Good 3 or 4 link guideline

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Im doing a 3 link suspension setup currently, and have been researching lots of different sources and stumbled upon this from the pro touring.com forums..seems to explain everything involved very well. it is a good read for anyone interested in a three or four link suspension design.

The most tunable & best suspensions for many applications are the 3-link & 4-link.

Basics:
The 3-link has two lower links … often called lower trailing arms or lower control arms … and one upper link … often called the top link, 3rd link or upper control arm. The top link can be centered, offset or angled. The reason to offset or angle the top link is to counteract the torque transmitting through the rear end housing.

The 4-link has two upper & two lower links. For racing & track applications the triangulated 4-link is not optimum, as you don’t want any forces pushing or pulling in a direction other than parallel. So we’re only discussing parallel 4-links here.

The design, set-up & tuning concepts are the same for both 3 & 4 link rear suspensions. The 3-link allows for more rear end housing articulation within the chassis. Both 3 & 4 link rear suspensions will bind at some angle different than the chassis … but the 4-link will bind at a lower angle.

For drag racing … high power, high rpm, side step the clutch launches … the 4-link is better suited to deal with the launch forces. While the bottom links “push” on launch … the top link(s) “pull.” Pulling the rod ends is more susceptible to failure than pushing. Two links with four rod ends are obviously twice as capable of handling this “shock” that can be so powerful, the 4-link suspension is lifting the front of the car off the ground.

So, either 3 & 4 link rear suspensions can be used for corner handling & drag racing, but the 3-link has a slight edge for cornering & the 4-link a slight edge for drag racing.

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I’ll outline a step-by-step method for the average car guy to build & install either system. This will be an adjustable system that can be tuned for optimum performance.

Start with the lower links when laying out either system. You want to position them as wide in the chassis as you can (top view), while still providing adequate clearance for tires, frame, etc. Wider placement gives the 3 or 4-link more control over the rear end housing … narrower is less. You can mount the front of the lower links under the frame rails, provided you can achieve the correct height for the front rod ends & enough “length” in the overall lower link to minimize angle changes as the suspension travels.

There is no magic number, but I like to see the center-to-center distance on the lower links be 24” to 36” long. If you can’t fit the bracket you want under the frame rail, then you’ll typically mount the front brackets inside the frame rails. You need to space the brackets far enough away from the frame rail to put a nut on the bolt.

Let’s talk height of the front & rear mounting points for the lower links. Again, no magic number here. But you’ll want to understand the concept of “roll steer” or “rear steer” … which have the same meaning. The lower links control rear steer. If the lower links are perfectly horizontal … level with the ground … when the car is at ride height … the car will not have any rear steer effect … as long as the rear of the car rolls evenly.

Rear Steer:
If you adjust either end of the lower links … and end up with the pivot points higher in the front (towards the cockpit) … the car will now have a rear steering effect as the chassis/body rolls. The link on the outside of the corner pushes the rear end housing back … while the link on the inside of the corner pulls the rear end housing forward, making the rear of the car provide a steering effect, helping the car’s ability to turn. This frees up the car throughout the corner, as long as chassis/body roll exists, including while trying to power out of the corner, which can make the car loose on corner exit.

If you adjust either end of the lower links … and end up with the pivot points lower in the front (towards the cockpit) … the car will now have a counter rear steering effect as the chassis/body rolls. The link on the outside of the corner pulls the rear end housing forward … while the link on the inside of the corner pushes the rear end housing back, making the rear of the car provide a counter steering effect, hurting the car’s ability to turn. This tightens up the car throughout the corner, as long as chassis/body roll exists, including while trying to power out of the corner, which can make the car exit better with more grip … unless it is too much, then it can push on corner exit.

I strongly suggest you design yours for level lower links … and have holes or slots for adjustment each direction for tuning. Rear steer can be a good tuning tool once you learn how to use it.

It’s easiest starting at the housing. You’re either going to buy or make brackets that weld on the rear axle housing tubes. Most tubes are 3” OD … but measure yours to be sure. There is no magic height number everyone should run. But remember, if your lower links are level … as you extend the imaginary line forward to intersect with the top link(s) … the height of the lower links is going to be the height of your intersect point known as the Instant Center or IC.

If you’re buying brackets, most are going to have holes or slots ranging from 4” to 7” below axle Centerline (CL). In my experience that is a pretty good range. My only rule of thumb here is … higher points help me get the IC we need for lower powered cars & lower points help me get the IC we need for higher powered cars.

To get an idea of where your lower link rear pivot point will end up … or points if you have multiple holes … requires doing simple math. Tire height divided by two … minus expected tire sag from load … tells us “about” where the axle CL should be. Example: 26-1/2” tall tire divided by two = 13-1/4” … minus ¼” expected tire sag for stiff sidewall, low profile tires on wide rims with 35+psi … puts the rear axle CL at “about” 13”.

