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Official P4 rear brake adapter thread
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So I have had this idea to build a brake adapter for the rear P4 brake but very much lack the skills in CAD or CNC. There are a few threads with people who may be interested, and I am sure even more people frustrated with the stock brake. So let's put our collective SlowTwitch minds together to get some of these bad boys built up.


See photo below for my measurements. Fellow P4'ers, please help to confirm.

The idea is to machine a horseshoe that bolts into the 2 brake bosses (the top 2 holes in the photo). Then, have a 3rd bolt going into the bottom of the photo hole, to add stability. Then, somewhere very close to that bottom hole of the frame, have a hole drilled out on the horseshoe to run a standard, single bolt rear brake (best options are TriRig or Magura).

Thoughts? Please share more details and opinions so we can hopefully have someone build this.





Alex Arman

Strava
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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In order to figure out where the hole for the caliper mount needs to be you need to measure at least one of those hole to the axle centerline.
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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I'd be interested to see where this goes and would be a buyer if an adapter was made (to allow use of an omega brake).

Three issues to consider:
1) Due to the location, the new (omega?) brake you end up using will be exposed to the wind (along with the new adapter). In other words, unlike the original brake, it will no longer be shrouded and the exposure to the wind will make it less aero.

2) Based on where the cable comes in, I don't see any way it could be anything other than center-pull.

3) The other quandary is the cable itself. Perhaps a hydraulic cable would be best, to avoid kinking. Magura RT6?

I wish you all the best in this and again, I would be a buyer if it was made with quality.
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Re: Official P4 rear brake adapter thread [NYSLIM] [ In reply to ]
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Agree, Maguras all the way.

ishi no ue ni san nen | Perseverance will win in the end. | Blog | @nebmot | Strava | Instagram |
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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You could look on Craigslist for someone local that has a 3D printer and can do the design for you to work out prototypes at a much cheaper than going to metal. A college student could probably do this for you pretty quickly.

ishi no ue ni san nen | Perseverance will win in the end. | Blog | @nebmot | Strava | Instagram |
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Re: Official P4 rear brake adapter thread [pyrahna] [ In reply to ]
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If I place the caliper bolt right on the smaller, lower hole in the frame, the brake pads line up very well with the wheel.

Alex Arman

Strava
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Re: Official P4 rear brake adapter thread [nebmot] [ In reply to ]
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Good idea. I'll just hang around the CU campus trying to not look creepy :)
But really, that is a great thought. Will try to find someone to make some mock-ups.

I am running Maguras, for sure. And I am okay with the minor "exposed rear brake aero drag" to have drastically improved braking.

Alex Arman

Strava
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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That will make placing a through bolt in that location very difficult.

It would probably be easier to use a 'Direct Mount' interface since that is two threaded holes on either side of the central bolt hole.

That would knock the maguras out as a possibility though. Omegas, shimano DM brakes, and EE would still be possible though.
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Re: Official P4 rear brake adapter thread [pyrahna] [ In reply to ]
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Quote:
It would probably be easier to use a 'Direct Mount' interface since that is two threaded holes on either side of the central bolt hole.


This is the adapter Orbea makes for the Ordu OMR to switch to direct mount brakes. The cabling is different also. Full housing from lever to brake, and also a new (larger) cover.


Last edited by: efernand: Oct 12, 17 12:44
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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If that is the case, why are you not just finding the right spacer and bolt diameter to mount an Omega right into the frame there, forgetting about the other two mounting posts?
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Re: Official P4 rear brake adapter thread [chicanery] [ In reply to ]
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Honestly, I don't completely trust that the frame hole and threads can handle brake force.

The rocker pivot bolt that hole is designed for is spec'd only to 3nm of torque.

http://cervelo.co.kr/...efe6&ckattempt=1

Alex Arman

Strava
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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I think it should be alright. What do think handles the brake forces, or lack there of, now?

You need to measure where the braking track is in relation to the current mounting holes. An easy way would be to install the P4 brake and take a measurement at the mid point of the pads.
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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I found a brand new 2011 frame...planning on building it up this fall...I too would love to find another option for the brakes...hope someone figures this out...
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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doublea334 wrote:
Good idea. I'll just hang around the CU campus trying to not look creepy :)
But really, that is a great thought. Will try to find someone to make some mock-ups.

I am running Maguras, for sure. And I am okay with the minor "exposed rear brake aero drag" to have drastically improved braking.

Let me know if you find a CU student. My daughter was supposed to make me some parts, but so far she is "too busy". With all the tuition I pay you would think she could help dad out, ...but no.
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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I've planned to make basically this exact thing for quite some time, but never seem to get around to it. I've got a set of Maguras waiting for the project. The problem as I see it is that the adapter plate creates a long moment arm between the brake bosses and the attachment point for the adapted brake. The whole thing will want to accordion away from the frame under braking forces, so you've either got to rely on the unknown structural integrity of that 3rd hole, or make the adapter plate beefy enough that it won't flex too much. Determining the proper plate thickness is the crux of the design, and I haven't investigated it enough to figure out the forces it will see. A good baseline would be to consider the thickness of the existing cantilever arms, but account for a moment arm 2-3 times longer-- it gets to be a pretty thick plate unless you go with steel. I'm really surprised by how thin the Orbea adapter looks.

The original design is basically a center pull cantilever with really poor mechanical advantage. Thinking about it now, I wonder if you could fit a lowly linear pull V brake to the existing bosses. It might not work well with traditional aero brake levers, but worth some investigation.
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Re: Official P4 rear brake adapter thread [mt2u77] [ In reply to ]
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Yeah, I'm all in using Maguras, so really want to figure out that design. You sound much smarter than me with an engineering mindset, so hopefully we are getting the right people on this thread to make this damned thing :)


Regarding a cantilever option, a Paul Motolite could work, since the arms are narrow and the springs are built into the boss bolts. Just not sure a TT brake would have enough cable pull.

https://paulcomp.com/...components/motolite/

Alex Arman

Strava
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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Maguras are MUCH harder than Omegas due to the fact that the post is threaded into the body rather than a bolt that goes through.

I have them on my Dimond, for which I figured out how to make such a thing work in a reasonable way, which was to put a very thin, very hard nut on the post.
Then screw the post (and brake) round and round and round until it's almost tight, and aligned correctly. Then unscrew the nut against the frame.

A similar thing could be done here. If I owned a P4 I would build them for you nerds, but I don't have a frame to work on.
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Re: Official P4 rear brake adapter thread [doublea334] [ In reply to ]
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I likely have access to a 3-d printer through colleagues. unfortunately, I have no idea how to create 3-d files for printing.
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Re: Official P4 rear brake adapter thread [mt2u77] [ In reply to ]
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mt2u77 wrote:
The problem as I see it is that the adapter plate creates a long moment arm between the brake bosses and the attachment point for the adapted brake. The whole thing will want to accordion away from the frame under braking forces, so you've either got to rely on the unknown structural integrity of that 3rd hole, or make the adapter plate beefy enough that it won't flex too much.

This was my first thought on reading the thread. I considered doing something similar for an FM087 Frame I had a few years ago, but I came to the same conclusion & figured that the 3rd hole (BB Cable Guide Hole on that Frame) wouldn't be worth the risk.

One solution might be to have longer "posts mounts" that "rib" into the main arch of the piece & avoid the 3rd hole altogether, but that would have to be analyzed (FEA).

I would think that a piece that adapts to the Direct-Mount standard would have less of that cantilever effect on the mounting points.


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