benjamin163

Hello,
I was always bought up to believe a higher tyre pressure on a thinner wheel meant more speed.
With all the new data available that now seems very old fashioned.
I was watching a video just now which was extolling the virtues of a 28mm tyres on 50psi pressure.
It seems so wrong when I've been on 23mm tyres at over 100psi pressure for years. But I can't wait to try it out.
However, I'm wondering how much weight has to do with it all.
The bloke recommending 50psi was whippet thin. I'm St Bernard fat!
Is the best tyre pressure going to be different for people of different weights?
I gather that what you don't want is for the tyre to get that 'squashed' look (he called it 'deflection') but I'm guessing that deflection point must be different for people of different weights.
Does anyone know if there's a sum or some sort of test to give you your optimal tyre pressure for your weight to achieve the best roll on a normal surface?
Any help gratefully received.

Litespud

the rule I heard is that optimal pressure equates with 15% reduction in tire height between loaded and unloaded. Probably easy to measure with an assistant

JohnDThompson 

It turns out rolling resistance in bicycle tires is largely a function of hysteresis losses in the tire casing. For any given tire width and casing design, a higher pressure will result in lower hysteresis losses. Conversely, a wider tire can be run at lower pressure with the same hysteresis losses as a narrow tire at high pressure. The advantage of running a wider tire at lower pressure is that it can give greater comfort, better control, and higher resistance to pinch-flatting than a narrow tire at high pressure. As a heavier rider, you will want to run at higher pressure than a light rider, but a wide tire will still give you more comfort, better control, and higher resistance to pinch flats than a narrow tire.

Sy Reene

I found this pressure calculator recently -- and think it's definitely one of the best available currently -- worth it to give your email/contact name to access the Pro version though. FWIW, 50psi seems way too low unless setting up tubeless for gravel riding conditions (in the below calculator 50psi comes out as result for category 3-4 gravel conditions and with an approx. 150lb rider).
https://info.silca.cc/silca-professi...ure-calculator

Maelochs

I find that low pressure in any kind of tire increases, not decreases, the possibility of pinch flats.

I am built like a whippet.. on the inside. On the outside I look more like an elephant. And I cannot run the kinds of tire pressures a lot of riders recommend ... my mass simply compresses the tires too much. I ride 23s at 110-120 psi, and 28s at 95-105 or so ... less up front. If I try these "at 70 psi the comfort is unimaginable" recommendations, I end up with snake bites after any hard bumps.

I make eyeball calculations somewhat similar to #Litespud's 15% compression standard ... if I see the tire bulge too much when I just sit on the bike, I know that the first bad set of railroad tracks will have me walking ... or actually, cursing as I change the tube and inflate to the proper fat-man pressure.

But ... I have a pair of bikes that weigh almost the same, one with 23s and one with 28s .... neither feels sluggish or sticky compared to the other.

People tell me that you can safely run a lot softer with tubeless ... 70 psi for road use is the lowest I had heard ... but I can pretty much guarantee that I have 100 pounds on most of the people recommending that, and I would be popping sidewalls at rail crossings at 70 psi.

biker128pedal

That’s a nice calculator but the recommended pressures are low for realistic roads. I’d pinch flat. Do you have the Pro version? That may have a more realistic number if the road selection is enabled.

phughes

No one said that lower pressures decrease the likelihood of pinch flats. What they said, is that you can run lower pressure in a wider tire, than you do in a narrower tire, and still not get pinch flats. You still need to run enough air in the tire to prevent pinch flats, but with a wider tire, the pressure to accomplish that will be lower, and you will have a better ride.

As for the pressures you are running, they seem to be in line with most calculators, depending of course on your weight. For 200 pounds fully dressed rider/bike combinations, on 23mm tires, and a weight distribution of 45% front, 55% rear, one calculator shows 95 psi front, and 115 psi rear.

Sy Reene

I have access to the pro version, but it generally would provide a higher PSI if you choose a better road surface -- the highest PSIs get returned for Track, followed by New Pavement. The simple calculator is likely using the "Worn Pavement / Some cracks" surface type. I might guess that if you're seeing too low of a returned value, it's possible the tires you're running are too narrow for your weight? That is, you're running a high PSI to avoid pinch flatting, but actually yielding a slower rolling tire as a result.

Also, the calculator doesn't differentiate between tubeless or tubed tires, so value may be possible without pinch-flatting running tubeless but not promising if running tubes.

phughes

https://silca.cc/pages/sppc-form?__h...a-c25cf17e117f

Kapusta

That came really close to what I run in my tires.

Kapusta

Talking about what pressure to run without knowing how heavy you are is like trying to fit shoes without knowing how big your feet are.

Any suggestions you hear on what PSI you should run that do not take tire size and rider weight into account are useless and should be ignored.

Maelochs

[QUOTE=phughes;21463104]No one said that lower pressures decrease the likelihood of pinch flats. What they said, is that you can run lower pressure in a wider tire, than you do in a narrower tire, and still not get pinch flats. I have a lot of cognitive issues ... too much fun in the '70s .... and the '80s ... and I enjoyed the heck out of the '90s ... and I guess I tore it up in the early part of this century too ... mostly relying on the memories of those who can remember ... I was having too much fun.

