Custom frame help – Brake mounts and thru-axles

I’m having a custom steel frame made for me and I’m stuck on a couple of small details. It’s going to be a rigid 27.5 drop bar mountain/touring bike. Similar geometry to Surly grappler/ Tanglefoot hardtack but with lower trail and sizing suited to my long legs/short arms/bad back.

I’m deciding between post mount and IS mount brake mounts. I’ve seen some frame/fork sets that have post mount on the frame but IS on the fork, does anyone know why manufacturers would do this?

For the thru-axles I understand there are a few thread pitch “standards”, normally I would just make do with whatever the manufacturer gives me but since I can choose, are any more solid or future proof than others?

Thanks STW!

For the brake mounts (of a drop bar build) why would you move away from flat mounts?

With regards thread pitch I’d be inclined to look at DT or Rockshox but quite honestly all are much of a muchness.

I’ve been lucky to have two custom frames built  – one uses DT RWS axles front and rear the other uses a Shimano thread pitch as thats what the dropouts (sliding) used.

Came here also to say why not flat mount for drop bars?

ISO on the rear is easy to make as the dropout face and caliper mount are on the same plane and flush – so one laser cut plate dropout does everything and is ready aligned. That is for qr / simple bolt in e.g solid axle singlespeed hubs / nutted axles eg hub gears. Probably needs a little bit of machining if a through axle hub – e.g. 3.5mm deep pocket for the end of the hub axle to sit in which is (142-135)/2. Presume just the same pocket for boost but wider dropout spacing, but I’ve never made a through axle or boost frame so no experience.  ISO rear also good if you want slotted caliper mounts for singlespeed / hub gear.

Post on the rear can be better from a packaging point of view as it can more easily fit the caliper inside the triangle gap between chainstay and seatstay (in some cases still needs a bit of seatstay curving to make a big enough space). More work to design and align everything. Same for flat mount but probably easier again to fit in the triangle.

Never made a fork so no experience to share. ISO is probably easiest to do but think the faces aren’t flush i.e. there is a few mm lateral offset of caliper mount to dropout face.

For the brake mounts (of a drop bar build) why would you move away from flat mounts?

I’m moving parts across from a different bike and have brakes ready to fit that are PM. I don’t think I would go for hydraulic brakes because I’m terrible at fixing them, especially if I’m far from home.

Perhaps having a set of brakes ready to go isn’t a good enough reason to avoid flat mount but aside from the ease of buying a groupset I’m not sure on the benefits of flatmount on a mountainbike, even if it does have drop bars. I’ll be running 180mm rotors which it seems is awkward with flatmount, certainly there are very few 160-180mm flat mount adaptors. Possibly the flatmounts could be set to 180mm from the outset so no adaptor is needed though.

Post on the rear can be better from a packaging point of view as it can more easily fit the caliper inside the triangle gap between chainstay and seatstay (in some cases still needs a bit of seatstay curving to make a big enough space). More work to design and align everything. Same for flat mount but probably easier again to fit in the triangle.

Makes sense, thanks.

My custom bike has post mount front and rear – I prefer IS mount but IS mount forks are unavailable in a sus fork.  I wanted both calipers the same for easy pares / exchange / swapping and I didn’t want any adaptors.

My custom bike has post mount front and rear – I prefer IS mount but IS mount forks are unavailable in a sus fork.  I wanted both calipers the same for easy pares / exchange / swapping and I didn’t want any adaptors.

Think I might go down this route. Have been looking at similar available bikes, some even come with a flatmount rear brake and post mount front, which seems bizarre to me from a repair/replacement/general symmetry point of view.

Paragon poly drops.
Gives you the choice to choose in the future.
Woyld definitely go flat mount for a drop bar build though. Rear axle pitch doesn’t really matter as you’ll always need to order a replacement anyway. Front axle diameter possibly be a more constraining decision on wheel choice in the future.

Woyld definitely go flat mount for a drop bar build though

Why is that? Is it the availability of groupsets or are there other benefits to flat mount I’m missing? It seems like it was brought in for road frames, particularly carbon ones that might have less space for a brake mount and want something sleeker without needing to run discs bigger than 140/160mm?

Rear axle pitch doesn’t really matter as you’ll always need to order a replacement anyway. Front axle diameter possibly be a more constraining decision on wheel choice in the future.

Thanks, I was thinking of going with 15mmx110 as that seems to be the most standard for mountain bike wheelsets, happy to be corrected on that though.