Bicycle Mechanics - BBs for triple to double on mtb

I was wondering, if I decide to drop the large ring on my mtb for a bash ring, should I use a longer BB axle to reset the chainline for a double? Is this the reason DH bikes have a 118mm standard axle versus 113mm for triple-equipped bikes?

It wiil make the chainline more optimum.

Nope.

DH bikes have a wider spindle to accomidate the wider chainstays. It's all about crank arm clearance.

If you are removing the large ring for a bash guard, you do not need to get a wider spindle.

It would throw your chainline off.

I can toss this slightly into perspective for you.

I ride a 2003 Banshee Scream (http://www.bansheebikes.com/bikes_scream.php) and it requires a 68 (the BB shell) x 118 (spindle) for the crank arms to clear the frame. I run a triple ring for the moment (Going to run an E-13 DRS once I get the money), and there is no problem.

Keep the spindle length as it is and get a bash guard.

Originally posted by BAC5.2
Nope.



I ride a 2003 Banshee Scream (http://www.bansheebikes.com/bikes_scream.php) and it requires a 68 (the BB shell) x 118 (spindle) for the crank arms to clear the frame. I run a triple ring for the moment (Going to run an E-13 DRS once I get the money), and there is no problem.

Keep the spindle length as it is and get a bash guard. Certainly,clearance is a factor. You don't know what kind of clearance he has. A shorter spindle could optimize the chainline for the double setup.

Chainstay-to-chainring clearance is fundamental. Unless you have a very poor chainline between your inner ring and smaller cogs, keep your 113mm spindle.

Thanks guys. To respond to a few points:

BAC5.2: surely I'd want the double chainline to align between the 2 rings (as per road bike), rather than on the middle ring as with triple, hence the question. But then again, maybe not.

Boudreaux: this isn't downsizing a DH bike - short spindle not an issue

John E : not an issue for me, I never cross-chain in this way.

Maybe I'll just stick to a 113mm, anyway.

Simply, replace the big ring with your bashguard. Road bikes and Mountain bikes are different. Aligning the chain between the two will keep you at a slightly canted angle, and considering most riders are in the middle ring more than either of the other two, I want my chain line setup in the middle ring.

Keep it how it is.

The stock chainline is optimized for a triple ring. Because you lose the big ring, doesn't make it any less optimized. 113 is fine.

Road bikes are setup with the chain line in the between the two rings because of the huge difference between the rings. a 22-32-BG setup is not a significant step up and a wider stance is only going to cause problems. Don't go wider than necessary.