cthenn

That's actually incorrect. "Bulk" air drag is by far and away the most impactful determinant to aerodynamics, but tire rolling resistance is second (a distant second). We did a lab experiment on this when I was in college, and though I only very vaguely remember the results, but rolling resistance in tires is a factor, albeit small compared to the overall profile of rider and bike.

cthenn

"most"

Campag4life

What is bulk air drag? Is this buying in quantity at Costco on a windy day?

McBTC

Another factor we probably might consider is that the body position needed to reduce the air drag of the body -- to the extent that achieving and maintaining it may be assisted by the comfort and security of having a wider tire at a lower pressure on the front -- may more than offset the aerodynamic drag of going with the bigger tire. For example, moving forward on the saddle and rotating the hips forward with the elbows up by the bars, as is possible using clip-ons, puts more dead body weight over the front tire where the ride can be a lot rougher than sitting over the back tire with your legs displacing a lot of your body weight.

cthenn

Funny guy! No, I just mean the overall air drag created by a particular profile in aerodynamic testing.

Seattle Forrest

To your point, my rims are made for 28 mm tires and have a max of 80 psi. Rim maker said "don't eff around with that max."

Campag4life

In bold...that is rich...lol.

Campag4life

Table exceeds mfrg' recommendations? How? The table is pretty accurate by and large. But as footnoted, a starting point. The smoother the road surface, wider tires can be run at slightly higher pressure because their benefit of lower pressure without flatting isn't as necessary on smoother road surfaces.

McBTC

Then, it'd be pretty accurate to say that by and large, there is at least a kernel of truth in your beliefs even if they're mostly, dead wrong.

McBTC

From Cycling Weekly article on Schwalbe’s recommended tyre pressure table...

Campag4life

Now I see why your thread has lasted so long..lol.

McBTC

...and creating a shortage of new disc road bikes that accept up to 32. All this wide tire talk is hitting the resale value of my 2015 road bike in the head-- damn!

ThermionicScott 

Hilarious. Keep the hits coming, @McBTC!

Kimmo

Tyre manufacturer's pressure recommendations can be safely ignored; the safety margin is close to 100%, depending on the bead interface with the rim - ie, it generally takes twice the max pressure to blow the tyre off, and nobody's going anywhere near that. As far as the tyres are concerned, all you need to think about is pinch flats and squirm, versus bouncing down the road.

Now, what rim manufacturers say OTOH, is disregarded at your peril.

....And regarding the question of whether discussing differences in rolling resistance is akin to counting how many angels fit on the head of a pin, perhaps it might be relevant to consider whether it's worth the bother of riding on the white line. I suppose as you go faster, it's a smaller percentage of total drag, but I've found under 30km/h it can be worth an extra gear, depending on the quality of the bitumen.

McBTC

Another to ignore then, that probably takes into account some of what has been discussed here. It is that, if you're looking at a table that is giving you a minimum psi based on total weight, tire width, etc., and if the recommended psi exceeds the maximum printed on your tire... just go to the next largest tire.

My guess is that any table that's been printed prior to 2015 based on recent discussions about rolling resistance, tire widths and psi... and, knowing OEM rims have gone from a standard of 17 to 19 -- probably can be at least mentally adjusted knowing that 23s nowadays are more like 25s, etc.

In practice though based my recent sojourn into the land of 28s-- it's not that much wider than my 25s but it's noticeably taller, despite supposedly having a supple casing as evidence by the 100 psi max pressure (my 25s have a max pressure of 130). And yet, the recent move to wider rims is to get wider not taller tires,

All of which leads me to believe anyone buying a new bike with the expectation of benefiting from wider tires probably should make sure the OEM rim width is >19 or all they'll be accomplishing is eliminating a risk that they may not have with 25s--i.e., a risk of pinch flats.

wphamilton

I can't tell. I'll probably have them at the same pressure, and I can't feel any difference at all from more air volume.

McBTC

For example, if you've got the cash to spend on an >$3K Trek Domane SL6 you get...

Rims Wheels[/i], Bontrager Paradigm Tubeless Ready

...and, if you look that up you get: 6061 alloy rim 23mm outer, 19.5mm inner width, 3 pawl hub Compatible with Shimano 10/11 speed, SRAM 10/11 freehubs

McBTC

This could be the future--e.g., Felt's VR4 at $3K but on sale maybe... OEM is Zipp 30 Course Disc Brake Clincher Alloy 28H Tubeless Ready ... which gets you-- a 21mm internal channel and right or wrong, they clearly got the message-- that's what you want your 28s mounted on. And, the bike isn't black! Trek better get up to speed for 2019

Kimmo

Do you mean tyre manufacturer's recommendations, or me?

McBTC

what's a tyre?

Dean V

Correct spelling! Just like red is the fastest colour.

ThermionicScott 

I don't know what you think magically happened in 2015, but I cheerfully ignore tire pressure markings with parts much MUCH older than 2015. And so do a lot of others.

Esthetic

The real world, best case scenario for tire size in relation to speed matters most in racing by top tier athletes/cyclists. And 25c has become the commonplace tire size it seems as rims have gotten wider.

Personally , 25c at 100psi and 28c at 90psi allow me the comfort to enjoy road cycling, in the real world conditions they roll on.

cyclintom

I sort of don't know where you're coming from. The difference in rolling resistance from a 23 mm tire to a 28 mm tire is only 2 - 3 watts. The total drag on a tire, say a Continental 4000 is around 20 w and there is twice that amount of aerodynamic drag increase going from 12 mph to 15 mph. If you can feel 2 - 3 watts you're a hell of a lot more sensitive than most people.

Seattle Forrest

These are 25 mm internal.