Yuba Mundo BB7 Disc Brake Installation

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teklektik   10 GW

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Yuba Mundo BB7 Disc Brake Installation

Post by teklektik » Mar 09 2012 2:25am

The base Yuba Mundo is equipped with Promax v-brakes with Tektro Io discs as an option – 160mm/180mm front/rear. A special 140/160mm adapter is required to mount the rear 180mm caliper due to a non-standard placement of the rear disc caliper mounting tabs. The adapter is part of the disc brake kit and is also available separately as a solution for mounting ‘any’ 180mm rear brake. However, mounting Avid BB7s is somewhat problematic since Avid uses a 185mm rotor which is not technically supported by Yuba and turns out to be incompatible with their standard adapter solution.

This post describes a means to install BB7s using standard Avid adapters and a small shim. The method employed to determine the shimming is described below and is admittedly only eyeball accurate, but the rotors and pads show the proper wear patterns and the installed brakes work very well – tested with 40+ mph stops. This installation was done on a Mundo V4 - you will need to verify with Yuba that mounting tab placement is unchanged for other frame models.

Components

Although Yuba uses a reverse 160mm/180mm front/rear arrangement with the larger brake in the rear, this installation is for a 185mm/185mm setup. The stopping power is excellent and the front/rear braking balance with equal sized rotors works well, in part because of the long cable run to the rear which softens that cable pull a bit. However, for a heavy cargo bike a 203mm/185mm setup would also work nicely – it’s about the same price.
  • EDIT - Here's a later ES thread that calls out difficulties using 203s - better stick with the 185s...
These items were purchased from Price Point:
  • (2) 2011 BB7 Front/Rear 185mm MTN brake kits (cheaper than the identical 2012 model)
  • (1) SRAM Pitstop Straight Jacket brake cable set
  • (1) 2011 pair Avid Speed Dial 7 levers
As it turns out, the Speed Dial 7 levers are too short to reach around the Magura throttle and have more than two fingers worth of accessible lever grip. The Speed Dials were returned and the original Promax v-brake levers retained. The outer pad adjustment of the BB7 is a fair approximation of the Speed Dial feature for adjusting lever throw and the original Mundo levers work well.

Front Installation

The front installation is a simple by-the-book BB7 installation. The cable sheath was cut with a Dremel with fiber cutting disc and the cable was cut with a standard cable cutter that was on hand. Lacking the proper crimper for the cable end ferrules, they were just glued on with Loctite Super Glue Ultra Gel Control (1109181). The cable end strands were super glued for ¾” in lieu of using an end cap so the cable could be easily disassembled. The outer rotor ring was wiped clean with isopropyl alcohol to remove machine and finger grease before assembly and again just prior to pad/cable adjustment.
26-40_iv250.jpg
A small bend is required in fender stay to clear caliper
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While the wheel was dismounted to bolt up the rotor, the black baked-on v-brake crud was polished off the rim with Mother's Mag & Aluminum Polish and the black polish residue removed with Simple Green. The rear wheel got the same treatment later.

Rear Installation

A comparison of the standard Avid 185mm adapter and the somewhat lower profile special adapter provided by Yuba is shown below. The Avid adapter positions the caliper further from the axle center and with a slight clockwise angular displacement.
26-11.JPG
Yuba 180mm adapter in front, std Avid 185mm adapter in rear

The Yuba Adapter can be used, but requires a couple of 5-6mm spacers and new, longer screws - not an attractive solution.
26-15_annotated.jpg
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Surprisingly, in spite of the non-standard mounting tabs, the standard Avid 185mm adapter works almost perfectly, requiring only a small 1.5 mm shim on the rear post. Simply install the brake normally and insert the extra shim as shown below - a common 6mm stainless washer (Ace Hardware) is the proper thickness. This shim provides necessary clearance between the bottom of the caliper slot and the rotor edge and aligns the outer edge of the pads to coincide with the outer edge of the rotor. You can check this alignment by looking into the rear of the caliper slot and checking that the pad and rotor edges are at the same height.
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The cable is routed through the braze-on cable guide on the seat stay and to the braze-on ferrule on the bottom of the top tube. The little soft plastic cable spacer was salvaged from the discarded v-brake cable and threaded onto the cable wire in between the fore and aft sheaths. As with the front installation, cable length was guided by Sheldon Brown's advice on cables.
26-44_iv250.jpg
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Other Shims

