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Educating the Debate – Part III

Keith Scott Circling the Numbers

Words by Keith Scott.
November 5th, 2013

Keith Scott of Banshee Bikes is back with another serving of reality for the wheel size debate – this time focusing on tires. The discussion in the comments in Part I and Part II has been great, so catch up with where we’ve come and dig in for another part…


In this post, we will continue with the wheel size theme, but focus on tire-related factors such as contact patch, tire pressure and tread. Check out Part I and Part II for some other considerations regarding the physics behind different sizes of spinning circles. In Part I, small wheels beat big wheels, but in Part II big wheels fought back… so which, if either, is going to come out on top for you?

educating-the-debate-large-3

We will discuss contact patch and related factors across the 3 common wheel sizes. Once again I will be taking the wheels and tires from Part I for consistency: the 2.3″ casing Maxxis High Roller II.

Contact Patch

What is the contact patch, and how does it affect grip and rolling resistance?

patch

Fig.1 Contact patch on simplified tire represented in blue.

The contact patch (shown in fig.1 in blue) is essentially the footprint of the tire that is making contact with the ground at any instant in time. For any given tire, it will change with tire pressure, as Pressure=Force/Area. So the lower the pressure, the more your tire will deform to the contours you are riding over.

A larger tire contact patch area represents more rubber on the ground, which increases friction and therefore grip (good). However, the larger the contact patch area the greater the rolling resistance (bad). So, as with most things, there is always a compromise, and you just have to pick the right balance between grip and rolling resistance to suit your needs.

Shape and Area

For this section on contact patch shape, let’s look at a basic representation of each wheel size (no tread, and no tire stiffness) each with 2.3″ width , based on 50kg of weight (assuming 50:50 weight distribution, and bikes + rider = 100kg), and 2Bar (about 29PSI or 200,000 N/m²) of a perfect gas on a flat surface for all wheel sizes. Since the pressure is the same in each tire, the contact patch area will be the same for this scenario as Pressure=Force/Area. This is not very realistic as pressures will change a bit with wheel size (I will go into that later), so this is just to give an idea of patch shape.

shape

Fig. 2 Contact patch shapes for same tire pressure.

In Fig .2 you can see the 3 wheel size contact patches overlapped for the same tire pressure and loads: the bigger the wheel size, the longer and narrower the contact patch. But the variation in shape is probably much smaller than you’d expect – or have been made to believe. So let’s look at this slightly differently…

One way of measuring optimal tire pressure is actually as ‘tire drop’, which is a percentage of original tire height (a little like suspension sag) as seen in Fig.3.

tire comp

Fig.3 Tire drop is measured by how much closer your rim rides to the ground under full load.

If for each wheel size we have a 6% tire drop when riding along a flat surface on a slick tire, then this will tell us a lot about required tire pressure, as well as contact patch shape and area for each wheel size.

As you can see in Fig.4, the contact patch area and lengths change as tire pressure changes, but the width remains the same due to same tire carcass width and cross sectional shape. So for the same tire drop of 6% the 29″ wheel has a 2.7% bigger contact patch than 650b, which in turn is 1.85% larger than 26″. The difference in contact patch area and shape is far less than most marketing would have you believe, but it is present.

area chart

Fig.4 Contact patch dimensions for 6% tire drop, and tire pressure for each wheel size.

This also shows that the larger the diameter wheel, the less tire pressure is required to achieve the same tire drop. Therefore you can get away with running lower tire pressure on bigger wheels if you wish. That said, the volume of the tire is the more significant factor, so the width of the tire will have a more significant impact on required tire pressure than wheel size.

These factors are the reason that mountain bike tires are wider than road bike tires. For road cycling, traction is less important than minimizing rolling resistance (and weight) and so they run narrow low volume tires at high pressure. Mountain bikes run lower pressure, larger volume tires to increase traction as well as shock absorption. It’s a case of picking the best tool for the job, by optimizing what you want, and compromising on factors that are not as important to you.

