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Author Topic: 1st Gen Driveshaft Design - Did GM Make a Mistake?  (Read 13174 times)
rich69rs
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« Reply #15 on: July 05, 2006, 06:09:50 PM »

Mike -

Thanks for the input - you pointed out something probably very obvious to everyone else, but I never much thought about it.  Wasn't aware that TH400 was only used on the BB engines - makes sense, just never thought about it - I have too much trouble just trying to keep things straight re: my SB, base RS coupe.

Since my experience with the BB cars is zilch, I'll defer to others to comment further.

Comments that follow below are just me "thinking this through".  I've never heard of any dimensional differences in rear ends for BB vs. SB cars.  If that is not correct, the following rationale is flawed.

Moving the engine over to the right in the engine compartment would shift the engine centerline horizontally relative to the differential centerline.  In line yokes on the driveshaft would accomodate this much the same way that they accomodate the centerline difference  in the vertical plane between the transmission and the rear end.  Only difference is that the horizontal centerline variance is essentially fixed, while the vertical centerline difference will vary as the rear end moves up and down.  Within design tolerances, a driveshaft with in line yokes can accomodate both variances simultaneously.

You may have it nailed as to why the TH400 applications had in line yokes - two centerline variables; both horizontal and vertical - no other option.

Still trying to understand the rationale for not using in line yokes on the driveshaft for all applications.

Richard

« Last Edit: July 05, 2006, 06:52:00 PM by rich69rs » Logged

Richard Thomas
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nuch_ss396
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« Reply #16 on: July 05, 2006, 07:32:12 PM »


 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

Mike,

Your right about the big block being offset about 1 inch or so to the right.  I'm embarrassed to admit that this fact
never entered my mind when considering the big block & THM400 combination in relation to the driveshaft yokes.

I wonder though why the 4-speed cars didn't use the in-line yoke driveshaft though.  The engine & Muncie would
still have been offset to the right.

Steve
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69 SS 396, Hugger Orange, D/80, D/90
Chambered Exhaust, N/66, THM400, 3:73 posi

Steve A.
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Mr12771
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« Reply #17 on: July 05, 2006, 09:47:03 PM »

Steve,

 You have a point about the 4 speed big block cars. Also the Th400 is a few inches longer the the PG or 4 speeds maybe that has something to do with it.


Mike
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nuch_ss396
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« Reply #18 on: July 06, 2006, 04:17:15 PM »

This topic is a real riddle.......
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69 SS 396, Hugger Orange, D/80, D/90
Chambered Exhaust, N/66, THM400, 3:73 posi

Steve A.
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rich69rs
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« Reply #19 on: July 17, 2006, 03:35:03 PM »

This is also currently being discussed at the following link:  http://www.camaros.org/forum/index.php?topic=980.0
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Richard Thomas
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My68SS
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« Reply #20 on: July 19, 2006, 02:18:21 PM »

This yoke alignment mystery has had me on the hop too. Let me really 'throw the cat amongst the pidgeons' here.
I have another car, a 1986 VL Calais built by GM-H [Aust.] equipped with a 6 cyl turbo and 4 spd auto which has the yokes in-line, but the exact same car in the 5 spd manual version has the yokes at 90 degrees!
Same diff and nose cone angle, same suspension, same shaft diameter and similar if not the same shaft length, yet totally opposing theroies on yoke alignment.

My nephew has a 5 spd manual version and it drives perfectly at all speeds and powers with the 90 deg offset shaft.
I have spoken to a few drive shaft specialists around the country and none of them can tell me why the manual VL has the yokes at 90 deg.
I've tried chasing GM about this but can't get past "level 1 support" as yet.
A couple of the driveshaft specialists did also mention that they had on file other GM-H cars with odd offsets like 20 deg and 33 deg. etc, though these may apply to cars with full IRS which may not have equal input/output included angles.

One thing that may need to be factored in to the theory is the shift in nose cone angle as the diff travels up and down, which is certainly the case with 4 coil suspension setups, so the whole thing with yoke alignment may be something of a 'best compromise', but 90 degrees is at the extreme and theoretically should compound the problem, not cancel it!! [Why did they do it and why does it work!!   Undecided ]

I'm going to chase this mystery some more, if I get some answers I'll be sure to post back here.
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Rob
1968 L34/M40 SS
12 bolt posi 3.55
Build - 12C
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