Ex DH rider trying to design and build a gravel frame, go get the popcorn

Happy new year to all you bike fab maniacs!

So the other day I had the opportunity to purchase a GRX groupset at a very good price and couldn’t turn it down. This obviously left me with no other possibility but to design and build a frame to mount it to.

But alas! Having only ever ridden (and built) squishy bikes before, that focus a lot more on descending scary rock gardens while stood up on the pedals, I have absolutely no clue what I’m doing, especially geometry wise.
So I asked chatGPT to give me a few geo numbers for my 189cm height/91cm inseam, and I also asked a few people who may or may not have some more experience with the geometry of a bike that is mostly ridden while sat down. This is what I ended up with so far: Feel free to criticise/suggest other numbers!

The fork I’ve taken the numbers from is a Ritchey 1 1/8" carbon gravel fork. I chose this one because it is less “meaty” than other carbon forks around the top end and I hope this blends in well with a paragon 44mm head tube and zs44 headset cups. Again, please correct me if this sounds stupid to you :wink:

Alright, a few more choices that are already pretty taken are the chainstays (Deda MPO240C2022) and Allotec C82 dropouts, because I already have them in a drawer… Apart from that, everything is pretty much up for discussion, especially tube diameters. So far, I’m shooting for a 35mm DT, 31.8mm TT and the seat tube, I’m actually a bit lost. Most tubes are too short for my size, and I don’t really understand the benefit of the external butting of many of the columbus tubes. Some advice here would be greatly appreciated!
Since I don’t have a tube bender, I will probably build wishbone seat stays (and because I think it looks cool). Also because it looks cool, I would like to have a dropped seatstay junction, which makes positioning the FM mounts a bit finnicky, but we’ll see…
So this is where I’m at so far:



Chainstay will probably require a bit of a dimple for clearance (thx @Daniel_Y for the measurements in ressources!), tire in this pic is 50mm wide and also probably a bit too close for comfort?

Excited to hear what you think! Oh and also, anyone in mainland europe ordering from paragon anytime soon? I may be in the market for a few things and shipping costs are less painful when shared!

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First thought is the geo looks solid but a bit conservative. I reckon if ya coming from a DH background I can imagine you’d want to be riding smoother blue trails on this bike as well as gravel/fire roads. If that’s the case I would look more at some of the “progressive” gravel bikes out there. This is a quick seach on gravel bikes with a 69deg (or slacker) HTA. Might be a good starting point. Personally I love the look of the @Amigo_Frameworks Bugout.

2 Likes

Looks good to me!

A few comments/questions:

  • With the UDH dropouts and those tapered stays, do you get enough “meat” on the CS to land the flatmount boss?
  • Tube diameter is probably would consider a frame that size
  • Seat tube:
    – I would reccomend a 27.2 seatpost. I think the majority of frame compliance comes from the seatpost and (anecdotally), 27.2 flexes way more than 30.6.
    – that limits you to a 31.8 topper
    – Since you have a dropped stay, you should probably use a straight guage 31.8x.9 seat tube.
    – The combination of the 31.8 topper and seat tube might make the seat tube very stiff and heavy… That is a total guess though.
    – If you go down to 28.6ST, the velospec seat tubes are nice: AH29/28110608-680
  • Segmented SS’s: since you have the CAD already, you should look into lasercutting the tubes! (this is my current homework project)
    Laser Cutting and Sheet Metal Services l OSH Cut
    https://www.ptlmfg.com/

Most tubes from Columbus and Reynolds are old school. They are too short for mountain bikes and big drop bar bikes. I would look at the velospec pro catalog: Velospec PRO (Cold Drawn, Air-Hardening Steels) to see if they have better lengths for you.

4 Likes

Thanks for all your input, greatly appreciated!

CS/FM meatiness is probably enough. I’ll fillet braze these in probably, so I can create extra meat :wink:

ST: Now that you say topper, that opens up new possibilities :wink: This method isn’t really common here in Europe I guess, haven’t really thought about it!

Laser cut SS:
I have done the data prep for a small batch production run of normal, bent seat stays in a previous job. Both mitres and a hole for the ss bridge were cut on the tube laser. It worked quite well, although bending after cutting leaves a lot of room for error :wink: There was no possibility to “3D-Cut” the mitres, so the cutting direction was always perpendicular to the tube surface. This has to be taken into account, especially with acute angles, small diameters and large wall thicknesses! The data prep was not easy, a lot of projections of cut lines onto surfaces etc…
For my home built single frame the process is probably not really worth it, especially since I’d like to use different diameters for all the segments.

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