Tuesday 11 October 2022.
Updated 02 November 2022.
Replacing the drive wheels on GRIT’s Freedom Chair family of leveraged wheelchairs.
- 1. Rim rims or rim‐rims?
- 2. What for? Or: Alright, but rims for rims for what?
- 3. Quirks of construction
- 4. Parts to coördinate
- 5. Sizing
- 6. Appendix: Tyre sizes, in order of my preference for immediate‐term usage (October 2022)
- 7. Appendix: Ordering parts
1. Rim rims or rim‐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”.
2. What for? Or: Alright, but rims for rims for what?
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.
The chair’s lever drive excels at forward locomotion. It cannot function in reverse. This has more implications than might immediately appear obvious. Every slight nudge forward on either lever is a serious commitment.
There also seems a profound risk, especially on upward slopes, of an engaged lever crushing the fingers, limbs, snouts, or little bodies of lapriders or inquisitive companions, against the wheel with horrifying force.
Thankfully, with moderate faff, the levers can be engaged or stowed from moment to moment, allowing the wheels to rotate freely backwards in settings that call for this flexibility (should the operator(s) have preempted it). Yet there are no pushrings on the stock wheels, and the tread is difficult and uncomfortable to grasp. Not only self‐propulsion, but self‐minutest‐adjustment is severely impeded.
As a result, manoeuverability is unnecessarily terrible indoors, in tight outdoor spots, going through gates and doors, when commingling with dense foot traffic, among tame animals or kidlets, travelling on public transport, and while waiting to cross roads.
Most pushrings are shaped to connect with the wheel rim at six evenly distributed points. For the stresses on the wheel to sustain this symmetry, the number of attachment points functions best as a factor of the number of spokes. 36‐spoke wheels, then, would make so much more sense than the 32‐ GRIT chose.
2.2. Worth of girth
Each wheel’s axle and quick‐release button project well beyond the tyre span — by about 45 mm.
Barely taking advantage of these nine enormous centimetres of additional width becomes extra frustrating at gates and other narrow pathways. Here, the button clips stuff seemingly way out. It often even becomes caught or depressed. A pushring or a wider tyre would better shield the axle from a greater range of angles, and diminish the surprise at the clearance required to skirt on safely by.
2.3. Grubbiness trouble
It is slow, hard work to clean the tread on the C.S.T. Patrol tyres after they meet gardens, parks, or wet streets.
The ultra macho colour scheme makes for an unfriendly appearance and poor visibility.
2.5. Falls risk
The chair is prone to tipping backwards or sideways on some crucial inclines.
Anti‐tip devices would ruin vital ground clearance.
Taller drive wheels at the back push the front wheel into soft surfaces with that bit more force, as well as raising the centre of gravity and lengthening the overall craft; but, the skew they create can buy precious degrees’ grace. That doesn’t directly help with the sideways factor, directly intensifying it — although a gentle‐textured, wide tyre can be braced against to more sophisticated effect when shifting upper‐body weight around, subtle though we’re getting. To me, sideways tilt has been most confronting on rare occasions where a second person is pushing from behind. Whereas in everyday life, I find myself doing a lot of ledge‐climbing, for which popping wheelies from given slopes is essential. Of course, the front wheel can be changed as well or instead, to compensate.
Managing falls risk well is all an intricate trade‐off, informed by very personal requirements.
Despite being the chunkiest GRIT offers, even these drive wheel tyres are not sufficiently wide/soft/low‐pressure for much beach or desert access, nor for some other desirable terrain.
2.7. Improving the above
The last point could be addressed with wheels in the environs of 100-507 ERTRO. Everything else is resolvable with any of a variety of sizes. Please see the following sections for specifics.
3. Quirks of construction
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.
4. Parts to coördinate
This project entails designing a pair of practically identical wheels, with each wheel consisting of the following:
4.1. Torque coupling
Including bolts for it.
GRIT sell sets of these on request.
As for a bicycle front wheel. And:
- I.S.O. six‐bolt disc brake mount
- 36‐hole forged aluminium spoking flanges
- 100 mm* O.L.D.
- to take 15 mm through‐axle
Hub, rims, and such need be nothing fancy, and best their materials’ normal, silvery colours.
A wheelchair axle, not a bicycle one. Can reuse original pair if need.
- 15 mm diameter
- quick‐release mechanism (or not, I suppose?)
- distance from thread to securing balls is around 135 mm*
4.4. Wheel rim
- 36‐hole aluminium
- drilled (precisely!) to accommodate push ring
- of dimensions to suit tyre and push ring
4.5. Spokes and nipples
- stainless steel spokes with brass nipples
- wouldn’t hurt to go for something strong, that can be hauled on in the event of a fall
- dimensions determined by the shapes and sizes of wheel rim, hub, and tyre, as well as the pattern according to which the spokes will be laced
4.6. Rim tape
- I prefer rubber, although other types work
- dimensions to fit wheel rim
Sized according to combo of push ring and wheel rim.
If possible, I choose lighter colours of urban bicycle tyres.
For a “fat bike tyre”, though, the priority is just something with widely‐spaced knobs, to make cleaning easy — though a reflective stripe will be a nice bonus.
4.8. Inner tube
- to fit tyre
- my personal preference is for Presta valve, but Schrader okay too 🙂 (and Schrader is generally more advisable!)
My preferred pushrings would be the likes of:
- thick‐tubed bare steel (an inch or more thick)
- Spinergy TC Max Handrim
- Spinergy TC Stainless Steel Handrim
- CarboLife Curve L
- CarboLife QUADRO
4.10. Hardware to securely attach push ring to wheel rim
Um, look let’s discuss wheel sizing…
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?
6. Appendix: Tyre sizes, in order of my preference for immediate‐term usage (October 2022)
- 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?
- 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.
- 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.
- 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.
- 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.
7. Appendix: Ordering parts
I’m still asking and scrounging around for other manufacturers’ data, but this is a start.
7.1. Pushring dimensions
Tab of tabs!
|Tab distance||Curve L||Quadro|
7.2. Pushring manufacturer headquarters
1709 La Costa Meadows Dr
San Marcos, CA 92078
United States of America
Phone: +1 (877) 291-4540 (Toll Free) or +1 (760) 496-2121
As for distributors? These products are sold on the Australian market…