James Bailey

At the Cal speedway the Porsche Driving Experience was allowing test drives in a Boxster and a Cayman S. So naturally I signed up to drive the little "slalom" course. They ONLY gave you one lap in each car! The instructor in the Boxster commented that I was a little harsh getting on the gas and could easily lift the front end and cause understeer....... The guy in the S simply said "Excellent" and I was pushing it even harder

So that got me thinking.....how much actual lift upward force does the front axle see ?? The typical wheelie comes mostly from the center of gravity being higher than the tire contact patch but that is not what I am questioning.

How much upward force is there given the limited traction of the tire ? Maybe assume 300 HP 270 ft lbs, 96 inch wheelbase. Then you have to make assumptions about gears.

Tony

que kibort in....

hacker-pschorr

I remember pulling the outside front wheel off the ground while autocrossing my 944S years ago. It was because my rear shocks where blown.

blown 87

I got to drive the Roller Skate a couple of days ago, and if he ever goes to a prepped track it is going to have issues with pulling the front end, but then again it is a Geo Tracker with a built 454 in it.

James Bailey

My old 911 racecar would lift the inside front tire AND CARRY IT OFF THE GROUND FOR FAR TOO LONG. But what I am trying to figure out is does the amount of energy being delivered to the rear wheel really amount to much lift in the front, BOTH front tires. The lifting of the inside front tire is more a function of too little rear spring and too much front roll bar. And three wheel vehicles do not corner as well as 4 wheeled.

Speedtoys

I dont think you were LIFTING..just causing excess weight transfer to the rear of the car..which causes, as stated...push/understeer.


Precision performance driving is ballroom dancing, few things, if any..happen quickly or harshly. Braking, acceling, turning the wheel..etc..etc...all at the right time, the right place..ballroom dancing.


Just guessing..thats what instructor #1 was sayin.

James Bailey

When I am cornering at/ near the limit in the old very brown 1980 more throttle causes throttle oversteer the front end does not wash out....the rear just rotates.. So I really wonder about this front end lift thing. Some of the instructors in POC talk about it in the clinics. I have a hard time seeing it. Maybe if I had MORE horsepower. I remember one rennlister who had a supercharged 928 screwed up a corner and jumped on the gas because the rear was BREAKING LOOSE....he ended up on his roof. He THOUGHT that more throttle would tranfer weight to the rear wheels and have more traction, he was wrong.

Speedtoys

Well, adding power -will- transfer weight, and will increase push TENDENCIES..but your rear is stepping out before the front pushes out.

Doesnt mean that the action/reaction isnt happening, just the front still stuck better.

Where & how you're doing that, is why that is happening to you right then & there.


*heh*..and..within certain limits..adding more power if you come out..is the right thing to do, but its pretty situational. Sometimes you dont want it to snap back tight on you..sometimes..youre just along for the ride.

RKD in OKC

Throttle push from the front end lifting depends on the balance of the car and the rebound of the shocks and the speed of the car (gearing and torque). I set my suspension ie., springs and sway bars so the car is balanced at apex with speed maintaining throttle. That means gradually increasing speed around a skid pad until the car breaks loose and both the front and rear break loose at the same time. Then I adjust the front damper rebound to control how much throttle I can apply before the rear breaks loose. Too firm and the rear breaks loose to easily, too soft and the front end lifts and the car understeers. This is so slight throttle adjustment can be used to steer the car in a 4 wheel drift through the apex and out of the corner. Lift a little off the throttle, the nose tucks or lowers the front gets more traction, the rear lifts a little and the rear gets less traction and the car rotates into a smaller radius turn. Add a little throttle and the front lifts gets less traction that the rears, the rears get more traction and the car pushes out into a larger radius turn. The highest speed around the corner is with all four tires slipping at an angle of from 7 to 15 degrees depending on whom you talk to. The angle of the front tires is controlled by steering input and the angle of the rear tires is controlled by the angle of the car. If the fronts or rears are not sliding just a little, you could be going faster around the corner.

Most balance of most cars is also speed/corner radius dependent. A well balanced car on high speed sweeping turns will push or understeer on slower tight turns. And a car set up balanced on slower tight turns will oversteer on high speed sweeping turns. There is a range a set balance works best in.

However, given enough torque you can throttle past the push and the rear will come loose.

When I had my BoxsterS the rear would try to come around if you weren't fairly aggressive on the throttle going around the corner.

Charley B

Jim, I'm guessing your front inside lift was more noticable on right handers......................................

bigmac

Carl, The road is called I-94. I once had my 86TQ Audi with upgraded Turbo up to 120mph, State Police Radar'd me, But I had an excuse, as I was on call to read Er-Xrays in the next town. He just told me "take it easy Doc". Nice guy. Find a local Doc who is a car nut, upgrade his 928 and go for it. Mac

anonymousagain

#1 gave you a psychologist answer --> generalization of behavior to force a standardized answer. Didn't apply to you.

Had he experienced the cars behavior under more throttle, he would have seen that the rear end steps out, negating his comment that you would encounter understeer...which would have indicated to him that you were already at limit/threshold, with adequate front grip, so you must be more intimate with the cars behavior than he anticipated, supporting your appropriate application of throttle quicker than he initially calculated.

He couldn't have gathered all that input from one lap...unless he had more experience under his belt and better reference points from other like-car behavior, which arrives at the response from #2

smiffypr

Another factor is roll centre heights.
Take a shoe box, and make a hole on the centreline of one end near the top, and at the other end near the bottom. Thread a rod through the two holes and hold the rod so that the shoe box is level.
Now tip the shoe box. You will notice that one bottom corner will lift much more than the other one at the same side.
This is what happens with an old 911. Front strut and rear swing arm suspension mean that the front and rear roll centres are at very different heights, and pure roll will cause the inside front wheel to lift. (Obviously spring and anti-roll bar stiffness also contribute).
Smiffy (ex 911 owner)

James Bailey

The 911 tendency to carry the inner front wheel was caused by a heavy front antiroll bar(sway bar) and too soft a rear spring. The rear torsion bar is limited in diameter (strength) due to how it attaches to the inner mount and the spring plates. Racing 911s went to coil overs to correct some of that. My very light weight 911 would carry a tire way too easily but with 70% of its weight on the rear tires it only needed three wheels. And it would nearly lift the front end off the ground in a first gear drag race launch at about 7,000 RPM, 12 inch slicks and a spool in the diff. Now it did push badly in slow corners due to the locked rear end with 24 inches of rubber trying to go straight So a quick downshift (anytime it got down to 6,000 it was time to grab a lower gear) turn the front wheel a bit and power oversteer until it got pointed the right direction.

tv

I have nothing to add to this thread except a-


screen shot of a Blue Bloods episode a month ago, where the bad guy was coming out of a garage and saw the cop cars coming to get him. IT WAS the act of acceleration that lifted the inside wheel up and kept it up for a while. wheel was on the pavement even given the slightly odd angle entering the street. The sudden acceleration lifted it and kept it up for a while, THAT is what made me do the screen capture a month ago.