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One reason to pick 26" over 27.5" is that 26" has shorter spokes which "require more external effort/stress to bend."

Is it possible to express in numbers (quantify) how much stronger are 26" compared to 27.5"?

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6 Answers 6

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I'm going to assume that the question is not "is it possible to measure strength" but what would the numbers actually be.
I'm also going to assume that by "strength" the question wants to know how much vertical weight it would take to buckle spokes.

According to bicyclewheel.info's wheel simulator
For a 36 spoke, 2mm steel spoke wheel with an alloy rim - etc. (I used the default variables on the page and only changes the rim size)

Wheel Strength by Rim Size

Wheel Size                      26 inch/559       700c/29er/622   
Lateral force to buckle spokes  115.1 lbs.        95.8 lbs.  
Radial force to buckle spokes   453.1 lbs.        425.8 lbs.

According to the instructions on bicyclewheel.info "radial" means vertical weight.

Radial stiffness: The radial bending stiffness is the resistance of the rim cross-section to bending within its initial plane (think of a rim being squashed top-to-bottom).
Lateral/Radial force to buckle spokes: The maximum force that can be applied before the first spoke buckles. Beyond this point the wheel will lose stiffness and strength.

The mathematic model indicates that all things being equal for the given specification - only changing wheel size would give 27.3 lbs. greater vertical load than a 700c wheel. This is a 6% increase in vertical load capacity.

Interestingly, all things being equal, a 16 inch wheel has a radial force to buckle spokes of 604.9 lbs.

I'm sure the model and the variables can be debated endlessly with great authority and validity.
All that aside - the model offers at least an indicator of how much of how much benefit there is to the smaller wheel's ability to handle radial forces.

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require more external effort/stress to bend

I'm sorry to have to say this, but such a statement means that its author has no grasp on the physics of a wheel with spokes. Please do not take other such statements by the same author seriously without cross checking them first. The truth is that spokes simply do not transfer forces by resisting to bending. They basically act like wires, holding the rim in place purely by their tension.

That said, spoke length does play a minor role in wheel rigidity: Assuming the hub remains the same width, a shorter spoke length produces a stronger angle with the plane of the wheel. Consequently, a lateral force on the wheel translates to a quicker increase/decrease of tension on the spokes, making the smaller wheel more stiff. Using a wider hub would have the same effect. As does using a hub with a larger diameter.

However, the easiest way to increase the strength of a wheel is to use more spokes. Thus, a touring bike should always be equipped with 36 spoke wheels, not 32 or less. This alone makes the wheel stronger by 12.5%, which is a lot more than you can expect from reducing wheel size. The next points on the list for wheel strength are

  1. quality of the wheel build (how evenly are the spokes tensioned, do they have enough tension?)
  2. spoke lacing (fixes the compromise between lateral rigidity and ability to transmit torque, the later of which is important for disk brakes and the driven wheel)
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    Any explanation for the downvote? Nov 29, 2023 at 17:44
  • People on these sites should explain their downvotes. +1 to compensate you.
    – Robusto
    Nov 29, 2023 at 20:47
  • @MindDBike I have now rephrased that leading paragraph. I'm not sure whether I made a good job of it, though. Feedback is still appreciated :-) Nov 30, 2023 at 8:35
  • Don't mind the downvotes - its the overall nett score that counts.
    – Criggie
    Nov 30, 2023 at 20:16
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    @Criggie I just like to learn from the downvoters. Sometimes they have very valuable corrections for me. Sometimes not, of course. But it's the valuable corrections that count to me, and I simply can't get them from the upvotes... Nov 30, 2023 at 20:35
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Is it possible to express in numbers (quantify) how much stronger are 26'' compared to 27.5''?

Lateral strength is what differs. Lateral strength usually doesn't matter in a bicycle, because lateral loads are minimal. In theory by misusing a bike you can subject the wheels to ridiculous lateral loads so if you planned to misuse your bike and do stunts tricks, 26" could be preferable.

