Singlespeed & Fixed Gear - Rear derailleur for a 5-speed freewheel cluster?

I'm planning to build a cummuter with a single-speed crankset up front and a 5-speed cluster in the back. The hub I have comes with a 5-speed, so I'm sorta working around that. It's also a mountain bike set-up.

My question is what derailleur is compatible with a 1x5 set-up? I will be using a SRAM MRX 5-speed grip shifter, but still unsure of what rear derailleur to use with a 5-speed. Looks like there aren't too many out there. Will a 6- or 7-speed derailleur work then? Or even a 9?

What about chain length for a single-speed crankset?

Thanks!

Your rear derailleur should be a SRAM one that works with SRAM's 1:1 ratio that most of their gripshift and trigger shifters use. I believe the exceptions are the SRAM Attack components, which are designed to work with Shimano rear derailleurs. You can use a 6, 7, 8, or 9 speed SRAM rear derailleur, just adjust the limit screws accordingly. It should work. Probably. As to chain length, put the chain on the small cog in the back, through the derailleur, and around the front chainring. line up the chain so that the rear derailleur pulleys are parallel to the ground and the chain isn't touching the derailleur cage. Hope this helps.

I'm planning to build a cummuter with a single-speed crankset up front and a 5-speed cluster in the back. The hub I have comes with a 5-speed, so I'm sorta working around that. It's also a mountain bike set-up.

My question is what derailleur is compatible with a 1x5 set-up? I will be using a SRAM MRX 5-speed grip shifter, but still unsure of what rear derailleur to use with a 5-speed. Looks like there aren't too many out there. Will a 6- or 7-speed derailleur work then? Or even a 9?

What about chain length for a single-speed crankset?

Thanks!

Having done several of these conversions...ANY modern rear deraillieur will work w/your 5 sp cluster provided the der cage is long enough to clear the largest rear cog. For example a Tiagra 27t road der will NOT have the clearance necessary for a 30, 32, or 34t cog, but will BARELY clear a 28t...I know. I've got this particular set-up on my 1x8 road bike. Refer to http://www.sheldonbrown.com for how to set your 'B' adjustment screw. Once you've established that the der has sufficient clearance then the 'lateral swing' part is easy. Turn both the 'H' and 'L' screws in as far as they will go. Attach your der to the frame.

Then place your chain on the front chainring and loop it around the LARGEST cog on the rear cluster. Bring the two ends towards each other w/o going thru the der and remove the 'overlap', but leave one extra link. This will assure you've enough chain to weave through the der pulleys and enough length to go around the largest cog. Use an 'Unramped Chairing' such as a Rocket Ring http://www.bikepartsusa.com as the teeth are all the same length. Ramped chainrings have a tendency to 'chaindrop' when shifting from bigger to smaller cogs. The BMX or Messenger type of CR all but eliminates this problem(Once or twice per year as opposed to once or twice per week). You can leave your front der to act as a 'chainguide', also. When you've got your chain to the proper length weave it thru the der pulleys and re-connect it. Then while rotating the chainring turn your 'H' adjustment screw out until the chain drops onto the smallest cog. Then pull on the shift lever until the chain wont go any further back on the cluster and turn the 'L' screw out until the chain goes onto the largest cog. This is called 'lateral swing' adjustment. Then run the chain/der back onto the smallest cog. While the drivetrain is static loosen the 'cable screw or bolt and pull the cable w/a set of pliers until it's tight. As you're holding/pulling the cable w/t pliers tighten the cable screw(or bolt) back down. Then repeat the 'lateral swing' adjustment process and you should be good to go.

My preferences for this type of conversion are as follows: Sugino 103mm square taper track bottom bracket from http://www.universalcycles.com Brings the cranks in real nice and tight to the frame and gives a nice straight chainline in you're climbing(low)gear. It's a 'sealed' cartridge type that is 'English' threading and should go right into your bb w/no probs. You'll need a Park BBT-2 bb tool, though. They're available @ http://www.bikeisland.com w/no shipping charges. Rocket Ring Chainring from 34t to 48t...you're choice of size. Bulletproof bmx crank set...from 140mm to 170mm w/a 110mm bolt circle. BMX chainring bolts and crankset bolts. Sachs/Sram PC-58 chain. All available @ BPUSA.

Good luck. I've been running 1x7, 8, 9 for years now. I'm, primarily a commuter and realized pretty early on the small chainring on road bikes is unnecessary for commuting once you've got your terrain dialed in. I've done the same w/mtbs, too when I commuted urban.

I'll check my parts box. I may just have an old Suntour GT 5 speed derailleur lying around. If so, I'd be happy to give it to you for the cost of mailing only. I'll check and PM you if I still have it.

Thanks for the info, guys! That's what I needed to hear! The build is gonna be a budget build but still with some decent parts. I'm gonna go with a SRAM SX4 rear derailleur to go with the SRAM MRX 5-speed shifter. The chain will be a KMC Z50 5-8spd chain. Crankset will be a Bontrager single speed with 32t.

Thanks, Tom, for the offer. I think I'm gonna go with the SRAM derailleur!

According to this:
http://www.sram.com/en/srammountain/compatibleshifters/mrx/twistshifter.php
the MRX is a 2:1 ratio, which means it's not gonna work with the SX4. Any index-able shimano derailer should work.

This:
http://www.sram.com/en/srammountain/components/sx4/rearderailleur.php
lists the SX4 as 1:1

It's crazy how the manufacturers put specs on their site!

Interesting that you should mention that, SV. I didn't order the SX4 rear derailleur because Jenson didn't have it in stock and it's not sure when they will get more in. I wanted to get it from them because they diid a price match and I could do a pick up. It would have been cheap for me. I instead got the SRAM ESP 3.0, but based on what you said, I guess it won't work with the MRX! Grrrr!