1" pullout for driveshaft measurement?

Curious why 1" is used for the measurement when ordering a driveshaft?
I know there's suspension moment, but for a typical car that DS angle is almost flat at rest.
So any movement of the suspension would only pull the driveshaft out more.
I get that a several pinion angle downward will have some affect, but not much.

So is it better err on a longer DS, 1/2" pulllout?

Reason I ask, is I had a drive line vibration, putting in a longer slip yoke helped a ton.
The pullout now is 1.5", and I still have some vibration.

Don't want to spend $600 and not get it right.

Thx

When i put my car together(many moons ago) The driveshaft guy asked me to put my car on a drive on hoist and measure from the trans output shaft tip to the center of the ujoint saddle on the diff. I gave him that measurement and the slip yoke and he built me a shaft. It's been in there 25+ yrs with zero issues. Could your vibration be caused by pinion angle and not the shaft? I changed ride height at one point and developed a vibration on light accel..... a shim under the leafs cured it.

Keep in mind the suspension goes up as well.

Ive had several Drive.Shafts built. And always use the 1inch out measurement. No problems

When I ordered my new driveshaft from Mark Williams, they will go down to 3/4" slip yoke travel (and their tech support recommended that). I think I ended up on the short side of the 3/4" due to rounding measurements.

I converted from a driveshaft with around 1-5/8" slip yoke travel and noticed a vibration improvement from the 1-5/8" to the 3/4". Leaving the U-joint hanging out there too far can be felt, at least with a four speed it can be felt in the shifter. The MW tech confirmed it's a common issue, too. People get worried about the output shaft seal dust boot, make the driveshaft too short, then have a vibration issue.

The old Mopar drag seminars would say to push it in all the way and slide it out 3/4 inch with all the weight on the suspension including the drivers weight and use that measurement from center to center on the length for the U joint cups centers, regardless of which tranny yoke length you had
That method has work well for me and a lot of others racers on here

You don't mention your setup but if the factory built the same body style with the same transmission and rear end, the easy way to get it right is to use the length from the FSM.

The amount that a driveshaft actually 'slips' through full suspension travel on these old Mopars is minimal. On my '66 Dart, the difference shaft length at ride height vs full suspension droop was about 1/8"-3/16" (actually shorter at full droop). After I realized that, I now file this is the "don't overthink it" category. 3/4" of pull out at ride height is plenty.

Keep in mind that the u-joint cups are are 1"+ in diameter and the cradle in the rear yoke that they sit in is a little over half that deep, so you need at least that much in order to push the shaft forward during installation before pulling it back into the yoke. 3/4" should cover all bases.

Appreciate the input, another question

Would changing the front yoke to 1350 from 7290 and using a conversion u-joint change the DS length?
1350 is a little larger, so maybe only 1/16 or 1/8" ?

Thx

The two different size or different brands of front yokes might be different lengths. Best to get the yoke and then measure for drive shaft. No guessing here.

3/4" here.

Some kind of weird pinion angle, leaf spring setup may need more.

I’ll answer my own question on this part, 1350 has 1/16” larger cup size compared to 7290.
So 1/2 of that 1/16” would make it 0.030” longer, nothing to worry about.

I’m going 3/4” when I get around to getting this done.

True.

But the hole is bigger. The C/L doesn't move. Does a larger or smaller crank pin journal change the stroke?

Yes but is the bottomed out length the same on all yokes? Different brands, different materials, oem, aftermarket forged or billet. All the same?

Very good question, I have 2 different yokes, 1 aftermarket cast, and a factory forged that came with the 518 when I pulled it at JY.