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Driveshaft diameter

jeepthrills01

jeepthrills01

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Strength difference between2.5, 2.75 and 3.0 diameter?
This is for a street/ ocassional strip semi aggressive driving 69 383/727/3.55 28" tire.
 
493 Mike said:
OEM should be fine.
Mike
Click to expand...

Thats what I thought too
until I twisted the drive shaft out of my car on the chassis dyno
Destroyed the custom exhaust too

As for the original poster's question
My research found that 3 inch was recommended for most street applications
From Doctor Diff's website:
"Street/Strip 1350 driveshaft assemblies are built with 3" diameter seamless 4130 cr-mo tubing, nodular iron weld yokes, nodular iron slip yokes, heavy duty 1350 U-joints, then dynamically balanced."

the best thing you can do is go with the larger 1350 U-joints
My twisted drive shaft was the result of a U-joint failure

I went with the Strange Engineering Driveshaft as recommended by Doctor Diff

IMG_20201012_172613689_HDR.jpg
 
A dtive shaft is a piece of pipe or seamless tubing. Measure the Outside Diameter, if its a pipe, the OD will be 2.375" for 2" pipe or 2.875" for 2 1/2" pipe or 3.500" for 3.00" pipe. Pipe is by OD X schedule (sch 5, 10, 40, 80).
Tubing is sized by OD x wall thickness such as 2.500" OD X 0.060"wall OR 0.125" wall.
Pipe or tubing can be EITHER welded (made from formed strip material) and continuously welded or drawn and pierced from a slug (no longitudinal seam). The universal joint trunions are welded on, in correct phase, on to the developed length of pipe, stress relieved then balanced. Just thought you might like to know...
BOB RENTON
 
The equation for the torque capacity of a driveshaft has D^4 in there. So doubling diameter = 16x torque capacity. Diameter matters. Just how much each of those listed can handle, when 50 years old, I’m not going to calculate but if you are doing much more than stock go bigger. The 3”, if the wall is the same thickness, can handle 2x the torque of the 2.5”.
 
In the sixties and seventies most HP shafts were 3 1/4" OD.
 
Nate S said:
The equation for the torque capacity of a driveshaft has D^4 in there. So doubling diameter = 16x torque capacity. Diameter matters. Just how much each of those listed can handle, when 50 years old, I’m not going to calculate but if you are doing much more than stock go bigger. The 3”, if the wall is the same thickness, can handle 2x the torque of the 2.5”.
Click to expand...
I believe the terminology you are searching for, deal with the section modulus which as you note is a function of
upload_2021-3-3_14-51-19.png
OD + the wall thickness which affects the moment of inertia of assembly. And yes. Its an area function.
BOB RENTON
 
RJRENTON said:
I believe the terminology you are searching for, deal with the section modulus which as you note is a function of View attachment 1078386 OD + the wall thickness which affects the moment of inertia of assembly. And yes. Its an area function.
BOB RENTON
Click to expand...
Exactly. For this forum I thought the actual math less important than the concept.
 
Nate S said:
Exactly. For this forum I thought the actual math less important than the concept.
Click to expand...
Perhaps, what you say has merit but....personally, I like to know the actual WHY. Not throwing stones at anyone in particular, ignorance is not bliss, as learning about new things, is what life is all about.....philosophically speaking.....
BOB RENTON
 
I probably pushed my stock driveline WAY farther than I should have. My car weighs 3870 and runs 11.2x’s, turning 6500 rpm at the stripe. This year I finally took some good advice from a professional and ordered a 3” Strange unit from Dr Diff. I’ll feel a lot safer this year, that’s for sure.
To the OP question: I can’t regurgitate what the guy at Drivelines NW told me about sheer stress at speed as it relates to diameter and RPM, but he said the stock driveline in my car was a ticking timebomb. If you’re thinking about it.... it’s probably time to upgrade.
 
LemonWedge said:
I probably pushed my stock driveline WAY farther than I should have. My car weighs 3870 and runs 11.2x’s, turning 6500 rpm at the stripe. This year I finally took some good advice from a professional and ordered a 3” Strange unit from Dr Diff. I’ll feel a lot safer this year, that’s for sure.
To the OP question: I can’t regurgitate what the guy at Drivelines NW told me about sheer stress at speed as it relates to diameter and RPM, but he said the stock driveline in my car was a ticking timebomb. If you’re thinking about it.... it’s probably time to upgrade.
Click to expand...
Did you get the Street Strip or the race piece?
 
You need to look at a critical speed chart.

I've seen more driveshafts come apart from exceeding critical speed than exceeding max torque capacity.
 
So this is a common thing, for even Strange driveshafts?
 
Every driveshaft has a critical speed, at which it becomes unstable and comes apart.

So doesn't really matter who made it, just that you don't exceed that speed.
 
The '71 Charger always had a high speed vibration with the stock driveshaft.
When I installed the strange S60 with 1350 yoke, I got a new driveshaft with the 1350 U-Joints and trans yoke.
Got a 3-1/2" x 0.083" wall Chromoly Driveshaft from Mark Williams. Nice driveshaft, and no vibration, but not cheap. The 727 yoke adds $204 to the cost of the $498 driveshaft, $702 total plus shipping.

MW driveshaft chart with critical speed per driveshaft size/type at different lengths:
https://markwilliams.com/servicebull/sb0049.pdf
 
Just wondering what the failure rate is.
The chart I saw lists a 20% safety factor.
I broke mine on the starting line, pretty sure the u-joint broke first. I've seen others break at the start.
 
From what i have seen, a failure on a starting line is usually u-joint related. When a driveshaft turns into a pretzel, its usually a critical speed failure on a chassis dyno. (Cars can reach high gear or overdrive speeds on a chassis dyno they could never reach on a highway). That could be a result of a bad u-joint being over-speeded too.
For my car, with a transbrake and 14/32 tires i wanted to make sure my driveshaft wasnt a weak link. Four inch aluminum shaft, billet 1350 yokes both ends, short shaft, light car. Critical speed is somewhere around 225 mph. 75 mph safety margin, at least.
 
When the drive shaft for my car got made they were told that the car would also have a 350 nos kit on it. Mii ok rod dynoe’d at 655 hp and 620 tq. It’s supposed to be good to lots more than these numbers. Kim
 
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