29 er vs 26 er – where's the math?

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  • 29 er vs 26 er – where's the math?
  • Premier Icon charliemort
    Subscriber

    given the debate, I’m surprised there dooesn’t seem to be more quantifiable evidence of the differences (that I have been able to find anyway)

    I know Dirt sort of did something a couple of months back, but this was predominantly downhill

    I’d have thought it would be prety easy to take 2 similar bikes (eg Transition Bandit 26 and 29 seem very close to me other than wheel size) and time a number of riders around different parts of a course

    Would have thought the manufacturers would have done this?

    rootes1
    Member

    I’d have thought it would be prety easy to take 2 similar bikes (eg Transition Bandit 26 and 29 seem very close to me other than wheel size) and time a number of riders around different parts of a course

    There have been a quite a few back to back comparisons posted on the web.

    Premier Icon wwaswas
    Subscriber

    a lot of the racers have.

    MattPage did a lot of experiments with 26/650/29 wheels recently.

    Not sure he wants the results in the public domain though 🙂

    Premier Icon mikewsmith
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    From the department that brings you the science bit in shampoo commercials

    Premier Icon ampthill
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    Easy in theory. Nightmare in practice…..

    For example do you give them the same tyres. Yes you say thats fair…

    But is it. If the 29er has more grip then it could run a shallower tread for the same traction. Or if the 29 er is more comfortable over roots it could trade this comfort away for a thinner tyre. Which is presuambly is what a racer would do. If we are not talking about racing who cares which is faster….

    Premier Icon charliemort
    Subscriber

    mikewsmith – yeah like Michael McIntyre – “my wife bought some anti-wrinkle cream. I’m going to take it back in 20 years and ask for my money back”

    Premier Icon roverpig
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    Blind tests are pretty hard to arrange on a bike and as soon as the rider knows what they are riding it’s pretty much impossible to control for their preconceptions. Basically, bikes tend to feel like we expect them to and whatever bike it was that you wanted to buy in the first place will probably feel the best.

    Cheers,

    Andy

    If you’re going to compare 26 and 29 you probably want to do so with tyres with equal contact patch size, so wider on the 26, and run the same pressures. Tyre theory would then suggest that the 26 roll quicker when it’s smoother and harder whilst the 29 roll quicker when it’s bumpier or boggier. So on the face of it 29 should be better offroad much of the time.

    thomthumb
    Member

    29-26=3.

    HTH

    Math-S- Are you an American?

    Premier Icon jameso
    Subscriber

    imo, there is no maths of value aside from physical comparisons of the wheels. And that tells you only a part of it. ie roll-over angles don’t also consider tyre deformation differences at impact point. Volume variances. Contact patch etc etc. Too complex to model with any validity.
    Would you try to balance variables between wheels, or keep tyres and pressures optimised for each wheel as a rider would? How do you test when off-road riding and riders themselves vary so much? I guess a rider, 2 or 3 otherwise near-equal bikes, a track and a HRM can help them decide what works best for them (many mags have done this – a feature rather than a test imo, but still, some result trends seem to appear in them), but I wouldn’t call that ‘doing the maths’ as it can’t be applied to riders or terrain equally.

    I’ve done back-to-back runs on 3 wheel sizes and the only thing of value was how I felt the bikes rode differently and of that, what I would attribute to the wheels as one of those ‘princess and the pea’ kind of riders. I’ve also done roll down / along types of ‘tests’ on different tyres and pressures.
    After all that, I only had more questions about accepted component designs and what we feel is important on a bike. Rim diameter isn’t as important as we may think / in the way the generalisations suggest imo. ie, why all the focus on just that aspect of the wheel? We’re happy to challenge one accepted norm yet not others, or we get distracted by one aspect and lose focus. Perhaps because (like suspension) it’s too complex to sell easily without being distilled down to easily digested ideas.

    drofluf
    Member

    Mountain Flyer did a test recently – last issue but 1 IIRC. They took 2 similar bikes a 29er and a 26er and did back to back loops over a range of loops. The net result was about even with the smaller wheel performing better in some areas and the bigger in others.

    As others have said there’s too many variables to be able to make a truly scientific test valid

    Frankers
    Member

    I’ve rode my Cotic Soul and Solaris back to back

    Premier Icon steveoath
    Subscriber

    If bigger = Better then
    29 > 26
    therefore 29 is better than 26
    QED

    Premier Icon scaredypants
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    It’d be easy to time a downslope roll over roughish ground (not DH, just mimicking xc riding without pedalling). Done over a few different types of terrain in terms of bumpiness, that should sort out “rolling”.

    Grippiness, much harder

    Handling, essentially impossible I’d think, as it’s too subjective

    So we’re left with making up our own minds (as long as manufacturers allow us to, i.e. continue to supply both/either/650/wotevva)

    Premier Icon charliemort
    Subscriber

    @ failed engineer – no I’m not, just using an American expression.

    In the Queen’s own we tend not to use the expression “where’s the maths”, whereas the American’s do tend to use “where’s the math” or “show me the math” so I thought that would be obvious to one so clearly intellectual as yourself!

    IHN
    Member

    Why do people think 29rs have a bigger tyre contact patch size?

    The size of the contact patch is related to the pressure in the tyre and the weight of the bike+rider, not the size of the wheel. Do 29r tyres tend to be run at lower pressures than 26r tyres?

    Mark N
    Member

    Why do people think 29rs have a bigger tyre contact patch size?

    The size of the contact patch is related to the pressure in the tyre and the weight of the bike+rider, not the size of the wheel. Do 29r tyres tend to be run at lower pressures than 26r tyres?

    Agreed, but if all things are equal then the 29er will have a very slightly larger contact area. One could then argue that this larger contact area would have a higher drag/rolling resistance.

