Being a dealer for many wheels, I can explain this. We refer to it as "Rain Wobble". It is NOT specific to iForged.
It is a rare occurrence that can happen with any brand of wheel, that is built using a smooth lip, or soft lip construction. These wheels have the drop-center (the raised hump on the outer hoop) on the backside of the wheel, which makes the wheel lip appear flat/smooth. The opposite construction would be a stepped-lip wheel, with the drop center towards the face of the wheel. So, the smooth lip wheel has a valley between that hump, and the backside of the face of the wheel. This valley can collect water, as there is nowhere for it to escape. Some of you might even observe that puddle form behind the spokes, when washing your car - if you have smooth lip wheels.
That water getting trapped behind the spokes is what throws the wheel balance off. This only happens during HARD rain, as it has to be raining hard enough for water to collect back there. Driving slower usually makes the wobble disappear. When it stops raining, it only takes a few miles for the water to sling its way out, or dissipate. It is just a temporary issue.
Soft Lip (aka "Smoot Lip") wheel shown below. Notice the drop-center hump on the backside of the wheel. The water gets trapped between that hump and the back of the spokes.
Stepped Lip wheel shown below. Notice the hump is directly under the edge of the spokes, and the backside of the wheel is smooth. No place for water to get trapped.
Many people prefer the Soft Lip wheel, because the center forging is 1 inch larger in diameter, so it makes the wheel appear larger to the eye. It also makes the lip appear deeper than a stepped lip. But, the Stepped Lip wheel is lighter, due to the smaller center forging required, and is better for performance. I run both types, depending on the vehicle. Also, monoblock 1-piece wheels generally have the stepped lip internals, with the drop-center hump on the outer face. Those don't trap water, either.
Stepped lip on my '06
Soft lip on my '13
Tony
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