So what is the actual weight difference between a 26" wheel with a given tyre and a 29" wheel that is built similarly and has a comparable tyre?

The ISO values for the diameters are 559 and 622, making the 29" 10% larger in diameter, and so also in circumference. Let's say you have a 1kg tyre and a 0.5kg rim on the 26" wheel, so one would expect that the equivalents for the 29" wheel will be 10% heavier, which makes it an extra 150 grams. You may use 4 more spokes (36 instead of 32) to build a 29" wheel as strong as the 26", plus every spoke will be 10% longer, but that will only contribute about 50 grams extra, and that is spread across the wheel, not all at the rim, but I'll simply add it to the 150 to get 200 grams. The hub should be identical. This applies to both of your two wheels, so that's a total of 400 gram difference that we're talking about.

I don't recall enough of my physics lessons to figure out how much power is required to accelerate a 400 gram mass placed at 35cm from a circle's center up to a given speed, but I'm going back to my spinning a wheel in a bike stand test. A decent road rim weighs about 400-450 grams, so see how much force is needed to get one of those wheels up to speed with no tyre installed. As I said before, I can do it with my little finger quite easily.

Now compare that small force to the amount of energy it takes to accelerate your body up to 15 kph.

I agree that the total force require to get two 29" wheels with heavy tires (and sometimes lots of mud attached) up to speed can be significant, but almost the same is true for a set of 26" wheels & tires. The important thing is that the DIFFERENCE between the force required for 29" wheels and 26" wheels is NOT significant compared to the force required to also get the bike and rider to accelerate.

There are certainly important differences between 26" and 29" wheels and tyres, but one of those is NOT the difference in power needed to accelerate/decelarate them.

This months German Bike magazine did a power tap test on a Scott 26er and 29er pretty much the same bike except wheel sizes. On average the 29er required about 10W more to keep the wheels turning on a climb. This is about 10W in 200W or about 5% more power.

If you calculate the power required to accelerate the wheel to about 8m/s the difference in work is 436J for the 26er, or 457J for the 29er (again pretty close to 5%).

The reduction in rolling resistance is supposed to be about 10% so if you are riding constant speeds where you are not accelreating or decelerating the moment of inertia of the wheels then its clear the 29er is better... But as I have already said Alpine riding is not XC riding. You do not travel the same speed the whole time and in places its more comparable to trials riding then it is to XC riding.

While that 5% may not seem very much think about how many times you will do it on one technical descent and that adds up, add into that tight switchbacks which the longer wheelbase of the 29er has trouble with then I'd still stick with the 26er for the kind of riding I do.

Also you'll remember from your school physics Moment of Inertia is not just related to the mass but also where the mass is placed. On a 29er the mass is located further from the hub then it is on a 26er. Think a longer or shorter baseball bat. This is more the reason for the difference in power rather then just the difference in weight of the wheel alone. Heavier tyres further from the hub is what I was getting at.

In favour of the 29er the German magazine actually seemed to be in favour of the 29er even though it required more power on the climbs. They reckoned it was more efficient everywhere else. This was on a TransAlp route though which consisted of a lot of fire roads and not too technical trails by Alpine standards.

Its horses for courses, if you're riding is more XC based or tours on fire roads or woodland riding then maybe a 29er is worth looking at. If you are riding on the hiking paths in the Alps then a 26er still seems to be the bike of choice.

If you enjoy trail riding where the uphill part is as fun(or as necessary) as the downhill part then I think a 29'er may well be the go. It comes down to the type of riding you enjoy and do the most of. There are a couple of very good 26" duallies with 6" of travel that are lightweight enough for climbing and won't bob like mad when you pedal them.

However:

I have ridden and raced both types of bike, am the owner of a Spark30 (don't like it) and am in the market for a 29'er dually, oh and I'm short at 170cm.

The only downside I can foresee is on switchbacks or in very tight situations, where the longer wheelbase 'may' make it more difficult to navigate these turns, and this is also where buying a 29'er for the Alps might be a little tricky, as you don't want a bike with a head angle that is too steep(and twitchy), of course this applies to 26" bikes as well, but not too slack where it feels like a truck to turn.

A brief look at the upside:
They "feel" right ... you sit in the bike not on it, making it feel more stable
Roll really really well
Don't get hooked up on roots and rocks like 26" wheel
Better grip climbing and through mud etc
Parts are as easy to get as 26

Put it this way, the pros outnumber the cons

There is a bike test in Gryon VD next weekend Oct2/3, costs CHF20 but there seems to be a good selection of brands turning up

http://homepage.bluewin.ch/biketest/index2.html

I'm hoping there is a 29'er there or two to try

PS: I'm not involved in the running of the Bike Test in any way

Do you grab the rim end or the hub end of the spoke? My guess (zero practical experience) is you can tell the difference if you turn it at the hub...which is the more realistic test.

And more generally, flywheels hold surprisingly high energy at relatively low masses, so I would go with Eire on this debate...

Interesting discussion BTW.

