El Davisimo

Can someone please tell me why the trend right now is 700x25, instead of 700x23? I always thought that 23s are thinner tires, which means less contact with the road, which means less friction, which means more speed.

TimothyH

Wider tires have more volume and so pressure can be reduced. This can increase comfort.

A narrow tire has a longer contact patch with more sidewall deflection. Sidewall deflection is what causes rolling resistance.





-Tim-

JasonD67

I've gone from 23s to 25s to now 28s on my new bike with disc brakes. As the tires have gotten larger the pressure has gone down and the ride quality has gone way up and the bike handles much better.

jon c. 

There's definitely a trend toward wider tires and lower pressure. And charts that seek to demonstrate optimal pressure. What is really optimal depends on the construction of the tire, surface of the road, average speed and a host of other factors, so I suspect the charts are a bit too general an estimate. Although I'm not sure it makes all that much difference in terms of raw performance for the average rider. Harder, thinner tires certainly feel faster. I'm convinced that isn't necessarily so, but I still prefer the feel of a firm tire so that's what I ride.

rubiksoval

Once you start dropping the pressure the wider tires are going to have higher rolling resistance, not less. You've still got to maintain the pressure to reduce rolling resistance. And aerodynamically, the narrower tires are faster.

I ride and race 23s. I do time trials (very infrequently) with 21s.

HTupolev

Contact with the road doesn't cause rolling resistance: the contact patch isn't sliding against the road at all, unless you're skidding!

There are two major sources of rolling resistance that have been identified:

1-When a section of tire reaches the contact patch, it deforms as it presses against the road. When it leaves the back side of the contact patch, it springs back into shape. But some amount of energy is wasted in this process; the amount of energy that sprung back into forward motion is never as much as much as was used to compress the tire; the lost energy was turned into a small amount of heat within the tire's casing when the tire deformed and sprung back. Higher tire pressures reduce this loss because the tire doesn't deform as much. The construction of the tire also has a big effect on this loss: tough stiff tires tend to waste a lot more energy than supple high-performance racing tires.
For a given tire pressure, wider tires don't deform as much, and so experience less of this loss. However, wider tires are generally run at lower pressures, which largely negates this difference.

2-The reason that bicycles use pneumatic tires is to serve as a suspension system. Roads are not perfectly smooth: they're made up of little irregularities, tiny bumps and valleys. If a tire is pumped really stiff, it will chatter off those irregularities and transmit them to the bike and rider, rather than smoothly deform around them. Even when this buzzing is subtle, it can still waste a meaningful amount of energy.
The rougher the surface, the softer the tire needs to be pumped in order to roll smoothly across the surface irregularities. But it's relevant even on very smooth roads: Silca did a test with a 190lb bike+rider on 25mm GP4000 tires, and even on a very plush fresh rolled asphalt road, the bike started to lose performance when the tires were pumped stiffer than 110PSI.

So, putting these two effects together:
You want to pump your tires soft enough that they're able to behave well as suspension. But narrower tires need to be pumped firmer in order to be well-behaved and minimize the losses from deformation. So, a wider tire can be pumped softer and achieve better performance as suspension, without suffering from greater losses to deformation.
Furthermore, a wider tire pumped softer can provide a more comfortable ride, and feel stabler on rough ground.

The compromises that wider tires face on road bikes are that they tend to be heavier and suffer from increased aerodynamic drag.

So, what's the optimal tire width? I dunno. In theory, it depends on the riding surface(s) and how heavy the bike+rider is (and a whole bunch of other things like the construction of the particular tire, and the wind conditions).
One reason behind the drive toward wider tires is that, in the last decade or so, a lot of people have been finding that the penalties for going wider are a lot less significant than they expected. In a sense, the reason behind the increase from 23mm to 25mm on racing bikes is that there wasn't much reason not to do it.

It's important to emphasize the importance of tire construction. My gravel bike is usually wearing tires which - aside from being 53mm wide - are built like high-performance road slicks. I occasionally take that bike to spirited road rides, and can largely ride with the same people as when I use a skinny-tired road bike. But I've also got a pair of cheap beefy ~46mm tires with very tough sidewalls: they slow the bike down by an absolutely massive ~2mph, despite being quite a bit narrower!
The point being: if you want a road tire to be fast, getting a tire that's fast is a lot more important than 23mm vs 25mm vs 28mm.

