Bash Guards for Unicycle Disc Brakes

Unicycles and Equipment
1

Quick Review / How-To: Bash Guards for Uni Disc Brakes

Why / Background:
After using discs for the past few years, I’ve been frustrated by bending discs when I UPD in rocky areas, which happens frequently on the dry rocky trails I ride. When I bail, the unicycle will fall with the disc hitting a rock on its face, which bends it out of true. This causes severe rubbing on the caliper and the disc has to be bent/trued back on the trail - which is difficult.
Lately, I have switched to using Magura rim brakes, which I now prefer. Seeing that the support for rims brakes is dwindling fast though, I decided I would try to solve the issue of bent discs with a bash guard. So far (2 weeks) this setup has solved my issue - I have taken several falls where the bash guard prevented my disc from bending.

Overview:
There are several disc systems available for unis - KH External disc, Mountainuni external disc, Nimbus Internal Disc. Each system has different options for bash guards.

I will cover 2 options here:

  1. Chainring bolt mounted bash guard - works for Mountainuni disc systems
    The mountainuni disc system uses standard bike cranks, but mounts a disc in place of chainring. This system made it easy to buy a bash guard (or chainring) - I just needed to buy something that has 5 bolt holes and uses a 110mm bolt circle diameter (110bcd).

  2. Axle bolt mounted bash guard - works for KH and Nimbus disc systems
    This option uses the ISIS bolt and axle to mount a guard with a central hole. Thick aluminum bmx chainrings are a good guard because they use a central hole that fits the ISIS bolt. On KH cranks , the chainring bashgurad needs to be cut to make room for the crankarm.

NOTE - There are other options of mounting a bash guard to the uni frame. I haven’t tried these yet. They potentially could be lighter and not add rotational weight, but they would also stress the frame

When mounting the bash guard, it is most effective to mount it in the space opposite of the crank arm where the disc is unprotected. Leaving some gaps between the bash guard and the crank arm seems to be OK.

Materials:

Dremel or similar tool with cutting wheels (to cut aluminum - needed for option 2)

Measuring caliper or tape / ruler

Metric Hex Key Set

Uni with disc brake setup

Bash Guard:

  • Option 1:
    -39 to 44 tooth thick (DH or BMX) chainring with 5 hole 110bcd bolt circle, or a chain / bash guard for 36 to 44 tooth 110bcd chainrings
    Examples that can be purchased online:
    – Odyssey Utility Guard BMX Bash Guard 39t size ( qty 1 or 2 - used here)
    – Girvin Rock Ring 110BCD
    – Salsa Tooth Fairy 110BCD
    – All City Crossing Guard 110BCD 44t

-Long / triple chainring bolts, nuts, and spacers (>= 30mm long)
– Example on EBay: https://www.ebay.com/itm/5-Pairs-Crankset-Bolts-Crank-Bolts-Bike-Chainring-Bolts-Crank-Screws-Nuts/332155658458?ssPageName=STRK%3AMEBIDX%3AIT&var=541186766143&_trksid=p2057872.m2749.l2649

-Chainring Spacers - I had to get enough of these to space the bash guard out from the disc so it cleared the caliper.
I needed about 10mm worth of spacers per bolt for my caliper. It helps to get some that can fit around the outside of the chainring nut as well.

  • Option2:
    -39 to 44 tooth thick (DH or BMX) chainring with center /axle mount hole, or Origin8 Power Disc
    Examples that can be purchased online:
    – Odyssey BMX Chainring 44T
    –Eighthinch BMX Chainring 43t (used here)

-Long (32mm) ISIS axle bolts (needed for option 2)
– Example from UDC that should work: https://www.unicycle.com/extra-long-isis-hub-bolts-m12-x-1-0mm/

Steps: Option 1 - Mountainuni

  1. Measure Disc Caliper Clearance:
    -Measure the distance from the where one of the disc mounting bolts meets the crank, to the outside of the caliper and 5mm or so for clearance. It is likely this will be at least 10mm.

  2. Get a bash guard and chainring bolts / spacers (see Materials section above) so that it can be mounted with enough clearance per 1.

