Originally Posted by
guy153
But why is it necessary to get the chain-stay length exactly right? Obviously if it's way off you will alter the whole geometry but a mm or so won't make any difference.
No 1 mm makes a noticeable difference. A bit off in chain or seat stay length is more important that other places. That is because of the ratio of length difference between the hub width and the distance to the rim/tire. That means 1 mm of stay length difference will move the tire/rim over almost 3mm. The tire won't just be 1mm closer to one side. That amount of tire offset is easily visible looking at where it is between the chain stays. If you off by 2 mm it is horribly off. I attached a picture with a 700C wheel in an old unpainted frame with Henry James horizontal dropouts. In one dropout I put a 1mm drill bit between the wheel axle and the back of the dropout. Notice how far over the rim is from being centered! In my opinion if I saw a frame with a wheel that far out of center, I would assume the builder is either lazy/careless or doesn't know what he is doing. If it was someone that sells frames, that is bad enough to give them an F grade. He is not yet good enough to hang out his shingle.
Getting the chain stay lengths exactly right is one of the biggest challenges in building a frame. That is why it is wise for someone starting out to choose horizontal dropouts so any length discrepancy can be compensated for by dropout screw adjustors. The problem can also take care of itself if using an IGH or a single speed in long slot dropouts.
I think every custom frame builder needs to hold at least a mm tolerance for wheels being centered. We should all take an oath that says we will hold to that standard. The problem is that a lazy/ignorant custom builder's work can tarnish the reputation of all builders. Even when they finally get to be acceptably good, their reputation (and by extension every builder's reputation) can be damaged by their 1st builds if they weren't up to standard.
By the way Guy153, this post isn't directed to you personally but rather is a general statement to all builders and potential builders of the tolerances we should all accept. We are one of the few trades that has no standards of knowledge or ability to enter. Except what we can do to encourage and complement solid workmanship online and call out poor workmanship as unacceptable.