    IHN
    Member

    if all things are equal then the 29er will have a very slightly larger contact area.

    Why (assuming that by ‘all things’ you mean the weight of the bike, the weight of the rider and the pressures in the tyres)?

    Premier Icon SimonR
    Subscriber

    IHN – totally agree.

    You often see comments about better cornering grip and how that’s related to a larger contact patch (shakes head in disbelief)….

    Different shaped contact patch maybe but definitely not a different size.

    [One of our standard training/interview questions used to be “how do you estimate the contact area of a tyre with the ground?” – some of the answers were hilarious bearing in mind they’d just spent 4 years studying engineering 🙂 …. my other favourite was “if you’ve got 1 newton in each pocket would your trousers fall down?”]

    Premier Icon njee20
    Subscriber

    Wow, the seconds must have absolutely flown by!

    IHN
    Member

    “how do you estimate the contact area of a tyre with the ground?”

    Er, my answer would be divide the weight of the thing by the pressure in the tyres (using same ‘weight’ units for each, obviously) and then by the number of tyres on the ground. Is there an easier way… 😐

    b r
    Member

    Give a Pro a 26, 650b and 29er – tell them they can ride any of the three and they’re paid on results, easy. Although they may choose different bikes for different days/course/weather etc…

    Premier Icon SimonR
    Subscriber

    Ah well spotted IHN 🙂

    Oh no, not again…
    Totally agree with jameso though. The tests are pretty pointless. Even if you had the same rider on two bikes, the one he rides first might be given an advantage or disadvantage, depending on how tired he gets and whether he’s had his Weetabix. And anyone can post quite different times on every run down the same trail on the same day. That fact that the first three riders in the Olympics, from a field dominated by 29ers, were all on different wheel sizes shows that it’s the rider, not the wheel, and that all these wheel sizes are perfectly valid sizes to have as a bike wheel. But it also shows, I think, that most of the claims are just pure marketing bull. If wheel size doesn’t make a difference, how else do you sell it? So it’s really just personal preference – and the power of suggestion.

    @Rorschach – best gif on STW for a long time 😆

    IHN
    Member

    Was I right? 🙂

    Now to question 2; how many pockets and what kind of newtons are they?

    Premier Icon jameso
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    The size of the contact patch is related to the pressure in the tyre and the weight of the bike+rider, not the size of the wheel

    Correct. 29ers and 650B don’t have a bigger contact patch than 26″, it just tends to be a different shape, elongated as wheel diameter increases. So it can feel grippier sometimes and is percieved as bigger. If the tyre contact patch actually is bigger for a given pressure it will feel firmer to ride ie you can use less pressure for the same ‘feel’ over bumps (pressure, force and area), I think that’s the only relationship. Sidewall thickness, rim width etc is a distraction though.

    Premier Icon charliemort
    Subscriber

    I don’t get why it is so complicated…….

    Okay – there is always a variable when a human is involved, but if there was a really significant difference between 2 bikes, same tread patterns, same tyre pressures, same shock pressures etc., the only real difference being wheel size, I think this would show. A few percent here or there could be put down to human variables, but a big difference would show through – imho….

    Static contact patch is proportional to the system weight divided by tyre pressure. Dynamic contact patch increases with wider and/or larger diameter tyres. There’s a reason supercars have bigger tyres than superminis.

    sbob
    Member

    Why do people think 29rs have a bigger tyre contact patch size?

    The size of the contact patch is related to the pressure in the tyre and the weight of the bike+rider, not the size of the wheel. Do 29r tyres tend to be run at lower pressures than 26r tyres?

    Contact patch is related to the size of the wheel. The bigger the wheel, the bigger the contact patch.

    sbob
    Member

    Static contact patch is proportional to the system weight divided by tyre pressure. Dynamic contact patch increases with wider and/or larger diameter tyres. There’s a reason supercars have bigger tyres than superminis.

    Yep.
    By reducing tyre width but increasing wheel size, contact patch is maintained but drag is reduced.

    Premier Icon njee20
    Subscriber

    Give a Pro a 26, 650b and 29er – tell them they can ride any of the three and they’re paid on results, easy. Although they may choose different bikes for different days/course/weather etc…

    Scott did just that.

    Nino Schurter chose 650b
    Florian Vogel chose 29er

    Both did perfectly well on 26″ wheels, so it’s not hugely helpful!

    Premier Icon jameso
    Subscriber

    I don’t get why it is so complicated…….

    Humna variables aside, imagine trying to set up a computer model that included all the terrain variations and riding dynamics over a 2 hr or 12 hr ride. It’s too complicated. I think the results of the fictional test would be very easy to tip either way just by changing a small variable ie there’s not a lot in it unless you know what you want the wheel for. There is no ‘better’, just ‘more fun for me’.

    If in doubt, pick the middle size )

    I had a look at this recently. One quanitfiable difference is that 29″ wheels and tyres weigh around 15% more than their 26″ equivalents. The tyres and rims are also further away from the hub so I am guessing that makes them around 20% harder to accelerate but that they have 20% more “momentum”.

    This would suggest that a rider doing a lot of techy climbs and descents with constant variations in speed would be better off on 26″, but someone favouring smoother XC type trails would be better off on 29″

    Premier Icon jameso
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    ^ True, but then consider roll-over and momentum benefit etc on those techy sections, and the ease of moving around a smaller wheel ..

    We get to the point of Rorschach’s gif very quickly )

    Premier Icon cRaNkEnStEin
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    Where does a 69er fit in this equation?

    Premier Icon jameso
    Subscriber

    and does a 69er = 650b F+R once it’s all equated?

    I’m out..

    Premier Icon cRaNkEnStEin
    Subscriber

    49er fat bike anyone?

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