Why is 29 faster than 27.5?

If there is a track difference it’s rolling rolling resistance in rough terrain. Basically a bump picked back less on a larger wheel. Think about a skate board off road. 0.5kg is not much weight.

However on less severe terrain my 650b gravel bike is faster than my 29er fs bike

White is about the slowest colour as well!!

Following my mate down Super Swooper at Dyfi, with him on a 27.5 and me on a 29er, on sections that we were freewheeling I had to brake to keep from overtaking him, purely because the 29er rolled better.

This effect is very obvious when your children move up through wheel sizes. My eldest has gone 14”, 20”, 24”, 27.5” and each bigger bike has been significantly heavier than the one before (and she’s light for her height). But the bigger wheeled bike is noticeably faster. I remember when my son went from 14” to 20” he was SO much faster!

Lower angle of attack over bumps, longer contact patch for a given tyre size, hold momentum better...

The higher weight and the slower acceleration downsides are/were far less of a problem than the nay sayers would have you believe, but the upsides of larger wheels are significant.

The new bike feels faster or I feel less tired.

You really need to use a stopwatch to compare, how you "feel" is often not a good guide. You also need to make sure that the bikes you are comparing are running the same tyres. My guess is that a 29er will be slightly faster, but that tyres will make a much bigger difference and that there will probably be trails where a 27.5er is slightly faster.

Fair enough! I was running Rekons on 2.8F and 2.6R on the Commencal.

Now Rekon 2.6F and Nobby Nic DD 2.4R.

Is the contact patch similar in area at the same pressure between the 2 bikes, albeit of a different shape?

I can agree that very tight and twisty trails might not suit the bigger bike.

I am mostly talking about effort rather than technical or handling ability so far.

But you're right, perception and reality may be quite different particularly as the Commencal was very stiff so perhaps I used to be more fatigued/battered than with the very comfy Big Al!

What I figured out when I first switched, was that when you think about the rollover thing you naturally tend to think about invidivual bumps, ie bigger stuff, but that's not really where it makes the difference I think- it's the actual amount of bumps that counts. Little ones get turned down from "a very small bump that you barely notice but that slows you down a little bit" to "a tiny bump that you barely notice and slows you down even less" and slightly bigger ones get turned down into little ones. It's just that this happens absolutely all the time unless you're on really smooth surfaces.

The big hits barely matter most of the time, because you deal with those by actually riding over them, moving your body, using the suspension etc. It does still help but it just doesn't happen enough to really add up on most trails. But washboard chatter on a fire road slows you down a tiny amount but a thousand times

There's more complicated stuff with tyre deformation, for hte same pressure you get a slightly different shaped contact patch on the bigger wheel, longer and narrower. So that'll tend to give a wee bit less loss from hysteresis since you're basically pushing a smaller, narrower "bow wave" of tyre. But then we probably don't use the same pressures so that's where it gets just messy. TBF actual tyre dynamics are super complicated and a bit unintuitive

Ride a 20" wheeled BMX off road and you will see the effect exaggerated.  They slow down so quickly it feels like you have the brakes rubbing.

Is the contact patch similar in area at the same pressure between the 2 bikes, albeit of a different shape?

At the same pressure they'll be the same area but a slightly different shape, larger OD wheel will have a longer thinner contact patch that may add some side to side stability in theory, but with tread and a loose/soft surface considered I think it's hard to say how much of a factor it is.

It's this -

But washboard chatter on a fire road slows you down a tiny amount but a thousand times

The larger OD wheel doesn't drop into holes so far so it reduces amplitude of bumps at the axle -  rolling resistance is lowered, lower movement of the mass of you and the bike over bumps and the effect that has on the tyre deformation.

On the acceleration and heavier wheels point, a larger OD wheel spins slower at a given speed anyway so all we're concerned with is moving along or up (rather than rotating) 6-7% more wheel weight if rim + tyre type is the same. When I first got used to a 29er I felt I was holding speed better overall so there was less need to accelerate back to speed. 'Felt' isn't quantifiable but the way the bigger wheels roll down gentle descents you know well is hard to miss and all in all I'd say efficiency was higher on a longer ride with a larger wheel despite the added weight.

Where I can really nerd out on this is road/gravel bikes for a mix of bad road surfaces and basic gravel use, where bump size and frequency is different and you can deal with a lot of it via tyre volume (eg 650B) on a smaller OD rim because the size of the bumps is generally much smaller.. but years on I'm still split on the 2 rim sizes there.

