PSI vs tire width?

[etu]

If the contact patch is determined by the psi and not the width of a tire, will a 700x33 cx tubular at 30psi ride similarly to a wider gravel tire, e.g. x'plor 700x40, at the same psi? I know the shape of the contact patch is different, but not sure how it would affect grip, rolling resistance, etc.

[spoonrobot]

Short answer is no.

What's the sag for each tire?

If you run a 33 and a 40 tire at the same psi for the same surface conditions and the tires are the same sidewall stiffness/suppleness either one tire is going to be overinflated or one is going to be underinflated.

Rule of thumb is that wider tires require less pressure than narrower tires to give the same sag percent, the defacto measurement of contact patch. This assumes the sidewall stiffness/suppleness is the same.

[ultraman6970]

Unless i got this wrong in the translation... the rule of thumb defies boyle's laws... ???

[cachagua]

If the contact patch is determined by the psi and not the width of a tire...

Huh?

Put 30psi in your cx tire, and 30psi in your car's tire. It will not be the psi that determines the difference in contact patch.

This example exaggerates the effect for clarity, but it's the same even with two fairly similar bike tires. Several different factors in play, as described above, but width is definitely one of them.

(Best way to compare the two set-ups is ride 'em.)

Unless i got this wrong in the translation... the rule of thumb defies boyle's laws... ???

Intriguing, say more?

[ultraman6970]

Well... less volume - more pressure inside (thin tubulars)... but from what I understood... more volume tubulars (extra wide cx tubulars) you have to put less pressure?? that contradicts Boyle's laws for gasses.

[jamesau]

Boyle's law does not apply in this analysis (because the number of molecules in one tire will not necessarily be the same as the other tire). Your higher volume tire may run at lower pressure than a smaller volume tire while having more air (as moles of air) in the tire.

[spoonrobot]

Here's a really interesting link: http://flocycling.blogspot.com/2014/11/flo-cycling-why-do-you-use-less-tire.html

[parco]

No one has mentioned how the weight of the rider will affect this.

[MattTuck]

Huh?

Put 30psi in your cx tire, and 30psi in your car's tire. It will not be the psi that determines the difference in contact patch.

This example exaggerates the effect for clarity, but it's the same even with two fairly similar bike tires. Several different factors in play, as described above, but width is definitely one of them.

(Best way to compare the two set-ups is ride 'em.)



Intriguing, say more?

No one has mentioned how the weight of the rider will affect this.

Exactly. If you put the weight of a car on a bicycle, the tires would have to deform enough such that the number of square inches of contact area (multiplied by the PSI) equaled the weight of the car, I think? Been a long time since high school physics.

[etu]

Here's a really interesting link: http://flocycling.blogspot.com/2014/11/flo-cycling-why-do-you-use-less-tire.html

thank you for this link, very informative.

however to make this discussion less esoteric and more practical and to really get to my point... I am reviving my gravel bike and I am debating whether to go tubular since nice clement CX tires can be had for @$50-60 through UK vendors. unfortunately they don't go wider than 33. the current frame can only handle @40 in the rear. So wondering whether to go with wider clinchers or narrower but more supple tubulars. recent convert to tubulars for road and I am really enjoying their ride.

[spoonrobot]

No one has mentioned how the weight of the rider will affect this.

Rider weight is just beating around the bush because you also need to know bike weight. Tire sag is what you want to talk about.

15% is 15% no matter what the weight or PSI. Or whatever percent you care to choose - I like 15% for road and hardpack gravel with small stones. 20-25% for rough gravel and mountain biking.

OP, for gravel, step into the modern era and go tubeless 40mm tires. :beer:

The difference between 33mm and 40mm on the gravel around here is significant as our gravel is generally rough and loose. I hated 32mm tires, even the supple ones, but really like 40mm. YMMV of course.

[cachagua]

Debating whether to go tubular since nice clement CX tires can be had for @$50-60 through UK vendors. unfortunately they don't go wider than 33. the current frame can only handle @40 in the rear. So wondering whether to go with wider clinchers or narrower but more supple tubulars. recent convert to tubulars for road and I am really enjoying their ride.

If you love road tubulars, chances are you'd love cross tubulars...

[Ken Robb]

I have more experience with car tires than bike tires but, in car tires if the weight supported by a tire is the same, changing from a tall narrow (70 aspect ratio) tire to a wider tire with shorter sidewall (40 aspect ratio) at the same psi the shape of the contact patch will get shorter and wider but the area will remain the same. The change in handling on clean dry pavement is crisper turn-in, firmer ride, more tendency to follow grooves in the road. When driving through standing water or snow the narrower footprint would have better traction as the tire would cut through the water, snow to get down to the pavement.

My observations riding bikes tells me that fatter tires float better across soft surfaces like sand/gravel ( I have almost no experience riding a bike in snow) while skinny tires tend to dig in, swerve, and sometimes just STOP. The softer the surface the fatter the tires that I prefer.

For instance I happily ride my Hampsten Strada Bianca by MOOTS on 700x25 tires on pavement and it's like a pure road bike. It's ok on hard pack but when the hard pack turns to sand/gravel I get sweaty palms wondering if/when the front wheel will dig in and develop a mind of its own. OTOH when I run 700 x30 or fatter tires it's ok on pavement but a LOT of fun in soft or mixed surface trails. Today it has 700x30 Michelin cross tires that measure about 33mm on Open Pro rims.

[Gummee]

If you love road tubulars, chances are you'd love cross tubulars...

Up side to tubulars: no pinch flats and a nicer ride than even tubeless.

Rode Devil's Backbone on some LASes Sat and had a blast. I'll probably ride the same tires at Hilly Billy Roubaix in June too. ...unless it's raining, then I'll be on my rigid Superfly 29er

M

[palincss]

Exactly. If you put the weight of a car on a bicycle, the tires would have to deform enough such that the number of square inches of contact area (multiplied by the PSI) equaled the weight of the car, I think? Been a long time since high school physics.

Put the weight of a car on a bicycle and it will be squashed flat.