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Link Arm Angles and Locations

illusha

Member
Long post warning!!! I could separate into 5 different threads. Decided to keep it all in one. Working on a 1994 SuperBurban nicknamed “Overkill”. Goal is to build an expedition vehicle that can perform great both on and off road. Running a turned-up P-pumped Cummins 12-valve intercooled diesel and ZF6 transmission with 271 tcase over Ford Dana 60’s with 37’s on 20’s. Reinforced the dinky factory frame by plating it inside and out with 1/4”-3/8” plates and strap-boxing around crossmembers. Will be a heavy rig close to 10,000 lbs with a 3-link front and a Triangulated-4-link rear.


Now I’m trying to design the best suspension. Primary consideration – being able to perform as best as possible in ALL conditions – going 80 mph on freeway as well as rallying around gravel roads with an occasional rock course or mud hole. Started learning suspension from zero a few months ago. Read a bunch by searching forums. Played with both Triaged calculators and starting to get the basic concepts. So, now, a few things that I am not yet fully understanding…


MAIN QUESTION - different sources say different things about links being parallel to the ground. On pavement-racing forums, the trend is to make lowers parallel to the ground, while angling the uppers to get desired Anti-Squat. Street cars are not going for wheel-travel or ground-clearance though. While rock-crawlers prefer to keep uppers parallel to ground and angle lowers 10 degrees up (or more) to get clearance. Although, lately, I see more pro trucks running flatter lowers with a “cage” built in the center of the rig to hold the frame-end link mounts. What are people prefer to run in the desert? Am I correct assuming that suspension built for going 80 mph over sand whoops would also perform well at higher speeds on pavement?


Another dilemma is the location of link mounts in horizontal relation to the axle center. I’ve seen designs that put link mounts right on axle centerline and some that are offset a few inches aft or forward of the axle. What factors should be considered when choosing how much to offset the link mount from axle center? How do I figure out the geometry? Is there software that could model this geometry for me or are there manual ways of getting it right?


What factors should be considered when selecting the percentage of Anti-Squat? I was thinking of shooting for about 60%, but everyone prefers it differently, so what’s best for my intended use? Additionally, is Anti-Dive selected by different logic, particularly regarding the third link location and separation?


Along the same lines go the link lengths. Some people say to make uppers longer than lowers, some say make uppers shorter, so far I figured best is to keep uppers around 75% of lowers, may affect pinion angle at large drops, but at least it will steer straight at ride-height down the road (correct caster). Also long-arms vs short-arms overall length. Advantages and disadvantages? Any opinions?


And I’ve read some discussions on mounts being above or below the axle. I see in calculators that the forces placed on links change significantly just from a 1-2” variation. So is the top of the axle still acceptable for lowers? I know not to mount them any higher than that. Or are there any significant advantages to keeping them at center of axle or even lower below? All link brackets will be custom made and angled to reduce stress as much as possible.
 

illusha

Member
It seems my post above was far too long to process.

Still trying to figure out the importance of running Lower Links parallel to the ground vs angled up 10* or so.

Also trying to figure out if mounting links forward or aft of axle centerline makes much difference.
 

Glamisfan

Member
I've never heard of trying to keep the lowers parallel to the ground. It seems like it would be hard to get full travel without the mounts at the frame hitting the ground first. Easiest way to build it is to just look at a trophy truck and copy it. Most have the uppers shorter then lowers.

I've built a prerunner with the same setup as tt's run, and also 5 linked (4 link plus a panhard) my super duty, and they both work great on and off road. I've studied a lot of suspension articles. But I'm no expert.

Packaging everything so it fits the vehicle, doesn't rub the tires, clears the frame, etc is a big part of designing and building your own suspension too. Use tape or pvc pipe to mock up the links to give you a visual.
 

illusha

Member
It seems there are many theories on where to position the links and why.

