Design considerations for 20" wheel build

[brewsmith]

Looking at building a new set of wheels for my bike Friday so I can run 11 speeds on it. Since the wheel diameter is so much smaller, spokes shorter etc. can I get away with a lower spoke count than a 700c build? Any other design considerations I should take in to account?

[steamer]

Yes you can definitely use fewer spokes. 20, 24, or 28 are all reasonable options.

Another consideration is the number of crosses. 1x or 2x at most.

[oldpotatoe]

Looking at building a new set of wheels for my bike Friday so I can run 11 speeds on it. Since the wheel diameter is so much smaller, spokes shorter etc. can I get away with a lower spoke count than a 700c build? Any other design considerations I should take in to account?

Yes, but using less spokes buys you what? Less $ for spokes? Sure, build a 24h 2 cross and how is that ohh so much better than 32h 3 cross? 8 spokes weigh 56g, about. For 20 inch wheel, less..so unless it’s about hub/rim availability, not sure I get it.

[Mark McM]

Yes, you can definitely use fewer spokes.

Another consideration with small diameter rims is that the spokes will enter the rim at a more extreme angle. It is best to avoid sharp bends in the spoke near the threads, so if possible use a rim with angled spoke hole drillings. For rear wheels, an offset rim will help reduce the angle of the non-drive side spokes. Hub flange size also affects spoke angle, so use a small flange hub if possible (and/or use a lower number of spoke crossings to reduce spoke angle).

For a rear wheel, make sure that the hub can accept small sprockets. That means a Shimano Hyperglide-C compatible freehub (for sprockets down to 11 teeth), or a SRAM XD freehub (for 9 and 10 tooth sprockets).

[Mark McM]

Yes, but using less spokes buys you what? Less $ for spokes? Sure, build a 24h 2 cross and how is that ohh so much better than 32h 3 cross? 8 spokes weigh 56g, about. For 20 inch wheel, less..so unless it’s about hub/rim availability, not sure I get it.

So, are all your wheels 36 and 40 spokes? I'm sure these were common when you started building wheels (as they were when I started), have you found any reason to change?

There's nothing magic about 32 spoke wheels.

[palincss]

Looking at building a new set of wheels for my bike Friday so I can run 11 speeds on it. Since the wheel diameter is so much smaller, spokes shorter etc. can I get away with a lower spoke count than a 700c build? Any other design considerations I should take in to account?

Yes, definitely. Even going to a 26" size like 650B lets you use fewer spokes to get an equivalent wheel strength: 32 spokes on 650B is as strong as 36 spokes on 700C. Also, too many spokes too close together on a small wheel make it very difficult to get a pump head on a tire valve.

[steamer]

Yes, but using less spokes buys you what? Less $ for spokes? Sure, build a 24h 2 cross and how is that ohh so much better than 32h 3 cross? 8 spokes weigh 56g, about. For 20 inch wheel, less..so unless it’s about hub/rim availability, not sure I get it.

No, you don't get it. Why not show some humility about something you obviously don't have any experience with?

This is a 36H, 3x 406 wheel.
https://lh3.googleusercontent.com/TTUak4jaIiOXteZ-QZjud7PLmexJ41Cb33v_ZAD4oAO5C_SrPa_Xs0vX6AWKAEU_jM Lr0nvSqGhOFN4uMag-qEPWvFhEO1uBgZnAYaC-AUxnLr1aYOaDfBe4ZDiH6k5v7gm6dcks7e77ppPFCHxoO0aV4c UeDjCXpEluxsvp0lN9KTDZikoH1sRX-8aNCHMX1WsEipVHoESEiv1XON2RAlD9J-BAznvMqcgYdTOEkWBFYcMffBJ2ufRA_yhJE45c_oTRXBbbiPPa 57Ui98Z2YMHHL-Nul3bSrLTa5ydMc-3y483HnVvZ8v4xxHbXREfi1gPdUfMmr_r6urNoR0HiTRSagqzJ NzUW48IwHzP8Vc3iEGeUxM1F-Ov36xIboxk-ZFW7PAFImxmQsb5u4jTKCvkA0pXl-xTgJEyDVDPLqLCUS2mvvX-sergK_pofBfjMKzfZq5teWN6rAY92Xvi1I1jiLvHkAOBRG8oW6 IG9uK9UCc1nkXhTCV7JT_BxAzt3mA8dcCTbEM7DcjeOwVu3o0w E9IrmPuHe3hiHo-jI4ZH0iHsxHAKaac5GnXFMgc9iApA7AjqRHdE-FqGaOhFzm4c3KQEdXbvpqkSYBhoVr5Kc7dCh0Tl3EUDTuL05qD AJHuJCF2MTJJd6CNwqFouGTVJBuBqOy8aDoRj5yiOXWlN4614Y 4hlQPr0oITHua-yndxsScjDD1ufuLjgyQV4NkFg_=w703-h937-no

