Calling out those of you who have been around the block with a driveshaft or two... It's pretty widely known that the NNBS trucks' aluminum shafts tend to break at high speeds. Some of them break just over 90mph, some take a bit more. I've taken mine to 115 with 31.9" tires.

So, I was reading up on it and found this little piece of information...

Critical Speed: This is the probably the most influential factor in driving the purchase of an aftermarket driveshaft. The critical speed of a spinning shaft is described as the lowest speed which excites the shaft at it's natural frequency of vibration. This will cause the shaft to bend under the stress of vibration coupled with the centrifugal forces due to the rotation. In english, this means the shaft will vibrate very badly and possibly take a permanent bend (if not destroy the car in the process) as the critical speed is exceeded. If you use the calculator to predict the critical speed of a shaft, you can clearly see that lighter materials with higher modulus of elasticity give higher critical speeds. So do larger diameter shafts with thinner material thicknesses. Unfortunately, there is a limit to the diameter of shaft you can put in the car, so stronger lighter materials were developed to make stronger and faster spinning shafts.

I'm not accepting that as scripture, but if thats true, then why do the 5" aluminum shafts have such a horrible reputation for playing jump rope under the truck?

 

cuz theyre like 7-8ft long...lmao!

 

I know lol, but length would be the control variable if you were to test this. A relatively skinny steel shaft versus a larger aluminum one...according to that information, the same length shaft made of aluminum would tolerate higher speeds. But dyno/track evidence suggests otherwise.

 

Hmm that is interesting.. I have heard of more Aluminum shafts letting loose then steel shafts.. Could it possibly be tha fact that they are longer?? In turn a longer area to flex while high rpms??

 

It has alot to do with the length of the shaft. But also, tire diameter and gear ratio changes that many guys to for drag racing, speeds up the driveshaft. Thus it reaches its critical speed at a lower mph.

 

Not too familiar with drive shaft tech, but would having it high speed balanced help?

I want to have mine high speed balanced and upgrade the U joints and yokes to 1350 u joints. I imaging this would help strengthen it correct?

 

Again with the length, a larger diameter shaft should be less prone to bending. I'm thinking that GM just decided to use it for the weight purposes, but decided not to balance them as well to compensate for the weaker material. An aluminum shaft will be a lot more vulnerable to a shitty or low rpm balancing job.

If they are equal in length...consider length irrelevant in this question. I know its not, but for the sake of this particular topic, assume it is. I'm talking about the failing point of two shafts made for the same truck, one being steel, the other aluminum.

 

I recommend a 2 piece driveshaft if you plan on gearing down, smaller tires, or plan on high speed runs.

 

Young's modulus for steel is about 3 times higher than alumiunum, in general, (30msi vs 10msi), but aluminum is slightly less than 3 times the weight as well. So strictly from this line of thinking and assuming both are equally weighted in determining critical speed, steel has a slight advantage. Aluminum is cheaper and lighter though...

 

Interesting topic. I had my 2 piece steel shaft balanced and spoke with the guy at the drive shaft shop and he said 1 piece aluminum is a lot better than 2 piece steel. He could go up to 78" or so with no problem, with higher gears and power levels he recommended at least a 5" or 6" dia shaft.