18" tyres

Matt Austin · #1 16-04-14, 14:29

Seven tyres. Anyone want to go in together? I'd be keen for two.

Cheers,
Matt

Quote:

Originally Posted by gjamo

http://www.ebay.com.au/itm/1100x18-b...item2ed0dcf911

Matt Austin · #2 03-05-14, 23:08

Please pardon my ignorance, but what is the difference between 10.50-18 tyres and 1100-18 tyres? The decimal point and number of digits has confused me. I'm still very much a neophyte at all this!

Cheers,
Matt

Tony Wheeler · #3 04-05-14, 07:25

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 · #4 09-05-14, 00:04

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 · #5 09-05-14, 00:32

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 · #6 09-05-14, 01:05

Quote:

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 · #7 09-05-14, 01:12

Quote:

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 · #8 09-05-14, 03:44

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 · #9 09-05-14, 04:20

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.