631 and 4130 are similar density, but 631 is stronger. So theoretically you could build equal strength with a thinner guage tube using 631.
I think the FactionBMX company uses 631 tubing to achieve a lighter frestyle bike than its smaller-sized 4130 competitors.
1 x 631, 631 x 1
1 and 631
631 = DCXXXI
631 to the nearest hundred is 600. 631 to the nearest hundredth is 631.00
631 = DCXXXI
0.0301
0.0294
631 to the nearest tenths place is 631.0
901, add 631 and 270 to get
631 mm equates to approximately 24.8 inches.
There are about 8/5 times 631 = 1009.6 kilometers
0.0032