Half shaft question
Le Mans Master
Joined: Dec 2006
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Melting Slicks
Joined: Jun 2003
Posts: 2,802
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From: Niles, MI
I'm certainly not a driveshaft (or halfshaft) expert but this thread prompted to me to do a little research. Here are a several informative articles:
The Science Of Driveshafts
Driveshaft Tech – Properly Shafted
IMPORTANT DRIVESHAFT CRITERIA
And here is a calculator Driveshaft Critical Speed
Bottom line, it looks like if the halfshaft material is the same, the 3" halfshaft will be lighter, stronger (torsional yield higher) and the critical speed will be higher (a good thing) than the 2.5" halfshaft. Am I wrong?
The Science Of Driveshafts
Driveshaft Tech – Properly Shafted
IMPORTANT DRIVESHAFT CRITERIA
And here is a calculator Driveshaft Critical Speed
Bottom line, it looks like if the halfshaft material is the same, the 3" halfshaft will be lighter, stronger (torsional yield higher) and the critical speed will be higher (a good thing) than the 2.5" halfshaft. Am I wrong?
Drifting
Joined: Jan 2012
Posts: 1,908
Likes: 129
From: Fredericksburg Virginia
I put 3" shafts on my 1970. As others have said, they are absolutely interchangeable.
VERY interesting conversation relative to the benefits of a solid 2.5" versus a 'seamed' 3". My engineering brain goes the same direction as the conclusion provided by Mr. KidVette...the wall thickness (to a point) plays a small part in the overall strength of the shaft. And for the ridiculously short length of these driveshafts, I would speculate that 'seamed' versus 'solid' would have a similarly small impact on strength. I would speculate that the increased diameter would have a vastly greater positive impact to the 'losses' of thickness and construction.
All that said, I'm in the process of planning for a Dart 400 block upgrade which will result in well over 500 hp through a 5 speed and 4.11 hears. When combined with the 325 tires I plan on running after flaring my car, I am working toward fortifying the rear end, which will include more scientific half-shafts.
VERY interesting conversation relative to the benefits of a solid 2.5" versus a 'seamed' 3". My engineering brain goes the same direction as the conclusion provided by Mr. KidVette...the wall thickness (to a point) plays a small part in the overall strength of the shaft. And for the ridiculously short length of these driveshafts, I would speculate that 'seamed' versus 'solid' would have a similarly small impact on strength. I would speculate that the increased diameter would have a vastly greater positive impact to the 'losses' of thickness and construction.
All that said, I'm in the process of planning for a Dart 400 block upgrade which will result in well over 500 hp through a 5 speed and 4.11 hears. When combined with the 325 tires I plan on running after flaring my car, I am working toward fortifying the rear end, which will include more scientific half-shafts.
Last edited by keithinspace; Jan 19, 2018 at 11:15 AM.
Race Director
Joined: Apr 2009
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From: Charlotte NC
From the second link you posted:
Seam Tube is the most common material used for OEM applications. It is perfectly suited for everyday driver or restored cars. Although stretching its limits is a possibility, it could be carefully used in circle track and drag racing applications.
Pros: This is the most cost effective driveshaft you will ever find.
Cons: It’s also the weakest of all the steel driveshaft and has the lowest critical speed survivability rating, and is still heavy. Once again, steel weld yokes are used.
DUB
Last edited by DUB; Jan 19, 2018 at 06:22 PM.
Thread Starter
Pro
Joined: Apr 2016
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I put 3" shafts on my 1970. As others have said, they are absolutely interchangeable.
VERY interesting conversation relative to the benefits of a solid 2.5" versus a 'seamed' 3". My engineering brain goes the same direction as the conclusion provided by Mr. KidVette...the wall thickness (to a point) plays a small part in the overall strength of the shaft. And for the ridiculously short length of these driveshafts, I would speculate that 'seamed' versus 'solid' would have a similarly small impact on strength. I would speculate that the increased diameter would have a vastly greater positive impact to the 'losses' of thickness and construction.
All that said, I'm in the process of planning for a Dart 400 block upgrade which will result in well over 500 hp through a 5 speed and 4.11 hears. When combined with the 325 tires I plan on running after flaring my car, I am working toward fortifying the rear end, which will include more scientific half-shafts.
VERY interesting conversation relative to the benefits of a solid 2.5" versus a 'seamed' 3". My engineering brain goes the same direction as the conclusion provided by Mr. KidVette...the wall thickness (to a point) plays a small part in the overall strength of the shaft. And for the ridiculously short length of these driveshafts, I would speculate that 'seamed' versus 'solid' would have a similarly small impact on strength. I would speculate that the increased diameter would have a vastly greater positive impact to the 'losses' of thickness and construction.
All that said, I'm in the process of planning for a Dart 400 block upgrade which will result in well over 500 hp through a 5 speed and 4.11 hears. When combined with the 325 tires I plan on running after flaring my car, I am working toward fortifying the rear end, which will include more scientific half-shafts.
Race Director
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From: Corsicana, Tx
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You might want to call a local driveshaft shop and get a quote on having your halfshafts retubed. I just called a driveshaft shop here in Tucson and got a quote of 300 bucks (150 per shaft) to have a set of 3 inch halfshafts retubed with .134 wall thickness seamless tubing and the price included balancing them as well (factory halfshafts aren't balanced for some unknown reason). That's a lot better of a deal than buying an aftermarket set of 3 inch halfshafts for 500 bucks or more.
