*2-2. Wheelchairs with large rear tires are most common, but models with large front tires and small back casters are available (Figure 21-12, B) (fig.20-11B) However , the large front tires can limit access to the environment , such as when transferring and sitting at tables. The wheelchairs with large front tires is also more difficult to push over curbs and uneven surfaces because of interference from the rear casters.