Seatmast Topper Spec & Discussion

Attached is the drawing for a seatmast topper which accepts the Enve single bolt saddle clamp. Because it’s so thin, I’d definitely recommend a heat sink.

Also interested to hear everyone’s thoughts/opinions/experience with seat masts in general and custom toppers.

I’ve made 1 frame with seatmast and custom topper and as far as I know it performs as expected. It did however blemish the paint where the clamping force was applied.

This was a 1.25"x.035" mast with 1.375"x.049" (maybe .058" i cant remember) topper.

I’d really like to try an epoxied carbon seatmast in the future.

Enve-Seatpost-Spec (1).pdf (49.3 KB)


I have more questions/comments on ISP style bikes. I have always wanted a bike with one.

Spencer Wright did a really cool series of articles on metal 3D printing that revolved around designing a 3D printed topper:

I highly recommend going down that rabbit hole.


I really like isp bike sand have done a handful of steel isp frames and one carbon.

I have a personal distaste for round head and/or single bolt seat post and do not trust them off-road. So I came up with a solution that uses Thomson hardware and seems to hold up super well.

If you want to use a carbon tube, Rockwest Composites sells 27.2 tubes in any length so you use normal seat tube material for the ‘lugs’ and topper.


Nice! Can you share some details of how you fab’d this?

Any insight as to wall thickness and layup? Filament wound, unidirectional, pultruded?

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Great idea/solution!

That part would be a great candidate for a stainless printed part. Probably $20 for the lower cradle


The cradle support is 1.125x.058 and the bolt holders are half of two m6 stem binder bosses. The Thomson conical washers sit in the bosses perfectly.

Uhhhhhh uni and 2.0 or 2.5mm wall? I think they only have one 27.2 option.

I have tried a laser cut part here but it folded pretty quick, printed would be really cool especially if it interacted with a straight cut tube.

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some lunch break CAD:

Its missing real dimensions, but the part is very printable

  • short height for cheap print
  • critical geometries are facing up
  • minimal support material

I love it. It’s beautiful. It’s a part that makes sense to be stainless as well, even for brazing.


If people like this design, I would be happy to finish engineering it, print a few out, and add the design to the open bike hardware project.

I would love to see more ISP bikes.

Question for the group, I know the Thomson hardware is nice and readily available, but it’s kinda pricy. Are there better options? For example, can we use this cheaper reverb clamp? Or will my ISP also develop some play (reverb joke).


I would love to grab a couple of prints! I think I’ve got 1 or 2 isp builds coming up this summer if not sooner.

I’ve actually built around the reverb top before.

It’s 100%… adequate. It’s heavier, less fancy and the m5 bolts are coated in so much thread lock that you pretty much need to run it through a die.

Building around this profile (at least for non-3D printing) was pretty hard. It’s designed to sit on top of a ~24mm dropper upper rather then a larger tube. Because of this, the interference between the topper and bolts is likely more likely.

(I’ll see if I can find a photo of that topper)


Edit - here’s the topper.

I built it so it it sits on the high spot rather then the low spot where the reverb would normally sit.


Ive done two. One was an internal stub post style and the second external uding Enve parts.


Put me down for a couple!

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How do people feel about this style?

  • pretty good price
  • hardware looks like pretty good quality
  • you will probably need to drill out/ream the 3D printed hole for the barrel nut

The greatest advantage I see is that it allows for a topper w/ saddle setback. I don’t use THAT much setback on my road bike, but I think most people have 10-20mm.

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Set back is good as it allows the seat tube centerline to pass through the saddle center but then have the rails clamped behind rail center which helps prolong rail life and lessen the bending on them when sitting on teh saddle properly. Visually it looks better and structural sits better. In my opinion.

That picture above of that topper is a badly positioned saddle within the clamp.

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Print one in ti and I can use it on my personal track bike project.

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To be honest at this point I would much rather use the Thomson hardware. You’re within a few dollars of the cost of a Thomson clamp and bolts (at MSRP), and Giant small parts won’t have as good availability over time. Plus, Thomson makes their clamp hardware for 7x10 carbon rails as well, which is a really nice option to have. It makes setback a slightly different problem to solve, but not too bad. I’m working on a topper using Thomson hardware at the moment (at 20mm setback), I’ll post some photos when the design is in a workable state.
I think it’s also worth mentioning that Thomson is a great company to work with, and has very good pricing for framebuilders on their parts. If you reach out to them and set up an account, buying their clamp hardware will be cheaper than most other options.


One-up sell their seatpost parts separate as well


@Daniel_Y the Giant inaccessible bolt design is pretty annoying to use as a fitter / end user. Your fingers being the only thing that sets the front bolt tension usually leaves you tightening the front, then tightening the back and realizing you didn’t tighten the front enough, undoing the back again and then starting over. You lose some of the infinite adjustability of a two bolt design because of this. With some slotted saddles you can get a 5mm through the top of the saddle, but not all.

In the world of inaccessible bolt designs, BBB does it the best. Their inaccessible bolt is cross drilled so you can wedge a 2.5mm key through the bolt and spin it that way. Not sure if they sell just the clamp tho.

@Pi_bikes The Oneup clamp is really good from a user perspective. Easy to use, easy to swap saddles. It’s the same price as the Thomson hardware though.

Like the reverb clamp it’s designed to sit on top of a narrow dropper shaft, which may or may not be an issue with a 3D printed part.

Oneup replacement parts seem to go out of stock often and stay out of stock for a few months at a time.

At least with Thomson, if they don’t have it QBP or LTP might on the wholesale side. On the retail side, it’s a part many LBS / internet bike shops (IBS) have in stock.


I machined an internal expanding seatmast topper for my personal bike a little while ago and it was designed around the USE Duro Clamps. I’ve found that they seat easily on a tapered surface and give a very secure clamping surface when torqued to spec.

Here’s the seatmast topper disassembled

And assembled

At 223g it is a VERY heavy prototype but the wall thickness was around twice what it should have been. There’s definitely room for lightweighting in the future. Likely through a machined and bonded design or a printed and post-machined design.

One thing that I did notice was that with it being machined to some very tight tolerances, it doesn’t take much torque on the tensioning screw to lock that post in. Definitely better than some of the quill stems I’ve used in the past.


Some very early carbon bikes (I owned a Vitus, it was pretty awful but I felt super cool at the time) had quill seatposts, pretty funny to see it come back!


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