why two-piece drive shaft???? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

double d · 05-20-2005, 06:31 AM

bear with me im new here. 1968 short box 2 wheel
im putting in a 454, 6800 rpm cam,march serps,
too much to list. truck has a slight rubber rake to
it. low in front higher in back. anyhow my driveline guy
keeps telling me i have to stay 2 piece because the
angles wont work out with a one piece. i want a one
piece. a little more here on set-up turbo 400, chetah
manual reverse valve body, 4:10. motor mounts are on the way, got em off ebay from a guy in texas, said they
are the best you'll ever buy, $175 whole set, yikes.
truck was a strait 6, 3 on tree, then sbc, 350 turbo,
now who knows where it can go. any pointers appreciated.

68C15 · 05-20-2005, 07:02 AM

the 2 piece is better in 3 ways from what I see. it distributes the load over shorter pieces & therefor able to handle more torque without twisting. its easier to get the less than 2* angle per joint. the frame flexes too much for a 1 piece.
JMO

FirstGen · 05-20-2005, 12:21 PM

interesting... 6.96's @ 102 mph in the 1/8 on my 210 dollar one piece drive shaft...

CHEVYE · 05-20-2005, 12:50 PM

i've never had a problem with a one piece. i have a big block truck and i give it alot of hell. i'm no "driveline" guy myself but i've seen 500+ horse engines on 1 pc. shaft in these trucks, and haven't heard of one failing. i'm sure someone has though.

68LSS1 · 05-20-2005, 01:06 PM

I'm also building a '68 short step and plan on having about 750rwhp and will be going with a one piece also. I don't see any reason on a short bed not to run one based on all I've ever read on this board.

FirstGen · 05-20-2005, 01:13 PM

I should be around 650 on the second stage, thats around 760 at the crank. No issues on the street. Haven't hit the second stage at the track yet...

Maximum Overdrive · 05-20-2005, 02:23 PM

I had a 68 Chevy shortbox and ran a short shaft with no problems.

There is an article about driveshafts in the Classic Trucks Dec 2004 Issue addressing this issue.

What determines use of a one piece shaft or a two piece shaft is critical speed. Critical speed is related to length, weight, diameter and speed of the shaft. If critical speed is reached the driveshaft comes apart. Longer, heavier driveshafts have a lower critical speed than that of shorter, lighter shafts.

**pjmoreland** · 05-20-2005, 03:41 PM

When I had my one-piece 4" drive shaft made for my SWB at the local drive-shaft shop, they told me that the critical rpm of the shaft was 4500 RPM. I've got 3.73 gears and 29" tires, so that means I can't exceed about 104 MPH with a 1:1 transmission. If you have a LWB truck and 4:11 gears, I'm guessing the top speed will be significantly lower than that.

**1970 CST Short Wide** · 05-20-2005, 04:33 PM

My 70 short bed has a 454 and a 1 piece shaft and my son has a long bed with a 350 and a 1 piece shaft. Both have NO issues at all. Just have it built by a shop that knows what there doing

toddtheodd · 05-21-2005, 10:31 PM

Quote:

Originally Posted by pjmoreland

When I had my one-piece 4" drive shaft made for my SWB at the local drive-shaft shop, they told me that the critical rpm of the shaft was 4500 RPM. I've got 3.73 gears and 29" tires, so that means I can't exceed about 104 MPH with a 1:1 transmission. If you have a LWB truck and 4:11 gears, I'm guessing the top speed will be significantly lower than that.

Question.
Doesn't the crank spin twice as fast as the cam? And isn't the flywheel / torque converter / transmission attached to the crank?
The distributor is run off of the cam, which means that rpms is counted off how many rpms the cam is spinning. So therefore, wouldn't the driveshaft in 1:1 drive be spinning twice as fast as the rpms say the motor is spinning?
Seems to me that "critical speed" would be reached a whole lot sooner than you thought.

Todd

**pjmoreland** · 05-21-2005, 11:05 PM

At 65 MPH my tach reads 2000 RPM. I have 29" tires, a 3.73 differential and a .7 overdrive transmission with a locking torque converter. Here's the calculation of the crankshaft RPM at 65 MPH:

(65 mi/hr) x (5280 ft/mi) x (12 in/ft) x (1 hr/60min) x (1/(29in x 3.14159265)) x 3.73 x .7 = 1967 revs/min.

The tachometer displays the crankshaft RPM, not the camshaft RPM, even though the distributor is turned by the camshaft.

Here's a link that has a table that shows critical RPM for driveshafts of different lengths and materials:

http://64.233.161.104/search?q=cache...speed%22&hl=en

double d · 05-22-2005, 05:51 PM

thanks guys, im putting the rest of my info on and some pics,
these were from last summer when i had the 350/350 set-up.
sounds like i can do the 1 piece, being my motor will spin less
than 6800 rpm and a 4:10 in the rear, tire diameter @ 29IN.
INPUT GREATLY APPRECIATED

05-20-2005, 06:31 AM	#1
double d Registered User Join Date: May 2005 Location: michigan Posts: 4	why two-piece drive shaft???? bear with me im new here. 1968 short box 2 wheel im putting in a 454, 6800 rpm cam,march serps, too much to list. truck has a slight rubber rake to it. low in front higher in back. anyhow my driveline guy keeps telling me i have to stay 2 piece because the angles wont work out with a one piece. i want a one piece. a little more here on set-up turbo 400, chetah manual reverse valve body, 4:10. motor mounts are on the way, got em off ebay from a guy in texas, said they are the best you'll ever buy, $175 whole set, yikes. truck was a strait 6, 3 on tree, then sbc, 350 turbo, now who knows where it can go. any pointers appreciated.

