Watts Or Panhard Location Help

[BRIAN]

This is on a truckarm style rear suspension. Question I have is does it matter if the panhard or watts is located in front or rear on the axle? I have a tight situation where I am probably have to run it in front of the rear assy. I am trying to keep the panhard at axle height.

Does the distance from the axle fore or aft have any effect on handling? Street car.

Thanks

[baz67]

Yes, it can be done(a phb at least, not sure how you would fit a watts in front of the housing), but I would not do it. IMHO, it would be to hard to get the RCH in the proper location. The more forward you move the lateral location the steeper the slope of the rear roll axis becomes and it will raise your RCH. This also depends where your front pivots are on the truck arms. In this case the higher they are the better your outcome will be. I would guess there would be driveshaft to PHB interferance concerns as well.

So yes it will have an effect on handling because it will change your RRCH.

[vcho455]

If you go to stockcarproducts.com, and look at their chassis tech section, they address placment of the locator bar, and how it will affect your handling. Good luck!

[wendell]

The SCP info is based on a road composed of a never ending string of left hand turns.

You've stated that you want the RC at axel height. The realestate in front of the diff at axel height is already spoken for by the drive shaft. A watts would have to be offset with a diff mounted bell crank and a p-bar would have to be a long J-bar. Neither of those are atractive options. I'd mount it in the rear.

[BRIAN]

I guess I should have also noted that the car has a quickchange rear. The problem is either running a 12" baracket to have the panhard bar clear the rear of the quickchange and keep the panhard at axle height.

You are correct in that mtg a straight bar from the front would obviously not be able to be done due to the driveshaft being in the way. But the pinion sits quite low so it really would not be totally above axle height but somewhere on it and above.There are tons of brackest pre fabbed but they are all to run the "J" bar style bar.


So my options are:

panhard bar 12" extended from housing to the rear at axle height.

Front mtd bracket with either "J" bar or slightly above pinion height if straight. If I run long panhard bar it really wouldn't sit no more than an inch or two above the axle height.


Once again this is street car. I just do not want any weird handling I am not concerned with the best calculated piece on paper that really wouldn't efferct normal road use.

Thanks

[Mean 69]

I believe Hoerr Racing Products used to sell a Quick Change Watt's bellcrank, take a look at HRPWorld.com, great place to do business with.

Mark

[wendell]

There are tons of brackets available for a QC. I've seen a bunch of Pbars and Watts run off the back. To install truck arms and a QC but then not really care where the RC ends up because it's only a street car seems a little counter intuitive. With the $/effort you have in this already I wouldn't scab a p-bar on where ever it's easiest. Resources like left hander, HRP, SCP, Howe, Coleman... will give you the flexability to mount a pbar/watts where you want it.

Under slung rear clip, horizontal chassi mount wattts under a QC... now we're talking! Some day...

[Mean 69]

Under slung rear clip, horizontal chassi mount wattts under a QC... now we're talking! Some day...

Kind of like this? Okay, so it's a 9", but you get the point. Hey, where are the upper links on that four link car!?!?! Oh!!! That's right, real race cars use three links for stick axle performance (shameless plug, naturally). This is the ACS Trans Am Mustang that Boris Said ran last year at Long Beach. Alas, no more Trans Am racing, pass the tissues, please. Oh well, at least we still have the Speed WC GT series.

I agree with Wendell, if you have all that good hardware on the car, regardless of the "intent" of usage, don't waste it by putting, of all things, a "J bar," or even a short Panhard, take the time to do it right. Please!

Mark

Darn, need to resize the file, too big......

[wendell]

Way to operate the computer there mark. Got my hopes up. I saw some older Riley Scott chassis with that set up. They were on 3ls and satchells. Very cool. With the surge in ASA and LMSC road racing I fear that the V8 tube frame stuff is dead for good.

Im interested to hear how the QC holds up on the street. With that set up you should run wide fives. If you're going to be a bear, be a grizzly.

[BRIAN]

I never said I was doing anything half ass'd. There are tons of brackets but they are for short off the pinion mtg brackets or for J bars. The quickchange rear housing sticks out about 12-13" so to mount it from the rear requires a bracket that sticks way out there and to be honest f's up the view of the QC.

