Question: As I understand a gravel bike, it's basically a road frame but perhaps with a slightly more upright riding position. It has wider tires with much deeper treads and a stiffer rubber than standard road bike tires. What is the theory behind these changes? I had always thought that the thicker and stiffer tires were to prevent small bits of gravel from pinching flats, but is the theory instead that the deeper treads and wider tires are supposed to increase traction? And what is the rationale for the upright riding position?

Answer: 
I think you are asking two separate questions:

1. What's a gravel bike and why are they popular?
2. Why are off-road tires designed they way they are? (Which applies equally to mountain bikes and hybrids)

Gravel bikes have longer seat stays and more relaxed steering geometry than road bikes, which makes them a little more stable on loose surfaces. They have wider tires that allow riding on rougher surfaces. There are many reasons why they are popular - filling a gap between road bikes and increasingly heavy and complicated mountain bikes, suitability for touring or bikepacking and suitability for general riding with increased comfort are some of them.

I think you are perhaps a bit confused about off-road tire design. Off road tires do not have to be stiffer but they do have to be tough to deal with more flexing and distortion. A thick tread is not totally necessary either, slick low pressure tires will actually grip pretty well on dry gravel. Lower pressure increases the contact patch side and does increase traction on loose surfaces.
**Q1: Riding position**

A1:

Do you know the riding position of a so-called gravel bike and a regular road bike could be 100% identical?

Let me show you my real case.

I just have a s-works diverge (so-called gravel bike) and a giant advanced tcr (so-called regular road bike).

I have carefully studied their differences which include geometry. Here is part of my study work.

[![enter image description here](https://i.stack.imgur.com/dNUch.png)](https://i.stack.imgur.com/dNUch.png)

And, let's put them together now and check it again.

[![enter image description here](https://i.stack.imgur.com/BpWNC.png)](https://i.stack.imgur.com/BpWNC.png)

They are almost 99% identical. Surprised? But, too see to believe, right!

The so-called gravel bike has slightly higher handlebar but it is under the range of fitting and adjustment. Actually, the riding position of my diverge (so-called gravel bike) is more aggreesive than the tcr (so-called road bike). If you like to, they can be adjusted to 100% identical easily.

**Q2: The tires which gravel bikes use.**

A2:

The key characteristics of a so-called gravel bike (here is the diverge) might include:

1. It is much more comfortable and enjoyable riding experience.
2. Slightly longer wheelbase.
3. It is also an adventure machine.
4. It is as fast as the regular bike or even faster. (I have multiple new PR with the diverge)
5. It can not replace the XC and DH MTB. For technical off-road, I will wish I ride a MTB or a specific purpose bike.

It is quite similar to the relationship of a SUV and a regular car.

Thus, the so-called gravel bike usually uses wider tires (>=38C).

But, some of your knowledge about gravel bike might be not so correct.

The thread of gravel tires is not necessarily deeper. There are gravel tires without thread (e.g. Gravelking)

Check here to find the thread of a gravelking plus tire: <https://www.panaracer.com/lineup/gravel.html>

Ok, it is my study work. It might not be your expected answer but I wish it will be helpful.

**Final words and it is an important reminder.**

The upper case is just only one of gravel bikes which are recoginized by the gravel players' community. Actually, there is not a single kind of gravel bikes but many many kinds.

That's the reason I use "so-called gravel bike" but not "gravel bike" in the article.

Cheers