One piece vs two piece drive shaft

[Fanchier]

I getting ready to put a 700R4 in my LWB 71 GMC C25. And of course it's 2" longer than my Turbo Hydro-Matic.... I want to do everything at one, and I am considering having a new driveshaft made. The price between a one piece & a two piece are negligible.

Other than only having two u-joints, & no carrier bearing, what advantage would I have with a one piece?

[Caddylackn]

Less rotating mass, less friction. You can go aluminum and go larger diameter and thinner wall for less weight.

The downsides is there are less maximum rpms you can safely run with a longer driveshaft. That is something you should check if you race or drive at very high speeds. There is a simple calculation for this.

There are some differences between setting your pinion angle but I don't know if that is an advantage or a disadvantage. I know that all three angles of a two piece driveshaft set up should add up to zero, and for a one piece driveshaft both angles add up to zero. So your pinion setup for a two piece may have to be shimmed to get the proper angle to get to zero on a one piece and have no vibration.

[pontiacvince]

I shortened my 2-peice in my '71 Suburban 3" when I went from TH350 to 700r4. I asked the shop that did the work if I could go with a one piece. They had a chart that said it was too long for a one piece.

[Fanchier]

Originally Posted by Caddylackn

Less rotating mass, less friction. You can go aluminum and go larger diameter and thinner wall for less weight.

The downsides is there are less maximum rpms you can safely run with a longer driveshaft. That is something you should check if you race or drive at very high speeds. There is a simple calculation for this.

There are some differences between setting your pinion angle but I don't know if that is an advantage or a disadvantage. I know that all three angles of a two piece driveshaft set up should add up to zero, and for a one piece driveshaft both angles add up to zero. So your pinion setup for a two piece may have to be shimmed to get the proper angle to get to zero on a one piece and have no vibration.

Thanks! Right now I have 4:10 gears in the rear end, but will be changing that to something in the 3:70-something in the near future. I want to be able to do interstate speeds (75 MPH here) at a reasonable RPM.

[Caddylackn]

My crown vic has a very long aluminum driveshaft. It is speed restricted to 122 mph.
I think you will be fine at 75 mph

[Accelo]

Short box I would convert it to a one piece.
Since it's a Long Box I would leave it as a two piece.

[Fanchier]

Originally Posted by Accelo

Short box I would convert it to a one piece.
Since it's a Long Box I would leave it as a two piece.

WHat's you reasoning on keeping the two piece on the LWB?

[Fanchier]

Originally Posted by Caddylackn

My crown vic has a very long aluminum driveshaft. It is speed restricted to 122 mph.
I think you will be fine at 75 mph

I'll be doing a little over 75, but nowhere near 112....

[tdangle]

One thing to keep in mind is not the road speed but the driveshaft speed. With 4.10 gears you are in the 3K rpm range for the driveshaft. 100 mph in a car/truck with 3.08 gears will have a much lower driveshaft speed. I would suggest leaving it a 2 piece and try out the 4.10 gears you have with the 7004r trans. In overdrive your engine speed will only be around 2K at hwy speeds. That's what I have in my C20.

[Jason Banks]

You should be able to talk to whoever makes/modifies your driveshaft about how long they are comfortable making a given diameter/length/material of driveshaft.

I bought my latest from
https://www.actionmachineinc.com/col...our-driveshaft

If you scroll through the different driveshafts they offer, the larger the diameter, the longer "Max Length CL-CL" they will make.

[Accelo]

Drive shafts will start to whip at higher speeds. It is why the front shaft is so small in diameter on 4x4 vehicles. Because it's short their is no need to worry about whipping. Increasing the diameter of the drive shafts is one method that reduces the tendency to whipping. As the diameter increases it becomes more difficult to fit the larger diameter drive-lines through the factory hole in the cross-member in out C10's. On some modern vans the drive shaft can be up to 6" in diameter. However, it is designed for the length and RPM. Their are published tables that one can look up the diameter, material and length and find the what is called the first critical of drivelines. This is the speed that whipping starts. Overdrive transmission compound the issue. Their is nothing inherently wrong with two piece driveshafts. A numerical high differential ratio and an overdrive transmission is one of the worse case scenarios for driveline whip.
For example;
Using data from Jason Banks' link a 5" aluminum driveshaft can be built to a max length of 84" and a 4" aluminum driveshaft can be built to a max length of 72"
Cheers.

