Jan 26, 2016 update to the Ford F-350 Ramp Truck project car

Here’s Why Our Ramp Truck’s Drive Shaft Fell Apart

This is what we would need to repair.
Mike Osteen of Gary’s Driveline gets the measurements right before building us a new three-piece driveshaft.

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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Comments

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DeadSkunk
DeadSkunk UltraDork
1/26/16 1:04 p.m.

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?

rslifkin
rslifkin Reader
1/26/16 1:07 p.m.
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.

Keith Tanner
Keith Tanner MegaDork
1/26/16 1:10 p.m.
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?

Fueled by Caffeine
Fueled by Caffeine MegaDork
1/26/16 1:13 p.m.

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.

mazdeuce
mazdeuce PowerDork
1/26/16 1:14 p.m.
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?

EvanR
EvanR Dork
1/26/16 1:53 p.m.

An overdrive transmission would lower the speed of the driveshaft, as would a numerically lower differential ratio. Just sayin'

rslifkin
rslifkin Reader
1/26/16 2:01 p.m.
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).

wae
wae Dork
1/26/16 2:03 p.m.
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?

iceracer
iceracer PowerDork
1/26/16 2:11 p.m.

Makes me wonder how the NASCARS spin their drive shaft so fast.

mazdeuce
mazdeuce PowerDork
1/26/16 2:15 p.m.

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.

rslifkin
rslifkin Reader
1/26/16 2:19 p.m.

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).

codrus
codrus Dork
1/26/16 2:25 p.m.

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.

ultraclyde
ultraclyde UltraDork
1/26/16 2:25 p.m.

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?

Knurled
Knurled MegaDork
1/26/16 2:25 p.m.
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 (!).

Knurled
Knurled MegaDork
1/26/16 2:26 p.m.
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.

Keith Tanner
Keith Tanner MegaDork
1/26/16 2:28 p.m.
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.

ultraclyde
ultraclyde UltraDork
1/26/16 2:34 p.m.

how does that cut down on frictional losses in the trans?

rslifkin
rslifkin Reader
1/26/16 2:39 p.m.
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.

Toyman01
Toyman01 MegaDork
1/26/16 2:59 p.m.

I might need to do some measuring on the bus before I put it on the interstate.

WOW Really Paul?
WOW Really Paul? MegaDork
1/26/16 4:10 p.m.
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.

NOHOME
NOHOME PowerDork
1/26/16 4:15 p.m.

That said, this not being a new truck, how many miles were on the shaft when it let go?

BrokenYugo
BrokenYugo UltimaDork
1/26/16 5:14 p.m.

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.

iceracer
iceracer PowerDork
1/26/16 5:19 p.m.

Or did the U joints let go ?

Knurled
Knurled MegaDork
1/26/16 5:19 p.m.
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)

Toyman01
Toyman01 MegaDork
1/26/16 5:49 p.m.

In reply to BrokenYugo:

The rear end is a slightly more modern Eaton two speed rear end. It's supposed to be good to about 65.

erohslc
erohslc Dork
1/26/16 6:20 p.m.

So how old was that truck?
Seems to me this failure would be a very well known issue by now ...
Just sayin'

wheelsmithy
wheelsmithy HalfDork
1/27/16 5:40 a.m.
Tim Suddard wrote: 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.

That is the kind of stuff that makes me love this forum. I always heard stories about how you could almost bolt a Granada 302, C4, and driveshaft in a Volvo. Now it all makes sense.

Thank You GRM.

TiggerWelder
TiggerWelder New Reader
1/27/16 7:30 a.m.

The heavier Volvo diff, used in V6 and diesel cars takes a standard "10-10" U-Joint. When I put a 350 in my '80 Diesel I used a Camaro drive shaft, shortened with a 700R4. No big deal.

ncjay
ncjay Dork
1/27/16 7:56 a.m.

The rulebook for Nascar stock cars calls for a driveshaft to be made of magnetic steel, a minimum diameter of 4.0", and at least .065" thick. Due to the design of the average stock car, the driveshafts really aren't all that long, but yeah, they spin like crazy at some tracks.

iceracer
iceracer PowerDork
1/27/16 8:47 a.m.

They also have to be painted white.

shadetree30
shadetree30 HalfDork
2/2/16 4:06 p.m.

Slightly off topic question on the ramp truck...

Are the frame rails a single piece unit or are they cut and a section welded in to get the extra length?

shadetree30
shadetree30 HalfDork
2/7/16 1:50 p.m.

bump^

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