BREYDON’s ART CHAI rims for rims

Typically, a drive wheel for a manual wheelchair has two different components that are known as rims. To plan a wheel, it is vital we distinguish them — hence referring to the loop of handrail as a “pushring”, and to the structural frame as the “wheel rim”.

In many situations, an aftermarket wheelset is needed on the GRIT Freedom Chair to improve performance, safety, and practicality.

I am working with an early 2020s Spartan model, which is fitted with GRIT’s Sand/Snow Tires option as standard. The stock wheels on the 3.0 and Pro models are very similar; although their tyres are narrower, and the Pro can be ordered with lurid colouring to its polymer spokes.

The drive wheels that GRIT supplied for the Freedom Chair Spartan are of a 32‐spoke structure, with 57-559 C.S.T. Patrol tyres, and nearly entirely in black finishes. I suppose possibly this could be adequate for certain recreational settings, but there are substantial downsides to GRIT’s selection.

Before beginning, there are some things to be aware of.

Great care will need to be taken in drilling the wheel rims to accept the pushrings. For unpredictable arms like mine, pushrings are to be nearly flush with the wheel, so that fingers don’t get caught and damaged.

Both wheels may be built without dishing the rims off‐centre, unless the choice of tyre is too wide to otherwise clear the lever‐drive.

The torque couplings get bolted to the disc brake mounts.

The wheels attach to the chair using the wheelchair axles and torque couplings.

One wheel will be mounted “backwards” (from a bicycling perspective), so directional tread is to be oriented accordingly.

This project entails designing a pair of practically identical wheels, with each wheel consisting of the following:

Parts here are described according to the size of tyre that they are designed to fit (or be), and expressed in I.S.O. sizing (also known as an ERTRO size). Note that the vernacular and commercial designations tend to use other, mutually conflicting systems.

Remember that the bead seat diameter referenced in the ERTRO sizes is not what ultimately determines the fit between a wheel rim and a pushring — because that’s not where they connect. Nor does the size in the other tyre‐oriented systems of shorthand. Wheel rims come in various shapes. On some wider wheel rims, pushrings can mount at a range of positions relative to the spokes.

If in doubt — and one should be — consult products’ actual physical form, or use unambiguous measurements according to a ruler.

Choice of sizes will be determined by availability of pushrings or possibly, to a lesser extent, adventurousness of technician.

Possibilities may include:

• 635 pushring on a 40ish-635 wheel — if pushrings built to fit 635 size are on market or affordably makeable.
• 622 pushring on a 40ish-635 wheel — might be possible with careful choice of screws and spacers?
• 622 pushring on a 50ish-622 wheel
• 590 pushring on a 50ish-622 wheel — again, might be possible, if sketchier?
• 590 pushring on a 37-590 wheel — though chair would be no less tippy than with stock wheels

Or if really desperate, the 57-559 CST Patrol tyres and their tubes could be reused for:

• 559 pushring on a 57-559 wheel

Last but not least appealing are:

• smaller diameter pushrings on a very fat‐tyred wheel — as long as dish to clear lever‐drives, and keep tyre‐width around 10 cm (give or take a couple)

As with all wheelchair parts, it is also important to consider what sizes and systems are readily available in the regions and circumstances in which the chair is to be used! In this respect, it is hard to go wrong with a 37-590 wheel. But I would prefer either around 100-507 or the 40-635. We’ll see, hey?

1. 100-507 or wider, on very wide rims to maximise volume (youth fatbike sizes): Tyre and rim choice is likely to be very limited, but otherwise this strikes me as ideal for the time being. I may need to trim knobs if tread is cluttered. I don’t know if 36‐hole rims are available in this size range?
2. 40-635: Worth looking into extra durable 12‐tab sports chair pushrings designed for 700C [xx-622] wheels, bending tabs and substituting longer screws with spacers as necessary. The match is not as critical as on a day or court chair, and the ease of replacing other parts on the road / in remote areas / overseas would probably more than compensate.
3. 37-590: Even easier to get parts for (particularly pushrings), although at my absolute lower limits on tyre width and seat elevation, for this chair.
4. Adult fatbike sizes [primarily 559]: Super interesting possibility, with a wide variety of options for rims/tyres/pushrings. Extra care would need to be taken around some of the larger tyre volumes possible; parking brakes could need abandoning or creative changes to articulation, and overall wheel width could substantially curtail the levers’ arc. However, a big boost to rear elevation would bring immediate, massive improvements in the supportiveness of GRIT’s way‐too‐reclined seatback, so I am super enthused to explore this approach whether now or as a secondary wheelset in the longer term.
5. 50-622 or smidges wider: Not amazing as poss for travel (in terms of sourcing spare parts, from either wheelchair or bicycle worlds), but a size ripe for cargo/city tyres with great visibility and load‐bearing. Note also that even in the absence of pushrings, a comfortable‐enough tread on a tyre even of this much width could still represent quite an improvement for short bursts of low‐speed hand‐wheeling.

I’m still asking and scrounging around for other manufacturers’ data, but this is a start.