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.
Thanks!
The gravel world is a wild west of chain lines, chainring sizes, tire clearances, and chainstays.
I will create a resource for all the different standards, but can you fill in some blanks:
What 2X standard do you want? GRX 2x, road wide 2x, or road 2x?
Long chain stays (430+) and a wide set of cranks (Easton EA90 with 136 spindle, White Ind Adventure, ect) will save you a lot of heart ache.
This is the chain stay I use on all of my gravel bikes.
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.
Gravel bikes are in-the-saddle, ride-in-a-straight line bikes. If I can get buy in from customers, I just make the chainstays as long as I can with the dropouts/stays I pick out.
-Walt
To play the devil’s advocate:
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.
If I ran the world, I’d aim for 50/50 weight distro on gravel bikes. Yes, that’s right.
But I’m not so yeah, you gotta look at all the standards and figure out your priorities and then cram the tire in there.
-Walt
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.
I’m exaggerating a little with the 1:1 thing, but I’d probably do ~600-650mm front center and then 500-550mm chainstays if I were building a gravel bike for myself, just as an example.
Now, I’ll build myself a gravel bike when hell freezes over, but if I were to do it, I’d make myself a freaking Cadillac.
-Walt
Going for GRX 2x.
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.
That’s really cool, thanks!
I have a few of these that I want use:
LR26A-1 Flat Mount Dropouts M12x1.5mm
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.
Sadly I could not find any CAD or documentation on those dropouts. I had to do my best guess on where the dropout would land:
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.
A STEP file is available for these dropouts if that helps… Thanks Andrew at BFS
LR26 R_200420 (1).STEP (3.7 MB)
@Swood Thanks for the CAD!
I was also looking at the wrong LR26A-2 dropout 
Here is the correct design:
Here are a few comments:
Does anyone know what chainstay the NDS dropout is designed for? It calls for a very specific departure angle and inner diameter
Do people just manipulate their chainstays to fit this dropout?
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 added an extra degree to the angle of departure of the NDS chainstay: 10.5°>11.5°
Now the chainstay lands on the BB in a good spot.
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.
This is amazing! Thanks for helping a novice out like this, much appreciated.
The T47 upgrade could do the trick it seems and I can get the Zona CSs.
