2BWise

I am not hugely familiar with the 928 so assuming it had a strut at the front was my mistake, but that doesn't change the understanding of front geometry. It simply changes your approach to it. It is a production car at it's roots, so there will be a general trend of the front suspension to produce understeer at high lateral.

As for the damper travel. The bump stop is simply a spring in series. It is not in use until it is compressed, but once compressed it then calculates into the overall roll and wheel rates of the car. So if you're riding the bump stops it will dominate in the equation relative to the spring and potentially the tire (depends on material and stiffness). Just because the only time you know you're in the bump stops is Laguna doesn't preclude you being in the bumps stops at every corner on every track. You need to understand your damper travel to understand where you are. The bump stop is not solely an element that acts during vertical accel. it is displacement based not acceleration based, so if and when you use up all the free travel the bump stop is active and increasing the wheel rate.

SeanR

Mark, do you know if your bump stops are in good shape or are they 20 years old. Most that get removed are nice and crumbly.

mark kibort

they are pretty old... i think you might be right... about 17 years old, but are not old and crumply... just old and wrinkled.

mark kibort

thats some interesting information. thanks .... would the speeds traveled have much to do with the movements? (150mph, the small bumps can cause more motion than at 50mph, right? not that it matters much, because i get the principles that you are referring too)

No problem about knowing the 928 geometry... that was just to eliminate the possibility of changing things via the" strut" modifications.

as far as your second comments, i know im into the bump stops in the corkscrew... i did mark it, but you can see from the video and the pictures in the high g loading areas, the wheel is no where near as high in the wheel well. its not a guarantee, but it sure points to me not being on the bump stops at any other point on the track. Ill have to mark the shocks.. go out and tip-toe through the corkscrew off line, and drive hard every where else for a session and see where the markers turn up on the shock throw. (or get the go-pro up in there.

I know street cars, basically, are set up to be more forgiving. sure, with all the lowering, stiffening, and lightening, with near double the power and 1000lbs less weight, they are different animals. its not to say that the root designs can be effecting the handling here. im still leaning toward the rear end toe as keeping things pretty tight and the use of the worst quality used tires ive used in a few years. (my buds are not tossing the A6s out after 5-6heat cycles anymore! ) . I dont mind a very loose car, so im wondering if to get the car to rotate , a combination of more toe out in the rear, and tightening the rear swaybar will help? I dont know if raising the front , as the mustang guys have suggested will do anything but raise the CG , if its not riding on the bump stops everywhere but the corkscrew. (or the hill at T-hill)
im not too extited about changing shock settings just yet, because on some of those smooth surfaces, the push is happening at steady state conditions.. sure, its nice to have some additional dive to load up the fronts with more grip for an aggressive trail brake approach, but if you cant complete the turn there, you are left with the push through the rest of the turn, and nothing i can do will fix it (driver input wise, via power or off throttle ) besides going into the turn slower... that works great, but its slower and that's no good.
what about loosening the rear swaybar? could that put more twist in the chassis and apply more force to that outside wheel? seems counter intuitive, but someone told me that too.

Earlierapex

Yes. As soon as you bottom the suspension for any reason, you get a dramatic change in spring rate. Bumps aren't required.

Correct. And it doesn't have to be infinite to be huge.

More or less, but there's no need for a distinction. The reason for hitting the bump stop (whether actual bumps or g loading) doesn't change the physics of relative stiffness. Actually, bottoming the suspension on a smooth track (implying g loading cause) is probably worse because you are close to the adhesion limit of the tire and dramatically change the roll couple nearly instantaneously.

it's usually easiest to start with the anti-roll bars. If it is understeering on turn-in and under power, then you have a LOT of understeer to dial out.

mark kibort

yes, there is a lot of oversteer and always has been, but like i said, with new tires, its near perfect and the car is very fast. but as soon as the end of the race comes near or the tires are cooked, there is no way to tune it out, and their should be. (i would think).. so, thats why the questions.

I cant really go softer on the front bar, as i already am near lifting the rear wheel to get rear wheels spin. ive gone low up front for rain races, and its worked but for dry and forgetting to set it back, i lift the rear wheel and it cooks the diff. fix it and that problem goes away. soften the front springs would do the same and more. So, im left with dive characteristics and the rear bar stiffening. im a little leary of that because ive had some real crappy tire in the rear, and it is dangerously loose and the push goes nowhere.. it pushes and is loose in critical , high speed turns.
this is fun, right?? again, i know i can solve the problem by cutting a big check for some new tires (and i might do that this year anyway) but there are those that think that i can tune this back to neutral and im trying to understand how. what about loosening the rear swaybar... intuitively that is supposed to get rid of oversteer and make a push worse, but could it work in some cases?

heck, i missed that free SCCA test day , but skiing was just too good! i want to just reduce all that rebound in the rear and rebound and compression in the front and see what it does.

You want to see tight.. this is the tightest car ive ever seen or driven. just put a wing with too high of setting on the car, no splitter crappy tires, and they had just repaved laguna. it was a joke.. only 1 second slower than normal, but a mess to drive.

