Idiot question: Why can't you use a cv axle as a driveshaft?

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mct75

mct75

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Reading about DC driveshafts, I wondered why driveshafts have a u joint to begin with. Why does nearly every vehicle on the road use CV joints on axle shafts but still uses U joints for the driveshaft?

Additionally, why are driveshafts hollow and cv axles solid? I'm guessing it has to do with the speed they spin at but I'm not sure.
 
Some vehicles do use CV (constant velocity joints) Just 1 comes to mind mid 80'S gm 4x4 . CV's are crazy high to replace. I just replace 1 on a piece of farm equipment, all total was $3600.00.
 
CV's tend to require total replacement when they fail, while driveshafts have parts that can be serviced/replaced. That's one theory, anyway :o
 
I'm guessing it is primarily because of the increased cost of the CV components, with a secondary (or 1B) goal of the driveshaft being an "engineered failure point" meaning that you blow up an easily and cheaply (by comparison) replaceable U-joint rather than damaging a more expensive drive-line component.
 
Simple answer here.

They work. They're cheap and serviceable.

Driveshafts are hollow for weight savings and because the larger tube is easier to balance and work with than a tiny piece of solid stock.

Solid shafts are needed at the wheels because of the added torque at the wheel. Your driveshaft "gears down" at the differential...amplifying your torque by whatever your gear ratio is ie. 4.10:1, 4:88:1 etc.

Drive shafts don't need the strength that axle shafts have...plain and simple.
 
Safety
Cost
Mass
Function

You really have to try to blow a U-joint apart. it is self-retaining except in very extreme circumstances.

CV joints are expensive compared to U-joints. look at all the precision cutting and grinding that is necessary.

Hollow drive shafts offer higher rigidity for less or equivalent mass to a solid shafts.

They work just fine most of the time. it is difficult to justify replacing a cheap, low-mass part with something more expensive and heavier.

Of course it can be done, there are just a lot of trade-offs.
 
I had custom drive shafts made for my 40 when I put the LS1 in it. I was told it was necessary because the stock drive shaft wouldn't be able to handle the torque (plus it needed to be shorter, IIRC). A buddy of mine owns a company that does precision milling of power train components. They milled a front and rear drive shaft. It was stupid expensive. IIRC it cost me around $3500 or so and that was with the "cool guy discount". I can't remember what they made it out of but it was stout as hell. Looking back, it was probably complete overkill and a waste of money. But I didn't know what I was doing (other than throwing obscene amounts of money for custom parts, doing an engine swap that had never been done before...).
 
LS1FJ40 said:
I had custom drive shafts made for my 40 when I put the LS1 in it. I was told it was necessary because the stock drive shaft wouldn't be able to handle the torque (plus it needed to be shorter, IIRC). A buddy of mine owns a company that does precision milling of power train components. They milled a front and rear drive shaft. It was stupid expensive. IIRC it cost me around $3500 or so and that was with the "cool guy discount". I can't remember what they made it out of but it was stout as hell. Looking back, it was probably complete overkill and a waste of money. But I didn't know what I was doing (other than throwing obscene amounts of money for custom parts, doing an engine swap that had never been done before...).
Click to expand...
Pics? I would like to see this
 
tacoma2002 said:
Simple answer here.

They work. They're cheap and serviceable.

Driveshafts are hollow for weight savings and because the larger tube is easier to balance and work with than a tiny piece of solid stock.

Solid shafts are needed at the wheels because of the added torque at the wheel. Your driveshaft "gears down" at the differential...amplifying your torque by whatever your gear ratio is ie. 4.10:1, 4:88:1 etc.

Drive shafts don't need the strength that axle shafts have...plain and simple.
Click to expand...

Why do Freight Trucks that weigh 10X+ more than cars have driveshafts instead of solid shafts with more torque?
I was going to make the same conclusion, then I thought about stake trucks and Uhauls.
 
Great replies so far! It just seems like considering the money we throw at our rigs, someone would have made a CV driveshaft and never worried about vibrations ever again.

A double cardan joint is a CV joint, but the U joint on the other side of a DC driveshaft isn't. Why not make a quad cardan driveshaft? Just thinking out loud here.
 
After they made mine he said "you've now moved the weak point in to the axle...". So maybe the drive shaft u joints are the "fuse" or weak point on purpose?
 
LS1FJ40 said:
I had custom drive shafts made for my 40 when I put the LS1 in it. I was told it was necessary because the stock drive shaft wouldn't be able to handle the torque (plus it needed to be shorter, IIRC). A buddy of mine owns a company that does precision milling of power train components. They milled a front and rear drive shaft. It was stupid expensive. IIRC it cost me around $3500 or so and that was with the "cool guy discount". I can't remember what they made it out of but it was stout as hell. Looking back, it was probably complete overkill and a waste of money. But I didn't know what I was doing (other than throwing obscene amounts of money for custom parts, doing an engine swap that had never been done before...).
Click to expand...
Whoever told u the stock 40 driveshaft wouldn't hold up to the torque was mistaken. 40 driveshafts are extremely strong. I have twisted the tube on a 40 driveshaft, but the tube is easy to upgrade. The factory 40 ujoints are ridiculously strong, they don't break ever and hold up to any power a v8 can dish out.
 
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White Stripe said:
Whoever told u the stock 40 driveshaft wouldn't hold up to the torque was mistaken. 40 driveshafts are extremely strong. I have twisted the tube on a 40 driveshaft, but the tube is easy to upgrade. The factory 40 ujoints are ridiculously strong, they don't break ever and hold up to any power a v8 can dish out.
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Correct. But at the time nobody knew for sure. An LS had never been put in a 40. Plenty of TBI builds. It was hypothetical.
 
Hornd said:
Why do Freight Trucks that weigh 10X+ more than cars have driveshafts instead of solid shafts with more torque?
I was going to make the same conclusion, then I thought about stake trucks and Uhauls.
Click to expand...

Freight truck shaft's are alot thicker than cars.

Driveshaft material around here is sold by the 1/4 inch. Just pulling # out of the air lets say it's $5 a 1/4" thats $20 an inch and lets use 40". So $800 just for the tube.
 
Can any of it be related to safety? i.e. during a severe impact the hollow driveshaft will collapse and not penetrate the passenger compartment, whereas a solid one would likely come up and penetrate the passenger compartment (and perhaps a few passengers?)...ouch.
 
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