john578

Hello,

I have a RoadMaster bike with 3 gears in the front and 6 gears in the back, which came from walmart or target a few years back. It's a pretty terrible bike and was left outside for several years, and no longer shifts well, so I bought a "SRAM MRX Comp Bicycle Front Micro Twist Shifter" and a "SRAM MRX Comp 6-Speed Rear Shifter, Shimano compatible".

Unfortunately, after replacing both shifters, no matter how I adjust the cable tension and High/Low set screws on the derailleur, the front shifter can only shift between 2 of the 3 front gears, and the rear shifter can shift only between 3 of the 6 gears. It's almost as if the shifters or derailleurs don't have enough range of motion.

Did I buy the wrong kinds of shifters? I tried figuring out what model or type of derailleurs shifters I had, but couldn't find any markings other than "TD One" on the rear derailleur. Now that I have new shifters, would it make sense to buy some derailleurs that are compatible with them and replace them too? Or would that not work because the deraillers might not fit with the gears somehow?

Or perhaps, is the "not enough range of motion problem" a result of my misadjustment somehow? I'm totally a noob when it comes to bikes .

Andrew R Stewart 

Without having the bike in front of me I really can't say much with any certainness. But here's some thought. First is that the low end ders that I see on the cheaper big box bikes are very poorly made. Lot's of slop in the pivots and pulleys. The cables are often not name brand grade, with brake housing for the gears. Often the chain is a straight plated design of the lowest construction possible. Then the cog and ring teeth lack shifting aid designs and usually have much run out/wobble. Taken as a whole the shifting performance has been nickeled and dimed away with every component. No doubt the shifting sucks.

To try to get the systems working the best you can starts with making sure the cable casings are the SIS ones. Then that the der range limits are set right independent of the shifter controls functions (pull the cables to check der range, do not use the shifter control). Replacing the likely low grade chain with an inexpensive SRAM or KMC will improve the tooth pick up action. Only then as you confirm all else is as good as it can be do you use the twist grip to shift the ders. Good luck, this bike needs it. Andy.

dsbrantjr

Did you place both the shifters and derailleurs in the smallest cog/chainring (low gear in front, high in rear) positions before attaching the shift wires?

Try following these procedures exactly, withour skipping any steps:
Front Derailleur Adjustment - Park Tool
Rear Derailleur Adjustment - Park Tool

Retro Grouch 

A bicycle is a simple machine but the devil is in the details. Cable routing and how the cables connect to the derailleurs is important. It's also important that the shifters be in the smallest cog position when you attach the shift cables. Everything works together so it's hard to make a very intelligent guess about what might be wrong with so little information to work with.

pressed001

This sounds like the problem. It sounds like your cables are either too tight or too loose.

Shift into as high of a gear as possible in the back (smallest cog), loosen the cable from the rear derailleur, make sure the limits are somewhat okay in that the higher derailleur pully is in-line with the smallest cog (derailleur pully nearest the cassette). That sets up your higher limit (somewhat). Then, making sure that your shifter is to the highest gear setting, attach the cable again to the derailleur. You should now have full spectrum shifting assuming that your lower limit is set properly, something you will now have to do by shifting into the lowest gear in back (largest cog) and making sure your derailleur pully again lines up with the cog. You don't want to overshoot it here because your chain will fall in and eat up your spokes.

For the front derailleur it is pretty much the same. Shift to lowest gear up front (smallest cog), remove cable, adjust limit, install cable, move to highest gear (largest cog), adjust limit. Front derailleur adjustment is not easy. You need to get the angle at which it is installed perfect, as well as the height.

Cheers!

CrippledKonaBoy

Derailleur limit screws -- check 'em. Make sure you're getting full travel sideways with both derailleurs. Your parts ARE compatible.

Having done literally thousands of these, I could have you going in minutes -- but my Star Trek transporter is broken (what I always said to people who thought they could tell me to "hurry up"....). Sorry.

There should be a short video on YouTube for that.