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Rear Wheel Diameter: ultralight wheelchairs

By Elisa Smith, PT, DPT, NCS. Last updated 2/28/2020.

During a wheelchair order session, the impact of the rear wheel diameter selection is often glossed over or confusing. Ultralight rear wheels typically come in 20”, 22”, 24”, and 25” diameters with 24" being the most common. The wheel diameter influences the rear seat to floor height, seat incline, arm alignment, camber and wheelchair total width, wheel spacing and anterior stability, clearance for lateral pivot transfers, rolling resistance, and tire options. The height of the user's torso and the length of the user's arms should significantly guide the decision between wheel diameters.

Rear seat to floor height: Increasing the diameter of the wheel raises the rear seat to floor height. For example, replacing a 20” wheel with a 22” wheel increases the rear seat to floor height by 1”. This is because the radius increases by 1”. Eliminating this change in rear seat to floor height via adjusting the wheelchair rear seat to floor height hardware on the frame is possible on some wheelchairs.

Seat incline with front seat to floor height and posture: Increasing the diameter of the wheel and raising the rear seat to floor height decreases the seat incline. This is because the rear seat to floor height is taller while the front seat to floor height remains the same. Dump (rear seat to floor height - front seat to floor height) is less. To avoid changing seat incline, which will impact posture, raise the front seat to floor height. For example, replacing a 20” wheel with a 22” wheel raises the rear seat to floor height by 1”. Therefore, raise the front seat to floor height by 1” to maintain the initial seat incline to maintain the posture of the user.

Arm alignment: When the user is sitting in an upright posture, place their hands on the top center of the handrim. The upper arm to forearm angle should 100-120 degrees, per the PVA’s Upper Extremity Preservation of Upper Limb Function Following Spinal Cord Injury.

Increasing the rear wheel diameter raises the wheel in relation to the user’s arms. For example, replacing 20” with 22” wheels increases the radius of the wheel by 1”. This means the top of the wheel is 1” taller relation to the user’s arms, causing more elbow flexion and a smaller upper arm to forearm angle (i.e. 100 to 90 degrees). To offset this change, raise the rear seat to floor height by 1” in addition to the added rear seat to floor height already caused by the new taller wheels. Remember to then again increase the front seat to floor height by 1” to maintain the seat incline.

After establishing the correct upper arm to forearm angle, have the user propel the wheelchair forward on a level surface. The user’s shoulders should be relaxed and not assume excess shoulder extension, abduction, elevation, and/or internal rotation. The user should not need to use significant trunk flexion-extension to push the wheelchair or use extreme wrist angles. The user should be able to propel with a semi-circular pattern with long push strokes (high “contact angle”), low push frequency (push cadence), and long glides or coasts between each push stroke. The total configuration of the wheelchair, including the rear diameter of the wheelchair, influences the push mechanics. This image on the semi-circular versus arc push-pattern is from the PVA’s Upper Extremity Preservation of Upper Limb Function Following Spinal Cord Injury.

Camber and wheelchair total width: ______________________

Wheel spacing and anterior stability:_______________

Clearance for lateral pivot transfers:

Transfers: Due to the larger radius of taller wheels, the wheel extends more anteriorly along the side of the wheelchair. For users who perform lateral pivot transfers, this creates a larger hurdle. These users scoot forward to perch on the front corner of the seat. Then they swing their buttocks to the target surface. If the rear wheel is larger in diameter, their path may be blocked by the large rear wheel so that they need to pivot higher to clear the wheel to transfer.

Rolling resistance: Based on physics principles, wheels with larger diameters tend to have a lowering resistance. This tends to make the wheelchair glide or coast more easily, causing more efficient propulsion and less strain on the upper extremities. The larger wheels also tend to roll over uneven terrain more easily. Other factors beyond the wheel diameter impact rolling resistance, such as the tire, air pressure level, and terrain. For example, soft thick tires on tall diameter wheels offer a lower rolling resistance over grass and gravel while hard thin tires on shorter diameter wheels handle level concrete with a lower rolling resistance. Imagine mountain bikes versus roller blades when thinking of the rolling resistance in these settings.

Tire options: Bike shops offer a wider range of tire and inner tube options than medical supply shops. Some of the cycling products cost less and/or are higher in quality than some of parts available from wheelchair manufacturers. Wheelchair wheels and tires are measured differently than the standard used in the cycling world. Be aware that a 25” wheelchair wheel is equivalent to a 26” bike wheel. Less confusion exists if metric measurements are used. For example, a 559 mm wheelchair wheel is a 559 mm bike wheel.

Wheelchairs with 20”, 22”, and 24” are common in the medical world, but not in the cycling world. For teens or adults, consider trialing and ordering 25” wheelchair wheels (559 mm) to have the greatest access to bike components for cheaper and more durable part options.

User's torso height and arm lenghth: Given the impact on upper extremity alignment and lateral pivot transfers with taller diameter wheels, smaller children often need 20” or 22” wheels. This requires looking at the configuration of the entire wheelchair, not just the rear wheel diameter. As a starting point, trial 20” wheels with typically-sized 2 to 5 year olds and 22” wheels with 6 to 11 year olds. Plan to start trialing 24” or 25” wheels as children enter middle school. Similarly, look closely at rear wheel diameter for people a short or compressed torso with typical-length arms. For example________________

Conclusion:_____________


Case example:

  • The 8 year old ultralight wheelchair user starts with 22” rear wheels, 15” rear seat to floor height, 18” front seat to floor height, 13” deep seat pan, and a 13 degree seat incline with good trunk alignment. The upper arm to elbow angle is 130 degrees, indicating the user is sitting too high between the wheels.
  • Using the current wheelchair, replace the 22” wheels with 24” wheels. Now the rear seat to floor height is 16”, front seat to floor height is 18”, seat depth is 13”, and seat incline is 9 degrees. This causes postural problems. To return to the initial seat incline of 13 degrees, raise the front seat to floor height to 19”.
  • Check the upper arm to forearm angle, which is now 120 degrees. Have the user propel and check for a long push stroke, low push frequency, and semi-circular pattern with an upright trunk and relaxed shoulders, elbows, and wrists.
  • If changing the wheel does not solve the problem, take a broader look at the wheelchair configuration and components. All wheelchair evaluations, trials, modifications, and orders need to begin with a full wheelchair mat evaluation.


References:

  1. Mason BS, van der Woude LH, Goosey-Tolfrey VL. The ergonomics of wheelchair configuration for optimal performance in the wheelchair court sports. Sports Med. 2013;43(1):23-38.
  2. Sauret C, Bascou J, de Saint Remy N, Pillet H, Vaslin P, Lavaste F. Assessment of field rolling resistance of manual wheelchairs. J Rehabil Res Dev. 2012;49(1):63-74.
  3. Mason BS, Van Der Woude LH, Tolfrey K, Lenton JP, Goosey-Tolfrey VL. Effects of wheel and hand-rim size on submaximal propulsion in wheelchair athletes. Med Sci Sports Exerc. 2012;44(1):126-134.
  4. Edelle C. Field-Fote. Spinal Cord Injury Rehabilitation. F A Davis Company; 2009.
  5. Mitchell, Steven (“ SCI_OTR”) on Care Cure Forum thread. Accessed May 6, 2014. Available at: http://sci.rutgers.edu/forum/showthread.php?131927-Simple-question-tire-sizes&highlight=wheel+diameter