I'm trying to understand wheels better and I googled Bead Seat Diameter and this image comes up a lot ...

enter image description here

There are two components that look to be the exact same with one flipped vertically.

Here is a picture of a 20 inch wheel for example:

enter image description here

What is considered the "bead"? Is it that little circle. A bead is normally just one small item on a necklace ( google ).

But in this case if it is that little circle, would the bead be something that creates a torroid and goes around the entire wheel?

From what I can tell the tire and the spokes are not depicted in the diagram, just the "metal" frame cross section.

  • "From what I can tell the tire and the spokes are not depicted in the diagram." Correct, the hub is omitted too and the cross sections through the rims ("metal frames" as you put it) are closer together than they would be on a real wheel. Nov 25, 2019 at 18:41
  • Seems like wheels and rims are used interchangeably but normally they are made of a metal structure of frame. I imagine sometimes they are made of some kind of fiber as well. Nov 25, 2019 at 23:47
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    Often people will say 'rim' when referring to everything between the tire and axle (presumably because that's the practice when talking about car wheels). On a bicycle wheel, it means the hoop that the tire sits on and spokes attach to. Also people will say 'tire' when they mean the entire wheel. You are correct that rims (and sometimes spokes) are made of carbon fiber composite. Nov 26, 2019 at 1:34

2 Answers 2


The bead on a tire is a raised tubular section that goes around the inside edge of the tire on both sides.enter image description here

The diagram you have attached has no bead. It is a cutaway diagram of a rim/wheel, which does have bead seats (where the tire beads "lock into" the rim). The diagram is a bit confusing because it cuts out the center of the wheel (spokes and hub) to just show the cross sections of the rim. The bead seat diameter is a measurement on a rim that determines what tires would fit onto it.

BSD is only one piece of making sure tires and rims will work together. Width is also an issue. For instance, both 29'r MTB wheels and tires and 700c road wheels and tires have the same BSD (622mm), however due to width issues (of both tires and rims) many different combinations of tires and wheels in these categories will not work together.

Sheldon's page on this may be a bit more enlightening.

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    Damn a picture is worth a thousand words! Nov 26, 2019 at 21:48

The bead is the wire at the inner edge of the tire, which creates a bulge in the edge that locks into the groove in the rim. The word "bead" here is more akin to the woodworking term bead than to the "small ornaments on a string" sense.


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