Probably.
I dug into the spreadsheet a little, found that it was adding the static pinion line angle which can be positive or negative to Angle 2 which is 180 minus the angle of the pinion line. That 180 is the source of the problem but the bottom line is that a pinion angle approaching either zero or 180 is perfectly fine. If the static pinion line entry could be formatted to read as positive numbers it might cure the problem but it's easy enough to just ignore it.

What I'm seeing on the drag forums indicate that they like Anti-Squat (AS) values from about 110 up to approaching 200 depending on the type of tires and other factors with the low end of the scale actually being used more with slicks to get a turn or so of rotation before hooking up. To me this indicates that that values for a street car are likely to be below that 110% value. The question being by how much.

I have found that in the circle track community at least, the lower links are not generally considered an adjustable item, and are held within 2-3 degrees of horizontal. In which direction I don't currently know, but they determine the rear steer characteristics, presumably roll understeer.

Apparently 100% AS means that all of the load transfer caused by acceleration is being carried by the upper links. A 6-8 degree angle is recommended.

This link has a very good explanation of how some of this works:

https://www.hotrod.com/how-to/ctrp-0406-antisquat-explained

What I'm seeing is that 100% AS means that the rear of the car neither goes up or down upon acceleration and over 100% induces what they call "separation" or body lift while under 100% induces squat. Equal length bars parallel to the ground have 100% AS it appears.

Getting back to the roll understeer/oversteer, roll understeer promotes side bite which helps with mid-turn traction, letting you accelerate harder out of the turn. Or so I've heard. Also suggested was having the frame end of the LCA 1" higher than the axle end, which moves you through zero halfway between them.

I'm trying to internalize all of this and come to some sort of a conclusion but I'm not there yet. Playing with the spreadsheet gives me some pretty widely varying setups and though I've gotten close a few times with the three numbers I'm not there yet. What I'm shooting for is less than 2 degrees of pinion change, less than 3 degrees of roll understeer, and close to 100% of anti-squat.

My control arms should be here Tuesday, I'll have a little better idea of my limits then.

And Carl, since taking more accurate measurements and finding that I have 1-1/2" minimum vertical clearance, I can go a little bigger on the tires as I had hoped. These 155/60R15s should work and still leave me an inch or more of clearance. That's about the most I can get by with but there is also a summer tire available.

https://www.tirerack.com/tires/tires.jsp?tireMake=Continental&tireModel=ContiWinterContact+TS800&partnum=56TR5TS800V2&i1_Qty=4&i1_Qty=4

Jim

Continental ContiProContact 400TW

https://www.tirerack.com/tires/TireSearchResults.jsp?zip-code=37663&width=155/&ratio=60&diameter=15&rearWidth=155/&rearRatio=60&rearDiameter=15&performance=ALL

Thanks for that link, Jim N.

There is also a very nice 3 Link Suspension calculator on that Crawlpedia page.

Check Ron Sutton's comprehensive 3 & 4 Link guide.  It is the 5th post down.

For someone like me that would not know where to start, this is a great first stop.

https://www.pro-touring.com/archive/index.php/t-100779.html

Don't think that link worked quite like you intended.

I wouldn't mind trying out a different 4 link calculator if it's a good one. I'm just feeling my way forwards on this.

About as soon as I can get the rear axle located I can make up airbag pads, connect some air lines and make up a driveshaft and then flip the car over and set it on its wheels. That'll be a pretty big milestone.

Jim

Oops.  Fixed the link.  Check it out.

https://www.pro-touring.com/archive/index.php/t-100779.html

Jim N.'s link that has both 3 & 4 Link calculators.

https://www.crawlpedia.com/4_link_suspension.htm

This whole anti-squat business is being needlessly overcomplicated by people picking different reference points and then acting like the final authority. We have one faction, of which Ron Sutton is a member that says if the UCA and LCA are parallel the lines will never intersect and anti-squat will be zero. This may be a limited use case where the LCA is horizontal or near horizontal, IDK. Then we have the faction that includes Crawlpedia that says if there is no anti-squat then the value is 100%. Obviously both can't be right.

