I need to replace the tyres on a trailer underneath a boat belonging to my father. The ones on it are, I think, cross ply and the size markings I have found say, as best I can tell "5 - 90 13". The marke between the 5 and the 90 is a bit longer than a dot but a bit shorter than a clear hyphen, so it could be either.
Can anyone suggest what size this is ... and what a modern equivalent would be? I plan to fit radials for economy, even though cross plies are reputed to give slightly better trailer handling.
5-90 is 5.9" measured across the diagonal, ie bead to tread edge. Metric is tread width. But a straight conversion isn't far out for standard profile tyres. So 155 or 165 will be ok.
Your advice to date has missed the most important issue. Your primary concern is not the size of the old tyres, which may not even have been correct for the rim, but the rim width which determines which sizes of radial tyre are deemed approved fitments on it. It's probably a 4" rim going by your crossply size and that can take radials from 135 R 13 to 165 R 13 with 145 being the ideal fitment.
If it's a 4.5" rim then either 155 or 165 R 13 are ideal but you can go up to 175. If it's neither of the above then let me know.
The other thing you need to watch out for is the loaded trailer weight and the single axle load index which determines how much weight each tyre size can cope with on a single axle.
145 R 13 - 825 kg
155 R 13 - 935 kg
165 R 13 - 1045 kg
175 R 13 - 1166 kg
There will be variations for tyres with reinforced plies. Usually a reinforced tyre has a load rating the same as the next size up non-reinforced tyre.
For best economy you want the skinniest tyre you can get away with.
All of this info will be in most tyre manufacturers catalogues and probably also on their websites. Mine happens to be an Avon catalogue about 10 years old although I'm damned if I can remember where I got it from. Give them a ring on 01225 703101 and I'm sure they'll stick one in the post for you.
Thanks Dave. Again, much appreciated. Loading shouldn't be too much of a problem: the boat (a Hunter 490) displaces 454kg but is stripped down: most importantly, 136kg of keel will be travelling separately. So about 300kg for the boat and roughly the same again for the trailer. I should remember, because I designed the trailer - it's a conversion of a gas trailer for met balloons, originally designed to carry a pyramid of 9 full sized gas cylinders. Again from memory, it as 3/4 ton Indespension units.
I have set my father the task of measuring the rim width in situ, if he can, but it looks as if 155 R 13s will be a pretty safe bet, since they will happily sit on 4" or 4 1/2" wheels and won't mind the load.
I'll have a sniff round for cross plies, though. I was idly looking at a small trailer in Homebase today and noticed it was fitted with 5.00
10 cross plies, so some sizes are clearly still available.
Well assuming the wheels are actually either of those sizes but I think they will be. It'd be odd for a little trailer to have anything wider.
Rim width and lip profile are usually stamped into steel wheels as I recall. You should see something like 4.00 or 4.50 and then either a J or B. The letter indicates the lip profile.
We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Dave Plowman (News)" saying something like:
Er, no. Nothing to do with diagonals, but straightforward width and height. From
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"Crossply Vintage Tyre Measurements The measurements for crossply tyres are given as the height of the tyre section and the width of the tyre (one measurement refers to both dimensions) x the diameter of the tyre in inches. For example, 5.00x21 describes a tyre with a diameter of 31 inches and a section height and width of 5 inches (to fit a 21 inch rim). Occasionally measurements for crossply tyres were given as the diameter of the tyre X the height of the tyre section in inches (as for beaded edge, straight-sided and split rim tyre measurements). Commonly today, tyre manufacturers do not have the demand to warrant making all tyre sizes. Crossply tyre measurements such as 5.00/5.25x16 indicate that one tyre has been produced to fulfil original fitments of tyres with 5 and 5 1/4 inch section heights (to fit a 16 inch rim). "
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