A frame builder did a recent comparison, and pretty much concluded that the only real tangible difference is the gyroscopic effect at speed. yes we know there is improved roll over but when you actually look at what it is like in the real world it would be unperceivable

Conversely, i am so much more a fan of the way my 27.5 bike climbs compared to the 29ers I've had. That being said, comparing my 29 hard tail to my Top Fuel, tyre choices, riding position etc, and the FS Top Fuel was a much nicer pedal, the hardtail being a horrible climber (Sonder Signal, dt xm481 with 2.6 enduro casing tyres)

“ A frame builder did a recent comparison, and pretty much concluded that the only real tangible difference is the gyroscopic effect at speed. yes we know there is improved roll over but when you actually look at what it is like in the real world it would be unperceivable”

I don’t know who this frame builder is but this sounds like a classic example of a poor executed scientific experiment leading to incorrect conclusions.

In the real world, off-road, bigger wheels roll faster if you keep all the other variables the same.

I'd agree with Northwind's comments about the cumulative effect of the bumps being what you notice.

When I first got used to a 29er I felt I was holding speed better overall so there was less need to accelerate back to speed. ‘Felt’ isn’t quantifiable but the way the bigger wheels roll down gentle descents you know well is hard to miss and all in all I’d say efficiency was higher on a longer ride with a larger wheel despite the added weight.

I'm not a physics person, but it feels to me like the way a larger flywheel holds momentum better and longer.

Not that I have a huge collection of flywheels 😀

It's because your wallet has been made much lighter.

It's pretty unusual to have a 27.5 and 29er with exactly the same wheel base, front and rear mechanical trail, front and rear centre, BB drop, suspension corrected for the improved roll over etc.

In fact, it's impossible to ever completely recreate all the same geometry between 27.5 and 29ers.  Especially when you start taking into account how the geometry changes as you turn the handlebars and the suspension moves.

I don't think bigger wheels are inherently faster.  If you took a 2011 29er and put it up against a 2018 onwards 27.5 you'd most likely find the 27.5 bike was much faster.

It also comes down to riding style.  A bigger wheel will roll over things better but if you are the kind of rider who is very active and constantly pumping to gain or maintain speed then you might find you get less out of a 29er than you would on a 27.5.

Probably with the majority of riders on the majority of trails 29er is faster (at least for the front wheel) but I don't think it's a universal truth.

it was a big UK? steel frame builder, can't remember who

yup, big wheels maintain momentum, but in mountain biking you can be constantly decelerating and accelerating. so additional energy is expended there. I'm pretty sure he wasn't poo pooing any of the arguments, just saying that they were a drop in the ocean compared to the gyroscopic effect bringing increased stability

It’s because your wallet has been made much lighter.

😂. That certainly should be a factor.

But the net cost was zero courtesy of the CRC crazy prices. I sold the frame for more than the new cost of the Big Al frame for starters.

In fact I think the new bike made a little money.

It’s because they make the trails come alive innit.

It’s because they make the trails come alive innit.

No, for that you'll have to wait for the soon to be announced goldilocks 28.25" wheels (3mm increase in BSD over 27.5)

I don’t think bigger wheels are inherently faster.

They do roll a bit faster, as explained above, but tyres will make a bigger difference than a 40 mm difference in wheel size.

For a while I had a 27.5 hardtail and a 26 FS. Going back to the latter after a few months it was noticeable how much forward momentum was lost on deep roots and larger step-ups.

A frame builder did a recent comparison, and pretty much concluded that the only real tangible difference is the gyroscopic effect at speed

..a drop in the ocean compared to the gyroscopic effect bringing increased stability

I'd say they were mistaken. Gyroscopic force is a classic Room 101 answer in bike stability and handling. It's got very little to do with it at average bike speeds. Maybe at full DH speed in the air there's some difference in feel. But on the ground, larger wheels are rotating slower at a given speed so the only difference in gyro force is from that 6-7% change in weight which will mean a similar 6-7% change in gyro force. If you can feel that you'll feel it equally as you change tyre weight within the same wheel size, like from an 850g to a 900g tyre. It's just not a factor esp. compared to the potential change in rolling resistance.

..Ahh, I'd missed the wheelsize chats : )

tyres will make a bigger difference than a 40 mm difference in wheel size.

However, you'd generally be using very similar tyres on either wheel size for the same application - whether that's gravel or technical MTB.

They do roll a bit faster, as explained above,

Yeah, but as was also explained above they also won't accelerate as fast and some riders use every slight dip they can find to maintain and gain speed.  You can't do that as effectively with bigger wheels.

Now admittedly not all riders ride like this.  In fact, maybe the majority of riders don't ride like this.  And of course, once you get into steep technical terrain it becomes less of an issue.

But even then, when things get really steep and technical, you are still looking for every minor dip to actively drive the wheels into so that you can gain enough traction to brake effectively (sort of reverse pumping).

