Brian Ratliff

Per the above analysis on my prior post, casing probably contributes about 1-3% of the total contact patch area. A super supple track tire will contribute almost nothing (it collapses under its own weight), while a heavy duty touring tire might be able to support maybe 3lbs with no pressure. The tire contact patch supports ~100lbs. So, 1-3%.

noglider 

Right, all things being equal, the wider tire is faster, but they are not equal since a wider tire is almost always heavier. The reason the discussion is interesting to many of us is that wider tires don't have as big a penalty as we previously thought. In fact, the lower rolling resistance is sufficiently low that the extra weight is not as much of a drag.

My narrow tires (~25mm) feel faster than my wide tires (32mm) but I'm not sure they are. I can't do a comparison because my wide tires don't fit on my racing bike. The differences will depend on the kind of riding you do. If you're not racing but are riding a big distance, say over 50 miles, the wider tire can, with its improved suspension, reduce fatigue in your body, which means it is more energy efficient, and that's even with its extra weight.

Doge

Do you lecture on physics?
It is basic stuff when static. Moving it is different.
I used to take my bike parts - wheels to class and we had lots of fun *trying* to figure things out.

Often quoted to me by my engineering mentors: “One good test is worth a thousand expert opinions.”

dmanthree

Great dialog in this thread. I had no idea tires were so complicated.

;-)

Doge

That is really a silly concept. Not your post, the concept. With any part you use it the way you think it is optimized.

The only question should be for a given rider, bike, weather and road, what part can make the rider go the fastest?
That is if fastest matters most (Or what is the most comfortable, or durable, or least cost....).

Compare the best you can do on a 30 (or whatever) to the best you can do on a 25 for the conditions you ride in.

Include the parts to support that tire. The rim, frame spacing etc. Then see what is the fastest and factor in other things of importance from durability to comfort to price.

I believe for tire width the pros are using what is the fastest for their conditions. I believe that because the same mfg offer a variety and generally withing a sponsors brand, unless they are pushing disc brakes or something like that, riders get to choose. For World Tour road racing they tend to choose 25 and narrower.

Brian Ratliff

And my experience as an engineer has taught me: "One crappy test not informed by fundamentals wastes everyone's time and costs a lot of money."

If you test, you must be informed by fundamentals. Separating the effects of different variables, the principle physics, stuff like that. Right now I am dealing with reworking a specification test that someone wrote 10 years ago which fundamentally cannot resolve the tolerance we are asking for on a part. The test essentially gives random answers relative to the tolerance range. A bit of time with fundamental study and reasons why this test cannot work just drop out of the math. Believe it or not, but this process works MOST of the time. There's a reason why engineers take all those classes in college.

Please, you are apparently an engineer... respond to the order of magnitude analysis I posted earlier. Let's get into some details and out of hand waving territory.

Abe_Froman

That sounds like throwing in the towel on understanding why something works and trying to refine things based on an understanding of physics in favor of just guessing, and going with whatever you randomly find to be fastest with a stopwatch.

Guessing, or just doing what others do is no way to improve anything.

SylvainG

My 35mm tires and 28mm tires fits on my Giant TCX bike. I can tell you the 28mm do feel faster, but they are Continental GP 4000S II while the 35mm are Giant PX-1 so this might have something to do with that 'perception'...

Doge

I do understand why. I just can't calculate it. I am result oriented. That is what engineers do (I were one). I'm not into research, more application. Some things are too hard to calculate. You can say why all the pool balls go where they do, but the pool player has a much better chance of getting them there than the physicist.
I guess based on understanding and experience, I test, find what I want and move on. I think you may find that is kinda common for the application folks.

The OP said he was faster on 25mm. The fastest in the world are not on bigger than 25s. I understand why.

Abe_Froman

50 years ago everyone was on smaller size tires. Did physics change?

Doge

Within the same order of magnitude. Say one tire takes 10W to move, the other 20W. If they are the same cross sectional area (size) and pressure (and same gas) I conclude the speed at which the tire deforms is 10W. At rest they both take the same W - Zero.
Short of slow-mo cameras below a glass surface of a bike at speed, I can't think how to measure the contact patch.
But it takes 10W more to deform on tire over the other one.
And the rate of deformation resistance (hysteresis) has to do with speed and tire.
My GUESS is the stiffer tire that takes 10W (100%) more energy to roll has about a 10% smaller contact patch.

I don't really have bad tires. My daily riders are Veloflex Corsa 25s with latex tubes. The low Crr tires are 25.5 mm silks. I have Francois of FMB make them for me with light tread and sometimes no covering on the casing. Then I have many others in between. As I posted above, I don't need to calculate it. I can measure it in speed/watt and feel and speed on rollers. I just know, as shown in video a few pages back, one tire deforms much easier than the other. It roles faster on everything.

Scarbo

I hear wisdom in this.

I have bikes that I regularly ride that have 23mm, 28mm, and 38mm tires on them (not counting my 2.2-inch MTB). In my neck of the woods a 7 percent gradient can be considered flatland. My typical rides encounter anything from 7 percent to 18 percent, and many times, more severe gradients. And I don't care how much pseudo-intellectual wanking goes on in threads like this because my experience tells me that the bikes I would rather, both climb and descend on, are those that have my 23mm tires on them (as I said earlier, these bikes won't even accommodate anything larger). Anything larger than 23mm--possibly 25mm if I could mount them on these particular bikes--feel sloppy, slow, and sometimes downright insecure on the most steep and technically challenging descents at high speed (in other words, these are not generous, Alpine sweepers); and slower on the steepest climbs.

