Let's Talk GMT900 DriveShafts !

[Foggy]

I'm aware of the issues with the GMT900 driveshafts that like to fail/pancake out at
higher speeds... (from what I've read, anything over 95 mph for a short distance)
Obviously we don't drive race vehicles. But I do tow heavy and when not towing have
gone a tad over the speed limit at times.

Anyway... Is there a difference from year to year ?? Mine is a 14 (last year of gmt900)
My driveshaft - REAR is made of steel and about 3.75" outer diameter. And I'm sure
it's the longest of the AWD/4WD since I have an XL model.
Of course a 2WD would have a longer one.

Let me know of info that you might have. I'm thinking I need to upgrade my shaft ! Just
don't know which way to go or IF needed for sure for my year.

 

[Marky Dissod]

What are the differences - if any - between the civilian driveshafts and the Z56-PPV driveshafts?

 

[j91z28d1]

I thought they were all aluminum. Definitely some differences then.


by pan cake, do you mean the shaft itself fails at 90? that's not great. the speed limits here are 85. I've definitely cruised past 90 before.

 

[intheburbs]

News to me. Guess I'm just lucky.

I've cruised for hours in the AWD Denali at 90-100 when I'm out west in those big states. Ditto for the Suburban, maybe even more so, and I had it as high as 116.

 

[Foggy]

I thought they were all aluminum. Definitely some differences then.


by pan cake, do you mean the shaft itself fails at 90? that's not great. the speed limits here are 85. I've definitely cruised past 90 before.

Yuppers. The limiter is also set around 90 mph due to the speed rating of the
tires. It's all about the lawyers !!!

I've seen pick of the bad ones. They kinda start flexing and then go oval.
That's what I mean by pancake. Then they just twist up like a pretzel

I thought they were all alum too (the rear driveshaft)
Mine is definitely steel cause it has some light surface rust

 

[Foggy]

News to me. Guess I'm just lucky.

I've cruised for hours in the AWD Denali at 90-100 when I'm out west in those big states. Ditto for the Suburban, maybe even more so, and I had it as high as 116.

View attachment 451055

I have too. Not for any amount of time/distance. Just short bursts.
But the more I've looked into it.... it's very very risky

 

[Doubeleive]

couldn't even tell you how many hours I have spent WELL over 100, more than almost anyone than a race car driver.
never had a drive shaft issue obs, gmt800, gmt900, k2
only time I had drive shaft issue's is when changing the original u-joints, ended up buying a dorman shaft, which they make on order brand new.
That being said can't say I have not considered having a "good" one made just never bit the bullet and did it.
lol I should have a "flight book" I certainly have a ticket collection.

 

[j91z28d1]

Yuppers. The limiter is also set around 90 mph due to the speed rating of the
tires. It's all about the lawyers !!!

I've seen pick of the bad ones. They kinda start flexing and then go oval.
That's what I mean by pancake. Then they just twist up like a pretzel

I thought they were all alum too (the rear driveshaft)
Mine is definitely steel cause it has some light surface rust

damn. that would be a hell of a reason to be stuck on the side of the road.


maybe less so with your steel drive shift?


I heard a ever so slight hum from mine at highway speed a few months back on a trip. I still fear it's a u joint going. it you find a good shop to make you one that's actually smooth. Definitely let us know. I'd spend good money to not have to deal with that crap. but mine being a totally different tranny and all. I'm sure it would be totally custom.

my next step was to find a used one somewhere to try

 

[Doubeleive]

I was super lucky this guy let me go I already had a 100+ ticket from October, here in Cali too may points in a specific time period and they yank your license

 

[Marky Dissod]

The limiter is also set around 90MpH due to the speed rating of the tires.
It's all about the lawyers!

Actually, sometimes, the tires, and the speed limiter, are selected in context of the driveshaft's 'critical velocity'.

I've seen pick of the bad ones. They kinda start flexing and then go oval. That's what I mean by pancake. Then they just twist up like a pretzel.

What actually happens is more like ...
How much margin does a driveshaft with a diameter of, say, 4.0" require around it for safety reasons, so long as it is operating properly?
Beyond a certain road speed, the driveshaft is spinning more RpMs than the engine.
Problem is when OE is not willing to spend enough on quality construction / balance of driveshafts.

Depending on how much evenly distributed mass a driveshaft is made of, if you spin a driveshaft too fast,
it eventually starts acting like a guitar string AND a jumprope, SIMULTANEOUSLY.

Often the driveshaft's inherent IMbalance is effectively less than the vehicle's potential top speed.
Apart from the thorium-footed Doubeleive, most of us almost never exceed 85MpH with our suvs (especially with anyone else riding along).

Those Z56-PPV Tahoes definitely have superior driveshafts ...