See how the spokes are actually touching the heads of the nearby spokes?


This is a 28H, 2x 406 wheel.
https://lh3.googleusercontent.com/LqF3yi9W-oHjCLRXI67KAjToRBAPL1ysrUcGoYukp1WvosGhNTgDOreH3JI OsYN9bcekG4FadiDXMNitZ1dZhZ2xE_DF3cYYq2GlMO1J6MP7b yhPMo1b4ts_sa99vi2b0zTtTqmhORWeN7IqlGPnoOC3wr3kZlX KVZpXDM55tNfY1TTHtjdFfcnlWRa9N1oR8YePWylZ3lD2rb2yI FyaslpxZtSINuxz4FRURW2JwMU5SkwAo2ekVSs3i4vQTYQKXne 8j6t4ZGlRgyHcznqql4hg82BFYJ7WgyEo8fNl5NmcBFaAr5obD u0luMPbZH1_lmvd9zwIRZBM9xZWrhBZFcoalgPF6p9ky6-H593C6-1PZ5N6OhiPqWbSWCH6NpSBkX7UCayGonOkeHDAE7i5fNWXOGCR wFE2IJBAivJkksy9z99Pe6EdlAYFwS1m35cuJ8ildswwGc1qn5 _Ln6L3LjPeWXprKlbTvWk_efd78REPzPm9SKZ-O8n2OX9hsgPwwuECptN02J5Sha1Of6YRes2PiSjvEJKHnSZEzI c_Dn6jHkhcSAijwp2GLyZh78TedimmzwvZhNP4IrcCL1nIrlji SzLCt-guujPjds2FQRbMR-lh7MNrEB7xO9YfJquBbsCxDhWa_plYXAF4_G_8P6eiihEOx4U= w1250-h937-no

Seems like plenty o' spokes doesn't it?

It's all about proportion and common sense, actually.

[brewsmith]

Thanks for the replies all.

As you mention op, weight reduction was not my primary concern with going for lower spokes, but rather angle concerns, pump clearance and ease of packing in a case with more room between the spokes.

[bikinchris]

A 26 inch wheel is 3 times stronger than a 700c wheel. A 24 inch wheel is 3 times as strong as a 26. A 20 inch wheel is....you guessed it. Strong as hell. That's why freestyle riders use small wheels with 48 spokes. You can run over them with a truck and giggle about it. ( well, not really, but hyperbole is fun)
24 or 28 hole wheels are plenty strong for loaded touring on a 20 inch wheel.

[oldpotatoe]

So, are all your wheels 36 and 40 spokes? I'm sure these were common when you started building wheels (as they were when I started), have you found any reason to change?

There's nothing magic about 32 spoke wheels.

Nope, for me, all are 36h and tied and soldered(put your complaint here-jobst).

My point is why fewer spokes? What's the rationale for using less spokes? In general..I hear this all the time..'over built', 'bomb proof'...considering that there is NO PENALTY for using say 32 vs 28 or 24. Oh, I forgot, that ounce or 2...or maybe the YUGE 'aero' penalty..yup, that's the ticket...
Nothing 'magic' about wheels at all but a 32h wheel is more reliable than a 28h wheel, same spokes, rim, builder..pretty simple, actually...:)

I build with fewer than 32(or 36) spokes all the time but I also don't make the '3 strikes yer out' mistake..

-too few spokes
-too light rim
-using too thin spokes

"There once was a French bridge builder..he bult many fine bridges..but he s__k_d one ___k..he was then kb=nown as the French ___k ___k_r..

