Exploding driveshafts?

[Doubeleive]

I agree with you all in theory, next time I take the truck out, I'll fire up the Tech-2 and stick it in 4LO and see what the driveshaft speeds are. Won't be the first time I've been wrong! LOL

Wes, there are more than a few pickup owners over on the GM-Trucks thread that have blown their driveshafts up once they hit the magic speed of 112 MPH or so (don't remember exact number but are several threads about them) Those were all K2s if memory serves. @Doubeleive

I've only had the new Silverado up to 134 max so far but I have had it in the 120's range quit a few times, takes over a mile to get it up above there and there has just been no runway, I get some vibration at about 90 which is probably from it being lowered.

 

[justinesky]

what year are these drive shafts in? mine has seen triple digits a lot without issue.

 

[gat0r]

^ you are lucky

'07-'20 for sure... realistically, probably all years. none were meant to go that fast on stock driveshaft.

lots have had the issues on dyno's, but read about a few that were racing on highway

 

[swathdiver]

Unless you are changing diff gear ratios, the driveshaft rotation vs axle rotation is always the same. 3:43, 3:08 etc. I can’t see how changing my TC to low does anything to driveshaft speed. It would only cause internal TC parts to change speed.

The transfer case is upstream from the driveshaft. The driveshafts are directly connected to their respective axles. Transfer case setting, whether LO or HI, has no impact on driveshaft speed. The driveshaft is turning the same speed at 30 mph whether you're in LO or HI.

x3 what Dana & Bill said

I agree with you all in theory, next time I take the truck out, I'll fire up the Tech-2 and stick it in 4LO and see what the driveshaft speeds are. Won't be the first time I've been wrong!

Yesterday, my youngest daughter and I and the dog went out into the country for some 4x4 time to do what I said I would do. We did comparisons at 20, 25 and 30 mph in both HI and LO gears and recorded the ISS and OSS from the Tech-2. For those of you in Rio Linda, ISS is the Input Shaft Speed of the transmission and OSS is the Output Shaft Speed of the Transmission.

When analyzing the data, the slower speed comparisons were done in different gears! Duh! But the 30 MPH comparison matched the chart more or less.

I agree with you that the driveshaft speed is only affected by the tire diameter and axle ratio. So what is GM doing here by involving the TC?

The math for figuring Driveshaft or Propeller Speed RPMs: MPH X 336 x AXLE RATIO / Tire Diameter (Inches)

So in the above example: 30 MPH x 336 x 3.42 / 31.04 = 1110.6 RPMS

They had a blast, driving on a narrow, muddy levy where we actually needed all four wheels powered, even with the worn KO2s to keep from getting stuck, with multiple blind curves made that part quite an adventure! The dog saw her first donkey and horse up close and went wild, spent a lot of miles cruising around in 4LO as well, gave the TC and Front axle a good work out. Needless to say, the truck is filthy now! Fun stuff.

 

[intheburbs]

what year are these drive shafts in? mine has seen triple digits a lot without issue.

Year doesn't matter. Driveshafts are not typically balanced to the precision needed to ensure non-explosiveness at super extra-legal speeds. Yours just might happen to be well-balanced enough, luckily for you. I would expect the vast majority are balanced enough, but it's not something that GM would check/test on every production part. Or if they do balance/test them, they're only doing it for 55 or 70 mph, not 100+.

ETA: And if your handle is any indication, you're doubly lucky because you probably have AWD, so you have two driveshafts spinning, not just one. That's one of the reasons I won't do a Blackbear tune on my Denali. Too many moving parts in the powertrain.

 

[Chert_Detective]

Aside from 60' and 330' testing I have only made one 1/4 pass on the street and trapped 105, the last little bit was pretty sketchy to say the least. Since the installation of the 4.11s I have only done 1/8 until I get a driveshaft. With the 4.11s and 29.5" tire the driveshaft speed would be over 5,300 rpm which is getting into the critical speed of even aftermarket driveshafts.

People are quick to forget buying a good tire before they start adding power, even more so overlooked is moving drivetrain components. Spend the money, don't risk it.

 

[Chert_Detective]

@swathdiver I believe the guys are correct. 4hi, 4lo, 6speed, 10 speed, 30mph is 30mph. With a fixed tire size and rear gear ratio the driveshaft will always be spinning at the same speed at 30mph because the reduction is done before the driveshaft. What will change is the necessary engine rpm needed in order to work with the tc to turn that driveshaft to the rpm needed for 30mph.

