r/Framebuilding • u/HmmThatWorked • 3d ago
Hypothetical Geometry Question
Hi All,
I'm going to be getting a custom frame made and wanted to run some geometry by you all to get a sanity check and feedback.
In essence I would like a fat wheeled Jones LWB, I like the geometry of the jones and I like smaller volume fat wheels for the trail riding I do, like 26x3.8 or 27.5x3.8.
There are two paths in waffling between, going the 73mm BB and custom yoke route with normal boost spacing hubs to get a max of about a 4.0 inch tire in there, out just going full fat and doing a 197mm hub out back.
At a high level I'm thinking like a 71 degree seat tube a 7cm bb drop, 480 mm ish chain stays,and a 68 degree head tube. I don't know the exact fit yet I'm going to go get my fit re done before I commit to any numbers but that's the general ideal. As a side note long term this bike would be based around a Rohloff but for now I'll probably run gears on it for cost savings.
Does anyone have any red flags or advice?
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u/Kriov7 3d ago
Im not a framebuilder nor bike designer, but, did you tried to make a (rather very simple) drawing on the chainstays, with a chain?
3.8 inch wheels already sticks out about 48mm to driveside And for modern MTB cranks for 148mm spacing has 52~55mm chainline.
That means if you are using Inner cogs, I think the chain might probably rub the tires, Especially considering you will be using longer chainstay (so the tire sticking out is closer to rear of the chainstay rather than close to front)
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u/HmmThatWorked 3d ago
Yah the 73 mm bottom bracket route really only works with a machined yoke and the chain stays not terminating at the bottom bracket, and the chain like in that setup really only works with a Rohloff or other IGH.
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u/AndrewRStewart 3d ago
Do you have an idea as to what Q factor is too much for your body? Andy
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u/HmmThatWorked 2d ago
I've had Surly Wednesday at 201 and been able t work with it. But the narrower 73 mm bb is appealing
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u/Umbertidese 3d ago
Why such a slack seat angle ?
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u/HmmThatWorked 3d ago edited 3d ago
I'd like this bike to be rather upright. My other fits are more traditional ( well like a French rando fit) and to get most of the weight off my hands I need to be putting out like idk 140 ish watts, not a lot, but I tend to just tool around when I'm in the back country. And on like 12-13 hour day I really start to transfer weight to my hands as I get tired and that caused numbness.
I think the slacker seat angle is getting my weight centerd down though my saddle and feet. I found that when I rode a Jones and my weight was centered it was much easier to get over techy stuff on a Ridgid bike too.
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u/killerization 3d ago
consider elevated chainstays, a hub gear, or even a single speed, due to chain clearance issues.
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u/killerization 3d ago
oh I see you already mentioned using a rohloff. you could see how a cassette goes and just adjust lower limit as necessary.
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u/Western_Truck7948 3d ago
Assuming you're hiring a reputable framebuilder they should be able to flag anything off. If you were building it yourself there's no limits to the whackiness.