aggravation

Just wondering if I'm understanding the physics correctly.

Is it a proper understanding that:
The rear engine design has weight hanging off behind (or so biased to favor if not "behind") the rear axle which causes the rear tires to lose grip because of a weight transfer to the front if you lift off the throttle. And if done while in a turn rear traction is lost so the rear end swings around because of the turn...

If I have that correct.... and you find yourself in a turn with a dire need to lift could you apply hand brake to reduce the weight transfer? Or would that not defeat any/enough weight transfer?

porsche0nut

Correct about part one... But pulling the handbrake locks up the rear wheels so while you may send weight back to them, you're going to continue spinning as they have no traction to push you out of the spin while locked up. Counter intuitive, but you have to apply throttle to give the rear the traction it wants...and keep looking where you want to go!

We practiced throttle steering at the skippad with PCA. No better way to under it than to do it!

Fast forward to 1:40... Where we start using the throttle to change the pitch of the car without altering the steering angle.

aggravation

thanks! I was thinking in terms of adding a braking force to rears only without so much that it locks the wheels might counter the effect.
Off to watch the video now...

Drisump

Sorry I'm not an instructor but I have done a number of enthusiastic miles in many flavours of 911's. With that disclaimer stated (LOL), typically when in the middle of a turn that you're sure is too fast, the car is understeering. When this is felt, a "controlled" lift will induce a certain level of oversteer which would be followed up by application of a controlled level of throttle (stopping the tendency for the car to radically oversteer). Obviously this all happens very quickly but on these cars, throttle control and proper gear selection before turning in seems to be even more important than with other layouts. BTW in many regions of the PCA they have an inexpensive day offered called "Skills". Amongst other things, they take you out on a skid pad where "throttle steering" is taught. When I took this course a number of years ago, a tanker dumped a load of water on a large patch of tarmac. Then you were requested to drive in circles at the limits of adhesion with as little steering input as possible....it was pretty cool doing fairly tight 360 degree turns on a very slick surface using very little steering but carefully manipulating the throttle. Cheers

Steve Weiner-Rennsport Systems

Great discussion here,....

If I may add,......

One of the main reasons why a 911 will spin if the throttle is released has more to do with its rear suspension design, rather than engine position.

Semi-trailing arm suspension systems change toe position with ride height. Under power, the car squats and the rear wheels are toed in; a stable condition. Release the throttle in any degree and the rear toe changes from that (stable) toe-in, to toe out; an unstable condition in which the outer rear wheel helps steer the car to the outside of a turn.

This phenomenon can be used for affect (as you gentlemen have written), to position the car and every 911 race driver masters this skill to take full advantage of the car's capabilities.

Just remember the dynamics of the rear suspension so you understand what the car will do when you back off the throttle in ANY degree.

TerrySmith

I had thought that lifting in an at-the-limit turn would cause engine braking on the back wheels that would take away grip (vector) needed to hold the turn.

Ed Hughes

No, it is weight transfer. A drastic change in balance will unsettle the car.

As to the original question, it makes no difference where the brake bias is, initial braking will always put weight to the front. That is until the weight is on the front enough, if you used your idea to only brake at the rear, to lose traction at the rear, causing the wheels to lock, and loss of traction-probably resulting in a spin. BAD idea.

oldskewel

Excellent point on the rear toe change there Steve.

From a physics standpoint, the "weight transfer" from rear to front is mostly a matter of the car decelerating. Whether that deceleration comes from 4 brakes, E-brake, or throttle lift ... all those create forces that act on the car as a whole. If it decelerates, it is equivalent to a d'Alembert force acting forwards through the center of mass. To maintain torque balance, the normal force on the front wheels increases and that on the rear decrease. So it is as if the weight shifts to the front, even though of course the mass distribution in the car is basically constant.

Skidpad tests like in the video above are very good training to understand this and get a feel for it in at-the-limit handling. Autocrossing is a good safe way to perfect it.

It's always the fine points that are the trickiest. Steve's point is an excellent one. Another factor I'm sure you'll find is the dynamic element - e.g., making slow vs. quick changes will give very different results. When you're at the limit, the best drivers know exactly when they need to stay steady and confident and ride it out smoothly, vs. something worse.

Jonathon Rolstin

Awesome discussion. +1 Steve

I love the dynamics of the 911... Coming into a corner too hot... Lift to rotate, punch when your on target... There is no better feeling than doing that! I have only done it a couple of times since I have lots to learn. But even after once or twice you just get an awesome feeling.

