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Unread 09-25-2011, 10:27 PM   #1
cjman15
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1986 CJ7 
 
Join Date: Sep 2011
Location: redding, califonia
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drive shaft and lift

will i need to do any modification with a 4 inch procomp lift, orv 4 inch, or trailmaster 4 inch? any experience with either of these lifts?
thanks,
Aaron

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Unread 09-25-2011, 11:54 PM   #2
BagusJeep
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You have a late model but you do not say which gearbox you have. You can work out how much change you will have on your driveshaft angle and length from simple trigonometry if you know how long your driveshaft is.

For example if you have the T4 or T5 you will be concerned about the rear driveshaft which is 22.9" long. The front shaft is nearer 32" long so not as much of a worry. These are the standard lengths, there is some slippage in and out allowed in the design.

If you take 20.9" as the length between the centres of the joints (allow 1" both ends) and drop one end by 4" the angle of drop is just over 10 degrees. The new overall length is around 23.5" so by itself length is not an issue but the added strain of having to flex every revolution by an additional 10 degrees will kill your prop joints after a while.

Having said that a lot of people seem to run 4" lifts without doing any work to their propshaft angles. I can only assume they spend money on replacing them more often than I do or just buy better ones.

If you look at the more advanced builds on this forum you will see that changes in pinion angle from revised spring perches are fairly common on the back axle to get it to line up. This removes the extra flex from the rear end of the rear driveshaft. Doing this to the front means turning the diff housing on the axle tubes but may not be needed as the change in angle on the longer front shaft is less than 8 degrees.

Another common feature is lowering the gearbox mounts to improve the rear angle at the expense of the front driveshaft angles.

There are other types of driveshaft that take these angles better.
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Unread 09-26-2011, 12:27 AM   #3
rainjeep
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Quote:
Originally Posted by BagusJeep View Post
You have a late model but you do not say which gearbox you have. You can work out how much change you will have on your driveshaft angle and length from simple trigonometry if you know how long your driveshaft is.

For example if you have the T4 or T5 you will be concerned about the rear driveshaft which is 22.9" long. The front shaft is nearer 32" long so not as much of a worry. These are the standard lengths, there is some slippage in and out allowed in the design.

If you take 20.9" as the length between the centres of the joints (allow 1" both ends) and drop one end by 4" the angle of drop is just over 10 degrees. The new overall length is around 23.5" so by itself length is not an issue but the added strain of having to flex every revolution by an additional 10 degrees will kill your prop joints after a while.

Having said that a lot of people seem to run 4" lifts without doing any work to their propshaft angles. I can only assume they spend money on replacing them more often than I do or just buy better ones.

If you look at the more advanced builds on this forum you will see that changes in pinion angle from revised spring perches are fairly common on the back axle to get it to line up. This removes the extra flex from the rear end of the rear driveshaft. Doing this to the front means turning the diff housing on the axle tubes but may not be needed as the change in angle on the longer front shaft is less than 8 degrees.

Another common feature is lowering the gearbox mounts to improve the rear angle at the expense of the front driveshaft angles.

There are other types of driveshaft that take these angles better.

can the rear axle pinion angle be changed by adding shims only? if so how do you know how big of shims you need? took my transfercase drop spacers out and now i have vibrations, daily summer driver and don't think i need an expensive custom made driveshaft for what i use it for.
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Unread 09-26-2011, 01:29 AM   #4
BagusJeep
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Quote:
Originally Posted by rainjeep View Post
can the rear axle pinion angle be changed by adding shims only? if so how do you know how big of shims you need? took my transfercase drop spacers out and now i have vibrations, daily summer driver and don't think i need an expensive custom made driveshaft for what i use it for.
With an auto your driveshaft is only 0.75" longer than the example above. I see you have a 4" lift so you will have the same issues with an added 10 degrees on your driveline angle. The maximum angle you should be running with a standard driveshaft is near 15 degrees. You started off with probably about 7 degrees so you are beyond the advisable angle.

You say "drop spacers" have been taken out so i assume the rear of the transfer box is now higher? This will add 1-2 degrees to those angles.

You COULD shim the perches but this may not be advisable, if those shims worked loose you would have a problem. However it is cheap, so assume the perch is 4" long and you need 1/2" difference between front and back to achieve roughly 10 degrees upward pointing, probably a bit more. The closer you are to pointing at the rear of the transfer box the better, it also raises the yoke a bit. Best thing to do is to measure it with a protractor and spirit level and work out what your angles are and where they need to be. How you are going to get the axle to sit squarely on the spring with shims is hard to say, but you will probably think of something.

Better solutions are rotation of the diff housing (more difficult than it sounds as it is not supposed to rotate) and rewelding of the perches.

If you are having vibrations it could be because your joints are worn. Have you checked them for play? With this amount of angle they will not last long and you will need good quality ones (say Spicer) and regular greasing or spend a lot of time under it. With a vice and BFH they are not difficult to chaneg at home.

When my wife first got her 1984 CJ I found a joint which had failed and not been changed. The circular ends of the cross bolt on one were worn about 35% and worn square rather than round. It had near 1/2" of play and it was only a question of time before t broke up or something else cracked. The noise from the exhaust drowned the noise and vibration from the drivetrain.

A custom driveshaft will not by itself change the physics of these large changes in each revolution. You are working the joints pretty hard. However a CV joint installed at the transfer box and a pinion pointed up will allow an angle of up to 30 degrees before it binds. This is pretty extreme.
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BagusJeep lives in Bali, the Land of Temples.
With a Jeep every prayer counts.

1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1995 Cherokee 4.0 - CHEROKEE
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