Engineering Question about CV Axles… sanity check.

Warning: I am not an engineer, but I play one while working on my own car(s).  I work in manufacturing and have a pretty decent grasp on what it takes to design and make stuff like this, but normally I rely on over-building rather than trying to actually calculate how strong an automotive component will be / needs to be.

Without going into too much detail:

I’m looking to adapt a rear differential from brand A to a CV axle from brand B.

The diff has 6 bolt flanges, as do the axles.  Great!

The bolt PCD is different… ok, not ideal, but fairly easy to overcome with some proper adapters that center on each flange.

The axles I would prefer to use are too short.  No…like WAY too short. We’re talking roughly 4-5” too short on each side (mockup needed).  That’s a pretty long spacer. 

I’d prefer to use 6061-T6 for the spacers because it’s loads cheaper, lighter, and easier to work with than steel. I feel like it should have plenty of torsional strength as a ~4” diameter slug…. But not really sure?  This diff will see the end result of approximately 200-250 lb ft from the engine… should be under 3,000 lb ft in 1^st gear (plus shock loads).

Assume that I have “only the best people” doing the machine work, so tolerances on these spacers will be controlled as much as reasonably possible.

Any thoughts on whether or not this is a Really Bad Idea?  (either in general, or the plan to use aluminum)

The other option of course is to have custom axles made, but that sounds both expensive up front and really annoying if I ever break one.

Considering entire driveshafts are made out of smaller diameter hollow tube, I think you'll be fine.

Cant you get some longer splined shafts from dutchman or something like that?

In reply to Robbie :

That was my initial reasoning as well. But then I started thinking about how the smaller diameter steel shafts are probably actually winding up and springing back during hard launches, and how steel is much better at the whole "springing back" thing than aluminum, etc…

In reply to Dusterbd13-michael :

The reasoning behind this idea is to have 4 identical CV axles on the car, that I can get from NAPA.

I think I'd feel better about having a 2" spacer on either end to not have the weight of the CV cantilevered so far out from the bearing support on one end. But as long as you're not expecting it to hold up for 100k miles it's probably fine either way. As far as material choice, just make sure that there's enough space that your can have enough wall thickness to be left with a decent amount of material around the threaded holes. Ie putting an M8 bolt into a spacer that's a tube with a 10mm wall thckness probably isn't going to end well.

In reply to dps214 :

Looks like the larger flange should be 110mm OD, so I’m looking at probably using 4.5” OD aluminum… either solid rod or maybe 2.5” ID tube if I can get it cheaply.

Unfortunately putting a spacer at the other end of the CV axle is darn near impossible as it’s a male spline end that passes through a wheel bearing/hub.  The weight of the CV being too far from the flange is exactly the sort of thing I’m not sure how concerned I should be about.  100k reliability is not a concern, but I would like it to be reliable enough that I can not think about it for an entire season of autocross/track days and maybe pop them off to check for cracks in the off season.

Put a swivel between two long extensions and try to turn the whole thing. You're going to create some unintended forces on the differential side bearings. Having said that, it will probably work for a long time in project car terms.

I'd also be faintly concerned about just how well (rigidity and accuracy) the diff locates the flanges; a small amount of runout could have the end of a 4" slug wobbling around and generating some pretty big radial forces (and vibrations) with its own mass and that of a CV joint and part of an axle doing an eccentric hula.

Would that spacer do weird things to the location of your axles' joints in relation to the suspension's joints and cause an issue with the axle's plunge?

Vigo said:

Put a swivel between two long extensions and try to turn the whole thing. You're going to create some unintended forces on the differential side bearings. Having said that, it will probably work for a long time in project car terms.

This was my concern as well. I’m not sure of a solution other than trying to support the spacer where it meets the axle and that sounds... difficult. Some transaxle cars do this with the longer of the halfshafts. Mazda did it with the 3 and Ford with the Focus. 

I've seen some cool stuff on a prerunner forum were CV axles were extended using DOM tube welded to a cut shaft.  Is it feasible to use the matching inner CVs to the differential, and the matching outer CVs to the hub with this method?

I have also seen some interesting things like solid axles cut in half and fit inside hollow tubes and welded at both ends. Have to get it *straight* though to make it work. I believe they were chucked up in a lathe to weld and welded in small sections to keep warping evenly 

Easy button is just measure what you really need and call driveshaftshop 

It could work, but as has been said, the loads on the differential bearings are going to be off. It worries me, but would likely work short term and at autocross speeds.

Another thing that you aren't considering are the rotational centers of the inboard and outboard joints. Those are located by the OEMs to correspond to suspension pivots to minimize the change in halfshaft length throughout suspension travel. This is just like steering inner and outer tie rod joints match ball joint locations to minimize bump steer caused by a relative change in length. Having said that most halfshafts have a little more stroke than needed in their original applications that might allow them to be used in another application.

