Mountain Biking - Mountain bike sizing question...

Son got a mountain bike for Christmas, supposedly a 18". We sized based on info we gleaned from the net. The seat tube is marked 16.5 inches, however, and measures that as well (middle of bottom bracket to top of seat tube). To what is the 18" referring? Thought the seat tube reflected the bike size (mostly). Bike is a Thruster 29er hardtail he found on Amazon. Works well enough but this detail is grinding my teeth. How does one measure the size of a mountain bike?

Also, are your shoulders, especially between the shoulder blades, suppose to ache after an hour of riding? The trail I rode to get into this condition is rooty, mostly flat with some short climbs, and much twisting and turning. But maybe I am too old to ride (52) mountain bikes and otta stik to the road...

Read this: http://sheldonbrown.com/frame-sizing.html

Perhaps your figure of 18" refers to this...

Some bikes with slanting top tubes are measured as if there were a level top tube, they use the length that the seat tube would be if it was as high as the head tube.

If you put a spirit level on the top tube and raise the end until it is level you will find that the point where the level intersects with the seat tube is 18". This is known as effective top tube.

http://www.abikestore.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=thruster-29er-bike&Category_Code=&Store_Code=abs

Hard to say. looks like the bike is made in three sizes, 16" being one of them. You very well may have gotten one the wrong size. Unfortunately though, it's not like it's a name brand and you can ask the Mfg, and I can't find the geometry charts online. Assuming you goot some documentation I'd look for the mfg contact info.

And depending on what surface you're riding on, a hardtail MTB can beat you up. I'm 52 also. Mine can kick my butt :)

52 is not too old. We have plenty of riders in their 60s and older. Talk to santiago, gastro, mtnbiker66 or dminor. dminor is approaching the century mark, from what I understand (and I'm not talking about 100 miles).

OK, all that stuff does work out. "effective tube length" is as you said.

Glad I am not the only graybeard on the trails...