But ultimately, if it was a universal truth that 29ers were faster than 27.5, the resale value of a 2011 29er would be a lot higher.

Wheels are just the final gear, so the bigger the wheel the better the ratio for transforming power into speed. This is why 1x with it's tiny front ring is a silly modish fashion.

Most of my long standing fastest times on Strava are on a 26'er

With a triple chainset....

Completely depends on the trail you are riding, and its the combination of wheel size / wheelbase / geometry

I have an old school 26er charge duster, on certain very tight twisty trails (like the labrynth at swinley) I'm faster overall on it that my ultra light scott scale 29er.

You also have 'total' speed vs acceleration.  I think I have a faster sprint on the smaller wheels, but overall the longer the section I'm timing the more the bigger wheels tend to win out.

You then have the 29 HT vs FS.  I also have a scott spark, which is pretty much faster everywhere than the scale, except up steep inclines.  But the FS feels slower and is less fun.  But the timings show its faster.

All bikes are black so its not the colour making a difference 🙂

Is that because you were younger and fitter when on the 26".  My fastest times on Strava are from 10 years ago when I was 46 rather than 56 and not sure the bike I was riding was as relevant.

Luckily I have just bought myself a 26" with a triple so will see how my times go over late summer when I am typically get the fastest times.  Although the bike is old I am running tubeless Mezcals which certainly feel fast enough to be in with a chance.

I don't think comparisons like, 'My 2010 26" FS is way worse than my 2020 29er hardtail' really tell us much, to be honest.  All that tells us is that bikes have improved over 10 years.

To really compare the wheelsizes I think you have to get something like a Geometron G1 with all the flip chips and get the geometry as similar as possible.  Even then it's not going to be identical and the bike was most likely designed with a particular wheel size more in mind.

There's been a few videos on YT where people have done just that.  I couldn't find the one where someone went full 27.5, mullet, and full 29er but the conclusion was that there wasn't that much difference but the 29er probably got less hung up on roots.  Don't think either one talked about how easy each version was to pump though.  Here's the PB one about mullet and full 29er:

I still remember first ride on a Niner EMD after a couple of decades on 26" wheels.
Oh.
My.
Word.
It just rolled like a juggernaut over the usual Houndkirk Moor trail debris and rocks. It just sustained speed where my 26" would have dropped into holes or square edge steps would have noticeably knocked back some momentum. Over the course of a longer ride, this added up to less effort to sustain speed but also more comfort.

I have 27.5 currently and plan 29 for my next bike.

(I will put aside an early QR SID and road 700c rims with a too wide early WTB tyre mounted, which flexed like crazy in more gnarly terrain and corners...eveything else was superb.)

BMX trails riders are on 20". 4X racers will be on 27.5". Long distance racers will be on 29". Fast DH riders tend to be on MX bikes. It's terrain, pedalling and handling-dependant.

What we can do is take all of those variables out and look at roll-down tests, if a larger OD wheel or higher volume tyre is seen to be faster there you have something that's more efficient and it's a passive advantage, it needs no input from a rider to have that advantage. So 29" has objectively greater rolling efficiency. Then we need to get into whether it's suitable for the rider and terrain, which is all subjective stuff to some extent. Or how the amount of suspension changes how a wheel rolls, or how a wheel size/weight influences suspension action.

  I couldn’t find the one where someone went full 27.5, mullet, and full 29er but the conclusion was that there wasn’t that much difference but the 29er probably got less hung up on roots.

I've spent a day at Ae forest, riding the same bike with the rear wheel switched between 27.5 and 29. Geometry corrected to an extent.

Yes - hanging up more on roots and in holes was the main difference descending.

Mullet more composed in steep terrain and more "flickable" (as we used to say), 29 just held speed better.

Also did a lot of comparison in the Alps last year, basically: Pila (fast & rocky) = full 29 better, Verbier (steep and twisty) = mullet better.

I don’t think comparisons like, ‘My 2010 26″ FS is way worse than my 2020 29er hardtail’ really tell us much, to be honest. All that tells us is that bikes have improved over 10 years.

[url= https://live.staticflickr.com/2346/2223390572_11f94e7a5e_k.jp g" target="_blank">https://live.staticflickr.com/2346/2223390572_11f94e7a5e_k.jp g"/> [/img][/url][url= https://flic.kr/p/4otsyd ]Niner Geometry[/url] by [url= https://www.flickr.com/photos/matt_outandabout/ ]Matt[/url], on Flickr

"So 29″ has objectively greater rolling efficiency. Then we need to get into whether it’s suitable for the rider and terrain, which is all subjective stuff to some extent. Or how the amount of suspension changes how a wheel rolls, or how a wheel size/weight influences suspension action."