What I meant was that I do not want to get into some heated debate over some issue that shows on paper but wouldn't really effect the handling. I know you guys know your stuff and that is why I am asking. I do not want to just slap it on there.


I think the front mtg is going to fit this application the best. The rear is narrow and there is not much space between shock mts and QC housing so off the shelf rear 13" brackets are a no way.

The front pinion hangsow where I think I can run a panhard bar almost in line with the axle line . THIS IS THE PREFERRED WAY TO RUN A PANHARD BAR??? Only difference is it will be front mounted instead of rear.

I will check the above web sites but I have been on just about all of them.


In regards to the QC it was $2650 built to my specs. It is a Winters Street Rod unit with there geared posi unit. As per Winters it will handle all the HP I can throw at it. Only downside is the gear wine. That has been lessened with gear polishing. Give them a call they are still family shop that only takes cash or money order.


Thanks for the help.

[BRIAN]

MEAN 69 you suck brother. Damn that watts bracket at HRP would have made it too simple. I will call Winters in AM to see about swapping onto mine. Thanks Brian

[wendell]

I didn't know the view of the QC was a priority. In that case I'd run a pinion mounted watts. Should be able to minimize the offset from centerline to a few inches.

[Mean 69]

Sorry, Brian, I just re-read your post in a bit more detail. Not trying to imply that you are not intent on doing it "right," sorry if it came off that way.

I read the word "J-bar," and although it's clear this is merely an option, my extremely strong advise is not to take this approach. You've probably read that the roll center height, and the corresponding roll axis will move up and down as the suspension articulates, which in most folk's opinion is not a really good thing (in my opinion, it is a really, really bad thing, by the way). That'd be the least of your issues with a j bar though. Biggest issue would be side to side axle migration in bump/droop because the bar is so short, never mind it isn't centered (i.e. pivot locations) which would introduce some funk into the setup, the roll center would migrate laterally as well as vertically (I "think," I reserve the right to reverse this statement if I take the time to think about it more and discover my hunch is wrong). The lateral movement of the axle assembly will migrate relative to the cosine of the angle that the J-Bar pivot points, in front (rear) view. By their nature, j-bars are short, so a given vertical translation of the rear suspension (and corresponding rise/fall of the axle pivot on the j-bar) will be relatively large.

There's nothing wrong with placing the Panhard bar 12" or so behind the axle centerline, as long as the brackets are rigid enough to handle the relatively high torque that this offset will create, keep in mind that virtually all of the lateral load is transmitted through these stuctures. If you have the real estate, and make the brackets strong enough, this is completely fine. Find a good mechanical engineer for structure advise if you aren't qualified to make this call, please.

The Watt's configuration with the bellcrank on the axle assembly is a decent way to go, it will prevent virtually all lateral movement, but the roll center height moves vertically the same amount as the suspension travel. Probably not a big deal at all on the street.

Of course, the absolute best answer to the question is to give me your QC rear end, sell the truck arms on e-Bay, and buy the Lateral Dynamics 3-Link setup! That rear axle assembly would work very well in our race car project..... Okay, just kidding, sort of.

One more thing, don't be afraid to drop the height of the roll center down a few inches. All things being equal, a lower roll center will reduce jacking forces (good thing) at the slight expense of increased roll, which of course is simple to tune out if necessary. Unless you have a really odd truck arm configuration, I'd suggest that roll steer issues will not be a problem at all. 9 - 10" is probably fine.

Best of luck!
Mark

[Norm Peterson]

Don't forget that a PHB also moves vertically at its midpoint relative to both the chassis and the axle (clearance issues with respect to the front of the pumpkin).


Expect a front-mounted bar to behave a bit differently. Among other effects, expect the geometric roll center to be located slightly higher than the PHB rather than slightly below it as is the usual case with a rear-mounted PHB.

The August issue of Mark Ortiz's Chassis Newsletter discussed this a bit further. Since reproduction for free use is actually encouraged . . .

(Reader question)

PANHARD BAR LONGITUDINAL LOCATION

What is the effect of locating the Panhard bar in a live axle rear suspension a) behind the rear axle, or b) just ahead of the rear axle, or c) even further forward in the car? For example, the Frankie Grill All-American Race Car chassis is now attaching the Panhard bar to the right-side rear door post. From there the bar runs across the car and about eight inches rearward, and attaches to a bracket extending about eleven inches forward from the left axle tube. These cars appear to be dominating at the current time.

