3 and 4 link rear suspension questions

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Jer73
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3 and 4 link rear suspension questions

Post by Jer73 » Sat Feb 20, 2016 1:36 pm

Hello I have gotten to a point on my build where it is time to mock up my 3/4link rear suspension and I have a few questions. The car is oriented at road race/canyon carving and the occasional trip to the drag strip. It is a street car and not a full blown racecar... yet.

1. How far should I space the lower control arms from the tire sidewall to prevent it from making contact under high cornering loads? I know spacing your control arms out as far as possible makes for a more stable rear suspension but too far and you start running into clearance issues. I have a 285/35/18 on a 10.5in wheel, 1 1/4in clearance looks good but I am unsure and looking for suggestions.
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2. How much of a difference will the 3 link compare to the 4 link in terms of handling and straight line performance? I have read that the 4 link is more prone to binding up than the 3 link. I am using 3/4 heim joints with high misalignment bushings and 24.5inch long control arms equal length upper and lowers, should I expect any kind of bind with the 4 link setup?

3. Offsetting the upper link on a 3 link can help cancel out driveshaft torque to help keep the car straight on launch/under acceleration. How does offsetting the upper link affect braking?

4. I am using a winters championship quick change rear axle and have really been scratching my head as how I could use a watts linkage that mounts centrally to the frame and the links extend out to the axle tubes. The spur gear housing is right in the middle of the works and kind of screws that all up. Is it worth the headache to make a watts fit instead using a panhard bar?
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Looking forward to some replies thanks. :D

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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sun Feb 21, 2016 4:31 pm

1. Personally, I'd call that good enough.

2. Here is the deal. Using 4 links with parallel and equal length links on each side will not bind (appreciably). But, it will also not have any anti-squat, which you will want. If you drop the chassis-end pivot of your upper arm, you will now have anti-squat but now imagine what tries to happen in roll. One end of the axle wants to twist one way and the other end wants to twist the opposite way. That's "binding" and it will either make the weakest thing in the system act as an anti-roll bar, or it will break something, or it will prevent the two sides from operating (sorta) independently in roll or in one-wheel bump. This is bad.

If you look underneath various rear axle drive OEM vehicles, some of them over the years have used 4-link+panhard rear suspensions. The current Ram trucks are one example. The upper and lower links aren't necessarily parallel. BUT ... they are using compliant bushings somewhere in the system in order to provide that deflection (and OEM vehicles use compliant bushings anyhow to cut down NVH). Also, typically, these vehicles aren't set up for hard drag-race launches, and often not for good cornering.

IMO you should keep your lower links and use one offset upper link positioned hopefully somewhere near where it should be for driveshaft torque cancellation, and now the upper link no longer needs to be parallel to the lower links because you will be free of binding, so you can build in some anti-squat (and even make it adjustable).

3. Obviously an offset upper link will transmit braking torque asymmetrically into the chassis, and it's not going through the drive shaft. BUT ... During acceleration, weight transfer is rearward and the rear wheels are doing (presumably) all of the accelerating, so it matters. During braking, weight transfer is forward, and the front brakes are doing most of the work, and the rears are just contributing that last little bit. So ... what happens under braking might be a little asymmetrical, but it's pretty much irrelevant since the front brakes are doing most of the work anyhow.

4. I'd use a panhard rod that is as long as you can make it and as close as possible to horizontal at nominal ride height. I don't think a watts linkage is worth the extra complexity. If your aforementioned lower link arms are parallel in top view, the axle-steering effect because of the axle being pulled a little to the side towards the limits of up and down suspension travel is going to be negligible.

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Re: 3 and 4 link rear suspension questions

Post by Jer73 » Sun Feb 21, 2016 7:36 pm

Okay so anything other than parallel upper and lower links with a 4 bar set up in side view will result in binding and worst case scenario breaking shit.

Now I am going to be building the three link mounts I just haven't gotten there yet. Any advice as to account for how much offset is needed to eliminate driveshaft torque?

