Start with the ground you skate most. If rough asphalt makes your board chatter, look at 93a Soft Sliders or 97a. If you split your time between street and park, 99a is a good starting point. If you mostly skate smooth concrete and like a firm, direct feel, try 101a.
What is Formula Four?
Formula Four is Spitfire’s urethane formula. It’s designed to resist flatspots, rebound quickly and slide smoothly with less sticking. Hardness is one part of how a wheel feels. The shape influences the width of the riding surface, and the diameter affects how high your board sits and how easily the wheel rolls over rough patches. Two Formula Four wheels marked 99a can feel quite different if one is a 52 mm Classic and the other is a 56 mm Radial Full.
Formula Four hardness scale
93a ●
97a ●
99a ●
101a
The scale shows the order of Spitfire’s hardness ratings. It doesn’t measure speed, grip or how different the wheels will feel under your feet.
| Hardness | Where I’d start | What to look for |
|---|---|---|
| 93a Soft Sliders | Rough asphalt and weathered street spots | The softest Formula Four option in this guide. It takes some of the buzz out of rough pavement while still being designed to slide. |
| 97a Formula Four | Street and park with a mix of smooth and rough ground | A little more give and grip than 99a. A good option if you like hard wheels but your usual spots are rough enough that you notice the difference. |
| 99a Formula Four | Everyday street and park sessions | A firm wheel and a useful starting point if you skate several surfaces and want one set for most sessions. |
| 101a Formula Four | Smooth concrete, clean ledges and skateparks | The firmest option here. It’s the one I prefer when the ground is smooth and I want the wheel to feel quick and direct. |
In my sessions, 101a felt faster than 99a on the smoother concrete and asphalt I skated. Both slid easily, and I didn’t get flatspots on either set. If you want to compare the two yourself, use the same shape and diameter at a spot you know so you can feel what the hardness changes.
How much difference does shape make?
Take two 54 mm, 97a wheels in the Nordic range. The Conical Full is 34 mm wide with a 22 mm riding surface. The Radial Full is 38 mm wide with a 25 mm riding surface and rounded edges. Both have the same diameter and hardness, so the Radial Full gives you a wider riding surface without going up to 56 mm.
You can also go softer without going bigger. The Reynolds 52 mm 93a Soft Sliders keep the compact Classic shape if you want a low setup for street. For a larger wheel, Nordic lists 56 mm and 58 mm 93a Radials as well.
Browse Spitfire Wheels at Nordic Skateboard Supply to compare sizes and shapes, or see the Conical Full wheel guide for a closer look at that profile.