Back end "breaks loose"

[bassjack]

Greetings,

In another thread on myths there is mention that the rear end will break loose in certain circumstances. I was reading Hemmings today and some one references it in article as "they all do".

I heard this over and over...can someone shed some light on how and why this happens. It hasn't happened to me yet, but I'd like to glean as much knowledge to know enough to avoid it, or know what to do when it happens.

My wrench tells me to take a DE course 'cuz that's where you want to learn how to deal with it, not on the street! I'm planning on that, but haven't done it yet. Been driving my 85 since last September, fwiw.

Thanks for any input.

Jack

[dentist90]

Any rear wheel drive car can break the rear end loose by lifting off the throttle in a corner (lift off oversteer)*. The lifting off of the accelerator causes the weight of the car to shift forward a bit, lightening the rear of the car and the engine compression causes the rear wheels to 'brake'. The result is that the rear tires begin to lose traction and slip.
The problem arises with our Porshes because the weight is in the rear and there is a lot of inertia there, so if the car gets sideways it is much more difficult to correct it. It's kinda like trying to throw a lawn dart backwards... it just wants to turn around. Cars with their motors in the front will tend to follow their nose more and it is easier to gather it up.
Oversteer under acceleration is different and much easier to control. If the rear wheels break loose while you've got it pegged you can gently let off and it will straighten out. The mantra is "slow in... fast out", ie get your braking done in a straight line and then power out of a corner, where the wieght transfer to the rear will only aid in traction and power-on oversteer is much easier to control.

* this can be done with a front driver as well, but it's easier to correct and a different topic... but the principles are the same: weight transfer, mass distribution and engine compression braking affect the handling.

[efhughes3]

There is no myth here. The 911 has about 60% of it's weight in the rear. When you are on the gas, you put even more weight there, so it is very well planted and offers a lot of grip. This is one of the reasons a 911 is legendary for its handling and has done so well in races. The problem is if you are in such a position, say in a turn, and you let off the gas. Your weight shifts to the front wheels, lessening the grip at the rear, and that mass in the back becomes a big pendulum and the rear lets go. Many 911's have been backed into ditches, trees, embankments, etc.

What usually happens is the driver gets in trouble, and the first instinct is to lift off the throttle, often resulting in the loss of control. The technical term is Trailing Throttle Oversteer.

This is why DE's ARE good training. You learn to use the gas pedal (and the brake pedal along with the steering wheel) to keep enough throttle on to keep the car stable in any condition.

[bassjack]

Thanks for the quick, enlightening replies! It would appear that this is one of those situations in which experience is the best instruction. However, both explanations are helping me to understand the physics of the situation.

Jack

[pgeorgeson]

Bassjack:
My recommendation would be to try autocross before DE. In my mind, there are two advantages to playing with the limits in autocross. First, the speeds are lower. A spin or a slide at 20-40 mph is a lot different than a spin or a slide at 60-80mph. Second, there is a substantially smaller possibility of damaging your car if you exceed the limits on an autocross course. About the worst that can happen with a spin on most autocrosses is some embarrassment and scuffs on the side of the car from a cone. At a DE, there is often the potential that exceeding the limits can cause real damage (concrete walls, tire barriers, and other cars lurking in the shadows). I spent a season autocrossing before I started DEs, and I think it was the way to go.

[Jack Olsen]

Originally Posted by bassjack

Thanks for the quick, enlightening replies! It would appear that this is one of those situations in which experience is the best instruction. However, both explanations are helping me to understand the physics of the situation.

Jack

Actually, the best instruction would come from an instructor.

All cars break loose, with enough speed and the right steering input.

[ted]

Join PCA and do some AX events, instruction is free and very helpful.
Pitch and catch is a 911 steering technique where you lift the throttle to get the rear end to rotate and then catch it by applying throttle.
Basically throttle steer which a 911 does very well.
Techniques like these turn a negative into a positive.
You can practice car control skills at an AX in 2nd and 3rd gear, much safer than learning in 4th or 5th.
When you get to the DE level you should already have car control skills, the DE is a bit more advanced with continuous laps in traffic.

[Scooter]

Before you get on the track for DE or even AX, you should participate in a PCA drivers skills day. I recommend driver skills to everyone, even if you never plan on getting on the track. You learn just as much about your car's skill level as your own. Look into it.

[spuggy]

Originally Posted by bassjack

I heard this over and over...can someone shed some light on how and why this happens. It hasn't happened to me yet, but I'd like to glean as much knowledge to know enough to avoid it, or know what to do when it happens.

Instruction is good. Wouldn't waste any money there, unless your name is Derek Bell or Hans Stuck etc.

Also take a look at Vic Elford's book "High Performance Driving Handbook", stocked by our host: http://www.pelicanparts.com/BMW/catalog/shopcart/BOOK/POR_BOOK_bkraci_pg9.htm - it's well worth it for the pictures, history and racing/rallying anecdotes alone, but the explanations of the physics of driving (very) quickly and the techniques and tips are excellent.

Huh. There's a 2nd edition now, I didn't know that.... Click click clickety-click...

[widgeon13]

Go to a Skip Barber one day driving course, get a better understanding of what the suspension is doing in weight transfer and until you understand what causes the ends of the car to swap directions, brake in a straight line. Smooth on the gas and brakes will make fast in the long run.

[efhughes3]

A day at Skip Barber isn't needed. One can get several track days in their own car where they can directly apply their skills to the nuances of their specific vehicle. PCA offers a great rout, and I've had some great instructors.

Skippy B obviously is a great school, but it ain't cheap, and there are other ways one can learn how to drive a 911.

