Tyre sizes

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Matchless?58
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Tyre sizes

Post #1 by Matchless?58 » Tue May 17, 2022 12:20 am

Hi all
A question about tyre sizes .
One of my front tyres together with the inner tube is both knackered and now flat so I would like to get it replaced .
I need a bit of advice on tyre size before I order it .
The size on the tyre is 12.0 / 12.5 - 18
I have sourced a tyre at a reasonable price on the internet .
The size is 12.5 / 80 - 18 .
These are both 12 ply tyres but can anyone explain to me what the 80 denotes ?
Will this tyre fit ?
Machine is a JCB 3c11


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Re: Tyre sizes

Post #2 by kaos » Tue May 17, 2022 3:14 am

My best guess is that these are two different methods of indicating aspect ratio, i.e. the ratio of the tyre height to width. If this is correct, your original tyre has an aspect ratio of 12/12.5 = 96%, and your proposed replacement has a height of 12.5*80% = 10 inches, meaning a diameter four inches less than the original. This might work for a 2wd machine, at least if you replace both tyres, but if it's 4wd you should be careful to keep to the original diameter.

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Re: Tyre sizes

Post #3 by gecko.cx » Tue May 17, 2022 8:03 am

Not aware of the naming convention, so I wondered off to find some info.
This table is from a Firestone catalogue
The difference in the rolling radius is only 13mm, so I think it'd be an acceptable substitute.


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Re: Tyre sizes

Post #4 by essexpete » Tue May 17, 2022 8:45 am

What type of tread pattern are you using? I ask as some tyres have more rubber on the shoulders and can foul the loader arms on full lock with arms lowered.


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Re: Tyre sizes

Post #5 by Matchless?58 » Tue May 17, 2022 10:09 am

Thank you all for your help . I will try and get a pic of the tread pattern today . I hadn't considered that the tyre might foul on the loader arms pete . Looking at the chart that gecko sent I think that there are some considerable differences in the tyre size I have chosen . It might make sense to renew both fronts at once although I really only want to get one side done . Cost against value of the machine as always is a consideration although it would only be one call out charge for the tyre fitter . I really don't fancy having a go at that ancient rubber myself and with limited equipment . Motorbike tyres are about my limit !


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Re: Tyre sizes

Post #6 by Matchless?58 » Tue May 17, 2022 11:01 am

Well I have just had a good search of the internet and it seems like I will not be able to get the 12/12.5 - 18 tyre size . The nearest is going to be 12.5 /80 - 18 . That probably means I will definitely have to renew both fronts . I have downloaded some measurements from the spec sheets and will have a bit of a measure up today just to make sure there is sufficient clearance for turns and to see if they will fit the rims . It's a bit of a minefield .


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Re: Tyre sizes

Post #7 by essexpete » Tue May 17, 2022 1:18 pm

You can get out of trouble short term with a 6 stud lorry wheel with the wheel fitted the same way as the outer wheel on a double wheel axle.


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Re: Tyre sizes

Post #8 by Matchless?58 » Tue May 17, 2022 9:01 pm

Thanks Pete . I will let you know how I get on . Cheers Chris


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