AJW2W11E

I dont think its 44 not 50. As for the PSI you're entitled to your opinion.

xroadcharlie

I use this calculator, PSI Calculator Which like many on line bicycle tire pressure calculators is based on the 15% drop principle. This is probably based on average paved streets. Gravel may be best with slightly less pressure (perhaps 22% drop), While riding along rumble strips on a mountian bike might be best at 40% drop The correct tire pressure minimizes unsprung weight which sends more energy into vertical vibration/motion. Too low pressure increases rolling resistance.

Tire construction (specifically sidewall compliance) and Tread will also effect the results obtained from the calculator. A more compliant tire might require slightly more pressure then a more rigid sidewall. If you have a deep tread, like on some hybrid bikes, Slightly more pressure might work best.

When the calculator's results conflict with the recommended pressure on the tire sidewall, I'm inclined to use one in between, Not exceeding the max recommended on the tire.

cyclezen

Like the chart! I don;t run anywhere near as many tire variations, yet... mostly 700 x 23,25,28 on road, but I will be adding a wheelset for dirt/gravel 700x32 & 35...
mtb I run only 26x2.25 and 27.5x2.25 - in both cases I'm still using tubes, so 20-22 psi frt and 25-26 pis rr... this is mostly rocky,rutted singletrack, when riding local open space I go 2 lbs more on each.
Ride on
Yuri

KerryIrons

Opinion? How about I rely on extensive research published in Bicycle Quarterly on the effect of tire pressure, tire width, casing design, etc. That's my preferred approach as opposed to "these tires feel really fast at this pressure" and other anecdotes.

blacknbluebikes 

hey, someone do us a favor... the question is "how much better?" ... has anyone seen the marginal values proposed? Like, if we set a baseline at 80PSI, one could run +20PSI or -20PSI. If you do that, how much wattage do you earn/lose by the adjustment? And, if we did have those numbers, is it enough wattage for me to care? If you tell me that my 15 mile ride could be 10 minutes shorter, that's interesting. If you tell me that ride will be 10 seconds shorter, it's not relevant - one stoplight erases that gain. Watts is watts - how much are we talking?

prj71

32mm road bike tires pumped up to 70 psi are faster than 25mm road bike tires pumped to 90-100 psi. Only for the fact that the wider tire with lower pressure absorbs the bumps better so don't have to slow down to avoid getting jarred around.

Sy Reene

You might find this article of interest. By no means comprehensive for all tire types, widths, rider weights, though directionally still informational:
https://blog.silca.cc/part-4b-rollin...-and-impedance

RChung

Watts are watts, but the actual number of watts depends on your actual weight and your actual speed; that's why we often compare rolling resistance in terms of a coefficient. Then anyone can multiply the coefficient by their weight and their speed to get the actual number.

You mentioned a 15 mile ride. Suppose it took you about an hour for that ride, so your average speed was 15 mph, or 25 km/h. Let's also suppose you weigh 80 kg, and that your bike and other equipment weighed another 10 kg, so your total all-inclusive mass was 90kg. Finally, we've seen elsewhere that even on good roads (not cobbles like one sees on Paris-Roubaix, or the rough chipseal one often sees on rural roads), there can be a difference in the coefficient of rolling resistance of around .0005 from a 20 psi difference in tire pressure.

So in this case, on a good road, total difference in watts would be right around 3 watts, on a baseline expenditure of about 100 watts. That is, the wrong pressure would cost you about 3% extra.

Is that "enough for you to care"? Hmmm. I spent the weekend with a friend with a very sensitive palate. He has a wine cellar, we drank fine wine all weekend, and he explained subtle differences in flavors in each of the wines he served. I could sorta tell a difference but I certainly couldn't identify the things he recognized as important; nonetheless, these were differences that were important to him and he cared about. For one lunch we had a beef stew that was made with an entire bottle of 2016 Chateauneuf du Pape. On the other hand, I know people who think if something isn't important to them, it shouldn't be important to anyone.

cyclezen

Great Article! Would be great to have the same test/data for a range of rider+bike weights !
I also compared the Silca tire pressure calculator with the ZIPP tire pressure calc. Silca calc comes in between 12 & 15 % hihgher than ZIpp, for same parameters, for my 700c road.
Although, both come close to each other in suggested pressure for mtb (26" & 27.5")
The ZIPP 700c calc results come closest to what I use, which are 15% lower than Silca Calc.
Ride On
Yuri

GhostRider62

It looks like -10 PSI from optimum costs 1 watt whereas +10 PSI costs almost 10 watts.

https://blog.silca.cc/part-4b-rollin...-and-impedance

ofajen

Interesting that I get essentially identical 700C figures from both Silca and Zipp calculators. That’s what I would have expected since Josh Poertner was working for Zipp when he started doing tire pressure measurements and research and he now runs Silca.

Otto

cyclezen

I get quite different. Silca : rider 150 lbs, bike 21 lbs, 700c, 25mm, worn rd surface, Rec ride , 48/52 weight dist. = 89.5 psi frt, 92 rr (I had ridden 85 frt and 90 rr after stopping racing - racing I had used 90-92 frt & 95-97 rr... on cotton tubular, silks for crits would be round 100, 105... track 110 psi)
ZIPP : 150 lbs, bike 21 lbs , 700c, 25mm,, 'Road' style, inner rim 19 & 'tube', std casing, dry road = 76.2 psi frt, 81.1 psi rr (I currently use 76-77+- frt, 82-83 +- rr, I also have v-short torso and more setback, so a bit more weight dist. to rear - relatively...)
When I'm on older, narrower wheels I go up 5 lbs for each... so 82-83 frt , 87 rr
Parameters for each calculator are different, so there is some different thinking going on with the 2 systems.
I do note a significant difference in the 'ride' (but no diff. in effort or speed) when I use my old/prior pressures as opposed to the newer, lower pressures.
Ride On
Yuri

AJW2W11E

Those studies are based on a "ideal tire" which behaves the same at 25 mm or at 44 mm . This is probably unrealistic. In fact every tire model out there has a unique characteristic behaviour curve as a function of rider weight, tire pressure, temperature , surface characteristics, tread pattern and other factors.

