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(Basically a follow-up to Why do all road bike sizes use the same wheel size?)

Why do most modern bikes have two identically sized wheels? I am aware that penny-farthing bikes have been a thing until the modern "safety" bike came around. And some recumbent bikes have small front and a large back wheel.

Why not a big wheel in front and a smaller one in the back for more torque? Or the other way around for that "chopper" look?

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  • Adding to the question: mullet wheels (29" front, 27.5" rear) are a niche thing on MTBs. Is anyone more familiar with MTBs able to say why some riders choose to do this?
    – Weiwen Ng
    Feb 10 at 15:11
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    @WeiwenNg You get the rollover benefit of the 29” in front, and the greater “flickability” of the 27.5” in rear. You also gain some butt-to-tire clearance on the rear, which is a big issue with long-travel 29” bikes.
    – MaplePanda
    Feb 10 at 15:47
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    @MaplePanda Well ... now I remember the commenter asked why bicycles have identical wheel sizes, rather than what rationales might lead to different wheel sizes and/or why they are so rare. So, good info at least.
    – Weiwen Ng
    Feb 10 at 16:13
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    Identical front and back wheel size makes it simpler to manage spare parts: you only need one size of inner tube, tyre, spoke, brake, rim, etc. Feb 10 at 20:47
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    @GaryE Why? Unless you are carrying a spare tire, an inner tube will easily stretch to accommodate any reasonable difference in wheel size.
    – MaplePanda
    Feb 10 at 21:48

4 Answers 4

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I guess because mixing wheel sizes doesn’t offer any advantages?

Some mountain-bikes came or come with “mixed” wheel sizes. Usually a slightly bigger 622mm (29") wheel on the front and a 584mm (27.5") wheel in the back. There have even been ones with 622mm and 559mm. It’s unclear if mixed wheel sizes like that ever really offered any advantages or if they were just a way for the marketing department to sell something new and different. Even the 622mm vs. 559mm debate for MTBs raged for years and decades. Only recently has 622mm finally “won”.

I think wheels on a bicycle tend to be as big as possible. A 622mm wheel is pretty much the biggest size you can accommodate in a normal diamond or MTB frame for riders over 1.6m without compromising on frame geometry too much. If you have to make one wheel smaller it’s probably better to do so with the rear wheel since it’s less important for rolling over obstacles. But then again, most of the weight is on the rear wheel …

Many recumbent bikes have smaller front wheels, but I think that’s only a compromise for frame geometry, not because the small front wheel behaves better.

Penny-farthings were direct drive. The only way to make them faster for a given cadence was to increase the wheel size (and they pretty much went for the maximum size possible for average men leg length).

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  • 4
    Many cargo bikes also have a smaller front wheel.
    – thosphor
    Feb 10 at 16:57
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    What implication are you going for with "But then again, most of the weight is on the rear wheel …"? It sounds like you're saying that's a reason to go big on the rear wheel, but smaller wheels are stronger.
    – RLH
    Feb 10 at 18:45
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    @Michael Those problems will be worse with a small wheel on the front than on the back (especially as on rough ground the rear wheel will be suspended by a shock or the out-of-the-saddle rider's legs), so I don't see that as being a caveat on "if you need to make one wheel smaller, make it the back one". (also see comments to the original question about why small-rear bikes get made for mountain biking)
    – RLH
    Feb 10 at 19:09
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    @Michael (and on the 1.6 meter comment, do you want to put an upper bound on the size range you're talking about? For a tall rider, it's quite possible to go to a larger wheel, so it seems like you may be intending to mean something like "riders of standard/average adult size")
    – RLH
    Feb 10 at 20:18
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    @Michael Regarding bumps and cobbles, that still doesn’t mean that you’d want to go “small front, big rear”, just that you want wheels as big as possible. Regarding rider size, “biggest wheel you can fit for a frame for someone over 1.6m” is still weirdly bounded — “about 1.7m” would resolve the upper bound issue (while sidestepping any questions about 36er bikes made for especially tall riders.
    – RLH
    Feb 11 at 13:57
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On top of Michael's answer, that addresses the aspect of bike design: I would guess that one reason for not using different wheel sizes is also linked to logistics: there's already a huge variety of tires, doubling them because you have different wheel sizes would add some logistic nightmare (some may argue that the tires already exists, but from some applications like trekking, road and city, anything other 622 is niche and is rarely available in stock) — and it's also true at home: you would need two sizes of spare tires for little practical benefits.

