Jethro Bodeen - Doin some cypherin on drive shaft RPM

[grzellmer]

With the last drive shaft thread off in the weeds somewhat, how about some math to determine the tolerances of the standard drive shaft, the effect of a switch to 4:10 gears and runnin the beast at 125mph.

Logic

1) Anyone know the drive shaft RPM's (with a 3:55) at 60mph? (Just curious)

2) Assuming FMC supplied a shaft capable of running at 117, the switch from 3:55 to 4:10 gears will result in 86.6% of the mph with the shaft rpm held constant. MPH with 4:10 gears is 101.

This says to me that anyone running 4:10's is on their own when running over 101 mph. If any of us want to go faster than that, then it is in***bent on us to buy a more capable drive shaft .

Also, some of the other threads on this subject mention PI shafts. Keep in mind that it is the RPM of the shaft, not the mph that matters. With the PI's running 3:27 or 3:55 gears, the drive shafts used in these vehicles will spin at significantly lower rpm for the same mph. Need to consider this in the MMX versus Reinhart debate.

Comments?

GZ

[MAD-3R]

Good point on the that. I forgot most PI's run 3.27.

[tetsu]

What about if we downsized to 16 inch wheels AND went with
low profile tires so that our standing tire height was only like 26 in
the back?

With 4.10s, short tires and 16inch wheels we can nibble down the
catastrophic failure MPH down to 55 if we really work at it! :)

But, man will we be able to make great 60 foot times!!!!

Johnny

P.S. do we need a different pumpkin to go with 4.33 or 4.56 gears? :rolleyes:

[MAD-3R]

Ok, now we're just getting silly.
BTW 4.33 should fit in the 8.8.....

[mensrea]

The PI in 99 had 3.55s (if you check the part number)...... I have 4.10s and have no problems.

The guys with the blown vics have 4.10s, other KB Panthers have 4.10s I think the PI shaft is fine. I blew my stock one, I haven't yet blown the PI shaft. If i do I will tell everyone. But I am totally comfortable that this solves the problem.

[Heavy351]

3rd gear at least on my car is 1:1 so drive shaft RPM = engine RPMs regardless of your rear end gear. My understanding is that the reason the police driveshaft is on the CVPI in the first place is because it is shorter than the stock CV GM driveshaft just to raise the frequency of the first bending mode. The tailshaft is longer on the tranny to make up the difference. Critical shaft speed is really not the reason for problems (I am sure it is much higher than the engine can spin it) the first bending mode would have the largest deflection ( and might have a frequency you could feel.)

I have at my disposal FEA software that I could use to get the natural frequencies of the 3 driveshafts in question here. Al I need is ID, OD, length, and material.

Anybody have it or know where I could get this information and at what RPM do you first feel the vibration at?

[SergntMac]

"2) Assuming FMC supplied a shaft capable of running at 117, the switch from 3:55 to 4:10 gears will result in 86.6% of the mph with the shaft rpm held constant. MPH with 4:10 gears is 101."

Thanks, Heavy, this is what I was asking in the other thread. The way I understood the techy stuff made me believe that the critical point, was somewhere in the 70-80 MPH range, where most of MM owners in America may find themselves crusing for long period of time. If so, I was concerned about the long term effect of even mild vibtation that can cause premature failure in connected systems, and how to prevent that.

My other questions, I will investigate on my own.

1) Can a dyno be helpful in detecting a problem here?
2) What drive shaft is stock, a standard GM shaft, or a PI rated shaft. I think a simple part number cross-check may answer this.

What I learn, I'll share here, and with my LM dealer if necessary.

Thanks all, for your input.

[SergntMac]

Whoops...sorry for the confusion on the quote, gentlemen, but thanks to both Heavy and GZ. Both have been quite helpful to me.

[tetsu]

Basically, I see it as whether the PI or the DR shaft is the way to go.

If I went with the PI shaft, I'd want to upgrade the Ujoints and have it
balanced first. So, that'd narrow down the price difference a bit.

Driveshaft would also be a good DIY project. :)

Johnny

[SergntMac]

Just incase you're interested...

http://neptune.spacebears.com/cars/stories/scdrive.html