I find the latter method somewhat lacking in that it takes a reference and assigns it an arbitrary value. I don't know why they did this but I just find it confusing. 100% of a zero value is zero. 500% of a zero value is also zero. 1% of a zero value is zero. Maybe it's an attempt to give percentages to anti-squat values but it seems to me it would be better to use negative numbers for negative values. It causes me to lose confidence in Crawlpedia, I can say that much at least. I can't speak to their motives but unintentional obfustication is obfustication just the same.

So I have gathered that holding the LCA parallel to the ground eliminates rear steer. A little lower in the front can help corner exit.
Parallel arms eliminate anti-squat and angling the upper arms down creates anti-squat. Don't think you want to angle them up.
Pinion rise on acceleration is a thing, changing the angle. So is angle change with unequal length arms etc. This might be where the balancing act comes into play.

Amidst all this I have to locate my air bags and shocks. Fun times. My rear air bags have 2/3 more surface area than my front ones and my front ones act on a roughly 2:1 ratio with the swing arm. Does this mean the air bag should be located 1/3 of the way from the body to the axle end of the LCA? I think maybe it does.

Jim

It's confusing but 100% ANTI squat is zero for reference. Here's a guy that uses an A arm and ball joint for upper link:

I am following with renewed interest,  until Jim started this thread, I was entirely happy with my choice of arm positions and angles, I had read a great deal about anti squat percentages and listened to those who said that for a street car 50% was about right. I n my own mind I think that with the limitations of space in a B to install a perfect and whether we  can actually utilise it (probably not in the U.K. anymore) except on track days then any better than halfway there solution will probably suffice.

QuoteHere's a guy that uses an A arm and ball joint for upper link:

Now, that is an interesting twist.  

My control arms came in yesterday and they are much stouter than what I would have built if I'd made them myself. Admittedly they were for a 3500 lb car where this one strives to be well under 2000. Maybe not with those sort of parts!  Anyway I will use them and be perfectly happy with them. I know they will not fail.



The threaded adjusters use a 15/16 fine thread. Those won't break. Looking at that photo above I see a weakness in the cross piece the a-arm attaches to. Being angle iron and having full acceleration forces going through the top arm, I think it might twist.

My lower arms are 21-7/8" center to center. Most likely I will have to extend them past the axle centerline to the rear and make the lower brackets accordingly. This will mess with the calculator but I'll just make adjustments and do the best I can. It's not going to be perfect.

The through holes in the LCAs are tubed and threaded. Overall they seem to be well made. Not sure what the yellow bits are for. Don't know which ends of the upper links were intended for the frame. Don't guess it matters. I will have to add spring perches to the LCAs. I can probably design a bolt-on mount. It will have to be offset to the side so it's a good thing those bushings are sturdy. They are urethane. Over time they will probably deflect with the twist, so if I can position the bags directly under the arms I will, but I don't think that will happen without cutting the frame rails. (GASP!) Like I haven't done that already. What load is being carried behind the spring seat? Oh yeah, shock absorbers. Either they get mounted behind the axle, or I go tubular. and probably still behind the axle.

Jim

Jim, The forked end goes to the rear end housing where stock has a large rubber bushing. On the lowers can't you move the front mount forward instead of the rear behind the axle?

QuoteMy control arms came in yesterday and they are much stouter than what I would have built if I'd made them myself. Admittedly they were for a 3500 lb car where this one strives to be well under 2000. Maybe not with those sort of parts!

Should have ordered them from Colin Chapman.  

Don't think Colin would approve of these, they're pretty stout.

Is that Crawlpedia 4 link calculator the same one I've been using? v3.0 by Dan Barcroft? Seems like I end up at the same place with every one I open.

Also, I really am pretty clueless about what 50% anti-dive would do. For all I know it'll drive and act just fine and the differences may only show up on the race track. What is 50% of zero anyway? Usually zero. I realize this is sort of a special case but the math doesn't work. There has to be another reference point. What is 0%? Usually zero but here it seems it must have a value. I don't think anybody knows what that is because it probably changes for every setup. You can't have a percentage without plus and minus limits, so what are they? Can they even be defined? Percentage requires a range.

