1st Gen Driveshaft Design - Did GM Make a Mistake?

[rich69rs]

About a year ago, on the old forum, we had a discussion ongoing concerning the phasing of the yokes from end to end on the driveshaft of first generation Camaros.

The topic was started by me and the following is my original post:

I have seen two examples where the yokes on each end of the center tube section of driveshafts from Norwood built 1969 Camaros were not "in line" with each other, but offset - one with respect to the other - by approximately 20 degrees.

One of these driveshafts was originally in my 69 RS coupe which I have owned since Nov 1991 (VIN# 124379N581767, 01C build date). The other driveshaft was from a 69 Camaro base coupe, 327, built around the 3rd week of November 68 (VIN# 124379N551248, engine code V1122FJ). Both cars are base V8's, my RS has a Powerglide, the other car had a 3 speed manual. The other car was being parted out back in March 1994, so I collected several items, including the 327 motor and the driveshaft - which I still have.
 
All other driveshafts that I can remember (other than these two) have the yokes "in line" (0 degree offset) from end to end.  
The reason(s) that the "as built" offset doesn't seem right to me, in that fundamentally, a driveshaft is designed to cancel out the inherent variations in rotational (angular) velocity from one end of the driveshaft to the opposite end and change those variations in angular velocity to essentially constant velocity. How well the driveshaft does its job is a funciton of several items.

One key element is to ensure that the included angle between the transmission and the center tube is the same as the included angle between the center tube and the differential. If these angles are not the same (+/- tolerance), the resulting non uniform angular velocity between the driver and driven ends of the driveshaft can result in potentially very high inertial forces, resulting in vibration, and eventual damage.

In addition to maintaining the proper angular relationships, the other key element is to ensure that the yokes on each end of the center tube section are in line (0 degree offset) with respect to each other so that the center tube can articulate by the same angular amount, but in opposite directions, from one end to the other.

So here is the crux of my question. The two driveshafts that I referenced above clearly do not have the yokes in line with each other with 0 degree offset. They are offset by approximately 20 degrees.  Consequently, it seems to me that it would be impossible to maintain the proper angular relationship between the transmission and differential during operation because the center tube cannot articulate properly.

Did Chevrolet actually design the driveshafts this way for 1st Gen Camaros?  If so, why - no other car that I know of ever had offset driveshaft yokes.  If Chevrolet did design the driveshaft this way, what was their logic?  What was different in 1st Gen Camaros that would require the offset?

The complete discussion can be found at the following link:

http://www.camaros.org/cgi-bin/webbbs4.33/cuscrg/cuscrg.pl?noframes;read=15977

In the discussion at the above link, please note the responses stating where: 1)TH400 transmissions had in line yokes, and 2) one example where a base 307 apparently had some sort of additional damper on its offset yoke driveshaft.  The second item is particularly interesting in that it sounds like a band aid applied to the drive shaft to dampen out the oscillations originating from the yoke phasing.

What GM apparently did with yoke phasing (other than the references to TH400 where the yokes are in phase) is contrary to any reference that you might pick up on double u-joint driveshafts, either in automotive or industrial applications. It would be interesting to note how many forum members have had drivetrain (transmission or differential) problems that could have resulted from the way the drive shaft was built (offset yokes).

My PowerGlide had to be rebuilt in 2004 (new planetary gear set) and I have wondered ever since as to whether or not the driveshaft was the culprit.

No reference I've ever found to any other car, other than 1st Gen Camaros, having driveshafts built this way.  Makes no sense.
There is nothing fundamentally different between 1st Gen and 2nd Gen (or any other car) as far as driveline dynamics are concerned.

My gut is telling me that either GM made a mistake or one of their suppliers made a mistake, and it was cheaper to "let it go" than to address the problem.  After all, warranty was only 12 months or 12,000 miles.

No obvious other answer. Matter of fact, the theory would tell you that the higher the HP/torque, the worse the problem.

[william]

The drivetrain warranty was 5 years and 50,000 miles.

All of my Z/28s had driveshafts with offset yokes and none had any driveline vibration.

In view of the fact that GM has made/purchased hundreds of millions of driveshafts I will bet they know all there is to know about it.

[rich69rs]

Got to be a fundamental reason why they did it for the 1st Gen Camaro only and not for later Gen Camaros, or Chevelles, or Novas, or Impalas, or ......

Also, previous posts indicated that the offset only applied to manual, powerglide, and TH350 transmissions and not the TH400 - why?  Driveshaft lengths may vary, but the engineering dynamics of the driveshaft are the same for any of the above applications.  Has anyone ever seen a high performance driveshaft built for racing applications, designed or manufactured with anything other than in line yokes?

From purely a mechanical engineering perspective, as it applies to cardan or universal joints, offset yokes makes no sense.

Are we aware of any other applications, other than 1st Gen Camaros, where GM did this?  I know my '66 Chevelle (283/Powerglide) has in line yokes, no offset.  Fundamentally nothing at all different in the driveline dynamics between my '66 Chevelle and my '69 RS. 

