Broke a CV Joint: This is how we will prevent that from happening again

Update by Tom Suddard to the Mazda Miata project car
Jul 25, 2023 | Mazda, Miata, Mazda Miata, lfx, V6 Miata

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We ended our Miata’s first race early, loading the car on its trailer with a broken CV joint and a missing transmission. (It’s a long story.) Now home, it was time to put the car back together and return to the track.

Let’s get to work.

[What you learn from your first time racing a new car]

A broken axle ultimately ended our race weekend, but before we could order a new one, we needed to figure out what caused it to explode.

In theory these axles are plenty strong–they’re custom-made by The Driveshaft Shop for V8 Roadsters, and they’re rated for way more power than our V6 makes. So we figured the problem wasn’t too much torque.

Instead, with V8 Roadsters' help, we arrived at a reason for the failure: A lack of grease.

Where did the grease go? It exited via the torn CV boot.

Why did the CV boot tear? Because it was overinflated from hot grease inside the joint.

Every CV joint is designed to expand and contract, but ours had been pushed beyond its limits. The car had been burping grease from its boots during test days, but we figured they were just slightly overfilled from the factory.

One cool thing about the V8 Roadsters swap kit? Its warranty, which applies even if you’re racing your car like we were.

We sent our old axles back and had fresh parts a few weeks later. But these weren’t identical axles: Instead, they were treated to better grease designed to mitigate this issue.

Once they arrived, we did one more upgrade suggested by V8 Roadsters: We shoved spray can straws inside each boot, which should let pressure equalize before it turns into an explosion or pushes grease out of the outer boot seal.

Axles solved, it was time to put the car together. That includes the transmission we pulled for no reason. Since we can’t lift a transmission alone–and love having company in the garage–we conned Very Cool Parts owner and team leader Wayne Presley into joining us in Florida for a few days of fun underneath the Miata.

With Wayne’s help, we put the transmission back in, installed the new axles, and ate a bunch of Mexican food. That’s what racing friends are for, right?

Car assembled, we loaded it up for a trip to our official test track–the Florida International Rally & Motorsport Park–for a shakedown.

Our goals for this test? See if the car still ran, figure out if anything major would fall off, and see if the new axles were belching grease like the old ones.

After a few hours at the track we had our verdict. It’s the same race car we’ve known and loved for years.

But a post-track inspection answered our real question: The wheels and suspension were grease-free, meaning our new axles were working properly.

Our Miata is officially back in action, so let’s keep developing it into the best endurance racer it could be.

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Noddaz GRM+ Memberand PowerDork
7/25/23 11:18 a.m.

Nice trick with the spray can straw.

Thank you for sharing it.

GameboyRMH GRM+ Memberand MegaDork
7/25/23 12:38 p.m.

One thing I learned from owning a car with rear CV axles for the first time is that you don't get as much warning before failure as you would from a FWD car, where you might expect to hear a ton of clicks while making tight turns long before anything actually breaks. Because the joints are running relatively straight and stable angles all the time, a single click when going between accel and decel is not only the first sign of trouble noticeable to the driver, but also the last - at that point the joint is on its last legs and could even blow up while just cruising on a highway...theoretically.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
7/25/23 1:05 p.m.

Interesting, thanks for the followup. I've been running my V8R axles on track for years without grease slinging or venting straws. The Driveshaft Shop can be a their production, I wonder if your originals were just poorly assembled.

Also, every time you post a picture of the Lightning with the frunk open I keep thinking it's broken down :)

obsolete GRM+ Memberand Dork
7/25/23 1:34 p.m.

I've done the straw trick on FWD race cars with good results as well. I used this stuff, which you can usually find at local hobby stores. A little more peace of mind vs. the plastic straw, don't have to worry it getting soft and closing up at high temperatures.

Also, aftermarket soft rubber CV boots are garbage compared to the harder thermoplastic OEM style ones. They turn into balloons at high speeds. Our solution was to tie a string around each rib in the boot (and put a little dot of super glue on the knot for security). That keeps the boot from expanding and rubbing or tearing on anything.

kevlarcorolla Dork
7/25/23 2:18 p.m.

I did the vent straw trick on our fwd endurance car for a bit,I simply stopped clamping the inner end.

 Also can't pull the outer end off if it wasn't quite right.

 Since I pulled,cleaned and repacked after every race weekend it also saved running out of straws :).

jimbbski SuperDork
7/25/23 10:45 p.m.

On my IT race car I used to not install the  smaller clamp  that went around the halfshaft. This worked fine at every track I ran until I went to Road America.  The sustained high speeds I saw there caused the rubber boot to balloon and sling grease out of the CV. Installing a zip tie fixed the problem. 

kevlarcorolla Dork
7/26/23 7:09 a.m.

In reply to jimbbski :

In our case we use redline grease,after 16 hrs on track its all still in the joint no chance of it coming out the small end.

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