If you bought axle brackets with 3 holes, one inch apart, at 4-1/2”, 5-1/2” & 6-1/2” from axle CL … that would put your lower link mounting holes at:
Top: 8-1/2” above ground
Middle: 7-1/2” above ground
Bottom: 6-1/2” above ground
* For PT cars with 500+hp, I’d start in the bottom hole.

Now, you need to place the front mounts (for the lower links) so you can achieve level links … and holes slightly up & slightly down. You need to find brackets to fit your application … with modification of course … that will allow you the mounting points you want. Don’t get lazy here. Cut, trim or modify them as needed to get the proper mounting points.

Tip: On one end of the lower links … either at the housing or frame … I like to have a slotted mount to fine tune the lower link angle. Because sure as the sun rises, when you have your car finally done and sitting on the ground at ride height, with driver weight in it …
a. It is easy for one side to be different than the other, if the car is not weight balanced side-to-side.
b. They will probably end up close … but not spot on zero.
If you have only holes for adjustment … a single hole change can be 1.5 to 2.5 degrees. But if you have slots, you can adjust the driver side that sagged to 0.6 down with your fat butt in it … back up to 0.0. And adjust the other side, that ended up 0.3 up … back down to 0.0. Now the rear end of the car will NOT be contributing to different handling effects on LH & RH corners.

Make sense?

P.S. The lower links are compressed under launch & acceleration on corner exits … so the slot is not at risk of being ripped out. Do NOT put slots on the upper links, as they pull and over time will cause failure if slotted.

P.P.S. If I have a bracket I like, that fits the application … and it only comes with holes (no slots) … I simply connect two holes & make a slot. I do this in a mill. If you do this by hand, sneak up on it, as you don’t want that rod end bolt loose in the bracket.

When you go to place the front brackets for the lower links, be sure:
a. The links are going to be level at ride height when viewed from the side.
b. The links are truly parallel with the car/chassis when viewed from the top.
c. And the rear end is absolutely centered in the car/chassis, level with the chassis & square to the chassis.

Now is a good time to pre-set your pinion angle … before you weld on the housing brackets.
d. Make sure the pinion of the rear end truly lines up with your driveshaft & transmission (top view) so the driveshaft is not running at a side angle.
e. Find a flat surface on the rear end that you believe to equal to the pinion … or 90 degrees to the pinion … where you can place a digital angle gauge (inclimeter).
f. Measure the driveshaft angle.
g. Roll the housing to place the pinion at a 2-3° downward ANGLE DIFFERENCE from the driveshaft.

* This is NOT a 2-3° angle from the ground, unless the driveshaft runs level with the ground. If the driveshaft runs uphill (uphill going from the rear end to the transmission from a side view) at 4° … the you want the pinion going uphill at 1-2° ... to achieve 2-3° downward angle difference.

The purpose of the angle difference is to have the pinion aligned with the driveshaft under hard acceleration loads ... when the pinion rotates up & all the slack or clearances are taken up ... then have a small angle difference when cruising. This 2-3 downward angle difference number is with rod ends. If you run rubber or poly bushings that will allow the housing to rotate more, you will need to start with more angle difference. I suggest a 4° difference and then go test.

We test two ways. One is with a GoPro camera mounted to watch the pinion & driveshaft relationship under different driving conditions. The second is on a chassis dyno, tuning the angle difference for optimum power.

Do not buy into the old school strategy of using pinion angle to increase traction. This binds the pinion bearings.

Make sense?

Bring this up & let’s discuss it if is not crystal clear.

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When determining your needed lengths for all 3 or 4 links … make sure the rod ends have AT LEAST as much threads in the link as the thickness of the rod ends … at the links’ longest possible length. In other words, if you’re using ¾” rod ends … make sure you have a MINIMUM of ¾” of rod end thread in the links at all times. If not, buy or make new longer links that get more thread inside … or put me in your will … I’m OK either way.

Spell my name R O N … just kiddin’ … sorta.

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Upper Links:
You need to shop for, or make, your front & rear brackets for the top links at the same time … because angles need to be worked out. If you’re building a 4-link, you may buy one-piece 4-link brackets that slide over the axle tubes … or cut them & weld on in halves … and therefore your upper housing brackets are already worked out. If you’re doing them separately, you’ll need to work out how high you want the pivot point holes to be & find or make appropriate brackets.

Note for 4-links: You do not “have to” place the upper links in line with the lower links (top view). They can be placed wider or narrower to some degree if that helps with packaging. Wider upper links provide more control & less articulation. Narrower upper links provide less control & more articulation.

