NAIO from the school of hard knocks...
Don't block the radiator flow...
Has nothing to do with weight. Has everything to do with blocking air flow.
In my case not much happened until the load got pretty big. Then one uphill climb in hot weather, stuck its tongue out at me.
This is why the "mount the spare tire on the grille" fetish came and went. Stuff like bicycles won't make a whit of different. Big, boxy items will.
No joke. Tape off 3/4 of your grill area then go for a drive on an uncrowded road. Keep a SHARP eye on the engine temperature gauge. If it climbs in mild weather, guaranteed it will blow the engine or transmission in hot weather.
