[Closed] Physics corner - 29ers v 26ers - am I right?

[IMG] [/IMG]

Black=white, you are a genius 😀

Oops I see I should also look at the linear kinetic energy required also - which would obviously be greater for the 29er - but enough to tip the scales?

I think I need a coffee first.

Angular speed decreases linearly with increased wheel diameter, angular momentum increases with the square of diameter, so the ease of acceleration decreases linearly in response to increasing diameter. Add increased mass and bigger wheels definitely require more force to accelerate at an equal rate but are better at maintaining their speed because of this.

Oops I see I should also look at the linear kinetic energy required also - which would obviously be greater for the 29er

And which tyres were used of course.

I don't understand the complicated maths above.

As a layman's explanation...
The two bikes are travelling at the same speed, therefore the surfaces of the tyres are travelling at the same speed.
However, any point on the 29er tyre (say, an individual tread block) has got to travel 10% further to get back where it started, plus there are more tread blocks.
So, moving more mass over a greater distance has got to take more energy hasn't it ?

Not necessarily, kinetic energy is half M V squared, so you get punished the faster you go, so 26ers need to rotate faster for the same speed so need more energy to achieve that speed.the trade offs favour 29ers, but it is very marginal.

The real benefits of 29ers seems to be the fact they roll better over terrain. So every pebble, stone, undulation - I.e thousands, even millions of imperfections over the course of a ride, takes out less energy from your forward momentum, and all these tiny, fractional amounts add up to something significant over the course of a long ride. And for the rider, less energy they need to input.

Done a few rides on my 29er now and don't notice much of a difference in acceleration between it and my 26er hardtail. Both bikes are lightish XC hardtails. The difference once the bikes are up to speed is far more discernable - 29er just keeps on rollin'

its quite sad reading all these stats when all most of us want to do is enjoy riding our bikes, which is something i wouldnt if all i was concered about was if im rolling better over this tree stump... 🙄

MidlandTrailquestsGraham, the surfaces of the tyres are the same, but the rpm of the wheel is lower for the 29er, therefore you need to accelerate the 26er wheel more to achieve the same speed, and the relationship between energy input and acceleration is not linear. The higher the speed you want to accelerate to the more and more energy you need to input. So the argument about accelerating the extra weight of a 29er is not necessarily true. It is either cancelled out by the lower speed of a 29 inch wheel, or benefits a 29 inch wheel.

However, any point on the 29er tyre (say, an individual tread block) has got to travel 10% further to get back where it started, plus there are more tread blocks.

Yeah it takes more energy for a turn of the wheel - but the bike has travelled further too.

I look at it this way:
* since energy can't be created or destroyed, 26ers and 29ers are equally efficient (on a flat surface). All the energy the rider puts in, they get back, tho perhaps the profile of how it is put in and got back are different. ie the 29er takes more energy to get to speed but rolls further once it's there.
* but trails aren't flat. All the trailcrap (roots, loose surface, stones) cost energy and impede progress. But they impede the 26er more. Since our trails are littered with trailcrap, this can add up to a big difference over a ride of any distance.

Make sense?

[b]cynic-al[/b], the intertia of the 29er wheel is higher by 10% because of the mass (assuming the rim and tyre weight the same per cm) and 10% [i]squared[/i] higher because of the distance from the hub. So overall, it's 10% cubed. While the speed of rotation is only reduced by 10%, as it's a straight linear relationship with circumference (or radius). So on balance, the 29er is worse by 10% squared.

For the same reason, a Brompton with 16" wheels accelerates really well, on a smooth road.

What's 10% squared?

It can't be 10x10 = 100% nah

It can't be 0.01 x 0.01 = 0.0001 = 0.1%

?

its quite sad reading all these stats when all most of us want to do is enjoy riding our bikes, which is something i wouldnt if all i was concered about was if im rolling better over this tree stump...

So don't read the thread? What were you expecting from a thread marked "Physics Corner"?

Fortunately for you, there are people who are interested in the physics of bikes, which is why today's bikes are so enjoyable to ride.

This is like one of those questions on university challene whee you can't even begin to comprehend the question to guess an answer let alone work it out.

What's 10% squared?

yeah of course, doh

1.1 x 1.1 = 1.21 or 21%

ta

Talk about over complicating things!!

