Panhard rod design: Attn Curtis

[BlownMGB-V8]

Sounds pretty good on your shocks Curtis, more travel will give a better ride and make the spring and damping rates less critical. Ground effects may become less but we don't really get much of that anyway. I base the needed stiffness on what it takes to prevent bottoming out the suspension after becoming airborne. A foot or so of air generally seems adequate for testing. More than that risks loss of control if there is any turning involved, and of course you can't accelerate while airborne.

So if the wishbone is attached at the bottom of the bulge this is generally going to give it an upward angle towards the front of the car. Sounds like that might not be a good thing. Or do the side links, being located higher then become the controlling element? I don't want any bracketry dropping below the body. Also, the closer the body ends are to the same level the more the pinion angle will change during travel. That one shown would effectively be like ladder bars. I'm beginning to see the attraction of the angled 4 link. Binding must be avoided of course.

GM was using an angled 4 link back in the 60's on their mid sized cars. (GM A body platform, from which all other platforms were derived.)

Jim

[MGBV8]

...from "Race Car Vehicle Dynamics" by Milliken and Milliken

I guess not, if you have that book.  I do not.  I have read over the years, that it is a very good one.  Quite expensive, these days.

Yes, much different results dependent whether the wishbone is on top or bottom.  I have read that for some reason it makes a difference which way the  wishbone is pointed.

Certainly not as easy to calculate as a panhard rod.

[MGBV8]

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

Sorry, Graham.  Your 4-link is fine for cruising & drag racing, but the 3-link is superior for road racing, track days, & autox.

[Moderator]

Jim, having the lower links angle slightly upward somewhat from axle-end to body-end is normal and desirable for both 3-links and 4-links. The convergence of upper links and lower links (in side view) provides some anti-squat.  There are four 3-link suspensions in the MG photo gallery section (Terry Schulte's, Dan Masters', Ivan Collins', and Ken Biermann's).  I think it's fair to say they're all similar in this respect, although Ivan's is more adjustable than the others. Here's a rendering Ivan provided with his HIWD:


The mechanics of anti-squat and a visual method for approximating the amount of anti-squat effect is illustrated in the write-up on Dudiak's McKee Formula 5000 car on the BritishRacecar site. (That car has independent rear suspension, but basic principles aren't so different because radius links function similarly...)

The dynamics of pinion angle is indeed another thing to think about. You'll note they're affected by the relative length of upper links versus lower links in side view. For this reason, upper links should be shorter than lowers. But how much shorter?

[BlownMGB-V8]

All good and much appreciated but now we are back to looking at the wishbone vs twin top links. If the wishbone is on the bottom then the side links by necessity have to become the top links. If using the spring hangers as the pivot points this will give a rather steep downwards angle I think and the convergence point will have to be somewhere near the spring hangers as well just because there isn't room to put them any lower. Unless I'm mistaken I think this will cause a jacking effect on acceleration due to counter-rotation of the  housing. Good coming off the line but not so good coming out of a corner.

Now if the wishbone is above the axle it can have a more gradual downward angle and the side links now being the bottom links the convergence point can be moved further forward, decreasing the jacking or anti-squat effect. This all depends on where the end points are located of course and is limited by interference between the body and the bulge on one end and the driveshaft on the other. With the 4 link the upper attachment points can be below the bulge and near the top of the rear shelf area, creating a line that can project almost level I think if that is desired. IIRC this is somewhat consistent with the A body rear suspension which was actually surprisingly good for an American car of that era.

The effect of a shorter upper link  upon articulation is to rotate the housing downwards. I think this should tend to increase the jacking effect or anti-squat whereas equal length arms, especially parallel ones should minimize it and some anti-squat being a desirable trait this perhaps explains current designs. Just throwing assumptions out there while trying to work my way through the theory. So raising and lowering pivot points changes the convergence point and allows tuning of the anti-squat, seen much more often in drag cars to control jacking off the line. I have ridden in a drag car that I would swear raised the rear of the body a good 3 inches when the owner hit the gas.

