DIY BBSHD 15t Chain Guide

[Retrorockit]

This chain guide came to be out of another DIY project here The Mid Drive Cassette Zip Tie Mod.
This guide solved some problems peculiar to that mod. But also solved some more common ones too. AS you can see form the flat 50t chain ring and no Wide/ Narrow feature this bike has chain line issues. It might have been a candidate for a short stacked cassette. But one of the plastic roller chain guides allowed the use of the 2 lowest gears with power off. It ran rough when touching the back of the chain.
The final version of this has the idler bolt back at the fornt edge of the fende and lower down. The factory bracket shown wouldn't reach that far.
The version shown here was used for testing, but the thin frame clamp proved too weak. The idler with a 15t metal cog iside is correct. It runs smooth and quiet and puts the chain where it needs to go just like a derailleur does.
Terra Cycles battery mount clamp is strong enough.

https://t-cycle.com/products/heavy-duty-frame-clamp

https://t-cycle.com/collections/all-battery-mount-parts/products/heavy-duty-frame-clamp
Their drive side idler is too

https://t-cycle.com/products/sport-power-idler-15

Their spacer kit is nice to have also.

https://t-cycle.com/products/idler-spacer-kit

The DIY part is making the 3/8" Aluminum plate to connect those 2 parts
The broken clamp and the newer one are shown here with the adapter plate.

image widget

[Retrorockit]

The clamp on the seat post and the spacers are the adjustments to position the idler back and down to around the 30-40* position. This catches the chain in mid air before it can gets to the chain ring.. One of the bolts is behind the idler. Use a hex head bolt there, To work on the chain remove the front plate bolt, and loosen the hex bolt. The idler can then pivot up without disturbing any of the adjustments. The plate shown above is 3/8"x1". if you go 3/8"x1 1/4" you can drill the plate at an angle to match the seat post angle for a level plate on the bike.
73* seat post (my bike) =17* angle on the plate. Those 2 angles and the 1.97" between holes lets you use a triangle solver program to get the rest of your dimensions.
if you have the plate cut to length for your bike drilling and tapping 2 holes at the end vertically instead of across as shown will help to clear any bosses or other mounts on the seatpost. In my case a torque arm bolt on the other side, and a water bottle boss.. If you want to work in inches set the 6mm bolts 1/4" in from the edge of the plate, and drill 1/4" holes. In fact drilling 1 diameter in from the edge is a good rule of thumb for this sort of thing.
The T Cycle short thread bolt kit is nice to have also.
https://t-cycle.com/products/m8-basic?pr_prod_strat=copurchase&pr_rec_id=61b508954 &pr_rec_pid=550719488059&pr_ref_pid=55071958636 3&p r_seq=uniform
During testing I broke the first version of this. The 2nd version broke my BBSHD doing wheelstands in the first 2 gears. It was a ver.A motor. It's now converted to a ver.B rotor and does wheelstands just fine. I thought I would pull this out of the Zip Tie Mod thread in case that is a step too far for some users. It wouldn't hurt to do this first if you want to try that later.
If you have the ver.A motor you don't need to torture test this. I did that for you.

[Retrorockit]

If you have a wide bike with a small chain ring. There are a couple options for this. One is a 10mm bearing/bolt option for this idler.
The other is a larger 23t version of the toothed idler. Terra Cycle is deep into recumbent trikes, and almost unknown outside of that. So they won't have a lot of answers for questions about this application for their products.

https://t-cycle.com/products/sport-23-tooth-idler

[Retrorockit]

The photo of the parts got lost. I'll try again.

On the left are the over the counter parts that broke. On the right is the stronger clamp with the DIY adapter plate.
In my case the plate is longer, and the hole is offset up to get the clamp lower on the seat tube.
The original parts were sold as an idler in the middle of a long chain, not as a chain guide onto a BBSHD chainring.
They're very good parts for their original purpose. I also broke a lot of stuff that was sold as a chain guide.

The only thing that costs more than doing it right the first time, is doing it right the 5th or 6th time.

[Retrorockit]

I may have found another application for this. My pedal bike now shifts quite differently than my Ebike. Using some Trials/BMX/ Tandem parts I came up with a front freewheel for the XC bike.
The tie in for Ebikes would be the possibility of a Zip Tie Mod for hubmotors. The BMX type freewheel is intended for single speed or IGH bikes, not derailleurs. This Idler has 2 bearings and can guide the chain straight onto the chain ring just like a single speed. This will provide a constant chain line even with a derailleur.

[Retrorockit]

I got the BMX front freewheel going.I added this to feed the chain into the chainring straight like a single speed (BMX freewheel). The 11s cassette on this bike is a little wider than the 8s. The idler is a little noisy in the lowest 51t cog. I almost never use that but it seems to be the limit of what this can compfortably handle. Dialing in the chainline, and squaring up the idler helped a lot, but it's still there.