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Toyota’s Fix for Disconnecting bZ4Xa Wheels is Way More Involved Than Expected

Jun 11, 2023Jun 11, 2023

Back in June, Toyota sent out a recall notice for its bZ4X (and subsequently, the Subaru Solterra) after the wheels began to fall off some of the new EV crossover in Japan. Because a solution wasn't in the works yet, Toyota also issued a stop-sale of any bZ4Xs in the U.S. until a fix was found. Now, Toyota announced its remedy and it's extremely involved. Try to hang on as we explain exactly what was causing the issue and what the remedy is for global bZ4Xs generally and and U.S.-bound models specifically.

The main cause of the wheels falling off the bZ4X, according to Toyota, comes down to a lack of clamping force between the wheel bolts (which Toyota calls hub bolts but are commonly called "lug bolts," too), the wheel hub, and the wheel. The seats on the lug nuts and lug bolts on your car keep the wheel attached to the hub, while the threads provide the clamping forces required between the seats of the fastener and the wheel. In order to make sure the right amount of clamping force between the seats of both the fasteners and the wheel face, those seats need to allow the bolts or nuts to rotate so that the threads engage properly and can be torqued to proper specification. Turns out, that was the main issue at hand: The bZ4X's wheel bolts and wheel seats had too much friction to allow for a proper clamping force.

You might be curious why, if the lugs are tight, the clamping force wouldn't be up to snuff? Well, there was just enough space left allowing the wheel to move around. Barely if at all discernible to the naked eye, over time the Toyotas were allowing enough movement to allow the bolts to loosen (which is why it's important to periodically check the torque of your lug nuts or wheel bolts). It was found that during certain common driving scenarios such as hard braking, the heat cycling of the wheel bolts, wheel, and hub was enough to expand and contract those components so as to reduce the bolt seats' friction and allow the bolts to loosen even quicker. As if both of those issues weren't enough, U.S.-spec bZ4Xs wore different wheels with not only a rougher seating surface on the wheel (increasing the surface friction even more), but also inaccurate bolt patterns, which misaligned the wheels to the hub enough to exacerbate the clamping force issue further.

With all of that now known, Toyota has its fix. First, there is a new wheel bolt for the bZ4X. It goes beyond a new seat design, though it continues to use a conical seat (which is different from the typical ball seat you see from most European cars), but it's no longer directly attached to the wheel bolt. It is instead essentially a washer with a conical shape that faces the wheel while the wheel bolt itself is ground flat where it meets the flat side of this washer. Before you ask, no, it is not a Belleville washer—that's actually a spring that's made into the shape of a washer.

Toyota's new design is similar to lug nuts on the aftermarket that have a "floating" seat. The key difference is the intent; for the aftermarket, the floating seat protects the wheels from damage, keeping the rims prettier without any scratches on the seat's surface. For Toyota, the goal here is to reduce the friction between the wheel bolt and wheel seat to allow for proper clamping forces. To maintain those clamping forces even through heat cycles, Toyota also remixed the bolts' alloy to increase the coefficient of friction during such cycling, resisting any movement of the bolts as they expand or contract.

Those changes are just for the global bZ4Xs. On top of those, the U.S. version also gets new wheels that meet the specifications of the hub so that the lug holes match up perfectly. The seats of the wheels are also finished better so as to reduce the coefficient of friction over the previous rim design, further ensuring the bZ4X's wheels have the correct clamping forces applied even though it is a different wheel design.

The worst part about the recall isn't the fact that the wheels were falling off. No one had a Toyota bZ4X in the wild, anyway, as they hadn't been produced in enough numbers yet to be on the road outside of Toyota's dealer models and ones we in the media drove. Toyota identified the problem, pulled the cars from sale, and came up with a fix. Even so, it couldn't have been a worse PR blow for Toyota specifically to suffer. The brand has faced criticism for belatedly entering the electric vehicle market, even though its millions of hybrids have made significant contributions to fuel efficiency over the years. To finally have an EV to sell and have, well, the wheels come off before it is even out of the gate is just terrible luck. You can bet Toyota hopes it can move past this snafu, as it is committed to introducing 70 electrified models by 2025, with seven full BEVs under the bZ brand and have 30 BEV models overall helping it with its goal of selling 3.5 million EVs annually by 2035.