Since I use washers inside the rear dropouts as part of a hub motor 14mm dropout mounting solution, the hub and rotor are positioned further inward than normal. To compensate, a 6mm SS washer is added between each end of the caliper adapter and the mounting tabs to move the adapter and caliper inward by a roughly similar amount. The Avid CPS system (below) will accommodate some difference between the axle and caliper washer thicknesses. These shims are not for accurate alignment, but rather just to get the CPS into adjustment range.
26-42_annotated.jpg
Shims compensate for axle washer inside dropout
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The need for this became apparent during installation when it was impossible to properly align the caliper with the rotor in the desired 1/3, 2/3 ratio (Step 5).
26-32.gif
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With caliper installation complete, the rear brake lever was given a good two-handed squeeze to stretch the long cable. The cable anchor on the caliper arm was readjusted and the squeeze/adjust procedure repeated several times until the lever firmed up and no more stretch could be introduced.

Here is a link to Breaking in Disc Brakes. Do it! :wink:

Eyeball Evaluation

The wipe area of the rear hacked installation is essentially identical to that of the standard factory bracket installation in the front. :D
26-48.jpg
Front rotor wipe area (standard Avid installation)
26-46.jpg
Rear rotor wipe area (hacked Avid installation)
Determining Adapter Shimming

The remainder of this post describes the means to verify the shimming - you can skip this if you just want to get on the road :).

A simple technique was used to determine shimming and verify adapter alignment with slightly better than eyeball accuracy. A rectangle of plastic (luggage tag/shopping card) was cut with the same width as a brake pad. The pads were pulled from the caliper and the outer pad replaced with the plastic pad which protruded from the caliper serving to indicate the exact orientation of the pad seat in the caliper. The caliper was then mounted up and a photo taken that included both the caliper and axle. The protruding plastic pad was then identified in the images and used to draft some guidelines to determine the caliper/pad alignment.
26-12.JPG
Hole punched in card clears pad retaining pin on pad adjuster
26-13.JPG
Blue tape made edges easier to find in photos
The front brake was tested first to verify that the edges of the caliper pad seat are indeed parallel to the radius connecting the axle center to the midpoint of the pad. This causes the force of the rotor against the pad to push it squarely into the side of caliper seat.
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Different rear brake shim configurations were tested to identify one where
• the radial distance of the pads from the axle center and
• the angular orientation of the pads relative to a radius of the rotor
were identical to those of the front brake.
26-26_annotated2.jpg
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To simplify brake adjustment, Avid uses a proprietary Caliper Positioning System (CPS) that is designed to allow the caliper posts to be tightened down regardless of the particular orientation of the caliper. Fortunately, this feature will also compensate for asymmetrical shimming, allowing a simple solution using shims in lieu of machining a new adapter with squarely aligned post faces.
26-30.gif
Slotted caliper mounting holes and mating spherical spacers allow variable post-to-caliper alignment
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Last edited by teklektik on Jul 03 2012 12:11pm, edited 11 times in total.
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Philistine   1 MW

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Re: Yuba Mundo BB7 Installation

Post by Philistine » Mar 09 2012 5:53am

Good job. I was running a 203mm rotor on my Yuba rear for a while (when I had a 9C on the rear), but since moving to an old X5304 with no disc attachment, I have just been running with a front brake only. I had intended to run regen braking through the rear, but even with Kiwi's awesome torque plates the sheer force of regen braking was shaking my axle loose. So I have just been getting around with a front brake (a 203mm with BB7s). It means I am putting enormous strain on my forks (they have sometimes flexed in ways that scares me), and makes riding in the wet a sphincter tightening experience.

Your Yuba is looking nice and tidy. They are awesome bikes, I use mine constantly.