Another oddity worth mentioning is that most people "feel faster" on tires that are pumped stiff. When you go faster on a bicycle, the vibrations you feel from the road increase in frequency. So, when you go really fast, you're feeling lots of high-frequency vibrations. If you then damp these vibrations with a tire that's pumped appropriately-squishy, that "feels" similar to going slow.
I think that this is something that you can acclimate to, though. The first time I rode my gravel bike with those 53mm tires, I felt as if I was moving slower than my speedometer said I was going, but that sensation went away quickly.

rms13

I thought the trend was towards 28s

HTupolev

28s and 32s are what people used on recreational road bikes in the 1970s, that's old stuff. The new trend is towards Rivendell 33.3333333333s.

Maelochs

When one wants to see where the latest thinking on absolute high-speed performance is leading, look to the pro peloton.

Not everything they do translates to regular road riding but some of it does.And the trend there is towards 25s and 28s---for all the reasons @HTupolev stated in his thorough post.

I can remember when 15-mm tires at 190 psi or some such was considered 'fastest" because obviously less drag and friction. Now we have gone to almost twice the width and half the pressure ... and the guys in the peloton aren't doing it for comfort.

Also, studies have shown that what really affects tire aero isn't so much frontal area but rim width and depth. An ogive rim profile with the rim and tire perfectly matched in width creates less drag--drag being a function of turbulence more than frontal area.

There is a ton of research out there, I am sure, for people who want to go looking. I have seen a few small bits here and there, including one great piece on rim width and shape and tire width .... but when the pros started racing on wider tires and deeper, ogive-section rims, I didn't need to see a lot more studies. Those guys really pore over the research because there are careers and sponsorships riding on it.

79pmooney

In line with HTupolev's post - the faster you ride, the smaller (percentage-wise) rolling resistance is to the total resistance the rider has to overcome. Rolling resistance increases linearly with speed, air resistance with the square of speed. To show this trend, look at this scenario (totally fictional). Say at 15 mph rolling resistance is 10% of the total and air resistance is 30%. And that the rest, like rolling resistance varies linearly. Say we have 100 units of total resistance so rolling = 10, aero = 30, the rest = 60.

Now look at 20 mph. Rolling = 20/15+x 10 = 13.3. Aero = (20/15)^2 X 30 = 53.3. The Rest = 20/15 x 60 = 80. Rolling = 9%, Aero = 36% of the total.
Now look at 25 mph. Rolling = 25/15+x 10 = 16.6. Aero = (25/15)^2 X 30 = 83.3. The Rest = 25/15 x 60 = 100. Rolling = 8%, Aero = 42% of the total.
Now look at 30 mph. Rolling = 30/15+x 10 = 20. Aero = (30/15)^2 X 30 = 120. The Rest = 30/15 x 60 = 120. Rolling = 7.7%, Aero = 46% of the total.

(I'm guessing my figures are high for rolling resistance and quality tires and the "Rest" by a lot but that air resistance is considerably higher. I'm not going through the math again, but I think you'll get the point.)

So you can see that as speed increases, the tires matter less and less. Yes they are important enough that you won't even get to high speeds with clunker tires unless you are a muscular bull, but you can see that the difference between a high quality 25c and the same 23c is going to be very little, but that at high speeds, you might want to look at the air resistance of the tires. (But also the possible cost of flats from poor road surface and the effort required to chase back on if you are racing - a very real cost - and the confidence of cornering a little harder and faster on wider, more supple (due to lower pressure) tires.

An example - the Sky team last year raced the Tour Down Under with the whole team on 25c tires, not the usual 23c's citing reconnaissance of the road surfaces.

Ben

79pmooney

As long as you match the rim width and cross section to the tire, the narrower tire will always win on air resistance. Air resistance is Area X Speed squared X Air Density X Coefficient of Drag. Keep that coefficient of friction constant by simply scaling down your super slippery fat tire and rim combo but scale down the size and width by say 76% (going from a 25c tire to a 19c) and yo9u have a wheel that has 76% of the aero dynamic drag. (Well no, the spokes and hub don't scale.) Yes, the new fat tires and rims are faster than the old skinny tires and very poor rims. but those old skinny tires on equally sexy but appropriately narrower rims would be fastest yet.) So if you keep the coefficient of drag of the combined tire and rim constant, the skinnier tire always wins the aero race. Always. Aerodynamics 101. Freshman year.

