The Mountain Bike Life

So remember when I sold my Carbon Force and said I wasn’t going to do a 650b this year. I Lied! It was killing me so it dawned on me I have a couple of extra frames laying around and why not use one of them as the 650b frame. This lead to me trolling the classifieds and usual sites for a set of 650b wheels. Couple weeks later I score a nice set on Ebay for next to nothing. Just missing tires now, but I have my eye on some right now. The big question then becomes how complex is a 650b build. Well let me tell you.



Let’s start with the frame. By now you must think I am a little crazy when it comes to GT. If you are thinking this you are just flat out wrong. In fact I am crazy about vintage GT ;-). Back in the late 80s when I started to ride GT was the rage for BMX and the first Mountain bikes. At my local shop I would go there and just look at them. Seeing the pretty Zaskars and Xizangs hanging up on the racks with their unobtanium price was heaven. I wanted one so bad. Well years later I have had several Zaskars and I am always on the lookout for a Xizang. One day I will own one. Anyway, back to this 650b…It is pretty well documented in the 650b circles (get to this in a minute) you can convert old and new Zaskars pretty easy. Problem is I don’t have a Zaskar anymore, or do I? I kind of do. What I have is a USA made Avalanche from 97 which is the same frame as a Zaskar with some minor differences.

This is a 2009 Fox 26″ fork.  There is about 5/8″ clearance from a 26×2.3.  Should be enough clearance for a 27×2.1 or 2.2 depending on tire.

OK I am going to digress for another minute here then get back to the 650b stuff, be patient. When shopping for a used Zaskar there are a bunch of signs that it is a Zaskar not some other GT frame. You can get Zaskar decal sets easy so there are a lot of mislabeled frames out there. Thankfully the markers are pretty simple to see, flat cap on back of top tube, 6061 and serial number stamped on rear triangle, big hand made welds and usually polished aluminum. All the rest have a 7075 on the frame, serial on the BB and round cap on the top tube. Now my Avalanche is a strange bird indeed. The person I acquired it from claimed to be a GT rep back in the day and this was a sample frame made for demo. He only assembled it and rode it a couple of times before stripping it back down and letting it sit for a few years. So how do I know he wasn’t pulling my leg? Well the frame has no stamping for material type anywhere, the serial number on the BB denotes a USA build made out of Alcoa (6061 aluminum) and the weight is exactly the same as several other Zaskars I have owned at 4lbs 1oz. To me the last clue is the most important. Back when I was restoring one of my cars I had a welding instructor over to help me with some body panels. Now this guys knows everything there is to know about welding. With my crap welder he did some amazing things I didn’t think possible. Then, on the wall he spotted my bike and commented about the art in the welds. He then said, “and look both of those frames are welded by the same person.” One bike a Zaskar and one the Avalanche. Huh……….guess that was made in the USA along side the Zaskars.

This is the clearance on a 2010 RockShox 26″ fork.  There is about 3/8″ clearance on a 26×2.3″ tire.  I have heard you can sneak a 2.1 650b in here, but it will be tight.  I would prefer more clearance.

So, where was I? Oh yeah I got a frame to use now and confirmed it will work over at the 650b forum on the mtbr.com 650b forums. There is a lot of good information over there which is helping me plan this conversion. The trick with the Zaskar/Avalanche frame is just staying with a 2.1” tire in the back for clearance. From my own measurements and images from the 650b forum I should have enough clearance for mud and the usual trail debris. Ummmm also don’t forget about brakes. Rim brakes on older frames will not work, so either use a frame with a rear disk or convert to a rear disk with some type of adapter. I used a modified A2Z adapter which works great Once I get this put together I will post some clearance pictures with the first ride.

Here is a 2010 Xfusion Enix and it has 1 1/4″ clearance to a 26×2.2″ tire.  Plenty of room for a 650bx2.4″ if you wanted.


Next up is the fork, you need something with enough clearance for the new tire size. The only thing I can say here is stay away from 26” Rock Shox older than 2012. The clearance on a 650b 2.1 tire is tight. So I am going to use my Xfusuion Enix for 2 reasons. First is the 26” Xfusions have plenty of clearance for a 650b wheel and tire. My plan is for a 2.2” width up front. Also, I shortened this shock to 80mm of travel for earlier frames…….perfect! The Enix is actually a pretty nice fork from a cost and weight perspective. It is pretty light and was very affordable. Best part is the ride is very smooth for such a low end shock. Down side is it is a little flexy and I wouldn’t trust it on any drops or really tough trails. So I am going to keep the riding a bit less aggressive during the test.

In my research about this I turned up these tips. You really want about 10mm clearance to the frame and fork with the new wheel and tire size. Less than this and mud may become a problem. Not to mention if the wheel goes slightly out of round from an impact it is nice to have enough clearance to ride back with minimal rubbing. Also, remember the entire bike will be raised up about 20mm so your bottom bracket height will go up. This will be good for clearance, but you may feel it a little in the corners. True 650b frames address this and position the BB a bit lower. On that note if you are going to do this on a long travel full suspension bike there are some bump stops you can install on your front shock to prevent the tire hitting the frame. You definitely want to do some careful measuring before throwing on bigger wheels and then hitting the mountain.

A2Z disc brake adapter on my1997 Avalanche frame.  Modified to bolt directly to the frame.

That is it for the 650b specific stuff really. Confirm your frame/fork has clearance and get some 650b whees/tires. The rest of the parts for my build are from the departed Force and are mostly Shimano XT stuff. When done this actually will be a pretty light rideable bike. I am really excited about this because it will give another option for a lot of people who want to move up to a bigger wheel size without re-buying everything. I know a new frame will have geometry specifically for 650b, but not everyone can run out and buy a new bike. Bonus is if you are a kook like me you can get some more miles on a 15 year old vintage frame.

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