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PostPosted: August 4, 2008, 3:41 pm 
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The conventional wisdom I have found on the net is "do not cut and weld axle shafts. Leave axle shortening to a professional". This is the opinion of at least 90% of the sites I have searched. My research shows that most axle shops will cut one end and respline that end. I have seen a few brave souls who have shortened their own with varying degrees of success. One cuts both sides at an an angle, v-grooves the joint and then welds them together. This leaves a serpentine weld around the circumference. Others use a straight cut, v-groove, weld and grind,and then slide a sleeve over the welded joint and weld it into place. I have seen one autopsy of failed welds using this method. They claim the failure is due to the change in axle rotational flex at the weld between the smaller diameter axle and the larger diameter tube.

Not being one of "conventional wisdom", I wonder if one could use a full-length tube, welded at both ends into the larger diameter section of the axle. Along the length of the tube, drill some 5/16 diameter holes, and plug weld thru the holes into the original axles inside. Maybe 2 holes located at the joining of the 2 halves and maybe 10 or more distributed around the rest of the joint. I am talking about an axle of .950" diameter and a tube length of about 7-1/2 inches long. The car will have a 160-200HP rotary engine.

Any comments or suggestions? Suggestions on wall thickness?


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PostPosted: August 4, 2008, 3:53 pm 
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Meh... it depends.

A buddy of mine worked in the dyno cell of Nissan Motorsports, back when they developed their P85 car. One day they pointed a strobe light at the axles while it was running at 6-800hp and were shocked to see the axles twisting nearly 90 degrees over their length. Yeah, different application and different hp... but still, it made an impression on me.

Do nothing but cruising and you'll be fine. Drag-racing with slicks will absolutely break them. Trackdays... it depends. I'm not going it risk it myself.

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PostPosted: August 4, 2008, 4:53 pm 
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Do you have any other options? How much would you need to increase the rear track to prevent them from being shortened?

Quite a few of us have used RX7 diffs with Miata halfshafts - does anyone know if the spline on the outboard end of the Miata halfshaft fit the RX7 hubs? If it would, you could convert the other way around; that should be good to about a 56" track with the stock Miata wheels.

Essentially, no matter where you weld it, its going to eventually fail. Its kind of a double whammy since you have a stress concentration from the different grains in the material but you also end up with a geometric stress concentration from the change in cross sectional area and moment of inertia.

I'd personally avoid it at all costs since theres no right way to do it.

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PostPosted: August 4, 2008, 4:56 pm 
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A broken half shaft would be a genuinely scary event. I would imagine the wishbones would shortly be history.

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PostPosted: August 4, 2008, 5:38 pm 
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Ahh, the conventional wisdom is here too. Not that that is a bad thing.

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you also end up with a geometric stress concentration from the change in cross sectional area


Welding to the larger diameter ends of the shaft elimintes this to some degree, no? As far as the axle width goes, I neede to remove 2 inches from each shaft to get them to fit. Widening the track is not a desireable option for me. Also the wheels I have are not negotiable (rx7 OEM wheels). As in any other engineering issue, there are tradeoffs and knowing the fixed constraints at the onset is required. If the Miata ends mate to the Rx7 differential and hub ends, that may be a viable solution provideing they are 2 " shorter than the Rx7 shafts. Having custom axles made may be more "locost" than re-building the suspension parts I have already fabricated into the system. That decision may change after it gets on the road and I have time to sort out the car. I still haven't identified a viable steel tube for splicing.

I agree, a broken halfshaft is undesireable but the ends will just flail about, at least for a bit until something else gives. The wheels won't fall off immediately. As to safety, every other part of these cars are designed by us and built by us. Many of us are not automotive designers. We sometimes make some un-wise choices and still tend to get by without incident. I wonder which way it will be for me?[/quote]


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PostPosted: August 4, 2008, 5:42 pm 
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Not really. The ends with the splines are larger than the rest of the shaft to allow material to be removed for the splining. If you look at the minor diameter of the splines, they're usually only slightly larger than the diameter of the halfshaft.

Welding a tube on will end up making the splined portion the weakest.

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PostPosted: August 4, 2008, 5:59 pm 
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rx7locost wrote:
The conventional wisdom I have found on the net is "do not cut and weld axle shafts. Leave axle shortening to a professional".

Any comments or suggestions? Suggestions on wall thickness?

I'm almost hesitant to present this info ....... but

I once built a dune buggy from a 1940 Ford sedan. I cut 4 FEET out of the drive shaft by using a plumbing cutter on the torque tube and a hacksaw on the drive shaft. I beveled both cuts and welded each part up with a stick welder.

I put the two drive shaft pieces on a 90° piece of angle iron and started tacking and then welding around and around the shaft. It turned out straight and never gave any trouble running 10" wide rims and the stock 85 Hp Ford V-8.

Conventional wisdom guys said the shaft would break. I used to hill climb by running the engine wide open and side slipping the clutch in low gear. Never had any trouble.

BUT ............... if the shaft did break it would have been contained inside the torque tube, so I felt safe enough. I'd be more leery on open drive shafts and probably would NOT weld them.

