I have also found that with ATV's and Motorcycles coming from behind, I would turn my horse to face it or at least sideways so it isn't directly behind them. Has worked very well, and to be truthful, I don't really have to do this anymore.
We got a tractor and now both horses are both following it and being a nuisance. But that will be handy down the road someday too.
I haven't spent as much time as I should with my mountain bike , but that is also on the list to do more. In the past they didn't really care much, but I need to really work with them on the trail to see if past lessons are in place solidly.
That's what I love about horses, never a dull moment and always something to work on. LOL. As a matter of fact, they are very comfortable with my snow shoeing around them and I got some great exercise doing it!