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air pressure

Old 08-04-20, 04:01 PM
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shoff535
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air pressure

Just curious, what air pressure do people run in their rims/tires (hooked rims/clincher tires), in particular if they are vintage? Case in point, I have mid 90's Campy Vento rims with Vittoria Corsa G+ tires which have a recommended psi of 100-145psi. I typically run them with 90-100psi, but was told recently that I ought to run them closer to the max pressure of 145 in order to prevent rim damage (tire deflection) due to pot-holes on the road (I got a snake bite flat after running over a pavement imperfection). In short, can/do vintage rims tolerate that high of a pressure? Finally, while I think the Ventos are pretty stout, what about other rims (double walled) Ambrosio or Torrelli Triumphs. How much does spoke number or lacing matter??

Thanks,
SteveH
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Old 08-04-20, 04:59 PM
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You get least rolling resistance if your tires compress about 15 percent at road contact. Tire pressure should be a function of rider's weight, plus any cargo typically carried, and should generally be 10 PSI or so higher in back than in front. There is no one-size-fits-all number.

On my 700Cx28s I typically run 90 PSI front and 100 back. That's about as high as I care to go.

Having said that, I do go with the rated maximum of 65 PSI on my mountain bike (26" x 1.95").
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Old 08-04-20, 05:02 PM
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I use a calculator based on total weight and tire width.

Bicycle tire pressure calculator
.
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Old 08-04-20, 05:04 PM
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+1 to using a bike pressure calculator. Once you've done that, raise or lower the PSI by 5 every ride to find the sweet spot.
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Old 08-04-20, 08:30 PM
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My go-to tires are Vittoria Open Corsa SC and CX "open tubulars"; pressure range on the sidewall is 115-145psi. I run them on 1990s Mavic Open Pros at 125psi and the ride is sublime

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Old 08-04-20, 09:01 PM
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Im 175 lbs, bikes are 25-30 lbs with what Im typically carrying. Both have modern 23mm wide tb14 rims, same Michelin Airstop tubes. I based the following on Frank Bertos 15% drop graph thats included in that blog post.

The RH 28mm are inflated to 80f/85r, topped up every week if Im riding that one regularly. So the average pressure is probably about 5 psi less. Notably, these are on my rando-bagged Miyata where the front load is greater, so I tend to be more careful with the front pressure.

The RH 32s on the Marinoni use 10 psi less, topped up every other week. They seem to lose less pressure, and tolerate low pressure better.

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Old 08-04-20, 09:20 PM
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I can't say about vintage. I suppose much of it has to do with whether or not the rim is hooked & whether or not the bead is intended for "hookless" rims or not.

Also, psi is literally "Pounds Per Square Inch." A larger tire is going to have more square inches of surface area. So the rim will be under a considerable amont of additional stress with a larger tire pumped up to the same pressure as a small tire.

A lot of rims will have inflation limits available. Seek out if that information is available. For steel hoolkess rims & vintage tires 1&1/8 to 1&3/8 tires I wouldn't go more than about 80 psi. There just isn't any bead to keep the tire on the rim & the ride would just suck above that anyways.

I run my tubed 700x25c tires at 100 psi, & tubeless at around 90.
My
700x38c tubeless tires at no more than 50 psi.
559x42 tubed at no more than 65 road, or 55 off road just to avoid pinch flats. It rides rough as h-e-double-hockey-sticks.
& 2 inch wide tubed mountain bike tires at 35 psi (the minimum listed)

The minimum pressure has as much to do with preventing movement & wind-up of the tube in relation to the tire as it has to do with preventing a pinch flat. With tubeless this is much less of a concern.

Ideally, the pressure you run should be the highest pressure that also provides the lowest rolling resistance for the surface you intend to ride on. For rougher surfaces that will be a lower pressure on wider tires in order to best preserve forward momentum.

Larger tires do provide for lower rolling resistance than skinny tires--at the same pressure--but that does not take account the surface being ridden on or the ride quality. Sure a 700x47 (or whatever) tire at 120 psi will roll better on a steel drum than a 700x19c at the same 120psi but the trade off is ride quality & energy loss due to deflection to any direction other than forward at the first irregularity in the surface. Likewise the 700x19 (old school, common size) will need a much, much higher pressure 140+ to avoid pinching, or an unduly large high rolling resistant contact patch to conform over rough concrete or gravel.

One trade-off of skinny tires is is the lack of height between the rim & the road will mean greater flat risk should the pressure be lowered to something more comfortable. Or, conversly under higher pressures the increased tension on the threads of the casing leads to easier cuts & punctures where a tire would normally conform over a sharp bit.

Skinny tires have a lot more risk & very little upside over normal road surfaces.

