Determining Proper Tire Pressure

In response to questions in our Tubeless Super Thread, we’re covering the topic of tire pressure. How does one decide on an optimal pressure to hit the sweet spot of comfort, speed, grip, and rolling resistance? Rolling resistance is itself a huge topic, and check out our detailed preamble to this article that explains the ins and outs.

Along those lines, we just ran a poll in our Reader Forum - in fact, a repeat poll that we ran over two years ago to see what tire pressures are being used by real people over time. We wanted to see if things had changed, as much of the industry has moved to promoting lower pressures – often with wider tires.

The results are in! (Note that this is specific to road tires… we talked gravel tires in a previous article).

The chart above shows the current state of Slowtwitcher tire pressure for racing no-draft triathlons or time trials. A few of you complained that the tire pressures we listed don’t go low enough – a fair point. However, when the original poll was run in early 2017, we didn’t think that anyone was running much less than 90psi for racing, so it wasn’t included in the poll (showing us just how much has changed in a couple years).

Let’s compare this to the older data set:

Clearly, pressures are going down. The biggest changes were 110 and 120psi usage dropping in great numbers, while 90psi-or-more bumped up significantly.

This new paradigm is emerging in-tandem with increasing tire sizes on tri and road bikes. 25mm tires are now the effective minimum, though this often materializes as a tire labeled 23mm installed to a wide rim (increasing the effective tire size – check out this article for more information.

Key Factors

You should be concerned with four key factors that your tire pressure will affect:

-Rolling Efficiency
-Ride Comfort
-Cornering Grip & Stability
-Puncture Resistance & Pinch Flats

Rolling Efficiency

Review our previous article dedicated to this topic for the full download. The short of it is that tire pressures beyond 110psi only make sense on the smoothest of roads, which can take advantage of the reduced rolling resistance at high pressure without overwhelming the tire’s ability to absorb bumps.

The image above from the Silca website does a great job of illustrating what’s supposed to happen with proper pressure – the tire absorbs the lion’s share of the road bump. It’s almost always safer to err on the side of lower pressure with respect to Crr & impedance, because the power cost of imperfect pavement is so great. I’ve taken to using the term ‘Rolling Efficiency’ from the folks at ENVE, as the easiest way to talk about this complicated picture.

Ride Comfort

Ride comfort piggybacks on the topic above, further reinforcing why you want to err on the lower side with pressure. In other words – it’s not just that there’s a cost to your power with rough pavement, but also your comfort and fatigue level (with compound interest paid over long ride distances). This becomes doubly important on aerobar-equipped bikes, because your elbows rest directly on the bicycle, compared to a road bike (which allows you to bend your elbows to help absorb shock). It can even make sense in some situations to lower your pressure beyond the optimal level for rolling efficiency, costing you a bit of power on smooth pavement, but keeping your body from getting beat up over time.

Cornering Grip & Stability

This is an area that’s not quite as cut-and-dried, but it’s generally agreed that a reasonably low pressure helps with stable cornering. If the pressure is too high, your tires can want to skip and bounce over rough patches in corners, often requiring you to slow down. A sufficiently low pressure allows you to stay planted and ride more quickly through the turn. Pressure that is too low can cause squirmy-feeling handling, and possible air burps with tubeless tires.

Puncture Resistance & Pinch Flats

Brands such as Schwalbe say that lower pressure reduces your chance of a puncture on the road. In short, the tire is more supple and able to flex around a potential puncture-causing object. I’ve never seen any clear data to support it, but the logic makes sense. The counter to this is that if you run your pressure too low, you increase your risk of a pinch flat (a.k.a. snake bite) puncture, where your tire or inner tube gets pinched as your wheel bottoms out on the road surface.

Setting Your Pressure

When it comes to actually deciding on a pressure, the main things to take into account are the (actual inflated) tire size, rider weight, and road surface conditions. It’s tough to capture all of this in one chart. Thus far, my favorite is from HED:

Note that the tire sizes shown are what’s printed on the tire label, but the inflated size changes on HED’s wide road rims (which have a 21mm internal width). For example, a “23mm” tire typically inflates to 25 – 26mm on HED’s rims – meaning that it’s effectively a 25 – 26mm tire. Check out our Tubeless Super Thread for detailed drawings from Tom Anhalt on how this works.

HED adds a footnote that says for every 20 lbs of body weight beyond the maximum on the chart, add 5 psi. For me, this lines up exactly with the pressure I ride on average road surfaces. Of course, if you weigh less than the chart shows, you'll need to drop your pressure a bit.

On the topic of surface conditions, that’s an X-factor that you’ll have to figure out through trial and error (and potentially a power meter if you want to get really geeky). If I knew a road surface was particularly bad, I’d drop the pressure about 10psi below the HED recommendations. If I knew the road was perfect (which rarely happens), I’d add 5psi.

Other Important Considerations

Finally, there are some other practical considerations that go beyond the largely performance-oriented picture we painted above.

Tubeless vs Tubed Tire Pressure

One of the benefits of tubeless tires is that there’s no inner tube to pinch flat at low pressure (however, it’s possible to pinch flat a tubeless or tubular tire with a hard rim strike). Interestingly, I see some people call for lower pressure on tubeless setups as a blanket recommendation, regardless of tire size, road surface conditions, or anything else.

