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Difference between a 3.5" and 4" Driveshaft?

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So I thought a smaller diameter driveshaft is better because of less rotational mass but MPT offers a 4" driveshaft thats 50 bucks more than the 3.5" variant (Both dds driveshafts). Would a 4" ds be able to handle more power or something? 

Also AM has a 4" ds from SR Performance thats like 44 bucks less than dds's driveshaft. Basically I'm not too sure whats going on here.

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From what I remember the larger the diameter the stronger the shaft especially regarding twist strength.  They usually rate the for HP ratings.   Weight is the deciding factor regarding rotation with diameter playing a lesser role.

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So, thicker will mean stronger, but will also mean more rotational mass, and rotational mass goes up QUICK with increasing diameter.

 

Rotational Inertia is I=mr^2, so radius plays a phat part of the inertia of the system... That said, what are we talking about here, .25" radius delta and a few lbs difference, its nothing you will ever feel, and well within the noise of a dyno. We are into the realm of theoretical gains lol.

 

You can do the math easily if you know the values (I don't), but I assume DS's are hollow, so you just need the wall thickness and the dia of the pipe. I=M/2(R1^2-R2^2). If it isn't hollow, which i don't believe........... but I could be wrong, I don't honestly know lol, I=M(R^2)/2

 

Or you can just make up some numbers and see what it seems to be, and see how small a change it really makes. I remember I did the math on my flywheel (which isn't perfect cuz they taper out as you get out towards the teeth, so more of the mass is closer to the center of rotation, but lets just say "whatever" and not care about that. I went from the stock dual mass flywheel which is 32lbs iirc to my fidanza 11.8 lbs unit. I think it saved me like.... 6 ft/lbs of torque? if memory serves. I don't remember the diameter of a flywheel at all, and don't care enough to look lol. But you can probs assume radius is.... 10 inches if you want to calc it real quick yourself for fun. Probs close-ish.

 

Calcing the shear of the two is a bit harder, cuz then you really do need to know some real values or your answers will just be really far off lol. So not really worth going into there, but im more than sure DSS wouldn't put out a DS that isn't up to the job. 

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CJ Pony Parts salesman has a 3.5" Shaftmasters 1 piece I am looking at. 14 hp 18 ft/lbs torque gain over oem 2 piece.

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17 hours ago, LIGISTX said:

So, thicker will mean stronger, but will also mean more rotational mass, and rotational mass goes up QUICK with increasing diameter.

 

Rotational Inertia is I=mr^2, so radius plays a phat part of the inertia of the system... That said, what are we talking about here, .25" radius delta and a few lbs difference, its nothing you will ever feel, and well within the noise of a dyno. We are into the realm of theoretical gains lol.

 

You can do the math easily if you know the values (I don't), but I assume DS's are hollow, so you just need the wall thickness and the dia of the pipe. I=M/2(R1^2-R2^2). If it isn't hollow, which i don't believe........... but I could be wrong, I don't honestly know lol, I=M(R^2)/2

 

Or you can just make up some numbers and see what it seems to be, and see how small a change it really makes. I remember I did the math on my flywheel (which isn't perfect cuz they taper out as you get out towards the teeth, so more of the mass is closer to the center of rotation, but lets just say "whatever" and not care about that. I went from the stock dual mass flywheel which is 32lbs iirc to my fidanza 11.8 lbs unit. I think it saved me like.... 6 ft/lbs of torque? if memory serves. I don't remember the diameter of a flywheel at all, and don't care enough to look lol. But you can probs assume radius is.... 10 inches if you want to calc it real quick yourself for fun. Probs close-ish.

 

Calcing the shear of the two is a bit harder, cuz then you really do need to know some real values or your answers will just be really far off lol. So not really worth going into there, but im more than sure DSS wouldn't put out a DS that isn't up to the job. 

You could have just re-posted my post above lol.....  

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5 hours ago, Bananana said:

You could have just re-posted my post above lol.....  

But he had to make it longer. Ligistx could you write my papers? Lol

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I mean, to be fair..... I did explain why diameter matters more than weight of the shaft when your talking about rotational inertia ;). lol.

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If you had an infinitely heavy shaft that was the diameter infinitesimally small, assuming you wouldn't get a black hole......... it would take no effort to spin ;)

 

I have calced how small you would have to make a white board eraser to make a black hole, I forget the answer but I think it was on the order of smaller than an atom or some stupid shit. #funstuffiguess? lol

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9 hours ago, LIGISTX said:

If you had an infinitely heavy shaft that was the diameter infinitesimally small, assuming you wouldn't get a black hole......... it would take no effort to spin ;)

 

I have calced how small you would have to make a white board eraser to make a black hole, I forget the answer but I think it was on the order of smaller than an atom or some stupid shit. #funstuffiguess? lol

AM should definitely start looking into providing a black hole driveshaft. lol

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I had the 3-1/2" DSS shaft and I believe it was rated at 900 hp so no worries there. You get a little more clearance from the exhaust system running a 3-1/2" shaft.

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I'm ready to "Pull the trigger" going from DSS 3.5" last pony to the Shaftmaster's 3.5" for the 16 GT.

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Wow, so I just contacted a local shop and they said they could make one for about half the price. Definitely think I'm going to go that route.

Yea, getting a shop to make one is an option, but depending on how you plan to use the car I don’t know if I would. DSS has a good record of making reliable units even when used under track conditions.

Not to say the shop can’t do it, just something to consider.


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Getting one made with proper balance right from the beginning would be a great choice. When you order a driveshaft online even if the product is top quality it might take a few install attempts to get it perfectly balanced to the vehicle. When I first installed mine it had a vibration at 60mph. Clocked it once at the diff and it was fine until the next time I took it to the strip where I found it had a vibration at about 115mph. Clocked it a third(and last chance) time and it has been good since.

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Getting one made with proper balance right from the beginning would be a great choice. When you order a driveshaft online even if the product is top quality it might take a few install attempts to get it perfectly balanced to the vehicle. When I first installed mine it had a vibration at 60mph. Clocked it once at the diff and it was fine until the next time I took it to the strip where I found it had a vibration at about 115mph. Clocked it a third(and last chance) time and it has been good since.


Hmm, mine has been on and off a handful of times without paying attention clocking and it’s never given me an issue. Guess for once with the car I was lucky lol.


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9 hours ago, LIGISTX said:


Yea, getting a shop to make one is an option, but depending on how you plan to use the car I don’t know if I would. DSS has a good record of making reliable units even when used under track conditions.

Not to say the shop can’t do it, just something to consider.


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What you say makes since but the head tech at this place has 18 years of experience and they use a Coetz SE2000 balancer so they look pretty trust worthy.

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I say go for it but see if they can use a CV on at least one end instead of u-joints at both end as is typical for shaft manufacturers. Should keep it a bit smoother.

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On 4/25/2019 at 6:44 PM, CCstang said:

For haft the price will it be a steel or aluminum driveshaft?

It will be aluminum. I never even considered steel. lol

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