How do you calculate beaded edge rim diameters?

[CDG999]

Hello,

I need a quick lesson in how to measure beaded edge rim diameters. I'm more familiar with American bikes, so I understand the 26"x3" (= x20" Dia.) and 28"x3" (= 22" Dia.).

What I'm not sure about is the 26"x2½" and particularly the 26"x2½x2¼", which I understand is a larger diameter than a 26"x2½? And how is the rim diameter calculated?

Craig.

[R]

Why don't you tell us where this is going - or coming from ?
And the particular bike this enquiry is centred on.
Which is likely well documented ?

Beaded edge rims/tyres appear to be a subject all on their own.
With enough foibles and strangeness that hard and fast rules don't apply !
And then there are the metric versions...
Cheers.

[cardan]

Yes it's a mess: there were standards but they were introduced quite late and not universally followed.

In the sizes you mention, using overall diameter (i.e. not the bead, but the outside of the rim), roughly:

26x2, 26x2 1/4, and 26x2 1/2x 2 1/4 (used to be labeled 26x 2 1/4 fits 2 1/2" rim) all go on rims a bit over 22". These rims are mostly Westwood pattern (dropped centre).

26x 21/2 BE rims were used in the day - a bit over 21" - BUT all modern 26x2 1/2 tyres are designed for 22" rims. You have a problem if you have 21" Westwood rims.

650x65 tyres also use 21" rims, but very flat base. No tyres available, but a 26x3 usually works OK.

26x3 goes on a rim about 20.5", almost always with a curved base - not as flat as a 650x65.

28x3 rims are usually about 22.5", and always flat base.

So there, a nightmare these days, but there was sense there once.

Cheers

Leon

[CDG999]

Thank you both, this answers my question. I was curious to know and wasn't actually after particular tyres to fit a motorcycle.

[vintage_keith]

Hope folks don't mind me jumping in on this one....I'm entering into my 1st foray with BE tyres.
I have a bike from 1912, which is a factory prototype/development bike, came to me without any rims.
The front forks will only pass a 2" BE tyre, the rear will comfortably accommodate a 2.5" tyre.
I approached a rim manufacturer in the UK, to check what size rims were needed, and he came back with same diameter rims for different size tyres (although different profiles I believe).
When I suggested this didn't appear to add up, could he expand, there was a stoney silence......
I can see how a 26 x 2 tyre needs a 22" rim, but how can a 26 x 2.5 also need a 22" rim?
What happens to the extra 0.5" (x2)?

[R]

If the tyre is wider than it is taller, this could account for this ?
I met up with the oddities in the BE world when hunting for tyres for an old Douglas.
25 x 3, a plump balloon tyre now totally obsolete are simply not available, but 24 x 2 & 1/4 will apparently fit.
But are rather skinny, and will give a hard ride.  Ayeee !!

Rickshaw and bicycle tyres in all manner of sizes abound - but may not be suitable for motorized use ??
Display only ?

May we ask what this beastie is that you have ?

[cardan]

Historically, all 26 x 2 tyres went on a rim that was a bit over 22" outside diameter - usually about 22 1/4" or so. There was no "standard" in the early days.

Tyres got wider, 26 x 2 1/8, then 26 x 2 1/4 and they still went on the same rim.

By this stage rims were getting a bit wider, but were the same diameter.

Next came tyres that were labelled "26 x 2 1/2 x 2 1/4" or "26 x 2 1/2 Fits 2 1/4" rim". Same rim diameter.

As mentioned above, there were some real 26 x 2 1/2" tyres, and these fitted a 21" rim, as you might expect. They were not adopted widely.

ALL modern beaded edge tryes labelled 26 x 2 1/2 should really be labelled "26 x 2 1/2 Fits 2 1/4 rim" - they go on a rim of 22-and-a-bit inches, but usually wider than a 26 x 2 rim.

Do tell about the bike. I assume it is a lightweight, because most of the larger bikes had gone to 26 x 2 1/2 tryes by 1912.

Leon

[vintage_keith]

So presumably modern 26 x 2.5 BE tyres could be classed as 'low profile' - wider than tall.....
Working on your info Leon, I have an OLD (and I mean old) reusable rim that is about 1.75" wide at 22.5 o/d that should be ok for a modern 26 x 2 BE tyre, and a modern rim same diameter, but shy of 2.25" wide that should be ok for a 26 x 2.5 BE tyre?

The bike is a Royal Enfield, 344cc IOE with their own engine (engine number 2). It is the predecessor of the 425cc twin with the glass oil tank.
Was going to upload a photo, but message comes up: Upload full?

[cardan]

Hi Keith,

1.75" is right at the skinny end of 26 x 2 rims - remember there were machines back to 1903-04 with rims this size. I checked what I'd call a "normal veteran" 26 x 2 Westwood rim and and it was 1.85" wide.

2.25" is right at the wide end of 26 x 2 1/2 x 2 1/4 rims. I measured a few originals that I'd be happy to put 2 1/2" tyre on and they ranged 1.95" to 2.125" wide.

So nothing really wrong with either rim, but the combination might be a bit "narrow-wide". A lightweight twin 1912 would probably have had 2 1/4" tyres; personally I think I'd be going for 26 x 2 front and rear. Very pretty.

I messaged Nigel about the "upload folder full" thing.

Leon

[vintage_keith]

Now that the workshop has come up to a manageable temperature, I've been back to check actual dims, both rims are within a gnat's toozle of 22 5/8" o/d. The ancient rim is 1 13/16" wide, the new one is 2" wide, so apologies for wildly inaccurate guestimations! I also found a newer rim that I got from the VMCC supposedly for 26 x 2, that also measures 22 5/8" x 1 13/16", but the welded joint leaves a lot to be desired.
It looks like I may end up ditching the old rims, reworking/improving the VMCC rim (so I can get it nickel plated for the stirrup brake) using the new slightly wider rim on the rear, and as you say, sticking with 2" tyres (my other 1912 little Enfield twin has 2" tyres, but they are the old delivery push bike tyres, on wired on rims). No doubt fitted in the days when BE sizes were even scarcer than today.....

2" tyres at 40+ psi are no joke - I know from riding the other Enfield! But great fun until the pain barrier is reached. 

Still seems odd that same o/d rims fit different section tyres........

[cardan]

Sounds like a good plan.

I went for a ride yesterday on my 1910 FN shaft-drive single. 45 psi in 26 x 2 tyres. Tiring, but serious fun!

Leon