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Discussion Starter · #21 ·
Thanks for your patients Bob, It finally all makes sense. I will stop with the 40 questions.... on this subject anyway!

Paul
 

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I'll try and add my 3 cents worth. That's 2 cents + 1 for confusion.

The distance to the ground is not what matters for the panhard bar. Rather, it is the point the panhard bar crosses the middle of the differential. If that point is at the bottom of the differential, the roll center is low. If it is at the top of the differential, the roll center is high.

RallyBob's design can be adjusted at either the chassis mount, or the axel tube mount. If you were to move both adjustments as far down (toward the ground), then the roll center would be at it's lowest point. If you raised one end (or both ends for that matter) the middle of the panhard bar would move in relationship to the differential. (Higher)

An important concept here is that the differential does move up and down. It stays firmly planted since it is on the ground and un-sprung. It is the car that moves up and down in relationship to the "rear-end". So, as the car moves up and down, so does the chassis mount for the panhard bar. So... as the chassis mount moves up and down, so does the roll center. (With me so far... remember, I said 1 cent for confusion factor).

When the car rolls in a corner, the "roll center" is the "pivot point" for the roll. So, when when you turn left (and the body rolls around the "roll center"), the chassis mount for the panhard bar gets closer to the axel... and therefore the roll center is lowered. The oposite happens in a right hand turn. (I am refering to the chassis mount on the passenger side of the car.)

So, what you have is a dynamic roll center. It is actually moving all over the place. :mad: Now that I have everyone completely confused, there's an easier way to understand this than my words... build a little "stick" model. (Straws, and balsa wood with straight pins for the pivot points work well.) Then move the "stick body" in relationship to the rear-end. Remember... the rear-end doesn't move up and down, so just set it on the table, and move the body around. After you do this, you'll see what's going on.

So, why is all this important? Well, for the average driver it isn't. The panhard bar just keeps the rear-end in approximately the center of the car... so it will go straight down the road. But for those who push their car to the limits (and beyond), the roll centers (front and rear) determine front to rear weight transfer of the car. A car with a high roll center in the rear (compaired to the front roll center) will transfer weight from the inside rear to the outside front.

An extreme of this is when a front wheel drive car lifts the inside rear wheel in a corner... the "rice burners" are bad for this. (Hopefully you've seen pictures of what I'm talking about.) When the inside rear is lifted some weight is transfered to the outside rear, but most of it is transfered to the outside front.

So, back to out Opels... what do we do about all this? If you're going to just get groceries, or just "crusin" (which is about 99% of you), then nothing. If you want to make it look "high-zoot", then just build a panhard bar like Rally Bob has shown and leave the mounts alone. Just be sure that when you adjust the length of the bar that the rear-end is pointing straight... you'll have to have an alignment shop do this.

For the other 1%, when you make new mounts, make everything very strong. I can't over emphasize this... if the new mounts flex, it will make the car handle very bizzare... and worse, the flex will eventually lead to them breaking.:eek:

Like Rally Bob said, adjust the bar so that it is parallel to the axel. (When the panhard bar is parallel with the axel, dynamic movement of the roll center is minimized.) If you adjust the bar, adjust both ends so that the bar stays parallel to the axel.

It is impossible to tell you how to adjust the bar. There are too many variables... like Rally Bob said, springs, shocks, sway bars, and weight distribution (even how much fuel is in the tank) all have an affect. So, adjustment needs to be done on a "skid pad" or other very safe place to do it. DON'T DO IT ON THE STREET. A sudden oversteer can ruin your day.

Rather than try to tell you which way to adjust, the best thing to do is set the bar to one exterme of adjustment, and then drive the car to just beyond the limit and note which end of the car goes away first (oversteer or understeer). Then adjust to the other exterme and repeat. Now you'll know the affect both extremes have and you can adjust to ballance the car. (If the the same end of the car goes away at both extermes of adjustment, then it's time for a spring change or sway bar change... that's a whole 'nuther' topic, but the "rule of thumb" is make the offending end of the car softer... or make the non-offending end of the car stiffer). For anything but a race only car, when you are done adjusting, you want to have a tiny bit of understeer. Understeer is safer, and easier to recover from.
 