Tire pressure is a game of competing nominals.

...And for what it's worth, not many people are saying that wider/lower pressure tires are significantly faster, they're saying they are just "not slower" & they generally ride better & mostly don't flat as much...all of which depending what you are comparing against, of course.

Last edited by base2; 08-04-20 at 11:02 PM.
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Old 08-04-20, 09:31 PM
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145 psi?

Nutso

My riding partner is also of the impression more is better. I have a good 50 lbs on her, and she runs her 700x25 Gatorskins at 115 or so.

I run my Vredestein 700x25 Gran Fondos at 95F/105R.

My tubeless 700x28 Schwalbe Pro 1s run 85F/95R. So smooth I often think I have a flat.

I do a fair bit of descending and the confidence in that psi was slow coming.

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Old 08-04-20, 11:29 PM
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I started following this Psimet's tip of the Day. So around 84 psi front, around 94 psi rear.
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Old 08-05-20, 01:44 AM
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Originally Posted by shoff535 View Post
Just curious, what air pressure do people run in their rims/tires (hooked rims/clincher tires)
What's the actual inflated width of your tires, what's your bike+rider weight, and what are your road surfaces like?
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Old 08-05-20, 03:46 AM
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[QUOTE=shoff535;21623988]Just curious, what air pressure do people run in their rims/tires (hooked rims/clincher tires)/QUOTE]
I run my tires at 100psi...that's it. Doesn't matter which bike, which tire, which clincher rim.
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Old 08-05-20, 04:51 AM
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@schoff535 Around 200 lbs and run same tires, 23mm at 135 r, 125 f. best ride at these pressures for me.
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Old 08-05-20, 05:43 AM
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Originally Posted by shoff535 View Post
Just curious, what air pressure do people run in their rims/tires (hooked rims/clincher tires), in particular if they are vintage?
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Old 08-05-20, 06:03 AM
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I’m certainly vintage, but I’m not sure how that would affect my chosen tire pressure.
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Old 08-05-20, 07:24 AM
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I use a combination of the tire preasure Indicated using the "Psicalculator.com" recommendations and the makings on my 50mm tires...40 - 65 psi.

According to the calculator I only need about 30 psi in my front tires. But I feel better getting them closer to the minimum recommend.by Kenda to start. So I inflate the front to 35 psi and the back to 47 psi with a 40/60 weight distribution and let them drop over a couple weeks to perhaps 30 psi front and 40 rear.

It seems like when freshly inflated to my full preasure targets, The tires roll much easier, although I'm sure some of that is perception due to a slightly firmer ride.

These are cheap, wide, Multi-surface tires, So it's understandable higher pressure may reduce rolling resistance.

On a road bike with 23 - 25 mm tires over inflating them (for a 8% drop vs 15%) has been proven to increase rolling resistance on smooth surfaces where compliance still plays a roll. Some even suggest a fine tread also increases speed, But probably won't last long.

Actual Tire drop test measurements with full weight to achieve a 15% drop is probably the most accurate way to inflate tires for general road use because even the same type tires will have different sidewall stiffness. But I think the calculator will provide a good baseline for 99% of us, Which we can fine tune to our liking. I believe these recommendations are for 700c wheels, so I increased them slightly to compensate for my 26" tires.

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Old 08-05-20, 07:53 AM
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Hmmm, not sure what to conclude, on one hand, I've been riding more or less to the Psimets rule.. except I have generally run the same psi in both front and rear. Based on the other calculator, my rider/bike weight is about 160 (140+20) so based on these parameters my optimal pressure should range from ~80 in the front to ~120 in the rear. I suspect my problem was a function of running a set of michelin 20mm tires and hitting a pavement imperfection. The front survived just fine (it was at ~100psi), the rear according to the calculations could/should have been closer to 120 (it was also at 100). Regardless, poor road conditions and a fairly high speed don't always end well.
Thanks for all of the comments and the links, it all really helped. I'm definitely going to give the asymmetrical psi approach a try, with 100/120 front and rear.
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Old 08-05-20, 08:21 AM
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Im riding a significant portion of the time on crushed stone and hard pack, so Im running lower pressure than those calculators indicate. About 200 lbs total and on 700x32s I run as much as 55 psi front and 57 psi rear, sometimes slightly lower. As much as 35 psi front and 37 psi rear on 26x2.0, sometimes slightly lower.

Also, Id suggest more nearly equal front/rear pressures than these guides suggest if you do either of the following:

1) use your brakes, or
2) ride out of the saddle.

Both of these will change the weight distribution towards the front, and bike performance will benefit from having front tire air pressure more like what you have in the rear.

Otto
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