I would prefer the lowers were angled 5-10* up in the center of the truck to have more center clearance. That’s how I initially set up Triaged calculator and got decent numbers. But then I noticed that most of Short Course trucks and Formula Offroad rigs do have lowers very parallel to the ground. So I did more searching and reading. Thus my questions to the folks who actually use linked setups in racing.

My understanding is the Lowers locate axle front-to-back and transmit the wheels’ thrust to chassis. Lowers also control rear steer. The Uppers locate axle side-to-side and control pinion angle.

If Lowers are at an angle, when truck is at ride height, the truck should have some rear-steer effect as the body rolls in a corner. The lower link on the outside of the corner pushes the rear end housing back, while the lower link on the inside of the corner pulls the rear end housing forward, making the rear of the car provide a steering effect (flex steer). If Lowers are flat to the ground, at ride height, there would be no front/back movement of the axle during cornering.

With any rear suspension, we need some balance of lift & push. More lift & less push plants the tires harder, but for shorter distances. Less lift & more push plants the tires softer, but for longer distances. Level lower links create a neutral rear steer scenario as the car achieves roll angle. Angling the lower links uphill increases “lift” but adds positive rear steer as the car achieves roll angle.

With Lowers angled at 5-10* towards center of rig, I calculated +1.6* oversteer on front and -0.28* understeer on rear. With Lowers flat, I calculated -2.0* understeer on front and -5* understeer on rear. If I am understanding correctly, the understeer direction is fairly benign, but the oversteer can increase the puker factor and Triaged is not recommending doing more than +5.7* oversteer at ride height.

P.S. – I clearly have a problem with expressing myself using simple one-line posts : )
 

illusha

Member
P259.jpg P260.jpg P261.jpg P262.jpg
OK, my question has been solved, see the attached screenshots from Gillespie's book. Apparently I got confused somewhere online while reading about Parallel 4-Link specifically, in which case the angle of lowers does seem to be very important, as the roll axis slope is the same as the lower links’ angle, which affects rear steer to a large degree, but Triangulated 4-link constantly changes the roll axis during cornering anyway. And with 3-link, the top link is ignored in analysis because it does not react to lateral forces, while the roll-axis remains relatively unchanged due to the location of the track bar which picks up lateral forces. Moral of the story – it’s best to study theory from official publications and use the forums for clarifying questions – otherwise it’s too easy to mix up what applies to what and start going down the wrong road.

Good reference materials available to download as PDFs online seem to be “Fundamentals of Vehicle Dynamics” by Thomas Gillespie, “Race Car Vehicle Dynamics” by William Milliken, “Tune to Win” by Smith, “Car Suspension and Handling” by Barstow, Howard and Whitehead, and “Tires, Suspension and Handling” by John Dixon.

I will probably continue most of the build design discussions on Pirate if anyone is interested in following the progress.
 

Giant Geoff

Well-Known Member
Links on a 4000lbs (and up) on a truck is not a tech as people say, you got to remember, with weight and length, links become very dumb. When cars are light and the arms are short, it become tech and finicky. Instead of talking through a highly educational route I like to help people understand in a deferent way of what not to do. There are mainly 4 “don’ts” & 1 “do”: 1 Don’t mount the lower link on the front of the housing, this puts a great torsional load on the uppers. They already have too much stress from the side loads. 2 Never mount the shock forward of the center line and bring them closer if you what a more aggressive feel. 3 proper weight behind the axle to balance the car. All the weight in the center is for road racing on asphalt. 4 Springs to hold up that weight and progressive shock for how fast you want to go. 1 “Do” Make the frame pivots close together.
 

illusha

Member
Thanks Geoff, much appreciated, those are all good pointers and they do make sense. As for my project, after much deliberation and for a number of different reasons (including tuning time), I’ve decided to get a set of four 8-inch 9-leaf National Springs and be done with this heavy rig for now. It will do just fine being what it is. Some other day I will build a much different car designed around its suspension and probably for offroad-use only. There simply is no way to build one rig that would excel at many different things.
 
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