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It occurs to me that I talked about oversteer and understeer like everyone naturally knows what these are.:confused:

Understeer is when you are trying to turn, but the front of the car just plows straight ahead. This usually happens after you have entered a turn near the limits of traction. A little faster and you go over the limits of traction and the front of the car starts to plow to the outside of the turn... turning the wheel more, just makes it plow worse. The same thing happens when are going straight, slam on the brakes... lock up all four wheels and then turn the steering wheel. The car just keeps on going straight. With understeer, you see what you are going to hit. NASCAR folks call this "push"... Understeer to road race folks.

Oversteer is when you are in the turn, the rear end starts to come around, and you spin. This will happen most often if you are cornering near the limit of traction and "romp down" of the throttle. The extra torque to the rear wheels (rear drive car) will break the tires loose and around you go. It can also happen without additional throttle if the rear is the end that exceeds the limits of traction first. With oversteer, you "back into" what you hit. NASCAR folks call this "loose".
 

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Discussion Starter · #24 ·
Hey Now! Let's not slam just RICE BURNER'S for lifting the inside rear tire in a hard cormer....... Us NEON driver's with rebound adjustable KONI's can do it too!!!!!!!

LOL
Paul
 

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Discussion Starter · #26 ·
Here's change back from your 1 cent for confusion:

An important concept here is that the differential does move up and down. It stays firmly planted since it is on the ground and un-sprung. It is the car that moves up and down in relationship to the "rear-end". So, as the car moves up and ......

I assume you meant to say the differential does NOT move up and down....since it stays firmly planted......

Thanks Again for the valuable insights. I really have learned a Lot

Paul
 

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You're right... I ment DOES NOT move up and down. Except that in reality it does move a little... the tires deform and then spring back so there is a little movement. In fact, on some full tilt "formula" race cars, the suspension is so rigid that the tires are as much the springs for the car as the "real" springs. In any case, for our "door slammer" cars, the rear-end essentially does not move.
 

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Another can of worms

Not that I want to add to the confusion any more, but you can help firmly place the roll center of the car with a Watts link. My water injected Kadett rallye had one installed and man was it a blast to drive! If I can find an old picture, I'll upload it.

A Watts link takes the place of the panhard rod with 3 seperate rods linked to form a "Z." The very center of the center diagional link is the only part mounted to the axle, and in my case it was to a rather simple piece of 1/4 x 3" bar stock run from the top of the differential cover to the bottom and bolted on with a ring of plasma-cut metal around the inside of the cover flange. The bar stock had several holes to attach the center to, but I really never took advantage of the tunability of the set-up. The ends of the "legs" of the "Z" are attached to the body and to the center link so they are roughly parallel to the ground. As the suspension moves up and down, the 3 links move together to keep the center of the center link centered with the axis of the car.

I know this is a terrible explanation, but when you start really thinking about modifying suspensions, you might want to at least look into it.
 

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You cant just knock front drivers for picking up tires. I managed to pick up a tire in my 99 Audi A6 Quattro (AWD) at least once. It was in an emergency maneuver that included essentially a really quick tight slalom.

I know a guy who races a Jag (70s vintage) that pickus up the inside front tire when acclerating out of corners.
 

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My old Ascona picked up the inside front tire in hard turns (two front sway bars and slicks will do that!). Looks pretty cool in photos!

RE: Watts linkage. They're pretty neat, but not as adjustable as the panhard bar arrangement. Also, in a side impact (example: sliding and hitting a curb), they are pretty much unyielding. Something will break. Panhard bars will usually bend, so you have a crush zone.

Bob
 
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