Higher Speed Transitional Handling, 911
 

I've searched here and found very little about this question.

There are three places on our local tracks where I brake (generally in a straight line) to near 90mph and then enter a long sweeping turn. In each case I'm sure I could enter (and come out of) the turn faster if I weren't spooked by the abrupt shift of weight to the outside tires on entrance. I should say that I try to brake then let the car stabilize before turning. One additional turn is worse as the transition occurs after you leave a right hand sweeper and enter one to the left, meaning that the loads move from one side of the car to the other, not just from the center to the outside.

My 911's suspension has been tuned only a bit for the track. The car is a 74, with 22mm/30mm sway bars, a bigger stock front sway bar and a bigger rear Tarett sway bar. All bushings are stock and in good shape. Shocks are stock Bilsteins. Alignment is close to stock, except for 2° front negative camber and 2.5° at the back. I'm running BFG G Forces at the moment but the phenomenon I described happens no matter what tires I've tried. And alignment changes in the past haven't done much either.

Is the answer bigger torsion bars/coil overs to reduce lean and equalize tire loading or is there something more involved?

You could try lowering the car to reduce both the total lateral load transfer a bit as well as have a greater effect on the roll center height. Things that speed up weight transfer are #1 roll center height, #2 stiff anti-sway bars, #3 stiff torsion bars.

If you lower the car, the front roll center at least will get lower pretty quickly and so put the load into the anti-sway bar. In the rear, the roll center does not drop as quickly so you could go softer on the bar. If that does not help, back off on both bars. Don't raise the spindles unless the roll center goes below ground level.

This should help if the problem you feel has correctly been attributed to weight transfer occurring too quickly.

I think the real problem is a lack of rear downforce shifting the balance forward and contributing to oversteer.

I have a '74 as well with 21/28 t-bars and Tarett sway bars. My bushings are ER polybronze and monoballs. Stiffer bushings may help, but my suggestion is to be as smooth as possible w/your inputs. Deliberate, but smooth inputs help keep the car from becoming unsettled. You probably know this already, but it's something I tell myself every time I enter the track. . .

Have you had an instructor or pro drive the car with you as a passenger? I am always surprised and find big gains in my own driving when I ride passenger with a pro driving. I alway spay attention to his entry and exit speeds and try to get close myself.

Steady state cornering balance is managed by springs and sway bars. Steady state cornering is static balance and is not impacted significantly by shocks.

Transitional handling (weight shift) is managed by your shocks. Once your springs and sways are setup properly, I'd take a look at what you have for shocks. Bilsteins are just a manufacturer. Are they optimizied? By whom?

Unless you address the static and dynamic balance(in that order), the transitions will continue to be scary... Bushings, etc.. only help hold alignments and reduce friction while the suspension pivots.



During cornering, Shocks manage the speed at which weight is transferred around the car. Too slow and the car won't hook up soon enough, too fast and the tires exceed their grip circle.

For left to right (or vice-versa) transitions in corners, try and make a "mini straight" between the two to settle the suspension for a short time before changing directions.

it almost sounds like you could trail brake a little into the corner and start the turn with your car not pitching from nose down to nose up.

or finish your braking in a straight line and be back on the power before you turn. power on helps plant the rear tyres.

Just from your question, and the analysis you have done so far, you are exceeding your car's capability. Your CAR probably needs a bunch of things. But remember, we are working with an older 911, whose inherent chassis tendancy is to change direction ABRUPTLY. Part of the solution will be to determine what is simply unsettling to the driver, and what is actually unsettling to the car itself.


Here you are identifying chassis ROLL. Most of the preceeding posts, including yours, have good advice on this: higher-rate springs (torsion bars), stiffer (and slower reacting) dampers, lower ride height, lower weight placement in chassis, and wider track width work to limit how much weight is transferred to your outside tires. The anti-roll bars serve to regulate the RATE at which that weight is shifted.

As usual, Flieger has gone straight to the heart of the matter, with the simplest and least expensive solution with which to start.


Here you are identifying TRANSITIONAL chassis ROLL. As Chris so aptly points out, dampers play the primary role (not ROLL) here, combined with very smooth steering input, as others have said. In the absence of adjustable shocks, very STIFF dampers, ones that react slowly, will help here. At a minimum, the valving in your Bilsteins needs to be in top condition, and matched to your torsion bars.

As you probably already know, tires play a huge part in the confidence factor. Ultimately, who cares if the car leans, as long as the tires stick? Great tires can take away your "abrupt" factor.

And even with all this, in order to go quickly with an old 911, one needs a certain amount of throw-it-and-catch-it driving style. Initially, it takes a leap of faith, but that, in itself, is part of the thrill

Ed LoPresti

With an overdamped car, would not the weight transfer be more abrupt? I would think softer dampers would smooth out the load transfer to the tires, though the driver would feel a little sick and like a boat captain.

Not necessarily. For roll, increased bump reisitance acts to limit transfer to the outside. Conversely, increased rebound resistance acts to "keep" more weight on the inside. If nothing else, the limiting of the spring compression/extension, slows the process.