The rider of the bike with the bigger wheel (assuming the same gearing) will have a lower mechanical advantage, so acceleration will be less for the same energy applied.

So you fit a rear cassette and/or crankset to mitigate this. I won't do the maths!

I would think that an individual should opt for a size of bike that suits their anatomy and that wheel size should be part and parcel of this. So if you are a short arse, stick to small frames, small wheels. If you are long legged and rather tall, choose a large 29er frame with the bigger wheel sizing.. Simples!

Spongebob, it isn't that simple as I already explained. When your bike is moving it has linear kinetic energy and rotational kinetic energy (the flywheel effect of your wheels). The bigger your wheels, the more rotational kinetic energy they contain. This means that it is harder to accelerate a bigger wheeled bike (because you have to add extra energy) but that the bigger wheeled bike is better at maintaining speed in the rough (because it also resists negative acceleration more strongly).

Ride a BMX or a Brompton after riding a MTB or roadbike and the pros and cons of different wheel sizes become way more obvious than from the more marginal difference between 26er and 29er!

As chiefguru + the increase in mass which i anticipate (id put money on it) increasing at a greater rate than linear in respect to wheel diameter to attain equal strength and stiffness.

[quoteSo don't read the thread? What were you expecting from a thread marked "Physics Corner"?

Fortunately for you, there are people who are interested in the physics of bikes, which is why today's bikes are so enjoyable to ride.

I think you are missing the point...second thoughts forget it I can't be arsed

Stan's crest rims:
26er = 340g
29er = 380g

Racing Ralph 2.1 EVO, 26" = 465g
Racing Ralph 2.1 EVO, 29" = 495g

(+Spokes? But they're not at the rim so matter less).

Total difference about 9%.

Cheers greybeard, I see my error now - should have had the coffee first d'oh! what an idiot!

andypaul I don't see your point either, but don't worry about it.

[quoteandypaul I don't see your point either, but don't worry about it.

Lol, we all have our off days.. 😳

😀

I am crap at Physics but I know ALL bikes are fun to ride, I don't think Physics comes into the equation 😉

TBH, the acceleration of the average bike is only limited by the flat blob of lard sat on said bike..............

if we really want to answer the 29er debate, someone is going to have to build a "terrain" simulator that means we can accurately measure the drag torque on a 26 and 29 inch wheel over various types of surfaces. Think of a treadmill, but instead of a smooth belt, one which can be swapped out to have various sized surface artifacts attached to it. So we start with a smooth belt, measure the delta torque, then say 1mm bumps, then 3mm bumps, then 5mm bumps etc etc till we are all absolutely sick to death of the subject! (What? you're sick to death of it already? me too!!! 😉

All this is well over my head!

My own experience has shown my 29er is faster on what I would call "xc" type stuff, I.e. no sudden acceleration, steep stuff or sharp turns. But on more technical riding, wheel flex and slower acceleration makes for slower lines and less ability to really attack aggressively.

Yes, bigger diameter wheels are heavier and require more force to accelerate , so unless you have unlimited power they are slower .

[img] [/img]

I'm disappointed - I expected to see something about ghosts

Yes, bigger diameter wheels are heavier and require more force to accelerate , so unless you have unlimited power they are slower .

No they are slower to accelerate but also slower to decelerate which makes them faster when carrying speed is more critical - but slower when changing speed is more critical.

No they are slower to accelerate but also slower to decelerate which makes them faster when carrying speed is more critical - but slower when changing speed is more critical.

but on virtually all courses, accelerating (to speed up, slow down or turn corners) is important. You need to think about what is the result of having heavier, larger diameter wheels in practical terms. Maybe if you want to achieve a speed record down a straight hill then OK, but back in the real world, the physics says bigger wheels slower / need more power

There is a good reason why, ever since the dawn of the bike, manufactures have tried to make wheels as light as possible. The momentum advantage from weight alone is never enough to overcome the extra force required to accelerate that wheel in practice. As long as you don't compromise on stiffness a lighter wheel of the same radius is always better. Where this gets complicated with 29ers is that the larger radius means that they are decelerated less by obstacles. The advantage comes from the size not the extra mass though and you'd still want a 29er wheel to be as light as possible.