[Ironangel]

A one piece drive shaft in a LWB trailing arm truck is inviting trouble! Caddylackn indicated that a one piece DS has "less rotating mass." Not true when a single DS has nearly twice the diameter as a factory 2" two piece DS. Somebody mentioned the Crown Vic drive shaft, Crown Vic drive shafts have balancers built in to the yokes, the aluminum P71 versions were shorter due to longer transmission output shafts and tail housings to accommodate the aluminum shafts limited length restrictions. Camaro's, Mustang's and many muscle/race cars use two piece drive shafts, why? Because they're stronger, they are less apt to vibrate, and they can handle higher rpm...There is a plethora of reasoning in why GM engineers used two piece drive shafts in these trucks. Sure a one piece will work, especially in a lowered SWB truck or a truck with a gear vendor added. But I wouldn't even consider it on a LWB truck with a low geared diff. "Jus Sayin"

[72c20customcamper]

A lot of new trucks are 1 piece . I just replaced the two piece with a one piece on my 2007 dodge quad cab 8 ft bed 5 inch aluminum tube truck from day one had a lunch shutter new shaft it's gone . If the shaft is made correctly it will perform as well if not better than the original 2 piece . This shaft is made for a truck with a Gcvwr of close to 25000 pounds . Truck has a 3.73 rear
I'm going to replace the 2 piece in my 72 at some point .
This is the company I used they told me they would set one up for the 72 too.
https://www.actionmachineinc.com/col...um-driveshafts

I know a lot of people talk about terminal speed of the shaft but I highly doubt that we will be doing anything close to it in a 50 year old truck .

[Accelo]

My current truck has a driveline speed of 6000 rpm at 140 mph in drive.
I know this because it was dynoed at this speed.
I have a factory short box that was originally built with a two piece driveshaft.
Now it has a one piece shaft in it's place. My driveline was designed by an OEM not me.
Considering a long box truck would have either a 12" or 18" longer drive shaft it would be up to them, and their intended usage, to decide it a one piece is the better idea or not.
Four wheel drive trucks typically do not have the same issues as the transfer-case shortens the driveline length requirements.
Converting to an overdrive transmission will typically shorten the driveline length requirement by two to three inches.
So the drive-line in my truck is almost 3" shorter than if it was a stock THM-350.
Larger diameters are safer and less likely to clear the hole in the frame.

[tdangle]

Easy way to think of it is in 1 to 1 drive (3rd gear in auto trans) whatever your engine speed is equals driveline speed. So 4.10 rear in 3rd with 33" tires would be right at 3000 at 70+.

[Ironangel]

I own two 72 LWB trucks, one tows the other to the drag strips and shows..."Nasty Sally's" solid roller 439 lives at "critical speed" and eats them HEMI's for breakfast...Both trucks still sport factory two piece shafts.

[garyd1961]

The longer the driveshaft the more prone it is to vibrate, I would go with th he 2 piece shaft.

[truckdude239]

i had mine shortened 3 inches and all new joints and and it balanced it has held up well behind my ls swap and i beat the daylights out fo it

[72c20customcamper]

Originally Posted by garyd1961

The longer the driveshaft the more prone it is to vibrate, I would go with th he 2 piece shaft.

I don’t agree . If it true and balanced the one piece has less things to wear out that cause vibration . The two piece has an extra ujoint and carrier that are prone to wear . Also if the to piece isn’t phased correctly it will cause a vibration .

[Accelo]

My day job was dealing with vibrations of all kinds. I thought I would share this:

Drive-line Critical Speed:
Critical speed is a situation where the natural frequency is the same as the rotating speed of the driveshaft. When those two coincide vibrations are multiplied and will eventually shake the driveshaft so hard it will fail. When considering a driveshaft, it is extremely important to make sure that the car will operate at speeds lower than the critical speed.
The three factors that change critical speed are, length, diameter, and the ratio of weight to material stiffness. Just like a wind chime, the longer a tube is, the lower its natural frequency is, and the lower the critical speed. likewise diameter and how stiff a driveshaft tube is affects the critical speed. There are a lot of factors that go into critical speed but in general, a larger diameter and lighter tube will allow a driveshaft to spin faster. before it reaches it's first critical.

