I noticed that the smallest blocks from Hiwin, the 15mm, have a dynamic load capacity of around 2300 lbs each (11K+ Newtons). Static load capacity is double that. Is this correct?

That's about equal to a 1-1/4" Thomson SMART bearing, and higher than that of a 1-1/2" Thomson SUPER bearing. Pretty incredible from such a small block.

So why do people even bother with 25mm+ blocks on lightweight routers? I'm thinking it's because of ebay.

you buy what you can find. ive got THK HSR12RM rails, simply becuase it was a killer deal at $60 each for 3 rails that are 26" long and come with 4 trucks each.

I managed to get two RSR 9 trucks and a 10.75" rail on Ebay for $36. This will be used on my small router for the Y axis. The smaller truck/rail combos are not that common. I'd love to get another one like it if I could.

Yep....that's the reason....what's available.....though....those 9mm rails.....well they still seem like 'toys'.......

I think a lot of people also don't believe that they'll carry the rated loads, so go with the "bigger is better" approach. Also, the smaller ones don't come in longer lengths, so bigger may become necessary just to get the lengths you may need. But yes, the biggest factor is what can you get from Ebay.

I've gotten 2 pairs of 26" long 15mm IKO's that I plan to butt end to end to make a 48" wide gantry. $130 for 4 rails and 8 blocks. And a pair of 18" 15mm THK's with 4 blocks, $40. And about 6 months of waiting on Ebay 'til they showed up. :)

I just decided to search for rails again after not looking for a while and ended up with two 22.5 inch 2omm thk rails with two blocks each for 25 bucks, they are in excellent condition. I was lucky to find them, they didn't say thk or anything in the title, but the blocks say it I was stoked.

Yep....that's the reason....what's available.....though....those 9mm rails.....well they still seem like 'toys'.......

They may be small but there is a set of them on the Y-axis already, I'm going to redo the Y-axis and make it longer. Heck the Z-axis has a 7 mm one. http://www.cnczone.com/forums/attachment.php?attachmentid=18614

This machine is going to be for the small, small stuff. The spindle motor is going to be a Bien-Air brushless motor and controller, just a leftover dental electric handpiece

I dont know what static and dynamic loads are or how to calculate required rail size.A friend recently purchased a5x10 Thermwood router.The rails are 20mm.The machine weights 8000 lbs I guess the gantry must be 3000 at least {10 Hp spindle and auto tool changer} Now I know 20mm rails are overkill for a DiY CnC .
Larry

One thing to keep in mind is that while these bearing rails are in fact very sturdy, the main loading you need to concern yourself with often is not the static or dynamic load, but the moment load. Particularly on the rails on a moving gantry, if you have a tall Z axis, there is a significant lever arm acting on your bearings. Under large cutting forces, with a long Z axis, you could exceed the allowable moment load of some of the "wimpier" rails.

This is not to say that any of the previous comments have been wrong -- most of the rails out there are super strong, and are more than adequate for DIY, especially for the rails supporting the gantry. However, moment loads should be considered, and are fairly easy to calculate "back of the envelope". Force X Distance. I usually use a higher number than I think I'll actually see, like 300 lbs. of force on my spindle, to make sure everything is beefy enough.