Monday, October 7, 2013

Wheel size facts Part 1.... Dimensions, Weight and Strength.

You may have read certain online and printed marketing strategies which talk about wheelsize with a significant bias towards one size. The size they promote will always be either the only size that the source company produces, or the size that they want to push. Intentional marketing spiels are often very misleading and can skew the purchaser's judgement.

I feel it is my duty to set the record straight by writing a series of blog posts that kick off with this one, which addresses two key components of wheel size: weight and dimensions (and little bit of strength thrown in for good measure!). I plan to give unbiased information that you may find useful when deciding what size hoops you want your next purchase to be.

I can offer nonpartisan information (actual facts, rather than marketing blurb) as here at Banshee we offer all 3 mountain bike wheel sizes. We let the customer decide what they want rather than force it upon them, so have no reason to promote one over any other.

Every wheel size has its pros and cons, so picking the best wheelsize for you really comes down to personal preference. The main things to consider when picking wheel size are your riding style, riding purpose (style or speed), the terrain you ride, and rider height, but there are also many other factors. I'll try my best to cover the main ones.

So read on if you want some real numbers... 

The following comparisons for this whole series are based on using Maxxis High Roller II 2.3" tires on each wheel size with same rim width for all sizes.

Any comparison I do will be relative to 650b wheels since they are the middle wheel size and so it makes the % change figures clear and constant.

Dimensions: (Outer tire dimensions taken from official Maxxis 3D files)

Straight away this table is likely to cause some confusion... because as you can see, none of the rims or tires match up to their name sake. You can find out why this is the case by reading from a master of bike knowledge Sheldon Brown.

However, one point to notice is that while 650b is marketed as 27.5", it is only 1" larger diameter than 26", and 1.5" smaller than 29", so it is significantly closer to 26" than 29.  The 650b tire (often marketed as the 27.5") does not actually fall equally between the 26" and the 29" tires, so the characteristics of the 650b are far more similar to 26" than 29" wheels.


Static weight

Obviously, tire and wheel build weights can vary significantly for all wheel sizes. So I'm sticking with 2.3" wide High roller II 3C/EXO/TR. For the wheels, I will use Stan's ZTR Flow EX wheels for each size.

Static weight (the weight of an un-rotating wheel) is often emphasised by marketing teams. But it only really matters when you lift the bike on and off a rack or carry it on your back. However, static weight does have an effect on the...

Moment of Inertia

Moment of inertia is resistance to angular velocity change about an axis of rotation. Basically, the higher the moment of inertia of a wheel the harder it is to accelerate (and decelerate). This is far more significant than static weight when riding a bike.

Moment of inertia is related to both radius and mass, as Moment of Inertia (I) = Mass x Radius². A low moment of inertia results in a fast accelerating wheel (easy to start spinning). The flip side of this is that a high moment of inertia is harder to decelerate (harder to stop spinning), and so the wheel will carry the speed better once rolling if all other factors are equal.

The below table shows approximate moments of inertia by using the BSD as the effective rotational radius for all wheel sizes. 

What these numbers illustrate is that if you ride flowy trails that do not require lots of braking and accelerating back up to speed, then a larger wheel might be a better choice. However, if the trail demands regular braking and pedaling up to speed again then a smaller wheel might be better suited.

If using the same effective components, then as the wheel size increases the weight and inertia increase accordingly (as you would expect)... but because inertia increases at a rate that is proportional to the radius squared, it goes up more steeply than weight as the wheel size increases.

What does this really mean?

Lets take these numbers and do some simple calculations to look at how much kinetic energy is theoretically required (ignoring rolling resistance etc) to accelerate each pair of wheels up to 10m/s along a flat surface.

The above table shows the following:

-Rotational kinetic energy (energy of a stationary spinning wheels with external velocity of 10m/s).
-Center of Mass (CoM) kinetic energy (energy of static mass traveling at 10m/s)
-Total kinetic energy (adding together rotational and center of mass values)

The kinetic energy contained in each wheelset rolling at 10m/s is then compared to that of the 650b wheel value.

What this shows is that these 26" wheels require 4.87% less energy to accelerate up to 10M/s than 650b, and that 29" wheels require 6.71% more energy than 650b.

On the flip side, once traveling at 10m/s each wheel requires the same amount of energy to come to a complete stop... so if we consider rolling and wind resistance forces equal for all wheel sizes, then the 29" wheel will continue to roll 6.71% further than the 650b wheel which rolls 4.87% further than the 26" wheel. This is due to the 29er wheels having the most momentum for any given speed.


A factor that is strangely often overlooked by marketing teams is that of the strength and stiffness of the wheel. I find this particularly strange as wheels cost a lot of money, and are subject to a lot of abuse, and personally the lifespan of a wheel is a significant factor to me when choosing what set to invest in.

