More T18 Discussion...

I'm running standard long stroke rear shafts from Tom Woods on both the T18 and SM465 (which is longer and in a flatty). Don't have measurements on hand, but from what I understand your building and length - you should be fine. It really is more important that you get the angle of the Tcase and Rear pinion lined up as that will drive the bind in the drive shaft.

 

It 5 oclock here....I am going to go drink whiskey

 

Thanks Ken - this is what I needed to know. As the D18 output gets pushed rearward, the shaft shortens and the angle will get steeper - In theory, I can push the output and shorten the shaft until the angle gets to 10 degrees or so.

I'll go take a few measurements - with the current inclination and shaft length (19") and some trig, it'll be interesting to see what the calculated shaft length would be at 10 degrees of inclination - the trig isn't quite as simple as it should be as I am guessing the D18 output/D44 pinon, while hopefully parallel, are not horizontal. I can also calculate the estimated shaft length and angle for my currently preferred T18 mount that will shorten the horizontal side by 3". Given that we are working with small angles here of less than 10 degrees, pushing the D18 output rearward by 3" means the shaft length will shorten by almost the same 3" - but its not exactly the same. The shaft length will be just about 16".

 

Tom woods suggests 7 degree maximum ujoint angle (for a traditional 2 joint shaft)....
https://4xshaft.com/blogs/general-t... equal or intersecting,a maximum of 7 degrees.

I am not sure how jeep mounted their engines, but IIRC with the sbc in the 58, the D18 output was down to the rear at 3 degrees (and yes the pinon should match that). So when you start at 3 degrees down, add maximum of 7 at the actual joint, you get 10 degrees total from horizontal. If using Ken's 10 degree joint maximum, that would be 13 degrees from horizontal. I am interested in the measurement from horizontal as that is what you directly measure with inclinometer on the shaft...I'll see if I can get a pinon/D18 output angle, but that isn't as easy with the yokes and everything connected.

 

but crunching the numbers is the fun part...and I'd rather make questionably bad decisions based on estimated calculations from sketchy measurements than whimsical decisions based on CW....
Here are the numbers - these are all done with the jeep sitting statically under its own weight on a level concrete floor.:
Current driveshaft is 19" at an angle to horizontal that is 12 degrees. To get the actual ujoint angle I need the a direct measurement of how much the pinon is angled upward, and that is difficult without pulling the driveshaft and yoke, but best i can measure its 4 degrees. Trying to use the D18 ouput was even more difficult. So anyway, that suggests ujoint angle is around 8 degrees at the pinon end.

Doing some calculations - with a couple of assumptions: first, only calculating these triangles in the vertical plane - not accounting for the lateral offset between the D18 and D44. secondly, assuming the vertical height of the D18 output doesn't change - only pushed rearward horizontally. If anything, in reality the D18 output would possibly be lower - which only helps improve the angle. So, if the D18 output is moved rearward 3," it will shorten the driveshaft to 16.1" and increase its angle by 2.2 degrees. That would increase the ujoint angle to 10 degrees.

Now for comparison, going with the shortest possible T18 mount which pushes the D18 back only 1.2", the shaft length would 17.85" and increase in angle only .8 degrees to just under 9 degrees. in a sense, going through the extra hassle of swapping the input shaft and using an AA bellhousing, would save 1-1.5 degrees of drveshaft angle....all with the understanding that I really need to get the pinon angle measurement more accurate to really know where the ujoint angle stands.

 

Understood....but also recognize what I am asking is not the best possible solution for a 4speed swap or whatever (or at least not yet)...its a very specific question with a very specific set of constraints. At some point I may very well look into other possible solutions (such as a double cardan, or heck, even a different 4 speed swap, lengthening the wheelbase to cj6 etc), but for now, this question is specific to a traditional double u-joint driveshaft, and determining what its limits are regarding length and angle, and how it relates to one of two specific set-ups. Once i fully understand these possible solutions in this scenario, then we can discuss other options. Sometimes the breadth of the discussion needs to be set aside for a better chance to delve deeper into a more narrow topic. This can be done for a variety of topics, and then a comparison can be made between these topics and potential end solutions that use different methods. There is a lot of info to be addressed in a deep dive on double cardan joints as well - as Ken has been documenting (like can you run a D18 brake drum etc) - we'll get to that stuff soon enough. It probably helps to think of most of my stuff as hypothetical thought experiments....I've been working on this 4speed swap for 6+ years, its right in line behind the saginaw steering and hanging pedals and everything else that I am supposed to be doing to make my jeep usable.