This is the only video of a direct comparison I could find between a 3 and 4 link cars handling. Mind you this is a triangulated 4 bar and not a parallel set up.
https://www.youtube.com/watch?v=Pq5LjV4lnV0

To me it looks like the 3 link is less twitchy through the slalom.

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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sun Feb 21, 2016 10:05 pm

Triangulated 4 link without a Panhard will not bind provided that the end links on each arm allow the necessary twisting motions. Not all of them do. Rigid aftermarket bushings on both ends of fox body Mustang upper triangulated links plus aftermarket rigid arms that don't want to twist will make them bind! There's a reason the stock upper arms are stamped sheet metal with an open (inverted U) cross section ... they're supposed to twist, and if you don't let them, it binds!

Aside from that situation (which is user self-inflicted) the usual trouble with triangulated 4 links with the upper arms diagonal is that the roll center is way too high, and you can't do the driveshaft torque-cancellation trick. There's a reason the S197 Mustang went to a 3-link with panhard from the Fox-body triangulated 4-link. Mind you, there's also a reason the S550 has gone to IRS.

At one point there was an on-line calculator for figuring out the geometry of an offset 3-link but I cannot find it; I found an old link to it but it's not there any more. An analysis of how much force is applied in which direction should have the same results. You will want to make up a spreadsheet to do this.

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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sun Feb 21, 2016 10:15 pm

As an aside ... if it were me in the position of figuring out those forces, I'd use your existing link arrangement on the right side of the car as a starting point and simply leave out the upper arm on the other side, and see where that puts you.

The chassis end of the upper link needs to be lower than the axle end of that link; this is what gives you both the anti-squat and since the force on that link now has a vertical component where it connects to the chassis and that point is not in the middle of the car, that's also what gives you the driveshaft cancellation torque.

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Re: 3 and 4 link rear suspension questions

Post by MadBill » Thu Feb 25, 2016 8:43 pm

I asked a top race car design engineer about the offset for the top link. He didn't give me the details, but said he once built a chassis with enough offset of the link to completely cancel the forces. He said the car was bad beyond belief and that he cut up the rear clip and started over. By observation, somewhere ~ 5-8" offset seems to work.

Re a Watts link, maybe you could find a picture of the rear set up on the Howe TA2 car. It uses one with the mounting bracket for the rocker on the nose of the quick change (A Tiger QC unit is standard, I believe.)

There was an ancient S/T post on "Traction Dyno" that shows how to test third link offset
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Re: 3 and 4 link rear suspension questions

Post by John Wallace » Thu Feb 25, 2016 9:25 pm

There was an ancient S/T post on "Traction Dyno" that shows how to test third link offset
Billy Shope made those:
Billy Shope Calculators

:)
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Re: 3 and 4 link rear suspension questions

Post by Jer73 » Thu Feb 25, 2016 10:34 pm

MadBill wrote:I asked a top race car design engineer about the offset for the top link. He didn't give me the details, but said he once built a chassis with enough offset of the link to completely cancel the forces. He said the car was bad beyond belief and that he cut up the rear clip and started over. By observation, somewhere ~ 5-8" offset seems to work.

Re a Watts link, maybe you could find a picture of the rear set up on the Howe TA2 car. It uses one with the mounting bracket for the rocker on the nose of the quick change (A Tiger QC unit is standard, I believe.)

There was an ancient S/T post on "Traction Dyno" that shows how to test third link offset
Do you recall any details about the car?
John Wallace wrote:
There was an ancient S/T post on "Traction Dyno" that shows how to test third link offset
Billy Shope made those:
Billy Shope Calculators

:)
Thanks!

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Re: 3 and 4 link rear suspension questions

Post by MadBill » Thu Feb 25, 2016 11:35 pm

Jer73 wrote:...Do you recall any details about the car?...
I think it was a CASCAR, now known as NASCAR Canadian Tire Series or N-CATS Stock Car.

