Double Chainguide, a Cautionary Tale...

Chain Gang

Losing your chain from the cranks a bit too regularly? Me too. I wanted to use a chainguide on my bike to fix that. I have a double chainring setup, so that means going with a guide like MRP's Long Range Patrol or the Blackspire Stinger:

MRP's Long Range Patrol - bashring and chainguide.

If you're running a single ring then you have the choice of the full-on DH type guides, but that's another article.

Standards!

There are standards for everything... including chainguide mounts. There are a few different ways you can fix one of these widgets to your bike, either by tabs welded to the bottom bracket shell (ISCG or ISCG05), or by clamping it on with the bottom bracket cups.

ISCG standards (originally posted on rotorburn.com)

Unfortunately for me, I don't have those ISCG tabs. I am limited to the other mount type - I have to have the guide clamped on by the bottom bracket.

Geometry time

Problem: you need your chainrings to line up exactly with the roller on the guide.


You're either lucky... or you need to move the cranks or guide around a little bit.

• A normal (68mm) bottom bracket fits on with either one or two 2.5mm spacers between the bracket cup, and the frame (drive side) and one spacer on the non-drive side.
• A 73mm bracket (that's mine) has one (or none) spacer on drive side, and none on the non-drive.

The normal mounting technique for a chainguide, is to remove a drive-side spacer and replace it with the chainguide. So you might be able to move the guide in and out a bit to match the chainrings if you have two spacers. Or you can add a spacer to move the crank out a bit, if the crank system is ok with that. Park Tool has some handy diagrams to illustrate the situation:

68mm shell + no guide = two spacers on drive side, and one on non-drive side

68mm shell + chainguide ("E-type bracket") - one spacers on drive side, and one on non-drive side 

73mm shell + no guide = one spacer on drive side, and none on non-drive side  

73mm shell + chainguide ("E-type bracket") - no spacer on drive side, and none on non-drive side  

Lottery Results

So, I fitted everything up, and the chainguide was too close to the chainrings. Not only did it not line up, the granny ring bolts scraped the chainguide plate. Not cool!

SRAM cranks?

Unfortunately for me, I have a Truvativ (SRAM) crank. These cranks have a chainline that is fixed by the location of the opposite bearing cup. And I couldn't move that around. No options to move the chainguide (only one spacer to remove) and no options to move the chainline (fixed by non-drive side). If I had a normal 68mm shell I might have been able to move the non-drive side in a bit, by removing that spacer. But, I didn't - I have a 73mm shell.


So when it came down to it, my only fix was to buy a replacement crank - or ditch the idea of using a guide. Shimano & FSA cranks use a preload bolt (its a lot like a bar stem & top cap), and the chainline is fixed by the position of the drive-side bearing cup (think adding or removing headset spacers). RaceFace cranks use drive side washers and can adjust chainline one or two mm.

"You're buying more bike bits?"

After hunting around down the back of the sofa and raiding the piggy bank, I've bought an FSA crank. Even with the slightly wider (73mm) shell, I should be able to get the chainguide in line with the chainrings.


That's it, this is all just a cautionary tale - you can't fiddle the chainline with SRAM. That's never usually a problem, but if you want to use a chainguide it might end up being a small and expensive nightmare!


Update: The FSA cranks did indeed fix the problem. They have a slightly wider chainline - so no problems with the small ring contacting the chainguide plate.