Wheelchair casters are available in a wide variety of sizes. Wheelchair caster sizes vary both in diameter, width, and tire types.
Wheelchair casters are usually 3", 4", 5", 6", 7", or 8" in diameter. Each diameter has both pros and cons.
Smaller diameter casters, <= 5", are usually considered to be more a "performance" caster. The smaller casters are lighter and usually have less rolling resistance than their larger counter parts. Small diameter casters will respond to turning and maneuvering more quickly and easily than larger casters.
One drawback to the smaller caster sizes is their difficulty in rolling over obstacles. Typically doorways, sidewalk imperfections, and road debris are some of the common culprits.
Most smaller diameter casters mostly have narrow tires as well. This narrow tire width makes for the very small contact area which results in lower rolling resistance.
The larger wheelchair casters typically begin at the 6", 7" and 8" diameter. Although the 6" caster might be considered by some as being in the small caster category.
Larger diameter casters are ideal for wheelchair users who have more rugged terrain to navigate.
Larger casters can roll more easily over door sills, sidewalk imperfections, as well as other debris.
The 6" diameter caster seems to be the most reasonable diameter for new users who want descent performance along with a little more ease when maneuvering outdoors or uneven terrain.
8" casters are the largest size caster available for most all manual wheelchairs. In many cases the 8" caster is not available due to clearance issues with footrests, frames, etc.
The 8" caster is often selected specifically for use in outdoor terrain where flat surfaces are few and far between.
Wheelchair caster tires have a significant effect on wheelchair performance and ride. Pros and cons exist for most tire selections.
Pneumatic caster tires are casters filled with air, like a bicycle tire. Although the ride of the air filled tire is smooth, the performance is greatly reduced due to the increased drag from greater surface area contact. Additionally, the pneumatic casters are more difficult to rotate / turn. ( spinning / turning the caster fork )
Semi pneumatic caster tires are composed of hard rubber, and require no air. While virtually maintenance free, semi pneumatic casters provide a slightly rougher ride but higher performance. The semi pneumatic caster tires have a narrower contact area which has less rolling resistance thus making them more responsive.
Soft roll casters help to dissipate vibration from the terrain.
Offering a smoother ride as well as providing a descent level of performance. Typically the tires have a convex shape that provides minimal rolling resistance on hard flat surfaces and provide greater surface contact on softer surfaces.