With the Jag IRS it seems the anti-squat is essentially neutral as far as I can tell and that's fine with me. The LCA pivot points are very close to parallel with the ground. I do not know what the effect is on roll center but the typical RC for a Jag IRS seems to be about 5-6" up which is an improvement for the most part I think. I don't know if that is dependent on half-shaft angle or LCA angle though.

Still and all I'm definitely leaning towards the 4 link just because there is more room to locate the attachment points. As Bill's kit evolved I understand he solved the early binding issues and following his approach might be the best tactic. I should probably take a closer look.

Jim

[Moderator]

Alternatively, you could build a traditional 4-link complete with a Panhard rod. Probably easier to avoid binding issues. Put roll center wherever you want it.

I cringe every time you mention 60s GM A bodies. Those aren't sportscars. First generation RX-7 is a sportscar with a 4-link incorporating parallel and widely speced uppers. Interestingly, it uses a Watts linkage instead of a Panhard rod though.

[BlownMGB-V8]

No. you are right they were not sports cars. However in some ways they were really quite good when push came to shove, and all of my education in the fine art of the 4 wheel drift came at the wheel of a somewhat prepped A body. Done right they were extremely controllable if a bit slow in response, could easily achieve neutral handling and gave great feedback to the driver. This leads inevitably to the old term of "setting the chassis" which is just another way of saying that you had to anticipate the handling and prepare in advance. Transitions were wonderful exercises in timing. But the cars were forgiving so that was a plus. Not being a sports car certainly did not mean you couldn't take everything it had to give and maybe then some. I suspect more than a few of us did just that.

Incidentally, although brakes were inadequate and the steering atrocious the suspensions were advanced enough to be considered state of the art at the time. All the right parts were there and even by today's standards they could be competitive. Curb weight was 3400 lbs which was considered heavy, not so much today. Separate frame and body was not especially stiff but wasn't bad. Mainly they were big, which it turns out isn't necessarily a bad thing either. It gave them the advantage of being able to blast over curbs and such with impunity. For a rugged driver of a car they were really quite good and an aggressive driver could make time.

So I think using their high points as an example is more than appropriate, and the double wishbone and 4 link combination did work well. They certainly were anything but delicate and were far more refined than the MGB setup. Most people never had any reason to wonder what was under there. Disc front brakes at the end of the 60's and into the early 70's certainly helped, but they never did get beyond the recirculating ball steering which was a pity, and then all the fun went away. Too bad, those cars were a blast.

I do not like the panhard rod. I feel it is adding weight, failure points and complexity when the same job could be done in a simpler way. If not running leaf springs (ox cart technology) I just don't see the sense in it.

Jim

[MGBV8]

I do not like the panhard rod. I feel it is adding weight, failure points and complexity when the same job could be done in a simpler way. If not running leaf springs (ox cart technology) I just don't see the sense in it.

Leaf springs?  I resisted using a panhard rod because, IMO, they weren't meant for leaf springs.  The leaf spring are supposed to provide lateral support. For the most part.     Panhard rods were meant for 3&4 link suspensions, not leaf springs.  Then, I grew tired of my butt feeling the rearend move around.  Bit the bullet & built a panhard rod.  Certainly does the job.

Failure points, Jim?  Don't think your cargo go kart has to worry about that.  

[BlownMGB-V8]

Perhaps. But why add another link if a properly designed 3 or 4 link will do the job? I CAN see adding one to an already existing setup, but since I'm starting from scratch I don't feel it is appropriate to start out by planning for more complexity when less can serve just as well.

I'm pretty sure there will be no room for a wishbone to attach to the top of the bulge. Could be a moot point once the wheel/tire comes in and I can start mocking it up and I may end up having to cut sheet metal there anyway to tuck the tires up into the body but I hope not. I'd like to preserve the package shelf so there is at least a little bit of cargo area. What's left of the trunk will be taken over by the gas tank I fear.

