Tire pressure: A definitive guide? (Road)

One of the cycling issues I’m kinda frustrated with – especially given the amount mathematical brain power orbiting around and within cycling – is there is no, single calculator that can give you an optimal front/rear tire pressure recommendation for asphalt based on tire width, internal rim width and weight – for both tubeless and tube-d.

For instance – why can’t I give someone the following data:

Rider: 175 lbs.
Tire size: 700x28
Internal rim width: 21mm
Tubes: yes

Given that, should I be able to a F/R tire pressure rec? Like, Rear-93psi/Front-80psi

The trial and error method is fine – and I’ve seen many wheel-specific guides to psi, but there seems to be quite a bit of variance in all of the one I read. But honestly – as noted by @Nate_Pearson and others on the podcast – harder feels faster.

Anyone else struggle with this?

We discussed some of this stuff a while ago @batwood14. There’s a couple of articles in there you might be interested in…

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The latest (2nd) Marginal Gains podcast talks extensively about rolling resistance and there aren’t too many people who are more informed on the subject than Josh Poertner

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I’m listening to this podcast – literally – right now :slight_smile: That is what inspired the post . . .

None of his info (at least so far) has taken into account ‘internal rim width’ - which is a significant aero component…or so I’ve been told.

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The actual pressure you’ll need will depend on the quality of your asphalt. If you’ve got a perfect new road, run higher. If you’re running on an old janky stretch of road, run less. It also matters if you care more about max cornering speed, or max straight line speed. Most of the time, people seem to be asking about straight line speed, because it’s easier to calculate. But for some riders/courses, cornering grip is at least as important, if not more so.

I would be hesitant to rely too heavily on feel, because bumpy sketchy rides can feel faster, while actually being slower. Smooth and easy feeling can actually be faster in some circumstances. Whenever practical, if you can compare actual times over a section and compare, you’re going to be better off.

I’m also under the impression that the specific tire you use will impact the results. Different casings, tread patterns, tubes/tubeless, the age of the tire, the actual vs advertised dimensions, etc. all play a role, in addition to just pressure and width.

I would love to see a comparison between otherwise identical setups, but over a smooth and rough set of road, to really quantify how much of a difference there is at various setups.

External rim width might be the more important factor for aero dynamics?

There’s a good graphic in the thread I linked to…

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As @roflsocks says there can be no right answer go any given setup as a large part depends on the surface being ridden on. A higher pressure would be better on a track surface than rough tarmac even with the same tyre/tube/rim combo.

Aero is a different matter. A good rule of thumb is that the rim should be at least 105% the width of the tyre for recapturing airflow.

Another link from Josh Poertner is worth a read and also part of the series of articles linked by @oggie41 above.

The rule of thumb as I understand it for road racing is if you’re getting pinch flats because of how little pressure you’re running… you’re too low. Otherwise you’re too high.

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Try these pressures. It works for me.
Paraphrasing Josh Poertner " Think smooth not fast when evaluating tire pressure…"
A hidden benefit of lower pressure is increased comfort which can contribute to more speed.

Max

Have you tried the MyMavic app for iOS? It has a tyre pressure calculator.

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I’d also suggest the tire wiz app from quarq. A tire wiz isn’t needed for you to be to put in your weight, bike weight, and tire width to have it suggest a front and rear pressure. It’s what I’m going to try this year.

My question in all of this about lower pressures is where do manufacturer minimums come into the equation, if at all? Why do the manufacturers advise a minimum? Should it be taken into account when selecting a lower operating pressure or is it to be ignored? That’s something I have yet to hear a clear answer on, whether from silca, trainer road or others.

I have a few questions I want to add here that I’ve been researching for a while with no resolution.

  1. It seems that ideal tire pressure is based on everyone’s feel. There are a number of calculators, and I use a mix of the figures from this one: http://www.dorkypantsr.us/bike-tire-pressure-calculator.html but I’ll see people say go as low as possible until the tire feels a bit…off. I think I know what that means, but I’m not sure, and testing a tire to go low as possible seems dangerous - you have to push the tire to know when the pressure is too low right, and the more you push it (high speeds, hard turns)…

It’s all very subjective and I don’t know if I’m doing the right things. I am fairly light (65KG) so I don’t ever need to run (25 and 28mm) tires at 100PSI so perhaps I’m lucky here.

  1. Related to the above, people agree that tubeless lets you run lower pressure. But how much? All calculators I’ve found are for clinchers, I believe. Am I just testing these out until I “think” it’s good?

  2. Should tubular tires be at the same pressure as clinchers? Technically, these are tires with tubes in them.

  3. On Weight Weenies, I read someone saying that tire pressure should be based on the actual width when fitted on the rim, not spec width because the same tire on a wider rim has higher air volume. Thus, if I have a 25mm tire that stretches out to 30mm on a wide rim, I can use the tire pressure suggested for a 30mm tire. Is this true? I can’t wrap my head around this - a big bag should hold more air than a smaller bag no matter how how wide you make the smaller bag when you put air into it, right? If it’s true, that means I can get the comfort of a larger tire with the aero and weight benefits from a smaller tire if I put the tire on a wide rim and the tire is stretched out.

Its been a few weeks since I listened, but I think in the podcast Josh brings up an important point. While the static weight distribution on a road bike may be skewed towards the rear, when it actually matters ie descending or hitting a big pothole the weight shifts to the front more, so I’ve be wary of running too low of a pressure on the front following those guidelines. Another option which I’ve used in the past is to stagger tire sizes, 23mm on the front where aero is more important and 25mm om the rear, then run the same pressures