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?
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 driveshaft should only have been allowed to spin at about 2200 rpm. More rpm than that—as we found out in a very catastrophic fashion—would cause the driveshaft to come apart.
When our shaft came apart, it knocked the sending unit out of the tank, damaged the exhaust system, completely destroyed the carrier bearing, sheared the front U-joint flange and chipped the rear driveshaft flange.
What a mess! Fortunately we have a long-time friend in the driveshaft business: Gary’s Driveline. Mike Osteen from Gary’s Driveline set about to teach us and make it right. To save money, he found some late model Toyota Tundra driveline pieces. The idea was to build a three-piece driveshaft arrangement. A truck doesn’t care what pieces are in it and there are only three or four companies that make these pieces for every truck manufacturer.
For well less than $1000 we had a new three-piece drive shaft that we could spin almost as fast as we wanted.
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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.
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