Back wheels faling off coaches - if they made the front one fall off at speed it might roll in the wrong direction and tangle with one of the back wheels causing an accident. The front of the cart would also drop down and would probably inure the horses, cause the whole shebang to smash and damage the actors or stuntmen. With the back wheel, it should be possible to balance the rig so that it looks good, but no parts of the carriage will actually touch the ground in an uncontrolled way. It should be a lot safer for the people who are on the coach. Was Nyquist the bloke in the black hat? How frequently did he get shot then? Rubber tired wagons - didn't they use them in Frankie and Johnne - "Bring out your rubber tyred hearses, bring out your rubber tyred hacks. There's seven men going to the graveyard, but only six men comin' back......."