I am trying to build a space frame bike from the n55 project. They have some pretty cryptic "designs".

What do these lines mean?

FRONT WHEELS 24”x1.75 equipped with a reinforced axle (M12, 8.8) for one-sided attachment,

from their deign pdf here.

I'm actually going to be using this wheel for a trailer and the trike.

I googled a bit and could not figure it out.

I believe 24 inches is the diameter and 1.75 is the width in inches.

But what about the M12 and 8.8 parts?

The axle is reinforced for strength because it only mounts on one side.

Commented Link Picture

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Eventually will be this:

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  • The axle sounds like a "wheel chair axle", where the wheel is cantilevered out and only mounted on one side.
    – Criggie
    Commented Nov 25, 2019 at 9:38
  • 1
    It is only mounted on one side. Added picture to Q/A Commented Nov 25, 2019 at 14:03
  • The 24” x 1.75 part is easy. The 24" is measured from the hub to the rim and the 1.75 refers to how fat or slender the rubber tire is. 24 x 1.75 is something you should have an easy time finding at BMX stores/websites. Commented Nov 29, 2019 at 7:58

1 Answer 1


Yes those are the tyre diameter and width in inches. M12 is a kind of metric thread, 12mm in diameter and 8.8 is a strength rating. I guess the axle is a piece of M12 threaded rod.

  • 1
    To elaborate on the 8.8 part: "8.8 is ISO 898–1 grading for Bolts and screws. It indicates a bolt/screw with tensile strength of minimum 800MPa and a yield strength of 80% of 800MPa = 640MPa." Commented Nov 24, 2019 at 20:25
  • 2
    And to elaborate more on the tire size--there are five different and incompatible wheel sizes that are referred to as 24". The one specified here has a bead-seat diameter of 507 mm. More here. Although in this case, you've got some flexibility on wheel size, and you might be better off with 20" (406 mm) wheels: the tires are much more common, and the spokes have better bracing angles, which is important for resisting lateral loads on a trike. I've never seen a recumbent trike with 24" wheels.
    – Adam Rice
    Commented Nov 24, 2019 at 20:38
  • To clarify my previous comment, I've never seen a tadpole trike with 24" wheels in front—or wheels larger than 20".
    – Adam Rice
    Commented Nov 25, 2019 at 1:56
  • @Adam - what is a "bead-seat diameter". I looked at the link you posted and re-posted that diagram they used in my question. I can't determine what part of the tire/wheel I am looking at. Can you describe that diagram please. Also are the terms wheel and rim used interchangeably? Commented Nov 25, 2019 at 14:20
  • @J.M. bead seat diameter (BSD) is the diameter across the wheel where the tires (specifically, the tire beads) sit. So, maybe tubular tires don't have beads, but clincher tires (i.e. the overwhelming majority of tires) have metal wire or kevlar beads which slip over your rim. BSD can serve as disambiguation. There are a number of tire sizes which start with "24 x ...", and they have different BSDs.
    – Weiwen Ng
    Commented Nov 25, 2019 at 16:03

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