Titanium custom gravel frame

[mirphak]

I am designing a custom Titanium frame with high stack and short reach/top tube (I am 175cm tall, but with 86cm inseam), for wheels of up to 700x50C (around 365mm radius ). The geometry I am thinking of is the following

Any thoughts? Do you think 430 mm for the chain stays at 74 deg seat tube is enough to accommodate those 700x50C?

https://www.bikegeocalc.com/#6Gravela0b4c788.40879d290e639.56462f809.08132g1020.55889h448.9244i365j365k1386.85808l891m1163.40879n725.04336o1218.9371p967.61903q1220.65498r1352.51523s970.65556t31.8u40v10w0x10y280z40A0B150C0D394.71974E45.00002F172.5G30H30Z11Flyxii+FE02a0b4c770.11626d273e612.54127f788.40457g999.79162h427.91377i340j340k1360.99736l841m1141.11626n698.86577o1188.67371p938.0636q1201.90652r1333.76677s941.10012t31.8u40v10w0x10y270z40A0B150C0D395E49.54881F172.5G30H30Z

[Wet Noodle]

Rear tire clearance looks tight on the drawing. I'd personally go for an even shorter chainstay (my personal kink), but a bent seat tube. Not sure, if they can do that with their tubing. In addition, the connection point between the seat tube and the bb shell can be tweaked a little forward. However, unless you go for an s-bend, it should be noted that your seat tube angle now changes with saddle height (doesn't matter if you know your optimal height and don't change it).

Also 395 mm a-c on the fork looks way too short if the wheel radius is 365 mm - you need some space for the blades to connect to the shaft (I'd guess 40...45 mm for unicrown and decent-sized blades), and then some clearance above the tire. I just checked my own custom rigid fork which is literally designed around a 55-622 Schwalbe Almotion (read: wide slick) and a permanent fender: ~ 420 mm a-c and it's on the tight side. With knobs that could pick up gravel and mud, I'd have gone for more clearance.

[Sakizashi]

With the design as configured it doesn't work unless you have some tube shaping done. Putting this into bikeCAD you have only 2mm of clearance. You could ask them to offset weld the BB which would help. You could also move to a 75 deg seat post angle and use a set back post or consider dropping the BB even more and lengthening the chainstay.

Plenty of ~400mm A-C carbon forks are out there that will swallow a 700c x 50mm tire. Bent seat tube is a good idea but might make getting the FD working a challenge.

IMO if you are going to try this, I would get a copy of the SRAM Frame Fit guidelines (they offer more detail than Shimano in terms of documentation for min chainstay length, angle between the CS and the FD mount, etc.) and work the design in bikeCAD until you are happy. Then reach out to XACD and Waltly to see what they can do. They have different levels of capabilities and different pricing models. Historically Waltly has been cheaper, but XACD can some pretty nice custom machining if you can be specific about what you are asking for.

[Wet Noodle]

Plenty of ~400mm A-C carbon forks are out there that will swallow a 700c x 50mm tire.

Oh ... I was being an idiot and didn't even consider anything else but a metal fork made with/for the frame (but I guess, even then you could get pretty low with a low crown and/or integrated bearing seat or something).

[jannmayer]

If you are going to run 2x, you may want at least 435 mm chainstays to ensure clearance between the tire and front derailleur. That is what I am running and it is fairly tight.

[mirphak]

Wow, thank you very much for the comments. Definitely many welcomed expertise here.

I am actually unsure what drivetrain to run, but I am more inclined to run it with my GRX with garbaruk cage (1x11) for simplicity, with a 9-46,9-50/10-50 on the rear and a 38 or 40 in the front, or something on those lines. Since it would be more of an adventure bike, I honestly do not care too much about having the optimal cadence at all times, I can adapt. So maybe 420-425 is enough then provided I can get the maker to tune a bit the seat tube?

A 75 degrees is definitely tempting, but I did not put it in the design because I saw no gravel frames with that (although it is a rather common angle in 29ers nowadays).

