Robot vacuums have tortoise legs — thresholds are their kryptonite

Picture a tortoise trying to climb over a curb — that’s your robot vacuum at a tall threshold. The wheels are small and sit close to the chassis, so when the front bumper hits a rise that’s too steep, the whole body wedges before the wheels can get any grip. Most models can handle about 20mm of height difference before they give up and start spinning their wheels helplessly.