this thread has it all: high level bullshit, math, wild speculation, blues traveler, a letterkenny reference and GILF's
stay gold FABO, stay gold.
stay gold FABO, stay gold.
Driveshafts: why is rear end tapered?
- Thread starter LS&S
- Start date
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TT5.9mag
Two atmospheres are better than one
Simple answer is it’s the weakest point. So what. Just because it’s the weakest point in the shaft doesn’t mean it was wrong for the intended purpose. They sacrificed nothing. The shaft they designed accounted for all variables in its intended purpose. You don’t see stock vehicles chucking driveshafts out from under them. It was sufficient to do the job, and keep vibration under control. Aftermarket driveshafts are usually built with a different intended purpose and need to be stronger because the engines in front of them are modified and bigger tires are behind them. In that case resonant frequency is not of major concern. BUT usually aftermarket driveshafts are larger in diameter to accomplish a similar goal as tapering.
TT5.9mag
Two atmospheres are better than one
Not to mention FAR easier to build. A tapered shaft takes tooling to create, where as 3" round tube and welded to ends designed for them are easy.
Cut, Stuff, Align, Weld, Done!
jbc426
Well-Known Member
On a relate note, I never though much about checking the critical speeds of a driveshaft. After the installation of an overdrive transmission along with running higher gear ratios 3.91's and 4.30's didn't raise any red flags. I figured I'd have wheel spin before loading enough torque to twist a driveshaft without running slicks on a track, so I used stock style driveshafts. I may not have used a tapered one, but a regular 3" one anyway.
I like to stretch the legs on my Mopars from time to time, and started feeling significant vibration as speed increased. Then I did the math, 6000RPM/.67 = 8955 RPM and well over 150MPH. Stock driveshafts, especially long, tapered ones could easily be pushed beyond critical speed.
When it came to ordering a new driveshaft, my choices narrowed significantly. I didn't realized how high the rotational speeds where getting with overdrive higher gears and higher speeds, and how close I was getting to the critical speeds of "normal" factory driveshafts.
I ended up with a 52" Mark William's 4" aluminum driveshaft with 1350 U-joints and forged yokes. The critical speed on this specific unit is 8549RPM, so that will more than cover it.
I like to stretch the legs on my Mopars from time to time, and started feeling significant vibration as speed increased. Then I did the math, 6000RPM/.67 = 8955 RPM and well over 150MPH. Stock driveshafts, especially long, tapered ones could easily be pushed beyond critical speed.
When it came to ordering a new driveshaft, my choices narrowed significantly. I didn't realized how high the rotational speeds where getting with overdrive higher gears and higher speeds, and how close I was getting to the critical speeds of "normal" factory driveshafts.
I ended up with a 52" Mark William's 4" aluminum driveshaft with 1350 U-joints and forged yokes. The critical speed on this specific unit is 8549RPM, so that will more than cover it.
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isn't 8955 greater than 8549???? Or do you NEVER get that fast?
jbc426
Well-Known Member
No, I generally try to keep it under 100mph on the day-to-day, so 197mph is above both my pay grade and capabilities in my Mopars. I've only maxed out the 150mph speedo's in both my Mopars a handful of times, but I'm not done yet!
Last year, I decided to sell my rice rockets, because it's too easy to flirt with 200mph on them.
Found this, ( Trigger warning, it could be from GROK)
"The critical speed for a factory 3-inch diameter steel driveshaft on a 1968 Dodge Dart, which typically measures roughly 48 to 54 inches in length depending on the powertrain, is generally in the range of 5,000 to 6,000 RPM.
Exceeding this speed can cause the driveshaft to enter a "whipping" motion, leading to severe vibrations and eventual failure."
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TT5.9mag
Two atmospheres are better than one
Now THAT is what I’m talking about. You sir win the cookie for today.
TT5.9mag
Two atmospheres are better than one
TT5.9mag
Two atmospheres are better than one
And a poop joke for good measure.
jos51700
Green Bearing thread connoisseur
I think about it. Then I get scared.
At least the math isn't high level, and only the speculation is wild.
sir. it was my understanding there would be no math.
TT5.9mag
Two atmospheres are better than one
In high school we were under a friends Camaro that was on the lift, engine running and in gear. I remember our shop teacher, Mr. Luft pointing at the driveshaft and the clutch and saying “imagine those fuckers spinning at 7500 rpm”. I’ve been scared ever since.
OneOfMany
Well-Known Member
My understanding is that this is true. It prevents a harmonic vibration (or lessens it) at a particular frequency.
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