Drive shaft critial speed ? what you guys running
#745865
07/11/10 09:02 PM
07/11/10 09:02 PM
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sycamore,ohio
BobsProFab
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i talked with dyno tech about the drive shaft for the Demon and what RPM range it will live at. Jim told me i needed a 5 inch Alum. shaft to over come the critial speed on the drive shaft @ the length i will need / 50 inch center to center or go carbon fiber shaft. just wondering what some of the high winding motors are running for drive shafts with my small block shift 9,400 trap 9,800 is the plan  Bob
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Re: Drive shaft critial speed ? what you guys running
[Re: BobsProFab]
#745867
07/11/10 09:26 PM
07/11/10 09:26 PM
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Romeo MI
MR_P_BODY
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Bob I have a 4" moly... but its 39" and I have been in the 9400 range a couple of times... normal is 8200 and its been in there for about 8 years now, still with the same U-joints and it looked like new when I had it out the other day
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Re: Drive shaft critial speed ? what you guys running
[Re: MR_P_BODY]
#745868
07/11/10 10:09 PM
07/11/10 10:09 PM
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BobsProFab
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Quote:
Bob I have a 4" moly... but its 39" and I have been
in the 9400 range a couple of times... normal is 8200
and its been in there for about 8 years now, still
with the same U-joints and it looked like new when I
had it out the other day
@ 39" moly 4 inch the critical speed of your shaft would be over 10,000
the length is the important key to the formual.
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Re: Drive shaft critial speed ? what you guys running
[Re: BobsProFab]
#745869
07/11/10 10:42 PM
07/11/10 10:42 PM
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Out West
408strokerdart
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Bob, Most of what I have found is that most driveshaft companies severely under estimate the true critical speed of a given material. I was suspect due to what I seen on the chart in Mark Williams catalog. I did the calculations myself and came up with much higher critical speed numbers than what was published. Had another engineer go over my numbers and verify them. I ended up with a 49" long 4" diameter aluminum shaft. Shift at 8000, cross stripe at 8200. No probelms at all. I will revisit this soon as I will be going to a glide and the shaft length will be 56" long. I might have to go to an MMC material or carbon. I would guess you will be very close to having to run a carbon shaft at that length and RPM. BTW, don't forget you will have lower shaft RPM than engine rpm (converter efficiency not 100%) so it will be just slightly lower than engine RPM. Maybe 500 RPM less? 
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Re: Drive shaft critial speed ? what you guys running
[Re: 408strokerdart]
#745871
07/11/10 11:21 PM
07/11/10 11:21 PM
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BobsProFab
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Brain, the MMC bonded MW's shaft @ 4 inches is rated at 8,766 for the length i need, its sfi 43.1 spec. going to make a call to them about the shaft. when i go to the Hemi and a 400 i will need a shorter shaft and need to find out if they can shorten the bonded ones dont want to spend a grand twice on drive shafts  dyno tech doesnt offer the MMC ones anymore
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Re: Drive shaft critial speed ? what you guys running
[Re: BobsProFab]
#745873
07/11/10 11:29 PM
07/11/10 11:29 PM
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Out West
408strokerdart
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Quote:
Brain, the MMC bonded MW's shaft @ 4 inches is rated at 8,766 for the length i need,
its sfi 43.1 spec.
going to make a call to them about the shaft.
when i go to the Hemi and a 400 i will need a shorter shaft and need to find out if they can shorten the bonded ones
dont want to spend a grand twice on drive shafts
dyno tech doesnt offer the MMC ones anymore
This is exactly why I went the budget route on my current shaft....probably going to have all of 25 runs on it when I swap to a glide and need another. $$ is the only reason I skimped/saved, but I won't feel as bad spending bigger money on the next one, plus it will be longer making the problem even worse.
I think they can cut the MMC shafts and bond in a new end as long as they shorten it at least 2-3". Good luck with it, can't wait to see how your car runs with that killer small block.
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Re: Drive shaft critial speed ? what you guys running
[Re: BobsProFab]
#745874
07/11/10 11:48 PM
07/11/10 11:48 PM
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Joined: Jun 2003
Posts: 52,972
Romeo MI
MR_P_BODY
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Master
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Quote:
Quote:
Bob I have a 4" moly... but its 39" and I have been
in the 9400 range a couple of times... normal is 8200
and its been in there for about 8 years now, still
with the same U-joints and it looked like new when I
had it out the other day
@ 39" moly 4 inch the critical speed of your shaft would be over 10,000
the length is the important key to the formual.
Bob I am aware that the length is a MAJOR factor
thats why I stated the length.... Dyno tech build mine
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Re: Drive shaft critial speed ? what you guys running
[Re: MR_P_BODY]
#745875
07/12/10 12:05 AM
07/12/10 12:05 AM
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sycamore,ohio
BobsProFab
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Raff, not sure why they dont build the MMC shafts anymore  they built the one i ran with the dana rear
Last edited by BobsProFab; 07/12/10 12:06 AM.
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Re: Drive shaft critial speed ? what you guys running
[Re: BobsProFab]
#745876
07/12/10 12:25 AM
07/12/10 12:25 AM
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Romeo MI
MR_P_BODY
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Quote:
Raff, not sure why they dont build the MMC shafts anymore
they built the one i ran with the dana rear
I dont think they sold all that many... they are pricey
so why buy all that stock and equipment to have it
sitting around taking up space... JMO.... Dynotech
isnt that far from me so I go and pick up my stuff
and that shop isnt all that big
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