Author Topic: need some opinions and info for trying to find a driveshaft length  (Read 245 times)

Offline Irish_Alley

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someone on facebook asked "can someone tell me the driveshaft length when swapping trans" he went from a th350 to sm465. an idea has been in my head for awhile now about using the pythagorean theorem, since we know two of the measurements (trans length and old driveshaft length. now we cant use the same info for all trucks because somethings will be different (i.e. short bed vs long, one vs two piece shafts and fixed yokes vs slip)

what i need from you guys is your opinion on this and if you could measure your set-up to verify this would actually work. once i get a base line of info i could theoretically make a formula and you could just simply look up the trans and it would show you what size shaft you need with your wheelbase 

http://forum.73-87chevytrucks.com/smforum/index.php?topic=32506.0


heres what i wrote on facebook:

you have to use the pythagorean theorem (one of these days ill make a write up with this info).

you have to measure your old driveshaft (yoke to yoke) however you measure it the first time keep doing it that way. Most do center to center and to do this accurately measure from the left outside edge of one of your u joint holes (in the shaft) to the left outside edge of your other u joint hole, this will give you an on center Measurement, you'll use this as the "hypotenuse" or "H" on the formula.

ill give you one of the "legs: or the "G" on the formula. now in reality the "G" would be measure from the tailshaft of the trans to the 90 angle right above the pinion yoke. but if you did it this way you'll have to measure the space between the tailshaft and the 90 and we can simplify this process just by saying the "G" is the length of the trans.

the "F" is the height between the yoke and an straight up to the trans yoke. Since we want a constant number well use one formula to find that.

Now for the measurements of our one leg, this is going to depend on your tailshaft, I think the 9 is most common so ill use.

TH350 2wd 6 tailshaft 27.68"
TH350 2wd 9 tailshaft 30.68"
TH350 2wd 12 tailshaft 33.84"

SM465 2wd (bell to yoke): 24"

There will be a couple steps because we dont know what leg F is and we have to figure that out. First and to do that well have to use the third image.

F=(length of your driveshaft)-(trans length 30.68)
So whatever your length of your shaft times itself - 941.2624
And that gives you F. now unsquare F by dividing it by itself and with finding the F we can move on to finding your new shaft length (from yoke to yoke) and to do that well use the first formula in the image

Take the F from the first step and square that then add the sm465 length (24) and square that (576)
So F+576= H (just divide H by H and youll have the length of your new shaft yoke to yoke
If you cant tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline VileZambonie

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Re: need some opinions and info for trying to find a driveshaft length
« Reply #1 on: February 08, 2019, 04:25:20 PM »
To figure out driveshaft length you need to use the tape measure theorem.
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Offline Irish_Alley

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Re: need some opinions and info for trying to find a driveshaft length
« Reply #2 on: February 08, 2019, 07:22:57 PM »
lol no thats too simple. i tried a couple years ago and got a hold of the company that made driveshafts for our trucks to see if they still had a listing for all the different driveshafts for our trucks but it was too old and they didnt have that info anymore. and kind of wanted to make a database of that information so we could have it on hand
If you cant tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes

Offline jimishortbed

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Re: need some opinions and info for trying to find a driveshaft length
« Reply #3 on: March 22, 2019, 07:04:03 PM »
I always just find a drive shaft, at a salvage yard,  that's longer than what I need and just cut around the end piece weld. I pull out the end piece, cut the shaft down to desired length, slap the end piece back on and weld it back up.

I have done a lot of them down through the years and never had one to vibrate yet

Online roundhouse

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Re: need some opinions and info for trying to find a driveshaft length
« Reply #4 on: March 22, 2019, 08:56:33 PM »
Gonna have to use a tape measure

Talk to the local driveshaft place and ask em how they want you to measure it

Offline Irish_Alley

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Re: need some opinions and info for trying to find a driveshaft length
« Reply #5 on: March 23, 2019, 07:05:27 AM »
this isnt for me to use as what i needed as much as trying to verify the formula was correct. what i was trying to get to was if we knew what trans someone had and axle we could then be able to plug in those numbers and come up with an driveshaft length. i mean if there was a way to have a measurement of each trans/wheel base then that would be better but i know there was no way to get that information and the company that made the driveshafts didnt keep that information as it irrelevant nowadays to them at least.

i could of went on facebook and asked them but theres so many "smart" people on there the responses would not be consistent as theres so many variables and all, here we have a smaller group of test subjects i figured we could filter out the ones that cant read a tape and keep the good ones from trusted users.

and jim thats what i did on my crew cab cummins. i cut a yoke off the rear driveshaft then took  3" off and re-welded the yoke back on. with no issues.

i want to create another database much like this one for lifted trucks and what tires they could fit
http://forum.73-87chevytrucks.com/smforum/index.php?topic=31301.0
If you cant tell yourself the truth, who can you tell it to?~Irish_Alley

When you have eliminated the impossible, whatever remains, however improbable, must be the truth ~Sherlock Holmes