TiHabanero

Having real world experience with 18, 20, 23, 25, 28, 32, 37 mm tires I can honestly say the rate of acceleration on wider tires is slower than skinner tires (think weight), however the 28mm is my tire of choice and has been for almost 20 years. I have found that outside of acceleration, including climbs, they roll easier over the nice smooth Michigan roads we have around here (sarcasm here) and have no speed disadvantage that I can detect. I will add to it that 37mm tires are overall slower than the 28mm tires, but ride perfectly on gravel roads and some 2 track trails. Outside of off-road use, I see no reason to ride anything wider than 37mm tires.

jimmuller 

Yes, well, terminal velocity of a human body fall due to gravity is supposed to be somewhere around 140mph. If you can bike that fast, the great!

I neglected those things because they weren't under discussion, as many other things were not either. I was correcting incorrect physics. If you don't start with correct fundamentals you can't draw good conclusions, with or without any other factors being considered.

Kontact

It isn't just Berto. Michelin makes their own chart, and every car has a recommended pressure that goes with curb weight and loading.


As far as volume and math, the main thing I was pointing out is that if you graph the behavior of a tire, like how the chart shows a bigger difference between 28 and 25s vs 25 and 23s, you'll find that referencing volume makes the graphs more linear. And since tire width predicts cross section as a square of radius, it shouldn't be surprising that comparing diameters gives non-linear results. I merely used wind resistance as another familiar geometric relationship, not saying they have anything to do with each other outside of plotting curved lines on graphs. It doesn't really matter if the actual math is a simple square or a quadratic function because the major player is the fact that something is being squared.


You can approach the explanation of how tires work many different ways, and I've tried to explain a couple to make myself understood. But the major idea here is that a 1" tire and 2" tire operate under the same rules and it makes sense to set their "preload" via rider weight and tire pressure by referencing the same sort of compression figure. You want your loaded tired to have the same cross sectional and contact patch shape regardless of size because they are the same shape and materials.

CVB

I like the feel and look of a wider tire. I ride 28s myself, since that's what my bike was spec'd with in 1984. But I have a feeling that in 10 the bike manufacturers and tire makers will have us all talking about how the latest research proves that skinny tires are actually faster, more comfortable, more efficient, etc. And then in 10 more years we'll be back to the other end of the pendulum's swing.

Kontact

No, you just don't understand the physics you're talking about.

Terminal velocity is the limit, but an object with higher terminal velocity is going to accelerate in air when dropped faster than one with lower terminal velocity. Wind resistance vs weight changes all parts of the acceleration, not just where it ends.

And if you "drop" an object on a sloped plane, they will still have a terminal velocity and an acceleration curve, it will just be smaller than dropping them straight down. Terminal velocity for a cyclist is whatever speed they top out on on a particular slope. I've personally hit about 50mph, but on a steeper hill it might have been higher.

Please stop "correcting" high school physics. Galileo even did all of his test with ramps and ball to slow things down, so it is funny you're puzzled about what bikes on hills having to do with terminal velocity.

ryansu

I went Phat and I ain't going back! I think of 28s as skinny these days

McBTC

… the ultimate conclusion drawn by many, under real world operating conditions being, the wider tire at a lower psi will have the same area of contact so no greater rolling resistance (the greater rider comfort a wider tire at lower pressure will provide being... freee!). Sounds too good so it invites skepticism. Even so, the OPs observation if reasonable should be that 25s will feel even faster and that he should chose 25s as the better tire for the same reason he's choosing 28s over 32s.

Kontact

It really can't work that way. If the larger tire has a contact patch as small as a the small tire, than it has proportionately greater internal pressure and less "preload", so it is going to take more force to make it conform to the road than the skinny tire at a proper pressure.

If you use the charts for tire pressure you'll get similar tire compression which equals larger contact patches for larger tires.

If you fill a 32c tire to 100 psi it will definitely be a harsher than a 23c tire at 100 psi, because the force the tire is exerting between the bike and the road is larger with the same pressure and more volume.


This is a lot like hydraulics. A hydraulic servo with a 1 square inch piston and a 100psi pump isn't going to exert as much force as 10 square inch piston with a 50psi pump. Overinflate a big tire and you're just making the tire more resistant to absorbing bumps, not getting something for nothing.

mountaindave 

Kontact

I don't understand what you're complaining about. No one has said anyone's tire width choice is wrong.

