Originally Posted by XR2

I believe you're missing the weight of tire/wheel when comparing wider tires to racing tires. I imagine dragging that fat azz 650X42 plus attendant hardware around the French countryside for 3 weeks would take it's toll.

Originally Posted by triplebutted

Just ride

Originally Posted by Zaphod Beeblebrox

If you want to do the same comparison at home you'd want to try a pair of 28mm Paselas at Some psi as a reference (call it 90 if you like), and then try a pair of 32mm Paselas at the Same psi, and at a lower Psi and see what you think the difference is.

Originally Posted by old's'cool

I don't know what the psi rating is on sidewall of the 32mm Paselas, but do be careful about exceeding that by any appreciable amount. The forces trying to separate the tire from the rim increase with pressure but also with cross-sectional diameter.

Originally Posted by Captain Blight

I don't believe anyone's done a really academically rigorous, quantitative/qualitiative analysis of rolling resistance vs. tire size vs. contact surface quality. I would have no idea where to start.

Originally Posted by Hydrated

Ahhhhh... As an engineer, I love these threads! There are many factors that impact your speed, but you have to make concessions to get there.



For decades aircraft designers have known that a tire's contact patch size is a function of effective load and tire pressure. Lighter loads or higher pressures yield a smaller contact patch... Tire width has NOTHING to do with contact patch size. Width determines shape of the contact patch, but not size.



The rotational weight of the tire won't impact top speed once the rider has overcome the tire's inertia and gotten it rolling at speed. That's why a heavier tire makes it difficult to accelerate quickly to grab the tail of that group... inertia puts the brakes on you. But that extra tire weight is far from irrelevant. More weight enlarges that contact patch and makes it harder to maintain speed, even on a flat stretch. Why?



We know that tire width has nothing to do with the SIZE of the contact patch... so where is the difference? It's in the SHAPE that the difference lies. A wide tire will have a short but wide contact area, and a skinny tire will use a thin but loooong patch. And both tires will deflect and squish down to put that rubber on the road lengthwise. In other words, the sidewall has to deflect and bend enough to let that tread patch touch enough of its length to complete the contact patch. The thinner the tire, the more the sidewall has to deflect along its length. So a thin tire with heavy loads (more weight) or low pressures will be very inefficient because that sidewall deflection eats up energy like crazy. That's why most thin tires run high pressures... to minimize contact area and consequently minimize energy wasted overcoming tire deflection.

This sounds counterintuitive, but: A wide tire will be more efficient than a thin tire running AT THE SAME PRESSURE.

This works because the wide contact patch of a wide tire requires less sidewall deflection along its length in order to create a given size patch. Less energy wasted. The problem is that wide tires normally run at much lower pressures... and what do low pressures give us? Bigger contact patches... and more sidewall deflection. As a matter of fact, most heavier riders will be able to ride faster and longer on 25's or 27's than on those 20 or 23C's that the high zoot racing bikes come equipped with.

If we could find 2 inch wide tires running at 140PSI we'd be in business. They wouldn't accelerate for crap, but you could push those babies at speed all day long!

So to sum it all up:

It's all about the weight and tires on your bike. The rotating weight prevents you from accelerating to catch onto the group... and then those fat low pressure tires suck your tanks dry trying to keep up at speed. And that heavy mountain bike frame creates big contact patches to suck away even more energy.

Originally Posted by Zaphod Beeblebrox

If you want to do the same comparison at home you'd want to try a pair of 28mm Paselas at Some psi as a reference (call it 90 if you like), and then try a pair of 32mm Paselas at the Same psi, and at a lower Psi and see what you think the difference is.

Originally Posted by triplebutted

Just ride

Originally Posted by noglider

I think the reason narrow tires ride better is that they had more supple sidewalls. Until recently, it wasn't possible to make a wide tire with a supple sidewall. But it is now. Now that it's possible, people are saying they're the bee's knees.

I'm about to try some 32mm Paselas. The sidewalls are very thin. If I could justify spending on the Grand Bois tires, I'm sure I'd love them, but the Pasela is still a darned nice tire.

Originally Posted by bobbycorno

As an aside, I've got "650x38b" (really 35.5-36mm wide) SOMA B-Lines on my brevet bike. They look like a 650b, folding bead Pasela (non TourGard, thank the tire gods!). Dunno why, but they impress me as significantly faster than my 700x35c Paselas. A lot of that could be the bike they're on, tho' (a Reynolds 531cs Trek vs a Kogswell P/R). Whatever the reason, they've got me thinking seriously of eventually ditching 700c all together.

Originally Posted by Hydrated

Maybe the bicycle industry hasn't done this kind of anlysis... but rest assured that auto and aircraft designers have. And the physics all apply to bikes too.

This thread reminds me of one that I read a couple of years ago in which a guy was wondering why he couldn't keep up with the roadies on his loaded mountain bike. The debate turned to tires... wide vs. narrow... and I had to chime in. I have deleted some of the thread specific information, so pardon me if the post seems a little schizophrenic in places:

Originally Posted by USAZorro

This is the most succinct, and plausible reasoning I've seen on the topic.

Originally Posted by randyjawa

It seems to me if wider, fatter tires are faster, then every participant, in the Tour de France would be running them. But they don't. Did anyone ever wonder why? Perhaps because they are not faster:-(

Originally Posted by andychrist

Quite aside from the issues of rolling resistance and acceleration, there is yet another real world difference between wide and narrow tires: wind resistance. A narrower tire will slice through the air more easily than a wide one. Perhaps that advantage, in combination with the easier acceleration gained from the lowered mass, is greater than any that might be gained though optimized rolling resistance. Dunno, but this could at least partially explain the general preference for skinny tires among racers if such were the case. Anyway, you certainly wouldn't notice as much of a difference indoors, working out on rollers, as you would out on the road, against the wind.

Originally Posted by tru

then why are the pros going from 23s to 25s as of late?

here's the thing: the pros ride whatever stuff their sponsors give them. give them wider tires and a reason to do so, they're going to use them.

Originally Posted by andychrist

Quite aside from the issues of rolling resistance and acceleration, there is yet another real world difference between wide and narrow tires: wind resistance. A narrower tire will slice through the air more easily than a wide one. Perhaps that advantage, in combination with the easier acceleration gained from the lowered mass, is greater than any that might be gained though optimized rolling resistance. Dunno, but this could at least partially explain the general preference for skinny tires among racers if such were the case. Anyway, you certainly wouldn't notice as much of a difference indoors, working out on rollers, as you would out on the road, against the wind.