Thanks Jim, I will take another look at it. As it turns out the bushing end has a spherical bearing with what looks like Delrin seats or else very hard urethane. The seats are held in with big snap rings so they would be replaceable if that was needed and you could find replacements. I expect this to be a lifetime use. The LCAs use the same setup on the frame end. I pulled the ball and turned the ends to narrow them about 5/16" to match the forward spring hangers on the tub. Since those are 2" off the floor at full compression that sets the LCA height and the distance from axle center. Giving 4" of compression, 10.6" to axle center minus 1/4" for the contact patch, leaves 10.6 - .25 - 6 = 4.32" below axle center. Less here might be better than more as it could allow lowering the ride height without shifting the turning characteristics much. I'll have another look at where I want the front ride height to be before deciding. A slight nose down attitude is probably better or at least looks better.

Realistically, here's an interesting counterpoint: This setup is nearly identical to what GM used in the 70s era muscle cars like the 442 and Chevelle. A great number of those were fitted with Gabriel air shocks in the rear and run an inch or more higher than stock height in the back. Mine was the same. The only thing that I ever did to the suspension that I found to make a noticeable difference in the corners was the addition of a rear swaybar. When you run the esses down the west side of Gauley mountain at better than 60 mph in one of those cars you tend to notice. Speeds that not only get the rear of the car sliding but the front also with rabid transitions back and forth (a typo but I'm leaving it since it fits) tend to highlight any weaknesses or differences. The point is, maybe this is more of a critical difference on a closed course where a second in lap times is a big deal. Not so much on the street, although I generally ran the Gabriels at minimum pressure. They still raised the bumper a bit. Could be a judgement call.

I'm not eager to move the forward mount point given that a perfectly good, properly braced and triangulated location exists already but maybe it wouldn't be that difficult. As it is the rear pivot will end up about 2-1/2" behind the axle centerline which isn't really all that much. It will hurt me on air bag location. It shouldn't affect roll steer, but may influence pinion rise and anti-squat. I'm going to concentrate on getting the LCAs done up first. It looks like I will be able to mount the air bags above the LCAs without compromising the frame. A little trimming of the inner well lip and it can be well centered under the arm. Making the seat perfectly parallel to the arm at full compression should only take a reasonably small dent in the bottom of the frame rail and I should be able to hit that 1/3 location easily enough. The swaybar mounting holes will stay in the stock location and be unused for now. The Armstrongs may even be able to stay in the stock location, it's looking favorable.

The UCAs will need a good weld-on mount for the bushings on the axle housing and those will have to be forward of the housing and below the body sheet metal so they will be positioned low compared to what looks like common practice. That will leave one pivot point to be determined and that's when things might get dicey. I took some photos and played around with axle position a little.



Measured distance the shock moves is 6-3/4". Might as well set ride height at 4" up and leave 2" for droop.



Some PVC gives me a decent gage for the fully compressed air bag. I can make it fit but need to do some trimming.



I have some 3/8" silicon sheet that I think will make decent bump pads if I can glue or fasten it securely enough to the tub above the axle ends and the diff. That and limit straps should protect the shocks adequately.



Today I'll look at front axle ride height, IIRC the LCAs should be parallel to the ground or point slightly downward so first I'll see what the ground clearance is in that configuration and how it compares to the rear. That will help me decide on the rear ride height. Then I should be able to make up the LCA axle mounts and shock attachment points, trim the tub for the air bags and get started on the body side seats. For the LCA seat I should get by with just drilling a couple of holes.

