DogInBlack

Looks cool to me

Bryan Watts

There's probably a lot more chassis flex than you realize, just not in the way that you think. You can see the difference in roll couple between the front and rear of your car in the pictures you posted. The front seems to roll more than the rear which puts a large twisting force on the chassis.

schvetkaaks

But isn't the front experiencing a different type of force since it is full throttle, with weight transfer all the way to the back? A stiff chassis would make wheelies (to use a childhood term) if there was enough speed/g-forces/weight transfer to the rear. A flexible one would remain planted, as demonstrated by the street cars, no?

schvetkaaks

There's no benefit at all, and frankly, at this corner, it's likely wheel hop based on the horrible conditions of the track - that corner is covered in bumps in that direction. It's likely a small jump, really, but it looks kick ***.

DrJupeman

How many pictures do you want to see of my car doing the "fire-hydrant pee" on an apex? I've got a crapload of them. Not much I can do to adjust the car to correct for it, either (spec class, basically).

stuttgart46

Great pic Todd. I love seeing the Boxster driven in anger.

rickdm

I couldn't quite figure out how to post a pic, so i thought I would put it in the avatar. The shot is at Eagles Canyon on turn 7 where it is very bumpy, I don't think my wheel lifts that much under normal paving. I do think it is somewhat caused by a 22mm front torsion bar fighting a 31mm front sway bar.

schvetkaaks

Did you see the vid? Talk about 'in anger', lol

Bryan Watts

You are cornering at 1+ G and only accelerating at a few tenths of a G. Most of the forces acting on the chassis are from cornering loads. Carrying the front wheel has a little bit to do with being on power, but most of it comes from the limited rebound travel that you have in the front suspension that can't make up for the amount that the front of the car is rolling.

schvetkaaks

Is that because of springs that are too hard (1300lb front) or lack of travel distance of the front rebound?

dbryant61

All chassis flex, just like all beams deflect. It looks like yours (Todd) has a very well thought-out cage and that will limit flex greatly. I think one of Porsche's, espcially the older version 911s, big advantages is the stiffness of the tub. The steel is much thicker than that of modern cars. I think wheel lift is much more related to suspension. Most of us run smaller ARBs in the rear, or at least have them set softer, so that we gain rear grip for earlier throttle in the corners. The result is that most of the roll resistance in the rear is being done by the outside tire's t-bar or spring because the t-bar is ont transferring as much force to the inside t-bar or spring. Coupled with acceleration throwing weight to the rear, and you have outside rear corner squat. That coupled with less weight on the front naturally, weight removed from the front by acceleration, and a stiff front ARB lifts the inside front. The line connecting the inside rear and outside front is like a fulcrum.

When I first began working on my suspension, I installed 22mm ARBs F&R with stock t-bars. I could feel the outside rear squat like crazy, and others claimed my inside front was 6" off the pavement and would actually come to a stop in the air. Sorry, no pics.

Bryan Watts

They are one in the same....the stiffer the spring, the less rebound travel you'll have. Let's think about it, what's your corner weight up front? I'm just going to pull some numbers out of the air...let's say your car weighs 2500 pounds with you in it and that your car has a 55% weight bias towards the rear (it may be more than that?). So, making an assumption that the car is R/L balanced up front, you have about 560 pounds of weight on the front-right corner at static.

You have a 1300 pound spring mounted on a strut, which means your wheel rate (using around a .90 conversion factor from BMW's) is something like 1150 pounds. So, at static, your spring is being compressed approx 1/2 inch (560/1150 = 0.49). This doesn't even take into account the wheel rate added by your anti-roll bar or the fact that the front spring is compressed even less when you are accelerating and weight is transferred to the rear.

So, you have less than 1/2 inch of rebound travel in your front suspension. Unless you car rolls less than 1/2 inch in the front, the inside front tire is no longer going to be in contact with the ground. Due to the crappy roll centers you end up with when lowering a strut suspension, even a 1300 pound spring isn't enough to limit your front roll to less than 1/2 an inch.

What does all of this mean? I think all of you Porsche guys run too stiff of spring rates.

himself

Nice pics! Great paint job! I think it's a matter of rear shocks having too slow rebound combined with front sway bar being too stiff. But what do I know.

At the last DE here, some of the pics look like I'm about to have 2 off the ground. Other pics (without tire lift) posted here.

-td







Well, not tire lift, but I like this pic.

sbarton

Bah, only one tire in the air? How about 2!



-Scott

Larry Herman

That's why you run smallish (or stock) sway bars and helper springs when you run really stiff main springs. Keeps the inside tire on the ground.