MondeoGhiaX Posted March 25, 2008 Report Share Posted March 25, 2008 evening, just looking on the toyo website for a suitable tyre to match a larger wheel size. for each tyre, they quote an overall diameter, and also a static radius. now if i learnt anything at school, it was that the diameter of a circle is twice the radius, but this doesnt seem to follow in this case. some tyres have a similar static radius, but the overall diameter is completely different. whats all that about then? more importantly, which measurement should i be looking at to keep the overall wheel+tyre size the same? thanks. Quote Link to comment Share on other sites More sharing options...
Tony Posted March 25, 2008 Report Share Posted March 25, 2008 Would love to have a link to the site ..... One issue with the conversion sometimes is the addition of the aspect ratio, maybe this is complicating things? Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted March 25, 2008 Author Report Share Posted March 25, 2008 http://www.toyo.co.uk/productdetail.php?id...mp;product_id=5 for example, a 145/80R12 has the same static radius as a 155/65R13, but there is 5mm difference in the overall diameter. not much i suppose, but why any? does it mean when the tyre is on the car and the weight is squashing it? Quote Link to comment Share on other sites More sharing options...
Tony Posted March 25, 2008 Report Share Posted March 25, 2008 http://www.toyo.co.uk/productdetail.php?id...mp;product_id=5 for example, a 145/80R12 has the same static radius as a 155/65R13, but there is 5mm difference in the overall diameter. not much i suppose, but why any? does it mean when the tyre is on the car and the weight is squashing it? Metric/ Imperial/ Metric will leave errors.... I think to keep things totally uniform the tyre dimensions would need to be 153/62R13 or thereabouts... By convention 5% differential is within tolerance. Quote Link to comment Share on other sites More sharing options...
Sagitar Posted March 26, 2008 Report Share Posted March 26, 2008 does it mean when the tyre is on the car and the weight is squashing it? In a word, yes. Different terms are sometimes used, but "static radius"; "static load radius"; "normal loaded radius"; etc all mean the same thing i.e. the distance from the centre of the axle to the road plane with the axle normally loaded. This dimension is always going to be significantly less than half of the overall diameter of a free standing wheel and tyre. Tyre dimensions change, depending upon their situation. A tyre standing free of a wheel does not measure the same as a tyre applied to a wheel and inflated. The dimensions change again when the wheel is attached to a vehicle and used to support it. The dimensions change further when the vehicle is in motion and the temperature of the wheel, tyre and inflating fluid are raised and centrifugal and other forces apply. You cannot be absolutely precise about the dimensions, they will depend upon the nature of the variables that apply - even a free standing tyre will change its diameter a little when the ambient temperature changes. Manufacturers therefore quote dimensions under standardised conditions so that practical comparisons can be made. If you are doing torque calculations associated with the overall gearing ratio for a vehicle, the normal loaded radius may be more useful than the overall diameter of the free-standing wheel and tyre. Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted March 27, 2008 Author Report Share Posted March 27, 2008 ah, i understand that now. most tyre size calculator tools on the web appear to just use the overall diameter of the tyre when trying to compare wheel/tyre size combinations. i am thinking of going from a 4x12 inch wheel to a 4.5x13 inch wheel. my current tyres are 145/80R12. looking at that toyo page, the best match seems to be 155/70R13, though the diameter is slightly more. is this a good choice do you think? thanks. Quote Link to comment Share on other sites More sharing options...
Tony Posted March 27, 2008 Report Share Posted March 27, 2008 ah, i understand that now. most tyre size calculator tools on the web appear to just use the overall diameter of the tyre when trying to compare wheel/tyre size combinations. i am thinking of going from a 4x12 inch wheel to a 4.5x13 inch wheel. my current tyres are 145/80R12. looking at that toyo page, the best match seems to be 155/70R13, though the diameter is slightly more. is this a good choice do you think? thanks. I would opt for a 155/65R13 personally. Quote Link to comment Share on other sites More sharing options...
MondeoGhiaX Posted March 28, 2008 Author Report Share Posted March 28, 2008 thanks tony. i did look at that size, but i was put off by the fact that it is right on the lower limit of the rim width, whereas the other size is right in the middle. if it fits, it fits i suppose, but would it look ok? Quote Link to comment Share on other sites More sharing options...
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