If comparing like to like wheel builds (same rims, hubs etc), smaller wheels will always inherently be stronger than larger wheels. This is due to wider gaps between spoke eyelets and poorer spoke triangulation etc. So strength to weight ratio is something that will always be won by smaller wheels.

It is however easy enough to compensate for this by getting stronger and stiffer wheels, but they do generally either weigh, or cost more. So something has to give.

It doesn't stop there....

Weight, dimensions and strength are obviously very important factors to take into account when considering what wheel size to choose. But... there are other factors too! And if this mini-blast of physics chat hasn't put you off too much, stay tuned for future blog posts about topics where bigger wheels have the advantage.


Mohd Haris Khalim said...

Hi Keith,

Thanks for this write-up. A wider, unbiased look at the major wheel sizes that would be very helpful to many mountainbikers. Kudos man!

I was sooooo moved by it that I took the trouble to translate it to my native language, bahasa melayu. Hope you don't mind.


BC Kelolo

Keith Scott said...

I'm glad that you found this useful, and thanks for translating and sharing! There will be another 2 posts in thie series, so I hope that you can translat the whole series so that it paints the whole picture for those interested.

Trey Richardson said...

Great data! There are a lot of factors to consider when deciding which wheel size best fits your overall needs. Have you come up with any data that deals with the various axle heights and how a bike pitches back and forth into turns (quicker vs. slower handling and angle of lean)? A person who's majority of trails are a 10 mile loop on 1000 acres of land vs. a person's who's trails are a 10 mile loop on 10,000 acres of land would likely pick different wheel sizes. And though speed is important to some, agility, (i.e. grin factor), should be as, if not more important.


Keith Scott said...

Trey there are so many factors that I have to pick from... I'll cover what I feel are the most important factors which are just about wheelsize in this series... but may move onto handling characteristics in future... we shall see.
Thanks for the idea!

Milky said...

Great stuff.. more please, looking forward to find out the relationship with the axle / BB height and how it effects stability. Is there any advantage to run different diameter wheels like motor cross bikes ?

Aaron said...

Hey Keith, great presentation of unbiased data. I'm curious about the Kinetic Energy. You list the Total Ek 700-800 J for a wheelset. What's the Total Ek of the entire bike/rider system? I guess what I'm getting at is that a 29er wheelset may be 6.7% hard to spin up than a 650B wheelset, but what we actually feel in our legs is the total Ek of the bike+rider, so that percentage difference is going to be much smaller when you consider the bike+rider, right?

Keith Scott said...

Hey Aaron, I've kept this whole post purely about wheels by themselves (so numbers are not taking into account bike and rider. I just wanted to make this focused on wheels. The % changes will indeed decrease significantly when rider and bike are added into the Ek equation.
I might write a post purely about weight in future... one which will show that saving weight from your bike is pretty insignificant compared to rider weight saving (shit before you ride!) haha.

Aaron said...

Well since you aren't cooperating as calculator b*tch, I had to do it by myself ;) Looks like a 100 kg rider+bike will have about 5,100 J of Ek at 10 m/s (pretty fast). That means that as a percentage of the whole bike+rider, a 29er will feel 1.6% harder to accelerate than a 26er at 10 m/s - not sure about slower speeds. Apologies for my inner geek!

PS - I know what you mean about the weight issue, but I still say that you will *feel* a 2 lbs difference in your bike much more than a 2 lbs difference in body weight, and that light bikes will mean less fatigue at the end of your ride/run, even if it doesn't make you faster from a pure physics point of view.

Simon said...

Hi Keith,
I love it! On top of that it's what I've been thinking all along but was too lazy to calculate myself...

There's sooooooo many ways to look at this wheel size thing (bigger wheels - more grip - so you can get by with lighter rolling tires - so it feels lighter - ...)

Anyway, I'm very interested in the following articles, 'cause in the printed press it always stays so damn superficial and unscientific!

About the "feel" of a small weight saving on bike / rider, I'm rather sure it has to do with the speed at which it occurs; lightening up your bike can feel as an instantaneous gain, while a change in your body weight will more often then not be gradual, so it's much harder to compare between the heavy & lighter situation. You could easyly test this with heavy clothes or a heavy / light backpack, but I never got around to doing that either!

Mathias Marley said...

Hi Keith

YOu need to recheck your calculations. Rotational Ek can never be bigger than CoM Ek, ref last table.

Atle K said...

> Apologies for my inner geek!

Please don't do that!

Never apology for what you are, and why should you feel sorry for what you can or do? Where do this crap comes from?

I am a geek too, and it's a unique skill to accquire new skills or knowledge, or use these to solve problems. Not many people seems to do that.

Thanks a lot, I got a revelation about
psychic, I had difficulty with it at school, but psychic is too important to be ignored, so I haven't gived up.

> the printed press it always stays so
> damn superficial and unscientific!

They're listening to marketing and social pressure of not being a geek.
Thus they're getting hollowned out and empty.