Tire Tread and Compound

All this marketing chat about contact patch actually ignores the most important factor. Tread patterns are massively relevant, because in reality, none of us ride around on fully slick tires on the trails. So when talking about contact patch, we really should be considering actual contact patch of the top of the treads on the surface, and also considering the extra grip provided by the edge of the treads biting into the ground.

Tread pattern and rubber compounds make a bigger difference than theoretical contact patch area; the tread pattern changes the actual contact area far more than wheel size will!

HR2-2

The High Roller II tread pattern, albeit in 2.4″ DH casing. Photo Tim Coleman / NSMB.

So when thinking about grip, rather than think too much about wheel size and exact tire pressures, you’d be better off spending that time and effort picking the best tire tread pattern and compound for the riding conditions and experimenting with different tire pressures.

A softer rubber compound (lower durometer) will not only deform more to ‘grip’ the ground, but will also help damp the ride by compressing more easily under impacts. If you use a new soft compound tire you will be able to brake later, accelerate faster, and corner harder; the tread will bite into the ground with nice sharp edges, and the soft compound will have a higher coefficient of friction, and absorb the shock to stay in contact with the ground better.

For Consideration

From all the information above, you can see that a bigger wheel will offer a slightly larger contact patch area due to the fact that you can run a slightly lower tire pressure. Therefore, a larger wheel will offer a bit more grip than a smaller wheel with same tire drop, but the increase in theoretical traction of larger wheels is probably less you were expecting.

With the larger tire contact patch comes more rolling friction, and efficiency is reduced. So smaller wheels are more efficient than larger wheels in this area for same tire drop. On a perfectly flat surface with a slick tire, smaller wheels with equal tire drop will lose less energy when rolling along than bigger wheels.

But let’s be real… mountain biking isn’t about just rolling along flat surfaces and we certainly don’t use slick tires! It’s about carrying speed through rough sections, cornering hard on the edges of tires, finding traction when climbing steeps and many, many more fun things. For most of these things, tire tread pattern and tire rubber compound are FAR more important than wheel size when it comes to grip. So my advice to you is not to get too lost in these wheel size numbers, instead pick a good tire choice and just enjoy riding your bike!


This conversation could be sealed off, but we have a feeling you might provoke Keith into writing a fourth piece with your comments  – fuel his fire, as it were. If you need to brush up on the discussion check back on Part I and Part II.

  • vorlaz

    How about jumping, can you make a part IV on the effect that larger wheels have on jumping?

    29ers sure jump differently, and BMXers still prefer 20ers, so maybe there’s some intersting thoughts in that questions.

  • builttoride

    Hmmm, if I can find time I might write a final part that talks about non rolling related factors like handling in the air, frame and wheel geometry stffness factors, geometry, and a few other factors that individuals might want to consider when picking wheel size. If there is demand, then I’ll try and write this.

  • Jerry-Rig

    on bumpy terrain, lower pressure actually has lower rolling resistance.
    Thanks Keith for the articles.

  • buzzes

    Keith makes thinking Fun!!—-no seriously….it’s nasty cold and wet, and when I pussy out and stay in, stare at my wheels and speculate………..and there’s nothing wrong with that!

  • taprider

    since 29″ is ~11% larger than 26″
    does the fewer rotations of the wheel per unit travelling result in less energy lost to flexing of the sidewall of the tire (less rolling resistance) on smooth road?
    I am guessing from what you wrote, that for individual bumps on the trail that with appropriate tire drop to wheel size considered, that the difference in flexing of the tire sidewall for different wheel sizes will be nearly equal (therefore equal rolling resistance).

    • Dirk

      Fewer rotations, but larger diameter. You’ll actually have the exact same amount of flexing. The amount of contact made is essentially the distance travelled (barring any really long skids).

  • hampstead_bandit

    Hey Keith

    thanks for the great articles.

    something interesting happening in road cycling, tires are getting wider (and larger volume)

    specialized is now producing their top of the range S-Works Turbo race tires in 24c and 26c, and the sportive tires are going up to 28c

    Independant testing in Finland at a world leading lab has shown the larger tires have lower rolling resistance, although this has to be balanced against increased drag from air resistance with bigger tires.

    From talking to those guys, they reckon 26c is optimium balance between rolling resistance and aero drag.