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PostPosted: August 4, 2008, 6:02 pm 
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rx7locost wrote:
I agree, a broken halfshaft is undesireable but the ends will just flail about, at least for a bit until something else gives.

The gas tank is right there...

rx7locost wrote:
The wheels won't fall off immediately.

They may not fall off, but they can lock up if the axle jams, causing some immediately-entertaining driving of the sort you may not expect nor want.

rx7locost wrote:
We sometimes make some un-wise choices and still tend to get by without incident.

This is a very bad "reason" to do something like this. Also, using the "thinking outside the box" expression conveys a sense of distain for what's been done before, as though something that only lemming-like people do. However, sometimes, what's "normally done" is something that was proved through very hard, expensive, and dangerous lessons.

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Last edited by Anonymous on August 4, 2008, 9:20 pm, edited 2 times in total.

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PostPosted: August 4, 2008, 6:19 pm 
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olrowdy_01 wrote:
... never gave any trouble running 10" wide rims and the stock 85 Hp Ford V-8.

I think that's why.

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PostPosted: August 4, 2008, 6:26 pm 
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olrowdy_01, I understand completely. I have done the same for now with my driveshaft on my locost. but it is just for fitting purposes. I WILL have a new driveshaft made. Something about having this running at up to 8000 RPM a couple of inches from my hip. Even with a professionally made driveshaft, I will have a cage to prevent personal injury in case of catastrophic failure. The axles will run at 1/4 that speed and are much smaller in diameter. It should be less of a out of balance issue.

KB58, Hmmmmm......... the gas tank. Hadn't thought about that since I don't really have one at the moment. Maybe I should've thought about the solid rear axle a bit more way back when. I am more concerned about personal injury than I am about the car's longevity.

a.moore, I'm not as concerned about the splined area being the weakest area. If it was strong enough in the donor car, It should be Ok in the locost. I'm not planning on increasing the torque in the Locost. I'm just trying to learn here. Doesn't the stress arguement apply to driveshafts too. There are welded joints in them. Or perhaps it is the relative torque stress of a fast, large diameter driveshaft vs. a slower, smaller dia halfshaft?


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PostPosted: August 4, 2008, 7:00 pm 
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You're right - the significantly larger diameter and less torque is a large part of why driveshafts are fine. Multiply the torque through the driveshaft by the ratio of your rear end; if this is distributed evenly through both halfshafts, that is probably the best case scenerio. I'm sure under some instantaneous circumstance, you could get almost all of it going through one side.

From a static strength standpoint, you are correct; they *should* be able to withstand the load from a Locost. Keep in mind though that you are adding a stress concentration from the weld and the change in the size of the shaft right at the root of the spline. This is what I would be worried about.

Also, it really doesn't matter where you weld your 'splice tube' on the halfshaft. The torque is always going to be the same and therefore so is the stress.

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PostPosted: August 4, 2008, 7:02 pm 
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I used Ford Thunderbird IRS in my miata. There are several companies that make custom axles to your specifications. It think it cost me about $200 for the pair from a place in Colorado. Mark something or other was the name of the company. They make a lot of axles. Custom made axles were much cheaper than anything they sell. Perhaps it is because there is no warranty.

There are standard axles and there are 300M axles, which would have cost me about $600/pair.

I contacted the company for permission to send it to them for analysis. I figured out how long, boxed up one with typed instructions and a point of contact at the company and shipped it. A couple weeks later, I get a call with a price.

Correct length, standard non-300M axles should last forever. Replacement axles for application can be disassembled to put the new joints on the custom axles. Some joints require special techniques but it isn't too bad.

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PostPosted: August 4, 2008, 7:18 pm 
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On my school's FSAE car we just cut the splined ends with about an inch extra shaft off of the donor axles, turned the extra length down in a lathe (using a lot of bits in the process as they were case hardened) to fit inside a piece of 1" cromoly tube and TIG welded them together with the correct length of tube.

I never thought about the consequences of a failure on one of the axles. In retrospect, it was a pretty risky way to do it, but they worked and they never failed (thank god). The school's team had been doing it that way for years...

Please note this is NOT a suggestion to do it that way.


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PostPosted: August 4, 2008, 7:32 pm 
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Miatav8,MstrASE,A&P,F, I found Mark Williams Enterprises in Louisville, CO. I have sent them an inquiry.

a.moore, Thanks for the reply. I wonder, what exactly would be the failure mode? Would the weld fail and break free from the original axle, or would the axle itself shear at the weld line?

Currently, I have the 2 shafts shortened and tack welded for a life-size model. I used olrowdy_01's angle iron method of alignment. I have verified an exact alignment using a straight edge along the shaft bridging the splice 3 places at 120 degrees with no gaps along the edges. Sure would be nice if it would be feasible. I'll see how the quote comes back from Mark Williams.


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PostPosted: August 4, 2008, 7:37 pm 
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KB58 wrote:
olrowdy_01 wrote:
... never gave any trouble running 10" wide rims and the stock 85 Hp Ford V-8.

I think that's why.


But that engine put out about 160 ft/lbs at 1500 RPM.


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