Crossfit Sit ups bum burn

[Rollerbird]

Multiple Sit ups with ab mat are causing a carpet burn type abrasion on top of bum
Has anyone else suffered?
Or have any good solutions for prevention?

Use a towel. The towel will slide slightly every time you come up and down and you won't be stuck always at the same point on your lower back. A bit like the same principle on why you use chalk on the hanging bars.

This said, I think you might come up with the wrong inclination. Are you holding your head with your elbows closing around your head , and doing the crunch in the typical C curve you would do in pilates? Try to come up at a 45 degree angle , with your back possible a little more straight and and projecting your chin towards the ceiling. There should be a bit of C, but only in your lower abs and for the first part of the sit up, once you have your shoulder blades off the mat, you should have straighten your spine.

What is your aim? To do a certain number of rep? In this case, try to add a bit more speed as well, the momentum will help, because your hip flexor will work more than your abs. If in the opposite, you want to work on your abs and core, forget the sit-ups.

The ab mat is meant to stop you using your hip flexors,and keep the spine neutral. Legs in butterfly shape, soles of feet together. Arms touch floor behind and then feet each time.
It's part of a workout so for example today it was 50 jumping pull ups, 50 sit ups, then 50 walking lunges.
Keeping my back straight is an idea that would make it slower but possibly work lower abs more?
It's the friction that's doing it!

Yes! I used to get this at CrossFit! Back went scabby and everything. Tbh those type of sit ups are horrible for your lower back. I just switched to doing McGill’s curl ups (google it!) in place of them.

If you are doing sit-ups against the clock in an attempt to beat someone else, you will probably perform them with bad form. At this point you need to consider what is your goal? Stronger, more defined abs or just one of the challenges of the WOD. If your aim is to have good looking and strong abs, sit-ups are just wrong.
Browse a bit this channel www.youtube.com/channel/UCZIIRX8rkNjVpP-oLMHpeDw to see alternative exercise.