My bike only has room for a 35mm tire between the chainstays, and easily a 45 in the front. it's a steel SSCX Traitor Crusade with swinging dropouts. I would love to run a 40-42mm tire in this bike (anything bigger and I might as well just ride my mountain bike which has 29x2.4 tires) but the chainstay space is the only limiting factor.
I've read several accounts of people successfully widening space between chainstays (as well as making room for certain chainring arrangements) by crimping and dimpling their chainstays. (do a quick search for "DIY chaintay dimple crimp for examples. frame builders have to do this to their chainstays, but they are smart enough to do this carefully, and do it before welding those tubes to the frame.) I started making a tool out of wood for this but I need to practice with some cheap tubing to make sure I can do it without cracking my frame. the frame already has slight dimples in the chainstays for this purpose; I just need to make them bigger.
The other avenue I am considering is a 650B conversion. switching from 700x35 to 650x42 would drop the BB of the bike something like 10mm - does that sound correct? more importantly, how can I best estimate if the switch would be enough to give me space for bigger tires. right now, the space between the chainstays at the widest part of my 35mm (Gravelking SK on a Arch EX rim measures more like 37mm) is about 40mm of clearance. the paint has rubbed off the inside of the chainstays, probably mostly from the frame and wheel flexing and the tire rubbing because it's so close. how far in would a 650x42 tire be from that space?
what else should I consider in a 650B conversion? I would be building my own wheels on a budget, so whatever decent aluminum rims I can get and probably Sapim spokes on ebay. I want to reuse my White Industries rear hub and singlespeed freewheel for this.
bigger question: am I really gaining enough by going from big 35mm tires to slightly smaller 42mm tires to make this all worth the effort?
mack_turtle wrote:how far in would a 650x42 tire be from that space?
bigger question: am I really gaining enough by going from big 35mm tires to slightly smaller 42mm tires to make this all worth the effort?
650b bead set is 292 mm in radius. The wide part of the tire is going to be somewhere between 24-36mm past that, depending on the side knobs. So measure the width around 316-328mm from the axle.
I have re-crimped a lot of bikes and never had a problem. It's easy if you have a big C-clamp, some scrap wood, and some basic woodworking tools. My only advise beyond what people have said on MTBR is not to use a big puck on the inside, but something more thinner like https://www.flickr.com/photos/feathercycles/6255347058/ . I think a more localized crimp will have less effect on your rear spacing.
I had this issue on my previous gravel bike. It was aluminum, so crimping wasn’t an option. I put a 40mm up front, 35 in the back, and a thudbuster seat post on it, and called it a day. Worked great actually. Also gives a touch more trail (slackened head angle). I need the traction and flotation up front – the rear width is just for my tookass.
The real advantage to going bigger (especially in the rear) is being able to run a lower pressure. You can (carefully) try running the lower pressure on the 35 to see if you really need it. But no, there isn’t a huge difference between 35 to 40 (especially in the rear).
I think we have talked me out of mutilating my frame and buying new wheels. I'll probably just put a bigger tire up front and call it a day. That's another topic though.
