I don't know how I did it?


Well-Known Member
I have been testing converting my Wart Hog to a single gear drive, getting the correct chain lengths, for several gears etc. This is being prepared for when I destroy the the derailleur while trail riding, going thru the heavy sagebrush that is along side of the trail.

I was testing down to the middle smaller gears, (11-13-15-17-20-24-26-30-34T) and all of a sudden I see a brief flash of a code 21, for (2 sec), when I started pedaling, which indicates a wheel sensor error?

I was adjusting the tension screws on the rear wheel, which slides the locked complete wheel assembly forward and or backward to tighten the chain for adjusting the slack, with just over 1" of movement etc.

I have been testing for the last week and never seen this code show up, before.
I haven't touched the wheel sensor for any reason.

What's going on?
Your thoughts and suggestions..........
I found that sliding the axle plate (which contains the locked axle) back and forth, as little as 1/8" difference makes a big difference on the speed sensor magnet of the rear wheel so it does not work correctly and throws an error code..


In the above photo I ripped off, Thanks PCebiker,
you can see the 2 silver bolts, and at 3 o'clock, the adjustment bolt for sliding the axle plate, behind the 2 silver bolt heads, to see what I am talking about.

The end of the Black axle bolt is just above the shift cable and holds the other side plate on both sides etc. (The red circle has no bearing on what is being talked about).

I went back to my log book and found the original setting, measured from the back of the plate to the screw shank and or just change the wheel magnet distance setting from the tire rim..

Reset this distance and NO code 21 showed up, moved about 1/8" and the code showed again, so now I know what and how this happened. ymmv
Hopefully, your magnet or sensor is adjustable so that you don't pop a code in the field when you deploy your chain tensioner.

It's probably good practice to try to adjust sensor/magnet to get a feel for doing it, and what tools you'll need.
And also to see what happens if you pop code 21.
Your power assist might shut down or something?

It's good to know if you can ignore that code so that you won't have to deal with it until you get back home.

PS,.. I stole the picture too. 😂
Yes, all I need to adjust/move either the magnets or the axle plate is a set of Allen wrenches that are always carried on the bike.
I am glad that I bought 2 extra chains as I have to add 8-10 links to match the OEM chain length, I'll have to cut several links off the chain to fit the big 35T (slowest) gear, as there will be more slop in the chain than I like, when using the single speed gears.

I have a couple more tests for the speed end, but I think I can get by with just 2 different overall chain lengths, to cover all the single gear selections, fingers crossed.

This means, I can precut the short chain lengths needed and just add a quick links to finish the connection, and 2 adjustments of the idler pulley arm length will cover all the 9 gear single selection.