  3. (Optional) Modify bash guard:

  • The bash guard can be cut using a Dremel or similar tool, or drilled to fit better or save weight. When using a chainring as a bash guard, I removed a portion of it because it interfered with the crank arm (see pics).
  1. Mount the bash guard:

  • Remove the existing chainring bolts from the crank arm disc mounting holes where the bash guard will be mounted.

  • Insert the longer nut into the 1st disc mounting hole on the crank arm ( make sure any disc spacers are also in place that were there previously).

  • Add the spacers on top of the 1st nut

  • Loosely mount the bash guard and tighten the bolt so it has enough play to mount the other bolts/nuts/spacers

  • repeat for all of the holes used for the bash guard.

  • NOTE: mounting the bash guard and the spacers was akward. It took patience. It was easiest to do with the wheel off of the uni frame, but I needed to mount at least the first nut/spacers/bolt with the wheel on the frame to check for clearance with the disc caliper.

  • The pictures for Option 1 show the process and results of 2 different bash guards (cut DH Chainring and Odyssey Utility Guard BMX Bash Guard) installed on a mountainuni disc crank with a Shimano BR-M486 Caliper

Steps: Option 2 - KH / Nimbus

  1. Get bash guard and long ISIS bolts
  • Bash guard must be about the diameter of the disc and have a central hole large enough to fit the ISIS bolt (bmx chainrings work well - see Materials section above).
  1. Cut bash guard to fit crank
  • The bash guard must fit flush against the crank where the ISIS bolt goes through the hole. Any crank (KH, Sun) where the crank arm protrudes out from the axle will require the bash guard be cut so it can mount flush. For an aluminum BMX chainring, it is easy to cut these to the correct shape with a Dremel or similar tool.
  • (optional) grind teeth off (if using chainring), drill extra holes to save weight.
  1. Mount bash guard

  • Remove ISIS bolt on disc side crank ( note on external disc / KH this will be the right side as shown in pics, but for Nimbus / Internal disc this will be on the left side)

  • Put new long ISIS bolt through hole in bash guard

  • Mount bash guard and ISIS bolt. Tighten long ISIS bolt down slowly, being careful to keep the bash guard center over the hole and aligned so it is rotated correctly on the crank (so it protects the disc opposite the crank).

  • NOTE: Make sure the long ISIS bolt has a good number of threads engaged in the axle so the crank stays on and the threads hold if the bash guard takes a hit. Personal judgement is required to determine if the crank is secure and the bolt is threaded to the axle sufficiently. I also put thread locker on the end of the ISIS bolt.

  • The pictures for Option 2 show the process and results of using a BMX chainring bash guard installed on a KH Spirit disc crank with a Shimano BR-M446 Caliper using a 203mm disc (the 180mm disc fits fine too and is covered even more by the bash guard).

Conclusion:

Both of these bash guard methods have solved my problem of bending discs. I have only tested them for a few rides, and will post follow-ups for long term durability etc. I have not had any issues with the bash guards hitting my feet or ankles yet. I also haven’t had the long ISIS bolt come loose (option 2).

Pros:

  • Prevents Bending of discs when the uni crashes.

Cons:

  • Adds weight 125g - 225g - rotational

  • Adds stress to chainring bolts (option 1) / Axle bolt (option 2)

  • Could cause issues with crank bolt loosening if not enough threads are engaged (option 2)

  • If bash guard gets bent enough it could also bind against caliper

Disclaimer:

This article was posted so I could share a shade-tree DIY solution that has worked for me so far, but will probably end in complete disaster for you or anyone else. I make no guarantees of its effectiveness or safety. If you damage your cranks or braking system with this it will likely void any warranty you have on those parts, so there!

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Bash guards for Unicycle Disc Brakes - Option 2 Pics

Pictures for Option 2 in above post.

NOTE - Disc shown loose on top of bash guard is a 180mm disc, but the disc mounted on the uni is a 203mm disc.

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This should satisfy the “disc brakes are a health hazard” crowd :stuck_out_tongue:

4

This improves protection of the disk. The teeth on the sprockets may or may not be problematic as decreases clearance between crank and pedal. A good start though.

