Before we made our new project cool, we would need it to run again. After surveying the damage, we found the truck originally had a two-piece driveshaft. The front shaft was 72 inches long and 3.75 inches in diameter. There is a nifty calculator on Spicer’s website to find the maximum safe RPM for a given driveshaft.
Bottomline: Our original …
How does one over rev a driveshaft? Was the original extended when the truck was converted to a ramp back and became a skipping rope?
DeadSkunk wrote: How does one over rev a driveshaft? Was the original extended when the truck was converted to a ramp back and became a skipping rope?
Based on its length, material and diameter, every driveshaft has a maximum speed it can safely spin. Above that, it'll start to deform and whip around a bit in use and can catastrophically fail.
Tim Suddard wrote: For well less than $1000 we had a new three-piece drive shaft that we could spin almost as fast as we wanted.
Almost as fast as you wanted? Is there another story here? 
It's about harmonics. Simply put. The longer the shaft the lower the natural frequency or easier it is for the forcing frequency to match natural frequncy. Once that happens.. the two forces become additive and the shaft whips itself into destruction.
Keith Tanner wrote:Tim Suddard wrote: For well less than $1000 we had a new three-piece drive shaft that we could spin almost as fast as we wanted.Almost as fast as you wanted? Is there another story here?
LSR ramp truck?
An overdrive transmission would lower the speed of the driveshaft, as would a numerically lower differential ratio. Just sayin'
EvanR wrote: An overdrive transmission would lower the speed of the driveshaft, as would a numerically lower differential ratio. Just sayin'
OD trans will have no effect at all on driveshaft rpm relative to road speed. Numerically lower diff gearing will, however. So a deeper geared trans and taller diff gears will help in a case like this (slower driveshaft rpm for a given road speed and deeper trans gearing to get the engine rpm / road speed relationship back to where you want it).
EvanR wrote: An overdrive transmission would lower the speed of the driveshaft, as would a numerically lower differential ratio. Just sayin'
I get changing the rear end, but how would an overdrive gear in the trans slow the revolutions of the output shaft?
Makes me wonder how the NASCARS spin their drive shaft so fast.
A short driveshaft can spin really really fast with no trouble. It's only when they get long that you have issues. This is why things like school busses and ramp trucks typically have two piece driveshafts.
Shorter and larger diameter are the keys to being able to spin it fast. My Jeep has a 33-ish inch rear shaft, 2.5" diameter. That leaves the tube itself capable of spinning a bit over 7000 rpm before it self destructs. The u-joints are only rated to 5000 rpm though (they shouldn't come apart if spun faster, but their lifespan may get very short, especially if there's any angle on them, due to heat buildup in the needle bearings).
My 2500HD pickup has a 2-piece driveshaft, the ramp-truck needed a 3-piece.
There are a bunch of factors that go into the critical frequency, and the material it's made of is a big one. I wouldn't be surprised if Sprint Cup cars have carbon fiber driveshafts.
Interesting.
So were you traveling faster at that point than you had the rest o the trip or was it just cumulative time over revved? How fast WERE you turning the shaft? With 31" tires and a 4.10 rear end, you're over 2750. What're the tires and gears in that beast?
Fueled by Caffeine wrote: It's about harmonics. Simply put. The longer the shaft the lower the natural frequency or easier it is for the forcing frequency to match natural frequncy. Once that happens.. the two forces become additive and the shaft whips itself into destruction.
This is why the racing RX-7s converted the car to a 2-piece driveshaft. Two shorter shafts (of different lengths for different resonances) versus the OE single piece unit that had a resonance at 80mph (!).
rslifkin wrote:EvanR wrote: An overdrive transmission would lower the speed of the driveshaft, as would a numerically lower differential ratio. Just sayin'OD trans will have no effect at all on driveshaft rpm relative to road speed. Numerically lower diff gearing will, however. So a deeper geared trans and taller diff gears will help in a case like this (slower driveshaft rpm for a given road speed and deeper trans gearing to get the engine rpm / road speed relationship back to where you want it).
This is generally why high end European cars would have a 1:1 top and no overdrive. More efficient and lower driveshaft speed.
Note the new Miata also has a 1:1 top and no overdrive. 2.87 rearend gears.
Knurled wrote:Fueled by Caffeine wrote: It's about harmonics. Simply put. The longer the shaft the lower the natural frequency or easier it is for the forcing frequency to match natural frequncy. Once that happens.. the two forces become additive and the shaft whips itself into destruction.This is why the racing RX-7s converted the car to a 2-piece driveshaft. Two shorter shafts (of different lengths for different resonances) versus the OE single piece unit that had a resonance at 80mph (!).
Ha! Whooops.
The new Miata has those ratios to cut down on frictional losses in the trans. Makes for some very high speed dyno runs, let me tell you.
how does that cut down on frictional losses in the trans?
ultraclyde wrote: how does that cut down on frictional losses in the trans?
They can design the trans so that when it's in top (1:1) gear, it simply couples the input and output shafts with no gears in between. An OD trans has to have gears spinning when in top gear, so it creates a little more friction.
I might need to do some measuring on the bus before I put it on the interstate.
rslifkin wrote:DeadSkunk wrote: How does one over rev a driveshaft? Was the original extended when the truck was converted to a ramp back and became a skipping rope?Based on its length, material and diameter, every driveshaft has a maximum speed it can safely spin. Above that, it'll start to deform and whip around a bit in use and can catastrophically fail.
This, I lost one on an s-10 due to it not being specified high enough.
I may or may not have been spinning 2500 extra rpms vs stock however. 
That said, this not being a new truck, how many miles were on the shaft when it let go?
In reply to Toyman01:
Considering the probable gearing of a 1956 Ford bus rear end, it might not like doing more than 55-60 to begin with.
Or did the U joints let go ?
Keith Tanner wrote:Knurled wrote: This is why the racing RX-7s converted the car to a 2-piece driveshaft. Two shorter shafts (of different lengths for different resonances) versus the OE single piece unit that had a resonance at 80mph (!).Ha! Whooops. The new Miata has those ratios to cut down on frictional losses in the trans. Makes for some very high speed dyno runs, let me tell you.
It might have been a complementary frequency, or half-frequency. For sure the driveshaft is a lot longer than a Miata's and it's quite small in diameter. And at 80mph, unless everything is 100% perfect, you can get a righteous drivetrain shudder in all gears but 4th. The resonance clatters the hell out of the gears.
Do you have a MPH limit for dyno testing? I don't have nor have I ever operated a dyno, but on one Project that I was involved with, the dyno shop had to tune it in 2nd gear because 3rd (1:1) was running the rollers at over 150mph and they didn't trust 37" mud tires to go that fast without exploding.
(It made 550whp. Through 2nd gear and multiple other points of gear reduction... criminy)
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