That being said, utility bikes do benefit from having different wheel sizes. Longtails can have smaller rear wheels (20in rear, 26in front for example), where smaller rear wheels allow loading large items lower down, which is beneficial for stability and makes loading easier (especially if the 'load' is a kid that can't climb in the seat on its own with a 28in wheel). Keeping a large front wheel has benefits from a comfort point of view.

Longtail with platform

Modern cargo bikes (the ones with a platform in front of the rider) also have (almost) systematically different wheel sizes: I guess that the combination of long wheel base, disc brakes, simpler design and suspension forks mitigate the inconvenience of smaller wheels.

Cargo bike

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  • 4
    For modern cargo bikes, the advantages simply outweigh the negatives (especially since modern cargo bikes are generally city bikes, and therefore have no need to worry about rolling over large obstacles in most cases) and there’s no UCI ‘cargo bike’ discipline, so no need to meet UCI rules like there is for almost all other widespread types of bikes. Feb 10 at 21:57
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    @AustinHemmelgarn IMO a city bike benefits greatly from being able to go over large obstacles. Only, with a laden cargo bike there's not much chance to that anyway, regardless of wheel size. Feb 11 at 0:45
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    On cargo bikes turning radius is a pretty big reason why you want a small(er) front wheel(s). On a "bakfiets" style two wheel like the Urban Arrow you would need to make the bike longer to accomodate a bigger wheel which would make the turning radius even worse. Bigger wheels also means you need to raise the load higher which makes it more unstable.
    – max
    Feb 11 at 16:02
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    "Modern cargo bikes (the ones with a platform in front of the rider)" - is also just plain wrong. Long tails are still very popular and have some advantages in that the cargo doesn't block your view and they are less bulky. xtracycle.com/blog/support/…
    – max
    Feb 11 at 16:08
  • @leftaroundabout OTOH a cargo bike isn't exactly easy to pick up and lift over an obstacle if you can't go round. Plus dodging those nasty plastic speed bumps in car parks isn't always possible with something wide
    – Chris H
    Feb 11 at 16:41
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I generally agree with Michael's answer, but will offer another: The UCI forbids it, and the bike industry tends to follow UCI rules even for bikes that will never be used in international competition.

Francesco Moser's 1983 hour-record bike

There was a "funny bike" fad in the 80s, with bikes that had small front wheels (typically 24" or 650C) and other novel design features. Wheels inherently have bad aerodynamics, and the front wheel has more aerodynamic impact than the rear, so minimizing the front wheel is a reasonable way to improve aerodynamics.

GT Superbike

This trend continued into the 90s, especially with the spread of carbon fiber frames, resulting in bikes that looked less and less like a traditional bike. In response, the UCI drafted the "Lugano Charter" in 1996, which required wheels to be the same size, and created an "envelope" for bike-frame design. That was the end of mismatched wheel sizes on road bikes and track bikes. There is one obscure discipline in track racing that's an exception: "derny" or "stayer" racing, where the rider is drafting behind a motorcycle. These have tiny front wheels and reverse-raked forks to allow closer drafting.

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Yeah, that short answer that UCI dictates the form is right. As someone who has ridden mostly outcast bikes for decades, it's interesting to see how uniform things are in "normal" bike land, aside from mountain, fat, and cargo.

Recumbents come (came) with different sized wheels in front and back not, I would argue, because of compromises in frame geometry, but in order to meet the different design goals recumbent designers and riders had. Freed from UCI- or mass market considerations, and not caring about fashion, recumbent builders did a lot of odd things to make what they wanted. Lots of it was home-build until the 1990s, and even then, very small production numbers from established makers.

Unless you wanted your feet really high (and many early designers clearly didn't and were looking for more of an upright, lawn-chair type position) then smaller wheel in front got that done.

Most common for a long time was a bigger rear wheel (559 or 622) in the back with a 406 up front. Tadpole trikes are still mostly built that way too. Short wheelbase recumbents were usually big/small or small/small until the late 90s early 2000s when 2x 559, or 571 (later 622) "hi racers" with stick-frame designs became popular. There are even some long wheelbase designs using dual 622s, and they are a trip to ride first time you try.

Personally, though I own some of all of these, I have settled on the small/small combo for almost all my riding and building. I like the snappy handling, light weight, compact design, and the ease of getting a really nice, low-end gear range with standard gearing on a 406 wheel. (I made some <23 pound CF touring bikes that will go in a standard suitcase and have an 18-100 GI range...) With 40mm tires, the ride quality is perfectly adequate on all but the worst surfaces, and a lot better than bigger wheels with skinnier tires (though not obviously big wheels with fatter tires).

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