Above image © ENVE

I say that this is a “yes and no” situation. I’m not the final authority on bikes, though I’ve ridden a LOT of different wheels and tires. If you’ve done a power meter analysis and determined that your optimal pressure for a given road tire size will result in frequent rim strikes on the pavement, that makes tubeless the only way to fly (or perhaps tubular if you’re old school). However, you’re also signing yourself up for potential rim damage, and still may pinch flat the tire itself. In other words, your pressure is low enough to be a financially risky proposition, so I’d recommend that you raise your pressure a bit, even if you’re on tubeless tires.

Even with standard clincher tires and tubes, my preferred pressures result in zero pinch flats. I’m about 195 lbs, and will run 25mm tires at about 85psi, 28mm tires at 70-75psi, and 32mm tires at about 60psi (these are all actual inflated tire sizes). I do not arbitrarily run a lower pressure with tubeless, nor have I had any manufacturer tell me that my chosen pressures are unreasonable. Schwalbe’s Sean Cochran said the following to me in an e-mail: “…the 75 psi you’re running in a 28c is about perfect in my mind.”

The video above shows what it took for me to get a rim strike on a 28mm Schwalbe Pro One – a sharp edge and about 45psi (note that the tire inflated to 29mm on this wheel). Higher speeds and sharper edges could result in a rim strike at higher pressure, but I’ve yet to pinch flat with tubes on 28mm tires. The only caveat is that this can be somewhat left to rider technique – and I do my best to lighten the bike or do a small bunny hop over sharp edges or big potholes whenever I can.

Front vs Rear Tire Pressure

Triathlon bikes have a close to 50/50 weight distribution, so I set tire pressure equally for front and rear tires. For road, gravel, and MTB, your weight is more over the rear tire than the front. As such, you should use a higher rear tire pressure. I normally shoot for about a 2-3% difference (say, 73psi front, 75psi rear for 28mm tires).

Tire Thickness

The thickness of your tire can affect the best pressure. In short, if I’m using a tire that’s burly and thick, I’ll drop my pressure about five percent to help gain back some ride quality. Thick tires aren’t as flexible as thin racing tires, and consequently suffer in comfort. This actually gives a slight nod to running lower pressure when using inner tubes compared to tubeless, because there is another layer of material to flex. I don’t have data to back this up; it’s just a practical recommendation from my experience as a mechanic and rider. Thin tires and tubeless tires seem to give way more easily over impacts, so I’m actually a bit more apt to bump the pressure up a tad to avoid rim damage.

Maximum Tire Pressure Ratings

It should go without saying – but I’m going to say it anyway – that you should never exceed the maximum pressure of your tire or rim, even if you think that a higher pressure is ‘optimal’ from a performance standpoint. Also, treat the lower number as the maximum (i.e. if your rim is rated to 130psi but the tire tops out at 115psi, your actual effective maximum is 115).

Rider Weight Considerations

If you’re heavier than average, you must use higher pressure. If you’re lighter than average, use less pressure. HED’s tire pressure chart up above shows this, but it’s worth emphasizing. If you weigh 130 pounds and your spouse weighs 200, you should not both be using the same tire pressure for a given tire size. Yes, that means that us heavier folks are more likely to get past the tire’s ability to absorb vibration – so you should consider a larger tire size that can be ridden with less pressure.

Frame Suspension Designs & The Future

I wanted to close with an interesting topic that I predict will become more popular in the next five years as a new frontier of performance. There is a growing number of suspension designs for road bikes, such as BMC’s MTT rear suspension, the Specialized Future Shock, and Trek’s IsoSpeed Decoupler. I have ridden all three of these, and they make a tangible difference in ride quality. This is coming from an old-school mechanic that doesn’t like mechanical complexity, but I cannot deny that some of these technologies make a real difference in comfort.

My prediction is this: The next phase of performance development will be dialing in these suspension systems so well that we can run higher tire pressure than on a fully rigid frame – so that our tires can flirt more with the break point where rolling resistance gives way to impedance. Say that your race course has 10% of its distance covered in rough pavement. On a rigid bike, you’d need to run your tires at 80psi to sufficiently smooth that out without a big power cost… but on the other 90% of the course, your tire pressure is lower than optimal (costing you some wattage). With a good suspension system, you might be able to raise that pressure by five or ten psi to get more out of your tires for the majority of the course, while the suspension can help keep ride quality and impedance losses in check for the short rough sections.

I’m not a physics expert, so I inquired with the always-helpful Tom Anhalt to see if my thought process made sense. Can frame suspension actually save us wattage that would otherwise be lost to impedance losses? Or could certain material and construction choices help for rigid frames?

He replied, “Actually, Jan Heine has a small bit of info on that. Basically, as long as the suspension adds compliance AND has lower overall damping than the human body, you should be 'ahead of the game' for a given tire width and pressure. Then again, one can always go lower pressure in the tires and not give up much rolling resistance (and one can also run wider tires to prevent tire bottoming if need be).

That test data above and my own experiences riding with the old steel fork on my '86 Bianchi (where one can SEE the fork flexing while riding, AND its performance during rough, paved descents is 'planted' and bump swallowing) have me considering having a lightweight (flexible) steel fork built up for my gravel rig to replace the carbon fork...”