Ben

canklecat

Tread thread.

Seriously, try 'em both. Conti Ultra Sport II are cheap and good. Grand Prix 4000 S II are dropping enough in price to try a set of 700x23 and 700x25. See how they feel on your favorite routes and compare times over several rides to average out wind and weather conditions.

700x25 were more comfy on my bike and rough rural chipseal, and most of my best times over distance were on those tires in 2018. But handling was a bit squishy on fast turns on rough roads with 700x25 on skinny old school rims on the front wheel. So this year I switched back to 700x23. But I had to run them softer to feel as good, without sacrificing handling. It's a trade-off. If my routes were all fresh blacktop I'd ride 700x21, if I could find any good clinchers in that size. Realistically, that means 700x23.

I doubt most of us will see much difference. Any differences I *think* I've experienced could be attributed to many variables -- human, weather, etc. The benefit seems to be for pros on grueling one day races and long stage races where comfort is a significant factor in overall human efficiency.

diphthong

so for the engineers and physicists out there, how does the weight difference between the same tire in a 23 vs a 25 or 28 play out?
we've gotten the aero and rolling resistance feedback but what about the rotating weight (difference)? what sense does it make to
get spendy for lighter weight rims, then slap on a heavier tire? seems like there is a perceived sweet spot (or maybe two) for the
combo of weight, resistance, aero, style of riding, comfort and durability/wear for every rider and no one size truly fits all.
i'm a grinder/masher and i find it easier to get 23's up to speed and more responsive than 25's or 28's.

Paul Barnard

What is the point in width (and the associated weight) of diminishing returns?

rubiksoval

A better question is what sense does it make to go through all this analysis for tires and then slap a butyl tube in there? You're likely leaving more significant watts on the table doing that than going one size over another due to weight concerns.

jimincalif

wphamilton

"Less contact with the road" does not equal "less friction", which in my humble opinion leads to a lot of the confusion surrounding the subject. The shape of the contact patch does alter the rolling resistance, to some degree.

As for the question itself, I personally prefer 25's to 23's but if you pinned me down about "more speed" I'd have to admit that I don't see any difference at all in practice.

JohnJ80

redlude97

I run 25s (that measure close to 28) all around now on the road bike but used to run a staggered 23/25 setup that felt faster. Can run similar pressures and the front tire sees more wind where aero effects are more significant. Wear ends up more similar as well, 2 rears to 1 front compared to closer to 3 rears with 23s all around

TimothyH

This is an important question.

My own thought is that it varies with each rider.

I'm not powerful and am very sensitive to tire, tube and rim weight. Light tubes, thin tires and light wheels help me keep up with stronger riders on group rides. I would definitely examine weight if going significantly wider.

More powerful riders might not be as sensitive to increased weight.


-Tim-

jimincalif

Probably true, and maybe not just powerful but also heavier riders. I'm around 208#. So for me a few more grams in the wheels is much less significant overall than it would be for a 140# rider. And I have a lot more, ahem, tissue than a lightweight rider to jostle up and down, making the hysteresis effect more significant for me as well.

Reynolds 

Corsi e ricorsi, as they say.

Maelochs

i did an experiment on my Fuji Sportif, going form stock wheels and stock, lightly treaded tires to a set of light metal wheels with Conti Ulrasport II, to a set of light (about 1400 gram) carbon rims with Hutchinson Fusion 5s. The bike was noticeably more lively with the lightest gear---but I must have been saving almost a couple of pounds. Going from 28s to 23s, I might be saving a few hundred grams. Would I notice it? Possibly. Would the ride be more comfortable? Almost certainly. Would my overall average speed over a specific route given similar rest and nutrition be quicker or slower? I don't care because every ride is different anyway ... but also because i don't race. A tenth of a mile per hour over a 20 mile ride doesn't matter to me.

What each rider prefers is probably more important than the mathematical data which is only imprecisely recorded anyway, in most cases.

That said, i run 23s on three bikes, 28s on two, and 32s on another. if I want to feel like I am going fast, i ride the lightest bike with the skinniest tires. My actual average speed for the ride is a reflection of so many other factors, no valid comparison can be made. How it feels is basically how it is unless I am racing ... which i don't do.

So, as with so many things, the "right" answer for what tire and what pressure is "right" for any rider is totally dependent on what that rider believes it to be.

Sy Reene

fify

Maelochs

Thank you.