[Rodsrsr]

This is probably a dumb question but, why did Porsche put the engine back there to begin with? Is there an advantage to the rear engine design? Is the car potentially faster with s skilled driver? Porsche doesn't do things by chance, it seems that everything about these cars is well planned and designed for maximum performance so the basic rear engine design had to be that way for a reason one would think. Anyone know why?

[RWebb]

just a couple of minor additions:

your 1985 is very unlikely to swap ends rel. to earlier 911s - the first cars were worts, followed by the other SWBs, and so on

John Cramer posted a near perfect summary of what the factory did when a week or two ago

I think the very best thing to do for experience would be to find an empty parking lot with ice next winter and mess around there - you can do so at very low speeds as there is so little friction on the surface

next best would be a wet (and empty) parking lot - sp. right after a long dry spell broken by a good rain. you'll then have a "nice" greasy mix of oil & water...

an instructor can explain things a lot faster than experience alone...

[widgeon13]

Originally Posted by efhughes3

A day at Skip Barber isn't needed. One can get several track days in their own car where they can directly apply their skills to the nuances of their specific vehicle. PCA offers a great rout, and I've had some great instructors.

Skippy B obviously is a great school, but it ain't cheap, and there are other ways one can learn how to drive a 911.

I think driving schools, particularly Skip Barber are beneficial. No it's not cheap but you also don't have to worry about the vehicles you drive and you learn quickly. I've had the opportunity to do both PCA DE and Skip B (multiple driving schools) and found value in both.

Since the Pcar is not the only vehicle I drive and you get to drive a number of cars at a driving school, the experience can be applied to safe driving habits on the public roads. I drive quite differently on the track and public roads.

I have also seen a number of folks swap end on the cars at various speed and under various conditions. It can happen easily on any 911. The problem is that if you don't recognize it early , and don't know what to do, you will be around before you know it. If it's on a public road, you will most likely do some damage to your car, yourself or someone else.

[Bill Verburg]

Originally Posted by Rodsrsr

This is probably a dumb question but, why did Porsche put the engine back there to begin with? Is there an advantage to the rear engine design? Is the car potentially faster with s skilled driver? Porsche doesn't do things by chance, it seems that everything about these cars is well planned and designed for maximum performance so the basic rear engine design had to be that way for a reason one would think. Anyone know why?

There are several advantages to having the VW/Porsche rear engine
1) packaging, leaves room for passengers etc up front
2) traction
3) braking
4) most improtantly, an oversteering car(within reason) is a faster car

as Jack says all cars can break loose, it just depends on the situation.

I like this def. An oversteering car will hit the wall tail first, an understeering car will hit the wall nose first

One of the great things about New England winters is seeing the proof of that

[safe]

Originally Posted by Rodsrsr

This is probably a dumb question but, why did Porsche put the engine back there to begin with? Is there an advantage to the rear engine design? Is the car potentially faster with s skilled driver? Porsche doesn't do things by chance, it seems that everything about these cars is well planned and designed for maximum performance so the basic rear engine design had to be that way for a reason one would think. Anyone know why?

My best guess would be "packaging" and that they had experience with rear engined designs.

The car is a very good compromise.

[dentist90]

Originally Posted by Bill Verburg

There are several advantages to having the VW/Porsche rear engine
1) packaging, leaves room for passengers etc up front
2) traction
3) braking
4) most improtantly, an oversteering car(within reason) is a faster car

There is oversteer and there is oversteer. The 911 factory road setup favours understeer while under throttle, as do almost all road cars. It is the trailing throttle oversteer that most often bites us, as when one enters a corner too fast and feels the urge to lift off or even touch the brakes while cornering. With experience one learns to discipline oneself to keep a steady, light throttle and hope the speed scrubs off before you leave the road.
The advantage of the rear weight bias is that it tends to resist throttle ON oversteer as you accelerate out of a corner... to a point.

[dentist90]

Originally Posted by safe

My best guess would be "packaging" and that they had experience with rear engined designs.

I think so too. They took a concept and ran with it. A mid-engined car would probably give the best dynamics and handling at the expense of passenger space. A powerful front engined car tends to oversteer more easily on throttle and suffers from the extra weight of the drivetrain and exhaust systems. The rear engine layout allows compact packaging of the drivetrain, weight on the drive wheels and good passenger space, but on ice or slippery surfaces would be just as content to go backwards.

[Dixie]

A lot of myth propagation is going on in this thread. Let's try to set the record straight.

• A 911 handles pretty much like all other cars. If you get on the gas, on will turn less. Off the gas it turns more. A 911 just does it better.
• Trailing-throttle-over steer can make a 911 spin. But then again, it can make any car spin.
• A stock 911 under steers. Quite a bit in fact.

And here's the biggie...

• When a 911 spins, it's analogous to spinning a hammer. The fact that the engine is at the rear can make it hard to stop the spin.

And one closing comment. A Z06 with stability control turned off is 10 times harder to control than an old 911. A Z06 can swap ends in a nano-second.

[longhornchris04]

Originally Posted by Bill Verburg

.

I like this def. An oversteering car will hit the wall tail first, an understeering car will hit the wall nose first

I've always been partial to these definitions:

Understeer - You see the wall you are about to hit through your winshield
Oversteer -You see the wall you are about to hit in you rear-view mirror!!!

On topic - the 911, especially the later ones, do handle better. The real catch is that when the rear lets go, especially in a lift-off or trailing-throttle oversteer its more like walking off a cliff compared to most cars. When she goes, you have an inverted pendulum which tries to correct itself. This is in contrast to most cars, with the weight biased forward who will slip out but the nose will naturally stay forward (for the most part).

Learn your car, go autocrossing or participate in a skills course. Finding an empty parking lot after a rain storm also works well.

Finally, keep your tires in good shape & proper pressure. You are more likely to have handling problems on crappy, underinflated (or overinflated, but that takes effort) tires.