Exactly!

"I’ve spent a day at Ae forest, riding the same bike with the rear wheel switched between 27.5 and 29. Geometry corrected to an extent.

Yes – hanging up more on roots and in holes was the main difference descending.

Mullet more composed in steep terrain and more “flickable” (as we used to say), 29 just held speed better.

Also did a lot of comparison in the Alps last year, basically: Pila (fast & rocky) = full 29 better, Verbier (steep and twisty) = mullet better."

And this totally correlates with everything I've heard about mullet vs full 29. Also, I suspect once you get to 200mm+ of rear travel and/or high pivot the full 29"s advantage on fast and rough gets lost somewhat under all the rear suspension action.

We have done it with two identical bikes. In the UK,  Alps, Lake Garda and Finale. One set up 27.5 and the other 29. Swapping riders between bikes. The red one was always faster. The blue one was more flickable. Mullet didnt feel right.

Wheels are just the final gear, so the bigger the wheel the better the ratio for transforming power into speed. This is why 1x with it’s tiny front ring is a silly modish fashion.

I don't understand this. If I compare the x1 12 speed gearing to my old triple, there is hardly any useful difference.

And that's before the benefits are taken into account.

To me x1, which I started in 2010, has massively improved my enjoyment of riding MTB. I would put it on a par with droppers

Yeah, but as was also explained above they also won’t accelerate as fast and some riders use every slight dip they can find to maintain and gain speed.

You can't gain speed by riding into a dip because you will lose it as soon as you climb out the other side, all this will do is throw you off your pedaling and slow you down. The acceleration thing is mostly a myth too. A bike with smaller wheels will be faster through tighter trails where you are constantly changing direction, but the slight increase in weight of a larger rim and tyre won't really make a huge difference to your acceleration considering that the bike plus rider generally weighs 70kg or more.

To me x1, which I started in 2010, has massively improved my enjoyment of riding MTB. I would put it on a par with droppers

I was a 1x person, even had it on a road bike when had one.  Now using a triple which I don't think I have used for well over 20 years but I am liking it, very much to my surprise.  The 7 speed cassette is less caring due to wider spacing and it just feels really smooth and shifts perfectly (pretty good seeing that it is almost 30 years old!) and I also get a 42/11 which is good for fast stuff.

Anyway, none of that has much to do with 29 vs 27.5.

You can’t gain speed by riding into a dip because you will lose it as soon as you climb out the other side, all this will do is throw you off your pedaling and slow you down.

It's this kind of comment that illustrates what wildly different understandings we all have about what mountain biking actually is.

"You can’t gain speed by riding into a dip because you will lose it as soon as you climb out the other side, all this will do is throw you off your pedaling and slow you down."

????!!!!

You pump into the dip and you suck/pull the bike out of the dip. You're not pedalling but you're putting energy into the bike with your whole body, to generate/maintain speed. Pumping is one of the most fun and flowy things in mountain biking and the allegedly reduced pump from bigger wheels is what put off moving from 26 to 27.5 to 29.

However, I think on all but the smoothest trails I prefer how a 29 pumps vs smaller wheels because the "noise" of little bumps/rough is flattened more, momentum is maintained more and you can get on with the fun of pumping the trail. 29" singlespeed hardtail (with a long dropper post so you can MOVE) is just so much fun on singletrack for this reason.

^^ this is literally pumping in and out to increase speed right?

if unsure, Ben Cathro did some nice videos on it

Pumping is one of the most fun and flowy things in mountain biking

Totally agree.

I find myself trying to pump everything on the trail (gigidy).  No depression or bump is safe and especially in corners I love unweighting on entry and then driving the wheels into the dirt and either drifting or making that nice schralpy sound (or crashing).

I probably look like a chimpanzee having a seizure but I'm having fun which is the main thing 🙂

I found in the transition from 26 (HT and FS) to 29 that the 29 climbs better, rolls better but takes a bit more thought and energy to turn. It's like there's more forward momentum keeping a straight line.

Smaller wheels can feel more able to generate speed from downslopes in places but you'll lose momentum more easily in others. You put in energy to gain speed that you lose elsewhere - zero gain on an average trail (no such thing but you know what I mean). It's very terrain-dependant outside of a BMX track. Or it's about how long you ride for and can keep up that bump-pumping riding style.

I'm not even sure if it's the wheelsize that does it, there's the reduced BB drop and rear-centre of a smaller wheel bike to think about. If the BB to axle relationship was the same and the wheel weight was the same, the pump could/should be the same - but who'd want 29" wheels on a pumptrack or 4X course because of the corners, or the longer forks, etc.