For 3-links, the upper link can be centered or offset to the passenger side to help counteract torque on acceleration. No one can tell you accurately how far to offset it. The formulas I’ve seen involve rear steer, which makes no sense for handling cars. My rule of thumb is 8-12% of track width. Sometimes in the real world packaging challenges play a role.

Upper link angles … act the same regardless if the car is a 3 or 4 link. Obviously, you want both upper links of a 4-link to be on the same angles. A quick “car guy guideline” is you’ll end up with the upper link(s) anywhere from level … to pointing downward in the front 15 degrees. That’s about the max range you’ll ever use. If you can build your rear housing mount(s) & front mount(s) with enough holes to achieve those two angle extremes … you’ll be good. If you have to have less range … I suggest a window of 3 to 8 degrees.

The upper link angles are the final determining factor of your rear suspension’s Instant Center location. Remember … you’re lower links are level. So if you have any downward angle in your upper link(s) … the imaginary projection lines of each set of links will intersect at the height of the lower links … somewhere forward of the axle. How far is dependent on the height & angle of your upper link(s).


Let’s talk Anti-Squat. If you run the upper links exactly level & perfectly parallel with the lower links … there is no intersect point … no instant center … and is considered zero Anti-squat. This will provide the most secure corner entry under braking … because the rear suspension of the car is not contributing to the pitch angle under braking. But it will provide the least grip to the rear tires on corner exit … because the rear suspension of the car is not contributing any leverage helping to load the rear tires.

If you adjust the upper link(s) … at either end … placing the upper link(s) angling downward in the front … there will be an intersect point with the lower links imaginary line … creating an instant center somewhere forward of the rear axle … and the rear suspension is considered to now have some percentage of Anti-Squat. The steeper the angle downward in front … the more Anti-squat the suspension has … and will mechanically contribute to the pitch angle of the car under braking … making the rear of the car lift more. But it will provide more grip to the rear tires on corner exit … because the rear suspension of the car will now contribute leverage helping to load the rear tires.

If you go too far … with the goal of gaining more grip for corner exit or hard drag style launches … and end up with too high of an Anti-squat percentage … you will make the car loose on corner entry under braking. The key is finding the right balance for your car & goals … and tuning it to achieve optimum set-ups for different types of driving if you become a serious competitor.

If you want to play around with different combinations, you should buy Performance Trends 4-link plus software. It works just as well with 3 links. See it HERE.

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Only as a guideline, a good starting point for many performance cars is starting with the upper link(s) pointing downward in the front with a 7 degree angle. This “should” put you “around” 40% Anti-squat … which is a good balance for corner exit grip without making the car too loose on entry. There are tricks to run more angle, but I can’t outline them here.

If you start here, and the car is a little too loose on corner entry & you have everything else in the car’s suspension “good” … you can just take some angle out of the upper link(s). If it is not affecting your corner entry under braking, you may put more angle in the upper link(s) and watch your corner exit grip increase. When you get too greedy … the car will tell you on corner entry … buy getting loose.

Upper link lengths:
Some think the upper link(s) is/are supposed to be shorter than the lower links. That’s not accurate. They often end up shorter, due to space challenges underneath many cars. But that’s not the goal. The biggest effect the length of the upper link(s) has/have … is on pinion angle change during rear suspension travel. If the upper links are shorter than the lower links … the pinion angle changes more during suspension travel. The bigger the difference in lengths … the bigger the pinion angle change.

I prefer to make the upper link(s) the same length as the lower links … space permitting. I have even used upper mounts on the housing that have the holes significantly (1-3”) behind the axle centerline to achieve this. If I can’t make the upper links the same length, I get them as close to the lower link length (say that 3 times real fast) as I can … simply to minimize pinion angle change during suspension travel.

Upper link mounting hole heights:
Again, no magic number. But the farther apart … think distance not angle … I have the top links from lower links … the more control the suspension has & the finer the tuning adjustments. Mounting them closer together makes the tuning adjustments coarser. I have mounted upper links 3” above the axle tubes (so 4-1/2” above axle CL) at the lowest … to 5” above the top of the housing with centered 3-links (about 13” above axle CL). If I’m running regular tube upper links, I prefer to be somewhere in the middle of that … 6-10” above axle CL. But space considerations often play a role.

Clear?

If not … digest this for a bit … and then let’s talk.

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Last edited by Ron Sutton; 08-02-2013 at 05:47 AM.
Feel free to chime in or ask technical questions.

Ron Sutton

After 35 years of racing, I have refocused my company "Ron Sutton Race Technology" to bring modern, cutting edge suspension, brakes, aero & powertrain to serious Pro-Touring, Autocross & Track Cars. I offer affordable technical services, chassis & suspension design, consulting & track support ... along with 600 brands of parts ... as well as Track Warrior & Street Warrior cars in stages from "Build Your Own" to "Rolling Chassis" ... up to "Turn Key."

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