That basically leaves me with two options, either the triangulated 4 link or the 3 link with the wishbone as the bottom link and parallel top links. The center pivot joint (Rose joint?) would then be either at the bottom of the bulge which I dislike for basically esthetic reasons or at the heel plate which is the low point of the body and on the RB tub sticks down about an inch further than on the CB car. I would prefer to put it there if driveshaft travel will allow it. This would also mean fabricating new top link mounts but I'd have to do that with a 4 link anyway. Putting the center joint under the diff would let me use the front spring hangers for the wishbone however, so that would be a plus. But now ground clearance is down to less than I'd like. I should take a closer look at that, it may be OK.

Those two issues are why I have to seriously consider the 4 link despite the binding issue. I should probably try to call Bill and ask him for pointers since he's been there and done that, but mocking it up will tell me a lot. I should only be days away from that.

Jim

[MGBV8]

I think the rubber bushing are what helped the Mustang 4 link work.

With rubber bushings & limited travel, binding may be a non-issue.

[ex-tyke]

I do not like the panhard rod. I feel it is adding weight......and complexity when the same job could be done in a simpler way.

Exactly!..The way I view this whole suspension argument is that a 3–link + panhard = a 4-link. Why add the complexity of a panhard to control lateral movement when another short upper link will suffice.

With rubber bushings & limited travel, binding may be a non-issue

Speaking of limited travel, you'll agree that panhards follow an arc which can impact axle lateral movement if the travel is too large.
When I installed Bill's 4-link, I articulated the axle before adding the coilovers to see if there was any linkage binding – I didn't observe any!

[MGBV8]

Speaking of limited travel, you'll agree that panhards follow an arc which can impact axle lateral movement if the travel is too large.

Yes.  With a long panhard rod, & less travel, it is a complete non-issue. I can't feel any difference left vs right.  I could feel the rearend move without the panhard rod.

My understanding is that the 3–link + panhard is more tunable & possibly easier than a 4-link.

Not many road racers use 4-links.

[waterbucket]

Jim, try using the calculator; from Irate 4x4 four link calculator  I am about to put my home built four link together for the last time in the next week or so. I can confirm that it has no binding at all despite still using the nylon bushes that came with the Speedway rod ends, with the axle hanging on the straps I can lift either end to the bump stops with no ill affect . Energy Suspension poly bushes are now going in.
Although most of the online info is geared to off roaders I have gone with the few recommendations for road cars;
Top bars  should be 2/3 length of the lower bars, I am almost there but not quite, I didn't want to do away with the battery boxes so went with a forty degree angle to increase their length.
Having the lower bars spaced slightly closer at the front than at the axle reduces binding and roll oversteer. The calculator is very good for juggling around with anti squat and roll steer, hope this helps you if you go down this road.

[BlownMGB-V8]

Thanks Philip. I can't seem to find the right link to download it though, not sure what's up with that. Sounds like it would be super helpful.

Jim

[waterbucket]

I will post the link in full but a moderator will have to inspect it for spam before it appears, I was hoping that you might find it from the info that I posted.

[waterbucket]

https://irate4x4.com/resources/4-link-calculator.4/

[BlownMGB-V8]

I'm suspicious of this Wave Browser Pro that wants to download it... it wants to install on my computer from the looks of it. That shouldn't be needed for a download I don't think. What was your experience?

Jim

[MGBV8]

I would not do that, Jim

It is considered to be an invasive & possibly malicious web browser.

https://www.quora.com/Why-would-my-husband-install-a-wave-browser

https://www.reddit.com/r/sysadmin/comments/mlkwuj/wave_browser_by_wavesor_software/

[waterbucket]

I didn't use it Jim, I found the download somewhere else. I didn't think of any consequences of downloading from what I posted. Search elsewhere for it because it is available.
I downloaded it from here; https://www.crawlpedia.com/4_link_suspension.htm and I have always used Open office to run it.

[BlownMGB-V8]

That looks pretty useful. I'll have to take some measurements so I can input the numbers and see what it does.

Jim