Is a mullet bike slower pedaling uphill compared to full 29?

I don't know how noticeable it would be, but the smaller wheel with the same gearing equates to slower speed at the same cadence.

Just faster. That's the only change I found when I was doing my mullet experiment on my Chromag Rootdown- in order to keep the same average speed on the climb of my test loop, I would either have to grab a taller gear or increase my cadence. It wasn't much, but it showed up in the times.

Yup, that's what I was getting at, but didn't articulate it well. I didn't feel any other speed difference or additional resistance with the 27.5" rear wheel on the smooth, hardpack climb.

Now, on chunkier climbs, especially square-edged ledges, I felt like the smaller wheel hung up more than on descents.

I hope this helps answer your questions!

The smaller wheel is like gearing down. So when you ask do you need a higher cadence or more power, the answer is yes. Either works.

If you have a 32/46 gearing and your buddy has 32/40, you'll have to put down a higher cadence at same power or more power at same cadence.

A 27.5 rear wheel can make your easiest gear easier, but it'll also make your hardest gear easier. I have a 27.5 rear wheel and pretty much never use the 50t gear anymore.

I have not experienced that a mullet bike is slower pedaling... however the biggest negative of mullet bikes vs full 29 for me is the difference in the way the bike maintains momentum through choppy/technical terrain.  You can feel the rear end of the bike hanging up ever so slightly when compared to a full 29r.  This is something that I adjust to quickly and then stop noticing... right up until I'm riding with a few buddies on full 29rs and I notice how smooth they can roll through edgy/technical/choppy terrain compared to a mullet.  I do also feel like bikes that were converted to a mullet have this sensation more then bikes that were designed as a mullet.  This may just be placebo, but a few of my buddies have noticed this so if it is placebo, it's group think placebo.  

Funny. I actually have a suspicion for a lot of us a mullet bike is going to be faster going up. Its something I will likely test for the youtubes in an XC/Trail/"backcountry" application. 

Here is the hypothesis...

*A lighter wheel goes uphill faster than a heavier wheel with the same power applied

*All things equal (tire, rim, etc) - a 27.5" wheel is lighter than a 29" wheel. 

*The benefits of a 29" wheel (for most mortals) are not applicable in most climbing situations on a MTB (Inertia is higher at speed, rolls over chunk better)

Before anyone says it, at the WC XC level its unlikely a benefit would be achieved for two reasons - First, they are running silly light stuff to begin with. Second, they are actually climbing at a speed where the wheel's inertia may come into play. 

I have found that my mullet bike pedals just the same as my full 29" bikes. If anything on certain situations it feels better since the smaller wheel spools up faster and easier. 

Well also the 29er with a 51-tooth is going to be a faster 51-tooth than 27.5. The 29er with the 10-tooth is going to be faster than the 10-tooth 27.5.

The XC folk are more than strong enough to push the harder gears, so not only do they get the benefits at speed but they can go faster to get more benefits at speed.

Your first claim is correct, a smaller wheel in effect is like "gearing down" (2 teeth). However, one could mitigate this by adding 2 teeth to their chainring. 

That said, your claim has nothing to do with outright climbing speed, but cadence. Some riders love pushing a big gear at slow RPM, some riders love spinning a low gear at high RPM. 

The latter part of your claim (XC riders are strong enough...) is very subjective. It depends on the rider, the steepness of the climb, the tire, the bike etc. If I'm really precise with my hypothesis, my claim is as follows: The steeper the climb (and slower the speed) the better the smaller wheel will likely do in a conventional trail setting (IE, you didn't just drop $3000 on 1200 gram wheels). 

At the pointy end of the spear (elite level) you are right, this is going to be less significant for the reasons I already made clear. 

Remember, we are talking about bicycles, not motorcycles. Your claims would hold up on the powersports side of things a lot more than the human powered side of things...

Obviously lots of variables to consider when it comes to climbing but if you simplify it like your pedaling up a paved climb. If you take the differences of the circumference of the two sizes you will technically travel roughly 4.7" further with each rotation of the wheel on a 29" tire.  

