I’m building my first gravel/cross bike. I have an issue in figuring out the best approach to BB size and how to manage chainstays.
Goal: 45mm 700c tire on standard 68mm BB.
Issue: I think its very difficult to make room for everything: pedal arms, 2x chain rings and the tire of course. What are some good ways you go about this? S-bended chainstays, cutouts or wider BB? Open to all ways of solving this.
Right now I can make room for a 35mm tire. But its tight.
I agree that longer chainstays are the answer. I don’t think there is much advantage to super-short chainstays on gravel bikes other than bragging rights. I see some production gravel bikes with dropped chainstays and a huge amount of clearance to the seat tube and wonder what their design criteria was that led to this.
Smaller bikes should have shorter chainstays if you want to keep the weight distribution the same
shorter is a relative term
I created this thread for gravel standards:
Short, long CS, 1x, 2x, is all fine. But in general, I think it’s a good practice to set the design goals first (handling, tire clearance, fit, q-factor, gear ratio) and figure out the fabrication, tooling, engineering, and compromises required to get there.
Walt, by 50/50 do you mean by ratio standards (FC/RC = ratio : 1) I believe another poster in another thread said they feel 1.75:1 is equal front wheel / rear wheel traction for them. Or do you mean literally FC = RC as in 1:1 ratio?
IMO, after a lot of off road touring experience on tires ranging from 33mm to 2.2", the longer chainstays, the better. I feel slower on climbs, but I ALWAYS make it up! And the descent is nice and planted.
Right now Im designing for 420 CS. Based on the this Factor I really like. Im more comfortable with standard 68mm BB as this is what I usually work with. I don’t know anything Q factors and how to do a fit for an expanded Q factor.
Everyone is different and has different Q factor preferences. I generally try to minimize the q-factor because I think it works better for most people, and you can always space out the cranks at the pedals.
FYI, Shimano makes a narrower XTR pedal to directly address the added 5mm of Q factor of GRX.
If you tell me what dropouts you plan to use, I can use CAD to see what kinda chainstay fits, and how much dimpling would be required.
I’m not sure of an off the shelf chainstay set you can buy that will fit the 47.5mm GRX Chainline and say a 50mm tire, especially with 420mm stays. Hopefully someone knows of one, or perhaps you’re going for a 33mm tire or something similar.
Perhaps an important thing to think about is if your tire will hit the GRX front derailleur. I’ve heard of that being an issue.
This is with a Zona CX chainstay, which is generally readily available and easy to manipulate for gravel bikes. Maybe someone can chime in who has used that dropout before.
Don’t know if it helps, but here is a 1:1 PDF of the GRX standard, 45mm tire, and 420cs GRX 2x 48-31t 45mm 420cs.pdf (185.9 KB)
If you print it 1:1 on a 8.5x11 paper, you can physically lay out your design.
I don’t have a good way to manipulate(bend) pre-bent stays.
But in my experience, you can gain a couple of degrees by slightly shortening/thinning the outer edge of the plug and cut the flat end of the chainstay with a slight angle.
Not the same dropout but the above method was applied, you can sort of see the chainstay meeting the plug at an unnatural angle (from the plugs perspective at least)
I’ve been experimenting with smashing chain stays with broad, wooden forms with the goal of creating a flatter tube without a traditional dimple. Initial results are satisfactory. I use the Dedacciai MTB stays, which already have an S bend. The dies I cut out of wood follow the existing curve of the tube on both sides. I put the whole assembly in my hydraulic press (arbor press would work too) and smash until it looks good. I don’t have a stop, so not great repeatability or precision so far - but it works!
I’m only building for 1x but there is still precious little room to work with. The GRX cranks or other combos in the ~50 chainline ballpark have been the answer for me to get 430 mm stays and 45 mm tires without using a yoke. Shifting could be better, though. 12 speed drivetrains seem to be optimized around a 45 mm chainline which has me thinking about using boost hubs for my gravel bikes. I have a decidedly un-Kansan approach to gravel, though - I need a gravel bike to handle steep climbs, descents, and tight corners, so I build them a lot like mountain bikes.