Here's the book he wrote (and I edited), but be warned, he calls it simple and basic but it's anything but for most of us: http://www.rowleyrace.com/publications.htm
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Re: 3 and 4 link rear suspension questions

Post by peejay » Sat Feb 27, 2016 1:03 pm

The amount of offset on the 3rd link will depend on the gearing in the rearend! A 2.77 final drive is going to apply more of a twist on the rear suspension than a 4.11 will, it needs more driveshaft torque for the same acceleration.

I wouldn't get greedy and go for 100%. In practice I don't think you would ever WANT 100% anyway because engine (driveshaft) torque is also trying to lift the left front corner off of the ground, and this pulls load off of the left rear in a roundabout way.

Anyway this is why I have bee nrunning 4.78 final drive and will be switching to 5.43s in the near future.

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Re: 3 and 4 link rear suspension questions

Post by Brian P » Sat Feb 27, 2016 2:48 pm

There's other wacky things the offset 3 link will do. In one wheel bump, the left wheel going over that bump won't rotate the rear-end housing, the right wheel going over that bump will. In itself, this is not a big deal. But it also means that if your springs and shocks and antiroll bar (if present) are not either mounted to the (symmetrical) lower trailing link, or transfer force directly in line with it (attachment points on the axle are directly above or below the trailing link attachment point to the axle), and these points are not in line (above/below) with the centerline of the axle housing, it will have a different effective spring rate / shock rate / antiroll bar rate when measured at the wheel.

The asymmetrical effect on what happens when the rear brakes are applied has already been mentioned, and there is no way around that, although hopefully the brake bias of the vehicle keeps most of that on the front.

To me, this seems like one of those things that might be of some benefit in the first fraction of a second after a hard drag-race-style launch but a liability all the time after that.

The S197 Mustang uses a 3 link but the upper one is in the middle - no attempt to cancel driveshaft torque.

Did some digging ... I knew I had seen an old car that came with an offset 3-link as original equipment ... and I found it. It was the GM X-framed full-size cars from the late fifties to the mid sixties. Here is the best pic that I found of the rear suspension setup.

http://www.xframechevy.com/wp-content/u ... rame-4.jpg

Note that they put the upper link on the right side. I'm not sure, but I think the spring perches are on the lower trailling links, and the attachment points for those links on the axle are pretty much straight underneath the axle. The down angle of that upper link (and the panhard rod) probably is not a good indicator of what position the link is in at nominal ride height. I don't know if GM was thinking about driveshaft torque cancellation back then.

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Re: 3 and 4 link rear suspension questions

Post by MadBill » Sat Feb 27, 2016 10:50 pm

peejay wrote:The amount of offset on the 3rd link will depend on the gearing in the rearend! A 2.77 final drive is going to apply more of a twist on the rear suspension than a 4.11 will, it needs more driveshaft torque for the same acceleration...
I believe it's the other way around: A higher number gear applies more axle housing nose-up rotational torque per lb-ft. of driveshaft torque and it is the former that 'pulls' the right side end of the axle down via the angle of the 3rd link.
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Re: 3 and 4 link rear suspension questions

Post by peejay » Sun Feb 28, 2016 5:01 am

We're looking at different axes. I meant the twist trying to rotate the rearend like a propeller, lifting the right side and pushing down the left side. (The effect that we would be trying to counteract by offsetting the 3rd link)

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Re: 3 and 4 link rear suspension questions

Post by MadBill » Sun Feb 28, 2016 12:02 pm

The offset link develops its 'propeller' resisting torque proportional to both the offset distance, the link's angle relative to the ground and the axle housing torque resulting from the pinion trying to climb the ring gear, the last being proportional to the gear ratio. If the link was horizontal, it couldn't offset any of the driveshaft-induced 'propellor' lifting force.
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Re: 3 and 4 link rear suspension questions

Post by John Wallace » Sun Feb 28, 2016 4:47 pm

I'd read this from the beginning to the end.
Has some very interesting data.

Shope Calculators

:)

Answers a lot of the questions I believe.

:)
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