3 Rim/spoke questions

[epidemiks]

1. Sizing confusing the hell out of me:

A 2.50 x 18 rim is not 2.5" wide, rather 3.14". So what does the 2.50 mean?

2. Relacing a new 36 spoke rim, from 23" to 18". Rim I'm looking at has similarly angled punched holes, but given the size drop is that good or bad? Wheel building is prohibitively expensive down here, so I'm looking to DIY it then have it trued by a pro. Confusing business all this.

3. And, finally, using an alloy rear rim on the front: is it feasible, flipping the rotation as you would a similar tyre swap, or should I steer clear of this idea completely?

Cheers!

 

[mydlyfkryzis]

Does this help?
The recommended width is bead to bead, not overall width.

The pics show how the bead is measured.

As to the second and third questions: The angle changes a lot on the spokes whebn going from 23 to 18 in. diam.
There is no rear or front rim specifically. Since hub diameters change, the rim spoke hole angles just need to be the correct angle for the diameter hub.

 

[epidemiks]

That helps a great deal! Thanks. Tried to google rim profiles but didn't come across ones with bead measurements.

So the tyre to rim table is referring to bead to bead, not outside to outside widths?

Re the front/rear issue, the rims I'm looking at are advertised as rear, but you're saying that shouldn't make a difference?

Thanks again.

 

[mydlyfkryzis]

The tire size is themselves overall width. A 3.50 x 18 tire has about a 3.5 inch overall width. t
the bead width on a 3.50 x 18 tire requires a 1.6 to 2.15 inch bead width. The wider bead width gives better handling than the smaller. I am using a 2.15 rim on a 3.50 tire and like it much better than the stock 1.85 rim.