This will be my first Tech Week contribution, I hope the members of the HAMB will find it useful. Finding out what length driveshaft you need when building a car is sometimes difficult. With so many different combinations of transmissions, rear ends and wheelbases most builders will need one custom-made to fit their car. After the car is very close to being finished (body and chassis complete, engine and trans complete and installed) it's now time to build the driveshaft. This little tech write-up doesn't apply to closed-drive cars with a torque tube enclosed driveshaft. I have no experience with those and I'm sure each type has its own important details you need to know about and watch out for. Nor am I going to show you how to do it yourself even though many here do their own, I'll assume you're taking it to a HAMB-approved driveshaft shop. Just because your buddy dropped a SBC and a TH350 into his '50 Ford (sorry hard-core Ford guys) and you're using the same basic drivetrain in your '49 doesn't mean the driveshaft in his car will be the same as yours, even though both cars are keeping the stock Ford rear end. Transmission pinion yokes vary in dimension, two TH350 yokes won't necessarily be the same length. There are dozens of different yokes that look close to the same but aren't. The same goes for the pinion yoke on the rear end, they vary sometimes as well. Of course different rear ends will have their U-joint locations at different distances from the axle centerline. You'll have to measure your own individual drivetrain in order to have a new driveshaft of the correct length made up. Get the car up high enough to crawl under and measure, wood blocks of equal height under each tire works well. The car must be on level ground with all the car's weight on the suspension. The car should be finshed or close to it so all of the weight of the car is there. Push the driveshaft yoke all the way into the trans until it bottoms, then pull it out by 7/8". This step is important, this insures that the yoke won't bottom against the output shaft in the trans (causing transmission damage) when the rear suspension reaches its lowest point of travel. Then get out your tape measure and measure from the center of the U-joint bearing cups on the tranny yoke and rear end pinion yoke. Take this measurement to the driveshaft shop and have the new shaft made to this dimension. It's a good idea to bring two different shafts to the driveshaft shop. One shaft that matches up to the trans and one that mates to the rear end, especially when mating and older more obsolete rear ends to more modern transmissions. If the driveshaft shop can't mate up the two shafts they can likely use the ends off of at least one of them to make it happen. Adapter U-joints are readily available with 2 different size cups on them. Lastly but very important, have the completed driveshaft balanced. This insures long-life from the U-joints as well as keeping the un-balanced shaft from beating up the transmission and rear end pinion bearing. Also helps you keep your sanity, driveline vibrations aren't fun to deal with out on the road.
Fantastic point on keeping the car level! First time I installed a driveshaft, it wasn't level... I'd like to say oh well, wasn't my car ANWAYS.........but it was. I pulled it back out and had to start all over. xxx Brandy
Easy to understand now watch next week someone will make a post "How do I figure out how long to make my drive shaft? and should I get it balanced?" I've screwed up more than my share over the years and only wished someone had been there to say, "No you dummy this is how you do it." Good tech New guys take note.
The best advice on this subject is talk to the shop doing the work & find out how they want it done. I got a pamplet from a well known west coast driveshaft shop. In it was "how to measure". Great! measured my project & drove an hour to the local shop only to find out they measure a totaly different way. If you want it done & with a warranty.... So 1 hr. back & re do. crap! This shops system was pull back a full inch & measure to the flats on the rear ujoint. I don't remember what the first style was, but it didn't work locally.
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