Just wanted to share this experience out quickly for all Dads (for all) out their finding their child can't turn their piece-of-shugar Sunrace shifters on their entry level MTB bicycle; so I apologize that this blog entry will not include pictures (at least yet) or much installation detail. I have done the research, executed the solution, and am now reporting the success.
I had bought my 10-year old daughter a second-hand 24" Diamondback 'Tess' and fairly soon after I realized that her inability to turn the shifters was not just related to learning to change gears, but that the shifters were indeed very difficult to twist, requiring even for me, considerable torque. Researching online, I did manage to find other Dads who were questioning these shifters; unfortunately, few were getting any really useful feedback, the right feedback being: the Sunrace shifters suck, change them.
On investigation, I noticed that a common Grip shifter model on other similar caliber 24" MTB bicycles - notably my son's - but many others well, was the Shimano Revoshift. These are about $21 on Amazon and from what I could tell with my son, they worked fine (and appear to get good, albeit few reviews). Another one that I saw recommended a few times and a few bucks more is the SRAM 'MRX' (about $25 from what I can tell). I ultimately bought the Revoshifts for 3x7 or 21-speed because they were in stock and I had (my son's) experience with them.
The Revoshifts arrived with their cables already installed into the shifters, which is good, since the removal of the cover to access the cable 'seat' inside is a teeny hassle (involving 'tabs' rather than screws). Even worse are the Sunrace shifters which are clearly designed to be cheap; they don't even have a removable cover and must be removed from the handlebars and completely dissembled to remove the cables without cutting them (itself quite a hassle; I did it before I realized that I didn't need to, thanks to the blog post http://www.mystoopidstuff.com/sunrace-shifters.html).
So first you got to remove the Sunrace shifters. If you're like me and just a very occasional bike mechanic, mostly just trying to save a few bucks on easy repairs (and avoid the likelihood that the bike mechanic pro won't let you leave the shop without overhauling several other items that you really didn't realize nor want to deal with), I recommend you do each shifter replacement separately in sequence and completely so to have less of a chance of screwing up the arrangement of cables in front of the handlebars and on the frame. This one qualifies as an easy repair IMHO.
Actually first, you need to remove the rubber grips. Unlike what I just said above, go ahead and remove both rubber grips (not both shifters). There are several videos on this on Youtube (https://youtu.be/cxkSXYKaDh4 is one); and is shockingly easy. Can't say if this is bad method in the long haul, but it sure makes removal painless and fast. Simply insert a longish thin tool, such as a small gauge allen wrench into the grip (not through the grip! not from the ends of the handlebars! But between the grip shifter and grip) and, by turning your bike wheel so the handle bar slopes down so that you can pour in some rubbing alcohol (I inject a tiny quantity using a syringe) into the space created by tools in the grip. Twist (initially hard) to work in the alcohol, and suddenly the grips practically slips off. Apply a little alcohol into the grips to replace them at the end. Once the alcohol evaporates the shifters will stick on again securely.
Set gears on smallest chainwheel and sprocket to release tension on the derailleur cables. Now go ahead and disconnect the cable from, say, the front derailleur (i.e. unscrew the cable clamp, and start to pull the steel cable out. Leave cable housings roughly in place or set aside as you prefer, assuming these are in reasonable condition and you intend to reuse them for the new shifters' cables. Go ahead and cut the steel cable near the shifter if you like (I just bought bought a cheap, well rated cable cutter on Amazon). Note the arrangement of the shifter and its cable on the handlebar; is the cable in front or behind the brake handle? Does the cable pass through or in front of the brake cable, etc? Use a very small Allen wrench to unscrew the locking screw on the Sunrace shifter (I think this was a 2mm wrench), and slide this off the handle bar. Now throw away this piece of trash with gusto! Feel yourself ridding yourself and your child of all this negative energy! Do NOT try to resell it - do NOT inflict this piece of garbage on some other kid!
Place new front derailleur shifter onto the handlebar in the same position as previously. take a rag and apply some teflon containing chain or similar oil to the cable. Thread cable through the housing(s) and around any frame bracket provided for this cable. Pay attention to the arrangement of the cable relative to the other cables so it's not restricting itself or another cable's free movement. Pull taught, insert end into the front derailleur clamp and tighten. Cut excess cable (I like to leave a few inches). Place crimp (I temporarily used a jewelry crimp, while I wait for bike cable crimps to arrive). Fold end out-of-way. Done with that side.
Now repeat this procedure with the other side. Replace rubber grips as describe above.
Test. Should be good to go. This should have/could have taken 30 minutes besides computer time reading and ordering parts. 60 minutes or longer, because you had to fumble through these instructions while doing the replacement and finding the tools.
Let me know if you have any questions.
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