My insured was driving and went around a corner and the front wheel came off the car. The wheel caused substantial damage to the front fender. We have comprehensive coverage, but no collision. I have have not submitted the claim because a claim adjuster (also a friend) for the carrier told me that his interpretation of it would be that it would fall under collision coverage because of "colliding" with the wheel is what caused the damage.

With all due respect to your adjuster friend (who may well be correct), you would best serve your client by submitting the claim and letting the company either accept or reject.  If you don't, you leave yourself exposed to possible error/omission claim from your friend in the event the company may have accepted the claim.

For what it's worth, if the only damage it to the vehicle caused when the wheel impacted it, you appear to have a collision:  the car collided with the wheel.  Or, if you prefer, the wheel collided with the car.  Either way, it appears to be a collision. 

Jim B.

Question - isn't collision normally defined as collision of the vehicle with "another object".  If so then how can the vehicle collide with part of itself?  The wheel was in contact with the vehicle the whole time.  As such I would consider this a comp claim - but exclude the mechanical part which failed as the original cause.  This is the same as the hood that for one reason or another flies up and damages the w/s or top of the car, or would you consider that a collision also?