Originally Posted by
unterhausen
.65mm is pretty good, if you have your decimal point in the right place. I think most people quote 1mm if they say anything at all. Cantilevering off of the BB shell contributes a lot of that, you are measuring a lot of errors. As point of reference, a large U.S. brand of bikes says 5mm is ok.
Steel tubes are often off at least 1mm over their length depending on your reference.
For low-tech measuring, I think putting the head tube on a rod between v-blocks is probably a better method. It's really easy to get off a little referencing to the bb shell and then where does that leave you? The bb shell distorts quite a bit during brazing/welding
Thanks, good info! Had no idea those kinds of tolerences were acceptable. I mounted a cheapo Jamis sattelite frame I had, and using this method it's all over the place. Several points well over 2mm out, but measurements seem consistent if I swap the frame over, or even mount it to other corners of the island. Not nearly as straight as that Croll frame I did in the pic. ...but, like you said, if the bottom bracket is wonky, everything will be off.