Val,
I just want to explain my above theory clearer. Imagine this hypothesis: 
I weigh 100 lbs. I have a dog that weighs the same, or more. I am standing still; the dog is attached to a leash I am holding. He suddenly bounds forward. Because of his "mass", plus the acceleration of the sudden burst of speed, a force is exerted against me, that is greater than my own "mass", and the result is that I am yanked forward.
No reverse this. Say I am stationery,holding a leash attached to my Mango, who weighs just over 4 lbs. If she sprang forward, because my weight is so much greater than hers, it would cause cause a reaction that would jerk her back to a stop. 
Now apply this to Babybear. If that jerk on the leash,  pulled in any way on the front of the harness, which goes around his neck, what would the potential for injury be? 
I am so sorry if my post causes you concern, but I am just worried for your little guy. There is always the possibility that I am wrong, I am certainly no expert, but perhaps you could ask someone that is, like your Vet AND the manufacturer of his wheels. 
Lauren