These cars are a variety of Super Late Model, running on paved ovals in the northeastern US. Longitudinal axle location is by a form of 3-link system, with dual, compliant upper links. One upper link reacts tension forces occurring under power. The other reacts compression loads occurring in braking, including engine braking. The two links generally have different angles. They both have rubber or urethane biscuits in them that can be varied to change the rigidity of the link. The rules prohibit compliant lower trailing arms, which are commonly used at the right rear where legal.

(Mark's response)

The effect of locating the Panhard bar forward or back with respect to the axle depends on the layout of the rest of the rear suspension system. In some cases there is little or no effect; in other cases there can be a significant effect.

To understand this better, it is helpful to introduce the concept of the rear axle axis of rotation in roll. This is sometimes called the axle's roll axis. There is nothing wrong with calling it that, provided one understands that it is not the same as the the car's roll axis, the line connecting the front and rear roll centers.

The axle roll axis is a notional line about which the axle moves in the roll mode of suspension movement. The point where this line intercepts the rear axle plane – the vertical plane containing the rear wheel axis – is considered the rear roll center.


The axle roll axis is usually constructed as a line connecting two points: the instant center of the links or arms that locate the outer ends of the axle, and a point taken as representing the height and longitudinal location of the Panhard bar or equivalent lateral locating device.

In some cases, the assignment of these points can be rather tricky, and may call for some approximation. For example, it is quite possible that there may not really be an intersection point of the lower trailing arms in a three-link system. The link centerlines may converge a bit toward the front of the car, and may have an intersection in plan view, but they may pass over and under each other at that location, rather than truly intersecting. Or they may be parallel in plan view and therefore have no intersection, even if they lie in the same plane.

In the former case, it is reasonable to take as an assumed front point for the axle roll axis a point midway between the two link centerlines, where they pass over and under each other. In the latter case, in side view the axle roll axis is parallel to the trailing arm centerlines, or an average of their inclinations if they are not parallel in side view.

In the former case, we need a second point to determine our axle roll axis. In the second case, we know the inclination of our line, but not its height, so we need a point to establish that.

In both cases, we take for this a point representing the height of the Panhard bar or equivalent lateral locating device. In a passenger car, with a Panhard bar, the usual practice is to assume that the car has close to 50% left weight, and take the point where the Panhard bar centerline intercepts the vehicle center plane. If the Panhard bar is centered in the car, this will also be the midpoint of the Panhard bar.

Things get a bit more complex when the c.g., or the Panhard bar, or both, are offset significantly to the right or left. Here, we have a choice of two methods. We can take the point where the Panhard bar centerline intercepts the sprung mass c.g. plane (the longitudinal, vertical plane containing the sprung mass c.g.). Alternatively, we can take the Panhard bar midpoint.

When the Panhard bar is significantly off center, and significantly inclined, as in many dirt chassis these days, the heights of the c.g. plane intercept and the bar midpoint can differ by as much as two or three inches. Which method is more correct? They are both reasonably correct, provided we apply them properly. If we use the c.g. plane intercept, we do not make an additional correction for the vertical component, or jacking force, resulting from the Panhard bar inclination, when modeling roll behavior and wheel loads when cornering. If we use the bar midpoint, we have to include the jacking force in our calculations. The former method is simpler, and yields a good enough approximation for most purposes; the latter is more rigorous and accurate, but more complex.

In any case, by some rationally defensible method we choose an effective acting height for our lateral locating mechanism. We now still have to assign it a longitudinal position. If we have a Panhard bar that runs straight across the car in plan view, this is presents no difficulty. On the other hand, if the bar has significant plan view angularity, we have a bit of a puzzle. I think the right

approach for assigning the x-axis, or longitudinal, coordinate is to use the x coordinate of the bar midpoint in all cases.

We now have vertical and longitudinal coordinates for a point that is a reasonable approximation of a lateral force coupling point between the sprung mass and the rear axle assembly. We can now draw our rear axle roll axis, or axis of rotation in roll, in side view. If we have an exact or approximated instant center for the longitudinal locating links, we draw our line from that point through the lateral force coupling point. If instead we know the inclination of our axis of rotation, we draw a line at that angle, passing through the lateral force coupling point.

