Explain driveshaft plunge. . .limitations???

[atomicjoe23]

I've been doing a lot of research into long travel 4-link rear suspensions and as I was reading a thread today someone referenced driveshaft plunge in reference to a LT 4-link rear. . .I hadn't seen this mentioned as a limitation to 4-link travel previously so now I need to learn a little about this so I don't fab a poor set-up for my truck. . .it was alluded that this was mainly a problem w/short wheelbase vehicles but they also mentioned short driveshafts due to transfer cases on 4WD vehicles. . .my truck has a 117" WB and it's a 4WD. . .I just measured the driveshaft and it's 47.75" long (from yoke weld to yoke weld at rest). I would like to be able to achieve 18-20" of rear travel without having driveshaft, third member, or transfer case issues. . .

One statement from a builder in the thread was that he considered plunge to be the most critical aspects of 3- or 4-link design. . .but I've never heard of it before. . .especially not in reference to a suspension system. . .driveshaft angle yes but not plunge. . .

By the way I am newer to the off-road scene. . .all of my experience previous to the past year has been on muscle cars and motorcycles. . .so I would really like to learn more about the off-road application of stuff. . .I've got two 4WD's that I'm itching to start working on but I want to make sure I have a grasp on things before I just start doing stuff. . .

My questions are:

1) What exactly is driveshaft plunge? I have a couple of ideas on what it could be based on the name, but I want to make sure I know because I haven't heard this particular term before.

2) What is an acceptable amount of plunge?

3) Along the same lines what driveshaft angles are acceptable in a long travel desert rig? One individual in the thread said his was 27* and the driveshaft builder said it would be OK. . .another questioned this and thought it was excessive. The vehicle in question was a Class 3 Wrangler with a 105" WB and a 31" drivshaft. . .he had 27* driveshaft angle at full droop (18").

Thanks for the clarification on this stuff!!!

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[1450-ranger]

1) What exactly is driveshaft plunge? I have a couple of ideas on what it could be based on the name, but I want to make sure I know because I haven't heard this particular term before.

Change in the distance yoke to yoke as the suspension cycles. The driveshaft must change length.

2) What is an acceptable amount of plunge?

Argued alot. Some builders think it is near the top of priorities, some do not. Personally prefer as little as possible. On a stock frame rail truck you are somewhat fixed on suspension geometry, main Instant Center, without getting too crazy. Little driveshaft plunge, 1/4" through 30" travel, helps keep the carrier bearing alive on our truck.

[atomicjoe23]

Thanks McClintock. . .that's what I thought it was but I just wanted to be sure. . .

I knew that the driveshaft lenght has to change and this is the telescoping part of the driveshaft correct? On my '79 F-150 there is a telescoping part between the mid-point of the driveshaft and the third member. . .

What can be done to limit plunge? Is there a way for me to determine how much plunge I will see. . .like if I was designing a 4-link set-up and I know the location of my components (transfer case, 4-link lengths and mounting points) could I determine ahead of time how much plunge I will see ahead of time on paper or is this something that is normally done be cycling the suspension from say at rest to full droop?

Thanks again. . .you have been very helpful on my posts!!!
My build will be on the stock frame rails. . .

[1450-ranger]

I knew that the driveshaft lenght has to change and this is the telescoping part of the driveshaft correct? On my '79 F-150 there is a telescoping part between the mid-point of the driveshaft and the third member. . .

Correct.

What can be done to limit plunge? Is there a way for me to determine how much plunge I will see. . .like if I was designing a 4-link set-up and I know the location of my components (transfer case, 4-link lengths and mounting points) could I determine ahead of time how much plunge I will see ahead of time on paper or is this something that is normally done be cycling the suspension from say at rest to full droop?


The plunge is fixed by the geometry of 4 link. Lower link length is the main driver; think of where your driveshaft pivots about and how your axle should. I saw that you have a CAD program, do a simple 2-D sketch of the suspension @ bump and droop.. D-shaft distance @ bump will be longest, @ droop will be shortest.

[Wild bill]

You can calculate your pinion angle change and plunge with a Computer program. I just bought this program and it will let you change mount points, arm lengths, etc. and see what it does to the plunge and pinion angles. The program can be found at www.performancetrends.com. It will cost you $130.00 but believe me, it will keep you from making some very expensive mistakes. From some rough calculations, expect 1.5" of plunge with that short of drive shaft with 18" of travel.

[Jess@HighAngle]

Its said that if there is to much driveline slip change it will cause issue's pulling on the trans and or differential - I believe we have actually made headway on this issue- but the hard core pro trucks usually have 1-2 " max of driveline length change - Our trophy truck shafts have up to 8" of slip available and the spline is crome moly with a dual sleeve and extra lube capability the best driveline on the market we feel- Jess

[atomicjoe23]

The plunge is fixed by the geometry of 4 link. Lower link length is the main driver; think of where your driveshaft pivots about and how your axle should.

So if I have fairly long links (right now I've designed my set-up to have an ~56" long lower link) and located close to the pivoting point of the driveshaft/transfer case output shaft (my design also has the upper and lower links pivoting at roughly the same point as the U-joints of the xfr case output shaft/driveshaft) that will help limit the amount of driveshaft plung I will have. . .correct???

I saw that you have a CAD program, do a simple 2-D sketch of the suspension @ bump and droop.. D-shaft distance @ bump will be longest, @ droop will be shortest.

That will be easy to do. . .I can just copy the drawing at ride height, set the base point as the pivoting point and adjust it up to full bump and down to full droop. . .I REALLY like CAD. . .makes things so easy to do once you learn how to do it!!!

You can calculate your pinion angle change and plunge with a Computer program. I just bought this program and it will let you change mount points, arm lengths, etc. and see what it does to the plunge and pinion angles. The program can be found at www.performancetrends.com. It will cost you $130.00 but believe me, it will keep you from making some very expensive mistakes. From some rough calculations, expect 1.5" of plunge with that short of drive shaft with 18" of travel.

Thanks for the link. . .I'm gonna check it out!!!

Its said that if there is to much driveline slip change it will cause issue's pulling on the trans and or differential - I believe we have actually made headway on this issue- but the hard core pro trucks usually have 1-2 " max of driveline length change - Our trophy truck shafts have up to 8" of slip available and the spline is crome moly with a dual sleeve and extra lube capability the best driveline on the market we feel- Jess

Could you explain "driveline slip change" please???

[Jess@HighAngle]

Slip change is the amount of driveline plung or what I call slip or length change thru the cycle of suspension movement -Just another way of saying it- Jess

[Giant Geoff]

If you are having trouble the basics you can just buy my WIY link arm and pivots that have all the technical aspect and geometry built in.

[philofab]

If your drive shaft has to be 47.75 inches just shorten your lower links a little... with a 50" lower link and an upper link that is ~48" total distance parallel with the frame (the actual links are physically longer on a triangulated four link since they also travel from the frame rail to the center of the vehicle) you'll find you will have less than an inch of plunge. Minimizing the plunge in my opinion is very important for driveshaft/carrier bearing/trans output shaft/u joint life.

This is of course providing that you place the pivots in the correct places on your chassis and axle.

Minimizing your plunge with a four link can skew your anti squat and other geometry though. It's all about making everything work together... which is difficult.