5

omg.
you don’t wan’t to get caught by that chain ring with your calfs. This could end the same way as it ends with the old chainsaw discs from Qu-AX.

I would chose a non rotating bah guard, a static housing of the discm attached to the bearing cap or even clamped between bearing cap and frame.
Maybe sth. like that could be modified:

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Yes - the chainring teeth are a bit risky looking. So far they have not sawed off my ankle, or even touched it, but I see your point. I was planning to grind them off of the bash guards eventually for aesthetics and safety. Of course, on the other hand, bicycles place multiple chainrings at the same location and there are a lot of people riding those with no issues. I think that for muni though, there are many more UPDs than for a bike on any given ride. That would make the chances of getting sliced by teeth higher.

I like the idea of a frame mounted bash guard for internal disc setups. A simple version that mounts to the end caps and disc caliper mounting points would be pretty easy to make with simple tools. I am concerned though that it could put extra stress on the frame, especially if an impact caused a lot of torque in the wrong place.

Thanks for the inspiring pictures! Are all of those bash guards custom made or do you have links where they can be bought?

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I just did a picture search for “disc brake guard”. Such guards can be bought, afaik. But some of the pics look custom made.

9

Cranks with large Q factor essential :astonished:

10

Not more than regular. As the bash guard does not need to be thicker than the caliper.

11

I think I definitely prefer the one from Eric’s pictures that are attached using the caliper posts, at least then they’re not spinning! :astonished:

EDIT:

If you’ve got your heart set on a spinning bash guard of doom, is there any reason you couldn’t just attach one to the KH Spirit cranks (where the disc normally goes) but use an internal, hub-mounted disc brake, rather than having both attached to the crank? So get an Oracle or QX disc uni, but put KH cranks on, and use that crank to attach your bash guard.

12

That would work, except that the inboard brakes mount to the left hand side of the hub and the KH mount is on the right.

13

So flip the hub upside down? Is it handed in any way aside from the disc mount?

14

And where do you mount the caliper then? To the front? Nono. Inside is always left, outside is always right.

15

Sorry if I’m being dense, I’m really not sure how this is an issue…

If you take a Nimbus hub, and flip it over so the disc mount is on the ‘wrong’ side, are you not still able to mount the caliper right where it normally goes (Either on the KH frame’s caliper mounts, or using a Nimbus d’brake, mounted to the wrong fork)? How would this result in the caliper being on the front? :thinking:

I suppose there are also those M4O frames that have a caliper mount on each side…

Then your KH crank will be on its ‘correct’ side, and you can screw a bash guard into it.

16

The caliper is not ambidextrous. I think the only way your plan would work is to take an inboard mount frame like a Nimbus and turn it around so that the caliper is on the front right, with the rotor inboard. That would look funny but should work ok.

17

Don’t do it. The calipers are not designed to withstand forces in this direction. While some may work, others will fail …
We had that discussion more than often …

18

Hmm, can the caliper go upside-down? :smiley:

And yes, at this point I’m being intentionally obtuse :roll_eyes: I still reckon the bash guards mounted to the frame (either as an extra bit under the caliper screws, or sandwiched between your bearing cups) would be the best bet, if you really wanted one. With the ‘controversy’ around discs in the pro peloton I can’t imagine it’ll be long before all the manufacturers are making them, and there’s bound to be one that’ll either work or be easily modifiable to fit on a uni!

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Yes, but then it points again to it’s original side :wink: You can turn it in any direction you like and as often as you like, but the mounting holes will still stay on the same side of the caliper, even if you close your eyes, shake it as fast as you can and say hokus-pokus … :wink:

20

It will be a while before I am unicycling with brakes. Not sure how much I will care for the external disk until I try it.

Sorry but these external bashguards look very in the way. Those teeth are definitley a no go. I think they should be fixed to the frame, non rotating.

Rim brakes are more out of the way, but I have read that they cause problems and dissatisfaction. In bike trials some prefer them, as they are simpler, lighter, but they want instant lock up, not so much as gradual controled resistance.