Does anybody make SIX INCH OD aluminum driveshafts?

Is there any solution at all for a 75" shaft that needs to spin 7000 RPM? Carbon fiber/titanium/beryllium hybrid composite with integrated active Eigen-mode compensators and costs $140,000?

The responses I've gotten from driveshaft shops are amazing.
"You will have to convert to a 2-piece shaft for that length. Just delete the factory fuel tank so you can fit in a crossmember for the center support, should be no big deal."
"No."
"We don't do trucks."
"I wouldn't recommend spinning any shaft that long of any size/material over 3000 RPM, even if we, a professional high performance driveshaft specialty shop, built it with the best of our cutting-edge tech and craftsmanship."

According to interwebs critical speed calculators a 5" OD/.125"-wall aluminum shaft that's 75" joint to joint is good for 5700 RPM, which with my gear and tire nets only 130 MPH. If there were such a thing as a 6" OD driveshaft, then at that length crit would be 6900 and a speed of 157. This would be good enough. It does not need to handle 3000HP, which seems to be the only criteria driveshaft shops are used to dealing with.

Hot rodding is officially dead. R.I.P.

These guys show they will make them all the way to 86 inch.

https://www.mrdriveshaft.com/drivesh...afts/aluminum/

Forgot to add. Here are some 2 piece conversion kits

https://www.alldrivelines.com/2piece

Finding someone to make a driveshaft that length isn't an issue. Finding someone to build a driveshaft that length that will handle 1000lb/ft isn't an issue. It's that any driveshaft that's 75" long runs up against the critical speed limit where everything converts to large chunks of high velocity shrapnel.

http://www.wallaceracing.com/driveshaftspeed.php

This is a long wheelbase 2WD Envoy. No GMT360/GMT370 was ever built with a 2-pc driveshaft, unlike the trucks where depending on year and options it could have been either 1-pc or 2-pc. My fuel tank completely blocks access to the left hand frame rail, it's snug up against the floorpan from the tailshaft all the way back to just ahead of the axle tube. The spot where a crossmember would need to be added for the center support is already spoken for and a fuel cell in the trunk isn't something I'm willing to live with in a stock-appearing daily driver SUV.

The biggest baddest driveshaft any of the shops know how to make (5" OD/0.125" wall) that is 75" long is only safe to around 5700 driveshaft RPM, which with a 28.6" tire and 3.73 gear gives 130 MPH.

If this were a short wheelbase Envoy with a dinky little 59" long shaft then a 5"/0.125" would be good to 9200 driveshaft RPM, or 209 MPH, with the same gear/tire.

So, to get a higher critical speed than what 5" aluminum at this length can do it either needs a different material (carbon fiber, though I haven't found anyone who builds anything that long in CF) or larger OD aluminum tube (6"/0.125" aluminum = 6900 driveshaft RPM, no carbon fiber required). Moving the engine back 15 inches isn't really practical. Neither is building a dummy transfer case or a tailshaft extension. Changing the rear gear is sorta an option - current 3.73 to 3.08 would be awful, but would allow the same 5700 driveshaft RPM to net 157 MPH...

Just spitballing some ideas. What about the 3.08 and sloppy up the converter. Or see if someone out there makes a better first gear like they do on the TH400's.

They do make a taller 1st gearset for the 60E, but that doesn't solve either problem - needing to reduce driveshaft speed or reducing impact of changing to stupid tall rear gear. The stock 1st gear is already too low.

I've been thinking about/researching/emailing driveshaft shops about this for more than a year, so don't think I'm just being a kneejerk contrarian dismissing solutions as soon as they're presented. Until someone is able/willing to build a driveshaft of this length that's safe at 6500-7000 RPM there are no good solutions.

It might drive around OK with the dumb 3.08 gear given that the plan is to add a LSA blower, but it is 5500+ lbs after all and the same WB as a frickin Suburban.

I'm not a 4WD geek, guessing somebody will already know this.

What transfer case would be best suited to a full-time 2WD conversion? No front driveshaft or other crap, using it only as a tailshaft extension.

Man you're in some territory there! and i feel ya... just went through that kind of thing with getting an 82" driveshaft made for a custom chassis we built for a customers vintage aluminum bodied bread van. When all you give the driveshaft shop is joint sizes and center to center lengths and they say "yeah we can build it but max speed on that length will be 100 mph" and you try to double check their math based on the critical RPM and realize without tire diameter and rear axle ratio they literally just threw out a BS random number!!! :bigshock:

As you know, the OD increasing helps critical speed. The mass of the tube works the other way, the heavier it is the lower the critical speed. So, carbon fiber would absolutely be the best bet to get you there. One other thing that you might find a driveshaft shop willing to actually talk to you about is passing through the crit speed. If you were open road racing and staying at or around that speed I wouldn't consider it, BUT if you are doing 1/4mi runs and just passing through that critical RPM quickly, you may even be able to lower it a bit so you pass though it at a slightly lower rpm with less overall forces. There are a ton of variables in play that determine which types of vibration you can drive out of to reach the other side, but maybe an old timer thats been there and done it would have some secrets.

Oh just thought of something from your 2wd transfer case idea.... maybe see if Gear Vendors will sell you an "empty" / overdrive-less case for reasonable??

Yeah. For instance this: https://www.qa1.net/custom-driveshafts

For these things that start at $1300, other than dimensional measurements they only need to know these 4 things...

Quote:

---

-Total vehicle weight
-Horsepower
-Torque
-Maximum engine RPM

---

...two of which are totally irrelevant to the driveshaft. And engine torque is irrelevant without knowing the gear ratios.

To top it off they don't make them longer than 65", you know, the kind of applications for which in some situations there is no option other than CF. Great.

Yeah thats scary.... could easily have an application that looked trivial on paper with those parameters but zing the shaft right up past 15,000 rpm with dinky tires and 6:xx rear gear! And if you called their "tech" line to ask about it theyd probably keep insisting you were the one that didnt understand :banghead:
You said it though; hot rodding is dead... or.... is it just that everyone that tries to buy "hot rod" parts nowadays likely ends up dead haha.

I try to only work with technically affluent customers, so when i run into that kind of crap at my shop, I can just tell the customer "i cant buy anything safe or worthwhile so I need to design, engineer, fabricate, and integrate it myself - bring more money. lol. Point being... if you feel comfortable with your own calculations on the 6" aluminum shaft..... can you buy 5" aluminum shaft ends and quality 6" aluminum DOM tubing with material cert? If you can find a machine shop that a fellow hot-rodder runs he may give you a good enough price on turning some precision adaptor sleeves with a light press fit. The welds on 75% of the aluminum shafts I see are utter garbage, kill the tube heat treat alongside the weld, and thus have failed a couple with non earth shattering power. Check out the crazy specs on permatex "shaft lock" compound, i use that a lot on machined fits. When pressing together with that the welds are just there for formality and to keep less informed tire kickers from arguing haha.
I wouldnt trust the driveshaft shop local to me to weld a light bracket on a bicycle frame, BUT they can balance shafts up to something like 15FEET long, so if you can get them to just shush, balance and take your money youd be home free

Crazy thing is I could buy a used NP2 manual transfer case and adapter and 4WD output shaft and stock 61" 4WD Trailblazer driveshaft and be good to over 150 MPH, or, for nearly the same money, buy a 5"OD 75" long driveshaft that's only good to 130.