[Crumpler]

I have a distant memory of GB saying all 80’s P-cars were designed to understeer because of the 911 oversteer but the thread escapes me. Something about progressive loading on the control arms or bushings?
I certainly have heard that all MY 928 tend to understeer but that may also be anecdotal.
Is it a real thing, any hopeful an explanation of why?

Mine certainly understeers, but that may be my current configuration.

[worf]

I have always written that the 928 understeers. Part of the ‘factory’ understeer is simply not enough front tire out of the box. That is easily expounded upon with simple math and comparison to other cars with similar weight distribution and with the 911.

All manufacturers “build-in” understeer, deliberately, all the time, for street cars, thanks to Ralph Nadar and the Corvair lawsuits(*.) PAG is, arguably, over-aggressive with non-GT cars thanks to the 911 Turbo “Doctor Lawsuits” from the 80s.

One means by which it is built-in, exclusive of the expediency of narrow front tires, is to design-in dynamic toe changes related to steering angle, side-loading and suspension compression. Weird bushings are very often a good way to do that. The other, old-school, way is with a big-assed front sway-bar or weaker rear sway bar.

(*) One can easily argue that GM got what they deserved since they were aware of the problem and a solution (which was a bigger front / smaller rear sway bar - I forget which and am too lazy to google…)

[Crumpler]

Thanks Worf.
That makes sense to me.

I found a check list for understeer parameters here:

https://www.elephantracing.com/tech-top ... oversteer/

[Gary Knox]

Great chart. Just needs to be modified for 928's - which understeer - to have the first column read
'TO DECREASE UNDERSTEER', and the 2nd column to read 'TO INCREASE UNDERSTEER'.

Gary

[Crumpler]

One thing I can’t escape right now is larger tire size in rear.
I have 305’s on the rear to hold the boost. 245’s on the front

[Crumpler]

What about those stabilizing bars?
Not to use Carl as an example but I think he marketed a moly bar that ran from the forward lower control arm housing, side to side to make frame stiffer? Real thing or Kool-aid?

[worf]

What about those stabilizing bars?
Not to use Carl as an example but I think he marketed a moly bar that ran from the forward lower control arm housing, side to side to make frame stiffer? Real thing or Kool-aid?

It’s real. It’s similar to making the front sway bar stiffer.

And that will … ?

You get to a point with stiffer springs and sway bars where the chassis *becomes* the spring.

A floppy, bendy chassis certainly limits what you can make a suspension do. The problem is that it’s a multi-variable system: you can’t change just one thing and drag a 1972 suspension/chassis design into the 21st century.

On other hand, *nothing* you can do to a suspension will magically turn bicycle tires into sticky rollers. You are ultimately, with suspension “sophistication”, just attempting to maximize the ultimate potential grip of tires under a variety of loading conditions on several axes. So… bigger front tires would be my first thought.

I’m not tellin’ya not to trip on the sway bar and fall down the suspension Rabbit Hole. Just’sayin’ there are a lot of bumps in the hole on the way to Wonderland, Alice.

That written… where does it hurt most? Under braking? Under acceleration? Mid-corner?

[Crumpler]

Mid corner.
I tried apexing later and later, still seems to be fighting me.
Throttle lift compensates.

[worf]

Mid corner.
I tried apexing later and later, still seems to be fighting me.
Throttle lift compensates.

Ok. Then …

With 245s on the front and 305s on the rear, right this instant I think the primary problem is not enough front tire. If you look at this from a tire contact patch vs weight(*) on the patch standpoint it’s easy to see that the front tires need more contact patch to ‘balance’ the rear.

But, the bendy front chassis is beginning to contribute to the problem. The more front patch you get the more the bendy chassis will fight you.

(*) Don’t get confused by Freshman Physics theory that says the friction force is proportional to loading. Roads and tires do not provide mathematically flat and undeformable surfaces which are the basis of the basic math.

[worf]

Basic question: Tire Pressures? Hot? Cold? Front? Rear? Weight balance?

[Crumpler]

Recent log:

932——898 53.7% weight
X
803——768. 46.2%


Camber
-2.3 -2.2degrees
-1.5. -1.5

Toe
0.0. 3.0mm
1.5. 1.5

I tend to keep all corners equal with tire pressure, 30-32 psi

[Erik N]

I tend to keep all corners equal with tire pressure, 30-32 psi

I haven't driven my car in a couple of years, but used to track it quite a bit. It is exceedingly sensitive to tire pressure. Even a 2psi difference could be felt, either in turn-in on the front or during acceleration in the rears.
I used to run 36/34 F/R religiously.
Tires are way smaller, though. 225/255's.
Also, on my car at least, understeer was not a problem. I could trailing-throttle oversteer into a corner and gradually apply power then push the front6 out of the turn. Or just stab it and spin

[worf]

I tend to keep all corners equal with tire pressure, 30-32 psi

I haven't driven my car in a couple of years, but used to track it quite a bit. It is exceedingly sensitive to tire pressure. Even a 2psi difference could be felt, either in turn-in on the front or during acceleration in the rears.
…
I used to run 36/34 F/R religiously.

And this -^ why I asked about pressure. It’s the first thing you mess with for balance.

An old gray-haired racer (who now owns a company that owns 200+ track-prepped cars and rents them out), when I ask him about tire pressure told me “as low as you can get away with.”

Doesn’t really tell’ya *how* to do it, but it’s not wrong. The lowest possible tire pressure will be dependent on the track and on average and peak speed. And of course car weight.

I don’t suppose Hoosiers (or whatever) come with a loading/speed/pressure table (as like the uniform load rating guide tables…)

[Crumpler]

Appreciate the tips. Easy enough to modify pressures. One more DE this season.

I did install a frame brace for the lower front end.
I did a few cloverleaf runs and it potentially helps.
The ground post on the aftermarket alternator prohibited a straight shot between the LCA hardware. I had to fab a small spacer to move it back an inch. Hopefully that does not compromise the physics.

[worf]

… I had to fab a small spacer to move it back an inch. Hopefully that does not compromise the physics.

Unfortunately, it will: the adapter bolts are pivot points. The force that’s supposed to be transmitted from one LCA front bracket bolt through the long axis of the bar to the other bracket bolt is simply going to be expended rotating your adapters around.

[Crumpler]

… I had to fab a small spacer to move it back an inch. Hopefully that does not compromise the physics.

Unfortunately, it will: the adapter bolts are pivot points. The force that’s supposed to be transmitted from one LCA front bracket bolt through the long axis of the bar to the other bracket bolt is simply going to be expended rotating your adapters around.

They are welded to whatever the piece is called that comes down from the nose frame and takes the two forward LCA Bolts.

Proprietary Information Dave ;)

[worf]

They are welded to whatever the piece is called that comes down from the nose frame and takes the two forward LCA Bolts.

Ok. Can’t see that from the picture; just looks like an ‘extension’ at a right angle with the bolts (aka rotation points.)

Welding good.

Proprietary Information Dave ;)

My lips are sealed and I promise not to copy it without your permission

[Crumpler]

Well, field data is in.
The bar works — to the point of oversteer.
It was crazy.

[worf]

Well, field data is in.
The bar works — to the point of oversteer.
It was crazy.

Cool. Now mess with tire pressure

[Geza-aka-Zombo]

I used to use a probe type tire pyrometer to regulate pressures. Measure inner, center and outer of tread right after coming off a hot lap into the pits (someone else measures/records). Also helps with alignment settings (camber, especially).