Two issues cause most of the side loading on the shaft:
1. Wobble of the wheel, because they aren't perfectly balanced or centered.
2. External force on the wheel, such as when the disc compresses the tire.
This thread addresses balancing the wheel. If you have a fairly well-balanced wheel, the wobble will be acceptable at CIM speeds.
This thread contains advice from Andy Baker on how well the CIM motor can handle the side load due to disc-tire compression.