18" tyres

[Tony Wheeler]

Strictly speaking the decimal point should be present when written, but we often drop it like we do in speech. Same thing happens with gun calibre, eg. 303, 350, 762, etc.

10.50 and 11.00 are so close as to be considered the same, eg. 10.50 x 20 and 11.00 x 20 were both CMP spec.

[motto]

To explain a little further. The first group of numbers i.e. 10.50 refers to the nominal width of the tyre in inches. In this case ten and a half.
Around WW2 this would also give a good idea of the height of the tyre as most of them seem to have a 100 percent aspect ratio. By that I mean the distance from the inner diameter of the bead to the tread surface would also be close to ten and a half inches.
None of this relates to the rim width which would probably be around seven inches across the inside of the flanges.
The second group of numbers relate to wheel diameter. A 10.50-18 tyre would go on an eighteen inch wheel so the hole in the middle of the tyre would be 18 inches across.
By this reckoning a 7.50-20 tyre would be about 35 inches in overall diameter. 7.5"+20"+7.5"=35" and this works out pretty close although there is some variation between brands. Some makers back then actually gave the tyre outside diameter as part of its identity i.e. 7x34 although that wasn't general practise and fell by the wayside.

David

The foregoing is only my observation and open to correction or addition.

[Lynn Eades]

David, For many years I was of the belief that the first number was the height of the side wall, ie 9.00 x 16 meaning an overall height 9+16+9 (total of 34 inches)
I was later told on this forum that the 9.00 was the foot print (width) and that it had nothing to do with the sidewall height.
As you have explained with the 100 percent aspect ratio, I was still right.

I do know that when the 70 series radials came out, that the size given (say 185 x 70 x 13) that the 185 related to the width, and that the height of the side wall was 70 % of the width.
I am not 100% convinced which is correct for the early cross plies.

[Richard Farrant]

Originally Posted by Lynn Eades

David, For many years I was of the belief that the first number was the height of the side wall, ie 9.00 x 16 meaning an overall height 9+16+9 (total of 34 inches)
I was later told on this forum that the 9.00 was the foot print (width) and that it had nothing to do with the sidewall height.
As you have explained with the 100 percent aspect ratio, I was still right.

I do know that when the 70 series radials came out, that the size given (say 185 x 70 x 13) that the 185 related to the width, and that the height of the side wall was 70 % of the width.
I am not 100% convinced which is correct for the early cross plies.

Think there were a few anomalies such as the 9.25-16 tyres as used on Humber 4x4 vehicles. Unavailable nowadays and replaced by 9.00-16 with is in fact larger diameter than a 9.25 !
Incidentally, on tyre sizes, the old crossply such as 11.00-20 has the "-" dash between tyre and rim size, but only uses the "x" cross on the older designations that used rim and overall diameters, eg 7x34

[Grant Bowker]

Originally Posted by motto

Some makers back then actually gave the tyre outside diameter as part of its identity i.e. 7x34 although that wasn't general practise and fell by the wayside.

I don't have a source to quote but am quite sure the 7x34 size designation was for a separate series of tires, sometimes referred to as "high pressure" tires. I think the "high pressure" was in comparison to "balloon" tires that were supposed to be lower pressure and thus give a more comfortable ride. For the "high pressure" tires the 7 referred to section width (and approximately height) and 34 was the overall diameter which would go back to a 20 inch rim.
Until they started putting belts on tires (either bias or radial) all tires were close to 100% aspect ratio as there was little to stop the tire from taking on the shape that would give maximum volume for minimum circumference - cylindrical. once the use of belts started, the belt limited the diameter of the tire which in turn permitted the tire to be wider than it is tall. Think of the long skinny balloons clowns manipulate into other objects for the amusement of the crowd. (animals, hats, swords etc.) (Actually, a sphere has more volume per surface area - but is awkward to use as a tire.)
Here I am, finally getting some use from the calculus courses I failed so often at university.

[motto]

The modern tyre designations as you illustrate Lynn follow a different format. I believe the first group of numbers is the nominal width in millimetres i.e. 185 or 205 etc. The carry over is that they still give the wheel diameter in inches.

Perhaps the reason for the 9.25 designation Richard was to differentiate the Humber tyre from something similar and was not meant to be a true indicator of size.

Grant, you've rung a bell with your mention of high pressure tyres. I think they were what my Dad referred to as 'hardwalls'. Dad grew up in the 1920s, was a vehicle enthusiast all his life and was familiar with all the old stuff. He passed away a couple of years ago at the age of 93. I sure do miss him.

David

[Grant Bowker]

Rim widths for pneumatic tires on CMP were 6" for both 16 (including the tapered beads used on HUP (that were another user of 9.25-16 tires)) and 20 inch rims.
9.00-16 pneumatic cross country tires had diameters from 34.8 to 35.7 inches while 10.25 had diameters from 41.4 to 42.3 inches. In general highway tread tires had slightly smaller diameters than cross country and runflat were at the small end of the range for pneumatics of the same nominal size.
The above is from the AEDB design record.
I don't have any listing for Canadian manufactured military vehicles of the period using high pressure series tires but did find a reference to a trailer used by Canadians that was manufactured by Taskers that used them.

[motto]

Thinking on that 9.25-16 tyre size (which I had never heard of before now). Maybe they inadvertently put a couple of numerals in the wrong place and it should have been 9.00-16.25
Now that would have saved a lot of confusion and would be damn close to reality.

David

[Grant Bowker]

I have doubts about that suggestion David.
By observation, it is considerably easier to mount modern 16 inch tires on 16 inch HUP rims than on "real" CMP 16 inch rims. If HUP rims were sized at 16.25 I don't think you would have an easy time at all getting a 16 tire onto them if it were even possible.
Original war dated 9.25-16 tires have been seen, still with original sidewall markings.
I have seen several different printed sources refer to 9.25x16 but don't recall ever seeing a reference to a 9.00-16.25 tire (they could exist but I haven't ever noticed them). The AEDB Design Record makes several references to tires 9.00-16 and 9.25-16. In all cases the 9.25-16 is listed as smaller outside diameter than 9.00-16 within category of tire (cross country pneumatic, highway pneumatic, runflat). It seems unlikely that if they were really referring to 9.00-16.25 tires they would consistently, 100% of the time, refer to them as 9.25-16 - not getting it right even once by accident.
For cars and trucks, I don't believe I've ever seen a rim diameter other than full or half inch. Tractors use a sizing system I've never taken the trouble to understand (city boy) but they don't seem to me to be a simple extension of car and truck sizing.

[lynx42]

Originally Posted by motto

Thinking on that 9.25-16 tyre size (which I had never heard of before now). Maybe they inadvertently put a couple of numerals in the wrong place and it should have been 9.00-16.25
Now that would have saved a lot of confusion and would be damn close to reality.

David

No David, They didn't. The Ford Lynx Scout Car should run 9.25 x 16 tyres and I have 5 here taken off my Lynx and a spare.

Lynx continues 195.jpg

Lynx continues 203.jpg

I have now fitted 9.00 x 16 tyres and they are quite a bit higher than the 9.25 x 16s I took off.

Regards Rick.