The discussion is really just about what the relationship between pressures, volume and rolling resistance.

ascherer 

Steve Whitlatch 

What do professional racers use on their bikes? It`s that simple.

Steve Whitlatch 

I did a shake down ride on my new to me Schwinn Voyageur with cheap 27 1/4 inch tires. Felt good on the seat but I lost 2 mph average speed and my legs felt like the 17 mile ride was 65 miles. I need lighter wheels and tires.

McBTC

Some of us apparently have different feelings about life in the real world-- e.g.,

Trend spotting: Why you need to switch to wider tyres | road.cc

Kontact

I agree with the Dave Taylor quote. That's part of what I've been getting at.

repechage

I like the precise handling of a narrower tire- say 22mm tubular or 23 mm clincher up forward, 25-28 in back.

HTupolev

Yes, but different charts aren't all determined exactly the same way.

I'm not sure what you mean. How exactly are you adjusting the data? Doing something like shifting the PSI for each curve according to the measured tire volume ratio does align the data much better than the original graph, but seemingly not better than by using the width or height ratios.
The difference between the 25 and the 28 being so big is partly because the actual size ratio between the 28 and the 25 was way bigger than between the 25 and the 23. Look at the measured values.

At any rate, this particular consideration isn't really relevant to my objection; I'm not saying that all aspects of how to set up a tire scale linearly with width. I'm saying that width seems to describe functional behaviors of a typically-set-up tire in use more proportionally than volume does.

Dave Cutter

My search says 25 and 26mm are the tires of choice for pros now. I've never seen a 26mm tire. Those must be limited to the pros. I've found that "lighter" wheels rotate up to speed faster (the speed is of course is determined by my legs). I don't care which tire is fastest... nor am I overly concerned about which tire is more comfortable. All my bikes are comfortable... and a few minutes faster or slower ride time.... makes little difference to me. On my skinny little light road bike.... I prefer skinny light tires (with light tubes too). It just feels right.

On my gravel grinder... I like fat tires.

dddd

One more variable (besides air pressure) when scaling up to a larger tire size is whether the rim width remains constant. A larger tire on the same width rim and at a suitably-lower pressure will tend to feel wallowy as the tire's sidewalls angle more sharply outward. This alone will make a bike feel sluggish and less responsive.

I've ridden bikes with wide rims and like the response to the combined inputs from pedaling out of the saddle and steering. The wider rims have always been heavier though, which makes comparisons more difficult.

I remember in the late 1990's I ran Mod58 rims having a 16mm inside width using 700x20 Michelin road tires that actually plumped out to 22+ mm on those rims, and despite the weight the tires subjectively felt responsive and efficient. I couldn't escape the weight penalty but really liked the feel of the tires. Since I didn't race on those wheels it was a good setup for me running about 100psi with my 155lb weight on board.

I later ran 23mm tires on Mod4 rims that were even wider, and again I loved the tire behavior despite the rather obvious added weight. The guys on the training rides were like "what's up with those fat rims?". I found it hard to explain, but it was just for training rides, though I also used the same heavy wheels with 32mm Michelins for a few CX races.

The 1' Pasela and other brand's 1-1/8" tires on my bikes plump out to 26mm on older touring-width rims, and that is a very good combo around here. Soft enough without being noticeably wallowy. So I can see why the pro's might like this width of tire on nice wide rims that also support this width optimally in aerodynamic terms.

Lascauxcaveman 

So, you're saying hidden electric motors are the answer? I get that; sounds good to me...

jimmuller 

Not even counting the "for now" part (which is a valid conditional), the pros have very different concerns than I and probably most of you have.

They train hard for hours per day, race for hours per day, day after day. Even with my daily commute I don't come close to that kind of consistent pounding on my body.

They can't afford to back off when they encounter a stretch of rough pavement, but I can and think no more about it.

At least in road races as opposed to criteriums they spend much of their time running top speed where wind resistance is the limiting factor, and much of their time running a nearly constant speed rather than accelerating hard, braking for a turn, then accelerating hard again. I can accelerate whenever I want and enjoy the sense of lightness, or you can enjoy your sense of smoothness if you wish. Our, or at least my, elapsed time is meaningless unless I have to catch a train (which I have had to do on previous commutes but tried never to have to cut it so close).

