Purdy's Rear Derailleur Tweak

Our favorite mechanic Mark Purdy isn’t just meticulous, he’s also creative and adventurous. He told me about a rear derailleur cable routing he’s been using for mountain, cx, and now road bikes, and being a tinkerer myself, I had to try it too.

Purdy’s idea is to replace the last loop of cable housing with rigid tubing. Why rigid tubing? First of all, it’s compressionless. Even though derailleur housing is supposed to be compressionless, it can still give, especially around curves. Secondly, rigid tubing can be bent to tighter radiuses, so you can tighten up the loop and run less cable. (Purdy adds that although Nokon housing can make tighter turns than standard housing, it compresses even more under load when curved.)

Purdy’s tried a full length piece of copper tubing on his own bike, but found that the tubing can get bent when knocked around. (This only works with Sram derailleurs because they’re fixed on the b tension stop on the hanger. Shimano and Campagnolo derailleurs float on spring tension, so they require some play in the housing.) For my bike Purdy will replace the last part of the housing with a mountain bike v-brake noodle.

Purdy’s a tool slut – he has a tool that pulls and cuts zip ties (he gets many of his specialty tools from harborfreight.com). Here he uses a tubing bender to tighten the radius of the noodle.

Matching the noodle to the housing path. Purdy could’ve routed the cable even straighter, but the housing has to go above the skewer to permit wheel changes.

After cutting the housing to length, Purdy grinds the ends square. According to Purdy, this step is essential for a smooth shifting system. An oblique cut can settle over time and throw off your shifting. It can also make the housing bend on the stop under tension.

All assembled. Note the excess cable coming out of the derailleur – that’s how much shorter this routing is.

So does it shift better? Well, I’m not going to time my shifts down to the millisecond, so I won't make any claims. It’s subtle at best, but my shifting is as snappy and precise as it’s ever been. This is no panacea – it won’t fix a bent hanger or a mucked up cable, and the greatest gains will be in the dirt – but it can make your shifting feel a little lighter and smoother, and it can make you a little more confident that your chain won’t hesitate as you shift and stomp on the pedals.

You can get a v-brake noodle here, and a tubing bender here. Or just go see Purdy already.

Comments

cable ties

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Sat, 02/04/2012 - 4:55am

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0 Up

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shimano mech?

By: Noah Polished

Wed, 07/20/2011 - 5:14am

i'd really like to try this on my Deore LX mech. ok i under stand the SRAM vs. Shimano b-tension concept, but wont putting a noodle on an shimano mech help a bit annyway?

harbor freight

By: Lorenzo Rivnut

Mon, 05/24/2010 - 6:24pm

There is a harbor freight store in NJ, so you don't have to wait for UPS to deliver. If you get any recent car magazine, you can get a 20% off any single item coupon.

You need to be careful buying their tools some are decent, some are not so great.

Here's the pulley:

By: Andy Shen

Thu, 05/20/2010 - 6:18pm

Here's the pulley: http://nyvelocity.com/content/equipment/2010/kcnc-ceramic-derailleur-pul...

It's only there 'cause the Sram one cracked.

Cool idea

By: jyalpert

Thu, 05/20/2010 - 4:27pm

Love the plug for Harbor Freight. They are the Nashbar of tools. They send you a 30 page glossy catalog every couple of weeks that makes for great crapper reading. "Oh man, imagine what I could do with a $400 off-brand hand-crank automobile elevator!" It's what Billy Mays would sell if he ran a hardware store.

My humble suggestion for would-be tinkerers is McMaster-Carr. They are god's gift to machine shops. They sell every possible kind of tubing, metal or otherwise, in any diameter, as well as benders. They also sell pretty much anything else you could ever want for building things, with the best online ordering platform known to man. They have saved my ass more than once.

If this really works, I'd definitely consider asking Mark to install it on my soon-to-be Spooky build.

Proven

By: jkornbluh

Thu, 05/20/2010 - 3:13pm

Mark used this set-up on my CX brakes and it works really well. Anyone who has raced CX knows that brake cable drag can cause a lot of consternation when making sharp moves on a course. This system eliminated the drag and my cantis feel much stronger.

Mark = Evil Genius