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PostPosted: September 7, 2018, 1:42 am 
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Joined: December 6, 2017, 2:20 pm
Posts: 125
Location: San Jose, California
Hi Guys,

I been building the vodou frame. I noticed today when I mounted the miata rear diff, its off center. Im not sure what to do now, but it seems its a bad fit. I wonder if anyone have any ideas of what to do. This is a bit frustrating sicne i was hoping for a magical moment today.


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PostPosted: September 7, 2018, 10:22 am 
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Joined: December 17, 2010, 1:24 pm
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Location: Gainesville, Mo.
The Miata pinion shaft IS offset to the right, that's why most of the Miata Locosts have narrower seats on the passenger side. Most of the builders just widen the tunnel out on the passenger side to clear.

Good Luck! :cheers:

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PostPosted: September 7, 2018, 11:43 am 
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Joined: April 22, 2010, 4:43 pm
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Location: Livermore, Calif.
Check the length of your rear half shafts. You'll find that one is shorter to accommodate the difference.

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PostPosted: September 7, 2018, 12:21 pm 
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Joined: October 24, 2008, 2:13 pm
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Location: Carlsbad, California, USA
With a modification of your tunnel structure, you may be OK, but you'll have to check some angles for your driveshaft, or at least simulate the shaft with tubing or some other means.

You can get away with some offset of your driveshaft. However, certain principles must be observed so the the aggregate of the offset in 3 dimensions isn't too much over all, or you'll have balance/vibration issues that can't be solved. I don't want to quote the number from memory, but I'll try to find my reference material later on today.

If the center of your diff input shaft is in the same horizontal plane as the end of your transmission output shaft, then you should be able to absorb 2 degree angle change in your offset of the driveshaft.

It's easier to explain in pictures, rather than English. There are some other potential solutions too like mounting your engine/transmission to point to the diff rather than run down the centerline of the chassis, assuming the angle is small.

The overall message is that you very likely have multiple solutions available. My own build has an offset 3/4" to the passenger side. Other successful builds here do too.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: September 7, 2018, 1:22 pm 
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"There are no mistakes in the Tango... You just tango on." -- from "Scent of a Woman"

Like Lonnie said, a little angle on the driveshaft in one plane is not a problem, it's actually preferred. A lot of old front engine/single seat cars offset the engine/diff a bit to make room for the driver, or they angled the engine in the frame a bit so the tailshaft of the trans was pointed at the diff.

You might have to mod the tunnel a bit to clear the flanges at the diff, but other'n that, it's not a huge issue. Tango on, my friend...

:cheers:
JDK

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PostPosted: September 7, 2018, 2:04 pm 
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Joined: December 6, 2017, 2:20 pm
Posts: 125
Location: San Jose, California
Thanks for the reply guys! I feel a lot better now about it.

Lonnie-S wrote:
With a modification of your tunnel structure, you may be OK, but you'll have to check some angles for your driveshaft, or at least simulate the shaft with tubing or some other means.

You can get away with some offset of your driveshaft. However, certain principles must be observed so the the aggregate of the offset in 3 dimensions isn't too much over all, or you'll have balance/vibration issues that can't be solved. I don't want to quote the number from memory, but I'll try to find my reference material later on today.

If the center of your diff input shaft is in the same horizontal plane as the end of your transmission output shaft, then you should be able to absorb 2 degree angle change in your offset of the driveshaft.

It's easier to explain in pictures, rather than English. There are some other potential solutions too like mounting your engine/transmission to point to the diff rather than run down the centerline of the chassis, assuming the angle is small.

The overall message is that you very likely have multiple solutions available. My own build has an offset 3/4" to the passenger side. Other successful builds here do too.

Cheers,


If I can get that information, it would be great! I think the goal I will focus is now to get a 3rd mount point on the diff, so for sure it is as far into the tunnel as it can be and make it as level it is to the transmission output shaft on the horizontal plane. I can point the engine slightly into the rear diff, so I can try to "maximumize" that angle to get something like 2 degrees.

I'm not too key in modifying the frame, so I think this is the best solution for me.


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PostPosted: September 8, 2018, 12:06 am 
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Location: Carlsbad, California, USA
I found one reference and scanned the 2 most relevant pages. They're too big to post here in your thread, so I'll PM them to you here. I also have an excellent PDF file which goes into some detail about all of the driveline setup. Its got more information than you need to solve your problem, but is worth reading through. You can't upload PDF files here. I'll need to e-mail that to you at your outside address.

Since your differential is fixed in place, and the tail of your transmission should move very little, you should be fine, but' you'll have to keep in mind the "magic number" is 3 degrees of angle maximum overall in the two major planes.

As a simplification to illustrate the point, one plane is horizontal and one is vertical, perpendicular to the horizontal one. Just to get the concept, assume your driveshaft is parallel to the road surface.
Then think of a horizontal plane parallel to the road surface through the center of your driveshaft as installed.

Think of the vertical plane as at 90 degrees to the horizontal one above. There is a formula in the scans that tell you how to calculate your total angle from the two angle variation of the driveshaft in the two planes above. It's simpler than it sounds if you're OK with squares and square roots.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: September 8, 2018, 12:35 am 
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Joined: June 13, 2014, 11:55 am
Posts: 84
The position of the flange is not that important as long as the flanges, U joints or prop don´t foul on the tunnel structure/skins.
What is really important is that the plane of the flange (plane of rotation of UJ´s) are parallell or have cancelling angles.
To me parallel (sp?) is easier, and that´s what I´ve done as 1995 Mustang Pinion is off center too.
HTH


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PostPosted: September 8, 2018, 9:33 am 
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Joined: September 4, 2013, 8:31 am
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Here is a slight modification i did to my Haynes frame to make up for the 20mm offset.


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PostPosted: September 8, 2018, 8:20 pm 
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Joined: September 4, 2013, 8:31 am
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Location: Manheim,Pa
Also here is my diff front mount using a couple of energy suspension polyurethane bushings.
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PostPosted: September 9, 2018, 5:43 am 
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I'd use mistered's example.

Every diff I have ever seen had the pinion offset in the housing, usually a 1/2 inch. The axles are equal length.

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