ciquta

I'm building a custom Ti frame for light gravel purposes.

I'm not sure where to put rear brake caliper, since I opted for external cable routing under the TT I thought it would be better to go for seatstay mount.

The frame builder suggests to put a vertical bridge to increase stiffness, is that right? Would it be better to switch to CS brake mount then?

Worth mentioning i'm a big guy 220lbs and chainstay is 470mm long here.

Andrew R Stewart 

It's a pretty well known design, having a brace between the stays at the disk caliper mounts. I would follow your builder's advise.

As to caliper location- that is often driven by other aspects. Like rack or fender compatibility. Or chain/seat stay angles and will a caliper fit between them. Unless having a certain cable routing is vital I would let the caliper location determine the cable path.

I think the opening line should read more like "I'm having a Ti frame built for me, for gravel riding". Andy

dsaul

It would be better to use a "flat mount" on the chainstay, since most road and gravel groups come with flat mount calipers. If the builder is still using "post mount" caliper mounts, then a bridge is definitely necessary between the seat stay and chain stay for a seat stay mounted caliper.

ciquta

At least here in EU the post mount it far more popular now. However the fork is post mount, i don't see why not going the same at the rear.

This is the 3rd frame I'm building with same specs (included post mount) and this is the first time I've been suggested to put the bridge.

Why is it necessary in your opinion?

dsaul

All of the force from braking is concentrated at the front edge of the post mount and that force is very likely to crack the seat stay at the weld joint. The bridge transfers that load to the chainstay and prevents the seat stay from flexing under braking. Maybe it won't happen with a 19mm stay, but it is good practice to use a bridge there.

ciquta

In my previous build I had rack eyelets even with SS mount, no problem

I'm gonna route under TT, hence the SS caliper, but i really don't have preferences. Or should I?

duanedr 

What Andrew said. Listen to your builder. They are the one who has to stand behind the product and it carries their name.