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PostPosted: Thu Jun 14, 2012 12:49 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 8202
Location: New Forest, UK
Measure the distance from the dropouts to the brake bridge, chainstay bridge and fork crown. That gives a starting point for working out what would fit. Unless that is done we are all guessing.


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PostPosted: Thu Jun 14, 2012 7:05 pm 
retrobike rider
retrobike rider
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Joined: Tue Jan 29, 2008 11:04 pm
Posts: 1799
Location: A wretched hive of scum and villainy...
I would have thought that dropout to brake bolt distance would be the limiting factor, as it dictates tyre clearance and brake drop.

You've got some decent clearance there at the moment, so probably mountainbike 26"rims with a smallish tyre will give you the largest workable diameter. However, you'll need at least new rims, tyres and brake calipers for it to work.

Your original size, 24" x 1 3/8" is still widely used in the wheelchair world. Plenty of choice of tyres, from 23mm slicks to 1.5" knobblies (even tubulars :shock:) in black, grey and colours. Just not in bike shops... 26" x 1 3/8" is also still pretty common in wheelchairs, as are narrow mountainbike-rim sized 26" tyres. I can help if you need.

ETRTO wheel sizing geek warning:D

The best way to work this stuff out is to use the ETRTO mm sizing, not the inch names...24" x 1 3/8" is 540mm diameter at the tyre bead seat (not the outside of the rim, but about 12mm smaller):
ImageImage

26" x 1 3/8" is ETRTO 590mm, so assuming you keep roughly the same depth tyres, new 26" x 1 3/8" wheels will be 50mm bigger diameter, with 25mm less clearance to the brake bridge, and 25mm shorter brakes. ((590-540)/2=25mm)

Similarly MTB 26" is ETRTO 559mm, so again assuming the same tyre depth, you'll get a wheel 19mm bigger overall, with 9.5mm less clearance and brake drop. ((559-540)/2 = 9.5mm)

To calculate the outside diameter of any wheel, take the ETRTO mm size and add 2 x tyre depth (in mm) and you'll be pretty close. Simples...

Some 26" ETRTO sizes:
Mountainbike 26" (26 x 1.95 etc.) = 559mm
Road racing 26" (650c) = 571mm
Old French roadster 26" (650B) = 584mm
Old British 26" (26" x 1 3/8") = 590mm

Some 24" ETRTO sizes:
Mountainbike 24" (24 x 1.25 etc.) = 507mm
Road racing 24" (600c) = 520mm
Old British 24" (24" x 1 3/8") = 540mm

All the best,


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PostPosted: Sat Jun 16, 2012 12:51 am 
Dirt Disciple
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Joined: Sun Jun 10, 2012 6:38 pm
Posts: 12
Okay, I've measured, but bear in mind my tape measure sucks so these may be off by a centimetre or two...

dropouts to the brake bridge: 33cm
dropouts to the chainstay bridge: 35cm
Front dropouts to fork crown: 32-33cm

So what would you recommend?

Thank you so much for the help :)


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PostPosted: Sat Jun 16, 2012 5:00 pm 
Old School Grand Master

Joined: Tue Oct 09, 2007 1:55 pm
Posts: 8202
Location: New Forest, UK
OK I went out and checked the diameter. An MTB wheel with Conti Sport Contact 1.2 came in at 31.5mm. So it should just about work.


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PostPosted: Sat Apr 20, 2013 2:40 pm 
Dirt Disciple
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Joined: Sun Jun 10, 2012 6:38 pm
Posts: 12
Sorry to bump an old thread, but thought I'd give a small update to the guys who helped me out.

For the longest time, I used the 24x1 3/8 wheels, which made the bike feel tiny. Yesterday I managed to get a lovely pair of Schwalbe City Jets and a pair of 7 speed alloy wheels, so decided to try the MTB wheel/slick tyre combo and guess what? They fit!

I need to change the brakes for a nice small reach pair, so I'm gonna pop down the local bike recycling shop this payday and hopefully find a cheap pair.

Here's a pic of the old wheels: Image

And here's some pics of what she looks like now:

Image

Image

The tyres are a bit fatter than they could be (26x1.5), and she's definitely got an unusual look, but I love her and she's ideal for the commute back and fore work.

Thanks for the help guys! :mrgreen:


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