Drive shaft critial speed ? what you guys running

Posted By: BobsProFab

Drive shaft critial speed ? what you guys running - 07/12/10 01:02 AM

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

Posted By: cbarracuda

Re: Drive shaft critial speed ? what you guys running - 07/12/10 01:20 AM

high rpm my is 4 inch at 7200

Posted By: MR_P_BODY

Re: Drive shaft critial speed ? what you guys running - 07/12/10 01:26 AM

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

Posted By: BobsProFab

Re: Drive shaft critial speed ? what you guys running - 07/12/10 02:09 AM

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.

Posted By: 408strokerdart

Re: Drive shaft critial speed ? what you guys running - 07/12/10 02:42 AM

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?

Posted By: cbarracuda

Re: Drive shaft critial speed ? what you guys running - 07/12/10 03:15 AM

nice formula my is only 36 inch

Posted By: BobsProFab

Re: Drive shaft critial speed ? what you guys running - 07/12/10 03:21 AM

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

Posted By: 408Dust

Re: Drive shaft critial speed ? what you guys running - 07/12/10 03:28 AM

4 inch Aluminum, like you said length is a big factor in the equation.

Posted By: 408strokerdart

Re: Drive shaft critial speed ? what you guys running - 07/12/10 03:29 AM

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.

Posted By: MR_P_BODY

Re: Drive shaft critial speed ? what you guys running - 07/12/10 03:48 AM

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

Posted By: BobsProFab

Re: Drive shaft critial speed ? what you guys running - 07/12/10 04:05 AM

Raff, not sure why they dont build the MMC shafts anymore

they built the one i ran with the dana rear

Posted By: MR_P_BODY

Re: Drive shaft critial speed ? what you guys running - 07/12/10 04:25 AM

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

Posted By: Zapp

Re: Drive shaft critial speed ? what you guys running - 07/12/10 12:06 PM

I thought MMC was hard to come by so it got dropped

Posted By: 340RICK

Re: Drive shaft critial speed ? what you guys running - 07/12/10 03:59 PM

Holy smokes 5" drive shaft

That wont even fit in the car will it?

Posted By: smokinwoody

Re: Drive shaft critial speed ? what you guys running - 07/12/10 04:04 PM

in my Camaro..
Lukavich Matrix 4"..I've been at 8600 with a blower

the Duster uses a moly shaft 6800-7200

Posted By: HEMIFRED

Re: Drive shaft critial speed ? what you guys running - 07/12/10 05:14 PM

FORMULA
http://www.wallaceracing.com/driveshaftspeed.htm

using a 4" Strange with moly ends