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Gravel, adventure, gnarmac or cyclocross. It doesn’t much matter which moniker you care to hang off it because when the gravel gets lumpier, the adventure becomes more adventurous or things get a little gnarlier than you expected … your re-branded cyclocross bike is going to get out of its depth. 

In some instances, it might be the rider at fault and their unwillingness to fully embrace the drops which results in the bikes full off-road potential never being realised. However, in the majority of cases, the limiting factor when deciding just how adventurous our adventure bike’s going to be – is tyres. People talk about 40c like it’s a big tyre, it isn’t. It’s just over 1.5″ and when was the last time you rode your mountain bike in any meaningful way with a tyre that size?

Rather sadly, most of us are limited to fitting 40c tyres to our gnarly gravel bikes because anything bigger just won’t fit. I say sadly because the racers out there know that a tyre with a larger volume will often allow them to travel across rough ground faster. The slow and steady amongst us already know that a bigger tyre run at a lower pressure will provide more comfort for their aching backs … and the tarts out there, don’t need me or anyone else to tell them that bigger tyres are just so much better looking. 

Braking, climbing and cornering can all be improved with some additional rubber but in 90% of cases, your off the shelf, drop-barred, all-road bicycle won’t allow it. You’re stuck with 40c or maybe 42c if you’re very fortunate and that’s that … or is it?

700c x 40c … that’s not a lot of tyre.
Unless you really have been living in a heavily fortified underground commune, you can’t have helped but notice the rising interest in 650B+, in fact you’ll probably find some exquisitely penned words hereabouts if you care to look. Anyway … one of the things often touted about by B+ believers, is the fact that in some cases, it’s possible to fit the wheels and tyres to an otherwise ordinary 29er without modification, special parts or hammers. The larger volume tyres deliver the benefits previously mentioned, while keeping the geometry as nature intended give or take a couple of millimetres.

Those of you still awake have probably already figured out where this is heading, and you’d be right … Gravel+ or Adventure+ or whatever else you want to call it +. What if, by applying the same principle as B+ but in a scaled down version, you could transform your previously skinny tyred, bone jarring ‘All Road’ bike into something a little more capable?
650b x 2.1″ … much more tyre to play with.
Okay, so it’s not exactly ‘plus’ but an increase in tyre size from 1.5″ to 2.1″ can’t hurt in the pursuit of speed, comfort and good looks – can it? In all honesty, I don’t know yet but I will be back very shortly to let you know how it rides … although, I have to say, I do think it looks better already.

Thanks to James Olsen for sewing the seed ;o)

10 Comments

  1. Unknown says:

    Interesting idea, hadn't given it much thought but finding how capable my Focus Mares is with the 40c nanos, but could do with a touch more clearance, a 650b with a 1.9 or so XC tyre has legs. Maybe just go the whole hog with some even fatter tyres on a 26" rim? Hang on, we've been here before….
    Pete

  2. That's exactly it Pete … an interesting idea. I'm not saying it's better than running a 40c tyre on a 700c rim (although on paper it ought to provide certain benefits) but trying new things is sometimes driven not by the certainty of it being better but by the fact that we don't know.

  3. Unknown says:

    It's an interesting idea, first time I saw it in a was on the Unbeaten Path: https://up.opencycle.com/ I asked James about it last year when I was reviewing the Pinnacle Arkose and he said he was testing it out, but it wasn't as easy as simply swapping wheels out, there was more going on that needed to be figured out, in terms of handling. Maybe he could elaborate here.

    Jack

  4. James will know much more than me Jack but I wish he'd said it was more complicated than simply swapping the wheels over ……. before I simply swapped the wheels over ;o)

  5. Unknown says:

    James was talking about bringing a dual wheel size Arkose to market. Anyone can swap the wheels over but you might not end up with a bike you'd be happy to charge people money for. There's more to consider than tyre clearance. Look at the Unbeaten Path page for more discussion on this. There's the interaction of crank, chainstay, tyre, which led them to changing the shape of the drive side chain stay, to maintain a reasonable Q factor, there's handling issues when the tyres deform under load, I'm sure James can elaborate. I'll be interested to hear how you get on with your experiment too. Jack

  6. I think you might be looking too deeply in this instance Jack. Unbeaten Path themselves say that there's very little difference in radius between a 40c cross tyre and a 2.1" 650b tyre, so geometry differs very little. By changing wheel size from 700c to 650b I've not altered the Q factor, BB width, gearing, chainring clearance, chainstay length or anything else … they all remain constant. The geometry will have altered slightly but only due to the small reduction in tyre radius, you could lose a similar amount by simply changing tyres to another brand of the same size or just by reducing tyre pressure. As long as the bike behaved well when fitted with 700c x 40c tyres, there's no real reason why it won't behave when fitted with 2.1" 650b tyres.

  7. Unknown says:

    Hi Jack, Stuart.
    I'm not totally convinced by the 'just swap the wheels' point from some brands -despite suggesting it myself- but that's because I'm picky about bike handling and tyre / wheel size is a big part of it. There's more than wheel OD in a bike's handling, the OD is part of the trail figure formula but there's other stuff going on, pneumatic trail etc, things I prefer to try to feel rather than get mathematical about. The Pinnacle Pyrolite was made to be 650B compatible a few years ago but the tyre options were few and hard to get hold of, even then the handling variation was noticeable even if the wheel OD was close. I ended up making a few frames and forks to experiment with those changes. Tyre tread's a part of it also, as those who've ridden fat bikes may have experienced.
    Adaptability in a bike is a good thing and the changes in handling may be a positive change rather than anything detrimental, depends on where the starting point is.

  8. Unknown says:

    Stuart, yes the frame geometry remains constant. The point is that a frame built with chainstays wide enough to give sufficient clearance for fat tyres may require the Q-factor to be increased beyond what would be needed for just a narrower tyre (the Unbeaten Path solves the problem with a funky shaped drive side chainstay, but for lower budget bikes manufacturers might not bother). Whether a wide Q-factor is a worthwhile compromise on a dual purpose gravel+ bike depends on how much a wide Q-factor bothers you and the balance of time you'll be using the bike in fat tyre mode versus narrow. For most people it's probably a minor concern, I agree that adaptability is good. One fine day we may all get by with just one bike! Jack

  9. Unknown says:

    …and I guess another question is be whether "an increase in tyre size from 1.5" to 2.1" gives you a big enough improvement in performance/comfort to justify buying a new set of wheels. Looking forward to reading your conclusions.

  10. Unknown says:

    Bit of a late comment but looking at this as a wider option for my pinnacle as got fed up fixing punctures in wales ! .. Got any pics of clearance around fork and stays ?

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