Ed

Wait a minute. If you take limiting the travel to the limit, then a car with no suspension would take an infinite amount of time to transfer weight. But the opposite is the case, the weight transfer speeds up as the suspension gets stiffer.

You want soft damping on rebound and hard on compression, but this keeps more weight on the tires at the expense of jacking up of the car as the left-right-left transition is made (not good).

The bottom line is that the more the body is allowed to roll and the smoother it comes to a static roll angle, the smoother the load transfers to the outside wheels.

Flieger,

I am with you. Jacking forces are pretty much immediate. The stiffer the suspension, the more quickly the forces reach the tires. Adding more compression damping will reduce the time it takes the forces to reach the tires.

Anything you do that reduces the roll has to reduce the time it takes the forces to reach the tires.

Scott

While what you say Flieger is true, the reality of it is somewhat different in the middle ground. A soft car is easy to drive but when we are talking about a car as soft as a minimally modified 911 that is under damped, it has enough suspension displacement to gain momentum as it rolls and then when it finally comes to its steady sate point and you have already asked it to turn the other way, by being under damped, it violently springs back the other way unsettling the rear. I would also imagine with dead shocks that the OP has enough body roll to get some significant toe and camber change in the rear which may also be a factor.

IMO you want very little compression valving on your shocks as in an ideal world this would be adjusted with spring rate, yet some substantial rebound valving to slow the weight transfer. How much of each on an nonadjustable shock is something you will have to research your self or trust one of the experts to get it close to right.

To the OP, I would recommend getting your shocks re-valved for your current suspension and weight as from my personal experience new off the shelf Bilstein Sports are marginal at best with 30mm rear T-bars and get worse with age.

I am unsure if you are paraphrasing my post here, but that is not what I wrote. By increasing the resistance to BOTH bump and rebound with dampers, one is limiting ROLL. In addition to limiting total weight transfer, it has to do with the magic of shocks - progression.

On a turn, increased resistance to bump effectively adds spring rate to the outside springs, progressively, limiting the tendancy of the chassis to tip outwards. Similarly, increased resistance to rebound effectively adds rate to the inside springs, progressively, "holding" the chassis from tipping outwards. Less "tip" = less weight transfer, and visa versa.

We all agree with the very rudimentary rule-of-thumb: a stiffer suspension transfers its weight more quickly. Adding damping certainly makes the suspension more stiff. So, Winders, when a turn is initiated, the outside (and inside) tires "see" the START of weight transfer more immediately. But what comes after that can be different, depending on anti-roll bar settings, and, yes - damper geometry and valving.

Ed

Evan,

You are swimming against the tide of popularity here! For (how many?) years now, I have contended that mono-tube, gas shocks, if they are kept fresh, are fine for the street. For anything except the most casual track applications, they are sadly inadequate. Yet, thousands of these things continue to live on Porsche and Corvette street-made-into-racecars.

If you make any headway with this argument, I certainly applaud you! Maybe it's an idea whose time has come?

[John: Sorry for the hijack. ]

Ed

The weight transfer due to the CG being above ground and shifting to the outside should be significantly less than the weight transfer due to lateral force on the ground plane acting on a mass above it and requiring the weight transfer to react the moment. Therefore, limiting roll will reduce the total lateral load transfer. Dampers cannot limit the static body roll, being fluid filled cylinders+ pistons.

I was not saying that stiff compressive and soft rebound damping was good. It causes the body to work its way up on the suspension with each turn input given when the car has not reached steady state. The higher CG means more weight transfer.

OK, so you are saying that the driving frequency is approaching the natural frequency of the system, so more damping will quell the body roll response at that frequency.

However, limiting the body roll speeds weight transfer.

I think what you are getting at is that the bumps stops come into play if the body roll amplitude is allowed to get too large, so by slowing it before the bump stops it limits the amplitude in a way that does not spike the load on the tires. It speeds up weight transfer on inputs that would not result in bottoming out but that trade off is acceptable if the track has a turn or two where the suspension would bottom out.

Where's James?

Yes, something like that. We will see how my theory works as I have now changed almost every single part on my car and semi tube framed it.

He's here, Max, and trying to absorb all this. I must say this has turned into a very useful thread, at least for me!
My take so far: a lower car would help (can't go any lower and still do the driveway.)
Wider tread would be good (not with these no flare fenders.)
Relocating the driver, motor, fuel tank or anything else lower to get the cofG down would be good.
Soft hands and trail braking are helpful (I'm not bad here though one can always be better.)
Stiffer torsion and/or sway bars are always good (This thing gets driven on the street, not sure how much further I want to go here.)
I need to stiffen up the shocks, though exactly how seems to be the subject of much debate.
I was particularly taken by Ed's statement that "... higher-rate springs (torsion bars), stiffer (and slower reacting) dampers, lower ride height, lower weight placement in chassis, and wider track width work to limit how much weight is transferred to your outside tires. The anti-roll bars serve to regulate the RATE at which that weight is shifted."
I don't understand (and there seem to be differences of opinion) what the relationship is between controlling the rate at which the weight is shifted (with sway bar size) and controlling that same weight shift with the shocks. I first read Ed's statement as suggesting bigger anti-sway bars would be the most important change I could make, but I think the consensus is that it is shocks. Comment?