For those who believe they have played each of the 4 hoppers but never got the achievement, there is one GOTCHA that might getcha, and it is not DNFs (as scntyboy's edit shows, that was addressed by Turn 10, so even DNFs count now, though it is tempting to want to blame them).
However, it is still possible to select/queue up a hopper and end up in a completely different hopper without knowing it. Note that this seems to happen most often in the endurance category.. I suspect this is because most people don't want to play the endurance races more than once, so while you can usually find ongoing races in each of the other categories, you will often be unable to find an active endurance race.
Here is what happened to me when a friend and I were trying to boost this one.
1. We went to each of the four hopper categories and ran a race. Neither of us was looking forward to "endurance" racing and were pleasantly surprised when, after a long wait, it started and that it was only three laps on a short track. (guess what.. that was not an endurance race, though that is what we both selected and what we thought at the time).
2. Needless to say, neither of us got the achievement at the end of the series, so we set this one aside to come back to. Later, and not knowing which hopper we didn't get credit for, we decided to select each of the four in turn from the left. we would do at least 2 races on each track this time to make sure that one of them counted.
3. So we complete two Class A monthly races. At the end of the second race, I voted on the next track (we'll come back to this later) but then quit the lobby to move on to the endurance races. Again, we had trouble finding an endurance race. We tried both S and R, finally settling on R and just waiting. If it timed out we reelected. After a short while, I was prompted to enter the race (but not my friend). However, when the race started, I noted it was an A class race and not an R class. Apparently, voting on the next track somehow locked me into the next Monthly Race in the A class (but not my friend, as he didn't vote). I was puzzled but started to race anyway, still not getting the achievement (naturally, I was not really IN the endurance race I had been waiting for).
4. So next time around I created a private match just to buffer ourselves from the prior race series, then we both went back to the endurance hopper. Again, we had trouble getting a race, even though both of us were selecting the same endurance race together.
5. Finally, I called in a third person to help. Once all three of us selected the same hopper together, it finally worked. A new endurance hopper was created with only the three of us (proving to me, at least, that A) not a lot of other people were interested in endurance hoppers, B) you need more than 2 to get one going, and C) It is easily possible to think you are in the endurance race and not really be).
BTW, the endurance race was 13 laps on one of the short Long Beach tracks (17.42 miles, hence the "endurance" part, and presumably the other way you can tell you are not in an endurance race).
