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Thanks to Blake's mention of Mark Williams........

Critical speed is the speed at which a spinning shaft will become unstable. This is one of the single largest factors in driveshaft selection. When the whirling frequency and the natural frequency coincide, any vibrations will be multiplied. So much that the shaft may self destruct. Another way to think of this is that if a shaft naturally vibrates at 130 times a second, and one point on the shaft passes through 0 degrees 130 times a second (7800 RPM) then the shaft has hit a critical speed. There are several ways to raise the critical speed of a driveshaft. You can make it lighter, stiffer, or increase diameter without increasing weight. This is the reason carbon fiber makes a good driveshaft, it is stiff and light and can be made to any diameter or wall thickness. Aluminum, while it has a very good critical speed is not quite as strong as steel. Steel, with good strength characteristics will have a lower critical speed.
http://www.markwilliams.com/Images/critspeed.jpg

Vince@Meanstreets

05-24-2014 06:08 PM

Quote:

Originally Posted by Track Junky (Post 552256)

Sheeeeet........Wish you were close enough to make the drive. Hate to say it but if its not the trans it has to be the amount yoke sticking out of the trans. My follow up fix would be the correct length drive shaft. My brother-n-law just bought a truck load of stuff from Amir Rosenbaum(old owner of Spectre) and one of the items was an aluminum drive shaft. I could talk him into given it to you for cheap if your interested.

Quote:

Originally Posted by Sieg (Post 552260)

Size matters!

Norwood is 45.25" from end of output shaft to bolt face of the rear yoke. :D

no bueno, it was the spare from the 2nd gen....too chort.

Sieg	05-24-2014 07:25 PM

Quote:

Originally Posted by Track Junky (Post 552256)

Quote:

Originally Posted by Sieg (Post 552260)

Size matters!

Norwood is 45.25" from end of output shaft to bolt face of the rear yoke. :D

Quote:

no bueno, it was the spare from the 2nd gen....too chort.

Appreciate the offer and confirmation. Probably best I keep it local anyway since there's a decent shop here. :thumbsup:

glassman

05-30-2014 09:11 PM

So whats the final tally? did it work? or should i say does it work?

Sieg	05-30-2014 09:36 PM

Quote:

Originally Posted by glassman (Post 553461)

So whats the final tally? did it work? or should i say does it work?

The final result is it's as close as it's gonna get. At the current ride height the trans/driveshaft is 2.2*/1.6* - 0.6 and the pinion is 5.0*/1.6* - 3.4. That combination produces the least vibes. It's fine at cruise speeds and low triple digit range. It's just noticeable coming off cruise throttle.

To get it right will require getting the pinion lower to get more working angle at the trans/driveshaft u-joint. The 1" blocks lift the pinion too high so they need to be eliminated by using reverse eye leafs or a 4-link. At least that what I've concluded. As is the engine/trans angle is high by .3*-.8*.

After 15 pinion angle changes and test drives..........have I mentioned that I hate leaf springs lately? :D

Sieg	06-04-2014 05:30 PM

In an effort to neutralize the little vibe that's left I'm going to replace the right side Moroso solid motor mount with an Energy Suspension urethane mount and see if that has any neutralizing effect. I'm leaving the left side mount as the header to steering box clearance is about 3/16".

Thoughts?

glassman

06-04-2014 05:47 PM

While you loose torque (a little) on one hand, i believe the polyurethane to dissipate harmonic (wave) energy.....I mean if you placed your whole driveline on a sponge, you'd probably have no vibrations....

Roberts68

06-04-2014 06:08 PM

Glass man, is that proven or just a seat of the pants "torque sensing" observation?

That sounds snide but It is a sincere question because I do not know. And you have sparked my curiosity.

Does Poly or a rubber mount actually lose, or absorb torque?

I would think not, that it would only dampen the inrush of torque being applied to the structure. All the torque, or rotational forces at work are going to get to their fullest extent of allowable movent in due time are they not?

Track Junky

06-04-2014 06:34 PM

I personally do not like or use solid mounts. I do believe that the poly's absorb harmonics which is why I prefer them.

Sieg	06-04-2014 07:37 PM

The most annoying vibration feedback I'm getting now is in my right foot which tells me motor mounts.

The concern is the steering box clearance.

Are the urethane mounts rigid enough to keep the header from contacting the steering box?

http://photos.smugmug.com/photos/i-p...p23Dmm2-XL.jpg

..........oops! Just realized I forgot the F'n idler arm! :sieg:

http://photos.smugmug.com/photos/i-s...sDgdTCG-XL.jpg

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