Any racecar mechanics out there? I have an interesting question about dynamics.
My 1991 Miata: if I lift my foot off the throttle during a turn, I will induce understeer.
A 1983 Porsche 911: if I lift my foot off the throttle during a turn, I get oversteer.
Both cars are rear wheel drive with a 50/50 front/rear axle static weight distribution and a curb weight of 2700 lbs. Why the startlingly different response?
If one lifts the throttle, the rear end will lighten > frictional force of the rear tires is smaller> max slip angle of the rear tires becomes smaller > rear tires begin to slide, and > you have oversteer. Simple enough.
But if the rear of the car is lighter, the front of the car must now be supporting more weight > centripetal force overcomes the max slip angle of the front tires > the front tires begin to slide, and > now we have understeer.
The last two paragraphs here seem to contradict each other. I am missing something. Can anyone find my mistake?
This probably has nothing to do with it, but is it possible the (newer) Miata has speed sensitive steering and the Porshe doesn’t. Even if it does, I’m not sure that it would take effect at speed slow enough to turn as tight as I picture you turning in the OP.
These aren’t slow turns - it has to be fast and tight (like a 270-degree-turn off-ramp) in order for this stuff to take maximum effect. And there is no speed-sensitive steering on my Miata, just a steady assist ratio all the time.
Are you real sure that the Porsche’s weight distribution is 50/50? I’ve never heard such figures applied to a 911. The 997 for example is 38/62. That would go a long way to accounting for the oversteer characteristics.
Anyway, there are many things beyond simple weight distribution that figure into the understeer vs. oversteer situation. Rear toe, tire size stagger, varying track widths front to rear, camber curves, polar moment, differing spring rates, differing compression and rebound settings, front to rear roll bar stiffness variation, and front to rear tire pressure variations all immediately come to mind. That’s why tuning a car at the track can be such a bastard.
I leave everything be except tire pressures and adjust front to rear to try to bias ever so slightly toward oversteer. Then if I’m tracking wide in a turn, I can lift off throttle fractionally and tighten my line. And I mean fractionally. Like just raising my big toe slightly. Any more than that and things get hairy real fast. But that’s cause I’m driving a mid-engined, low polar moment of inertia car with a rear weight bias.
::: Rick checks to make sure he has his Nomex underoos on:::
Yup, OK.
When Porsche first came out with the 911 they ran a print ad that showed a smiling driver in a 911 and the headline read “You never see a Porsche driver looking bored” This was true, because if he was driving it near the edge of the envelope he was terrified that it was going to swap ends. :eek:
With the Porsche you have a huge hunk aluminum out behind the rear wheels that really does not want to be there. The Porsche engineers have done a huge amount of work to try and make a car that should oversteer like no tomorrow handle either neutrally or understeer slightly. Look at the rear tires on your Porsche, they are what twice as wide as the fronts? This was done to decrease the oversteering tendencies of the Porsche. If you think your car has handling issues, picture a 1967 911S with 4 skinny tires on it. :eek: :eek:
When you lift your foot on the Porsche you get a weight transfer to the front tires, you lose grip in the rear. That great big chunk of aluminum then decides to go visit the front of the car.
Don’t get me wrong, you can drive a 911 really fast if you know what you are doing. If you watch 911s race, you will see the really fast guys wag the tail (oversteer) just a bit on the exit of the corner. This is telling you that they were right on the limit, to the point the car starts to break away on exit of the corner.
Now your Miata is a different story. The big hunk of metal is already in the front. It does not want to swap ends. Also the tires are the same size front to rear, unlike the Porsche. With the throttle on you are either neutral or getting throttle assisted oversteer. When you lift the car goes back to understeering.
I’m sure that Stranger on a Train will be by shortly to explain polar moment of inertia and how it relates to what we are discussing.
In the 80’s I ran a Porsche hop-up & refurb business. As Rick said, 70’s & '80s 911s were notorious trailing throttle oversteerers.
That produced a sizable fraction of our business when somebody entered a curve too fast for their comfort & eased off the throttle – WHAM they were in the guard rail and then in my shop. Ka-ching.
I don’t follow the marque in detail any more, but I see from cornflakes that in 2007 they still haven’t repealed the basic physics inherent in the chassis layout.
My bad about the weight distribution! I was thinking about a Boxter that I recently drove at a dealership. I was “window shopping,” like, when you know you aren’t going to buy it but you pretend anyways that you will. What a ride.
Anyhow, yes, the weight distribution problem would go a long way towards addressing that little issue.
Pork Rind, the rest of your answer is really what I was looking for, thank you! That’s perfect. Now, in the future, I can figure this out on my own for other cars, knowing some more of the factors that go into this. Thanks all.
That’s a couple of great links!
On the second one the oversetter page oversteer, I don’t see this part
Did you lift off the throttle (yes)----> Did you hit the guardrail (yes)—> DOH! :smack:
What was the weight distribution on the 911SC? I had a one, and never felt a tendency to oversteer, even though I was rather aggressive in turns. (OTOH, I tended to step on the throttle in turns; not let off.)
IIRC the 911 got a longer wheelbase in 1969 or 1970 to help improve handling.
You were not at the limit. When you exceed the limit in a 911 the back end breaks away first. When you are close to the limit you get trailing throttle oversteer.
This has mostly been answered, but you have oversteer when Wf/Cf<Wr/Cr (and under when it’s reveresed), where Wf and Wr are the weights on the front and rear tires, and Cf and Cr are the cornering stiffnesses of the front and rear tires. That’s using a pretty simple model, but it illustrates the mechanism by which you switch from under to oversteer when you ease off the throttle.
This is, unfortunately, what happened with the Firestone/Explorer debacle. When the tread separated from the tire the Explorer changed from under to over steer. However, instead of having a nice stable Porsche, you had a top heavy relatively unstable SUV. When the tread would separate, the driver would turn the wheel to compensate much further than necessary because the SUV was now an over steer vehicle. A sharp turn at high speed in a top heavy vehicle=vehicle goes rolling down the highway.
What a ride.