WonkoTheSane
WonkoTheSane Dork
9/6/18 9:06 a.m.

Let's just say hypothetically your star driver put your Miata backwards into a wall and the built-in failure point did its job.   I see that it's spec miata legal to weld up the arm, or even to box that area with a reinforcement plate before you have a problem.

We're debating boxing that area on the replacement diff before the next race and having this one fixed as spare.   What do I break instead if I get rear-ended or into a wall backwards?

Trying to find an answer on the wild world of the intarwebs means that I'm sorting through thousands of "I wanna drift and weld up my diff.  Is me gud?"  "No!  Search N00b!"  which isn't exactly what I'm asking :)


Thanks!

WonkoTheSane
WonkoTheSane Dork
9/6/18 9:07 a.m.

Whoops, I meant to say "you've welded your.." in the subject.  Or get rid of the "your."   Any moderators want to take pity on me?

Keith Tanner
Keith Tanner MegaDork
9/6/18 9:41 a.m.

The purpose of that break is to let the drivetrain move relative to the chassis, part of the crumpling of the car. I'd guess you'd be likely to bend the tub if the diff wants to move. The notch in the arm weakens the part enough that really aggressive driving (like Spec Miata levels of aggressive, not "brah I totally carve the canyons" aggressive) can break it.

WonkoTheSane
WonkoTheSane Dork
9/6/18 9:57 a.m.
Keith Tanner said:

The purpose of that break is to let the drivetrain move relative to the chassis, part of the crumpling of the car. I'd guess you'd be likely to bend the tub if the diff wants to move. The notch in the arm weakens the part enough that really aggressive driving (like Spec Miata levels of aggressive, not "brah I totally carve the canyons" aggressive) can break it.

Right..  Sorry, I should have specified that this is on a fully caged Champcar racecar, not to be driven on the streets.

We were just trying to figure out if we'll take out the rear subframe first, or the transmission/engine via the PPF kinda thing.

Keith Tanner
Keith Tanner MegaDork
9/6/18 10:31 a.m.

Are we using the stock PPF, or some homebrew substitute? It is designed to crumple in a front/rear hit to let the engine submarine under the passenger compartment. When we were doing our salvage work, almost every hit was on the nose so it's obviously what most of the crash testing is based around. Besides, there's lots of empty crumple space behind the seats. Now that I think about it, I'll bet the snapping diff arm is to let it drop below the fuel tank instead of spearing it.

You might take out the subframe. You might take out where the subframe mounts to the tub. You might put the nose of the diff through the fuel tank. I'm not sure anyone outside Mazda has performed this particular simulation. Maybe check with a really big Spec Miata shop and see if they've had any of their drivers crash cars with the welded up diff arm.

WonkoTheSane
WonkoTheSane Dork
9/6/18 12:08 p.m.
Keith Tanner said:

Are we using the stock PPF, or some homebrew substitute? It is designed to crumple in a front/rear hit to let the engine submarine under the passenger compartment. When we were doing our salvage work, almost every hit was on the nose so it's obviously what most of the crash testing is based around. Besides, there's lots of empty crumple space behind the seats. Now that I think about it, I'll bet the snapping diff arm is to let it drop below the fuel tank instead of spearing it.

You might take out the subframe. You might take out where the subframe mounts to the tub. You might put the nose of the diff through the fuel tank. I'm not sure anyone outside Mazda has performed this particular simulation. Maybe check with a really big Spec Miata shop and see if they've had any of their drivers crash cars with the welded up diff arm.

We're running the stock PPF, and have no plans to change that at this time.   

That's a really good thought about the fuel tank.   I'll ask around the shops and see if anyone has any insight.

Thanks!

Ram50Ron
Ram50Ron Reader
9/6/18 12:25 p.m.

If you do end up welding the arm back on I'd expect a failure at or near the weld joint.  The heat affected zone (HAZ) around the weld will have roughly half the strength of the original alloy.  I'd anticipate a fatigue related failure from extended normal use and in a crash you have a nice weak point built in then.  But if it is just going to be your spare I'd say weld it and stuff it in the trailer.

WonkoTheSane
WonkoTheSane Dork
9/6/18 2:03 p.m.
Ram50Ron said:

If you do end up welding the arm back on I'd expect a failure at or near the weld joint.  The heat affected zone (HAZ) around the weld will have roughly half the strength of the original alloy.  I'd anticipate a fatigue related failure from extended normal use and in a crash you have a nice weak point built in then.  But if it is just going to be your spare I'd say weld it and stuff it in the trailer.

I don't believe that's a problem..  I can't find anything about what happens after you get it welded, only that "get it welded up and go racing next weekend" is the common SM sentiment about this.   There's never any followup which leads me to believe there's no further complications directly related.    

Perhaps any accident which would cause an issue with this is also bad enough that the car is a write-off anyway, so there's no specific failure attributed to the welded diff arm.

Ram50Ron
Ram50Ron Reader
9/6/18 2:30 p.m.
WonkoTheSane said:

I can't find anything about what happens after you get it welded

You go racing... not sure what other answer you are searching for here.

Unless you damaged something else that you are unaware of the only thing I foresee breaking is the repaired diff either due to poor welding or reduced metallurgical strength.  

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