Urine contains ammonia, which is death on aluminum, steel, brass—most common metals—if left and not rinsed off promptly (~24-48 hours).

Feces is another matter. Slower but still corrosive.

I do NOT recommend riding it. Urine (regardless of species) is sufficiently corrosive over time to compromise the structural integrity of a wheel, especially if the tire loses enough pressure to allow it inside the rim. Compared to the cost of a new wheel, what is the cost of a hospital visit if the wheel fails catastrophically, especially in terms of riding time?

Yes, I'm pessimistic. I have a lot of experience as a materials scientist and a geologist. If the (your device/structure here) fails while you are on it, how much is it worth?