Jim

Fox Mustang lower arm is shorter:

https://www.ebay.com/itm/285293485961?_trksid=p2332490.c101196.m2219&itmprp=cksum%3A285293485961ff4c4364d6e14078bb06782556156c59%7Cenc%3AAQAKAAABgGYW2MKgIjX98w%252BOSqwbH5XuxFI2yPdYNOWJEsUBF3zDTfVmJ4LQkvIZkS2UwrNlErhNqtjwLGYNb4LSK%252BIItvmEHRHNqj4CZLbWeqM%252B%252F9KCNC6%252BOvXRdCyOX8v3q2SQ5BDAHN2Tf1LO%252FBblCAK%252BycKmHjWur0ru21JsijM1wq%252Fo1hEl02LpaAcAC%252FUwAyKTSc4pT8A6a7YcfzoZEy054CZDGqdwQzlZVs600X3Tr9tWIw5q%252FYVwfL4Q6TjwfR%252FpXlrHTeCbWHc5MPg2z7jytdL5mn340sBEqjGFXZHgWBqm55C61MuC%252FK5ccxW1xwqcISHoWHOMoyIDqF3M4XXUSpBGiycMAQrz9JRXbfKamCC6NqRJhtok%252Ba2QXslqGWKiKCTLHF7TBh5%252F9kU4ZpjZbnw0WruS%252FVJ3kxfZe8efjhWW7zMKUIBLQRku32iLrc7gDmFKCQZAIjzdYI%252BEm3xzBRoZGRHK5arm5sg%252BQ%252B8CmOuGg3pWObLfkjXV73JUdH9SxA%253D%253D%7Campid%3APL_CLK%7Cclp%3A2332490&epid=3021611761&itmmeta=01KA1HPM8WXY3RFY6HE6F2AXRQ

Quote:
Here's a guy that uses an A arm and ball joint for upper link:

Now, that is an interesting twist.

 Colin Chapman did it sixty years albeit unsuccessfully and upside down; https://www.classicfordspares.co.uk/plates/374-rear-suspension-lotus-cortina-mark-1
IIRC the problem was the axle mount didn't move as freely as a ball joint would and caused distortion leading to oil loss. I can just remember my fathers veterinarian having one, it was just too unreliable for his work though.

I doubt Colin approved of the MGB but then again I doubt he ever foresaw our current horsepower levels either.

OK, tell me what you guys think here. Would it be better to shorten the LCAs and if so how much and why? I can easily cut and weld to get whatever length I want and now is the time to do it. One or two things though. First, the calculator doesn't work exactly right unless the bottom axle pivot is forward of the top one. I'm not exactly sure why that is but it seems I can ignore the 180 degree flip. Not sure what it does to the pinion angle though. This is made much worse by having to put the top link in front of the axle housing so it won't hit the body at full compression. The effective length of a 12" top link at 30 degrees is about 10-1/3" which will pull the pinion down relative to the LCA if they are parallel. I do not think a LCA of that length is likely to be a good idea. Second, equal effective lengths on parallel arms gives zero pinion angle change but will probably contribute to squat since the pinion will want to rise. I probably need to go back to the calculator again to see if that balance becomes easier or harder with shorter LCAs.

I did some work on the frame rails to fit in the air bags. They are ready for welding and seats I think but my welder has the .045" wire right now so that might not work so well.



I have adequate clearance with the air bag placeholder at full compression. I need to give some thought to the mounting bolts and air lines.



If I tried to move the front mount forward this becomes a real issue and I don't feel all the extra work to do it is justified for this car. A race car, sure but this isn't.



Checking front ride height showed that with 4" of clearance between axle and body everything is just about right. Close enough that minor adjustments and variances won't hurt anything. I need to use a 3" spacer between the axle housing and the LCA for mocking up the mount.

Does anyone sell a bushing mount for the GM axle top mount? Should be easier than making them so probably worth buying if available.

Jim

Jim, I think the lower arm should be the same length as Bill Guzman's and Phil's. Middle of axle leaf spring mount to front leaf spring mount.