Good deal. And y'all thought I was making that up!! They can go ahead and install solid Spicers while they have it.
JIM
Melting Slicks
Joined: Jun 2003
Posts: 2,802
Likes: 241
From: Niles, MI
But the tubing in the half shafts are not the same material. One is seamed and one is not. They even mentioned is somewhat in one of the article you posted a link to.
From the second link you posted:
Seam Tube is the most common material used for OEM applications. It is perfectly suited for everyday driver or restored cars. Although stretching its limits is a possibility, it could be carefully used in circle track and drag racing applications.
Pros: This is the most cost effective driveshaft you will ever find.
Cons: It’s also the weakest of all the steel driveshaft and has the lowest critical speed survivability rating, and is still heavy. Once again, steel weld yokes are used.
DUB
From the second link you posted:
Seam Tube is the most common material used for OEM applications. It is perfectly suited for everyday driver or restored cars. Although stretching its limits is a possibility, it could be carefully used in circle track and drag racing applications.
Pros: This is the most cost effective driveshaft you will ever find.
Cons: It’s also the weakest of all the steel driveshaft and has the lowest critical speed survivability rating, and is still heavy. Once again, steel weld yokes are used.
DUB
And it isn't clear from the pictures in this thread whether the 2 1/2" tubing is seamed or not. We can only view one side of the ID.
Last edited by Kid Vette; Jan 19, 2018 at 11:52 PM.
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Race Director
Joined: Mar 2016
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From: Napa Valley California
You might want to call a local driveshaft shop and get a quote on having your halfshafts retubed. I just called a driveshaft shop here in Tucson and got a quote of 300 bucks (150 per shaft) to have a set of 3 inch halfshafts retubed with .134 wall thickness seamless tubing and the price included balancing them as well (factory halfshafts aren't balanced for some unknown reason). That's a lot better of a deal than buying an aftermarket set of 3 inch halfshafts for 500 bucks or more.
Race Director
Joined: Apr 2009
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From: Charlotte NC
Yep, got it but the point I was making is that wall thickness is not the only factor in the strength or performance of a halfshaft. In this case, if the same type tubing is used in construction; the thinner walled 3" halfshaft would perform better in every category than the thicker walled 2 1/2". That's good to know if you are having custom halfshafts made for your application.
And it isn't clear from the pictures in this thread whether the 2 1/2" tubing is seamed or not. We can only view one side of the ID.
And it isn't clear from the pictures in this thread whether the 2 1/2" tubing is seamed or not. We can only view one side of the ID.
when you are talking about the wall thickness and diameter. Those variables can make a significant difference.This is an area that can raise a debate ...that is for sure.
DUB
Instructor
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Yep, got it but the point I was making is that wall thickness is not the only factor in the strength or performance of a halfshaft. In this case, if the same type tubing is used in construction; the thinner walled 3" halfshaft would perform better in every category than the thicker walled 2 1/2". That's good to know if you are having custom halfshafts made for your application.
And it isn't clear from the pictures in this thread whether the 2 1/2" tubing is seamed or not. We can only view one side of the ID.
And it isn't clear from the pictures in this thread whether the 2 1/2" tubing is seamed or not. We can only view one side of the ID.
Last edited by super charged; Mar 17, 2018 at 06:49 PM.
Melting Slicks
Joined: Dec 2009
Posts: 3,229
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From: Thunder Bay
So if I have this right, the strongest half shaft would be a 3" re-made with .125" or .134" wall seamless tubing, and combined with non-greasable u-joints. I've driven the new 427 for a few blocks (waiting for the snow to melt) and I think I'm going to have to start worrying about the strength of the driveline! 
Safety Car
Joined: Nov 2008
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From: Seattle Washington
True or False ? Big Block 2 1/2" Half Shafts were Shot Peened at the Factory to make them stronger ? I have herd this from time to time but never seen any proof. And even if it is true we can assume the production line used what ever they had handy at the time.
Race Director
Joined: Mar 2017
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From: Virginia
Assuming the steel is the same in both, the late stock 3" at 0.095" wall thickness (2.81" ID) will be slightly stronger (torsional stiffness) than the early stock 2.5" at even 0.135" wall thickness (2.23" ID). It's only a slight difference, so if there is a significant difference in weld strength, that would be the deciding factor.
So, yes, 3" OD, with a thicker than stock wall thickness would be the best of all.
So, yes, 3" OD, with a thicker than stock wall thickness would be the best of all.
Le Mans Master
Joined: Oct 2010
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
I think mine we're built by PST in Tampa I think I got the letters right. They are all new with solid joints. If I remember correctly they we're only a few bucks more than paying someone to install new joints.
They are beautiful! I called the owner and asked how strong they would be if I stepped up the power and he said if you twist them or break them ever he would replace them.
Unless you have a really stock car or a ncrs car I'd go after market. Bear in mind there are other weak links in the rear.
They are beautiful! I called the owner and asked how strong they would be if I stepped up the power and he said if you twist them or break them ever he would replace them.
Unless you have a really stock car or a ncrs car I'd go after market. Bear in mind there are other weak links in the rear.
Race Director
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From: Charlotte NC