There are tales of innocence and tales of experience.

Doge

35 years ago (1982) I rode Clement del Mundo 28mm silks.
The TdF winners are still riding 23-25mm.

In the 80s I had a MASI track bike with about 3 mm clearance between tire and fork. Through the 80s and into 90s it was popular to have tire/fork clearance very small. Due to wind tunnel testing, it was found that it was faster if more gap allowed more air to pass in between tire and fork than around. No physics changed - things were tested.

As tires get bigger, that gap gets smaller, or forks get wider. There is a sweat spot, that depends on speed, terrain etc, but for speed, on pavement I don't think it is over 25 for fast riders.

tomato coupe

There it is, in a nutshell ...

Brian Ratliff

Tire casings got better.

tomato coupe

You're going to have to translate that last part, because it makes no sense.

Same here ...

Brian Ratliff

"Say one tire..."

Lost me at that. Why 10W? Why 10%? Support the numbers.

HTupolev

Vulcanized casings have improved a lot in recent years. Cold-glued casings haven't changed much at all, and building wide tires out of them was perfectly viable in the distant past.

(It's also not like tires have gotten monotonically bigger over time.)

wphamilton

You could roll through paint, I'd suggest slowly, and see the width. Or sit on the bike and have someone measure using very thin calipers. Or use a drop of ink on the tire and roll over a piece of paper.

To rephrase, more or less the rolling resistance depends on the rubber's resistance to deformation, and on how much deformation occurs. Hence a linear function with speed.

Doge

Paint/width won't help because the circle and ellipse could have the same patch. We want area.
I guess if I knew they were the same shape, it might work and in the same width, they might be. So it could be something.

Have a puddle of paint on the bottom of a hill and ride through on tire 1 and tire 2 and see if you can measure the different width in lines? Hmm.

Slowly, still is not what I was saying. I'm saying it changes with speed.
When two tires are at rest we are not fighting the resistance of the tire to deformation.
When you roll them, you are and the faster you roll them more so.

Sites like I am asked for numbers or proof. use the drum test. Tires at the top of the list measure ~10W less than those at the bottom. It was my guess, because I don't know what else to do, that the contact patch area at speed with the same PSI would be about 10% less on a tire at the bottom of the list than at the top. In that video I posted pages back with the read tire, you can in a near static situation at the same PSI/width the two tires deform differently. The soft silk "wraps" around the red cotton tire. The Red tire is a thin case (Veloflex Corsa 25) with latex tube and is significantly stiffer. I'm saying it has a smaller contact patch area at speed than the silk does, and as speed goes up it gets smaller yet. All due to the resistance to rate of deformation (hysteresis) .

I do need to clean up the wording on my other posts. I was not on my device last night. I can't understand me. Sorry.

Abe_Froman

Glass/clear plastic roller with a camera inside. The equation for area of an ecclipse is very simple...

But you really dont have to mess around with measuring at speed. This stuff has been figured out already...you can make a simple adjustment for the centripedal force of the tire mass at speed without have to measure everything at speed. There's no need to reinvent the wheel.....er..tire here.

noglider 

That's a reasonable position but my intuition tells me that the happy medium may prove to be 28mm. Road racers do need low aero drag and minimum weight, so they don't want their tires to be too big or heavy, but the suspension benefit of going up to 28mm may prove to justify the increases in aero drag and weight. Time may tell.

wphamilton

It takes a certain amount of time for the rubber to deform, so plausibly the contact area would be smaller. But sniff test, assume it's a spring, hooke's law second derivative or look it up and do some ballpark math, it's not likely to see enough difference to be of a concern.

Doge

Yea - no idea, I was remote...
See post above this one from me.

I think I was trying to say as the difference between those two tires is 10W (measured Crr) then the one tire has 10W more resistance to deformation at that gas/air PSI and speed. It is the rate that the tire resits deformation that we couldn't see in a still example. Materials have rates they deform. A "stiff" tire resists deformation more than a supple one. At standstill both would get to their rest position. At speed I have no reason to believe both would deform at the same rate because the wheel is rolling. If they don't fully deform the same as when still.
A wheel with 100lbs @ 100PSI ~ 1SI area contact patch. That is .56inch width if it were a circle, and we know it is a bit longer/elliptical so I'll use .6in in contact with the road in a static situation. At 30mph the tire is going (if math is right (30*5280*12in/hr) / (3600sec/hr)) 528 in/sec. So start of that patch to end occurs in .6in / 528in/sec ~ .0011 sec ~ 1ms.

For a tire to fully drop the same amount in 1ms as it would in a static situation is something I don't assume, rather I assume it won't and because materials resit deformations at different rates the contact patch would be different too.

This was in the news lately at kid's school. https://www.airforcetimes.com/news/y...stops-bullets/ That is a "goo" that at slow speeds offers little resistance. At high speeds it stops a bullet.

Doge

That was all I was saying.
But how would you measure or calculate that. From the post above we have about 1ms at 30mph for it to deform.

It is more a shock absorber than a spring. That deformation happens and creates heat. And the resistance is tied to the rate of deformation (like the bullet stopping goo linked to above). We get much of our W loss there. When it "springs" back it is springing into the air, so we don't get the push from it.