In my opinion this makes he 911 more desirable (challenging I guess?) to drive hard at the limit. You really need to practice throttle steering with it before you hit a track in my opinion. If you don't have the right reflexes you can get yourself into trouble

Drisump

Good video nutter! A picture is worth a thousand words, a video...who knows?

Drisump

Like I said in an earlier post, I'm not an instructor but I'd just like to add that real "engine braking" happens when your foot comes right off the gas. There is more finesse than that, going from gas "on" to abruptly less gas (as opposed to right off) will induce the rear to come around. "Fine tuning" the neutrality of the car is maybe a better description. A 911 heading into a corner "hot" will be radically understeering,a gentle lift will improve the placement of the rear wheels (vs the front) but has to be "caught" at the appropriate time with application of some power. A driver may be gas on and gas off a few times in one corner to "fine tune" where the car is pointing. As Ed says, the weight bias shifts forward allowing the front tires to bite and the rears to slip toward the outside of the corner. At the proper time, application of some power will shift the weight to the rear and it will squat and bite. If done correctly, the car will be far closer to perfectly neutral for the last portion of the turn, allowing for early acceleration out of the corner. But hey...driving a 911 in an enthusiastic manner is like skiing, despite similar training everyone finds little personal touches that work for them. Cheers

TerrySmith

OK, and so why is the weight-transfer-induced loss of rear traction worse with a rear-engine car than a front engine car in this situation? I'm thinking that when the weight transfers to the front, the car is pitching (nose down, tail up) about the front wheel axles and having most of the mass at the back at a larger radius from the pitch axles causes more angular momentum and more pitch angle / weigh transfer. Does that sound like a good guess?

Charles Freeborn

If I'm not mistaken it's called "trailing throttle oversteer". I use it all the time to rotate my car around tight corners, as I don't have enough horsepower to do it with throttle. Everything Steve says is spot on. The other factor is with the lift and subsequent weight shift forward it gives the front tires more purchase so the car won't push. Essential part of driving a P'car at speed through the turns. It's a delicate dance, but once you've got it mastered it's a lovely feeling.
-C

oldskewel

BTW - I edited out my original post on the extra non-driving forces - that was not correct and not really relevant here.

Your guess does make sense. Causing more pitch cause more rear toe out. But I'm not sure if the front-biased car would pitch less than a rear-biased car. Depends on spring rates, height of the center of mass (so the low boxer engine design helps here), etc. The moment of inertia about that axis (greater for the 911 with the engine out back) would slow the pitch rate.

But the more basic issue is simpler - if you decelerate and have less weight (= less grip) on your rear wheels, but that mass (not weight) is still in the back and you're trying to use your lateral grip to make the car turn, it's unbalanced. You still need the lateral force back there to push the mass sideways, but the grip has moved to the front. (oversimplifying all this). Suspension, wider rear tires, etc. can help all this.

In a car with a balanced or front-biased mass distribution (e.g., front engine), if you decelerate and move the weight (and grip) to the front wheels, where the mass is, you still may not be able to make the turn, but you won't spin out when you lift or brake.

oldskewel

There are some basics involved here, but it really does get very complicated when the finer points start to matter. What happens at 6 tenths is different from 9 tenths or 10 tenths of the limit.

Here's an interesting viewpoint on the 1967 911S handling from Jan 1967 Car and Driver back in the day:

"Oversteer is back—and Porsche's got it! Early Porsches had it too, and now it has come full circle. Barely three years ago, Porsche employed a device called a "camber compensator" to curb the oversteering tendencies of the 356 series. Then the completely redesigned suspension of the 911 and 912 models made Porsches behave like normal, front-engined cars, and the purists started to carp. Porsche had even hidden an iron weight behind the 912's front bumper to keep the back end from coming around. Sure, understeer is safe—great for the masses—but oversteer makes driving fun . . . if you're expert enough to handle it. Fanciers of the marque yearned for the good old days when they used to wischen their Speedsters through turns, tails all hung out, arms sawing away like mad on the steering wheel.

"Porsche is making a car for these drivers again, offering a sportier version of the 6-cylinder 911 dubbed the 911S."

http://www.caranddriver.com/reviews/...ad-test-review

Reading that article makes me want to go and buy a '67. "The 911S surely must be the all-time high. Where can Porsche go from here? "