This phenomena isn't limited to drive-lines. Turbine generators to Airplane motor are all affected and need to be accounted for in the design phase.
In general terms, it is difficult to get into design issues with two piece driveshafts.

Specifically on our C10's the hole in the frame creates two issues. One, is it limits driveline diameter because of the hole diameter in the horizontal. The second is it limits the suspension travel because of the diameter in the vertical plane.
A two piece is definitely more expensive to build. Would have GM used a one piece if they could, darn right they would have. What else do we need to know?
All this is with a caveat, I have a one piece on my truck. Can you do it, yes you can. Easier to pull it off on a Short Box. Even easier on a lowered Short Box.
Can it be done on a long box? Sure it can. Use as large as possible drive-shaft, stay under 140mph, don't go jumping your truck off road and stay away from 4;11 and lower gears especially with an OD transmission.

Driveshafts actually take on the shapes listed below. It can be seen visually using a stroboscope, while it is at speed. However, I don't recommend getting close enough to do it in person!
It is also the science behind why driveshaft loops are required on many drag cars.
Cheers.

[AllGoNoShow]

I had a 4” one piece made for my LWB 68 with a Tremec TKO. It replaced a Muncie which is shorter that had a 3.5 one piece in it from previous owner. When I did the calculations, the critical speed for the 1 piece 3.5 was around 80 or so, hence the reason I went to the 4” which gave me 100mph+ (but the guys at the driveshaft place said they weren’t really worried at that speed and said it was conservative).

I had to raise the hoop on the trailing arm crossmember a 1/2 inch to get the 4” to fit at max suspension travel.
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[Keith Seymore]

Originally Posted by Accelo

My day job was dealing with vibrations of all kinds.

Mine, too (well - technically: designing the drivelines that you all are discussing).

As a result, I try to stay out of the noise and vibration discussions because they make me crazy, but since I'm here I'll make a few comments:

Originally Posted by Accelo

Drive-line Critical Speed:
Critical speed is a situation where the natural frequency is the same as the rotating speed of the driveshaft. When those two coincide vibrations are multiplied and will eventually shake the driveshaft so hard it will fail. When considering a driveshaft, it is extremely important to make sure that the car will operate at speeds lower than the critical speed.
The three factors that change critical speed are, length, diameter, and the ratio of weight to material stiffness. Just like a wind chime, the longer a tube is, the lower its natural frequency is, and the lower the critical speed. likewise diameter and how stiff a driveshaft tube is affects the critical speed. There are a lot of factors that go into critical speed but in general, a larger diameter and lighter tube will allow a driveshaft to spin faster. before it reaches it's first critical.

etc

Specifically on our C10's the hole in the frame creates two issues. One, is it limits driveline diameter because of the hole diameter in the horizontal. The second is it limits the suspension travel because of the diameter in the vertical plane.
A two piece is definitely more expensive to build. Would have GM used a one piece if they could, darn right they would have. What else do we need to know?
All this is with a caveat, I have a one piece on my truck. Can you do it, yes you can. Easier to pull it off on a Short Box. Even easier on a lowered Short Box.
Can it be done on a long box? Sure it can. Use as large as possible drive-shaft, stay under 140mph, don't go jumping your truck off road and stay away from 4;11 and lower gears especially with an OD transmission.

....

etc

Driveshafts actually take on the shapes listed below. It can be seen visually using a stroboscope, while it is at speed. However, I don't recommend getting close enough to do it in person!
It is also the science behind why driveshaft loops are required on many drag cars.
Cheers.

This is good. Very good stuff.

