| Author |
Message |
Greg Reinacker (Gregr)
| | Posted on Friday, November 02, 2001 - 9:22 am: | |
Just to add a couple of things to what Charles said...I also race a rear-engined car regularly.
There are generally 2 reasons you might get into an oversteer situation. The first is a power oversteer, where you are applying too much throttle during the turn, and your rear tires do not have enough grip to turn at the rate you're trying to turn, and accelerate at the rate you're asking for. So, the rear end slides out. This one's easy and instinctive to correct - you back off of the throttle.
The more difficult to control for the average person is the case that Charles describes, where you take too much weight off of the rear tires, and they lose their grip. This typically happens when you brake or lift while turning - weight shifts to the front, and the rear end comes around.
When this happens, though, your instinct says "oh my god, I'd better back off" and you lift. Wrong thing to do - this will seal your fate. Remember, there's not enough weight on the rear tires - that's why it's sliding in the first place. What you need to do is get more weight back there; and the way to do that is to get on the throttle. How much depends on the situation, and it takes lots of practice to "feel" this; you need to be careful, because too much throttle at this point will transform your slide into a power-oversteer slide, and you're no better off than you were a second ago. And of course, as Charles says, you're correcting the steering all this time as well to keep yourself on the road while you're getting out of the slide.
Just for grins, you can feel some of this by simulating a skidpad in an abandoned parking lot or somewhere (and when the cop asks, no, I didn't suggest this ;-). Start turning in about a 100 ft circle or so, and keep accelerating until you start to feel the car understeering a bit; i.e., your turn radius starts to increase just a touch while you keep the steering wheel in the same position. Now lift off the throttle. In a typical front-engined car, typically your turn radius will abrubtly decrease, and the car will drive more towards the center of the circle. In a mid/rear engined car, your tail will swing out, and you'll need to correct with the steering. Your speed will be slow enough that you probably can correct this situation just with steering - but it's good to slide around in your car under controlled conditions to get the overall feel.
Greg |
Charles Gault (Knox_Charles)
| | Posted on Thursday, November 01, 2001 - 1:55 pm: | |
I just reread my post. When I said "release the steering", I didn't mean to release the steering wheel! I ment that you should turn the wheel back a little in the direction of the skid or turn a little less into the corner.
In racing, you are always trying to "release" or straighten the wheel as soon as possible after the apex because the car will accelerate faster with the wheels straight. |
chris cummings (Entelechy)
| | Posted on Thursday, November 01, 2001 - 12:37 pm: | |
I've been studying a number of books & videos on high performance driving over the last few months. Charles' post is dead-on from what I've read and practiced. Regarding steering, there are a couple different methods out there, but I prefer Bondurant's technique. He advises to always keep both hands on the wheel as that is how the car & road surface communicate through the drivers hands. You can feel tiny minute changes that you'd miss if you took your hands off the wheel. Also, you have an incredible amount of control and balance staying at the 9 and 3 positions. I like to hook my thumbs over the notches at these positions for added strength when really pushing it through the corners. What's brilliant about this technique is you always know where "straight ahead" is wherever you are in the corner. Finally, if a corner is in excess of 90 degrees and you need added turn input beyond crossing your arms, you just slide one hand up to 12 before going into the corner, steering through in that position, then after the corner, slide back to 9 or 3 (depending on the direction of the turn). I've only encountered 2 or 3 turns in months of driving where you needed additional steering input beyond this. Hope that helps.
~Chris |
Charles Gault (Knox_Charles)
| | Posted on Thursday, November 01, 2001 - 8:57 am: | |
Here are my thoughts on the mid/rear engine oversteer issue. Unfortunately, it is hard to put some of this into words because it is somewhat a matter of feel or instinct.
First let me say that I have driven mid/rear engined cars for over 30 years and have raced several of them, including formula ford and Porsche 911 and 914-6. I also am an instructor at PCA driver ed events.
A car is connected to the road by its tires at four points called the contact patches. This is a relatively small area of contact to control a 3,000 pound mass changing direction at high speed. The overall goal of the driver is to keep the chassis balanced on these patches, and to not break the adhesion between the tires and the road surface. To do this, all of the inputs into the car's controls must be done as smoothly as possible. Accelerate hard, but don't spin the tires; break as hard as you can without skidding; turn in decisively, but don't jerk the wheel.
Also the driver must take into account the effect of the suspension on the balance of the vehicle. When you accelerate hard, the weight shifts to the rear wheels, unloading the front wheels. This is good for extra grip for acceleration, but bad if you need grip on the front for turning into a corner. When you brake hard, the weight shifts to the front, and if you need to turn the wheels for a corner, it might overload the front tires and cause a loss of adhesion. The driver must continually compromise between his inputs to the controls to keep the car balanced and keep all four contact patches in adhesion with the road.
Considering the effect of having the engine weight in the rear, look at an arrow. The arrow head mass leads the way and makes the flight stable. Try putting a weight near the rear of an arrow and see how straight it goes. The reason a car can be made to go straight where an arrow would not is because a car grips the road with its tires and an arrow is in a relatively frictionless environment.
But I digress, when you go into a corner with a mid/rear engined car, you need to get the vehicle mass to change direction. The mass wants to continue in its direction of travel, and it will take an application of force to change it. You break, shifting the weight to the front, and turn the wheel. This puts more weight on the front tires which is good for their adhesion, but, if you do it too abruptly and cause the rear to be too light, the rear contact patches will start to slide. When the rear has lost adhesion, it will want to continue the direction that it was going and will not follow the front in the direction of the turn. It will also want to rotate around the front because the front tires are still griping. This causes a spin.
