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What is the purpose of the top tube on bicycles? To neuter guys? My brother crunched his balls so hard on his bike, poor guy.

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2 Answers 2

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Same basic purpose as the roof on a unibody car. To prevent the back end of the bike folding towards the front end when vertical forces like your brother are applied to the cranks or seat.

Very simplified, without the top tube all of the bending moment needs to be resisted by the junction of the down tube with the bottom bracket. While some cruiser style bikes and city rental bikes do eliminate the top tube it’s at the cost of the weight of extra metal and larger tubes.

Without top tube and seat stays

Minimum static vertical forces on frame not taking into account dynamic riding effects.

enter image description here

With seat stays and top tube

enter image description here

If your brother is getting injured by his bike it might indicate that the bike is too large or maybe he just needs to accept that jumping off things comes with an element of personal risk and responsibility.

A rough rule is that there should be about a palm width between crotch and top tube when standing over the bike feet flat.

The Bicycle SE community extends its sympathy to your brother.

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  • 51
    +1 for the drawings
    – Berend
    May 13 at 21:47
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    I always thought the roof is there so I don't get wet when it rains. May 15 at 1:15
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    I can affirm that the top tube also resists torsional forces on the bottom tube. When a tube tore on my step-through, it ripped, and the front and back twisted/rotated back and forth as it tore.
    – david
    May 15 at 5:25
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    Man, this is a long way of saying "physics". +100
    – FreeMan
    May 15 at 17:57
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I don't have an engineering background, but I believe the top tube makes the bike stiffer laterally and torsionally (i.e. resisting twisting movements). You could build a bike without a top tube. Many bike share bikes don't have them, perhaps to totally eliminate clearance issues. You would have to really over-build the other tubes. This would make the bike heavier and probably stiffer to ride. We can't attribute the entire bike's weight to the lack of a top tube, but I think they weigh in the range of 40 lbs. They're really intended for trips of 1-2 miles, and most people aren't racing, say, the Nation's Triathlon on them - although one DC triathlete did this in 2012 and instantly became a legend.

The photo below is a Capital Bikeshare bike (Washington DC, US):

enter image description here

The down tube is much thicker than it would otherwise need to be. Similarly, you could build a bike without a seat tube, but you then have to over-build the other tubes. Superstrata tried it, although I think they only tried it for novelty. The bike was a flop, although again we can't solely attribute it to the no seat tube design.

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    In the Netherlands (a very bicycle-rich country) these top-tube less were traditionally ridden by women, because they wore dresses/skirts that wouldn't fit over the top tube. Swinging their leg over the back of the bike would also compromise their decency when wearing a skirt, so the top tube was omitted to allowed easy mounting.
    – Jacques
    May 14 at 9:24
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    Those women's bikes often have two down tubes that may even have braces between them to make up for the missing top tube.
    – ojs
    May 14 at 10:09
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    Shared bikes are deliberately heavy to make stealing them more annoying. If you search for generic "womens bike" you will find a lot of lighter examples without a top tube,
    – OrangeDog
    May 14 at 10:28
  • You don't have to believe, those downtubes need to be really heavy and wide to get anywhere near the stiffness of any diamond shaped frame. Any women's bike rides like it has a slight bit of suspension. I don't know about those shared bikes, but they need to be very robust and I guess they are really heavy on purpose anyways, so they are probably as stiff as a racing bike frame. May 14 at 15:22
  • I can believe that one of the design goals is to reduce theft, but they still can get taken, and on flat ground the weight is manageable.
    – Weiwen Ng
    May 14 at 15:37

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