Of course there is also an argument that, due to the different shape of the contact patch, 29ers have more grip so you can run a narrower tyre (reducing the weight penalty). Then you have the fact that people often fit lighter rims to 29ers, but that's often just giving up stiffness and longevity to save a bit on the weight. And of course you can always buy yourself out of the problem by fitting more expensive wheels so that you get the advantages of the extra radius with hardly any increase in weight.

world, the physics says bigger wheels slower / need more power

No it doesn't. Physics says you can't create or destroy energy so whatever power you put in you get back out.

No wheel size is 'slower' than an other.

but on virtually all courses, accelerating (to speed up, slow down or turn corners) is important. You need to think about what is the result of having heavier, larger diameter wheels in practical terms. Maybe if you want to achieve a speed record down a straight hill then OK, but back in the real world, the physics says bigger wheels slower / need more power

I have thought about that (and incidentally ride a 26" MTB and 20" BMX). When going downhill you have gravity as a power source and a bigger wheel can store more energy to pull you through the rough stuff, but requires more work from the rider to manoeuvre. I think the 29er's contact patch is a better shape in the mud but the bigger wheel carries more mud weight and it's harder to achieve good enough clearance around the BB. So for every pro there is a con. Choose what suits you and go and ride!

The 29'er was created to get rid of all the un sold short travel forks

I think that we should now discuss the merits of beards, pipes and slippers!

Gravity's not a power source. Moving an object upwards stores potential energy in it, that is converted to kinetic energy when the object comes down.

2 bikes of the same weight at the same place have the same potential energy, regardless of what wheels they have on.

But that's a bit pedantic too, sorry.

Gravity's not a power source. Moving an object upwards stores potential energy in it, that is converted to kinetic energy when the object comes down.
2 bikes of the same weight at the same place have the same potential energy, regardless of what wheels they have on.

Yes and unless you never brake then you have a surfeit of potential energy to convert into kinetic energy when descending, in which case wheels with more flywheel effect are incidental. Ride what you like, just don't make out that either 29ers are the Emperor's New Clothes or the ideal size for everyone. They're just another option.

a few things strike me as odd in this thread and the 29 er debate in general

Firstly I'm amazed that suddenly a wheel string more energy is better. That really flies in the face of everything I'v ever read or experienced about bikes.

Would sir like the kevlar beaded tyre or for a few quid less the steel bead. Oh steel for me i find a benefit on storing extra energy in my wheels. Do you have any lead rim tapes.

Its true that you might some times benefit from the energy stored in your wheels but I think the most likely out come is that you will waste it as heat in your brakes

The supposed benefit of a larger wheel is its ability to role over bumps. Which makes sense and is hard to deny.

However the same is true of suspension. In my mind I always thought that the 29er would win out for hard tails but FS would remain with the smaller wheel size.

The bike fit 29er argument for the tall makes sense. But i think mainly it tells us that tall people were sold/ bought bikes with chain stays that were to short for them in the intrest of style. Bizarley tall people only got the chain stays they need when FS and larger wheels hid the fashion crime of longer chain stays.

A thread on MTBR a while back with a couple of physics types' input said that if 2 different size wheels that weigh the same they have the same rotational inertia regardless of size. The formula shows the radius part of the rotating and forward momentum calcs cancels itself out.

So assuming the average 29er wheel is a bit heavier for a 'type', they need a bit more energy to spin up and once there they resist decelleration equally, but it's nothing to do with size, only mass. I could have guessed that from riding different wheels, but it was good to see it proven. There's not that much extra weight in a bigger wheel, not when you look at the total system weight.

So you may feel / notice the added wheel mass but the difference in performance is very, very minor, however noticeable the difference in response / feel / grip - something that may help you ride differently enough to be worth it. Or not.

Please bear with me if this is a silly question, but why is rolling better over all the bumps a desired thing in mountain biking? If you want to smooth out the trail, why not ride on a towpath ... Or Tarmac?! I'm sticking to my 26er hardtail cos I enjoy all the lumps and bumps!

Please bear with me if this is a silly question, but why is rolling better over all the bumps a desired thing in mountain biking? If you want to smooth out the trail, why not ride on a towpath ... Or Tarmac?! I'm sticking to my 26er hardtail cos I enjoy all the lumps and bumps!

