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If I look at the frame, and I see a nice "beaded" weld, how can I know if the rest is "bad"? Is there a way to tell from the geometry of the frame somehow? How can you tell by looking or with a measuring tape?

My friend has a Fuji bike, expensive. And I have looked at it long enough to know what good welding is supposed to look like, what the steel looks like, no rolled together sheet with a crease, all that kind of stuff.

What I don't know is about the geometry. I have read what Geometry is, but recognizing it is different. Is is something you can feel? See? Both? Beyond a good weld.

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  • The geometry part is really completely unrelated to the quality of a weld. Consider separating geometry part into a separate question.
    – Criggie
    Apr 13, 2018 at 21:14
  • Geometry has to do with design. A "good" geometry for a commuter road bike is not necessarily a good design for a long haul touring rider. A good geometry is one that fits the designed purpose.
    – mikes
    Apr 13, 2018 at 21:20
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    Conceptually linking 'geometry' to weld quality makes me believe that you perhaps misunderstand what bicycle frame geometry is. Apr 13, 2018 at 21:26
  • 1
    I don't understand your question. Geometry is the shape of the bike -- the sizes and angles of the triangles that make the frame, the shape and angle of the forks and corresponding position of the front wheel, etc. The quality of the welds is a measure of how well the bike has been manufactured. These are two completely separate issues. Apr 14, 2018 at 9:28
  • 2
    It probably helps if you read the text and realize that the explanation about welds isn't a question but more like OP explaining what they already know.
    – ojs
    Apr 14, 2018 at 10:08

3 Answers 3

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Assuming you are asking about the design and proportions of the bicycle frame which affects steering and handling; and the relative position of crank, seat and handlebars...

There is no 'good' or 'bad' geometry, only geometry suitable for a specific purpose. To give an extreme example: a tri-bike geometry would be a disaster for down hill mountain bike (and vice versa).

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  • Bad geometry certainly exists, but tends to be confined to BSOs: one size fits none and with dodgy handling
    – Chris H
    Apr 13, 2018 at 21:39
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    @Chris H, well yeah, I guess the Schwinn Stingray exists Apr 13, 2018 at 23:32
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I'm expanding on the answer by Argenti Apparatus, who says

tri-bike geometry would be a disaster for down hill mountain bike (and vice versa).

By looking at the illustration,

tri-bike vs downhill bike geometry

(source: url)

you can see that:

  • Riding downhill with the black-lines bike would make it too easy to go over the handlebar.
  • The white-lines bike would be several kilograms too heavy, and far from sufficiently agile, for use as a tri-bike.
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  • Why does a triathlon bike need to be agile? Arguably that's more important for a downhill bike. A triathlon bike is usually ridden in a straight line, after all. And I'm not sure that the weight argument follows from the geometry alone. Dec 6, 2022 at 4:50
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Bicycle frame geometry is a complicated topic with lots of numbers, opinions, and passion.

Frame geometry affects 1. how a bicycle feels/handles and 2. how a bicycle will fit. Frame geometry is something you can see and it is something you can feel.

  1. It's best to describe bicycle frame characteristics using words like "stable", "unstable", "agile", "fast steering", "slow steering", "stable with a load" etc. rather than value judgement words like "good" or "bad".
    One might describe a frame by its use - touring bike, racing bike - the challenge with this is that there is a range of designs inside each category.

  2. It is possible to look at a frame and guess at how it will ride or fit. For example, wheelbase, head tube angle and seat tube angle are easy to see once you've developed a mental reference for the characteristics of frame geometry. Guessing is nice as a rule of thumb but to really know a bike you have to ride it.

  3. Frame design - like most things on a bicycle - is all about compromise. As one example, for road bikes, at one extreme there touring bikes that are usually smooth riding, comfortable and stable under heavy loads. At the other extreme is the nimble racing bike. Bikes that fall into the middle - sport bikes - compromise on the two extremes in different ways that are determined by the design goals.

Here is a drawing illustrating the different measurements involved in frame geometry.

enter image description here
www.cyclingabout.com - Understanding Bicycle Frame Geometry

Just to get you started here is a touring bike and a racing bike with one easy to recognize characteristic highlighted.

Touring bike.
Ignore the racks. You can hang racks on almost anything if you really want to. enter image description here
Visually it's easy to pick out the wheelbase length - especially if the wheel size of the two bikes is the same. Longer wheelbase bikes tend to have the rear wheel further away from the seat tube. For a 48 cm size frame this bike has a 104.1 cm wheelbase. Complete frame specs are at the link.
Trek 520

Racing bike.
enter image description here A shorter wheelbase usually means that the rear wheel is closer to the seat tube. For a 48 cm size frame this bike has a 97.2 cm wheelbase. Complete frame specs are at the link.
2023 Trek Madone SLR Gen 7

Here are two more measurements to think about. Seat tube angle and head tube angle.
enter image description here
Compare the seat tube and head tube angle on this bike with the two bikes above.

Think about how seat tube angle affects fit. On the racing and touring bike your body will be positioned more vertically over the crank. The Schwinn angle gives a more upright riding position.

The Schwinn bike has what is referred to as a "slack" at about 68 degrees. The touring and racing bikes have a "steeper" angle than the Schwinn at about 74 degrees.

Seat tube angle affects seated rider weight distribution. The further back the rider sits the more weight is on the rear wheel.

enter image description here
bikeinsights.com
Notice in this chart that seat tube angle has a range for each category. For example, Triathlon seat tube angles range from 75 to 80 degrees.

Seat tube angle also affects handling. I knew a guy who was comfortable hauling 75 pounds of newspapers on the back of his Schwinn like this and 25 pounds on the front rack. The bike handled and turned like a slug even when not loaded but it was wonderful for its task.

Head Tube Angle
bikeinsights.com

Head tube angle plays a significant role in how a bike handles. In general a steep head tube angle results in quicker-feeling steering while a shallow angle results in more stable straight-line tracking. However, this is a generalization and the Fork Offset, Wheel Diameter, and Tire Width are all factors at play

Traditionally, performance road bikes have had steeper head tubes while recreational and utility road bikes have more moderate angles. Mountain bikes are commonly more slack than road bikes and have a greater range of angles depending on the use case.

The head tube angle on the Trek 520 is 70 degrees, the Madone is at 72 degrees, the Schwinn is approximately 68 degrees

This is a complicated topic and I've focused on a simplified explanation of just three characteristics. No one (or two) metric will tell you everything. To gain more understanding I recommend reading articles on the subject -some examples are linked in this answer - this will help you learn the vocabulary and what some of the controversies are.

On top of what is measurable - frame angles, wheelbase, etc. there is personal preference.

  • How does it feel to you?
  • What do you like?
  • What will fit your body best?

There is also what works for a particular task. Bikes are kind of like shoes. We have hiking shoes, running shoes, court shoes, etc. Just like many people have more than one kind of shoe there is a tendency to have more than one bike for different for tasks.

Riding different bikes and developing an understanding of what works for you will help you get the best tool for your needs. Otherwise, you are reading numbers and trusting someone else to tell you what will work.

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