3 replies to this topic

[bluesprite]

Hi, I am thinking about refurbishing some cooper alloy wheels and it says on the back they are 12B4.5 and I was wondering what the B meant because I always see J everywhere.

http://www.theminifo...es-it-all-mean/

This says that J is standard so what does the B mean? will I still be able to fit normal 145 tubeless tyres?

Thanks for your help!

[bluesprite]

bump for 'B'

[Dan]

This is a truly awful subject to try to get into. Essentially the J or B (or JJ or K etc.) defines the wheel flange shape that the tyre bead presses onto, according to a table. I've copy and pasted someone else's attempt at making this easy to understand from a place called Car Bibles. http://www.carbibles..._bible_pg4.html.

Like so many topics, the answer as to which letter represents which profile is a long and complicated one. Common wisdom has it that the letter represents the shape. ie. "J" means the bead profile is the shape of the letter "J". Not so, although "J" is the most common profile identifier. 4x4 vehicles often have "JJ" wheels. Jaguar vehicles (especially older ones) have "K" profile wheels. Some of the very old VW Beetles had "P" and "B" profile wheels.
Anyway the reason it is an "awkward topic to find definitive data on" is very apparent if you've ever looked at Standards Manual of the European Tyre and Rim Technical Organisation. It is extremely hard to follow! There are pages and pages (64 in total) on wheel contours and bead profiles alone, including dimensions for every type of wheel you can think of (and many you can't) with at least a dozen tabled dimensions for each. Casually looking through the manual is enough to send you to sleep. Looking at it with some concentration is enough to make your brain run out of your ears. To try to boil it all down for you, it seems that they divide up the rim into different sections and have various codes to describe the geometry of each area. For example, the "J" code makes up the "Rim Contour" and specifies rim contour dimensions in a single category of rims called "Code 10 to 26 on 5deg. Drop-Centre Rims".
From the tables present in this manual, the difference in dimensions between "J" and "B" rims is mainly due to the shape of the rim flange. This is the part in the diagram defined by the R radius and B and Pmin parameters. Hence my somewhat simpler description : tyre bead profiles.
Note that in my example, the difference between "J" and "B" rims is small but not negligible. This area of rim-to-tyre interface is very critical. Very small changes in a tyre's bead profile make large differences in mounting pressures and rim slip.

Which is all very helpful in that it at least describes what's going on. Bascially you really need tyres designed for the type of rim you have or they may well slip. They won't be making enough contact with the rim to grip it properly, the speed and pressure ratings will not be accurate I would imagine. There are some tubeless tyres in Mini friendly sizes made for a B rim, Yokohama do some for B and B/J as do others. You will have to look into it with the manufacturer you are interested in.

(The above paragraph is copyright Chris Longhurst / www.carbibles.com, used with permission)

[bluesprite]

Damn I was hoping to just put on the cheapest tyres I could find because I don't really drive fast and am quite miserly. I guess that might bump up the price of the cheapest tyres I can find to fit the wheels then

Thanks for the help! I really couldn't find anything about it online :)