Needing some help designing 3 link

Hey everyone I'm new to the forum, ive always creeped around on here from time to time but finally decided to make an account. I'm starting a full tube chassis street car that will be licensed as a hand built car when completed. So I've been looking pretty hard to find info on exactly how to design a 3 link rear suspension but haven't found anything very clear and to the point. Just to give you guys a little background on me, I'm 29 and have a welding and fab shop and I've been in the welding industry since I graduated 11 years ago. I can build and do just about anything but I'm not a scholar when it comes to suspension design. I do have some racing experience so I'm pretty mechanically inclined. I just need some info on how to determine how long I should make my 3 link bars. The only real info that I found was that the top bar in the system needs to be around 70% in length as the bottom bars. But I can't find anywhere where it says how long to make the bottom ones. Is it based off of the wheelbase? Like is bottom bar links determined by how long the wheelbase of the car is? From driving and setting up dirt late models I know that usually the longer a bar is the smoother the reaction is. I'll be running a watts link on the rear to keep it centered up. Adjustable coil overs front and rear as well. Since the car isn't built yet I basically have an unlimited platform to work with as far as mounting locations and chassis design go.

Btw my build is going to consist of a 5.3 engine and a 4l65e trans with either a aluminum or carbon fiber driveshaft with a quickchange rear end. Front suspension I'll be double a arm unequal length (have to start designing that as well) the chassis will be mostly 1 3/4"X .120 DOM and I'm looking to be around 100"-105" wheelbase maybe a little longer. Any kind of body panels will be handmade. I plan on having a ride height of somwhere around 4-5 inch ground clearance.

if some of you guys could get me pointed in the right direction I would so greatly appreciate it!

Welcome!

get a copy of Herb Adams' "Chassis Engineering", he goes through a bunch of suspension design stuff without overwhelming the reader with really in-depth maths.

If you want the "right" answer, Milliken's Race Car Vehicle Dynamics is a great resource. It has a couple chapters on suspension geometry. However, it is quite engineering-based... you may be better off taking measurements from a well-established suspension of similar design (late S197 Mustang, for example, is frequently praised for its well-executed 3-link design) and using those as a baseline.

what they will say is keep the lowers parallel,  the upper bar is going to point to the Center of Gravity, also the fwd. point of the upper needs to move up and down to adj. for CG changes and in a drag car more than a street car it needs to be able to move to the right some to account for TorqueSteer.

as far as length's go, the lowers should be equal and I have seen some DARN long uppers. the key is the housing must stay stable and move up and down not being in an arc that changes the Pinion angle, I saw a rear set up from Red Farmer back in the day that had rear and forward links to keep from moving in an arc at all. I didn't know WHERE to start to make changes. but read those books. as long as you have been doing it you know already, but it's good to have that info at hand and great knowledge if you haven't.

Also read Billy Shopes . one of the Original Ramchargers, this will be Online.

Thanks for the responses everyone. I’ve been looking around on the interweb and found a few articles that have given me some more insight but still nothing in black and white. From what I’m reading and I kinda already figured that the all 3 links should be level (or parallel at ride height) but having that top link in the same plane I guess as the CG kinda makes sense now that you mention it. As far as determining how long to make the bottom links it says in an article or post on Barnes4wd.com that the lower links should be twice the length of whatever your wheel travel is. So I guess if I had 7” of suspension travel then my bar length would need to be 14”? That seems awful short to me. But it’s a off-road website so it may not be the right info for what I’m trying to build. 

I used the Performance Trends suspension program to design my 3 link and it was good but not great. I paid Ron Sutton Race Technology to analyze my suspension and once I followed his advice, it transformed the handling of the car. He has a cheap service where he can advise you on your set up before you start cutting/welding so you can get it right. Otherwise, figure out how much space you have and make the bars as long as you can

Loosecannon thanks so much for chiming in! If his pricing is reasonable enough I will for sure go that route that way this thing handles well and will be enjoyable to drive. Just curious what does his services cost roughly?

Grain of salt: I'm not a pro, or vastly experienced, I just study things and try them; I might be a two-bit hack.