I would expand a bit on the driveshaft criteria (length, diameter, ratio of weight to material stiffness). As an OE vehicle manufacturer, we can also control wheelbase, shaft material selection, vehicle top speed, rear axle ratio, part number count and (of course) one piece vs two piece design. All of these criteria are mixed together by the engineer to allow program management to come up with the "right answer" from a cost/complexity standpoint for the specific model combination being discussed. And - as a result - the answer might be different for a 1/2 ton 2wd 350 cu in 700R4 equipped vehicle with a 3.73 rear axle and P235 tires, vs the same vehicle with 2.73 gears and P215 tires. That's why you see so much variation when surveying vehicles in the field.

Additionally, there may be driveline system resonances discovered on full vehicle shaker durability tests that are not intuitively obvious and that drive the selection between a one piece shaft and a two piece shaft (or, that drive the release of added pencil braces or other conspicuous band aids to help control the resonance).

The only nit from above that I would pick is this: I don't think the driveline loop requirement for drag cars is driven by any kind of shaft critical speed issues. Most failures on the drag strip occur at launch, when the loads are highest and speeds are low, the failure mode being either blowing a U joint or twisting the tube. The loop is intended to keep the front of the shaft from dropping down and pole vaulting the vehicle (and - truth be told - likely an attempt by track owners at minimizing any gouging damage to the racing surface itself).

K

[Keith Seymore]

Originally Posted by Ironangel

Somebody mentioned the Crown Vic drive shaft, Crown Vic drive shafts have balancers built in to the yokes,

.

That's not a "balancer", at least in the sense of controlling first order rotational imbalance issues (like a dead weight on a driveshaft).

That's a tuned torsional damper (like an engine "harmonic balancer" - which is a misnomer), intended to attenuate rear axle whine.

The "non conjugate meshing action" (I look for opportunities to say that in daily conversation) of the spiral bevel hypoid gearset causes a torsional resonance of the shaft. In other words, the shaft, as it is spinning, is bouncing back and forth rotationally, and the damper is tuned to the natural frequency of the shaft in order to knock down the amplitude of the disturbance.

Interestingly enough, you can put the damper on the pinion of the axle as well, but it is not as effective. The shaft is acting like a big rubber band and so the amplitudes are greater at the front of the shaft, making that a more effective location for placing the damper. Having said that, I would guess you could put the engine "harmonic balancer" at the rear of the engine, but would need to determine if it is effective there (and it's probably a really crummy packaging location).

I've tuned a few of these. You can also find them on Astro/Safari vans, and GMT400 C20 pickups. Normally the noise is structure born, transmitting up through the rear suspension mounting into the body cavity, but the noise transmission path on the C20 was airborne: you can hear the rear axle whine from outside the vehicle as well as inside (if not better).

I think I have a couple old reports around here somewhere if anyone would like to see the math. It's kinda cool.

Originally Posted by Ironangel

There is a plethora of reasoning in why GM engineers used two piece drive shafts in these trucks.

That would be "me". You can feel free to ask any questions you might have.

K

[Keith Seymore]

So - back to the original question:

Originally Posted by Fanchier

I getting ready to put a 700R4 in my LWB 71 GMC C25. And of course it's 2" longer than my Turbo Hydro-Matic.... I want to do everything at one, and I am considering having a new driveshaft made. The price between a one piece & a two piece are negligible.

Other than only having two u-joints, & no carrier bearing, what advantage would I have with a one piece?

...ask your shaft builder what he would recommend based on your configuration.

But - as for me - I always go one piece if I can. That's the starting point for all designs. It's simpler, with fewer moving parts, less clunky, more elegant, less expensive and what God intended. The two piece configuration is, by definition, a step down that the designer is forced into because you can't make a one piece work. All of the trucks I ordered originally were 2wd short wheel base vehicles and as a side benefit I would get a one piece shaft.



Heck - I'd figure on just driving slower if that meant I could have a one piece shaft. That's one of the knobs we turn as a manufacturer, as stated in my first post above, through top speed limiting by pulling fuel/spark. For me personally I'd just keep my foot out of it.

K

[Keith Seymore]

Originally Posted by Keith Seymore

I think I have a couple old reports around here somewhere if anyone would like to see the math. It's kinda cool.

K

Found it, on this very site.

If Accelo gets a kick out of this then it will have been worth it.

I think it is pretty self explanatory. Let me know if you find any mistakes.

K