I, for one, will not build a wheel I KNOW will be on the edge of reliability..

rantage off..I need more sleep...;)

[palincss]

I build with fewer than 32(or 36) spokes all the time but I also don't make the '3 strikes yer out' mistake..

-too few spokes
-too light rim
-using too thin spokes


Do you build many 17 or 20 inch wheels?

[oldpotatoe]

It's all about proportion and common sense, actually
Why not show some humility about something you obviously don't have any experience with?


See post above..I was 'questioning' the trend among people looking for wheels to use the minimum number of spokes(or less)..and often ask for a inherenty unreliable wheel..BTW-I have built a couple of sets of BMX wheels for a gent...36h, 20 inch(I think)..they were no fun to build. a couple of 24inch..lotsa 26 inch..yes, smaller wheels can use fewer spokes..my thread drift was about the trend of 'fewer is better' spokes on wheels...

[brewsmith]

Yes, you can definitely use fewer spokes.

Another consideration with small diameter rims is that the spokes will enter the rim at a more extreme angle. It is best to avoid sharp bends in the spoke near the threads, so if possible use a rim with angled spoke hole drillings. For rear wheels, an offset rim will help reduce the angle of the non-drive side spokes. Hub flange size also affects spoke angle, so use a small flange hub if possible (and/or use a lower number of spoke crossings to reduce spoke angle).

For a rear wheel, make sure that the hub can accept small sprockets. That means a Shimano Hyperglide-C compatible freehub (for sprockets down to 11 teeth), or a SRAM XD freehub (for 9 and 10 tooth sprockets).

Great info, do you have any rim reccomendations? I've been looking at the kinlin nb r 451 but do t think they have offset or angled drilling. As far as the smaller sprockets, I assume that is to compensate for the lower gearing of the smaller wheel? This can also be addressed with larger chain rings, correct? Thinking about 53/39 v compact with an 11-28 cassette

[palincss]

See post above..I was 'questioning' the trend among people looking for wheels to use the minimum number of spokes(or less)..and often ask for a inherenty unreliable wheel..BTW-I have built a couple of sets of BMX wheels for a gent...36h, 20 inch(I think)..they were no fun to build. a couple of 24inch..lotsa 26 inch..yes, smaller wheels can use fewer spokes..my thread drift was about the trend of 'fewer is better' spokes on wheels...

I understand that. However, since the wheel in question is 20", the standards appropriate to 27" wheels simply do not apply. A number that would be sturdy and reliable for a 27" wheel could be vastly overbuilt and difficult to live with in practice (you have no idea what a pain in the rear it can be trying to put air into a wheel where you just can't fit the air chuck because the spokes are so close together) for a much smaller wheel. I speak from the experience of having owned a bike that uses 17" wheels for 15 years.

[vqdriver]

I cant help with the building technicalities but I will say that with small wheels and many spokes it can be a pita to get a pump head on the valves. Depends on a few things but just to be aware of.

[Mark McM]

It's all about proportion and common sense, actually.

And this is the heart of it. It is not a question of, "how few spokes can I get away with," it's a matter of, "how many spokes are appropriate."

That smaller wheels need fewer spokes has been known for a long time. Notice the wheels on this 130 year old bicycle:

https://maroon-prod.s3.amazonaws.com/media/multimedia_photos/High_Wheeler_Bro.jpg

The very large front wheel has many more spokes than the very small rear wheel. The purpose of the spokes, afterall, is to support the rim, so depending the strength/stiffness of the rim, the spokes should be spaced a certain distance apart at the rim. As the wheel scales up and down, that means that the number of spokes should change to maintain the same distance apart for similar support of the rim.

While it is true that 20" wheels meant for jumps and other trick riding may be better with 32 or 36 spokes, that also means if the same riding was done on 700c wheels, then the wheels should have 44 or more spokes.

[Mark McM]

Great info, do you have any rim reccomendations? I've been looking at the kinlin nb r 451 but do t think they have offset or angled drilling. As far as the smaller sprockets, I assume that is to compensate for the lower gearing of the smaller wheel? This can also be addressed with larger chain rings, correct? Thinking about 53/39 v compact with an 11-28 cassette

Sorry, I don't have enough experience with 20" wheels to recommend a rim. If the rim holes aren't angled, it can be useful to pre-bend the spokes at the nipples, to reduce stress concentrations at the threads. This technique is covered in the Jobst Brandt's book, "The Bicycle Wheel". If you google around, you can find a .pdf version of this book online.