If GM had a reason (other than a manufacturing mistake), it would be very informative to understand what it was.

As an aside,I was not aware that GM had a 5 yr / 50K drivetrain warranty "back in the day".  I didn't know that any US auto manufacturer in the '60s offered anything other than 12 month/12K miles.  I always thought it was pressure from the imports much later that caused US manufactures to move from their standard 12/12 warranty.  Learn something new everyday.  One of the things that makes this forum so valuable.

[Farm Boy]

My "out of phase" original drive shaft sits in the corner of my garage. I retired it last year when I had a Nova shaft shortened to fit. The yokes on the new shaft are in line and it has less vibration than the original one did.

Some engineer at Chevrolet must have thought it was a great idea. I am surprised it lasted for 3 years.

[elesquan]

It would be hard to believe that GM would not correct a mistake like that before making over 500,000 driveshafts, so I have been very intrested to find out why too.  My '67 has always had a vibration, and I think it must be the driveshaft.  If the original design engineer would just visit this fourm and tell us all what the H***.  We may never know.

[rich69rs]

To some extent - this sort of thing happens all of the time.  Manufacturers are very reluctant to admit any responsibility and proactively fix a problem unless it is a potential safety related or regulatorily mandated recall issue.  And to some degree I feel for them.  As soon as they try to be proactive, someone will try to sue them or otherwise take advantage of the situation.  So the name of the game is never admit fault.  In terms of pure economics, no manufacturer is going to fix a design flaw, on his own nickel, unless he is forced to by regulatory or public / economic pressures.  Our legal system spends a lot of time blurring the lines between a design flaw and an improvement in the desgin.  If you can convince the masses that you are changing the design in order to improve it, you are not admitting that the original design was flawed, simply that your improved design is better.

Look how long Ford and Firestone bickered over their little issue regarding Firestone tires on Ford SUV's and rollover problems due to tire tread seperation a couple of years ago.  Somewhere there was a design flaw and everyone did everything they could to mitigate their responsibility - whether or not it was the right thing to do never entered the equation.  Alll about $$$$.

Just went through this with my daughter's 2002 Ford Escape.  Ford knows that they have a design issue with two connectors that are a part of the air bag / restraint wiring harnesses under both front seats.  It has not been deemed a Safety issue by the Feds, so there is no recall.  However, for $100 diagnostic charge, $95 in parts, and $200 in labor the local Ford dealsership will fix you right up.  And the fix . . . a replacement connector kit.  Existing connectors and wiring are cut off from the harnesses and the replacement connectors and wiring are soldered into the existing harnesses.  Why replace the original connectors?  To correct "high resistance" in the wiring due to the inherent design of the original connectors.  The "high resistance" could prevent the air bags from deploying.  Design problem corrected at the customer's expense.  What a deal.

What is hard for me to get my arms around is how GM could make this change.  It wasn't like a driveshaft was a new invention in the late '60s.  They had been around for a long time.

[nuch_ss396]

This topic has been on my radar for many years also.  When I started to look for all the correct, original drivetrain components for my big block '69 Camaro,
I became aware that THM400 equipped cars used the only "in-line yoke" driveshaft of that Camaro period.  So many of the THM400 driveshafts I found out
there were non-in-line, so I passed on them.  I own three of the THM400 driveshafts today and all are in-line.  Not at all sure what this indicates though.

Is JohnZ following this topic?  I'd like to hear his thoughts on this.  If any such Engineer from that period is still with us, I'd love to hear from him. 

Steve

[JohnZ]

I have no idea why they were offset. I seriously doubt if it was a "mistake" - Chevrolet didn't create, release, manufacture, and inventory unique configurations unless they needed them; most likely some development engineer found a driveline harmonic condition he didn't like and found that offsetting the joints with that combination took care of it. The driveshaft U-joints on my original '69Z are also offset 15*-20*.

[nuch_ss396]

Hey John,

Thanks for chimming in.  I tend to agree.  It's doubtful that the Engineering Group would have let something like this ride for three years ( at least ) if it was of
no significance.  I would like to know what makes the THM400 so unique to require an in-line yoke configuration though.

Steve

[rich69rs]

Whatever the reason, I personally would like to know what it was.  This is a very unusual design that at first glance can not be justified by fundamental engineering principles.  Although not an automotive engineer, I have a mechanical engineering background and for the last 30 yrs or so have worked with vibration related  / rotating machinery problems in industrial applications - many of which were direclty related to OEM design issues.

I concur with JohnZ that there must be a reason why this was done.  As John stated, creating this configuration created unique part numbers, inventory, and costs.  The key is understanding what Chevy thought was "unique" to the 1st Gen Camaro that resulted in this design.  To me it would be a mistake to simply accept this driveshaft design because GM/Chevrolet (the OEM) designed it that way back in 1966 for the '67 Camaro.  Not saying that Chevy didn't act in good faith - just that with the additional insight of the past 39 yrs it is appropriate to ask why? ..... why not apply this design to the TH400? ....... why dismiss this design configuration for the 1970 model year? 

