A buddy just picked up a cheap vinyl cutter, so I wont have to make ugly blue tape car numbers for track days anymore. 
I've talked to some guys at the track. It sounds like a lot of magnetic numbers don't stick to the car at track day speeds. Is there a super strong flexible magnetic backing that will stay stuck to the car at 100 mph? What should I use?
um, forgive my ignorance.... but why would you need numbers for a track day? Its a non-comp event by definition.
For Autocross, decent sheet magnetic stuff from Michael's worked great for me. The key is making sure it doesn't get warped so that it conforms to the door and that it and the car are clean.
I didn't even use a vinyl cutter, just used cheap stencils, a pencil and a good pair of scissors. Of course with a black car, the white lettering worked great and I put some of the bumper stickers on magnetic backing as well :)
Some HPDE's recommend or require numbers, depends on who's running it.
Around here the tracks want numbers so that the corner workers can identify a car on the radio. Otherwise calling in "Red Miata" for a black flag will bring in half the group. :)
Noted.
Rare earth magnets on top of the standard numbers?
Mine worked on my Miata. Then again at Hallett in a 1.6 Miata it only hit 90mph a couple of times per lap. 
But plenty of the other fast guys use the same stuff I did and the big boy cars would see 110-115 down the front straight.
RossD wrote: Rare earth magnets on top of the standard numbers?
That's actually a really good idea. I was planning on just using packing tape on the leading edge of my numbers.
My wife has a Cricut (read: craft/scrapbooking CNC cutting machine). The magnet I use was purchased at Hobby Lobby. It was thicker than what the other stores had. I've been using my numbers for about 3 years now with no failures, at speeds up to 110ish (probably-Whatever 6700 in fourth gear is).
I make sure the car and the magnet are clean to avoid scratches, and I make sure that the numbers are warm and pliable and that, if there is a curl, it's toward the car when I put them on.
Neodymium magnets on the leading corners is a good idea.
I bought a sheet of magnetic vinyl at the local sign place.
Easy to cut with a pair of shears. Made many sets for other ice racers. some rubbed them off on the snow bank.
Mine stayed on at 115 mph, not on ice.
Are their companies that you recommend that can make a nice magnetic race car number ?
The sheet magnet lifts if air gets under it. A good protection from this air getting in is to use one strip of masking tape to seal the leading edge.
In reply to mr2s2000elise :
Autocrossdigits.com
thatsnowinnebago said:In reply to mr2s2000elise :
Autocrossdigits.com
Thank you kindly
iceracer said:Mine stayed on at 115 mph, not on ice.
Was really hoping, given your name, that you were doing 115 on ice.
Not numbers but just gonna throw it out there. The guys I go with all use a small sign company in town that cuts single-use numbers in the size, font, and color you ask for. The only problem I've had with them is forgetting where I put the envelope. I clean the area before putting them down and they last a whole weekend and longer. One guy leaves his on semi-permanent. For 2 sets of numbers I paid less than $20 which I think isn't bad when adding <$10 to the cost of a track day. The advantage here being they don't come off at speeds exceeding 125mph and leave no residue.
Thin magnetic sheet from McMaster-Carr has been proven to stay attached at over 100mph.
Thin is key. The thicker the sheet, the less likely it is to conform. Nonconformance leads to flying off.
It's only available in brown, but spray paint sticks to it really well.
Duke said:RossD wrote: Rare earth magnets on top of the standard numbers?That's actually a really good idea. I was planning on just using packing tape on the leading edge of my numbers.
That's what I've done. I only Autocross, but when I started I forget to remove the numbers before my 100 mile drive home, and they didn't come off.
Max speed of about 75 while on the interstate, about 25 miles, buffeted by traffic the whole time.
Same set of numbers, both cars.
Trackdecals.com is having a sale right now. 20% your entire order. The magnetic panels are only $20. I have used them on multiple cars and they last at 50mph-100mph no problem. Never had one come off.
Use code: 2019July20 for the 20% off.
I did my numbers on Vistaprint. Quick, affordable, stay on at 130mph, no complaints. They seem to have a 50% off sale every other month or so.
Also I should add that individual cut-out numbers have a lot more "edge" per square inch of "flat", which is why they are so prone to blowing off. A nice big rectangle, circle, or oval with numbers printed on it is far more likely to stay on.
In reply to therieldeal :
In my experience, big square is more likely to blow off because it can't conform to complex curves. Thin numbers can.
Think Mercator projection.
All my numbers have been cut from bulk material from SPS. That stuff is awesome, available in lots of colors, and really not any more expensive than the thin craft store stuff. I've had them up to 130+ repeatedly and only ever lost one. And that was on a car with an extra layer of paint on a cold day when the magnets weren't super happy about taking the shape of the panel they were on.
some where I read that tapering the edge would help.
Make sure the surface is spot less.
In reply to _ :
Nope, 85 has been the top.
A longer strait might have allowed 115.
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