I’m confused about what defines a dirty lap. In some cases, I have been getting flagged for a dirty lap when I feel sure it was clean. I have even saved replays and studied them, but can’t figure out why I didn’t have a single clean lap in a five or six lap race. My leaderboard times suck thanks to all these flagged races. Am I just not seeing where I messed up, or is it possible that the game is giving me dirty laps by mistake? I’ve even noticed the ! symbol popping up when I’m driving straight down the middle of the track and nowhere close to another car. What’s going on?
Dirty laps are caused by:
Contact with others cars or track objects (walls and the like).
Leaving the track boundaries - these vary a bit and can take a bit to learn.
Using rewind.
Drafting or being drafted by another car (only happens above 160km/h or 100mp/h).
If you do any of these (aside from using rewind) in the last timing sector of a lap, the next lap will be dirty as well.
Where and on what track has this happened? Not likely to be a random wrong indicator, there is usually a reason and people just miss it (turn rewind indicator on and when you do something that dirties your lap it will bring the rewind reminder up so you know you have just dirtied your lap).
Is this no-drafting provision enforced on circuit races only? Hard to imagine it being against the rules on a speedway oval. You have me convinced that I have missed this violation in my own racing. Totally unfair that a driver is penalized for being drafted or bumped by another car…but that’s a rant for another time…
The drafting provision applies always - the leader boards are supposed to be a fair and level playing ground and in the past people have used some extreme measures to get to the top (a mini being pushed by a something fast like a Veyron). Doesn’t matter when or where, drafting will dirty your lap, even if you are being drafted as it is an artificial speed boost. It is still fine as far as racing goes but your lap won’t get to the top of the leader boards if you have drafting involved - which is all that is affected by the dirty lap.
I’ve noticed this happen a few times recently as well (currently playing through to get as many achivements unlocked), first time was on the full Le Mans course, was in the BMW M3 racing car, 30 plus seconds in the lead, went over the start finish line with no one in front, cars 30 seconds behind, no debris, no going sideways, in the middle of the track and immediately the start of the lap is flagged as “dirty” : again, it’s just happened on the small Catalunya course, in a Lancia Integrale, 17 seconds in the lead, no cars in front, over the line, immediately flagged as “dirty” … really don’t understand why this is happening 
Did you dirty your previous lap in the last TIMING sector? If so the next lap is dirty as well.
I’m so glad I asked this question on the forum. Since this thread, I’ve saved myself plenty of frustration, wasted effort, and time. For example, I get quite a few dirty laps on Silverstone that were previously puzzling. At a point just past Turn One, I would get the “dirty” indicator. I used to try various lines and techniques when coming out of this turn, all ineffective. No matter what I did, I’d get a dirty lap even though I was nowhere close to the edge of the track.
Turned out this was happening because the car behind me had gotten close enough (near the end of the previous lap) to trigger the "drafting’ thing! I still get my laps dirtied by another car, on lots of tracks, in lots of different classes, and I hate it. But at least I now understand what’s going on.
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Ah ha, now that makes sense, yes on a couple of occasions, thinking about it, yes I did … thanks for your input and I’ll bear that in mind from now on 
Another quick question, when you do have an off the triangle appears to the right of your time, sometimes, I’ve noticed a triangle appear to the left of the time and then disappear, what does that denote?
TIA
Can’t answer that one off the top of my head - would have to get FM3 out and have a look.
Hello, pls dun spread false facts! Get em right! It’s 160km/h or 100mp/h! That’s when drafting occurs…
You are misleading ppl…
Thanks for pointing out that typo from months ago - have fixed it just in case someone feels misled.
Thanks for the info. There’s something else you can clear up for me. When looking at the starting grid, there is a number in parentheses following the gamertag, and further to the right there is another number -just before the identifier of the car. What do these numbers stand for?
I will have to dig out FM3 and have a look for you - should know but haven’t had FM3 in the 360 for quite a while. Will get back to you.
EDIT.
Not sure about the number in parenthesis after the GT - I don’t see it (in later games inside the parenthesis would be the Club Tag).
The Letter and Number combo immediately before the car is its PI and what class that PI puts it in. This is a way to assess how evenly matched a base car is to other cars (too a certain degree - it isn’t 100% accurate but is close enough for most cars).
I see 3rd funkydunk1977 (with crown for VIP) D 337 Jaguar D-Type car image showing drivetrain layout which is FR in this case.
Otherwise if you could post a pic might help.
Question is not referring to the leaderboard but to the starting grid (like the screen you see when the lobby is getting ready for a race).
That was the starting grid - although in single player.
In multiplayer I think it is like driver level for online and the other figure is points (like for a championship ranking).
K thank you
two points:
about the “triangle” question: the (triangle+exclamation point) symbol to the right lets you know that you had a dirty lap (last lap, best lap, or current lap). The same symbol appears to the left at the point where your lap went dirty (either because of your driving or because a car behind you got close enough to be considered drafting). That’s the one which fades out after a few seconds.
concerning the number he was asking about (in multiplayer lobby starting grid): it’s the tally of lobby points (10 for each 1st place, 7 for each 2nd, etc,)–these accumulate the entire time that a driver is in a session. even if you change cars, as long as you stay in the same lobby, the count continues. this matters because one of the parameters which can be selected in game setup is grid position based on lobby points, you can have the higher lobby-point-score cars lead the grid, or you can also set it up so that the lowest scoring driver gets the pole and this leads to some interesting races as well.
added on Feb. 17–
funny thing about those numbers:
Looks to me like the one in parentheses (wins against “real” opponents in online racing) doesn’t update right now, but the one for lobby points does- I guess because of the server being down (speaking here of Forza Three server which handles leaderboards and other things).