SoundGuyDave

Of course, being the somewhat absent-minded guy that I am, I forgot to take some measurements, but I think we have enough to piece together all the relevant data...


Situation: I was installing a handful of BMR goodies on a buddy's '08, and took some measurements, all before starting the install. During the install, some issues popped up, and to be honest, the final measurements fell by the wayside.

ITEM ONE: Stock, the panhard rod has a 4.9 degree slope from the passenger side frame mount down to the driver's side axle mount.

ITEM TWO: The stock ride height (with factory 18" bullets and BFG tires) measured at the center of the panhard rod mounting bolt is 14.75" from the floor.

ITEM THREE: Stock, the wheelwell to rim clearance on the passenger side was 1-5/16", and on the driver's side it was 1-1/4". During the installation, I did shorten the panhard rod quite a bit to get it to drop in place with the axle and frame both on jackstands, and the final wheelwell-to-rim measurement was 1-9/32 on each side, with 165lbs weighting in the driver's seat.

If someone can come up with the stock panhard rod length, and ride-height to the center of the axle-mount bolt with stock tires and rims, that should tell us all we need to know.

That said, however, I think it can be said conclusively that adjustable panhard rods are conceptually valid, and are useful for applications with lowered ride heights. The springs we installed are the BMR drop springs like I have, with 1.25" front drop, and 1.75" rear drop. If we had not installed an adjustable unit, my estimate is that we would have had an axle offset to the driver's side by approximately 3/8", which would have been very visually noticeable. With the FRPP kit installed on the Shelby GT having (I think) a 1" drop all around, the axle appears to be offset by about 1/8", so this tracks.

If anybody would like to chime in with dimensions and trig, I think we can wrap this up!

itchy

Ok with some power point engineering and excel I came up with this.

I hope my math is right, otherwise this would be quite embarassing.

Given a stock lengnth between 40-48 inches we are looking at a shift of about 1/8 inch.

I tried to attach the excel sheet, but we aren't allowed. Here is a picture of everything.

SoundGuyDave

That all looks about right to me, and the result matches emperical data, so I would say CASE CLOSED!

Any dissention?

Rebel73

yah,

what setup are you running?

Wheels/tires/springs etc?

RRRoamer

I don't think I am eating any crow today...

This morning, I grabbed a tape measure and took a few measurements on my car. The panhard bar is right at 41" long and the bar mount point is 1/4" higher than the axle mount, this is with the estimated 1 1/2" drop with my Eibach Pro springs. That means that stock, the bar mount would be 1 3/4" higher than the axle mount.

Once again, we need to resort to two right triangles to calculate the differences. Here are the terms I am using, so everyone can follow along with me and check my math:

1) Before "long side" = a, before "short side" = b, before hypotenuse = c
2) After "long side" = A, after "short side" = B, after hypotenuse = C
3) We know c and C (panhard bar length: 41") and b (1.75") and B (0.25")
4) We have to calculate the long side of both before and after triangles, then subtract them to get the DIFFERENCE of side to side axle position before and after.

So...

A^2 + B^2 = C^2...
A^2 = C^2 - B^2
A = sqrt(C^2 - B^2)

And:
a = sqrt(c^2 -b^2)

Let's calculate the before position:
a = sqrt(41^2 - (1.75)^2) = 40.9626"

Now after:
A = sqrt(41^2 - (0.25)^2) = 40.9992"

So the shift from the stock position after dropping the back of the car 1 1/2"???
Shift = 40.9992 - 40.9626 = 0.0366"

Which, if you will check, is slightly more than 1/32". And no, you are NOT going to be able to see that and would probably have a hard time actually measuring it without good tools and good technique.

A 2" drop would have the exact same offset as a 1 1/2" offset as the panhard bar would drop below horizontal and start sloping up (from mount to axle). A 3 1/2" drop would put the axle pretty much exactly where the factory set it...

So, I'm not eating any crow here. Dropping the car just doesn't move the axle that much. Wear in the bushings will move it a LOT more than the change in angle on this panhard bar. It looks like the absolute worse case drop should be 1 3/4" drop which would move the bar from slopping down from the mount to the axle to being absolutely horizontal. And even that case, you would only move the axle less than 1/16".

