. When they are free rolling, they WILL track, and not slide sideways at all, unless the sideways slide began before the rear braking was removed or unless some force external to the vehicle pushes the vehicle sideways (such as a significant road slant perpendicular to the direction of travel, or in a hard cornering where there is significant lateral force on the car). Only locked up tires are likely to loose directional tracking. When front wheels lock up, the car will want to go straight, even if the front wheels are turned. When the rear wheels lock up, the car will want to spin around (as easily demonstrated by the toy car experiment).

Fishtailing while going forwards is always caused by sliding rear tires, not sliding front tires.

PLEASE TRY THE SIMPLE EXPERIMENT BEFORE REPLYING. The results are very dramatic! I really don't care that a published author got it wrong; the physics are quite simple, and apply to real cars as much as they do to Matchbox cars.

Four wheel braking is best, assuming the rear braking is light enough to avoid rear wheel lock up. But, if you must stop or slow with only two wheels, pray it is the leading wheels used to slow, for both directional stability, and for maximum stopping ability.

-- Alan

 

Sorry, I don't have a toy car around but I did the experiment two winters ago in my old car (FWD, ABS, no stability control).

I was driving on a flat straight road at 30 miles per hour. It was icy and I wasn't changing lanes or braking. Just driving slowly and cautiously because my car had already had trouble earlier that evening going down the ramp of a parking garage. I took my foot off the gas and coasted and at that moment, the rear of the car swung around. I corrected but it swung the other way until I spun around a few times and across several lanes. After it was apparent that I wasn't going to stop by hitting the median barrier or another vehicle, I decided to apply the brakes lightly so that I could come to a stop. With traffic backed up behind me, I did a 3-point turn and crawled home without further incident.

This was the scenario exactly described in the book. Remove your foot from the gas pedal in a FWD car and the front wheels stop pulling the car and engine braking kicks in.

Your experiment assumes one set of wheels is locked/skidding but that isn't the case in real life. With front wheel engine braking, all wheels can be turning, not sliding, and the rear end can come around.

 

This is a slightly different scenario. The problem here is that your engine braking was unevenly applied to your two front wheels, inducing a yaw. My guess that that the road was more icy (slippery) on one side of the vehicle than the other (while extremely slippery all the way around!), which led to one of the two front wheels locking up, but not the other (which the TCH stability control system will prevent, with or without B mode engaged). Older FWD cars that had bad torque steer issues were also more likely to unevenly distribute engine braking left/right.

This is more likely to happen with engine braking than with real brakes, but in the TCH there would be no difference between engine braking with B mode, or regular braking in regen mode (no use of the disc brakes, where all engine and/or motor deceleration is acting through the front differential).

All modern ABS systems are extremely sensitive to exactly this scenario, regardless of whether the left/right braking difference is caused by uneven road conditions, or differential induced wheel speed differences (front or rear). Unfortunately, an ABS-only stability system cannot correct for this since the correction is to apply the brakes to the non-skidding side, while ABS-only systems only modulate brakes off. The TCH system can modulate brakes on or off, as can most advanced electronic stability systems common to more roll-over prone SUVs.

With the toy car, this scenario would be equivalent to locking up only one of the four wheels, and then a lot will change with this experiment. If you want to try this experiment, your corner Wallgreens/CVS/whatever is likely to have a Matchbox like car for under a buck! A tilted phonebook is a long enough incline to see the results.

Given this scenario, I can now see the wisdom of the earlier referenced book (which was written before stability systems more sophisticated than basic ABS were available). I apologize for not previously considering this scenario. However, I suggest that this is not an issue with the TCH, or most modern vehicles with anti-skid control, and modern anti-skid controls make the FWD/RWD/4WD difference in this respect meaningless since they can force an even distribution of braking forces to each side of the differential.

It is still true that rear-only braking on slick surfaces is the worst possible scenario with regards to yaw spins. The most important aspect of maintaining yaw stability depends on the rear wheels tracking and not slipping. The best way to keep them tracking is by keeping them from locking up. The more brake force applied to a rear wheel, the more likely for that wheel to begin a sideways slide. Both front wheels locked up is most likely to keep a car going straight ahead. Both rear wheels locked up will cause a car to spin around. All four wheels locked up will cause a car to spin if the weight is greater on the rear wheels (the heaviest side of the car will want to lead in a four wheel slide, in spite of the fact that there may be slightly higher friction with the wheels on the heavier side) as the "center of drag" will trail the "center of momentum" (which is the CG).

-- Alan

 

I looked up the Michelin "Owner's Manual" to which I referred (dated 2005 January 01). It says:
"As a general rule, whenever only two tires are replaced, the new ones should be put on the rear."

alan_in_tempe — I like your clever experiment with the toy car! I hereby retract the last sentence of my post #44! What your experiment shows is that, when the front tires lose grip, it's the better grip of the rear tires that keep the car stable. (Or equivalently, that the end of the car with the poorer grip will move to the front.) What this is saying is that the tires with the better grip should be on the rear and not the front. That's what the book and the Michelin Owner's Manual are saying. Your experiment confirms this! The tires with the better grip will start to slide last, and so should be on the back wheels. If only the front wheels are braked (whether by regenerative braking or otherwise), the front tires might lose traction, but the (unbraked) rear tires will retain traction and so stabilize the car from fishtailing. If only the rear wheels are braked, they will probably lose grip first, and the car will fishtail. If all four wheels are braked, it's more difficult to say which tires will start slipping first (it will depend on the front/rear braking ratio and the tires' condition), but it will likely be the ones with the poorer grip. For stability, from the above arguments, the poorer tires should again be on the front wheels. You say much of this in your post #45.

Stan

 

Thank you!
A properly working braking system will apply far more braking to the front than the rear. Most of that is to accommodate the weight shift to the front wheels during braking, nearly 80% on hard braking, but there should be a bit more front braking than what is needed to accommodate that weight shift to ensure the rear wheels do not lock up first, even if there is a slight difference in condition of the tires. Rear wheel lock up is that dangerous!

-- Alan

 

Another way to look at it is that a locked up tire looses grip in all directions, and can no longer track; it can slide sideways as easy as it can slide forwards. Non-slipping rear tires keep the rear tracking, following the front tires.

If the non-slipping rear (or front) tires are leading, then the slipping front (or rear) tires are not so likely to follow the rear. Any side slipping of the trailing tires will yaw the car, steering the leading/tracking tires in the opposite direction which increases the yawing, which leads to a very rapid 180 spin until the tracking tires are then allowed to follow. Even if the rapid spin causes the previously tracking tires to begin to slip, there is a good chance they will regain traction as the yaw realigns those tires back to the direction of forward motion (although those tires will then be turning in the opposite direction as the car will have spun half way around).

It takes extremely good reflexes to control a car with a rear end lock up, requiring almost violent steering inputs.

-- Alan