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I now have some Kenda 28c tires on my bike, and i got a pair of continental gp 4000 in 23c. I want to swap them because of the superior quality of the continentals, but I'm afraid that they could be a little too stiff. What do you think ?

I assume by "stiffness" we are referring to comfort, and that the Kenda will have a cheaper tire construction. In that case which tire is more comfortable depends on how you set up the tire and your riding conditions. If you are riding on very smooth roads, then a properly set up 23 mm GP 4000 may actually be more comfortable due to the combination of a higher quality carcass construction and the smooth roads not requiring large pressures to avoid bottoming out the rim. If you ride on very rough roads or mixed terrain then the 28mm Kenda's may be more comfortable, as you can run a lower pressure than the GP 4000, which will better absorb impacts therefore feeling more comfortable and less "stiff."

Background

"Stiffness" relates to tire construction and casing tension (i.e., tire pressure). Cheaper tire construction tends to result in a stiffer casing and therefore will feel "stiffer" for a given volume. The continental GP 4000 will have a higher quality construction than the Kenda, therefore if all things were equal (e.g., size and pressure) the Continental would actually be less "stiff." The Kenda is also larger, and a larger volume tire will actually have more casing tension and therefore be "stiffer" for the same tire pressure, but these two tires shouldn't be run at the same pressure, which makes the comparison more nuanced.

So Why Do Larger Tires Have a Reputation for Being More Comfortable?

The key issue is that all things are not equal, including pressure. The main advantage of larger volume tires is that you can typically run them at a lower pressure than smaller volume tires without risking issues such as bottoming out the rim. If the roads are rough enough, the comfort gains from being able to run a lower pressure can out weigh any differences in carcass construction.

As such, we need to consider the environment you will be riding in.

If you are on rough roads you may not be able to lower the pressure enough in a smaller volume tire (e.g., 23 mm) to achieve the same level of comfort as with a larger volume of tire (e.g., 28 mm) which can be run a lower pressure for the same level of rim protection. Because you will need to maintain a relatively higher pressure in the smaller tire to avoid pinch flats or damaging your rim, the tire can often ride more harshly (i.e., feel "stiffer") under these conditions.

If you ride on very smooth roads then a 23 mm tire can be run at relatively lower pressure, which will provide more comfort in addition to there being smaller bumps to absorb. In this scenario it will therefore not feel as "stiff," in fact it may even feel more comfortable than the larger Kenda as the casing will be more compliant than the Kenda tire casing adding to the ride quality.

Interplay Between Tire Pressure, Tire Size, Road Conditions and Comfort

The real key is whether or not the tire volume is sufficient for the the road conditions. If the volume is too small, you are likely to hit what has been termed the pressure "break point", where suddenly more vibrations will be transmitted to you, which can be measured as an increase in real world rolling resistance (Figure 1).

Figure 1. Real world example break point pressure measured by Tom Anhalt from Part 4b Rolling Resistance and Impedance. Red arrows and text were added to the original figure for the purpose of clarity.

Tom Anhalt nicely demonstrated that an increase in tire pressure lead to less rolling resistance (as predicted by theory), until suddenly it didn't, because the pressure was too high for the tire to effectively absorb road imperfections. This resulted in increased vibrations transmitted to the rider, moving the rider up and down, which absorbed forward momentum, which was measured as higher real world rolling resistance.

From a comfort stand point the rider would likely interpret the extra vibrations as a lack of comfort from the tire being too "stiff."

Lowering the pressure below the "break point" will return comfort and the tire feeling "less stiff." This assumes however you can safely do so. If the roads are too rough, then you will start bottoming out the rim and risk pinch flats. As such you will need again raise the pressure to keep things safe, if this increase in pressure sends you over the "break point," then suddenly the tire will feel too "stiff" due to the extra vibrations begin transmitted to the rider.

Predicting where the break point lies in real life is difficult. Tire construction and size that will work best for your application will likely involve trial and error. That said, experimenting with pressure can be an important first step. Your loading weight (i.e., rider + bike + load) also is an important consideration as a higher load weight requires a higher pressure to ensure the rim doesn't bottom out.

I don’t think an objective, quantitative answer to this question is possible.

Personally I consider even the difference between 25mm and 23mm width to be surprisingly noticeable. I run the 25mm tires at much lower pressure (5 – 5.5bar) than the 23mm tires (6 – 7bar) because in my experience a snakebite puncture (pinch flat) is much less likely with the increased volume of the 25mm tires. The above pressures are what I (66kg rider weight) use. I try to go as low as possible because the increase in rolling resistance with good tires is surprisingly low. Depending on your weight and riding style you’ll need to go higher or lower.

Lower pressure is especially advantageous on rough tarmac, cobblestones and gravel.

In your case I’d use the 23mm GP4000 if speed or weight is important and the roads in your area are good. If you occasionally ride on cobblestones or gravel I’d stay with the 28mm tires.

I suggest measuring the width of both tires to see how large the difference is in reality. As David points out in the comment, tire width is also affected by rim width, so for accurate results you should measure on the same rim.

As a side note: Obviously the “stiffness” of the tires depends on pressure. If you run both at maximum pressure neither of them will be comfortable. You’ll only notice a difference when you start experimenting with lower pressures.

“Too stiff” sounds somewhat subjective, so you should try it and find out for yourself.

The Contis are a well respected and high end product, you may find that their suppleness makes up for the reduced tire volume.

Or you might not, and you could sell the 23s and buy something larger. You won’t know until you try it.

You mention specifically in a comment that you're worried about your rims being damaged by the smaller tires, which is a reasonable concern. "Stiffness" per se is not really the issue, but smaller tires require higher pressures which the rim may not be designed to handle, and the rim may also be wide enough that the tires will not be able to properly seat themselves.

With regards to the former issue, the rim should have some printed indication on it as to the maximum tire pressure it's designed to withstand. As long as you don't exceed this, you'll be safe regardless of size (keeping in mind that if that maximum pressure is too low to prevent pinch flats with the skinny tire, you'll still have other problems).

For the latter, Sheldon Brown's tire sizing page has a nice table in the "Width considerations" section that outlines safe tire width/rim width combinations. Of note: 28c and 23c tires share only one standard rim width: 15mm (internal). Sheldon also notes that the chart leans on the side of safety and that "many cyclists exceed the recommended widths with no problem."

700 x 23c tires have a typical 2096 mm circumference, while typical 700 x 28c have a circumference of about 2136mm - which means that, typically, a 28c tire will have a "tire height" of 6+ mm more (a quarter of an inch) than a 23c tire.

That means that, when using a lower pressure in the tubes, the 28mm tire can "eat" larger road irregularities - it's safe to say larger by 6mm or 1/4 inch.

Unlike @michael, I have moved from 20c to 23c tires, and from 9.5-10 atm to 7.5-8 atm pressure, and it made a difference.

But again, on shop store cement or very good asphalt the 20c used to give the impression of "bike on rails" - you felt able to follow a chalk mark and stop on a pinhead, so you lose a bit of precision in handling. From my point of view, the increased comfort from 20 to 23 was totally worth it, 700 x 20c shake your hands like a hammer drill.

Edit: Also, Paris-Roubais (one-day Tour de France with a lot of cobblestones) was won in 2018 by someone riding on 700 x 30c tires, and pros usually ride 25c tires even on good roads.