Yes, to compensate the gearing for small wheels you can use smaller sprockets, bigger chainrings, or a combination. But there is lesser availability of chainrings big enough to compensate for the smaller wheels, not to mention the other issues they create (such front derailleur height adjustability, shifting with larger chainring tooth differentials, and more chainring rubbing when cross-chained). Because 20" wheels are only 3/4 the size of 700c wheels, a 53/11 combination with 20" wheels has the equivalent gear size as a 700c wheel with about a 53/15 combination (or about 50/14 when compared with a compact). For higher gearing, you'll either have to go with extra large chainrings (56 to 62 are common for 20" wheel road bikes), or a smaller sprockets (Moulton has used sprockets as small as 9 tooth).

[brewsmith]

Yes, to compensate the gearing for small wheels you can use smaller sprockets, bigger chainrings, or a combination. But there is lesser availability of chainrings big enough to compensate for the smaller wheels, not to mention the other issues they create (such front derailleur height adjustability, shifting with larger chainring tooth differentials, and more chainring rubbing when cross-chained). Because 20" wheels are only 3/4 the size of 700c wheels, a 53/11 combination with 20" wheels has the equivalent gear size as a 700c wheel with about a 53/15 combination (or about 50/14 when compared with a compact). For higher gearing, you'll either have to go with extra large chainrings (56 to 62 are common for 20" wheel road bikes), or a smaller sprockets (Moulton has used sprockets as small as 9 tooth).

More great info, thanks! I'm toying with going 1x on this and it does look like there are some large narrow wide rings available. What's your method for calculating this gearing or are you ball parking with the fraction size of the wheels?

[NHAero]

Consider posting on bentrideronline, a recumbent bike web site and message board, you'll get more specifics because many bents use either 406 (more common) or 451 rims. When I was riding 'bents, there were more choices in tires and rims in the 406 size. I had one each in Bike Friday, preferred the ability to find wider tires in the 406 size.

[oldpotatoe]

Great info, do you have any rim reccomendations?

Velocity..A23, Cliffhanger, Dyad.

https://www.velocityusa.com/product/rims/a23-406

[Mark McM]

More great info, thanks! I'm toying with going 1x on this and it does look like there are some large narrow wide rings available. What's your method for calculating this gearing or are you ball parking with the fraction size of the wheels?

Bicycle gearing is measured in gear inches (equivalent to if the cranks were connected directly to the wheel, like on old high wheeler bicycles) or development (distance travelled for each revolution of the cranks, usually in units of meters). Gear inches is an archaic system, and development is easier to measure and calculate. Development is calculated by multiplying the wheel circumference by chainring size, then dividing by the sprocket size:

Development = Circumference x Chainring / Sprocket

Tables of wheel circumference for a variety of wheel sizes and tire widths can be easily found. Here's one:

https://www.cateye.com/data/resources/Tire_size_chart_ENG_151106.pdf

As can be seen in the chart, 20" wheels are roughly 3/4 as big as 700c wheels (1545mm circumference for 20"x1 1/8" vs. 2136mm for 700x28c), so for the same development with a given cassette, you'd need to use a chainring 4/3 larger on a 20" compared to a 700c wheel.

[NHAero]

As long as he chooses the 451 version of the 20" nominal wheel size instead of the smaller 406.

Bicycle gearing is measured in gear inches (equivalent to if the cranks were connected directly to the wheel, like on old high wheeler bicycles) or development (distance travelled for each revolution of the cranks, usually in units of meters). Gear inches is an archaic system, and development is easier to measure and calculate. Development is calculated by multiplying the wheel circumference by chainring size, then dividing by the sprocket size:

Development = Circumference x Chainring / Sprocket

Tables of wheel circumference for a variety of wheel sizes and tire widths can be easily found. Here's one:

https://www.cateye.com/data/resources/Tire_size_chart_ENG_151106.pdf

As can be seen in the chart, 20" wheels are roughly 3/4 as big as 700c wheels (1545mm circumference for 20"x1 1/8" vs. 2136mm for 700x28c), so for the same development with a given cassette, you'd need to use a chainring 4/3 larger on a 20" compared to a 700c wheel.