A lot more could be said, but it would take a book! When you drive, drive as if you were dancing. Be graceful. If you lift abruptly going into a corner, you will unload the rear and it will step out. Done too extremely, you will spin. To correct, you need to keep steady pressure on the gas and release the steering a little. You need to rebalance the car. If you over correct, it will go the other way. This is called a "tank slapper". To get out of that, keep steady gas and don't turn the wheel as far as it feels like you should. Let the car settle down without violent control inputs. I hope this helps and is not too confusing. This is really best done on the road live. |
Octavio Mestre (Alfab4308)
| | Posted on Wednesday, October 31, 2001 - 12:29 pm: | |
I also have heard that coming off the gas in a turn is a bad idea. |
magoo (Magoo)
| | Posted on Tuesday, October 30, 2001 - 10:22 pm: | |
Hey, That sounds like something Ralph Nader said. Just a joke Fred. |
Frederick Thomas (Fred)
| | Posted on Tuesday, October 30, 2001 - 10:02 pm: | |
The problem with letting out of the gas when your back end starts to come around is that when you get off the gas the weight in the rear continues to carry speed which can cause the back to want to swap ends with the front. I do know that any sudden changes in the throttle while turning hard in a rear engine car is a big no no! I would like to say that in no way do I claim to be a great driver so if I am wrong on this please jump in and point it out. |
magoo (Magoo)
| | Posted on Tuesday, October 30, 2001 - 9:56 pm: | |
Just as Fred says in his profile , "Man that first curve comes up mighty fast at 170 mph." |
magoo (Magoo)
| | Posted on Tuesday, October 30, 2001 - 9:52 pm: | |
Arnaldo, Many times when you are taken by surprise you can't concentrate on 3 or 4 different things all at the same time. You may be trying to control the steering wheel while at the same time be pushing on the accell. and not know it. Until all your senses are focused as they should be you can be doing something very wrong and working against yourself. Who knows what happened, just be aware that anything can be around the next curve. Glad you are OK. |
fred hundt (Fhundt)
| | Posted on Tuesday, October 30, 2001 - 9:37 pm: | |
My driving instructor has warned me that coming off the gas completely on my 355 can cause oversteer that can spin the car in a turn...I believe it has to do with the quick change of torque from the rear-mounted engine. We've experimented with it a little, though not to the point of losing control. |
Arnaldo Torres (Caribe)
| | Posted on Tuesday, October 30, 2001 - 8:26 pm: | |
I have ridden the road many times in both my bike and the TR, and I have been able to perform controlled slides of the rear in several ocassions while on these roads. The TR has always been very controllable at or near the limit. Nevertheless, somehow I guess I applied to much gas, and the TR had no problem delivering, which is what got me into this situation. I just wish I would have been able to gain control quicker. Thank God nothing happened, but it made me realize that there is still a lot to learn and I would appreciate any input from experience drivers, in particular TR drivers/racers, on this regard. |
Tim N (Timn88)
| | Posted on Tuesday, October 30, 2001 - 5:21 pm: | |
I really couldnt help you except tell you a rule i use to avoid getting into these situations. Never go more than twice the advised speet (the yellow signs) on an unfamiliar road if you live in the northeast. In the west, at least where i drove, the advised speeds are pretty much as fast as you want to go on an unfamiliar road. I also thought that using the brakes would name the whole situation worse. I think you are supposed to just countersteer, but dont over correct or you will spin the other way. I dont know how well this will work in a mid engined car though. Also, letting go of the wheel works well in my friend's camaro as does countersteering. I dont know how easy the back of a ferrari would be to control, but in the camaro, you can punch it around a turn, countersteer, and slide the back the whole way through the turn. The car also has alot of suspension work though. |
BretM (Bretm)
| | Posted on Tuesday, October 30, 2001 - 3:40 pm: | |
I've only gotten the 308 into this type of situation when I was playing around in the rain without a lot of room to work and it was a deal to get it back like Arnaldo said. With power steering it's so easy to get a car back under you, without though I've done things like Arnaldo.
PS This post means I'm really interested in the magic way to get things straight again like Arnaldo. Someone please help us. |
Arnaldo Torres (Caribe)
| | Posted on Tuesday, October 30, 2001 - 2:28 pm: | |
I posted this in another conversation earlier, but I thought it deserves its own thread.
Going back to the mid-engine car behavior, I was on one of my runs on the Santa Monica mountains here in L.A., and I came to an almost 180 degree turn, increasing elevation and barely any camber. I braked hard before going into the turn and shifted into second for a fast takeof. I guess I was trying too hard because the TRs rear end came loose like never before, and it took me a few back and forth steering corrections to get it in line again. I did let off the gas and that seemed to help. I did also slightly apply the brakes but I did not feel that I got positive feedback out of that. Having no power steering, I had to steer into the slide and then let go of the steering wheel (wheels automatically return to dead-center) so that I can switch fast enough to start steering into the opposite direction as the rear was coming around the other way. It took me about 4 to 5 such sequences to get her under control.
The questions I have for the real pros out there are:
a. Having to let go of the steering wheel seemed natural to be able to steer into the slide fast enough, but was it the best thing to do?
b. I did not think keeping the gas on was the right thing to do since it was what started the whole process in the first place, would it
have been better to reduce gas input without letting go completely?
c. When, if ever, should brakes be used in an oversteer case like this one?
d. Ultimately, what is the driving school-approved approach to getting a case like this under control a lot quicker?
Thanks for any input,
Caribe. |
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