Tuesday, October 2, 2007

650B...

Why a new tire/wheel size? Well, for starters, the 650B size is anything but new. Second, if it were not for the Russian Army, all mountain bikes might, to this day, be equipped with 650B wheels and 26” and 29” might never have existed. Back in the infancy of mountain bikes, Tom Ritchey, Joe Breeze, and some other early pioneers were experimenting with 650B wheels because they could get lightweight aluminum rims in that size. The klunkers were built with heavy, steel 26” rims.

Fast forward to 2007 and 650B is coming back. It’s always been around in the form of randonneur bikes (comfortable, fast, sport touring bikes) with 38c tires. Over the past year, there have been a few out-of-the-box souls who have experimented with 650B mountain bikes – Rivendell and Kirk Pacenti. Kirk even went so far as to make his own tires. He now has Panaracer making a 650B x 2.3 tire.

Why 650B? It makes a lot of sense on a lot of fronts. But why 650B instead of 29 inch or 26 inch wheels? Here’s my take.

Twenty-nine inch wheels are a hoot to ride. And at 6’3” tall, the big wheels make my bike look very proportional. But, there are some problems that need to be overcome because of the big wheels. When designing a bicycle frame, I start with a general concept of what I want my seat angle, head angle, effective top tube length, and bottom bracket height to be. Then starting with the bottom bracket, I lay in my seat tube. The seat tube/bottom bracket relationship is crucial to the rider’s position on the bike because the position of your butt and feet is what gets the pedals rotating. You can’t fool with that relationship beyond a small variance.

With the bottom bracket and seat tube in place, I figure out what my bottom bracket drop (bottom bracket height) will be. Then creating a horizontal line above the bottom bracket that represents my drop, I draw a circle with the center being the center of the bottom bracket and the radius representing the chainstay length that I want. Where that circle intersects the horizontal line of the bb drop, that is where the rear axle of the wheel will be located. Then from this spot, I draw in the diameter of the rear wheel.

Oops, those 29” wheels don’t fit. I’ve either got to increase the seat angle or lengthen the chainstays. Altering the seat angle is not an option, in my opinion. So, here comes the first concession to designing around 29” wheels – lengthening the chainstays. Lengthening the stays isn’t actually a bad thing. It’s what helps a bike climb steeps. Getting the rear wheel back further helps to keep the front end of the bike down when climbing. But because the big tire also takes up space that is also occupied by the front derailleur, the stays have to get pushed back even further.

So, I lengthen the chainstay so the rear wheel clears the seat tube and the front derailleur. Then with the top tube, head tube, down tube, front wheel in place along with the fork and fork offset, I fine tune the head angle to get a decent amount of trail which aids in the stability of the bikes steering. All is good until I get to the smaller sizes and realized that toe overlap is a possibility.

On the smaller sizes, you have to push the front end out to where the wheel and toe doesn’t overlap. You do this by also increasing the seat angle and reducing the head angle to also try to preserve the riders’ position. What you end up with is a “small” sized frame that has virtually the same front center as the “medium” sized frame.

Okay, that didn’t seem too hard. But this is a hardtail. When you throw in suspension, the design takes on a whole new variable. Now, you’ve got that big ole rear wheel moving in an arc around a pivot. Seat tube angles just got ridiculously steep. Some are over 74 degrees. Talk about being over the front of the bike!

So, here’s where 650B wheels come in. This mid-sized wheel “almost” fits into existing 26” wheel frames. Minor tweaking is all that is needed to fine tune a suspension frame for 650B wheels instead of a complete overhaul that is required of 29” wheels. A full-suspension bike can be designed with bigger (than 26”) wheels without having to make many concessions to accommodate the wheels.

But why 650B wheels and not 26 or 29? I don’t think that’s the question. There’s no reason why all can’t co-exist. After all, road bikes are available in 700C and 650C wheel sizes. BMX bikes are available in 12”, 16”, 18”, 20” and 24” tire sizes. All of those tire sizes (700C, 650C, 26", 24", 20", ... and even 650B) are also available in a huge variety of widths. And further, cranks and stems are available in a wide range of lengths that allow riders to tailor their fit on the bike. So why not wheel sizes?

Personally, I enjoy riding all of my 26” and 29” wheeled bikes. I like the light weight of the 26” wheels. I like the smoothness of 29” wheels and their ability to roll over obstacles easier. I like the “snap” that 26” wheels have when you need that extra punch to get up, through, or over something. I like harnessing the momentum of 29” wheels.

I think the 650B size will take each of the benefits of 26” wheels and 29” wheels and make them its own. As I develop the brand of my bike shop, I will be developing frames that I will offer under the Black Mountain Cycles brand. The first frame will be a 650B frame. The design is done and I can’t wait for the first sample. It just makes sense to me.