ChrisB

Is it just for packaging reasons that the panhard rod is angled slightly? Wouldn't it make more sense to have the panhard rod as close to horizontal as possible when the car is fully loaded?

Also do you think there would be any benifit from lowering the mounting point of the panhard rod? I think the roll center of the rear suspension is going to be determined by where the panhard bar connects to the rear axle - and isn't a lower roll center better? (for handling - it seems like it should be)?


Chris

Patrick G

iTrader: (14)

Yes to all your questions. Yes it's not horizontal because of packaging and yes, it would handle even better with a more horizontal placement. Clearances with a full load on bumpy roads get a little dicey so GM plays it conservative.

TomBusby

Having a perfectly horizontal panhard bar cuts in half the amount of left-right movement resulting from the up-down movement of the panhard bar. The stock panhard is higher on the right side, so as the rear squats in acceleration, the center of the car moves slightly over to the right side, which is good because axle twist is tending to unweight the right side.

You can analyze the effect of having a non-horizontal panhard this way: If we rephrase "higher on the right side" in this example, to be a panhard bar being a 40" line, with the left side being fixed at the origin of the x-y axis (we are in quadrant I on a graph but at the axle-attachment point on the car) and with the panhard starting at 4" above the x-axis on a graph (this is estimated to be the maximum suspension-unweighting height), and then we rotate the panhard down to full rear suspension squat (where the panhard is now right on the x-axis) then the left-right movement of the panhard bar will be (more or less) the short side of a 3-4-5 right triangle, or 3". (The arc of the panhard is the 5" hypotenuse -- I am ignoring the arc and describing it as a straight line.)

If the panhard were exactly horizontal at static ride height then the total movement would be 50% to the left for occasions of unweighting the rear axle and 50% to the right for weighting the rear axle. This would not necessarily be a better setup, because upon acceleration the rear of the chassis would first move left and then move right (assuming that the axle-tires can not slide -- the car must move). The back end is loose/light anyway, and with way too much weight in front on braking, I am guessing that stock panhard moving the rear of the chassis in only one direction, to the right, is more manageable and safer. I also believe that it is quicker and more predictable in corners for me too, but I can understand that it might be slower for some other drivers.

But, how would this affect roll center?

ChrisB

[quote]Originally posted by TomBusby:
<strong>Having a perfectly horizontal panhard bar cuts in half the amount of left-right movement resulting from the up-down movement of the panhard bar.<hr></blockquote></strong>

I think it does more than that though - let's say our up and down range is of a fixed amount - e.g. we need a certain amount of travel.

Now lets look at an instance where the panhard rod is like stock. Draw an imaginary circle from the frame mounting location with a radius of the panhard rod. These are the possible Locations of the panhard rod.

For a given vertical displacement we will also have a horizontal displacement (connect 2 points on the arc with a straight line (chord) and then reduce that to it's component vectors.

If we have the panhard rod at an AVERAGE 45 degree angle then (say it goes from 40 to 50) our vertical displacement will equal our horizontal displacement. So a 3 inches of suspension travel up will equal a 3 inch shift to the right.

Now if we start out at perfectly horizontal you can see that for the same vertical component (3 inch displacement) the horizontal component is going to be very small.

So it's not a "cutting in half" - but a different situation entirely.

[quote]
If the panhard were exactly horizontal at static ride height then the total movement would be 50% to the left for occasions of unweighting the rear axle and 50% to the right for weighting the rear axle. This would not necessarily be a better setup, because upon acceleration the rear of the chassis would first move left and then move right<hr></blockquote>

Ahh, but let's look at your frame of reference - if we start out at horizontal and load the car we move to the right by a very small amount.

If we start out at stock and load the car we move the the right by a large amount

The effects and movement will be the same - just more exageratted in the stock setup - with one exception. If you go from loaded to unloaded or vice versta in one sweep then you go from shift to the right to neutral to shift to the right again.

If you do this with stock your just go from a *large* shift to the right to neutral - but the total displacement is still much more.

As to how this actually effects the driver I have no idea - as I have only driven the stock type. I would think reducing lateral motion of the rear end would be a good thing though.

