Drive Shaft: Driveshaft yoke depth

[wittsend]

Hello,

I have a '65 Daytona and have replaced the B-W automatic with a GM 700R4. Prior to removing the original B-W transmission I measured the output shaft centerline to the tunnel. I did this to ensure the 700R4's output shaft was following same trajectory as the B-W automatic when I set my transmission mount.

Yesterday I went "Pick A Parting" for a driveshaft. I found a shaft that is virtually the same difference in length as the B-W transmission is from the 700R4. Though it is customary to measure for a driveshaft with the wheels "up" I also measured for the wheels down as most of the cars in the yard have the wheels in a dropped position.

Anyway, the crux of my post is that the distance between the rear of the tailshaft housing and the center of the rear U-joint cap (customary measuring points) is greater with the wheels up (normal ride height) than it is with the wheels dropped downward. I had always assumed that the opposite was true. (See pictures)

1. The first picture with the yoke deeper in the tailshaft is with the wheels dropped downward.

2. The second picture with the yoke extended outward is with the wheels at normal ride height.

Is this common? Can anyone enlighten me???

Further, If I follow the rules of setting the yoke depth at 3/4" short of hitting the output shaft with the wheels up (normal ride height) I run the risk of the yoke banging into the transmission output shaft if I encounter a situation where the wheels drop fully moving the shaft inward.

So, basically I'm befuddled because the driveshaft yoke movement seems opposite of what I assumed normal. Likely 99% of the time the car will have the yoke depth overextended (I'm at about 1-1/4" and the general rule is no less than 3/4" no more than 1"). Yet if I don't have that shallow yoke depth I run the risk of damaging the drivetrain because the yoke can possibly hammer the transmission output shaft in situations where the wheels drop fully (though admittedly rare).

Any feedback would be appreciated. thanks, Tom

[Alan]

The spring length is shorter from front to pin at 20 1/2", than from pin to rear at 30" So the rear travels in a tighter arc. That drove me nuts on my Avanti. I would set the angle of the pinion and when you put weight on it it would flatten out.

[junior]

Tom, your findings are consistant with what I found on my car (54 coupe). IIRC, the tech. that built my driveshaft told me it all depends on which end of the spring the shackles are on. I don't know if he was kidding me or not, but never the less I figured the shafts would move together during suspension compression, and move away from each other on suspension extension, but the tech kept telling me I had it backwards. I went back to my garage, and made the following measurements: these measurements were made at the slip yoke at the center bearing, but IFAIK it is still consistant with what would happen with your car. Full suspension droop (extension) was 1/8"gap, 5/8" gap at ride height with no passengers in the car, and 3/4"gap with 3 people sitting in the trunk to simulate compression over bumps. Hope this helps you out. Regards, Junior.

[brian6373]

Just a thought here, unless you plan on doing a Dukes of Hazard thing with your car, just when will the rear axel be fully extended with the wheels off the ground while driving?

[wittsend]

Thank you everyone for your input. The Daytona does not have a center bearing, but the lengthening issues might still be the same.

No, I,m not planning on any "Dukes of Hazard" stuff. BUT..., the problem is that during the condition I'm most likely driving the car..., the driveshaft will be extended about 30%-50% more than is considered acceptable.

Then..., on that rare occasion where the wheels momentarily drop - fully..., at that brief moment the driveshaft will be where it is suppose to be during all of normal driving. That's the annoying part of the issue. It's all "sdrawkcab" (backwards).

Tom

[junior]

Tom, trying to figure out what is backwards, according to my driveline guy, nothing is backwards except for your (and my) thinking the way things should be, not the way things are. Suggestion: get a driveshaft that will allow the yoke about 1/8" clearance from hitting the trans. tailshaft housing when the rear suspension is at full drop, and all else should be well. When weight is applied to the rear suspension, the yoke will pull away from the tailshaft housing. When ever the rear suspension is at full drop, such as on a lift, or the car supported on jack stands by the frame, there will be no interference of the yoke and trans. Good luck , Junior.

[wittsend]

Thanks Junior. As it is I'm 3/8" from bottoming out with full wheel drop. I'm thinking of loosening up all the bolts and see if the engine/trans will move back any. Also, at 1-1/8" I'm only slightly out of that 3/4"-1" "window" at normal ride height.

This driveshaft was from a 1976 Buick Skylark, 4 door, V-6, GM 350 transmission. $23 out the door for a shaft that is 95% correct is good enough for me. There was also a 1995 Chevrolet Caprice Classic (cop car) that had a shaft right about 1/4" longer. Probably ideal. However, at Pick A Part they lift the cars with a fork lift and a LOT of driveshafts were dented/bent (including that one). Also, the rear caps were too large for the Studebaker. My understanding is that there is a U-joint that will fit the larger capped Chevy driveshaft and has smaller caps for the Dana 44 on the Studebaker. So, I may look more into that.

With the cost of shortening the shaft at about $150 I can buy about five different shafts at Pick A Part (50% off sale) and still come out ahead. Hopefully it will end at two.

Tom

[junior]

Yup Tom, I'm thinking you're good to go with this set up. Junior.

[Alan]

.125" is not enough to worry about. Drive it and see if there are any clunks when driving it hard.

[wittsend]

I contacted a driveshaft service and put my questions to them. Wow, they replied back early on both my emails (and this, over the Memorial Day weekend!). This morning I received this favorable rely:

"You're good. The length in the photo is not a problem and when hanging down
it really does not matter.

Thank You
Tech Dept
Denny's Driveshafts"

https://www.dennysdriveshaft.com

So, I'm thinking I'm good to go. Hopefully this will assist others with the same issue. The seventh post from the top has the most pertinent information regarding the driveshaft size. When I get this project finished I hope to get a little "how I did it" posted.

Thanks again to all the responders, Tom