So all you folks seeing 1/8", 1/4", 1/2" changes in axle positions, well, it's not because of the change in geometry. Time to find out what part is not installed correctly or make sure those aftermarket parts are actually the stock length as the factory parts.

itchy

We know that the bar is 41" long, and I thought the angle was measured as 4.9 degrees. Going off of those numbers, the stock height "b" is ~ 3.5" no the 1 3/4 inches in your calculation.

If the angle is not 4.9 degrees, the value of b is different. How did you measure the height difference? Not calling you out, but obviously one of the measurements is wrong.

Its raining here, so that rules out getting unde the car and measuring. I'll just have to wait until tomorrow

SoundGuyDave

Okay, something is off here... I'm absolutely positive of my measurements, and I'm sure RRRoamer is as well, but there are pretty dramatic differences.

If an independant observer could also take measurements, it would be appreciated. I got a rod length of 41-5/8" measured bolt-hole center-to-center on the stock piece (MY2008). I also got an inclination angle of 4.9 degrees (measured in the center of the panhard rod, with a digital level magnetically attached to the rod), which, with the formula (thanks, Itchy!!)"=SIN(4.9*PI()/180)*41.625" plugged into my spreadsheet, returns a vertical separation between the ends of the panhard rod of 3.555", more than double the 1.75" separation indicated above. The formula "=COS(4.9*PI()/180)*41.625" returns a horizontal separation value of 41.473". If we adjust the vertical difference to be 1.805 (3.555 stock height, less 1.750 drop), and use RRRoamer's formula of (A^2 = C^2 - B^2), we thus have ( A^2 = 41.625^2 - 1.805^2) or A^2 = 1729.381, with a sqrt of 41.586. 41.586-41.473 yields a difference of .113", roughly 1/8".

The math is right, but I think the "given numbers" are off.

Conceptually, I agree, but there is no way this is possible. If you simply measure the height differential between the bumpstop and frame, and compare it to the height differential between the panhard rod mount bolt centers, I think you'll find that there is less space for the axle to move up than there is vertical distance between the mount points. Therefore, the panhard rod CANNOT be parallel to the axle housing, but MUST have a slope. This would be like an imaginary number, if you follow.

We need some more measurements... you and I are coming to the same mathematical solutions, but with different given numbers, and radically different results.

In the case of our "test" being discussed here, the ONLY change being made are the springs, with the only effect being vertical ride height. Even if the LCAs were "wrong" in length, that would have no measurable impact on the lateral location of the axle. In fact, the only suspension component that would affect axle location (laterally) would be the panhard rod. Nothing else could do that. So, all the people with offset axles would have to be the victims of excessively long aftermarket rods. Occam's Razor would preclude that one...

On my car, the rear suspension change consisted of adjustable LCAs, adjustable UCA, adjustable panhard rod, 1.75" drop springs, LCA relocation brackets, and D-spec shocks. As the stock parts came off, the adjustables were set to factory length, by inserting threaded rod through the ends of both pieces. Once the whole kit was installed, it was instantly apparent that the axle was offset visably towards the driver's side. I loosened the whole thing up, and then started tuning by centering the axle laterally, then fore-and-aft to center the springs in the towers, while simultaneously setting pinion angle. On my buddy's car, we installed fixed LCAs, relocation brackets, the same 1.75" drop springs, and an adjustable panhard rod. This time, we put the car on stands, pulled the lower shock mounts, calipers and panhard rod, then lowered the axle onto stands while swapping parts. The LCAs and brackets went in with no problem, the spring swap was fine, but when we loaded the axle to connect the panhard rod and torque the LCAs, it was obvious that I had to shorten the rod to get it to fit with the axle centered under the car.

RRRoamer

The only measurement I am willing to give you is the panhard bar length. It is pretty difficult to measure while laying on the floor with my head stuffed under the rear valance holding a tape measure. Of course, that extra 5/8" only decreases the offset, not increase it.