[quote]<strong>
But, how would this affect roll center?</strong><hr></blockquote>

Roll center for the rear suspension is going to be the point where the panhard rod attaches - simply because of the way the panhard rod works (assuming no binding from the LCA's, etc. - which yes, I know isn't true, but still it's primarily determined by the panhard rod mounting point on the rear end).


Chris

LS1MONSTER

iTrader: (1)

WOW! ChrisB and TomBusby, you two make the Panhard rod sound the like most high tech part on our cars! I learned quite a bit just from this post. Great info guys.

LG Motorsports

The roll center at the rear is at the center of the PHB. As the car goes around a right hand corner, the roll center rises as the right side of the car rolls in the middle of the corner. As the roll center rises, the car gets loose.

On the contrary, as the car goes through a left hand corner, the right side of the car goes down, and since the left side of the PHB is attached to the rear axle, it stays constant on the left, and the roll center lowers. This causes the car to get tight, (understeer).

On our World Challenge Camaro, on some tracks, we would flip the PHB and mount the left side of the bar on the chassis and the right side of the bar on the rear axle. We would also mount the bar lower to get more bite, coming out of a corner, and use this as a tuning device just like springs, sway bars, or shocks. It just adds another tool to the Chassis tuning fight.

For example, if the most important corners on a particular race track,were right hand coners, we would mount the PHB on the chassis on the left. This would allow me to charge the right hand corners harder. *** the car entered the corner I could pick up the throtle erlier. This would cause the left side of the car would drop, and the car would feel more stable due to the drop in the roll center.

Consider what would happen if the PHB was in the normal location. The left side of the car, on a right hander, would still drop, but it would have no effect on the roll center. But the right side of the car would lift, which would raise the right side of the PHB and thus raise the roll center. When this happens the car would tend to oversteer, and at the least, would feel unsettled, and reduce confidence and delay the moment that acceleration could begin.

In oval racing, NASCAR, they tune the car with not only PHB hight (roll center height), but sometimes they will mount the PHB on the chassis on the left to reduce "Mid Corner" push(understeer).

In road racing, on our Trans Am C5, we use a "Watts Link" which is a 2 part lateral locator, that piviots in the center. The roll center stays constant relative to the piviot. If we mount the piviot of the rear axle, the roll center stays constant. And if we mount the piviot on the chassis, the roll center goes up and down with the chassis. So if the car squats 1 inch, the RC moves 1 inch. As was stated in a post above, on a PHB car, if the car squats 1 inch, the RC only moves 1/2 inch.

So there you have it, another long but basic discription of a very simply part of the car, that has a major impact on how the car handles.


Thanks,

Lou Gigliotti www.lgmotorsports.com

1dirtyZ

iTrader: (1)

great info guys , now what about if the car has a two lower control arms and one triangular upper (with two points on the frame and one on the top of the diff)?? how would that affect handling compared to a panhard bar system?

Crazyquik

Thanks Lou, makes a lot more sense now <img src="gr_images/icons/cool.gif" border="0">

Jonathan

LG Motorsports

(From 1dirtyZ)
>>> great info guys , now what about if the car has a two lower control arms and one triangular upper (with two points on the frame and one on the top of the diff)?? how would that affect handling compared to a panhard bar system? <<<

Here is what the Triangular upper would do.
The line drawn between the upper mount on the rear axle and the attachment points on the chasssis would determine the instant center, with respect to the lower trailing arms.

The roll center is determined by the locating point, of the triangle, on the rear axle. Since this point is at the top, the roll center is at the top. This is a higher roll center than the PHB, so the car would be "Loose as a Goose". The high roll center would reduce rear roll but also reduce rear traction in a corner.

Some race cars have used this system but with the "Triangle" mounted on the bottom, and the rear control arms on the top. This works to increase traction, but induce increased roll that has to be controlled with a large sway bar. This system was outlawed in the Trans Am series that I race in.

Later,

Lou Gigliotti LG Motorsports

[ January 02, 2002: Message edited by: LG Motorsports ]</p>

Jacobi

Ummm yeah...this ****'s pretty deep!

Have another before you start to think this one through!!! <img src="graemlins/gr_cheers.gif" border="0" alt="[cheers]" />