Same here when it comes to moar suspension being "better" too.

Why not go rigid-forked if you like the bumps? )

Well there is an argument that says that mountains are a scenic and exciting, more so than tow paths. So a bike that cope with the terrain makes sense to me

So VickyPea and deafnbm I assume you ride bikes with 20 inch wkeels or even 12 inch wheels. That way you'd feel the bumps even more

Now you're just being daft!
I don't want rigid forks as I think my arms and wrists would complain, although I have on several occasions done some rocky descents with my forks accidentally locked out!
I still have a lot of improving to do before I consider anything other than a hardtail anyway.

Nothing wrong with rigid forks, they are much lighter so don't hit the bumps so hard;-)

I don't want rigid forks as I think my arms and wrists would complain

According to you the solution to not being able to cope with the bumps is top ride a canal tow path, why doesn't this apply to you

But rigid forks are much lighter, so they don't hit the bumps so hard

My better half has a fully rigid Kona and he loves it, but he says a couple of hrs riding on rocky trails hurts his wrists.

Extra energy required for the additional 29" wheel mass is minor compared to the energy saved in rolling resistance. However, the wheel size isn't where the real benefit lies with the 29".

However, the wheel size isn't where the real benefit lies with the 29".

Say what?

Geometry.

I assume you ride bikes with 20 inch wkeels or even 12 inch wheels. That way you'd feel the bumps even more

I do my short bridleway commute on a 20" BMX. It's much more fun/challenging than on the MTB, especially on the bumpy bits (where the key is to drop your hips low and back and pump the bike through with the front wheel barely on the ground). My hardtail feels like a long-travel full-sus in comparison, which really helps with confidence.

grantway - Member

The 29'er was created to get rid of all the un sold short travel forks

So manufacturers had a shed load of unsold short travel 29er forks before 29ers came out .

oh geometry! Guys, guys.... hang on a minute weve all missed the point - 29ers arnt to do with wheel size

RN I think he was kidding. I hope JCL is too!

My view is the whole acceleration/decelaration thing isn't significant in real terms. It would be great if some real world testing could be done on this stuff.

JCL has a point, there's aspects of 29er geometry that can't be easily done with a smaller wheel, eg BB drop and the amount of rise/fall of the front for a given trail is lower ('flop rate vs trail'?). Both are positives, one is a lot easier to explain than the other. But that's another thread/been done before.

Yep, zero BB drop. Can't be replicated with a 26" unless you want useless ground clearance.

Not saying there arnt some positives, but the idea that the industry has been selling on those points is pretty laughable

I still want to see a real work test. Common wisdom says that the a 29er accelerates more slowly but rolls further (whether through momentum or AoA). So, get 2 bikes (say giant Anthem 26er and 29er) put hope hoops shod with racing Ralph's on both and find an reasonable slope that leads to a flat field. Get the same rider to go down the hill repeatedly and see how far each one gets. You'll the see if the slower acceleration is offset by better rolling. Then start to add in a twisty course, so the directional changes come into play.
Of course, this isn't going to show which feels the best, and that's the one I'd buy, but at least I'd know in the next enduro I do, if I really was at a disadvantage on my 26er!

Edit, should just have said Al+1

I think the actual difference between riding a 26er and 29er equates to one decent turnout, say 500g faeces.

Double post deleted

willc9999 : ) "is that a smile of joy from riding your new wheels, or are you just constipated?"

Not saying there arnt some positives, but the idea that the industry has been selling on those points is pretty laughable

I don't think many in the industry really understand the positives. The fact that they've stumbled upon definite advantages over a 26" from BB drop and chainstay/wheelbase length etc hasn't been realised by half of them.

WTF? testing mountainbikes on lumpy treadmills or rolling downhill in a straight line?
Do you lot get off and walk when you encounter corners?

No drudh it what's know as wanting facts to back up the magazines latest fad. Doesn't seem too unreasonable. I'd actually like to see it linked to hrm's and power meters over long xc rides and downhills to check as well, but would,start with the simple test first.

JCL - Member
Yep, zero BB drop

What is this (29ers don't have 14.5" BBs?) and what is the benefit?