I actually have (with my current 73 deg frame) a problem: the horizontal distance from the saddle to the BB is just too long for my body / legs (thinking purely about optimal geo for road bike usage). My bikefit guy recommended me to even find a seatpost that brings me 20mm closer, so I currently use a saddle with 20mm setback but reversed, looks ugly and silly as hell but works.

So maybe a 75 deg seat tube can solve both issues at once. It brings me 25mm closer, I could even lengthen a bit the top tube so that there is less toe overlap, and it helps a bit with rear tire clearance. Together with some shaping in the tube that may actually work, right ?

Here's the updated link. Shadowed my current carbon frame (flyxii fe02)

https://www.bikegeocalc.com/#6Gravela0b4c783.33001d290e643.56773f811.59995g1024.10363h456.62754i365j365k1390.94764l890m1162.5605n723.72262o1218.19613p966.61903q1219.80669r1351.66694s969.65556t31.8u40v10w0x10y280z40A0B150C0D395E50F172.5G30H30Z11Flyxii+FE02a0b4c795.11626d298e637.54127f813.40457g1024.79162h452.91376i365j365k1385.99736l866m1166.11626n723.86577o1213.67371p963.0636q1226.90652r1358.76677s966.10012t31.8u40v10w0x10y270z40A0B150C0D395E49.54881F172.5G30H30Z

[mirphak]

Oh ... I was being an idiot and didn't even consider anything else but a metal fork made with/for the frame (but I guess, even then you could get pretty low with a low crown and/or integrated bearing seat or something).

Yes, the idea is to use a carbon fork. Plenty of 395mm and 400mm forks out there which are advertised for around 45-50mm.

[Sakizashi]

Wow, thank you very much for the comments. Definitely many welcomed expertise here.

I am actually unsure what drivetrain to run, but I am more inclined to run it with my GRX with garbaruk cage (1x11) for simplicity, with a 9-46,9-50/10-50 on the rear and a 38 or 40 in the front, or something on those lines. Since it would be more of an adventure bike, I honestly do not care too much about having the optimal cadence at all times, I can adapt. So maybe 420-425 is enough then provided I can get the maker to tune a bit the seat tube?

A 75 degrees is definitely tempting, but I did not put it in the design because I saw no gravel frames with that (although it is a rather common angle in 29ers nowadays).

I actually have (with my current 73 deg frame) a problem: the horizontal distance from the saddle to the BB is just too long for my body / legs (thinking purely about optimal geo for road bike usage). My bikefit guy recommended me to even find a seatpost that brings me 20mm closer, so I currently use a saddle with 20mm setback but reversed, looks ugly and silly as hell but works.

So maybe a 75 deg seat tube can solve both issues at once. It brings me 25mm closer, I could even lengthen a bit the top tube so that there is less toe overlap, and it helps a bit with rear tire clearance. Together with some shaping in the tube that may actually work, right ?

Here's the updated link. Shadowed my current carbon frame (flyxii fe02)

https://www.bikegeocalc.com/#6Gravela0b4c783.33001d290e643.56773f811.59995g1024.10363h456.62754i365j365k1390.94764l890m1162.5605n723.72262o1218.19613p966.61903q1219.80669r1351.66694s969.65556t31.8u40v10w0x10y280z40A0B150C0D395E50F172.5G30H30Z11Flyxii+FE02a0b4c795.11626d298e637.54127f813.40457g1024.79162h452.91376i365j365k1385.99736l866m1166.11626n723.86577o1213.67371p963.0636q1226.90652r1358.76677s966.10012t31.8u40v10w0x10y270z40A0B150C0D395E49.54881F172.5G30H30Z

I think this is buildable, but would likely be 1x only.

Ive played with 650b x 2.2 and shortest stays possible even with a seat tube bend and 85mm BB drop are 410mm for 2x. The yoke would have to be a work of art to make that work. Most likely, I think you would need to up to 415m. With a 700c x 50mm but 1x, I think you could shoot for 405mm but again chainring clearance will be the challenge.

[Wet Noodle]

With a 700c x 50mm but 1x, I think you could shoot for 405mm but again chainring clearance will be the challenge.

Off topic: If you pull out all the stops, you'd certainly get below 390 mm. But that is only really for fetishists and would indeed be a challenge ...