I tend to agree. It's moment of inertia that counts, not mass. With a constant torque exerted on the hub, the acceleration of the wheel is inversely proportional to the moment of inertia. The latter grows like the cube of the diameter of the wheel (assuming that all the mass is at the rim and neglecting the spokes). Roughly speaking, this means that a 10% increase in wheel diameter leads to a 33% decrease in acceleration.
Whether this is negligible compared to the acceleration of the rider himself is another story (and I guess this is where the type of riding come into play).

Isn't the 28" on a normal bike the (outer) rim diameter while the 29" of a MTB is the (outer) tire diameter ? Which basically comes down to: it's the same size.

Hm, a 26" or 24" being easier for climbing than a 29", isn't this just because the gearing is different ? The amount travelled per crank rotation is proportional to wheel size, having 29" instead of 26" is like having a 28 tooth sprocket instead of a 32 tooth, no wonder the 29" wheels feel 'heavier' for climbing

The 24 has nothing to do with climbing... Some riders used them for downhill a few years back (not sure if the trend is still going). AFAIK the argument being that it was easier to accelerate (and brake) the wheel, and still get the ability of the 26 wheel upfront to roll over stuff. (A bit like why they make 69ers!).

As for the sizes... I know that a 26er with a 2.4 tyre has almost exactly the same roll out length (and therefore diameter) as a road wheel with a 23c tyre. So the 29er with the same size tyre should be a wee bit bigger.

Either way as someone above very astutely pointed out the best bike is the one you have the most fun on. This is going to be hugely different amongst different people. For me it even changes with the seasons, so ultimately if you test it and it puts a smile on your face then its a good bike.

I just checked out the Canyon Torque Alpinist, sweet machine with well thought out spec and the Horst Link suspension, but at 14kg is still too heavy for my liking, perfect for lift assist but not to pedal uphill for a few kilometres

At the moment he's racing the Trans-Provence on it. 7 days, with between 1500m and 2000m vertical each day. Jekyll and I ride together a bit and we have no problem putting in 1500m days on these kind of bikes if we have to. We'll use the lifts if they are there, and if not we'll ride up. A bit like the 29er they require a bit more effort but they do get there... and on the way down the eat up the trail.

I don't have one, but I have looked into them. If and when I ever get a new hardtail to use for XC I will go for a 29er. I wouldn't use one as a primary bike here in Switzerland though, that I would leave to a 26" All Mountain/Enduro bike.

I find the fact that Jekyll is "racing" a 14kg Enduro bike in an event with that much climbing very interesting.

Also for the record, yes a 29er takes a little more effort to climb on, mainly due to higher gearing and accelerating from slow speeds a little harder, however once at speed it is much easier to maintain this speed due to increased grip and the bigger wheels' ability to track over obstacles, having you a lot fresher at the top of the climb to enjoy the downhill afterwards

That very much depends on the race.

http://www.trans-provence.com/

Everyone in the race last year when I did it was using bikes that weighed between 13 and 16kg. Its not an XC race but a an "all mountain" enduro race. You'd be quite uncomfortable on some of the specials on a pure XC bike... and yes you still have to get those bikes uphill under your own power to start the specials.

Actually jokes aside... a 29er might actually be an ok bike for this event. There are only a couple of stages that are really tight and techy the rest is quite fast and flowy blasting over roots and rocks etc.

It looks like an interesting/fun event, I checked it out when you made reference previously, something different to numerous races out there already. The footage of the riding makes it look pretty enticing.

I did wonder if the timed stages were more oriented to gravity being a friend (as opposed to a foe), it also explains why you could get away with an All Mountain/Enduro style bike.

I can assure you it is awesome. Its much more physical then the event format would have you believe though. The trails are so much fun that you forget the suffering of the climb as soon as you tag your timing chip on top and drop in.

I'm getting fairly regular updates from friends down there at the moment and really wishing I was back down there.

@ChrisW: That's all good in theory but as Eire point out, if you throw on a pair of 1.2kg downhill tires and ride anything which is not pointing downhill then it's like riding through mud. I have a Stanta Cruz heckler (26" with 5" suspension F/B). I also have two sets of identical wheels. One with 700g Kenda Nevegals and the other with 1.2kg downhill Nevegals. The difference is so huge. Riding the downhill tires even on the flat is punishing as hell and requires about twice as much effort... But the extra rolling inertia once it's on the slope is so much fun.

I think the physics and statistics conversation is interesting, but in the end, for me at least, it's not about the power difference, the grams, and the technical comparisons. it's about the ride, the feeling--something which doesn't translate well into numbers. you can ride two mtb frames with identical head/seat tube angles, top tube and chain stay lengths. but they may feel dramatically different when cornering, when flexing the bottom bracket, when...

similarly, you can compare land rovers to land cruisers but it's best just to drive both and see what feels better!

not knocking these posts, there's a lot i'm learning, but i guess i'd be best convinced of what bike is better for me by simply riding one before the snow comes...

Personally, I would just try and ride the bollocks off whatever you have ...

15 minutes into the ride and you will have retuned yourself to whatever steed you are on. You may not be as fast as you could be ... but unless you are racing, is that an issue?