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650B explained

Ok, so after my next to last post was, erm, posted, I got 2 emails from readers asking what the hell I was talking about. So here goes with a little bit of explanation of what 650B is.

First we need to do a little refresher course on wheel sizes. For this I’m going to stick with just a few popular wheel sizes. Here’s a sample of some dimensions from popular tire sizes:

common name ISO
20″ (BMX) 406
26″ (mountain bike) 559
650C (small road bikes, tri-bikes) 571
650B (mountain bike) 584
29er (mountain bike) 622
700C (road bikes) 622
27″ (older road bikes) 630

The common name is obviously what most of us use to describe the wheels, or indeed, the bikes we ride.

The ISO size is the diameter of the bead seat of the rim in mm. This doesn’t match the common name unfortunately. For instance a normal 26″ mountain bike wheel has an ISO size of 559mm. 559mm converted to inches is 22″. So where does the 26″ come from? Well some time ago the industry thought it’d be fine idea to use the outside tire diameter as the primary dimension. However, over time, the link between stated size and actual size has become looser and looser. This is mostly down to a wider range of tire widths (and heights) being available to fit standard rim diameters. Got that?! Sheldon Brown has a more indepth article describing the fun that is tire dimensions and that can be found here.

So, back to the point of this. As many of you will be aware, a new mountain bike wheel size has been becoming more and more mainstream over the past couple of years. This wheel size has been become known as 29″ and the bikes they fit are known as 29ers.

This is confusing for 2 reasons, one is that of course as we now know from the lesson above, 29″ tires aren’t actually 29″ in diameter and also the more astute of you who studied the above table would have noticed that 29er tires actually have the same bead seat diameter as 700C!

This second point is actually understandable as although cyclocross bikes start to blur the lines between bikes using 700C and 29″ tires, most tires will be designed, sold, bought and ridden for two very different bikes. One a road bike and the other a mountain bike, so this distinction does actually make sense even though the ISO standard rim dimension is the same.

But what about this 650B thing? Well, before we get to that, we need to address one issue first. Namely, why do we need 29″ as well as 26″ mountain bike tires. This is a point that has been argued in many, many different spheres and I’ll stay away from whether I think it’s a good idea or a bad idea in the interests of keeping this simple! Many argue that the 26″ wheel size is not a result of good design but is merely an accident, an historical throwback. When the original clunkers were being ridden offroad at the inception of the sport, the beefiest, strongest bikes around were old cruiser style bikes which happened to use 26″ wheels. As these frames were broken, crashed, lost, new frames were built to replace them but were constructed so that the original parts could be used. They needed to be able to work with 26″ wheels. And so this continued until today when most mountain bikers are riding around on 26″ (ISO 559) wheels.

Proponents of the 29er movement argue that a bigger wheel rolls over obstacles easier and is more suited to offroad riding. While this can’t be argued against, 26″ bikes offer better acceleration, stronger wheels (usually) and some other minor advantages that can be worked around for 29ers. One downside to bigger wheels is that it’s harder to design a bike for offroad use that will use bigger wheels. Main problems include keeping the stays (and wheelbase) relatively short while still providing adequate tire clearance and keeping the handlebar height relative to the seat height reasonable, while preserving a sensible bottom bracket height. Other minor issues include things like toe overlap (on the front wheel wheel turning the bars). These design issues are exaggerated when the rider is smaller.

Now, along comes the 650B wheel size. This sits somewhere between the 26″ and 29″ sizes which has prompted some to christen it the 27.5er (catchy!) or my favourite designation, the ‘tweener’ (‘in between’). It’s argued that this size might just allows us builders to overcome the design challenges faced with 29ers but still keep some of the advantages. Personally I think it’s a good idea. I think it has merit and I think we will se more and more custom builders as well as bigger brands starting to offer 650B bikes. In fact it’s already happening. At the industry trade show Interbike this year, there were a number of bike manufacturers showing 650B product including Carver, Haro, Pacenti, Soma, Sycip and Vicious. Also showing forks/wheels/tyres were White Brothers, American Classic, Velocity and Panaracer.

Having said that, there’s only really one tire choice and a couple of rim choices at the moment. And those are pretty thin on the ground generally and almost non-existant here in the UK. Currently, I’m leaning towards waiting a while but writing this little post has got me a bit more excited about the concept again. Maybe I should just get something built up and try it.

And before anyone jumps on my head over this, I feel I should mention that although I’ve described the 650B size as new, it’s not. It’s been around for years. There’s mailing lists, forums, flickr groups and all sorts. Eh? I hear you say… Well the 650B size has been used as a size for touring (type) bikes for many years now. Rivendell have been a strong proponent of this wheel/tire size for a number of years and it was/is popular in older French camping bikes. But I’m talking about 650B (27.5, ‘tweener’) specifically for mountain bikes. This normally means stronger (heavier?) rims and much higher volume tires. It’s important that there’s some kind of distinction.

Anyway, I’ll keep you posted with what I end up doing. Meanwhile, If you’ve a desire for a 650B custom frame, let me know!

posted by steven - October 24th, 2007