From the start I’ve been all about full disclosure on this blog….so you get to share in my triumphs and face plants. Here’s one of the latter.
The last two bikes I made were singlespeed cross frames that were nearly identical. One for me, one for a friend. The main difference was that mine had a wishbone seatstay and the other had 16mm straight (half)fastback seatstays. I have been wanting to make a little Nifty Tool to check chainring and crank clearances on the drive-side chainstay for awhile now…but just never got around to it. But really, that would show me that i messed up after the fact (after mitering and tacking the stays). I really should just drawn up the crank and chainring clearances…I already had a drawing for rear tire clearance after all. Double DUH!
Well, what happened is I completely overlooked this one thing on BOTH these frames and had to put a huge dimple in the outside of the drive-side chainstay to get the SRAM Rival crank and chainring to clear. What a stupid dummy…lesson learned the hard way. I always mock up the tire clearances on paper and in BikeCAD, but just have neglected to mock up the cranks. For one, the info is hard to find online, and when you find it, it sometimes doesn’t have the measurements you need (it’ll have the distance to the inside chainring but not to the inner crank bolt that sticks out from the chainring). Chainline is even hard to find sometimes.
It’s the component part of building bikes that has turned out to be the most frustrating for me. The diversity in parts and their clearances, all the new “standards” popping up seemingly yearly (i.e,. For bottom brackets you have: BB30, PF30, BB86, BB92, PF30, eccentric BB, English threaded, Italian threaded, Bombay threaded (Ok, that last one was a joke)). But see what I mean? It’s silly! The advent of carbon mass-produced bikes has led to all the new ‘new standards’ for various reasonable reasons. But for steel frames, the good ole’ threaded BB is pretty much great. Anyways, where was I? Oh yeah, self-deprecating myself into the bottom of a pint glass.
So here are some photos of how I post-dimpled the outside of the driveside chainstay. Not pretty, but it works. I used some old oak blocks for this and a 4″ clamp. The inner is a 1.25″ block cradling the inside of the stay, the outside is block that I whittled down to an elongated oval shape for dimpling for tire clearance (before I got a small arbor press for the tire clearance dimples instead). The chainstays on frame 14 (orange one) are the same wall thickness as the stays on frame 15 but a different oval profile. They are both OX Plat True Temper. Holy crap it was hard to dimple 15’s stay. I actually had to change methods and put it in a table vice with a Paragon 1.25″ block on the inside of the stay and an aluminum formed dimpling block on the outside to get it to dimple enough. Never again (hopefully). Luckily, both frames are still fine to ride but they aren’t quite as pretty as before. What was interesting to me is that the dropout spacing didn’t change at all before and after dimpling on frame 14, but it did on #15. I just had to cold-set the driveside dropout back into 130mm axle spacing and it was right as rain.