The measurements of the axle pivot height and panhard bar frame mount height are a lot more precise. They were taken with a tape measure with the car in my garage (which is level side to side, but it has a drainage slow front to back) from the garage floor up to the center of each pivot. As I'm able to get my head under the rear valance right at each pivot point, I have a very clear view and it was very easy to center the end of the tape on the bolt at each pivot. The frame end was only 1/4" higher than the axle end and the bar was very nearly horizontal visually.

With my car at least, there isn't any problem with the panhard bar being horizontal. It has definitely been horizontal (and beyond) at times when I have been going on road trips with a lot of baggage stuffing the trunk or when hitting bumps while going down the read. No "imaginary number" here...

The only thing that might explain this discrepancy is my assumption that the back dropped 1 1/2". I know the front dropped that amount as I measured the drop at 1 3/8" when I first installed my springs. At that time, my floor jack would JUST squeeze under the center hump in the k-frame to jack up the car. Since the springs have settled, the bottom of the hump is 1/8" below top of the saddle on my floor jack (with the jack all the way down of course...). If I push up on the nose of the car, I can ALMOST get the saddle to clear the hump. But not quite.

I originally measured the drop at the rear as well. My memory says it too was 1 3/8", but that has been a couple of years ago. The front I remember because of the clearance on the floor jack. If it settled more, or was dropped more, than it would change the "before" measurement and increase the offset.

Oh well. Either way, I'm happy with the axle centering on my car with my drop. At one point, I tried to measure the offset on my car using a plumb bob referenced from the center of the wheel well arch and a tape measure to measure the distance from the line to the center cap on both left and right wheels. They were pretty much exactly the same on each side. Again, this measurement was taken with the car in my garage which is level side to side.

And I just ran the numbers... Assuming the rear dropped 2" instead of 1 1/2" and using the more accurate 41.625" panhard bar length, I STILL only get an offset of 0.0601", which is slightly LESS than 1/16" A 2 1/2" drop would have only offset my axle 0.0902", or about 3/32".

I'm not sure what was up your car or your friends car, but I do know that with my car and the geometry of the panhard bar it has, lowering the car just didn't move the axle enough to be noticed. I'm also quite sure of my measurements and my trig (my background is BSEE and BSME and this is first year engineering math...).

Of course, what REALLY matters is if YOU are happy with how well the axle is centered on YOUR car. I'm happy with mine.

itchy

Roamer, I am also a ME. I don't really care what the actual number are, It was just an interesting problem since more than one person looked at it and we all came to slightly different answers. It's actually just highschool trig.

It's still raining outside, so I am still unwilling to crawl under the car and take a look. Some day I will get under the car and measure myself. Odd that the height measurement and the angle measurement don't jive?

SoundGuyDave

If the end result is that everybody's happy, then I think all is good. My background isn't in EE or ME, but I do have a BS in Music Business, with two years of Honors Calculus, and a very strong science concentration, and I also find it odd that the angles and measurements simply don't agree. The ultimate point here, however, is NOT who is right, or who is wrong, but rather whether adjustable-length panhard rods are a potentially necessary part for proper suspension alignment when lowering an S197. On that point, at least, the jury is still out. I did take pix of the frame-side ride height stock, and the angle measurement on the stock rod, which I will be happy to post if someone wants them, but I would still appreciate somebody with a stock vehicle taking measurements and pix so that we can gather more data for the purposes of putting the debate to rest. There appears to be an overwhelming amount of subjective and objective information pointing towards the need for adjustment with larger drops, and if that is going to be refuted, we need to have solid, hard measurements. To all who would be willing, please post the following data:

Axle-side panhard rod bolt height to a flat floor: To the nearest 1/8" if possible.
Frame-side panhard rod bolt height to a flat floor: To the nearest 1/8" if possible.
Panhard rod length (if adjustable), measured bolt-center to bolt-center: To the nearest 1/8" if possible.
Wheel-arch plumb to rim or centercap distance on each side, to the nearest 1/16" if possible.
Tire size and wheel diameter.
Brand and "drop" amount for springs, if not stock.
Wheel-arch to floor (largest measurement to bottom of arch lip)

I'm particularly interested in measurements for Shelby GTs, as they run a drop with the stock panhard rod.