If a wheel fails laterally, it is immediately apparent: it will take the shape of a taco. So the big question is: how often are touring bicycle wheels tacoed? Not often.

Typically, tacoed wheels occur in accidents when the wheel encounters loads it doesn't encounter in any reasonable normal usage.

I heavily recommend not selecting your wheel size based on some theoretical fear of tacoing your wheels, but rather by frame size and availability of good tires. For most users, this would mean bead seat diameter 622mm wheels (700C, sometimes marketed as 28", sometimes as 29"). However, I have to say that for smallest frame sizes, there could be some benefit of 26" or 27.5" wheels since it's hard to build a very small frame with 622mm bead seat diameter wheels. Although then, there's the drawback that 26" and 27.5" wheel sizes have a mountain bike history so availability of good road wheels is limited.

As for wheel design, prefer rims that have 36 spoke holes (although for 26" wheels 32 is adequate as it has the same spoke density as 36 has for 622mm wheels) and only use double wall rims with double eyelets. Use only triple butted stainless steel spokes. The nipples should preferably be spherical head brass ones, not conical head ones. Spokes should be tightened to a good equal tension measured using a tensiometer and stress relieved. With these tips, you get a very high quality wheel.

When selecting a frame, about the only thing you might want to check is that if it uses quick release rear wheel, prefer frames having 135mm over locknut dimension and not ones having 130mm over locknut dimension. 130mm may be adequate for road bikes but the angle of the drive side spokes is too steep for touring use especially on the newest hubs that accept 11-speed cassettes. However, by far the most preferable modern wheels are 142mm thru-axle wheels.

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    The nipples should preferably be spherical head not conical head ones. If strength matters, the nipples should also be brass and not alloy. Nov 29, 2023 at 18:17
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    @AndrewHenle To be pedantic, brass is an alloy. Nov 29, 2023 at 21:47
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    @MichaelRichardson True, but I was using the term in the "bicycle wheel" vernacular, where "alloy nipples" mean "soft, weak, but weight-weenie light aluminum that strips if you look at it sideways". ;-) Nov 29, 2023 at 23:04
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    "and only use..." - don't be so absolute. Sure double-wall w/ double eyelet is a sensible choice (though single-wall rims can actually be very good, cf. motorbikes and trials bicycles!), but butted spokes aren't in any way mandatory. I would second the strong recommendation for brass vs aluminium nipples, though. Nov 30, 2023 at 0:36
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    Round uniform thickness spokes are completely fine for most people. Nov 30, 2023 at 7:39
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For this case, the quality of the wheel, the number of spokes (their lacing, their tensioning) are much critical than a "simple metric" such as the diameter. You can make any wheel strong, but then you'll compromise on something else, for example weight because of the increase of number of spokes. Smaller wheels have a higher potential than larger ones, but a well built large wheel will be more resistance than a small poorly built one.

Now for a touring bike, my impression is that if you take a quality bike with a very high permissible weight, it's unlikely that a 26/27.5/29 diameter will make a difference, as there are other components that will become the most likely failure points if you load the bike beyond the requirements.

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It is for sure possible to quantify how much strength you gain by making wheels smaller. I am just not aware of any data attempting to do so.

However, I fully accept that all else being equal 20" BMX wheels are stronger than 26" MTB wheels, which in turn are stronger than 27.5", 29", ... wheels. There is just shorter lever arms in smaller wheels, making them generally harder to wreck, as well as decades of "common sense" from generations of BMX bikes, cargo bikes built with 20" wheels, etc. This is my charitable interpretation of your idea that wheels with shorter spokes require more external effort to bend.

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Take the taco wheel off the bike and smash the appropriate high spot against the ground. I’ve seen pictures in the press so it must be effective or else they wouldn’t put it in print would they?

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  • Please explain how this answers the question about a numeric quantification of the difference in strength between wheel sizes.
    – DavidW
    Dec 9, 2023 at 17:49
  • It’s humor not science. Not a proper answer ? Sorry for silliness.
    – HVAC
    Dec 9, 2023 at 19:28

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