27.5"---86.4"

29"----91.10

Guys, you're killing me. The number that actually determines your forward speed relative to your pedaling cadence is what's called  "gear inches," which measures how many inches forward your bicycle rolls for one turn of the pedals.

Gear inch = A/B * D, where:

A represents driving chainring tooth count.

B represents rear chainring (“sprocket”, “cog”) tooth count.

D represents drive wheel diameter in inches.

You can counterbalance a change in any one variable by adapting one or both of the other variables. Bigger wheel diameter? Use a smaller chainring, or bigger rear cog, or both. Smaller wheel diameter? Use a bigger chainring, or smaller rear cog, or both. On a modern bike most of this is managed by simply shifting gears. Because we're talking about 11 or 12 speed bicycles that we shift sometimes hundreds of times per ride, your brain is going to automatically optimize your gearing for your preferred RPM and uphill speed throughout your ride.

No one in the world pedals an EXACT rpm all the time, which is why the typical ratio between gearing jumps on a cassette is 17%. You're never in "the perfect gear" because your body can easily absorb and adapt to an "imperfect" RPM if it's close to your ideal. And ideal RPM for any given rider changes multiple times within a ride due to variations in heartrate, lactate, humidity, rotation of the earth, your lucky Powerball number, etc.

Really good points. All things being equal that smaller rear wheel is a not-insignificant amount lighter, and a lot of evidence shows that a higher cadence is more efficient.

On an ideal (in my mind at least) climbing trail, with turn/switchbacks, grade variations, etc., that would translate into a faster and easier climb with the smaller rear wheel. Our trails have a lot of those features, but also have pretty gnarly rock sections, which I feel the 27.5" rear wheel requiring just a bit more effort to get through.

The only experimentation I've done was on a hardtail- I am considering building up a 27.5" wheel for my Chilcotin to see what difference it would make with 167mm of travel. Unfortunately this isn't an ideal platform so I'm a bit hesitant to invest in a rim and spokes for the sake of trying this out.

Regarding wheel size and rolling speed, it all comes down to how rough or slippery your riding surface is. The UCI had to ban smaller wheels for indoor velodrome track racing and institute a 700c wheelsize because tracks are so smooth and grippy that riders were able to run tiny wheels with tires at astronomical pressures (200+ psi). 20" wheels required enormous chainring sizes to balance out the RPM and speed, but they allowed the bike to be much lower and more aerodynamic, and they were also significantly lighter. Bigger wheels produce a bigger contact patch at the tire, but grip doesn't matter much on a velodrome where the steeply banked walls produce additional g-forces to crush you into the track. Advantage, smaller wheels and tires.

By contrast, road race bikes have increased in tire size and seen pressures drop as research has shown that road surfaces are significantly rougher than previously thought. Racers used to run 23mm tires at 130 psi and now it's not uncommon to see 30mm tires at 70 psi in a grand tour or monument, although continental teams are pretty secretive about their tire pressure. Advantage: bigger wheels and tires.

For climbing on a mountain bike? I don't think it makes any difference whatsoever, except in fringe cases. If anything I'd agree with Jeff that a smaller wheel would be a slight advantage on most MTB climbs, unless they were extremely rough and/or slippery in which case a 29" wheel would be advantageous. Nino Schurter is the winningest male cross country rider of all time (tied with Julien Absalon), and he didn't switch to 29" wheels until 2016. He was the only dude on 27.5" wheels for 4-5 years, and it didn't seem to bug him. I think he racked up 4 rainbow jerseys and 15-ish World Cup wins as the only dude on 27.5" wheels, and it's widely suspected that his change to 29" wheels was more about marketing and sponsor pressure than performance.

Incredible depth of knowledge here. I am a nerd and knew none of the track stuff, nor did I realize Nino was on 27.5" stuff for that long. 

nino is tied with no one. 

Hey Jeff, did you get around to testing this?  Really curious about these results