    • builttoride

      Acrually I believe that the reason they are going wider is due to longer contact patches resisting rolling more than short contact patches. and since area remeains constant for any given pressure, a wider tire will have a shorter wider contact patch.

      There is no such thing as a constant optimum… it will depend on the road surface, the climate conditions, rider weight, rider speed etc etc… never beleive anything that tells you something is optimum for anything where conditions are constantly changing, as ‘optimum’ changes with conditions. Question the marketing!

  • leverfingers

    Good stuff here, but there’s so much more. How ’bout leverage and gear inches? Bigger wheels = more gear inches for the same ratio. Also, I’ve heard that you get more traction because you have less leverage – like starting out in second gear in your car on the snow/ice. I think there are 2 or 3 more articles here Keith. It’s winter now, lots of time to write. Right?

    • builttoride

      Having lower torque will prewvent wheels spinning out for sure (just like never driving a car in first gear on snow)… but people will change gears as they change wheel size to get similar gearing range so I think this is an irrelavent argument when it comes to wheel size (although valid for chosing gearing).

      I’m glad you want more of this… although not sure how many more weekends I’m willing to sacrifice writing them! haha. I may start doing a regular article once a month or so. I’m educating myself while reseaching each one which is good for me!

  • biggles604

    Maybe Keith or someone with more knowledge than myself can chime in, but does the 29er’s jumping/manualling feel come from the fact that the BB drop is much higher on the big wheels, forcing the rider CG lower relative to the wheels? How does the BB drop interact with handling?

    • Dirk

      This is one of those things I don’t really buy – i.e. that sitting lower relative to the axle height improves handling. BB height affects cornering in two ways:

      1) Changes your center of gravity.
      2) Changes the height and distance you need to travel to come back to center.

      Theoretically, these numbers should be independent of the wheel size. Same thing with manualling. But perhaps there is some effect that I’m not thinking about here.

      To take this to extremes, what would happen if you created a mountain bike with 20 inch wheels but identical geometry in every other sense (bb height, chainstay length, etc.). Would it corner and manual terribly?

  • builttoride

    Ability to manual can’t be put down to any one thing. The important factors are…

    Chainstay length – longer = harder to pull up to manual

    BB drop – more drop = harder to pull up. Not only rotate back further to find alance point, but also the horizontal distrance between rear axle and BB will lengthen til point where it is same height as rear axle.

    Reach – longer reach generally makes manualling harder, as it requires more forcefull pull back to lift front.

    Stack – lower stack = harder to manual as you initally have more weight forward and low to pull up (similar to longer reach)

    Axle path – the more rearward the axle path, the harder it is to pull up to manual, because as the suspension compresses as you shift weight back, the chainstay effectively lengthens.

    - Suspensions setup obviously has a big factor too.

    So lots of things, but you’ll also notice that nearly all the things that make a bike harder to manual are things that racers want to have to go faster… once again… it’s all about compromise!

  • boomforeal

    that was great keith. really enjoying this series, hope you can keep it up

  • El-Train

    Why did you model only a 6% tire drop? real world has got to be more like 30%+ with the low psi mtn bikes run. I would think the shape of the contact patch would be greater as drop increased. Bicycling Quarterly mag has had some interesting articles about this subject as applied to road/touring bike tires. Their conclusion was lower rolling resistance is the product of high quality supple tire, at a moderate width, with a carcass that deform the easiest. They talked about tire drop extensively and claimed 25% is optimal for road bikes.

    • builttoride

      The reason that I went for 6% tire drop was that calculations shows that in ideal conditions (whole thing is based around ideal theoritcal numbers)without tread (adding tread patterns changing things massively) that 6% tire drop represented a pressure in the 26PSI range. Hopefully a tire designer will write some artciles like this sometime using real world data.

  • craigsj

    There is no reason to compare two wheel sizes at different tire pressures and then conclude that the smaller wheel is “more efficient”. There is nothing about a smaller wheel that is more efficient other than the contrived advantage it gets from running articficially higher pressure. On the trail that is unlikely to be an advantage anyway.