 

[Danny3737]

Some of those “Hot Hatches” are surprisingly quick off the line. I got to drive a tuned WRX STI about 4 years ago and was amazed at how fast and nimble it was. Back in my younger days, we used to hop up Datsun 240Z’s, 510’s, Rabbit GTI’s and BMW 2002’s and take them to Summit Point Raceway and run them around the track. Most of those wouldn’t hold a candle to what you see these days. Although, the best tune to any vehicle ( in my opinion) is to upgrade the drivers ability to handle any vehicle.

Civic weighs half your tahoe does

 

[Doubeleive]

so if the driveshafts are an issue at above say 100mph, do the PPV/SSV get something different? (I suspect not) and obviously they are sold and marketed as a pursuit capable vehicle so???

 

[iamdub]

Y'all makin me wanna fab up a driveshaft loop while I'm under it.

 

[Chert_Detective]

so if the driveshafts are an issue at above say 100mph, do the PPV/SSV get something different? (I suspect not) and obviously they are sold and marketed as a pursuit capable vehicle so???

That's a good question, I asked my buddy who has a ppv. He said ppv is lowered 2wd, ssv is standard 4wd, both stock drivetrain but ppv is rated to 130mph. It is not unheard of to have a factory driveshaft balanced to a higher rpm, the factory tolerance pretty low. Perhaps during the conversion to police use the driveshafts are rebalanced.

 

[swathdiver]

@swathdiver I believe the guys are correct. 4hi, 4lo, 6speed, 10 speed, 30mph is 30mph.

So care to take a stab at why GM changes the OSS when in 4LO? I don't have a clue.

Y'all makin me wanna fab up a driveshaft loop while I'm under it.

Back in the day that was SOP for a 13-second or faster auto.


The SSVs/PPVs have a larger diameter aluminum driveshaft if memory serves and that combined with their shorter wheelbase raises the critical speed and therefore, top speed of the vehicle.

Hey Wes, does the Silvy have a 2-piece driveshaft? @Doubeleive

 

[Chert_Detective]

So care to take a stab at why GM changes the OSS when in 4LO? I don't have a clue.

Ya got me there bud, I have no idea. I may just be overthinking my simple thinking, but the driveshaft speed should be the same at a given mph in one gear vs another. Otherwise the ciritcal speed formula would involve other variables besides mph, final drive, and tire height.

Unless you have a super special diff or reduction planetaries after the driveshaft. In which case I want those haha.

But your tire height isn't changing, the rear gear isn't changing, and the mph isn't changing. So the shaft speed shouldn't change either otherwise your mph would change as well. The only, main, thing that's changing is the engine rpm needed to turn the shaft at the speed necessary for that same mph as before because of the change in gear sequence before the shaft.

 

[randeez]

you are in charge of your own safety .... if YOU think you need it and will forever be hesitant about going over 100 then just order a custom shaft and be done with it if you plan on going over that point often

it could happen, sure. it could be a fluke thing, or it could be driveline angles, vibration, bent shaft, etc. saying that a perfectly good one, in the perfect installation will still explode over *** mph, comes from critical speed+length of the tube material.
https://pstds.com/critical-speed-chart/ for custom shafts atleast. the stock steel shaft sounds like a soda can when you tap on it.

personally never had any problems with stock driveshaft on mine (short awd yukon) the shaft is like 41" long iirc. had it balanced for like 80bux with a local guy. had him shorten, cut to custom length and replace the stock material with something a little thicker throughout its life.

 

[Doubeleive]

So care to take a stab at why GM changes the OSS when in 4LO? I don't have a clue.



Back in the day that was SOP for a 13-second or faster auto.


The SSVs/PPVs have a larger diameter aluminum driveshaft if memory serves and that combined with their shorter wheelbase raises the critical speed and therefore, top speed of the vehicle.

Hey Wes, does the Silvy have a 2-piece driveshaft? @Doubeleive

No it has a big fat ass single aluminum one on the rear

 

[randeez]

So care to take a stab at why GM changes the OSS when in 4LO? I don't have a clue.
@Doubeleive

i dont know about changes, but i think the oss is just used to relay mph info when you're in 4lo (after the tcase reduction) any other time it just looks at the ISS to know speed . its probably physically a different reluctor gear so it doesnt read linear to the ISS