Survey aftermarket components for ultralight wheelchairs, and one trend is clear:  Trick rear wheels are in.  And, if you're like most consumers, you've likely had your eye on a set, whether carbon fiber 3-spokes, aluminum spinners, or fiber spokes. But, before plunking down anywhere from $450 to $1,200 for the latest, greatest, wild wheels, how do you know they will properly interface with your chair?  Sure, wheel diameters are readily understood, matching 24" with 24", for example.  And, axle diameters are clear, with North American chairs using ½" axles, while international chairs use 12mm axles.  However, more in-depth, how do you know that the new hubs - the most vital and complex factor in fitting new wheels - are compatible with your chair?  Indeed, understanding how hub width affects compatibility goes a long way toward ensuring that your new wheels not only fit your style, but also your chair. To understand the role of hub width, start by considering how rear quick-release wheels interface with a chair.  To place rear wheels on your chair, you pick up the wheel, press the quick-release axle button, and slide the axle into the frame's axle sleeve until the axle engages.  The interfacing components, then, are the wheel hub, quick-release axle, and axle receiver sleeve.  Now, here's the critical aspect:  The dimensions of all three components must coincide for the wheel to properly engage on your chair.  For example, if your chair has a hub width of 2", and an axle sleeve depth of 2", then the quick-release axle must measure approximately 4", as it must pass through the hub and axle sleeve to engage (the axle has two tiny balls that pop out the back side of the axle sleeve to lock it in place).  If one of these dimensions changes - as with using a wider or narrower hub - the quick-release axle will not properly engage.  Therefore, put simply, when shopping for aftermarket wheels, hub width is a critical measurement, one that you must maintain or correct.

On the technical side, wheelchair hub spacing falls into two width ranges, "narrow" and "wide."  Narrow hubs, which are most common on spoke and mag everyday wheels, range from 1.87" to 2.06" (outer bearing to outer bearing).  Wide hubs, sometimes called "sport" hubs, typically range from 2.23" to 2.31", and are a bit more obscure than the narrow range.   Although the width difference among narrow and wide hubs may not seem drastic, it is usually greater than the adjustment range of a particular quick-release axle, requiring a new axle length when switching hub widths - this is why it's preferable to ensure that the hub width of your new wheels is as close as possible to the width of your existing hubs.   To easily determine the width of your existing hub, remove the wheel from your chair, and mark on the axle where it extends out the back of the wheel, against the bearing.  Then, remove the axle, and measure from the mark that you made, to the inside of the adjustment nut.  This measurement provides your hub width (of course, calipers can be used, but marking the axle and using a ruler or tape-measure is sufficient for most consumers). Practically speaking, if your hub measures right around 2.0", and you have adequate thread on your quick-release axle for adjustment in both thread directions, you can most likely use a narrow 1.87" to 2.06" hub.  If your hub, on the other hand measures somewhere around 2.25", you'll want to look for a wide hub within the 2.23" to 2.31" range. While it's ideal to maintain your hub widths among wheel sets, there may be an occasion where you have a different hub width than that of your existing configuration.  In that case, you may need a new quick-release axle of appropriate length.  To determine the length needed of the new quick-release axle, determine the width of the new hub and the depth of your axle sleeve, and the cumulative length dictates the approximate axle length (note the term "approximate," as, again, some tolerance is factored in via the axle adjustment range).

Axle Length Equation:   Hub Width + Axle Sleeve Depth = Axle Engagement Distance (+ tolerance)

In selecting the right quick-release axle length, the critical measurement is the distance in-between the adjustment nut and the detent balls - this is the "engagement length."  However, while you really should know the engagement length of prospective axles, most online retailers simply specify the overall length of the axle, or the distance from the detent balls to the top of the threads.  To use such a simple means as the overall axle length as a guide, take the increase or decrease of hub width, and correlate it to your existing axle to determine the length of your new axle.  For example, if your existing hub width is 2.0", with a 4.5" overall axle length, a new hub width of 2.25" logically dictates a new 4.75" axle length.  In this way, it's important to fully understand how the retailer measures the axles, and carefully apply those measurement standards to your needs.   No one ever said that looking cool is easy.  And, when it comes fitting cool wheels on your ultralight wheelchair, it can prove tricky.  Nevertheless, by noting hub widths - and understanding how they interrelate with axles and sleeves - you can roll in style, with your wild wheels screaming coolness in any crowd.