05-20-2005, 07:02 AM	#2
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	the 2 piece is better in 3 ways from what I see. it distributes the load over shorter pieces & therefor able to handle more torque without twisting. its easier to get the less than 2* angle per joint. the frame flexes too much for a 1 piece. JMO __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card

05-20-2005, 12:21 PM	#3
FirstGen Just Don't Stop! Join Date: Feb 2004 Location: Dallas Posts: 1,239	interesting... 6.96's @ 102 mph in the 1/8 on my 210 dollar one piece drive shaft... __________________ 1968 C10 build in process TT LSA/6L90e/9"

05-20-2005, 12:50 PM	#4
CHEVYE my gas saver Join Date: Sep 2003 Location: Clarksville, TN Posts: 2,046	i've never had a problem with a one piece. i have a big block truck and i give it alot of hell. i'm no "driveline" guy myself but i've seen 500+ horse engines on 1 pc. shaft in these trucks, and haven't heard of one failing. i'm sure someone has though. __________________ 85' SWB, 4.3/TH350, getting ready for paint 84`SWB, 462ci./TH400, cowl hood, 15" billet spec., flows, blazer buckets/console, flat black 71`SWB, project ebay feedback

05-20-2005, 01:06 PM	#5
68LSS1 Registered User Join Date: Dec 2002 Location: Tampa Posts: 1,191	I'm also building a '68 short step and plan on having about 750rwhp and will be going with a one piece also. I don't see any reason on a short bed not to run one based on all I've ever read on this board. __________________ '68 Short Step LS1/T56, Hydratech, Fatman Fabrications Stage III, Baer, Hot Rods to Hell, US Body, S&W, etc

05-20-2005, 01:13 PM	#6
FirstGen Just Don't Stop! Join Date: Feb 2004 Location: Dallas Posts: 1,239	I should be around 650 on the second stage, thats around 760 at the crank. No issues on the street. Haven't hit the second stage at the track yet... __________________ 1968 C10 build in process TT LSA/6L90e/9"

05-20-2005, 02:23 PM	#7
Maximum Overdrive December 21, 2012 Join Date: Apr 2000 Location: Black Eagle, Montana Posts: 1,634	I had a 68 Chevy shortbox and ran a short shaft with no problems. There is an article about driveshafts in the Classic Trucks Dec 2004 Issue addressing this issue. What determines use of a one piece shaft or a two piece shaft is critical speed. Critical speed is related to length, weight, diameter and speed of the shaft. If critical speed is reached the driveshaft comes apart. Longer, heavier driveshafts have a lower critical speed than that of shorter, lighter shafts. __________________ *"!Ama Sua, Ama Kjella, Ama Lllulla!"* Last edited by Maximum Overdrive; 05-20-2005 at 08:08 PM.

05-20-2005, 03:41 PM	#8
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,559	When I had my one-piece 4" drive shaft made for my SWB at the local drive-shaft shop, they told me that the critical rpm of the shaft was 4500 RPM. I've got 3.73 gears and 29" tires, so that means I can't exceed about 104 MPH with a 1:1 transmission. If you have a LWB truck and 4:11 gears, I'm guessing the top speed will be significantly lower than that.

05-20-2005, 04:33 PM	#9
1970 CST Short Wide Senior Moment Join Date: Mar 2002 Location: Pittsburg Ca Posts: 4,145	My 70 short bed has a 454 and a 1 piece shaft and my son has a long bed with a 350 and a 1 piece shaft. Both have NO issues at all. Just have it built by a shop that knows what there doing __________________ 1970 CST LS 1 6 speed Ford 9 inch Detroit Tru Track, Dakota Didgital, Vintage Air, QA1 Coil Overs Front & Rear Lots of FUN 2013 GMC PU, Oldest Son 71 GMC LWB. QA1 Suspension, Angry SB. Youngest Son 2019 GMC Diesil Dually. Youngest Son 2017 Toyota SUV Daughters car 2018 Traverse , Wife’s Ride Pittsburg Ca 94565

05-21-2005, 11:05 PM	#11
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,559	At 65 MPH my tach reads 2000 RPM. I have 29" tires, a 3.73 differential and a .7 overdrive transmission with a locking torque converter. Here's the calculation of the crankshaft RPM at 65 MPH: (65 mi/hr) x (5280 ft/mi) x (12 in/ft) x (1 hr/60min) x (1/(29in x 3.14159265)) x 3.73 x .7 = 1967 revs/min. The tachometer displays the crankshaft RPM, not the camshaft RPM, even though the distributor is turned by the camshaft. Here's a link that has a table that shows critical RPM for driveshafts of different lengths and materials: http://64.233.161.104/search?q=cache...speed%22&hl=en

05-22-2005, 05:51 PM	#12
double d Registered User Join Date: May 2005 Location: michigan Posts: 4	thanks guys, im putting the rest of my info on and some pics, these were from last summer when i had the 350/350 set-up. sounds like i can do the 1 piece, being my motor will spin less than 6800 rpm and a 4:10 in the rear, tire diameter @ 29IN. INPUT GREATLY APPRECIATED



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