On my V8 Firefly (4-link, but the same idea) I ended up setting it up like a drag racer's 4-link by drawing up a side view of the chassis, guestimating where the center of gravity likely would be (one rule of thumb is the camshaft height in a pushrod V8), drawing the "Anti-Squat Line," and figuring out what angles I wanted.

Now, you might not be building a drag-only car, so you will likely want less anti-squat.

Packaging is also going to be a concern.

LONG arms mellow out the reaction of the suspension. I think.

SHORT arms react a quicker.  I might be wrong.

I think I read somewhere that the Nelsons shoot for a 48" instant center, but I might be mistaken.  Perhaps they will chime in. I would think you'd want the instant AT the center of gravity to make the most of the vehicles weight on the suspension, but I really have no clue.

To get some really good anti squat (sweet for drag), I really need to angle the lower bars such that the front mount is higher than the rear mount, BUT that induces some "roll steer" where the car becomes -really- tail happy in corners. I set my V8 Firefly with the lower bars horizontal with the ground so there is still some roll steer, but that is probably what makes my nose-heavy car actually turn well.

To get the Instant Center on the Anti-Squat Line (that was my goal, so what I thought would be good balance between handing and hooking), I then had to lower the upper arm front mounts (they are triangulated to locate the axle laterally). But the lower I moved them, the shorter the instant center became, and the more into "Gimme Squat" it went. I ended up with what I think is my best compromise given what I had built, and it works pretty good. I have no complaints.

As far as I understand, you may not want much anti-squat at all on a light car, or for road-racing, and the general recommendation on a Lotus Super 7 replica "Locost" is to run zero or very little anti squat. I built my Locost with zero anti-squat, and it seems fine.

Keep in mind, I'm not an engineer, I don't play one on TV, and I didn't stay at a Holiday Inn Express. I do have a Sawzall and a Welder, and I'm not afraid to use them.

Hey thanks for the info that really puts a lot of it in layman’s terms for me, I’m kinda like you I’m no engineer but I do have a pretty good sense of anything mechanical and I’m not afraid to use my tools and equipment. This car is basically going to be a cruiser but I wanna be able to auto cross it and stuff. Not big into drag at all but I love a good handling ride. I live in KY and recently got married so hopefully me and my old lady can see the west coast in this thing. Btw loosecannon I checked out that Ron Sutton link and wow that guy explains things in such a way that is super easy for me to understand! Thanks so much everyone I think im starting to get some traction now that you all have led me in the right direction. 

Btw Idk if any of you all are familiar with the Sire Custom Performance Twin engine V8 car. But this is kinda where I got some of the inspiration for my build. Here is the link check it out. https://www.hotrod.com/articles/hrdp-0909-custom-speed-buggy/

cvfab said:

Loosecannon thanks so much for chiming in! If his pricing is reasonable enough I will for sure go that route that way this thing handles well and will be enjoyable to drive. Just curious what does his services cost roughly?

Looks like his services for rear suspension starts at $99 but what I think you want is the #18P 

Rear Suspension 3-Link & Adjustable Panhard Bar – Tailored Design: $450

• Tailored Dimensions to Fit Your Car
• All key X, Y & Z dimensions Provided
• All Link & Mount Dimensions spec’d out
• Utilizing Your Choice of RSRT TT or TA2 3-Link Mounts
• Utilizing Your Choice of RSRT Adjustable Panhard Bar Mounts
• Car measurements provided by client
• Optimum Anti-Squat percentage
• Optimum Ratio of Tire Loading for your application
• Optimum Pick Up Point length & height
• Optimum Instant Center/Swing Arm length
• Optimum Roll Steer for your application
• Optimum Rear Roll Center NOT included without Suspension Setup
• “Optimum” is within limitations of the Car Dimensions You Provide
• Top Link can be Centered or Offset to cancel out Torque Steer
• This is a great design package for a tunable, high grip, good handling, track car
• This design does require rear seat elimination, as the Top Link mounts to the back side of the roll bar/cage main hoop
• Includes Performance Trends Geometry Files or PDFs with all Dimensions
• Includes Blueprints if purchased with #17 Rear Frame Clip Design

Click here for services

the best layout for a cruser, a car that does ALL things well is a truck arm I Say that because it does not use a top link, BUT I like a three link.

A good read from Ron Sutton...

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