Numbers are used by the corner worker to aide in identifying crashes and last bike on track for the end of the session. Make them big, with contrasting colors. 4" tall gray numbers on a black background is not a good idea.
I work at race events and, when corner-working for motorcycle track day events, I can honestly say that bike numbers on about 40-50% of the track bikes fail the visibility test. Don't be in that half.
The best plate and number configuration is:
- yellow or white background plate
- black numbers
6 inches high, 1 inch wide and spaced about an inch between numbers
- there should be at least an inch of plate area between the numbers and the plate's edge (for more contrast)
- plate numbers should be visible while rider is sitting on the bike
These are actually rules for some motorcycle racing orgs. Just a suggestion for track days.
High contrast is what makes your bike number readable from afar. Standing 6 feet from your bike doesn't cut it.
That is no joke. And, please, don't use the gold "mailbox" number plates from your local hardware store.
A corner worker or other track official should be able to read your plate numbers from 7-10 car lengths away (about 150 feet). Sun, glare, reflection and bike at lean angle make that task more difficult.
Use other high contrast colors for tracks if you wish. It's your right to deviate. I will just ask that you do the readability test standing 150-feet (10 car lengths) from your bike.
Thank you in advance!