Drive Shaft Basics


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	Drive Shaft Basics


		by Jim Clark, Hot Rod M.D.

	What: A solid steel or hollow steel/aluminum tube that transmits rotational power from the output shaft at the rear of the transmission to the input/pinion shaft in the rear differential. The solid steel or hollow steel shaft with outer housing (closed driveline) is usually housed inside a tube bolted to the rearend at the pinion shaft and pivoting around a round socket connection at the rear of the transmission (example early Ford driveshaft in torque-tube). The solid or hollow shaft is usually connected to the pinion shaft at the differential by a coupler and connected at the rear of the transmission by a universal joint. Hollow steel/aluminum tube (open driveline) is usually connected at both ends by a universal joint. Rear differential movement up and down changes the spacing between the trans and rearend so the splined front yoke at the rear of the transmission slides in and out of the transmission to allow for this movement. Longer driveshafts are split into two sections and supported by a bearing assembly bolted to the frame of the vehicle. How: The closed driveline that is most commonly seen on early Ford hot rods is mainly a swap meet or wrecking yard item. It bolts up to the back of the crossmember and connects to the rear of the transmission. The other end of the outer tube bolts to the rearend at the pinion/ring gear housing. To use a closed driveline in a different than original vehicle, the outer housing and center shaft can be shortened. The outer housing is cut and welded back together at the desired length. This is a relatively simple process. Shortening the inner shaft is more challenging though. This is a job for a professional driveline shop that has experience with this process. Some early shafts (33-34 Ford) will need an adapter (old Ford part #48-4684) with 6-splines to replace the early 10-spline end. An early Ford specialist will be able to provide the proper conversion.


		At the rear is a complete early Ford rear end with the stock closed driveline

	Internal shaft in early Ford closed driveline is connected to the pinion via this splined and pinned coupler.


	This is a typical open driveline in a 1932 Ford chassis.




	To figure out the length of the new driveshaft you will need to determine a number of dimensions. For accuracy the vehicle will need to be resting on all four wheels at it’s running ride height. If you have the correct transmission yoke, insert it into the rear of the transmission until it bottoms. Then pull it out ¾ to 1-inch. Some portion of the front yoke should still extend through and beyond the rear tailhousing bushing at this point. If in doubt mark the yoke and remove the tailhousing, insert the yoke to the mark and verify that some portion of the yoke still extends beyond the bushing.

After establishing the yoke location you can determine the driveshaft length by measuring from the centerline of the front U-joint cup to the centerline of the rear U-joint cup (dimension A).

If you don’t have a yoke, measure from the rear of the tailhousing to the centerline of the rear U-joint cup (dimension B). In this case the driveshaft shop will need to know the transmission spline diameter, number of teeth (C) and the transmission seal diameter (D). This will allow them to supply the right yoke with the new driveshaft.

In both cases you will have to supply the driveline shop with the diameter of the U-joint cup (E), the distance to the retaining clips, and whether those clips are on the inside (F) or outside (G) of the joint. You should also check both front and rear joint as they may not be the same.


	Driveshafts are created from heavy-wall tubing with yokes welded into each end. Then spun balanced to make them run true.


	Shown is a handful of balance weights that are welded to the driveshaft tubes to achieve dynamic balance.


	Shown here is a typical output shaft yoke and universal joint. Small round item welded to the shaft is a balance weight.


	Rear of shaft yoke is fitted with a universal joint that bolts into the yoke in the rear end pinion shaft via horseshoe clamps.


This is a new driveshaft created for a roadster with a Turbo 400 trans and big Buick rear end. Front yoke is ‘60s Buick item, shaft and yokes are new items supplied by the driveline shop.

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