Custom Frame - Adjustable seat post angle - experiments with seat masts

Hi,
I thought you may be interested in my curiosity project. I realise it's a bit odd, nobody will buy it and the execution isn't perfect yet, but hopefully it's a fun discussion point :)

I initially built a frame last year or Grinduro with a removable seat mast - The idea was to be able to change it in the future for something with a bushing or made from carbon or whatever the future would bring.... It was a prototype.
This was mainly build as a rigid MTB as below

After Grinduro I tried a more road-orientated fork on this frame and it obviously dropped the front by loads. I measured some of the geometry and it was actually all quite nice, apart from the seatmast almost upright, so it got me thinking about making an adjustable seatmast so that one frame could (in theory) cover all bases - run with a short suspension fork or shorter road fork and still be nice in both situations.

So I resprayed the bike and made some aluminium bits and bobs to enable the seatmast to adjust forwards and backwards. I bought a bit of titanium tubing and made a seatmast from it, stealing the hardware from something else. It worked a treat at grinduro this year with drop bars but still with the long fork, it's very strong and stiff.
I call it the 'Deck chair' because the bracket underneath changes angle to prop the seat angle.

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I was keen to refit the road fork when I got home. See below. The seatmast is then adjusted to lay back further.
This obviously changes all the geometry, but I think it still works really well.
The headtube angle is now 71 deg. The BB height is now 278. It feels good. Although it looks a bit odd maybe.

I haven't measured the steering and fork geometry or checked back to the CAD, but it feels really nice. The Columbus forks have adjustable rake too, so there is a tiny change I can adjust there if I felt it was needed.
I'm playing around about with the seating position over the BB and changing the stem length a little, so small tweaks as I am a bit fussy with reach.

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So that's where I'm at for now. I want to make a more refined version. The aluminium clamps are a bit crude. My neighbour CNC'd four clamp brackets for me same-day (I hate using boring heads to make a 28.6mm hole) , and I manually machined all the rest in a day, largely making it up as I went along.

Also, the way it bolts to the frame is very crude. There is a brass threaded bush at one end. It basically just tightens with bolts. There are no creaks though, no flex either.
It would be fun to fit a little damper and spring. ha.

I realise that creating a bike to do mild MTB work and then be efficient enough to ride 30-40 miles is maybe a bit of a compromise for both disciplines, but I don't think it is really. If you rode the bike in MTB set-up (if I had a suspension fork on there), I think you'd like it, and the version with the road fork is actually a really great to ride on the road and towpaths too - I'm doing my 12 mile commute on it tonight :)
It's not a bike for heavy MTB use, or doing Time trials on, but I don't do that... and not many people actually do.

It also has a bolt-in yoke. Just more fun really, and a bit of adaptability.
another picture of it with 650B slicks. But I prefer with 29/700 wheels to be honest.

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A reason to have less bikes? Sorry I'm out.

No in all seriousness that is very interesting. I actually think the execution of your prototype is very impressive and once painted actually looks pretty cool.

Wild

awesome, love it

Thats awesome

Great stuff - can you apply for copyright / intellectual rights / design (or whatever they call it on dragon's den) so that no-one nicks your ideas?

Thanks!
Yeah, I haven't quite convinced myself that I'd be wanting to swap forks very often.... but maybe once or twice a year offsets the complication and weight penalty, I'm still not sure - Although the bike was bang-on 10kg with pedals in it's first rigid MTB guise, so isn't disgustingly overweight.

I think the main benefit is the ability to tinker, swap and change parts. It's another element of geometry I can ply around with. Which is something I really enjoy.

Ideally I'd cling onto this design and prevent any copying, but I don't think it's really workable without throwing cash at copyright stuff...?
I've got a small stand at Bespoked this year where I want to make 2 identical frames and built them up differently, to demonstrate that versatility. I've got a bit more prototyping and testing to do, especially how it fixes to the frame... and how it looks. Ha.

Love this. I love messing about with seat angles as well. I'm sure high 70s are the sweet spot and this is a great platform to experiment on and adjust to suit styles. Nobody is doing it as well so it's great to see someone having a go.

Ingenious!

