Author Topic: Bsa round tank  (Read 233 times)

Offline Bear640

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Bsa round tank
« on: December 04, 2019, 04:14:46 AM »
Ok guys, I am having a little trouble getting the tyres not to rub on the guards. I have just about fixed this problem but double checking what other guys with RT,s have for tyres on their bikes?

Other guys I have spoken to all are running 26 x 3 front & rear, books I have read have said the same & then I find one book that says 24 x 2 1/2.

Cheers

Offline R

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Re: Bsa round tank
« Reply #1 on: December 04, 2019, 05:18:33 AM »
What year is your round tank ?

Offline Bear640

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Re: Bsa round tank
« Reply #2 on: December 04, 2019, 06:41:08 AM »
I have 2, 1924 & 25.

Offline R

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Re: Bsa round tank
« Reply #3 on: December 04, 2019, 08:19:36 AM »
If you use your x-ray vision (or a magnifying glass) on this 1924 brochure page,
it looks to be 24 x 2&1/4

https://i.ebayimg.com/images/g/tysAAOSweHtdkHLs/s-l1600.jpg

Didn't BSA change to drop centre rims not long after that ?
Although the pic for 1925 looks rather similar (?)

Edit. Amended to the correct  1/4"
X-rays weren't working too well...
« Last Edit: December 04, 2019, 10:43:02 AM by R »

Offline 33d6

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Re: Bsa round tank
« Reply #4 on: December 04, 2019, 08:56:23 AM »
I’ve always taken an interest in RT’s because they ran the same size 24x2 1/4 be tyre as my Model R Matchless. This has always been a “fringe” size even in the beaded edge world and certainly isn’t fat enough to rub on anything.
I’d do a lot of double checking if anything is rubbing. There has to be something amiss somewhere.

Offline Bear640

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Re: Bsa round tank
« Reply #5 on: December 04, 2019, 09:08:01 AM »
Bugger bugger,, I have fitted 3”. & we all know how hard beaded edge tyres are to fit

Offline cardan

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Re: Bsa round tank
« Reply #6 on: December 04, 2019, 10:03:30 AM »
The Round Tank (and it's mates the Triumph Junior and the Levis Popular) popularised the 24 x 2 1/4 tyre.

The rim diameter for 24 x 2 1/4 tyres is about the same as those for 26 x 3, but the rims are way narrower. And since the nominal rolling diameter of a 24 x 2 1/4 is 24", and the 26 x 3 is 26", it's hardly surprising that the tyres rub.

If the rims on the bike are the originals (Westwood or "dropped centre" profile) the 26 x 3 tyres are massively undersupported and there's a real danger of the tyre catastrophically leaving the rim. People have died, as a failed BE tyres usually fall off the rim (no wires) and can jam between the wheel and the frame.

The EW Douglas used a rim of very similar measurements, but it had a flat base and it can better support as 26 x 3 tyre. The original tyre size for the EW was 25 x 3 (often branded 25 x 3 Fits 24 x 2 1/4 Rim), but these are no longer available.

In summary, the only safe currently available tyre for a Round Tank is a 24 x 2 1/4.

Cheers

Leon

Offline mini-me

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Re: Bsa round tank
« Reply #7 on: December 04, 2019, 10:27:55 AM »
Cardan, can you enligthen me as to the correct rim profile for a 1918 Douglas? asking for a friend who speaks no english,thanks#

Offline cardan

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Re: Bsa round tank
« Reply #8 on: December 04, 2019, 10:57:26 AM »
Yep. Douglas used "Westwood" (dropped-centre) rims, to suit modern 26 x 2 1/2 tyres (correctly called 26 x 2 1/2 x 2 1/4, or "26 x 2 1/2 fits 26 x 2 1/4" rim"). These rims are usually about 22 1/4 to 22 1/2" overall diameter and about 2" wide. I say "about" because there were no generally-accepted standard rim sizes in the teens.

(Beware the 21" Westwood rim: these were built in the day for "real" 26 x 2 1/2" tyres, a size that never caught on. All modern 26 x 2 1/2" tyres fit the larger daimater rim.)

Cheers

Leon

Offline Bear640

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Re: Bsa round tank
« Reply #9 on: December 04, 2019, 10:58:26 AM »
So what you are saying Leon is that 24” tyres will fit on the same size rims that also take 26”?

Offline cardan

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Re: Bsa round tank
« Reply #10 on: December 04, 2019, 12:23:08 PM »
So what you are saying Leon is that 24” tyres will fit on the same size rims that also take 26”?
The rims are about the same diameter.

In the olden days, tyres were labelled with their (nominal) rolling diameter. So (very roughly), a 24 x 2 tyre has a rolling diameter of 24" and fits on a rim of diameter 24 - (2 x 2") = 20", and a 26 x 3 tyre is 26" overall, and fits on a rim 26 - (2 x 3") = 20". (In practice the rim sizes are usually about 20 1/2" and 21".)

However a 26 x 2 tyre fits on a rim about 26 - (2 x 2") = 22". Historically, 26 x 2 1/4 was an oversize tyre for a 26 x 2, so it also goes on a 22" rim. 26 x 2 1/2 was another oversize, still on 22" rim.

Of course a 28 x 3 tyre also goes on a 22" rim, since 28 - (2 x 3") = 22".

There were attempts to introduce standards for beaded edge rims in the early 1920s, but in practice rim and trye sizes varied somewhat from maker to maker.

Make sense?

Leon

Offline Bear640

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Re: Bsa round tank
« Reply #11 on: December 04, 2019, 12:25:17 PM »
I think I have you.  I will measure the width of to rim tomorrow & post a pic.
 Thanks

Offline R

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Re: Bsa round tank
« Reply #12 on: December 04, 2019, 09:25:57 PM »
There was also the minor matter that tyres (and rims) went metric somewhere about then too.
I notice some models of BSA in that brochure list the 650 x 65  metric size.

How does this relate to 24 x 2 1/4  and the matter of rims ?
Is it interchangeable at any level, or rider beware stuff...

Metrics didn't ever really catch on in the tyre world  ?

Offline Bear640

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Re: Bsa round tank
« Reply #13 on: December 04, 2019, 10:22:33 PM »
I noticed that on your link R but thought to myself that that’s another day question.

Offline R

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Re: Bsa round tank
« Reply #14 on: December 05, 2019, 12:03:47 AM »
I've got an EW Duggie thats going to need tyres at some point ( although probably not soon-ish),
so I thought I'd ask - today. Forewarned is forearmed, as they say....