Mountain Gear

Tech FAQ: 2x, Head Angles and Bleeds

More on 2x drivetrains; why twitchy or slow and a thought on clean brake fluid

Dear Lennard,
I am currently in the process of converting my mountain bike from a 3×10 set up to a 2×10 setup. I have purchased new chainrings and a bashguard to replace the outer chainring as well as 2×10 shifters. However I have not purchased a new front derailleur, and was just going to adjust the hi/lo limits on it. Do you think that will cause any problems with shifting?
-Patrick

Dear Patrick,
Yes, it should work. I’ve done the same thing before on one of my bikes, and it worked fine. Keep in mind that 2x mountain bike front derailleurs did not even exist when riders were converting their 3x9s to 2×9, and those worked okay—well enough to inspire SRAM to come out with 2×10 groups. Let me know how it works.
-Lennard

Patrick’s reply… and photo

Dear Lennard,
Yeah I actually did try it and it worked; my local bicycle shop just adjusted the Hi/Lo limits on the derailleur. You have to hold down the shifter to get it onto the big ring but it shifts very smooth and is not much of an issue. I attached a picture of the finished product. The conversion was much cheaper than buying a new crank. All in all it was about $200 for the new shifters and chainrings.
-Patrick

Converting a 3x9 to 2x9 can look like this... bash guard in place of the big ring (slippers optional). Photo by Patrick

Dear Lennard
Am a fan of your column and would like to ask about headtube angle on mountain bikes. On what conditions of trail would you want a slack headtube bike and when do you want a steep headtube?
-Leonard

Dear Leonard,
A slack head angle provides more stability, slower steering and a longer wheelbase. A steep head angle is more twitchy and less stable, faster steering and shortens the wheelbase and hence the turning radius. And, of course, the more fork travel you have, the more degrees the head angle steepens by on compression, so you want to start with a slacker angle to not end up with the wheel underneath you on big front impacts.

Also, because of the larger wheel diameter, a 29er will need a steeper head angle for the same amount of fork travel and type of riding as a 26er, since the fork trail (the distance the intersection of the steering axis with the ground is ahead of the tire contact patch) increases with increasing front wheel diameter (assuming the head angle and fork offset stay the same). As fork trail goes up, the bike’s stability goes up and its maneuverability goes down.

So, open, fast riding, and fast, rough descents lend themselves to a slack head angle. Tight, technical trails involving lower speeds and requiring lots of maneuvering lend themselves better to steeper head angles. And the longer the fork travel, the slacker you want the head angle. Finally, bigger front wheel diameter begs for a steeper head angle.
-Lennard

Regarding the Tech FAQ column on bleeding Avid brakes:

Dear Lennard,
I’m sure you know about this, but regarding Jeff’s question about bleeding Avid brakes I’ve found this out recently.

I had done a few bleed jobs and when in the stand everything felt good with a firm feel at the levers. But after just a few rides it didn’t feel good anymore with a spongy feel at best.

Getting tired of bleeding the brakes, I pulled the brake lines off and drained everything out of the system. Filled it back up with fresh fluid, following Avid’s instructions exactly and my brakes have never felt better. A number of hard rides on them and they feel as good as that first bleed.

I think sometimes no matter how hard you try, getting all the contaminated fluid out just isn’t going to happen without starting over.
-Chris

Dear Chris,
Great point. Thanks.
-Lennard

Readers can send brief technical questions directly to Lennard Zinn.

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

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