RChung

At some point, many of these threads come down to some version of "I don't know how to do a test and I'm a smart guy so you can't possibly know how to do such a test either."

We've looked at temperature of both the ambient air and the riding surface, the weight of the rider, the actual power of the rider, the roughness of the pavement, the tire width (though all of the tires we've tested have been 700c), tread patterns, the type of tube, and tire pressure for several different models and brands of tires. Statistical analysis can be your friend, if you know what you're doing. Then, to validate the analysis we make predictions based on what we learned, and run another experiment. If the prediction is borne out, that's good. If it's not, that's even better, because it means we learned something new.

GhostRider62

I have tested quite a few different tires under varying conditions. For my purposes, I generally do not care about the precise Crr of a tire; rather, I am interested in its relative performance against its peers. None of the factors you mention has any meaningful effect on this relative ranking although some tires perform better in the cold than others and some perform relatively better at high speed than others.

My down and dirty testing (vs using the Chung method) is a rolldown test. I have a hill with defined start (sewer grate) and roll until I stop, where I make an x with chalk on the pavement. I repeat three times per tire. The results are very repeatable. The hill is steep but short, resulting in a top speed of 10 mph and a long flat rollout. I have never seen the relative rankings change, they line up with the roller testing, and Chung testing. On a cold day, none of them roll as far as they normally roll but their ranking vs other tires is the same. A simple way to test is to have a control tire (Vittoria Speeds in my case) during any testing. You might say, why bother. Because sometimes you get a bad tire. Or sometimes you mount it poorly (rare) or maybe you are curious the effect of butyl vs latex tubes. Or you buy those expensive Hearse tires and want to see how slow they really are.

AJW2W11E

Mr. Chung, I respectfully do not concur. I've seen statistical analysis skewed many times.

vespasianus

That is really interesting. Does the silica calculator determine the optimal point "before the break point"? I wonder what it would look like at different weights - not just a single 190 lb point. I also wonder if these results could be examined using other methods.

RChung

"At some point, many of these threads come down to some version of 'I don't know how to do a test and I'm a smart guy so you can't possibly know how to do such a test either.'"

GhostRider62

Did you read the Silca blog post?

It was not about their calculator.

They measured. You could contact Josh for more detail. The very important point of that article? There is very little lost being slightly under pressured but a lot lost being slightly over pressured for the conditions (lousy roads).

I found that 100 psi was the perfect pressure for my bike and my weight using 26 mm wide (actual width) GP5000 tires. Silca says 89 psi. Low and behold, I found that my pressure gage on my pump was reading 11 psi high. According to their online calculator, their suggested tire pressure is not dropped all that much with lower weight.

GhostRider62

You have been riding for 2 full years and are on probably one of the worst tires imaginable.

Talk about skewered.

Sy Reene

It's not hard to get the 2 calculators to align much more closely, at least for a tubed setup. We don't know the unsaid assumptions each is using in their calculator. Example input changes using your scenarios above:

1. Use "thin" casing on the Zipp calculator -- I actually am not sure how a user of the calculator determines the appropriate 'thin' vs 'standard'. Using Conti tires as an example, does thin = GP5k, Std = 4Seasons, Reinforced = Gatorskin, and a DH Casing = ?? (Schwalbe Marathons or somesuch)? Likewise, since Silca's is a "Pro" calculator, is it default assuming you're using a tire meant for race-day?

2. Is your 25mm tire really 25mm when inflated? The Silca calculator asked for actual measured casing width. Likewise the Zipp calculator unfortunately (IMO) is guessing at an inflated width for the tire by asking for your rim width. For a 19mm rim and 25mm tire, I'd hazard a guess most 25mm tires are going to 26mm+ and Zipp is building in that assumption. So try entering 26mm on the Silca calculator which will lower the Silca's returned PSI targets.

ofajen

As someone who rides at least down to freezing temps, it definitely feels like my very nice Conti MTB tires get substantially more sluggish and less efficient at colder temperatures. That is of course consistent with the general behavior of rubber materials. Have you done tire measurements at temps of 40F and below? Roughly what kind of changes have you observed?

Thanks,

Otto

vespasianus

Yes. That article has couple of big things for me. The first, is that after a certain point, higher pressures have a high negative cost. Second, you could argue that up to that point, higher pressures have benefit. You could argue that that benefit is minor and a 10 PSI difference on the low side has only minor downside while a 10 psi difference on the high side has a big downside.

They state they want to look at other weights and I would like to see that data and data generated using an orthogonal method as well before I believe it 100%.

As slica also makes a pressure guide that they promote, I was wondering if they actually have taken this data into mind when they recommend a tire pressure.

GhostRider62

Ok then.

Collect that data and share it.

It is not as easy as you think

Sy Reene

Perhaps more of a factor in your circumstance is whether you're inflating and measuring your tire psi after tire and air inside is at the ambient cold temperature, or doing the inflation indoors. eg