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.
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:
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).
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.
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
- 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.
-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
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.
Get a bash guard and chainring bolts / spacers (see Materials section above) so that it can be mounted with enough clearance per 1.
(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).
- 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
- 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).
- 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.
- 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).
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).
- Prevents Bending of discs when the uni crashes.
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
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!