Karl's V20 variable seat angle solution

[Karl42]

The idea of modifying the V20 so that the seat angle can be adjusted while riding has come up in this forum quite a few times, and some people have actually built some working versions of this. I've been wanting to build my own ever since I saw those other projects, and now I'm finally getting around to do it.
With the unmodified V20, climbing very steep hills in the fully reclined position is much harder than climbing on my upright bike, mainly for two reasons:
1. My power output in the reclined position is lower, perhaps because the heart is at the same height or even lower than the legs.
2. Front wheel traction is suboptimal because the steeper the climb is, the less weight is on the front wheel, and hence, less traction.

While I can get up 20% grade hills on my unmodified V20 if they have a good and clean road surface, I noticed that when I sit upright, both power and traction are much better. However, on the standard V20 I have no back support in the upright position, so I can only sustain that for very short amounts of time by holdying myself up with my arms and my abs. Doing a long Brevet with many climbs will have me climbing in the leaned back position due to exhaustion, and then I have even less power than when sitting upright.
As I prepare for a 600 km ride this summer with 8000 m of climbing, I have a good motivation to finally advance this project and build my own variable seat angle solution for the V20.

The two existing projects that I have studied in depth for inspiration are those:

"The seat of the problem on my new V20" by @Pcmousley: https://forum.cruzbike.com/threads/the-seat-of-the-problem-on-my-new-v20.8182/

Here is his video:

V20 "seat adjustment" by @Salaamaleikum: https://forum.cruzbike.com/threads/seat-adjustment.14231/
Here is his video:

Sitzverstellung V20.mp4

This is "Sitzverstellung V20.mp4" by Salaam Aleikum on Vimeo, the home for high quality videos and the people who love them.

vimeo.com

There are various other threads that reference this, for example:

Adjustable seat angle ideas

I got my Thor seat for my V20 in the mail. Since it's the off season I am not in a hurry to mount it, so I have some time to consider a way to make the seat angle adjustable. I just want something that when I am going slower I can sit up and have the advantages of that: little better climbing...

forum.cruzbike.com

Climbing

I did a test run today on one of our local, tough, steep climbs. Bear in mind it was too hot, and my legs are wasted from riding this week with hard efforts yesterday. Climb is short about a mile or a little over, averages 10% with portions 15-20%. I completed 3/4’s of the climb which...

forum.cruzbike.com

The thread by Pcmousley is quite old, and it appears that it used to contain images that may have gotten lost when the forum software was upgraded or something like this. So it is only text, and a very short video that is so close-up that it's hard to understand. But after reading his description a few times, I think I fully understand his design.

The other design is by Salaamaleikum. I have been in personal contact with him to ask questions and get CAD files and more pictures, and while his solution is quite clever, it requires custom machined parts and has some aspects that I don't like.

So I'll be mostly copying the design by Pcmousley, with some inspiration from Salaamaleikums work. In particular, I will cut the original V20 seat, join it with a carbon-kevlar hinge, and then support it from behind with a padlock hasp. The mechanism to adjust the seat while riding will follow the design from Pcmously exactly: a bungee cord will pull the hasp towards the front, and a shifter cable will pull it back, operated by a small lever that I will attach under the front end of the seat.

 

[Karl42]

This is a work in progress that I want to document here. The first part was building a proof of concept, and then finding out where to cut the seat.
For this I found an old aluminium plate which is about 3 mm thick, and I made a mock seat with that to test the position of the upright seat.


The V20 stock seat has some convenient holes in the perfect location for some screws to rest my alu plate against. With some old packaging material to prop it up, and a bit of duct tape to hold it together, I was able to experiment with various hinge positions and seat angles and ride around with that contraption to get a feel for it. And I have to say that climbing the steep hills nearby is a lot easier with the more upright position.




