Props To The Lowly Driveshaft

spikesputnik · September 1, 2009

I've been rolling around under my truck alot recently checking stuff out, installing goodies etc. And one thing that always catches my eye is the rarely mentioned driveshaft. What a work of art.

Seemingly no more stout than a Fosters Lager can, yet able to handle ridiculous mounts of torque. You rap on it and think it would dent with just the "dead squirrel bounce". It obviously receives alot of attention at the factory judging by the tiny balance weights and machined multiple taper ends. You often hear about problems at either end of it, but the shaft itself seems to be more reliable than, well, you name it!

From a stying standpoint, it is a bit plain, but I'm sure someone here has addressed this "weakness"?

Anyone have any photos of what they look like when they do get "overwhelmed"?

Stevens03 · September 1, 2009

Can't locate them right now, but I've seen 1 or 2 snapped like a piece of paper. Not to pretty. Being all aluminum, I've also seen guys shine them up pretty dang nice too for a little bling at a show. They will dang near mirror up like chrome when polished.

2005LT · September 1, 2009

What helps the driveshaft handle the torque is the relatively large diameter which increases the polar moment of inertia (ability of the object to resist torque and not twist). As you noticed, it doesn't have to be that thick once you go to larger diameters which helps out a bunch in weight reduction.

It's the next link in the drivetrain that surprises me the most...the axles. This came to me this summer when my dad had the rear brakes/axle seals looked at on his 2003 Duramax. Long story short, the shop had the wheels off and slid the passenger side rear axle out a few inches while they were replacing seals. I am amazed by how much torque those ~1" diameter shafts take.

With a light tune in that truck, the engine puts out about 600 lb-ft of torque. In 1st gear, the torque output from the transmission is increased by ~3 times. Therefore, 1800 lb-ft is transferred through the driveshaft. The 3.73 ratio rear diff then steps up the torque to 6714 lb-ft which is split between both axles so 3357 lb-ft per axle

Of course this assumes no powertrain losses and that the wheels wouldn't slip in 1st gear at full engine load. But it gives a guy an idea of the magnitude of power transmitted through those axles.

brims · September 1, 2009

Never thought about it that way with the torque mulitplication and everything. Amazing that the axles stand up to so much.

Don't these drive shafts come apart at higher speeds? Or is that some other vehicle I'm thinking? High speeds are different than 1,800 ft-lbs from an Allison from a dead stop.

Stevens03 · September 1, 2009

Thats the video I was thinking about

2005LT · September 1, 2009

I'm thinking the "coming apart" comes from an imbalance at high speed. That's probably the more plausible failure mode for a driveshaft in conjunction with the torque being applied as well.

And a vid:

Not a GM or close to stock but somewhat entertaining.

spikesputnik · September 1, 2009

A breif 'jack- I was at my insurance agents office and noticed an aluminum "driveshaft" (about 3" diameter as best I recall) hanging on his wall with a 1/2- 3/4" hole right through the center. I asked what it was from, he said it was the tail rotor shaft from a Heuy he was on in 'Nam. They never knew it had happened untill after they landed.

2005LT · September 1, 2009

That's definitely pretty lucky.

pwall · September 1, 2009

Thats the video I was thinking about

Hard to see the video....it's like there is a blue screen in front of the video.

P71_CrownVic · September 1, 2009

Never thought about it that way with the torque mulitplication and everything. Amazing that the axles stand up to so much.

Don't these drive shafts come apart at higher speeds? Or is that some other vehicle I'm thinking? High speeds are different than 1,800 ft-lbs from an Allison from a dead stop.

No...they break. It's all got to do with 'driveline harmonics'. At very high speeds, the driveshaft will look like a piece of wet spaghetti (well...sorta).

That is why the police versions of the Crown Vic have a longer tailshaft on the back ofthe transmission (resulting in a shorter driveshaft), and the driveshaft itself is made of aluminum (or a metal close) rather than steel.

And despite these changes, the 3.27 RAR Crown Vics are limited to 129 MPH and the 3.55 RAR Crown Vics are limited to 119 MPH.

brims · September 2, 2009

Never thought about it that way with the torque mulitplication and everything. Amazing that the axles stand up to so much.

Don't these drive shafts come apart at higher speeds? Or is that some other vehicle I'm thinking? High speeds are different than 1,800 ft-lbs from an Allison from a dead stop.

No...they break. It's all got to do with 'driveline harmonics'. At very high speeds, the driveshaft will look like a piece of wet spaghetti (well...sorta).

That is why the police versions of the Crown Vic have a longer tailshaft on the back ofthe transmission (resulting in a shorter driveshaft), and the driveshaft itself is made of aluminum (or a metal close) rather than steel.

And despite these changes, the 3.27 RAR Crown Vics are limited to 129 MPH and the 3.55 RAR Crown Vics are limited to 119 MPH.

That's why I've heard people getting their drive shafts high speed balanced or the 2 piece drive shafts.

Starrman · September 2, 2009

the driveshaft isn't the weak link, it's usually the U joints.

Steel driveshafts are smaller diameter but weigh more than the aluminum units....

..then there are the very high dollar carbon fiber driveshafts that are even lighter but can handle much, much more power.

a lighter driveshaft has less rotational mass for the drivetrain to turn, so theoretically should free up some hp that is lost through the drivetrain. Same thing goes for the flywheel....an aluminum flywheel will be lighter to spin and will let the motor rev up quicker than a steel flywheel.

spikesputnik · September 2, 2009

Come on- someones gotta have an exploded drive shaft pic.

Correction to my original post- the tapers at either end are just hydroformed it appears, not machined. But still I bet the R&D mishaps on these were interesting.

jrrod2004 · September 2, 2009

Some random driveshaft failures:

http://images.google.com/imgres?imgurl=htt...l%3Dus%26um%3D1

superjim · September 2, 2009

Come on- someones gotta have an exploded drive shaft pic.
Correction to my original post- the tapers at either end are just hydroformed it appears, not machined. But still I bet the R&D mishaps on these were interesting.

I have broken (twisted) two driveshafts, but I do not have any pics of either one.

One was on a 1958 ford that I had dropped a 406 CI engine and a 4 speed into.

The other one was on a 1973 C65 dump truck with a 427 CI that I got stuck.

When the driveshaft in the dump truck broke, the backlash into the rear end broke some teeth off of the ring gear.

Ended up having to put a new ring gear and pinion in it, in addition to a new drive shaft.