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teklektik   10 GW

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Re: Yuba Mundo BB7 Installation

Post by teklektik » Mar 09 2012 9:55am

Philistine wrote:... I was running a 203mm rotor on my Yuba rear for a while, ...
What did you do about mounting it? The whole business with the non-standard mounting tabs is a pain - Yuba has no good advice on how to deal with the BB7, although I did Google up a bit on a special Avid adapter Yuba seems to have made and sold back in 2010 for $30.
Philistine wrote: So I have just been getting around with a front brake (a 203mm with BB7s). It means I am putting enormous strain on my forks (they have sometimes flexed in ways that scares me), ...
Hmmm, I'll have to keep that in mind - maybe I will stay with the 185 up front instead of a later upgrade - it works great and this may be a case where more is not better.
Thanks for the heads-up! :wink:
Last edited by teklektik on Mar 09 2012 10:29am, edited 1 time in total.
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Build Thread: 2WD Yuba Mundo V4

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ohzee   1 MW

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Re: Yuba Mundo BB7 Installation

Post by ohzee » Mar 09 2012 9:59am

Wow great write up.. I actually get my Yuba Mundo dropped off today via Fedex.. I ordered a mid drive kit from urban , but one of the mods
I plan on doing is the disc brakes so this will help me very much.

Thanks for your hard work. Cant wait to get my new toy.

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teklektik   10 GW

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Re: Yuba Mundo BB7 Installation

Post by teklektik » Mar 09 2012 1:15pm

Thanks - much appreciated!

I just updated the leading post with a couple of shots showing the rotor/pad wipe areas of the standard Avid install up front and the ad hoc installation in the rear ... an easy check when you do yours.

(You're going to love your new toy - every ride is like a little vacation :mrgreen: )
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Farfle   1 MW

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Re: Yuba Mundo BB7 Disc Brake Installation

Post by Farfle » Mar 09 2012 4:03pm

The avid bb7 is my favorite brake, they can take SO much more heat than any bike hydro, because they don't have seals that melt and blow fluid everywhere.
Test Review Revise Repeat

botz244   10 W

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Re: Yuba Mundo BB7 Disc Brake Installation

Post by botz244 » Mar 16 2012 5:13am

i bet if you turn those adapters in the right direction you wont need those spacers lol and farflie i had wanted an axle but you dont answer your in box

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teklektik   10 GW

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Re: Yuba Mundo BB7 Disc Brake Installation

Post by teklektik » Mar 16 2012 11:06am

botz244 wrote:i bet if you turn those adapters in the right direction you wont need those spacers lol and farflie i had wanted an axle but you dont answer your in box
You don't have to trust me on this - check for yourself - examine the pictures and you will see that reversing the adapters cannot work. Please read above - Yuba consciously put the Mundo mounting tabs in a non-standard place because of the geometry and stays in the rear; this solution is serendipitous, not the result of design by either Yuba or Avid.

There's nothing in my inbox. I have no clue about an axle :?
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teklektik   10 GW

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Re: Yuba Mundo BB7 Disc Brake Installation

Post by teklektik » Mar 25 2012 1:17pm

250 Mile Update

All is going well - no issues at all - the installation continues to brake smoothly without grab and with no unusual wear. Brake modulation is essentially identical front and rear and is very controllable with a wide linear range and firm lever action. Happily, there is not much else to say.

Although the original v-brakes worked fine, there was always a certain lack of immediacy or urgency to high speed or steep downhill stops. It's probable that different cables/pads and tweaking could have improved the situation, however, the available BB7 stopping power is fierce by comparison. The ride experience is quite noticeably improved.... :)
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ohzee   1 MW

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Re: Yuba Mundo BB7 Disc Brake Installation

Post by ohzee » Mar 25 2012 1:58pm

great - just ordered me a set of these along with the cable's. Will let you know how the install goes , but sure
it will be fine with your well written instructions.

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Re: Yuba Mundo BB7 Disc Brake Installation

Post by GMUseless » May 06 2012 11:11am

Again Tek, thanks so much for your Mundo write ups. I've referenced them again and again. I just ordered my BB7s. I was curios about the rear cable length, checked your threads, and bam!: SRAM pitstop 93".

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teklektik   10 GW

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Re: Yuba Mundo BB7 Disc Brake Installation

Post by teklektik » May 12 2012 10:04pm

You're quite welcome - I'm happy to hear your Mundo build is coming along - a few little weirdnesses to address but a great bike when you get them sorted out.
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