Someone in Chevy Engineering may have thought he had a better idea of how to eliminate a potential problem with the new Camaro for '67.  Was there a resonance problem with the 1st Gen Camaro driveshafts?  Both torsional as well as lateral natural frequencies (resonances) are functions of driveshaft material, length, thickness, etc.  Did someone try to correct a resonance problem by stiffening the driveshaft with offset yokes?  Off setting the yokes would tend to stiffen the driveshaft since it wouldn't be able to articulate properly. (In general the resonance frequency will increase by increasing stiffness, decreasing mass, or both).  Very hard to believe though that a fundamental driveshaft resonance problem could make it through the design process and then have to be "fixed" in such a manner.  It would be easier to change the ID/OD of the driveshaft to stiffen it appropriately.

However, the driveshaft, as designed, can create other porblems (including driveline vibration harmonics).  The as designed configuration does not allow for synchronous running of the ends of the driveshaft (unifrom angular velocity from end to end).  By uniform angular velocity, I meant that at any instant in time, the instantaneous angular velocity is the same at both ends of the driveshaft.  Just because the driveshaft is "solid" tube, doesn't mean that its instantaneous angular veloicty will be the same from end to end.  It is an elastic element that will twist under load (torque).  The amount of twist can/will be different end to end and so will the instantaneous angular velocity.  The main function of a driveshaft,  besides delivering the power, is to maintain uniform angular velocity (in an ideal situation constant velocity) end to end.

To achieve uniform angular velocity through the driveshaft, the ends of the driveshaft must:
    1.  have the same included angle between the driveshaft and the input (transmission) and the output (differential). 
          (One of the reasons why setting the pinion angle correctly is so important when installing a rear end.)
    2.  have the yokes lie in one plane or in-line with each other so as to constantly maintain the same included angle between the drive and driven ends.  If the yokes are not in line, it will be impossible to maintain the same included angle between ends of the driveshaft.  If the included angle is not equal, the instantaneous angular velocity will aslo be different from one end to the other.

So you might ask, why care about uniform angular velocity between the ends of the driveshaft?  Because to have non-uniform angular velocity literally means that at any instant in time the applied torque load will be amplified through increased twisting (angualr deflection) of the driveshaft, increased torsional stresses, forces and vibration - not a good thing.

May never know why, but it sure seems like something we all need to fully understand.

[nuch_ss396]

Richard,

Your thorough analysis of the dynamic forces involved in driveshaft design is an interesting read.  That leads me to question all the more why THM400 driveshafts
were in-line and all others were out-of-line.  When you consider the revving capabilities of the Z/28 and the L/78-L/72, you wonder why this condition existed at all.

Steve

[rich69rs]

Steve -

This whole issue has been something I have tried to understand for over 10 yrs.  Back in the mid-'90s, I removed my original driveshaft (as others have also done), set it aside in the garage, and replaced it with a driveshaft with in line yokes (donor driveshaft from a 69 Camaro that was being parted out).

Higherr rpms, higher torque/Hp just make the situation more problematic.  I wonder what Roger Penske did "back in the day" with the Sunoco #6 Camaro in the Trans Am Series.

Still trying to understand the rationale.

Richard

[nuch_ss396]

Maybe the guys over on the yenko.net site can shed some light here.  Also, some of the NCRS members were racers from back in the day.

I may post on both sites in an effort to enlist help.

Steve

[rich69rs]

Received the following today from a reputable high performance driveshaft manufacturer:

"You are right it makes no sense but GM did it anyway...........They did it for road racing applications is what the rumor is."

Interesting in that if the driveshaft was modified specifically for Trans Am (someone's still got to explain what the benefit was), then Chevy had to build a minimum number "production units" in order to be homologated per Trans Am rules.  Some parts were homolgated as over the counter items (cross ram), others were homologated as a part of normal Z28 production.

One would think, though, that if homologation was the issue, that the driveshaft would only be modified as a part of normal Z28 production and not modified for 3 spd, powerglide, etc.

Another good question would be did Pontiac make the same mod to 1st Gen Firebird driveshafts?


Richard

[Mr12771]

 
    2.  have the yokes lie in one plane or in-line with each other so as to constantly maintain the same included angle between the drive and driven ends.  If the yokes are not in line, it will be impossible to maintain the same included angle between ends of the driveshaft.  If the included angle is not equal, the instantaneous angular velocity will aslo be different from one end to the other.

 

I'm no engineer but I think you just answered your own question. You say why not on TH400 cars? Th400 transmissions were  behide the 396 engines. If i remember right the 396 engines were offset to the right about an inch. If I understand what you stated, the yokes are not in line, and the instantaneous angular velocity will be different. Would that be the same as the offset yokes on the dirveshaft?

Mike