With a good idea where I want the hinge to be, I used the existing holes of the V20 seat to screw on the carbon-kevlar hinge. With the seat still uncut, I drilled the holes on the other side of the hinge also. Then I cut the seat with a carbon saw from Park tool.
So now my seat has a hinge and is already adjustable. For now the hinge is only attached with screws, but I will also glue it to the seat to make the connection stronger. But even now, it feels a lot sturdier than I expected.






I found that padlock hasps come in various standard sizes. The one in the picture below is the original one that Pcmousley recommended and is 96 mm long (4 inches). I also got one that is 125 mm (5 inches), but that lifts the seat too much, so I'll probably be using the shorter one.



The back of the seat will be supported by a small 7075 alu plate that is 2 mm thick. I actually ordered two plates, one 2 mm thick, and the other 3 mm, but even the 2 mm one feels thicker than necessary. But first I want to glue the hinge to the seat and then work on the mounting point for the hasp that is supporting the seat from behind.

to be continued...

 

[Frito Bandito]

I too saw the hinging seat and think it is a great idea for climbs. Looking forward to seeing your progress.

 

[bromclist]

Nice. So you now will have the speed of V20 and the comfort of S40 (each when required)

 

[Karl42]

Cutting the seat was the scariest part of this operation, because I was afraid of making the cut at the wrong place. I'm pretty sure that what I've done now is workable, but only long-term ride experience will validate that.

The second scariest part is gluing on the hinge, because I'm afraid of getting glue on the hinge part and making it inoperable. I've already practiced using this glue on some smaller carbon-kevlar hinges for the lid of my tailbox, so I have a little bit of experience with it.

To prepare the glue, I used masking tape to protect the surrounding areas on the seat, and I put a thin 4mm thick strip of tape on the hinge to protect it from getting glue in the hinge section. Then the contact patches were cleaned with alcohol and then sanded:




Applying the glue (2-component MMA glue recommended by the carbonhinge company) was straightforward, and having the screws meant that I didn't need to worry about finding the correct position for the parts, and also didn't need extra clamps:


The result looks clean, and the hinge still works perfectly:




The next part is mounting the hasp and working on the mechanism to lock and unlock it. One of the two hasps I got fits perfectly to the screw hole where the seat-holder clamp is in the frame:




Attaching the bungee cord is also very simple and works well to pull the hasp towards the front of the bike, thereby raising the seat:


Next up is the shifter cable to pull the hasp in the other direction. Here I hit a small problem. I need to drill a hole into the hasp for the shifter cable, but drilling through this steel proved surprisingly difficult. I ruined all of my 3mm drill bits and almost made it through, but not quite. So I need to get a better drill bit before I can continue.

 

[Karl42]

With the drilling problem solved, I have now routed the shifter cable from the under-seat lever to the hasp and have made a basic proof-of-concept demonstation to show that it works:

This is still a little rough and needs more fine tuning, but the basic idea seems to be working.
The hasp shown in the video is a bit too long, but its base fits better than the other hasp I have. I may have to shorten this one a bit, or buy another shorter one with a different base.

So the next steps are to work on the back plate of the seat, and then to fine-tune everything.

 

[Frito Bandito]

Excellent job so far Karl! When the seat is reclined how flush does it sit on frame? If it isn't flush how do you think is a good way to stabilize it? Perhaps a pair of metal strips glued on the back of the seat then some kind of dampening material on those where they rest on the frame?

 

[Karl42]

In the reclined position, the end of the seat will be raised by 5 to 10 mm, depending on the details of the construction, so I will attach some spacers to the mounting points on the frame to support the seat.

For now I have to work out how to make the back plate of the seat. The hasp pushes against the seat with a very small pointed contact area, so the material there needs to be protected against the hasp punching through the seat. It seems that my 2 mm 7075 alu plate would be more than adequate for this, but it is flat, and the seat is not, so I would have to bend it a little bit, or make it very small and use some other material to protect the seat where the hasp slides against it. Such a material could be a thin POM plate.
Or I could strengthen the seat by laminating a few layers of carbon fibre to it, perhaps followed by a POM plate for the abrasion resistance.
Since I already have the alu plate, I'll start experimenting with that.