Panhard rod design: Attn Curtis

[MGBV8]

I have always heard that part called a wishbone, as in, wishbone 3-link.

I have seen the wishbone oriented in both directions & top or bottom.

[MGBV8]

Question for Scott & Curtis:  What was your reasoning for mounting your panhard rod that distance below axle center line?

[BlownMGB-V8]

What is the reasoning behind putting the wishbone under the diff giving better handling? Also is there any reason why the single pivot point being at the differential instead of at the body any better?

Jim

[MGBV8]

I do not know for sure, but I do not think it matters.  Certainly not with your chopped up grocery getter & your emergency tires.  

[Scott Costanzo]

Question for Scott & Curtis: What was your reasoning for mounting your panhard rod that distance below axle center line?

I wanted to play around with reducing the roll axis inclination to see how things would react. The car seems to corner really flat now with minimal body roll so I think it was a positive change. It's basically a stock suspension with a 3/4" roll bar up front. I was enjoying it this afternoon going around a roundabout.

[MGBV8]

Good suspension tuning, Scott!

Without a rear sway bar, that is correct direction.  Lowering the panhard rod below the axle centerline will more the car towards understeer.

Curtis can likely go a little lower since he has more weight on the rear wheels.

[Moderator]

Even among MGBs, we have a great variety of set-ups and great differences in how we intend to use our cars. These cars left the factory with front roll center height close to ground level and rear roll center height close to axle centerline. (Front roll center was defined mainly by the relative inclination of upper vs lower wishbones. Since MGB didn't have Panhard rods from the factory, rear roll center was defined mainly by leafspring mounting points.) It's been roughly 45 years since the last MGB left Abingdon, so things have changed. At the front... if a particular MGB has been lowered by sagged or deliberatly shorter springs, its front roll center height is likely significantly below ground level. Fitting Killer Bees style dropped spindles (as I did) will move that upward, but not as much as Headley style dropped spindles. Entirely different front suspensions (e.g. FastCars) likely place the roll center even higher. But at the rear, regardless of whether you've got leafsprings or coilovers, until/unless you deliberately move the rear roll center it's probably still near axle centerline. A line drawn from front roll center to rear roll center defines roll axis. On MGB, it's pretty steeply inclined compared to other cars and particularly ones known for good handling.

Carl's first question: (paraphrased) "why did I choose to lower mine 3.5 inches?"   I've long felt that if you're going to the trouble and expense of fitting a Panhard rod anyhow, it'd be silly not move the roll center down significantly. It doesn't cost any extra! I expect the bigger roll-couple (i.e. height difference between center oif gravity and roll axis) will provide me more feedback and the flatter roll-axis should make handling more consistent in various cornering situations. You can read a hundred better explanations than I'll write about how roll center height affects load transfer and tire grip. Most credible sources are however discussing "real racecars" or more modern sporty cars that have some sort of independent rear suspension. Where class rules allow, roadracing cars usually have much lower rear roll centers than MGBs. Nearly all cars with independent rear suspension much lower rear roll center heights too. Even so, you'll see people discussing the relative merit of shifting roll center lower.

I'm gonna decline to digress into discussions of spring rates, anti-sway bars, etc.

Jim asked "Why put the pivot on the diff rather than the car's body?" The most obvious reason is that doing so puts the pivot point a very predictable and stable height. A pivot on the body would bounce around, relative to tire contact patches. (Jim: you could use your project car's existing leafspring mounts to mount the wishbone. Installing the wishbone would thus be easy. And don't you want to ditch the leafsprings anyhow?)


(edited to correct grammar and improve clarity)

[Moderator]

Carl wrote: "Curtis can likely go a little lower since he has more weight on the rear wheels."

We'll see about that;o)

[BlownMGB-V8]

I was thinking the simplest lower links would duplicate the front half of the leaf spring with a tubular link to a pivot below the axle. Since I'm using a banjo housing welding on brackets is easy of course, but I have to be mindful of vertical limitations since the suspension has to collapse into the body for loading. To the point where there is about 1/2" clearance between the body and the ground and I may even need to revert to CB front spring hanger locations and shave the jacking points and driveshaft pan. With a 22.3" tire this means using every fraction of available space. We had already removed the filler panels over the axle on the '74-1/2 tub and it'll need every bit of that space and possibly a bit more. Plus I'll need perches for the air bags. The original plan of mounting the bags where the bump stops went doesn't give enough compression, so they will probably need to go outboard if the tire allows clearance. I have one of the wheel/tires on the way.

So the wishbone would set the roll center height? Is that what you are saying? If that is the case I can see the point. And a bracket on the bottom of the banjo bulge doesn't really affect anything important here. But if that's the case why is it common practice on a 4 link to triangulate the top links rather than the bottom ones?

Jim

[MGBV8]

I don't know if the wishbone always set the roll center.  Best I can tell, nothing is simple about suspension design.

I don't think any of this much matters on your little clown car.  

This looks to be right up your alley, Jim:

https://www.lotuscortinainfo.com/?page_id=1799

High tech wishbone:

https://youtu.be/cjZRhL0WMlg?si=JRt2qs3DzF6ChYrf

https://www.youtube.com/watch?v=SibLgsG__-I

[Scott Costanzo]

Curtis, funny how similar our approach to the panhard bar is.

Scott

[MGBV8]

My panhard rod is adjustable on the body side.  I see Scott's is, as well.  Can't tell on Curtis'.
 