This was Randy Pobst in Sofros Grand Am GS BMW M3 with the exact same HP as me and the same weight (me on used tires and him on brand new hoosiers). times were 1:38s for randy and 1:39s for me.

danielyonker

Mark - Your hand positioning leaves something to be desired - i see you turing with one hand, with the other hand presumable resting on the shifter, and the hand on the wheel turned all the way around to max range (on a left hand turn). Reference 1:18, 1:38, 1:54, 2:14....after that I had seen enough. You'd be better of shuffle steering, or presetting your hands for the radius, or anything! Seems to me your driving style in itself is inducing understeer. A wise man once said slow in and fast out....

mark kibort

the shifter had a problem and needed to be held. the entire reason you keep the hand or hands in one spot is to know where center is without looking at a mark. the push was so bad, it was almost worth shuffle steering.
anyway, you can contrast this when the push was cured with a splitter, hood vent, and better tires. when you are used to running a certain speed, you dont want to sacrafice lap time, with getting rid of a push with slow in, fast out. towward the end of the race, i did that for a lap and it actually looked hooked up...`1 seconda laps slower.

Veloce Raptor

A fact which has been repeatedly made in this thread starting on the first page, to no avail

Earlierapex

Mark,

The push problem is driving style related. You are trying to carry too much speed into corners, apexing a little early and having to "fight" the car on the backside of the turn. You can see this in the video because the steering angle required after apex is WAY more than it should be.

The reason you are having a problem with lifting a wheel in the rear is because you are asking the front end of the car to do all the work. You've tried to compensate by adjusting the suspension. There's only so much you can do with the suspension.

The oversteer problem you have is actually understeer related. When you are having to use that much steering angle and fight understeer after the apex, you end up inducing "pinch" oversteer. While this seems like oversteer, it's actually a function of fighting the front end with all that steering angle. When you finally do get it to bite, the extreme steering angle snaps the car into pinch oversteer on the backside of the turn.

We all have to go back to basics sometimes. I found myself doing the same thing recently at Sebring and had to remember to get the car rotated earlier and take a later apex. This is ALWAYS good advice to fall back on.

Cheers.

ProCoach

^^THIS^^

Superb and astute post...

T&T Racing

Going back to basics, the first question always is driver induced or set up induced? Always easier to blame the set up over looking inward.
Having built race cars and crew chief for a TransAm, bump steer should be minimized and tire temperatures are important in set up.
My $0.02

mark kibort

I get that.. again, if you read 1st or 2nd page, when that happens, i have no ill effects. im worried and want to change handling characteristics on the turns where im no where near the bump stops. so your point is say, moot!
Mk

mark kibort

usually, true, and can be true depending how you look at it. one thing I do know clearly is what the car can usually do around all the turns at laguna. sure, in that day, based on changes to track, tires and aero, the car couldnt do what i usually would ask of it. clear unbalance of grip up front. tire temps did show the fronts were over worked but still even across the tire, with warmest point on the inside edge.

all you said, is all i worked with that day on the track... you have to compare to the next day i visited the track with the better tires and proper aero. none of these huge problems were present.. .... if you look at the end of the video, i was experimenting with going slower in and trying to throttle steer out, still there was such a lack of grip up front, i couldnt even do that, around constant radius turns such as turn 2, which at one point doesn't matter if you entered too fast or not (thats the acid test).

also, as far as the sway bar goes, when i disconnected it, it lifted the rear of the car inside tire and got wheel spin. there was no exceeding the front of the car capability, the grip was even better due to weight transfer, so much so that it lifted the rear of the car. putting the bar back on, settled the rear of the car and grip in the rear was restored and the ability to oversteer when needed. (we are talking a different day)

the ONLY reason i posted the over steer video , was to think about how to cure it. its never been that bad and ive run much faster, on pure handling , not power since, through and exiting the turns. you have to give credit to elements of the system that could have been failing. yes, was i driving too hard into the turns, yes, for those tires and the set up. at the cost of 0 control on the exit, the lap times were not far off, but it was a dangerous set up had i been driving with a lot of close traffic and it chews tires up fast . what i was looking from the group here was things mechanically i could do to improve the grip as you can see Randys car totally hooked up in the turns and our cars (ive raced these cars for many years )usually handle near the same....

the point is and the question i have is what can you do to get more weight transfer to the front outside tire to get more grip. driving slower is not answer, set up is. again, i know when the car is not handling the demand and when it is. In thinking about that day, it could have truely been a tire only problem... i remember later in the race (on the raw video, not what was posted) , doing things , like really aggressive braking straight and then trail braking with as much weight transfer as possible to the front (at a slower speed) and things not changing much. the only time it really handled well was on the cool down lap, at about 75% of the speed.

that original video was from 2006, here is the same car at the exact same lap times (slighly faster) with no suspension changes , ONLY tire (better used tires).
pay attention to the greatly different steering angles (compared to the push video) for faster turn in speeds and slightly faster lap times.

mmuller

I just can't believe a thread on 928 handling issues on track could go 4 pages. That's what astounds me......