Thanks for the positive responses!
I genuinely though everyone would rip it to shreds.
It's been an idea that been rolling around in my head for about 8 months.
I originally designed it to just be these bits below, but obviously that wasn't stiff enough so I had to add the deck-chair prop last minute - literally finished on the mill the morning I left for the event.
The seat tube also takes a bit more force than usual.... the original Titanium one was 1.1mm thick and wasn't 100% happy, so I've gone for a 1.5mm thick Ti tube next. I suppose the clamps could be wider to spread the load... but the whole thing needs sorting to improve the way it looks. That weird tapering top plate looks a bit strange from some angles. I'll probably get the whole lot CNC'd and then anodised.

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Would be nice to see a gif with the changes in geometry

Yeah, I need to get my act together and do this.
I think I originally built it as an MTB with a headtube angle of 67.5 or 68... one of the two.
It's now 71 degrees.
I want to share the complete geometry, anyone with bikecad and some idea will be able to work it out anyway I imagine.

I think you're talking about a patent. My understanding is that you can only apply for a patent before your idea becomes public knowledge, because after that point it's essentially a "free" idea at nobody can claim ownership of.

Patenting something is also fairly time consuming and expensive, and they expire after a while. So as a private individual it's only really worth pursuing if you think your idea is going to make bank.

This is super cool btw!

Love this, fantastic work!

Cheers!
Yeah, I think there is a very slim chance of anyone ever ordering a frame like this, even once fully resolved and looking good. Who knows, I don't mind.

I'm not really building frames to make it a profitable part of the business anymore, that idea faded after so many people told me how toxic it can be (I'm a bit worried about how tough Bespoked can be for exhibitors, so I've heard!). Although it's a very good way of marketing the rest of the business - my art/design customers wanting metalwork regularly want to talk about the bike frames I'm making, which is nice.

Thanks again. I'll update with some pictures and geometry when I get a chance.

99% of brilliant. Needs a little bag in the triangle.

Ha. Thanks. Some kind of concertina bag that is able to change dimensions if the frame geometry changes!

I'm working on a bike to take to Bespoked in a couple of weeks.
I've been doing a long commute on the previous version that went to Grinduro, and while it's nice and sturdy it's a pig to adjust.

I thought it would be interesting to clamp the seat tube by tightening through and into a mandrel that slides inside. This will obviously mean that the seat tube will need slots machined into it - 2 separated slots with 2 M5 fixings. I've limited the slots to 28mm long centres - so that is the amount of height adjustment that the seatpost will have - Enough for me I think, different shoes/pedals and angles.

I've also gone for a lower clamp onto the frame that is a bit inspired by thru-axle dropouts with aluminium bosses that fit into the steel bosses that are brazed in the frame.
There are bronze bushes on the front mounting and stainless pivot axles.
At the moment the whole assembly (not including seatpost) is around 500grams, so I'd like to get that down by hollowing and removing material as I go.
The seatpost is 28.6 with a 1.5mm wall thickness - straight gauge. It's possibly a bit over the top... but let's see!

It's not quite fully detailed so some surfaces may change shape.... but it's got to be on the CNC machine next door in a couple of days so I've got to settle soon.

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Other side with a few parts pulled out.
I'm hoping to build this with a completely vertical aero 'seatpost' below this.
I think it's going to get complicated joining all these parts up.... but I'm not going to 100% plan that.. it'll become apparent when I've got the bosses suspended in the jig hopefully.

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I got this made and displayed at Bespoked at the weekend.
It was a mad rush, I genuinely made and sprayed the frame in 2.5 days, so the BB area and drop outs didn't get much sanding, and the brazing could have been smoother... but it's very square, straight and stiff.
The adjustable seat mast works really well, clamps tight and feels very rigid. I've got some weight saving to do on the pivots/axles/bolts and I'm brewing a new frame that is a bit lighter too.

There was alot of good feedback in general. Some journalists on the Friday saw it and just asked WHY scornfully (haha), some thought it was interesting, a few liked it so it may get a feature, we'll see. Most of the public enjoyed seeing something different and wanted to chat - I suppose we don't go to bike shows to see 200 of the same gravel bikes do we.

The framebuilders are a very positive bunch and there's alot of respect for each other. Amazing skills and work on show, genuinely incredible metalwork. It was nice to feel part of it for a few days.

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These kids bikes have a mechanism that you might find interesting:

https://blackmountain.bike/products/skog

(Basically, the bike is expandable and when you make the frame bigger you also take off an outer part of the sprocket to make the gear bigger, then to tension the belt and extend the reach, the seat tube rotates a bit to move the bottom bracket forwards and the the seat back.)

Ah nice, thanks, that's genius!
I like things like that... a bit like the old softride beam with sliding seat clamp. The adjustment seems to make sense to me.