Physics corner – 29ers v 26ers – am I right?

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?