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alloy wheel width

I've read a few threads on here about wheel sizes, getting the right offset/PCD etc. and it all went over my head. decided to look into it, and i kinda grasp it all now. glad to hear it you say, now get to the point.

one thing that i still don't understand, and haven't been able to find any explaination of, is when talking about the width of an alloy. sometimes they're listed as 7, 8 etc, and sometimes they're followed by a 'j'. for instance, new wheels for my brothers golf, the recommended width is 8j. if someone could tell me what that stands for i'd be much obliged.

  RenaultSport clio 172 mk2
Look at the cross section of a wheel and you see there are lips on the outer edge of the rim that the beads of the tyre are inflated out against. And there's deep trough in the centre of the rim so that you can fit the tyre to the rim.

The problem has always been with that way of making wheels is that if your tyre went flat while you were driving the beads of the tyre wouldn't continue to be pushed out against the outer lips and they'd immediately collapse inwards, fall into the trough in the middle of the wheel. You'd have no braking. The tyre could come off the rim completely. A blowout would result in instantaneous catastrophic loss of control.

So someone had the idea of machined a small lip into the rim inside of where the bead is normally seated against the outer lip of the rim.. That lip would stop the beads of the tyre from falling into the trough of the middle of the rim if deflation occurred. That way loss of control when a blowout occured would be less catastrophic. The tyre would lose pressure but would stay pretty much in place. And that's what the "J" stands for. That the wheel is machined to have those retaining lips there there. Of course ALL wheels for road use are required to have theme these days, so all it really tells you is that its not illegal for road use. They make it harder to fit a tyre unless you have a fitting machine because the beads of the tyre have to be stretched over the retaining lip, but it makes blowouts safer.

There have been further advances since "J" was thought of. You'll find some wheels are labelled JJ. You can guess what that means.