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  • Driveshaft angle questions

    I have a 360/T18/Dana 20 combo driving the D20 rear and d30 front. I have completed the SOA with the YJ spring conversion giving me about 5.7" of lift. Now that I am back from Afghanistan, I need to get the drive shafts modded. The rear needs to shortened due to the transmission swap, and the front needs to be lengthened because of the lift. I did a bit of measuring and this is what I came up with.

    The rear pinion is angled up 4 degrees, and the transfer is angled down 5 degrees (allowing the axle to torque up during cruise torque). The driveshaft measured 27 1/4" long with an angle of 24 degrees (or an operating angle of 20 degrees). This is rather steep. Could I get away with a single Cardan shaft, or am I going to have to get a double Cardan?

    The front axle is much less severe. I have 6 degrees up from the pinion, and 6 degrees down from the transfer case with a 33 1/4" driveshaft. This will be sitting at 17 degrees or 11 degree operating angle. I am pretty sure I can get away with a singe Cardan on this one.

    What are you thoughts?

    One last question. Any suggestions of shocks?

  • #2
    I can't say in your particular case, but I had to go with a dual cardan and it worked well on mine.


    • #3
      Anyone else? I'm ordering them today sometime, so let me know if the extra money I'm spending on the double cardan is worth it!


      • #4
        I geeked out again.

        If my rear driveshaft angle is 24 degrees from level, and is 27 1/4 inches long, then it calculates that the distance from the rear of the tranny to the pinion is almost 24.89 inches and the drop is 11.08 inches. I'm trying to figure if I need to allow for more splined length on the driveshaft to allow for greater expansion/contraction of the driveshaft due to the increased angle that it is running. The factory driveshaft (depending on which one I was playing with, so I went with the longer of the two) expands/contracts 2 1/2 inches. So, since the operating length of the shaft should be around half the expansion/contraction length, I started calculating the numbers that I needed.

        First off, I needed to know how much axle drop and rise the factory allowed for. So I calculated the factory numbers. I rounded off a few to make my math easier, so I used 25 inches from tranny to pinion horizontal length, the pinion being about 5.5 inches down from the tranny, to calculate the factory driveshaft length of 25.6 inches. Then I added half the expansion contraction length to see how much rise/fall the factory allowed in the axle. It was 4.29 inches. I rounded this to 5 inches for safety.

        Now I went back to my application again and calculated that the pinion sits about 11 inches lower than the tranny, so I added the 5 inches of up/down movement of the axle. This gave me 16 inches of total drop at full extension. I know already that the horizontal distance from the tranny to the pinion is about 25 inches, so the result is a driveshaft length at full extension is 29.68 inches from the original static length of 27.25. This equals 2.43 inches of expansion, for a total driveshaft spline length travel to be 4.86 inches.

        I understand that there are some errors already assumed in this calculation, and someone may already have provided a graph of these needs in driveshaft modifications, but I haven't seen them. Someone check my logic on this please.


        • #5
          A phone call to Tom Woods would probably save all sorts of frustration and gray matter destruction, 47.


          • #6
            yes call tom woods they will tell you what you have to do and what shaft you will need


            • #7
              Tom Woods. What a patient crowd. I explained everything as I had calculated before they chuckled at me. After asking me how long it took to calculate all this, he told me he could have given me the answer in the time it takes to call. Well, it was nice to now that my trigonometry still works! Their answer and my answers were only about an 1/8 inch away from each other. So, the drive shafts are ordered and should be in on Friday or Monday. I am hoping for Friday, as next week I leave for California for two weeks.

              I got the Jeep running yesterday after about a year in a barn. She sounds great! Put new belts on, new oil, new tranny fluid, new gear oil, new fuel filter, fresh tank of gas, new fuel line (don't ask) and she started up after a little priming of the fuel system. It looks as though I need new front brake calipers, and maybe a new master cylinder (again). I also ordered the new shocks and steering dampener yesterday, even though I had new ones in a box before I left (wife cleaned the garage out). With any luck I should be on the road in a week.

              I did end up with a double Cardan drive shaft with a 4.28 slip. I'll post some picks when I am done. What a shame to do all this work and sell it!
              Last edited by 47driver; 09-16-2010, 09:03 AM.


              • #8
                Why do you have to sell it? I don't know the story?


                • #9
                  We are moving to Germany and it isn't street legal, and I am pretty sure I won't pass the emissions test. We will probably pick up an '09 or '10 JK and put a conservative lift on it.


                  • #10
                    Got it all put together and drove it home today. I should get my shocks tonight...but I leave for California in the morning, so no play time. I will attempt a backyard alignment, as soon as I get back from Cali. It was all over the place coming home tonight. She needs a bath real bad also; again, has to wait till after Cali. Then I'll be home for a week, then off to South Dakota for a hunt. I hope I can get the alignment dialed in enough to drive around.

                    Any suggestions? I know there are old threads on this, so I will search also.