I think you will be better off getting two halfshafts per side (one that matches the wheel side and one that matches the differential) cutting the solid shafts and welding them together, probably with a sleeve. Or getting a custom halfshaft made. I know you want something in production that you can get at Napa, and it might still be out there given some more research.

Regardless of which option you choose definitely make sure you've got enough stroke in the halfshaft so that it doesn't bottom out or pull apart during suspension travel. Either will cause premature halfshaft failure. I've seen it happen on improperly setup test rigs at my last job (as a halfshaft design engineer).

I've learned that the hard way on my turbo caravan which coincidentally is also an improperly setup test rig. Got the cracked transmission case to show for it when that cv shaft split the inner cage and started flopping around. 

Robbie said:

Considering entire driveshafts are made out of smaller diameter hollow tube, I think you'll be fine.

i think driveshafts get away with it because they're on the high-speed / low-torque side of the ring and pinion.

Can you share what you're trying to do, and what parts you've chosen so far? Perhaps the hive can piece together a better solution to your problem using off the shelf parts. There's a ton of axles out there, and a 6 bolt inner joint is a fairly common feature so this helps with potential interchange.

JesseWolfe said:

I've seen some cool stuff on a prerunner forum were CV axles were extended using DOM tube welded to a cut shaft.  Is it feasible to use the matching inner CVs to the differential, and the matching outer CVs to the hub with this method?

Generally considered a 'bad idea' in theory, but has been successfully implemented in practice. At least two Locosts I'm aware of have been running axles shortened this way for a number of years now...One of which is a rotary powered street car and the other a dedicated autox car running a SBF and 14" wide slicks.

Here's some discussion on the matter, starting off with why it's not commonly recommended and viable alternatives, but getting into how it was ultimately accomplished too: https://www.locostusa.com/forums/viewtopic.php?f=5&t=4245&start=0

i think driveshafts get away with it because they're on the high-speed / low-torque side of the ring and pinion.

Well, i read somewhere that the mechanical limits of a 1/2" square of steel (like a 1/2" ratchet or breaker bar) is limited to roughly 1000lb ft.  CV axles are many many times bigger than that. Most low power cars won't make more than 3000 lb ft at the wheels in 1st gear, although i dont know how to ballpark the actual torque amount of shock loads from clutch dumps etc.  Anwyay, that's the wheel torque the OP estimated anyway, so one way of ballparking the requirement is 'is this thing 3 times stronger than a 1/2" ratchet?'  

I've noticed that you generally don't see any CV axles with notably huge diameters even on extremely powerful cars. Even when people spend money buying upgraded CV axles you're still limited by the stock spline diameter at the wheel end which is usually the same size or slightly smaller than the stock shaft. I guess I'm just saying CV axle strength is less of an issue than we might assume and most people that break them are doing it with shock loads and geometry issues. 

Alright, here are some more details on the proposed setup.  This is part of a long term project which is currently in the brainstorming/parts gathering/design phase. 

Car: Ford Escort GT

Diff: BMW E30 188mm LSD

Axles: FRONT axles from an ST185 Celica Alltrac (flange inner, 26 spline outer)

Uprights: 1990 Mazda Protege 4wd

Reasons:

My Escort is already running a FWD Toyota E153 gearbox with ST185 front axles.  These are identical left to right and fit the stock mazda hubs.  This means I only need to bring one spare axle to autocross.  Continuing to only need one spare axle would be pretty neat.

BMW diff is stronger than the Alltrac diff, has the right gear ratio, plus it was $80 all-in.  (This is roughly 10-15% the cost of a torsen alltrac diff.)

The E30 diff is a clutch type lsd which, for this application, is better than a torsen.  The escort tends to lift the inside rear pretty easily, and it seems unlikely this tendency will magically go away with AWD.

Adding one more make/model to frankenstein’s parts list is always fun!

Issues:

ST185 front axle is too short as I’ve already established.

The BMW diff is wider than the Alltrac diff, I forget exact dims but at least 1” wider.  I already tried modeling up an adapter to fit between the BMW flange and the unobtanium 4-bolt REAR alltrac axles, and the assembly will end up way too wide. The alltrac axles are usable with no spacers and the alltrac diff, for reference.  1g Rav4 short side rear axles MIGHT work, but the shafts are pretty thin and the bolt pattern is even smaller/more problematic.  Adapting 6 bolt to 4 bolt is kind of a pain, it turns out.

I’m not opposed to making things a little more complicated up front, if it makes the entire package stronger/more reliable/faster in the long run.  See: Ford Escort with Camry gearbox.

Thanks for all the comments so far.  The mention of intermediate shafts on FWD have my brain juices flowing.  Might be able to make these spacers into sort of… stubby intermediate shafts that bolt to the subframe above the diff…

With tarmac coilover suspension I don’t think there’s enough shock travel to worry about the CV joints not having enough travel.  However I would definitely prove this out just like I did in the front.