Once we do that, we can see where this axis of rotation intercepts the axle plane, and we can take that as our rear roll center when modeling roll and wheel loads in cornering.

Now, returning to the original question, what happens to the roll center when the lateral force coupling point moves forward or back? It depends on the rear axle roll axis inclination angle.

If the axle roll axis slopes down toward the front, then moving the lateral force coupling point forward while keeping it at constant height raises the rear roll center. If the axle roll axis slopes up toward the front, the effect reverses: moving the lateral force coupling point forward while keeping it at constant height lowers the roll center. If the axle roll axis is horizontal, then we get no change in rear roll center height from moving the lateral force coupling point forward or back.

There are other effects as well, when we move the Panhard bar forward or back. If the axle rotates under power or braking, as it does when the upper link is compliant, the end of the Panhard bar that attaches to the axle rises or falls as the axle rotates. That means the roll center rises or falls with power or braking. The further the Panhard bar is from the axle centerline, the more it rises or falls, and the more the roll center rises or falls.

When the Panhard bar is far ahead of the axle, as the questioner describes, the roll center rises under power and drops under braking. That makes the car tighter (adds understeer) on entry and loosens the car (adds oversteer) on exit. I don't see how that would make a car faster, but it would make it different. There are other ways of controlling the car's balance during entry and exit, so with the right combination overall, such a car could win races.

One advantage of having the Panhard bar really far forward, if you’re going to have it ahead of the axle at all, is that it's easier to keep the Panhard bar out of the way of the driveshaft, without putting a bend in the bar. There are other packaging implications as well. It becomes harder to find room for the oil tank and the battery behind the driver. Overall, I would have to judge this idea a mixed blessing, and ascribe any success to users having the overall combination dialed in.

Norm

[wendell]

Awesome. Norm, you have a talent for taking very graphic ideas and explaning them verbally clearly. Mind if I ask what you do/did for a living? regardless, that was a great post. Interesting that a forward mounted locator can make a car tight in and loose out. That's rarely the fast way around a track yet the majority of dirt cars I see run a forward mounted j-bar. I guess they can conpensate for these attributes with spring links and all the other crazy stuff. Again, thanks for taking the time.

[Norm Peterson]

I can't take any credit for the above quote, as that's straight from Mark Ortiz. I'll edit the previous post to make this more apparent.

Me? Sometimes I'm an engineer, sometimes I'm not . . .

[Mean 69]

Mark Ortiz is one heck of a gifted engineer, and does a terrific job explaining things in written form. He writes a column in Racecar Engineering called "The Consultant," really terrific stuff. I have to get on that darned distribution list he has, lots of terrific articles like the one posted above.

Mark

[wendell]

I thought just the bols was the Ortiz quote. Either way I'm still impressed with your ability to communicate knowlege.

[Norm Peterson]

Fixed it some more (work "interfered").

[BRIAN]

Thanks guys, tons of good info in the above posts.

I have to say this is why I usually order a system and go with it. The problem is I was tired of the poly 4 bar systems where the bushings just do not hold up that well. Also I honestly feel that some of the MFG'S are simply using fit as their guide to placement rather than using theory. I understand that this applicable and even so to some degree in my situation.

I see tons of products on these pages directed towards the 1st gen Camaro guys. I just can't understand the not wanting to expand into the so called Street Rod crowd that is using basically the same components but in their set up. Most Camaro subframe and rear rails arrangements are close to common used parts. I understand there wood be some reengineering or maybe even some generic parts that would require adaptation. Open up Street Rodder and by the time you hit 1/2 way through you have seen 20 adds for Mustang 2 stuff. Not one of the clip MFG's advertize there??? I have a Wayne Due 1st gen frame with all DSE components as I just do not like the quality of bushings and components in the M11 stuff. It worked out fine and is the basis for the front of my frame. Ask Heidts how how many kits they sell. There is a big need for improved parts not to say Heidts parts are bad.

Oh well back to my application. Either a front mounted Panhard bar mounted 8" from axle centerline sitting level with the axle centerline at ride height. or a watts set up just above the pinion mounted to the face of the QC rear. I have to see what the custom watts bell crank will run. The one at HRP just sits too low as the brackets would require a length that I am uncomforable with. The whole idea is to stop side to side movement and I feel their may be some flex using that bracket in my application.


I appreciate the help. Brian