I believe it is important to have the panhard rod level with the full weight of the car & driver.  To accomplish this, I drove my B up on Les Shockey's trailer that has an open center.  Les sat in the driver's seat while I crawled underneath & leveled it up using a bullet level.  Trailer was very close to level.

[Moderator]

Coilover shock absorbers...

[MGBV8]

What shocks are you using, Curtis?

[MGBV8]

Sometimes, I wish I had studied mech. engineering with slant towards vehicle dynamics & suspension design, instead of electronics.  It can get complicated in a hurry with trigonometry & calculus.

As a starting point in suspension tuning, the front roll center height should be about 15% of the height of the center of gravity.  The rear roll center height should be about 30%.  

Raising the rear roll center height from there aids turn-in, but also starts inducing oversteer.

Lowering the rear roll height moves towards understeer.

Everything affects everything dynamically. So, each vehicle will likely need slightly different tweaks.

The above is for RWD cars with no aero aids.

[Scott Costanzo]

My panhard rod is adjustable on the body side. I see Scott's is, as well. Can't tell on Curtis'.

I believe it is important to have the panhard rod level with the full weight of the car & driver. To accomplish this, I drove my B up on Les Shockey's trailer that has an open center. Les sat in the driver's seat while I crawled underneath & leveled it up using a bullet level. Trailer was very close to level.

Carl, mine is adjustable on both ends. You can kind of see it in the picture if you look closely.

I tried to adjust mine so the rod is level at the halfway point of suspension travel. I think that is supposed to minimize the deflection the rod causes.

[BlownMGB-V8]

I wonder where the roll center is for the Jag IRS?

I also wonder how a triangulated 3rd link affects roll center? Might as well optimize it if I'm building it.

Jim

[MGBV8]

"I tried to adjust mine so the rod is level at the halfway point of suspension travel. I think that is supposed to minimize the deflection the rod causes."

With our long panhard rods & limited suspension travel, any lateral deflection is minute.  We will never feel it.

[Scott Costanzo]

I believe it is important to have the panhard rod level with the full weight of the car & driver.

By the same token, this approach is a waste of time  in my opinion. You will feel no difference with it in that position.

[MGBV8]

Everything that I have read indicates the panhard rod is to be placed level at ride height.  Adding weight to the driver's side, tends to make the bar not level.  Not being level will increase the lateral deflection.  As you said, probably will not feel it with such a small amount.  A larger angle with certainly move the rearend.

[ex-tyke]

A better approach is to ditch the 3-link set-up and go to a 4-link and scrap that problematic panhard rod!

[MGBV8]

"I wonder where the roll center is for the Jag IRS?"

You will have to find it like for the front suspension.

"I also wonder how a triangulated 3rd link affects roll center?"

No easy answer for finding that roll center, either.

[MGBV8]

Problematic panhard rod, Graham?  I have been using my DIY panhard rod for 20 years without issue.

Your rear roll center is fixed at what ever Bill Guzman designed into his triangulated 4 link. They are known to bind.  Probably not an issue with the limited articulation of your rear suspension.

As Scott has shown, rear roll center can be adjusted with a panhard rod.

[Moderator]

"I also wonder how a triangulated 3rd link affects roll center?"

No easy answer for finding that roll center, either.

Not really that hard. First I'll point you in the general direction:

from "Race Car Vehicle Dynamics" by Milliken and Milliken

And then I'll give you my 2 cents:

In practice, whether (a) an A-arm converges at a spherical bearing (or equivalent) that's secured to the rear axle housing as shown in the figure, or (b) a pair of links converge toward some point in space behind or ahead of the axle (as on many 4-link suspensions) doesn't need to confuse the issue. Technically, roll center height is found by projecting the convergence point back to the vertical plane the rear tire contact patches are on.  Other links that constrain the axle's movement affect the correction. However, that correction is small beans! On any car I care about, the height of the point of convergence is very nearly equal to the actual roll center height.  And, because it's a close enough approximation, it's fair to generalize that:

(1) Attaching an A-arm to the axle ABOVE the 3rd member (as on some dragracing cars and some rock crawlers) would put the roll center VERY high. Bad for cornering.

(2) Using converging links above axle center line (as on many 4-link suspensions) puts roll center height above axle centerline. Not great, although other aspects of the suspension might be good.

(3) Attaching an A-arm wishbone to the axle BELOW the 3rd member (as on a Lotus Seven or on Jerry Richards' MGB GT V8 racercar) puts the roll center height quite low.

[Moderator]

What shocks are you using, Curtis?

Shock travel seems to be my more pressing concern. In the photos above, jackstands held the axle at static ride height. I bumped the axle straight upward, and found that the shocks would compress 3.25" before the spherical bearing of the top link contacted the (unmodified) body. (If I modified bodywork there as done on Ken Biermann's car, there could obviously be more suspension travel.) Then, however, I put brakes/wheels/tires on and bumped one wheel at a time upward, simulating roll. In this check, my hypothetical shock on the bumped side compressed about 4.25" before contact between axle and body. Based on this - not thinking yet about extension requirements - my initial reaction is that 7" shocks probably would be more suitable for my suspension than the 5.2" ones I already have. I'm investigating some longer spring options accordingly. The good news is that I don't have clearance issues between the coilovers and anything else. They'll be pocketed up neatly into the back seat area in any event, and on account of my roll cage I've got lots of options for attachment.