Screwing around with cv shafts

New rabbit hole of research: miata differential swaps.

I know my challenge car is a peg leg, and wounded. (1999 miata)

I forsee 325 at the wheels

I firsee a ine tire fire from here until the cows come home. 

So, a diff swap seems to be in order.

To avoid torsen money, im looking into ford 7.5 or 8.8 irs centersection swaps.  Lsd, plenty of gear choices, and a hundred bucks after tax.

The mount brackets look simple enough. 

The axles, on the other hand, are eye watering expensive. 

So, since ive never messed with cv axles other than replacement with remans from the parts store, how doesone go about making them for custom applications? Is it like a driveshaft where cutting and welding is ok? Or are there conversion joints readily available?

My tbought is take tbe ford inners, miata outers, cut to proper lengths in the middle, weld and sleeve. But that almost sounds too simple, to be honest. If it were that easy, there would be a market for tbe 1k axle sets for this.

Is the 7.5 in the Merkur the same as the base MN12 tbird? Xr4ti axles are cheap and short-ish.

In reply to Dusterbd13 :

Dana 36 from a C4? They're pretty much worthless to Corvette peeps & use u-joints instead of cv joints. 

Nah. Around here tge valueless danas still bring good money. Ford, and japanese are about it for cheap options.  But i dont think the japanese will be geared high enough. 

So is it possible to cut 2 CV shafts and a half weld them together and sleeve the weld joint for strength? Or is this a hand grenade?

Dusterbd13 said:

So is it possible to cut 2 CV shafts and a half weld them together and sleeve the weld joint for strength? Or is this a hand grenade?

I've heard both that it works great and that it is a hand grenade, and I have experience with neither. However, I think if you find a long enough shaft it should be cheapish to cut the length and have someone machine the correct spline into one end to match the CV you are trying to use. The other option is to find CV axles that use flat flanges on both sides and build adapter plates. These are from an old 7 series BMW:

We are going through this exercise right now on our bike engined build, and the book we have says welding and sleeve-ing works, but the books is focused only on low weight bike engined racecars, so the torque applied to the shafts will be much lower.

To tag along and ask deeper questions, how important is balancing them as opposed to driveshafts?  Not as important because of the shorter lengths? Not as important because they're spinning 1/3 to 1/4 the speed? Or both?

Good quetion stampie. 

Also: how lonk should the sleeve be?

Robbie: how would i go about dissassemly from cv to splined shaft? 

Explorer 8.8 inner stubs, miata iuters, whith a shaft in between is the plan im considering 

There's this kind: You would remove the circlip on the end of the shaft and press or knock the 'tri-bearing' thing off the end to expose the splines.

Or this kind: which I think works mostly the same way

In reply to Robbie :

Yeah, most cars use the first style on the inboard side and the latter on the outside.

Personally, I'd look at machining the joints to accept the style that bolts in place like many VW, Porsche and BMW's do.

The joints are less expensive and you can often find shafts in various lengths to match your needs.

Balance is not as critical due to the speed and where the mass is located relative to the center of rotation.

This has crossed my mind with my exocet. Same   cut, machine, sleeve, and weld approach. I look forward to everyone’s thoughts.

This blog is about the most Grassroots I know of when it comes to V8 Miata swaps.

http://miatav8.blogspot.ca/2014/01/rear-subframe.html

The go-to cheap method is or was to use axles from Factory Five and get the Miata hubs broached to accept the Ford spline. Rumor mill is that FF no longer sells the axles. Worth asking.

Found another source that will make them to whatever length you want, and the cost is about $600. Handy if you are messing with track width.

As this had turned into an analysis paralysis thing in my project, I cut the gordian knot by ordering axles and hubs from Martin at Monster Miata.

In reply to NOHOME :

I found that stuff as well, but it quickly became cost prohibitive.  Which is when i went to thinking about cutting and welding. 

I will have to find a cv shaft fr o m a miata to take apart, and take apart a ford one in the junkyard.  If we can cut and respline, that may be the solution, assuming that i can find a machinist willing to do it. I dont think my amgle grinder is precise enough. 

Dusterbd13 said:

But i dont think the japanese will be geared high enough. 

really?  are you planning on running something taller than the {aforementioned in another thread} A7's?  Go much higher than the 4.1 miata diff on the 205/50R16's, and 1st will be useless and with your power levels you'll might get pretty close to redline in 5th?

In reply to sleepyhead :

I think he meant higher the other way, as in taller. Or numerically lower.

Yep. Somewhere in 3.08-3.55 Would be better than the 4.10s i have now. Rx7 diff swap is still on the table as well, but 3.90 gears are as high as they go.

Want to do your spreadsheet wizardry with different gears and a 3.8 camaro t5?

man, those inverse relationships always through my brain around keeping them straight... it's like math dyslexia for me

and, yeah, I'm still working on the "sheet for general consumption"... decided to try the 'walk away from it, and come back with fresh eyes' approach, figured I had a little time to do that... reading this, I'm realizing maybe not 

Bumping this thread.  Duster, what have you done for axles?

I have a pair in use now but was researching replacements in case something broke it'd be nice to have a spare around.  It's feeling like buying one each ford and mazda axles and cutting/sleeving them.  Not ideal but Monster Miata wants big money for their parts.

I know that on Roadkill's Rotsun, for strength they cut the stock WRX axles in half, put a sleeve with two holes drilled 180 degrees out over them, welded the axle together, slid the sleeve in place and welded it, and then rosette-welded the insides of the drilled holes to the axle. Those axles held up to the ~400hp/450tq turbo Chevy 4.3

What year Miata?

If all this info is true: http://www.solomiata.com/Drivetrain.html

You could get a set of resplined axles from Ronin Motorsports.

These are my Ford 8.8 inner to FC TII RX7 outers. I also upgraded to Mazda 929 Turbo outer CVs, which aren't available stateside, but can still be found.

A good design as the above should work for your app.  If you want to experiment with a lighter shaft you can just cut each halfshaft short, say 3 inches long and then use a length of 4130 steel tubing to connect the two stubs. If the the shafts are different diameter you can have the larger stub machined to match or have the tube bored/drilled out to match. Then weld as described above.

I know that VW used this method on some of their halfshafts, the longer one of the two.  I guess to keep the weights equal?  Of course the power they had to deal with is not what you're talking about.

I actually haven't done a thing on rear axle swaps or cv shafts, guys. 

I decided to get it up and running on the stock stuff and then decide what needs done.

Well, for future reference anyways:

• a halfshaft info database that I saw here on the forum a few weeks ago.
• don't worry about precise balancing the halfshafts. It's not done at the factory because small rotating radius and short length
• disassembly of halfshaft isn't too difficult. I'm sure there are guides and videos to be found online
• Cutting and welding the halfshaft bar: it depends. I'm sure it would work fine done right, but there are a lot of variables. Heat treat of the bar, diameter of the bar and splice, torque, etc.
• Also keep in mind that you'll need joints that can handle the power and torque, not just the bar. So try and pick donor cars with similar power to your needs.

Not sure if it helps at all, but HC Mazda 929's could be worth a look. Should be irs and have a ratio of about 3.58 with the 3.0v6.