You are correct about the two different 24" rim sizes, but they are NOT wheelchair and cycle specific.... If you look at the tire sizes, you will see two different ways of specifying the width, fractional (24 x 1 3/8) or decimal (24 x 1.375) At least in the US, both size standards are common on pedal bikes. However the fraction numbers are used for the 540 rim diameters, and the decimal numbers are used for the 504 diameter....
If I'm remembering Sheldon correctly, wheelchairs normally use 540 rims, and so do most pedal bikes. One of the popular manufacturers used to be Schwinn, who tended to do non-standard parts, supposedly because they were better, but more realistically to try and lock people into buying their parts from their dealers.... One of these non-standards was the 504 size tires, now used on their bikes and some other brands of 'cruiser' bikes.
The Sheldon Brown website mentioned elsewhere is almost the 'Bible' (except for being fact-based) of all things pedal-bike, and is a fantastic reference... Here at Artisan's Asylum we have the home base of "SCUL" Somerville's friendly bicycle chopper gang, which builds some of the worlds strangest bikes.... Sheldon is their number one reference source....
I suspect your biggest challenge will be finding rims with the same number of spoke holes as your motors, at least that is what I found to be the case.
I did an out-door conversion on my manual chair, and this may be a way for you to think about... I went from 540mm 24 x 1 1/4 solid tires to 504mm 24 x 3.75" in the back, and from 4" to 6" casters up front - net result is the chair sits 1" higher, but still level....
The tire case went up about 2.5" in nominal diameter, but the rim shrank 36mm or about 1.5" so the OD of the tire only went up a little over an inch, and I get some of that back by being able to run very low pressure.... Changing the front casters gave me the same amount of lift on the front, so the chair stays level, just taller....
My biggest problem was finding hubs and rims with the same number of spokes - if they aren't the same, it isn't going to be possible to make a safe wheel as the spoke spacing and angles will be wrong... I couldn't find any 504 rims that matched the spoke count of the existing hubs on my spare set of wheels, so I ended up having to buy a set of SUN wheelchair hubs with the right spoke hole count.
Sheldon has a very nice spoke length calculator which I used to pick my spokes. Most manual chairs use a 'radial' spoke pattern which is simple and works OK, but is on the weak side. Since I was starting from scratch I decided to go with the strongest 3-cross pattern which is used on most pedal bikes. It requires longer spokes so the wheels are heavier - I calculated both, and it was almost a 3 GRAM difference (I decided it wasn't very important....) I also opted for the strongest 'double-butted' stainless steel spokes....
One of the big issues when increasing tire diameter is clearance between the tire and the chair frame. I tried to increase this by 'dishing' the wheels to the outside as much as I dared when lacing them, but I still found that I needed longer quick release axles and a couple of spacers in order to shift the hubs out enough to clear the frame... Also it obviously becomes necessary to readjust the brakes.... On my chair I found that if I set the bar of the brake into the mounting bracket as far as it would go, and set the position on the indoor wheels with the mounting bracket, I could set the brakes for the outdoor wheels correctly by pushing the bar all the way forward so the end was flush with the bracket...
It works really well, although it is a bit of a pain since I need about 20 minutes to make the swap between modes in either direction....
ex-Gooserider
grazy+1949 wrote:Hi Woody,
There are 2 sizes 540 mm bead which I think are wheelchair specific nominally 24" and 504 mm dia bead which are cycle specific 24" also nominal. I discovered this when I excitedly bought 2 x 24" x 2" tyres from a cycle shop and then found out that they had a 504 mm dia bead whilst mine had a 540 mm bead. I am quoting these measurements from memory. I looked at a friends new wheelchair and it had 2" wide tyres and the diameter of the 24" rim was smaller.
The main point though , is where can I get Fat rims and tyres that is to say 20" x 3"-4" at a reasonable price?
Graham
