Rear tire clearance is hard
I’m almost done clearing out old projects and finally getting ready to build my first full bike frame. It’s going to be what Jan classifies as an urban bike. Kind of a light touring bike, drop bars, front porteur rack, 650B wheels. I want it to fit knobby tires without fenders or 40mm wide Hetre tires with fenders. To make this work I’m placing the fender mounts around 60mm from the rim. Here is a line drawing which gives the basic proportions:
The hardest place to fit these wide tires is the chainstay/bottom bracket area. I was playing with BG101 (an Excel spreadsheet) on the bus this morning and it shows this very nicely. This is what my bike might look like using a 55mm tire, 9 degree bend chainstays from Henry James, a 44t single chainring, Ritchey cranks (150mm tread) and a Rohloff hub:
Each grid mark is 5mm. Everything just doesn’t fit (the crank arm and chainring are both too close to the chainstay). There is a thin path through the chainring, crank arm, and tire which will let everything fit. A chainstay which makes that ideal path isn’t available off the shelf, so I’m going to have to modify (bend) what I can get. This is the hardest part of the bike for me, and the first thing that I’m looking at when I see other fat tired bikes on the road.
I’m glad that I avoided it on my first bike by using an existing rear triangle borrowed from another bike.
fatties fit fine, eh?
Can”t you just use duct tape or something?
Do you know any bike which has not chain stays –uhh deformed (sorry for m,y english I am not native speaker) in places which you are pointing to.
Most bike makers resolve these clearance issues by simple cold bending (means simply hammering) a bit of tubes inside so everything fits well.
Ladia — Many bikes have chainstays which are ovalized in that region. That is called a round-oval-round chainstay. Here is an example from my old Bridgestone XO-1:
http://phred.org/~alex/pictures/bikes/xo1/xo1-chainstays.jpg
You can also intent the chainstay as Alistair did on his 650B bike:
http://www.flickr.com/photos/duncancycles/2689389729/in/set-72157604494651833/
Most traditional steel frames have straight chainstays which aren”t manipulated this way. You don”t need to when the frame is built for tires up to around 32mm. It’’s when you get into 40-60mm tires that things get more challenging.
Longer chainstays make this easier (because the tire is farther from the bottom bracket) and shorter ones make it more difficult.
Mountain bikes typically have smaller chainrings and wider cranks which also makes this easier. It’’s not impossible to nail it and get fat tires, skinny cranks, and large chainrings, but it does take more thinking and measuring. It’’s these small things that make the difference between a good frame and a great one in my eyes.
iSn”t thiS wereS “S” chainStayS come in? Hardly classic looking unless you say it is an old English design.
So, have you considered a slightly longer wheelbase (silly question, of course you have)? It wouldn”t take much; even 1cm would help, and would make the ride more comfy and stable, as well as being able to handle loads on the rear a bit better. I know you”re well aware of all this, and a longer chainstay would probably still require some manipulation to wind up in all the right places, but it could make the whole issue a bit easier to deal with.
Andre — It is where S-bend chainstays come in. I think they are overkill for 50mm wide tires though, and don”t really care for the look.
Val — I already lengthened the chainstays 1cm to make life easier. Not sure I want to go 2cm. Part of this is the challenge, life isn”t fun without some good challenges.
I wish I had titled this post “Rear tire clearance is harder than it looks”. Guess I could change it. The goal was more to show some of the challenges in building fat tired road bikes rather than complaining.
Oh, and for the record a 5 degree bend will solve the issue. I can”t buy prebent stays with that bend, so I”ll have to do it myself. That isn”t too much work.
Very interesting article, i have bookmarked your blog for future referrence. Best regards