Jim, Maybe Currie would sell you these brackets or you could fabricate similar brackets:

https://www.currieenterprises.com/64-66-gm-a-body-9-inch-housing-and-axle-package

Triangulated weld on upper tabs:

https://www.speedwaymotors.com/Triangulated-Rear-Four-Bar-Weld-On-Upper-Tabs,63235.html?gad_campaignid=23022589191



Many weld on tab options:

https://www.amazon.com/OywhC-Universal-Mounting-Bracket-OywhC-7/dp/B0F8C5BN5S/ref=sr_1_6?s=automotive&sr=1-6

Well I found these:
https://www.butlerbuiltperformance.com/Upper-Control-Arm-Mount-Metric-Pair-p/ubm46-1091a.htm

So at least I now have a better idea of what the mounts should look like. Seems it needs a stepped bore. Now, with the force being applied by the pinion lifting and the housing pulling away and with the angle of the UCA that means the force vector comes in from the outside so the larger diameter ought to be towards the outside to make sure the bushing stays seated. So two lathe turned tubes and two pasteboard templates for four tabs should do it. Or I could buy the Butler tabs for $45 + shipping, which are in single shear and trim them to fit...  I guess I'll make them. I don't really trust the single shear.

Effective lower control arm length based on the MGB springs would be 20-1/4" by my tape measure. My current position puts the axle at 20-3/16" which is surprisingly close. My measurement of C-C distance for the LCA at 21-13/16" means a pivot position just 1-5/8" behind the axle center.

With those two points fixed and the upper axle pivot estimated my first attempt on the calculator gives me one degree of roll understeer, less than a degree of pinion movement, and minus 65% anti-squat. That is with the UCA angled upward to the front. If I move the body mount point 3/8" lower than the axle pivot I get zero AS and 6 degrees of pinion movement. That would have to be with the pinion angling downwards.

Which brings up two questions.

First, does 0 AS mean that there will be NO acceleration forces pulling the rear deck down when power is applied? I can live with that. Or does it mean that the suspension itself does not contribute but has no effect on the counter torque attempting to rotate the pinion and pull the body downwards against the instant center?

Second, since there is going to be roughly 3 degrees of upwards pinion movement when power is applied and bushings flex, if pinion angle is set 2-3 degrees down doesn't 6 degrees of pinion movement seem reasonable? Does it make this perhaps the best compromise?

Jim

I may have gotten that backwards... If pinion movement is downwards, and with the shorter top link it would almost have to be, That added to 3 degrees of initial pinion down angle would not be good. -9 degrees at full compression and no power. Pinion at zero might be better, 3 degrees under power and -6 at full compression with no power, -3 at full compression and under power. Under load +- 3 degrees. I think it'd handle that ok.

Here's my PAD mockup (Plywood Aided Design). Note the 2x2 sits a bit above the end of the LCA which is intentional.



Here it is with the steel parts ready to be welded to the axle once I have zeroed the pinion and checked the longitudinal and cross measurements. Some adjustment will be possible afterwards using the top link turnbuckles but it's best to get it right to start with.



Upper link attachments should be a treat, note the stepped bushings. The forward ends of the links all have spherical soft joints to help avoid binding.



Jim

Check out this guys:

https://www.youtube.com/watch?v=BXLEG_2NS8s

That bushing is stepped because on a Chevelle (like the '68 SS I had), you press it in until it stops (on that step.  The mount is a wide, single shear that is part of the casting.

His links all attach behind the axle centerline but I don't have room above the axle to do that so the top links will have to go in front and will attach lower. They will be a bit longer than the ones used on the stock GM cars though. The geometry seems not perfect but OK.

Today I expect to weld the lower brackets to the housing. Looks like my upper links will be pretty close to parallel with the lower ones, only slightly angled down in the front. Not sure yet what else I may do, mondays tend to be sort of a slack day. I switched off to the bus for a bit yesterday and got some work done on the propane line so that's still coming along and the worst part of that job is now complete.

I might fire up the lathe and work on the axle mounts for the top links. I think I have some round that I can use to make the tubes. Happily I have some new carbide tooling that will help a lot with the amount of metal that will have to be removed.

Here's something that doesn't get mentioned though. What is the effect of moving the control arm pair down relative to the axle centerline? Accel/decel forces are now concentrated on the top link rather than balanced. One obvious effect would be that the pinion will not have as much of a tendency to climb, at least I suspect that to be the case. But maybe not. Just curious.

Jim