I am converting my Sonders eBike from a single speed to a three speed and I need some help finding the best Derailleur. I would like to use a Shimano Altus as I understand that they are a step up from the Tourney line. My rear freewheel cogset is 16,19,22t and my front chainwheel is a 48t. This gives me the same speed ratio as my original when I'm using the 19t rear cog. The original chain is 1/8" and I believe that my new chainwheel is also 1/8". My derailleur uses a SunRace Shimano Compatible Index Hanger Plate. The question is, do I use a short, medium or long cage and what is the model number? Thanks for your help, I am converting 2 of these bikes and would like to only buy the parts once.
Yes - Altus are a step up from the Tourney lineup, but is considered a very entry level MTB product. I would encourage you to consider a higher spec model if you can afford it - I suspect the price difference will be small if you shop around.
Derailleurs are rated by the capacity and maximum tooth size. As you are run a x1 setup, you capacity requirement is 22-16 = 6, and the maximum tooth size of 22 is smaller than a typical road cluster, so any derailleur will do the job. As such, using a short cage will give better performance.
You don't say what spacing the three cogs are - If you are using an indexed shifter make sure it matches the cog spacing of your cluster. As always Sheldon Brown has all the info you need
I ran across my old question after 3 years and found that there were over 700 looks so I thought I would share the outcome of my project.
I have 2 yellow Sondors originals and I participated in the 3-speed conversion blog on the Sondors user site. https://sondorsforum.com/topic/20-3-speed-conversion/ This site has more information, pictures and how to's about Sondors 3 speed conversion than anyplace on the net. As for derailleurs, I used a bit higher model than most because I got a great deal on two of them. I used a Shimano 105 SS short cage model RD-5701-SS with a 52 tooth chain wheel. The Shimano 105 is considered a touring model and is very robust, falling at the top of the mid-priced models. The slightly larger 52 tooth front sprocket gives me the original ratio on the lowest 16T cog instead of the middle 19 tooth, but the original cog was too low, to begin with greatly limiting my top speed. Now I have two taller gears, more important to me so I can go faster and keep up with light traffic.
The biggest problem I had was finding a shifter that would shift the odd spacing that the Chinese cogs have. Most of the other converters on the blog used a Shimano friction shifter or a click shift. The click shift usually had to be shifted up or down one extra notch then back shifted to make a smooth chain ride. I solved this problem by building an electric shifter powered by a small stepper motor and an Arduino tiny and selected by 3 switches on the handlebars. Details can be found on the Sondors user's blog or search youtube for DIY electronic bike derailleur.
This question is old but I just cannot resist adding my advice concerning the shifter. This kind of setup would work great with the friction shifter. With only 3 speeds, the highest and lowest gears would be a cinch to shift in and out from. It would take a bit of skill to reliably hit the middle gear but it would get easier with practice. Just for future reference for anyone doing such a conversion that wants to keep it simple.