Aero 250 wheels and tyres

[Tazmantic]

Hi all does anyone know what size wheels and tyres the aero 250 should have? my tyres say 3.25-19 and my speedo is reading wrong saying about 36 mph when I'm doing 30 (according to gps)

Cheers

[Doug]

The 1936 catalog states "25 x 3 Firestone" for the 350cc tyres and I would presume the 250cc was the same. Which I gather was old-speak for 19" rims fitted with 3" tyres.

I would leave it. It is probably the easiest 6mph boost in speed you will ever get from it! 

-Doug

[Tazmantic]

I would leave it. It is probably the easiest 6mph boost in speed you will ever get from it! 

-Doug

  how rude  but probably right, so are 1936 catalogues available? and sorry but why would size say 25 x 3 but tyre saying 19 would be the same size  not into tyres 

Thanks Neil

[Doug]

Back in the pre-wire bead tyre, pre-drop center rim days they used clincher or beaded edge tyres that were typically sized by the outer diameter and section. Not sure if the section was actually the height or width of the tyre (though generally a one to one ratio then), but the general (but not always precise) rule of thumb was to take the outer diameter, subtract twice the section, and you had the rim size.

So a 25 x 3 tyre is 25-(3*2)=19 or same thing as a 19 x 3 tyre.

I do not have a full 1936 catalog, just excepts:


And a Pride & Clarke leaflet for the 350, which was pretty much the same as the 250. Curiously, it specifies 26 x 3 tyres. Must be a mistake, as I doubt it has 20" rims!



And an advert for the 250:


-Doug

[Tazmantic]

Cool thanks for that Doug, but am I correct in thinking if my speedo has no faults causing it to read fast I would need bigger diameter tyres to correct the speed error (bigger diameter doesn't spin so fast for same road speed)

Cheers Neil

[EW-Ron]

There were indeed once 20 inch rims, they were quite common at one stage.
Norton Internationals used them on the back wheel  into the 1950s even.

I'm presently chasing a 20" tube for one, WW2 Enfields used some on the front wheels.
You can still buy 20" tyres too, Michelin and Mitas have both supplied them in the recent past.
But the tubes seem to be a scarcer item, and I'm reluctant to use a 19" stretched to fit.

Cardan will probably chime in here maybe, since the subject of Douglas using odd sized wheels
and optionally different sizes seems to be a persistent theme through their history !

Quite an elderly 20" tyre, marked in both systems, well worn.
Bit hard these days, but suitable for display - and that diamond tread pattern is now rarely seen ?

[Tazmantic]

Great info thanks, but where is the measurement of the rim taken? On the edge as mine measured 20” or bottom of the groove where the tyre fits 

Cheers

[EW-Ron]

Where-ever you can get the right measurement !?

If I put a tape measure across a bare 19" rim, it measures a neat 20" rim-edge to rim-edge.
So its obviously (?) down in the well base.
Its stamped as a 19" rim, and a 19 inch tyre fits it, perfectly.
I took one off it.

If I sit a 20" rim beside it, it stands a near perfect inch taller.
And its stamped as a 20" rim, so I'm assuming they knew what they were doing...

[cardan]

If you look inside the rim, you'll see a shoulder where the wire of the tyre sits where it's fitted. This is called the bead seat, and the diameter of the rim at this point is called the "bead seat diameter" (BSD). This is the part of the wired-on rim that determines the rim diameter: it will be 19" for a 19" rim, etc. The over-all diameter of a 19" rim is larger than 19" - usually 20" or so.

Wired-on rim profiles usually conform to a standard that goes from WM0 (smallest - 1.5" wide inside at the bead seat), to WM3 (largest 2.15" wide). There are tables that show the various tyres that will fit on a a particular rim, for example https://www.maxxis.co.uk/know-your-tyres/motorcycle-tyre-safety/wheels-and-rims . Maxxis are happy for you to fit anything from a 275-19 to 360-19 tyre on a 19" WM2 rim.

So measure the BSD, measure the width of the rim (see here https://burtonbikebits.net/wheel-rims/ or read the WM number usually stamped into the rim), and select a tyre size from the table. Much easier than for Beaded Edge rims!

Cheers

Leon

[eddie]

Neil,
        Back in the early 30's, Douglas produced their own speedo drive that had enclosed gears and fitted inside the end of the hub. I see from the photo of your bike (on ebay) that it is fitted with an 'after market' speedo drive that is clamped to the spokes. These were available in a variety of ratios to suit various speedo heads and tyre diameters - so you may just have a mis-match of drive and speedo head. Bear in mind that when your bike was produced speedos were a luxury, not a requirement, and therefore, were not required to be accurate within 10%.

  Regards,
                   Eddie.

[Tazmantic]

Hi Eddie,

yes it has a smiths speedo so Ive been doing some research online and going to check the ratio with the speedo etc and see where the error lies 

cheers Neil

[Tazmantic]

Adding to this does anyone know of a pushbike speedo that would work ok? I bought a cheap one from home bargains and installed it ok (span front wheel and speedo read ok) but as soon as I started the engine it either didn't work at all or froze at some stupid speed  no matter where I mounted or held the speedo but within seconds of the engine stopping it worked fine again...

Cheers

[Keith]

I've been using Cat Eye Velo speedos for over 10 years now and they have proved very reliable on 5 of my vets and vintage rides. They even have wireless versions now which means you don't have wiring to worry about. As well as speed they add a clock, run time, distance (great for the oil pump every 10km), max speed, etc.

Cheers, Keith...

[Tazmantic]

Cheers Keith,

Looks like there are several models to choose from does it matter which one?

Thanks Neil

[Keith]

I've bought the Velo 5 in the past which has all the functions I need, but just recently I bought the Velo 9 which has a slightly larger screen to help these aging eyes