If we can get a big enough sample, we can come to a definitive conclusion.

My car is coming back from paint on Tuesday, and I'll measure and post my data. I'm willing to bet a six-pack of my favorite adult beverage on the outcome...

itchy

Roamer,

I am also a ME. I was just interested in finding out what the actual numbers are. It makes a nice trig problem to solve. It's odd that the angle and the height measurement don't jive.

Sorry for the double posting, my internet was acting funny, and wouldn't update the page.

RRRoamer

My 1 3/4" is an assumption based on a MEASURED 1/4" height difference between mount and axle mount position. The 1 3/4" is only an assumption because I have to assume the drop was actually 1 1/2" instead of 1 1/4" or 1 3/4" or what ever the actual might have turned out. But is not a calculated value per se.

If you go by the angle of the bar, then you also have to get the angle of the axle to get the relative angle between the two parts. Something to keep in mind when you do get back under your car.

RRRoamer

It is a bit strange that things are not lining up. With mass produced cars, these parts are, for all intents and purposes, the same, so we should all have the same geometry when stock.

On thing you might want to keep in mind is that when you are working with very small angles, the precision of the measurement becomes critical. We have two sides of a right triangle (or more to the point, two sides of two different right triangles after we lower it) and it is very easy to calculate the third side, so I'm not sure why you would want to introduce potential error by using a more "sensitive" measurement like angle.

One thing to keep in mind folks, the people that designed these cars are not stupid. They know about the geometry changes as the ride height and axle position changes. There is no WAY they would design a system that pushed the axle all over the place as the car went over bumps. That would kill the handling and give the car a very weird feel going around corners that were not perfectly smooth. They just wouldn't put something on here that was super sensitive to axle height.

itchy

Roamer,

What we need are the stock distances from the mounting points to the ground. I agree that knowing 2 of the three sides in the right is the best way to calculate the third if you are going off measured values.

My car is still stock for the next two weeks. I'll have to find a flat surface or use a level and make a flat surface under the car. unless someone wants gets to it first.

RRRoamer

Itchy,

That is what I did. I used a tape measure from the concrete up to the center of the mounting points. The car was in my garage which is level side to side (to within 1/4" across the span). The tape measure was sitting on the concrete with the blade locked and adjusted until it was sitting right in the center of the mounting point. I would then pull the measure out and read the measurement. With my car, the bar mounting point was only 1/4" higher than the axle mount.

Hopefully, someone has a stock car that is at least 6 months old or so so the springs have yielded all they are going to yield.

On a (slightly) related note, my D-spec struts and shocks from the group buy came in today, so I know what I am going to be doing tomorrow...

itchy

Roamer, I got under the car this weekend, Just wanted to get a rough estimate, so I didn't check if the ground was level, but the height difference was about 2 inches. I would say that lines up with your 1 3/4 inch estimate based on your lowering spring drop.

On a side note, what did you do about the bumpstops when you lowered the car?

Glenn

don't know about all your math and measurements but my rear end was off after lowering. The adj panhard bar fixed it . I can't remember by how much but it was off. maybe when I get a few minutes I'll throw the stock bar back on to refresh my memeroy.

blkstang06

Yep! their making this much more complicated than needed! when I dropped my car, 1.6' in the rear I had a 3/8' shift to the driver side (left) thats not total variance that what was needed to bring it back to center!

RRRoamer

itchy, I didn't do anything about the bump stops. I'll take a look at them today (heading out to install the D-Specs after I type this).

Glenn, blkstang06, The whole reason I first replied to this thread is because the geometry of the panhard bar would NOT allow the rear end to move that much while lowering it a couple of inches. But something is definitely going on as there are way too many people that need an adjustable panhard bar after lowering, but there are a lot of people that DON'T need one either... I'm fairly sure we all have the same stock geometry... I hope!

SoundGuyDave

I'm sorry I can't get the measurements to you yet, the body shop was supposed to have my car ready yesterday, but it's STILL not done... the paint is on, though, so it's just a matter of curing... AS SOON as I get it back, I'll get a complete set of measurements posted up.