(Here is a somewhat extreme example of what could be done with a wide(ish) t47 bb shell with tire cutout.)

[Sakizashi]

Off topic: If you pull out all the stops, you'd certainly get below 390 mm. But that is only really for fetishists and would indeed be a challenge ...

(Here is a somewhat extreme example of what could be done with a wide(ish) t47 bb shell with tire cutout.)

Does that fit a road or gravel crank with 50mm of clearance? The issue is space for a 44t or larger chainring on the driveside.

[Wet Noodle]

Does that fit a road or gravel crank with 50mm of clearance? The issue is space for a 44t or larger chainring on the driveside.

Short answer: It kind of can be done, but you'd need rather special road cranks for it. The issue is not chainring size; it's cranks being generally weird.

At the absolute minimum chainline position, there is space for a 46 or 47 t chainring no problem. The issue is not chainring size, but that there is virtually zero lateral space between the ring and the stay. Meaning, you'd need a crank with the spider offset to the right side of the chainring. That may sound very trivial from an engineering standpoint ... because it totally is. But being bound by what you can buy off the shelf, you're now basically looking at frankensteining together a very custom crankset from modular crank parts. Or having stuff machined. Or go very wide (which nobody wants, because it sucks ass, in a bad way).

[Sakizashi]

Short answer: It kind of can be done, but you'd need rather special road cranks for it. The issue is not chainring size; it's cranks being generally weird.

At the absolute minimum chainline position, there is space for a 46 or 47 t chainring no problem. The issue is not chainring size, but that there is virtually zero lateral space between the ring and the stay. Meaning, you'd need a crank with the spider offset to the right side of the chainring. That may sound very trivial from an engineering standpoint ... because it totally is. But being bound by what you can buy off the shelf, you're now basically looking at frankensteining together a very custom crankset from modular crank parts. Or having stuff machined. Or go very wide (which nobody wants, because it sucks ass, in a bad way).

I see, that's another tradeoff that I wouldn't make. It would change the chainline and again violate the frame fit specs from the groupset manufacturers even for 1x. At shorter chainstays you have even less margin of error to play with in that respect. You have a min 395mm CS length to be compatible anyway. IME violating the frame guidelines quickly degrades shifting quality and leads to dropped chains, ghost shifting and all sorts of fun.

For a single speed setup like in the photo; sure it doesnt matter and it looks like it would be an interesting bike to ride.

[mirphak]

Amy (Waltly) got back. Most of it doable it seems, but she suggested to not go very aggresive with short stays because it would require heavy yoke-like chainstays which would make fully internal cabling difficult or impossible.

She made some suggestions: 52-52mm of head tube for internal routing and T47 bottom bracket.

[mirphak]

We have a first design, and overall it seems to address most of my request.

The only missing piece, and I wanted to ask you, is this

From the drawing, I believe that is the chainstays looking up. In my understanding, the only way they could design that to fit the 42T chainring and the 50C at the same time is by putting a flattened part in the right chainstay. That means that the cable needs to go out around that part, correct? Do you have any ideas or suggestions of how can I avoid this (if at all possible)?

[Wet Noodle]

I missed that. You want internal routing? I mean, I get that it looks nice and all, but do you assemble your own stuff (or do you pay some poor sod to do it, in which case it's no problem)?

I assume, we're talking mechanically actuated rear mech (thick Bowden tube and all, is my point)? With that particular flat-piece half yoke thingy, there is no way to go 100 % internal. But why bother? Nobody will ever see those 20 cm of exposed cable behind the chainring / under the bb. I personally wouldn't even bother with reentry/exit on the chainstay, but I digress.

It could be done with a hollowed/tubish yoke piece (as Waltly suggested, for other reasons (and also advised against)), probably something they need to machine (that sound right now is a spinning $ wheel). You would probably lose a bit of chainring clearance (most of it except for the outermost position or so). Alternatively with flat bar, a split design (think: channel from bb shell to chainstay) might also just barely work, if you're a bit adventurous. Any of that would be it's own medium-level pita for very little gain.