Easiest option for most bikes is to buy a cheap windscreen and get some nice vinyl numbers for it, and then just swap windscreens before a TD.
I think the single digits aren't allowed. At least they didn't used to be. It'll be stated in the Riders' Manual if that's the case...
When I had a bike that did street and track duty, I got a white vinyl sheet and put the numbers on it. Then I taped it over the headlight. Easy to put on an off. Or, you could put numbers on the windscreen and just leave them on. They should be close to 6" tall, and in a contrasting color so they are easily readable by the corner workers.
Gaffers tape is awesome! A little pricey but sticks well, resistant to speed and water. Comes off easy and leaves NO residue.
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They should be checking. Intro doesn't need them but the other 3 groups do. I've sent people back to get them and provided duct tape on many occasions
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I was looking at that, it's black like my ride too. Good for lights? I was worried about sticky residue...
Great for lights! But, make sure you take your fuse out or your bulbs. You don't want them on all day as that would heat the tape up. That tape literally leaves nothing behind!
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