So, possibly, those words don't say what i think they say ......

Anyway, yes ... wider tires at lower pressure are better in about every way except aero ... and apparently so long as the rim and tire are the same width, there isn't much loss there either ....

biker128pedal

Guess I need to go tubeless. Have the wheels for it. Would like 28mm but 25mm does not fit my old Madone.

benjamin163

This is great, thank you for sharing.
I'm no elite athlete, so I think using this as a general guide is perfect for my needs.

c0rbin9

I have heard this but don't quite understand it because wouldn't it mean that a giant fat tire at low pressure has the same rolling resistance as a skinny tire at high pressure? And if so, why would anyone use skinny tires if they have the same resistance with less comfort? Is it just because of the weight?

Russ Roth

I checked based on what I like to run and it seemed to be in the right area though I tend to make the front 5psi lower and not 2psi like it said.

I wish I weighed 200lb let alone me and the bike and I run my 23s at 90psi front and 95-100psi rear. I used to run at 110/115 but the ride just wasn't as good. For a race I can bump it back up there but I don't feel any slower running 90/95 and the ride is much nicer. Also haven't pinched flat in years.

Something gets lost somewhere and wider tires to me have always felt slower, its why I've stuck with 23s, just didn't like the sluggish feeling I always had when running 28s. Maybe the butt-o-meter is wrong but I still like feeling faster even if it isn't and I haven't found 23s to be uncomfortable unlike the 18s I rode in the 90s that also always felt slower to me.

Maelochs

After a certain point the tires start to weigh a lot more, and are less aero (which is minimal except at speeds above about 18 mph, I hear.)

You can get a 38- or 45-mm tire, but you generally cannot get a light, supple tire in that size. most of those are made for gravel or dirt, and even the slicks are going to be heavy.

As I understand it, with a really hard tire (necessary to prevent pinch flats with a really narrow tire, as the overall air volume and thus shock absorption is low) the tire actually chatters over the road surface, bouncing clear and slamming down in tiny micro-chatters. This wastes energy. A softer tire will absorb the chattering and stay on the road ... but if it doesn't have a larger air volume, it will pinch-flat a lot.

Theoretically you could run a very tall very narrow tire at lower pressure and have enough volume to absorb shocks, but then you would run into rigidity issues when cornering---imagine a tire 23 mm wide bit 50 mm tall---plenty of volume to absorb the big hits, but it would flop all over the place.

Theoretically one could build a superlight 38-mm tire with really thin sidewalls, really thin tread, and a very low aspect ratio (38 mm wide and maybe 20 mm tall) and get low weight, sufficient rigidity for high-speed handling, and maybe high enough volume to survive pinch flats ... but it might have too severe a profile, sort of an "edge" like a square-edged MTB tire (great in the back, Terrible up front .... ) and get really low rolling resistance .... but why? at some point you have width for no purpose. it is possible to run a narrower tire (28, say) at a low enough pressure to mellow out rad chatter, so why go wider?

Usually people who run really wide tires (38 or whatever, are riding mixed surfaces or mostly gravel, and minimizing rolling resistance on pavement isn't an issue---traction on softer or shifting surfaces is key, so the tires are designed for that.

Kapusta

At some point aero and weight become an issue.

However, there are a few very light and very supple larger tires out there that are indeed very fast at low pressures.

I’ve been riding Compass tires (now called Rene Herse) in 32-38mm sizes for a few years and would never even consider going smaller than 35s again. Even if I never saw a dirt road again, I would still be riding the 35s.

greatscott

If you calculated your weight clothed ready for riding and your bike's weight ready to ride, and take that total you shouldn't be pinch flatting unless your gauge is inaccurate, in fact the pressure recommended should give you at least a 10 psi range from under to over, but even 10 pounds under you shouldn't be pinch flatting unless maybe you're running on some very rough roads then you do need to up the pressure.

pdlamb

The pressures the (lite) calculator give are about the minimum before I notice the tires are squirrely. I'll take another 10-15 psi, please.

Just out of curiosity, how many people regularly stop and adjust tire pressure mid-ride when you hit a different surface?

asmack

I'm a big guy -- 266 lb -- and always ran about 100 psi on 700x38 tires. Even then I still had an occasional pinch flat. Since going tubeless I've run at 60 rear and 50 front and haven't had a flat in two years. Based on the review at rollingresistance.com I sprang for the Schwalbe Marathon Almotions and have been completely satisfied. It appears to be true, rolling resistance is all about the sidewall construction and minimum required pressures are all about preventing pinch flats in tubed tires.

Josh206

I weigh 250 and was advices to run my tires at 90 psi...

adalah

interesting thread, I had the same question

DomaneS5

I'm 187 lbs... and I run 90psi on the front and 100psi on the back.

Kapusta

For what sized tire?