Driveshaft question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**425HP409** · 05-17-2023, 05:37 PM

I am converting my 68 short bed from small block, TH700R4 and 2 piece driveshaft to a 396, TH700 R4. Since I have to move the transmission back 1.5" to accommodate the engine, can I use a one piece driveshaft that will be 58" long?

LS short box · 05-17-2023, 10:16 PM

I always rely on my some what local drive shaft shop for their direction. Length, diameter and max engine rpm all factor into the design of the drive shaft. I would upgrade to 1350 u-joints.

cornerstone · 05-17-2023, 11:03 PM

I went from a long bed to a short bed and I'm running a driveshaft a little longer than 58", based on several other builds I studied. I'm not on the road yet so take that for what it's worth.

**68bowtie** · 05-17-2023, 11:53 PM

I have a swb 396/700r4 with a single piece ds. I want to say it was 58.5” but that’s just from memory.

**HO455** · 05-18-2023, 10:36 AM

Part of what determines the limits of drive line length is the intended usage along with gear ratio, tire diameter, transmission top gear ratio, engine horsepower. These factors determine the maximum RPM of the driveline as well as how much torque it will be required to absorb.

There is a maximum RPM that a drive line can turn at before it starts it reaches critical speed. The length, diameter, and material used in the driveline determine its critical speed. The critical speed is basically the RPM at which the drive line starts to whip or bend so it resembles a jump rope which can be felt as a vibration in the vehicle. Exceeding the critical speed will lead to driveline failure due to fatiguing of the driveline material. The longer time and/or the greater amount of RPM you exceed the critical speed causes the failure to happen sooner.

Spicer has calculator to help determine what the critical speed of your driveline would be.

https://spicerparts.com/calculators/...rpm-calculator

Then you need to calculate what the maximum RPM your driveline would turn at to see if you're in danger of exceeding the critical speed.

https://www.dennysdriveshaft.com/pot...alculator.html

05-17-2023, 05:37 PM	#1
425HP409 425hp409 Join Date: Oct 2008 Location: Prosper Texas Posts: 834	Driveshaft question I am converting my 68 short bed from small block, TH700R4 and 2 piece driveshaft to a 396, TH700 R4. Since I have to move the transmission back 1.5" to accommodate the engine, can I use a one piece driveshaft that will be 58" long? __________________ Charles 68 SWB, Dad bought new, now 396 & factory ac 64 Impala SS, 409 425HP, 4 speed AC 64 Bel Air Wagon, 400 Small Block 57 Bel Air all original 283 w/Factory Air 77 GMC Sprint, 454/425HP 67 Olds 442, all original 90 ZR1 Corvette

05-17-2023, 10:16 PM	#2
LS short box Registered User Join Date: Jun 2015 Location: Carlos MN Posts: 1,923	Re: Driveshaft question I always rely on my some what local drive shaft shop for their direction. Length, diameter and max engine rpm all factor into the design of the drive shaft. I would upgrade to 1350 u-joints.

05-17-2023, 11:03 PM	#3
cornerstone Registered User Join Date: Apr 2014 Location: Azle,Texas Posts: 2,248	Re: Driveshaft question I went from a long bed to a short bed and I'm running a driveshaft a little longer than 58", based on several other builds I studied. I'm not on the road yet so take that for what it's worth. __________________ Brian 1972 C10, "Loyd", LWB to SWB, 5.3, L83/6L80e, 4:11 Tru Trac, Air Ride, VA, DD, 20" Coys, 4 wheel disc, A quick LS swap turned into a 6 year frame off resto-mod.

05-17-2023, 11:53 PM	#4
68bowtie Senior Member Join Date: Jan 2007 Location: Turlock, CA Posts: 8,344	Re: Driveshaft question I have a swb 396/700r4 with a single piece ds. I want to say it was 58.5” but that’s just from memory. __________________ I live in California, but identify as a Texan "If a job's worth doing, it's worth doing well" Project "Little Sister" (70 c10 blue original paint refresh) Project "C10 Fever" (68 factory black 396 swb) Project "Blue Bomb" (70 c30 blue original paint refresh) Project "Vitamin C" (71 c10 orange original paint refresh)

05-18-2023, 10:36 AM	#5
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 10,805	Re: Driveshaft question Part of what determines the limits of drive line length is the intended usage along with gear ratio, tire diameter, transmission top gear ratio, engine horsepower. These factors determine the maximum RPM of the driveline as well as how much torque it will be required to absorb. There is a maximum RPM that a drive line can turn at before it starts it reaches critical speed. The length, diameter, and material used in the driveline determine its critical speed. The critical speed is basically the RPM at which the drive line starts to whip or bend so it resembles a jump rope which can be felt as a vibration in the vehicle. Exceeding the critical speed will lead to driveline failure due to fatiguing of the driveline material. The longer time and/or the greater amount of RPM you exceed the critical speed causes the failure to happen sooner. Spicer has calculator to help determine what the critical speed of your driveline would be. https://spicerparts.com/calculators/...rpm-calculator Then you need to calculate what the maximum RPM your driveline would turn at to see if you're in danger of exceeding the critical speed. https://www.dennysdriveshaft.com/pot...alculator.html __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban the WMB,1991 S(stink)-10 Blazer,1969 GTO, 1970 Javelin, 1952 F2 Ford 4X4, 29 Model A, 72 Firebird. 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.