How one finishes is probably dominated by aerobic and overall conditioning more than by the raw mechanics of the bike. My finishing is determined by how much food I brought with me.

The point is, what the pros are doing "for now" or at any time really doesn't apply to my choices. That's the real message of all the notes here and of Grant Petersen.

FlMTNdude

jimmuller 

Well, actually I do. For any given speed wind resistance is whatever it is and is not directly related to mass. The force of gravity will depend on the slope of the hill, and wind resistance is simply added to, or subtracted from, that. But the inertial effect of both static and rotating mass is independent of both of those factors. If you added, say, 200g to your bike the effect on acceleration will depend on where you put it, wind resistance being constant. (We'll get to that point in a moment.) If you add that 200g to your wheels your acceleration downhill at any speed will be lower than if you add it to the frame instead. If you want to go downhill faster you should get a heavier (actually denser) bike, but heaviness in the frame will get your more acceleration than heaviness in the wheels will get you. (I'd rather not have to pedal the heavier bike uphill just to get a bit more acceleration downhill, regardless of where on the bike that extra mass is located.) Now the only way you can add 200g to the bike is to put it somewhere. Unless you put it inside the frame (thicker tubing walls or denser material) or tuck it out of the wind stream, it will likely affect the wind drag some. But that's a different question.

Wind resistance is usually modeled as velocity squared times frontal area times an empirical coefficient. It does not scale with mass beyond the question of where on the bike or rider you put the mass. However bike and rider are such a complicated and continually changing shape that the usual drag calculation methods are fuzzy at best. The only valid way to think of acceleration in the presence of drag is to start by taking drag as constant. It isn't, of course, and in the real world it varies over time as the rider pedals and it varies a lot with speed. By the time you get going fast enough that the wind resistance of that extra two hundred grams shows up, it's effect on your prior acceleration will have been significant. Wind resistance at that speed will be determined by so many other things that where or even if you have that extra mass will be undetectable. Providing you don't use it to build a sail.

In real life I, and probably most of us, spend far less distance (and even less when scaled per time) going at terminal velocity downhill than in accelerating on reasonably flat roads or minor hills. That's where you will feel the extra mass, and where it matters where on the bike it is located. But the effect will always be to make you accelerate less when the weight is on the wheels instead of anywhere else on the bike or rider.

speedevil 

Agreed on the Compass Bon Jon Pass 35mm tires. I put them on my Cannondale ST600 and it rides really well with them. Not cheap, but worth it to me.

I just wish Compass had narrower tires with the same construction, because some of my older steel frames just don't have the clearance for wider tires. 23mm tubulars fit OK, but wider might or night not depending on the mounted real-world width. I checked and compass does make a 26mm tire, need to check their return policy if they won't fit - but I would have to mount them to know for sure. A lot would depend on the rim.

63rickert

It's always apples and oranges. This subject is plain hard to discuss. As a small example the two most popular gofast clinchers of the moment are Continental GP4000iiS and Vittoria Corsa G+ Isotech. The 700x25 version of either of those tires (700x25 on the label) measures 27mm when inflated on current norm 23 or 24mm rims. So when racer boy claims to be riding 25mm is he really riding 27mm? Only way to know for sure would be to walk around the starting area with calipers. In a forum like this where many if not most are still on vintage or vintagey 19 or 20mm rims the new tires are just plain different than they are on new style rims.

Racers, real racers as in pro peloton, are still overwhelmingly little skinny guys. They weigh 140-160 pounds and 120 pound climbers are more common than 200 pound bruisers. A tire with actual width of 25-27mm gives a 140# rider a lot more flotation than same tire under a 200# civilian. The racer is also on a 15# bike and carries absolutely nothing. Team car is there to carry the load. To get roughly the same loading per volume of air as a racer has with 25-27mm a full weight rider with a modest load is going to have to ride 28-32mm and might go to 35mm just to be in the same ballpark.

When Jan Heine cuts loose and starts in on tires he is mostly addressing a little club of engineers and aficionados with infinite discretionary income who all have the same latest and greatest gear. Who do put calipers on their tires and know that you cannot possibly equate a fat Gravel King with a fat Compass. It's rare air. The discussion makes sense in its own terms. Trying to apply that discussion to what ordinary cyclists do is kinda hopeless.