Inline speed skating wheels are typically made of polyurethane and are available in a range of hardnesses, or durometers. The durometer of a wheel is a measure of its hardness, with higher numbers meaning a harder wheel. In the UK, our safest bet for inline racing is an F1 / 86A Wheel. This will insure that you have the perfect balance between grip & roll.
In general, harder wheels are faster and more durable, but they can be less grippy and less responsive to changes in direction. Softer wheels are slower but offer better grip and a more responsive ride.
For indoor racing, many skaters prefer softer wheels because the "slippier", flat surfaces of indoor tracks in UK that offer less grip than outdoor surfaces. Softer wheels can help to maintain stability & reliability on these surfaces. However, some skaters may prefer slightly harder wheels for indoor racing because they offer a more roll and can help to increase endurance and rebound on tight, technical tracks.
Ultimately, the best wheel hardness for indoor racing will depend on your personal preference and the specific characteristics of the track you are skating on. It may be helpful to try out a range of hardnesses to see what works best for you. I hope that SKATE13 can help with all your skating requirement and wheel testing with all the best quality wheels and equipment.