Myth 11: Rear tires should run at (significantly) higher pressure

To celebrate Bicycle Quarterly‘s 15th anniversary, we are looking at myths in cycling: things we used to believe, but which we’ve since found out not to be true. Today, we explain why your bike’s weight distribution does not directly translate into your tire pressures.

We are partly responsible for the myth that front tires should run at significantly lower pressures. When we first started researching tires, we published Frank Berto’s tire pressure chart, which lists inflation pressures to achieve a ‘tire drop’ of 15% with average tires. That pressure depends on the width of the tire and on the load on the wheel.

Most bikes carry roughly twice as much weight on the rear wheel as on the front (above). So we reasoned that it makes sense to inflate the rear tire twice as hard as the front one. Except it doesn’t work that way.

During hard braking, the entire weight of the rider is on the front wheel. Now the front tire carries three times as much weight as it usually does. If this isn’t factored into the tire pressure, then the tire can collapse during hard braking: Suddenly, the sidewall no longer holds up the tire. The tire loses the ability to transmit the forces from the road to the bike – braking and steering are seriously impaired. It’s like riding with a flat tire.

In recent years, the potential for trouble has increased as we now run supple tires at lower pressures. Back in 2010, when we published the chart, we inflated our 23 mm tires to 120 psi. Running the fronts at 80 psi was fine, since the stiff sidewalls of the tires most of us rode helped with holding up the bike.

These days, many of us are on supple 42 mm tires inflated to 35 psi. Dropping the front to 23 psi is fine when rolling along, but during hard braking, the sidewall will collapse.

The solution is simple: Use Frank Berto’s chart (above) to calculate the optimum tire pressure for your rear wheel, based on the weight distribution of your bike. But for the front, assume that 50% of your weight rests on the wheel, even if the real number is less. In our experience, that will prevent the tire from collapsing.

When you are just riding along, your front tire will be a bit harder than necessary, and you’ll lose a little comfort, but it’s better than risking problems during hard braking.

If you use a handlebar bag, that puts more weight on the front and works toward equalizing the weight distribution – a randonneur bike’s weight distribution is 45:55. This means that you don’t have to overinflate the front tire by much.

If you carry a heavy front load, your weight distribution may be heavier on the front than on the rear. In that case, you obviously want to inflate your front tire based on the actual weight it carries.

In any case, the pressures in the chart are just a starting point for your own experimentation. We’ve found that these pressures work well for supple tires and for riding on pavement. On gravel, you’ll want to reduce your pressure (above), but not so much that you bottom out frequently.

This week’s myth shows how our understanding of bicycles continues to evolve – and also why on-the-bike observation is more important than theoretical reasoning. What seems right in theory often overlooks factors that are important on the road.

Further reading:

  • The original post from 2010 that discussed the science of determining your tire pressure.
  • Other posts in this series:
  • Myth 1: Wider tires are slower
    Myth 2: Titanium is lighter than steel
    Myth 3: Fenders slow you down
    Myth 4: Stiffer frames are faster
    Myth 5: An upright position is always more comfortable
    Myth 6: Tread patterns don’t matter on the road
    – Myth 7: Tubeless tires roll faster
    – Myth 8: Modern components are lighter
    – Myth 9: Fork blades don’t flex
    Myth 10: Stiffer forks steer better
    Myth 12: Disc brakes work better than rim brakes
    Myth 13: Leaning without Countersteering
    Myth 14: More lumens make a better light
    Myth 15: Marginal gains

 

About Jan Heine, Editor, Bicycle Quarterly

Spirited rides that zig-zag across mountain ranges. Bicycle Quarterly magazine and its sister company, Compass Cycles, that turns our research into the high-performance components we need for our adventures.
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23 Responses to Myth 11: Rear tires should run at (significantly) higher pressure

  1. Johan Larsson says:

    Why not put an optimal pressure for the actual load in the back tire, not making it a bit too soft? And then use a safe pressure in the front, which means calculated for half your weight?

  2. Dr J says:

    I think you’re just stretching it here. Arguing that the rear tire should not run at higher pressure because the front tire may collapse under hard braking makes no sense at all. If anything, it simply says that the front tire should not run at too low pressure. That’s all.

  3. the coasting frenchman says:

    I remember a few years ago, when I told my father-in-law that the front tire should be inflated much less than the rear after studying Frank Berto’s chart ( I actually came across your blog after discovering the chart on Sheldon Brown’s), he told me that it made little sense to him; he was born in 1933 (and died two years ago), and he had been an amateur racer, and then taken part in many “cyclosportives”, and when we rode together, despite the age difference, I was not the one who needed to slow down and wait for the other; basically he said that when racing, especially in criteriums where you needed to brake hard and take sharp corners, you needed the front (tubular) tire to be inflated enough that the bike would react immediately, and not risk having the tire come off altogether. Apparently he was right on that. I also tried to convince him that wide tires could be as fast as narrow ones, but I unfortunately didn’t have enough time for that …

  4. Derek says:

    We need an updated Berto Pressure chart with 42/45/48/55 tire widths!

    • Berto’s chart was done in the days of narrow tires and stiff casings. He gave me his original data… Most tires back then were quite stiff – the one somewhat supple tire Berto tested, the Michelin Hi-Lite, actually had a much larger tire drop than all the others. Nonetheless, we’ve found that his values work well for supple tires, even though the actual tire drop may not be 15%.

  5. Andy says:

    Is collapsing the front tire actually an issue people encounter? This isn’t something I’ve ever experienced, perhaps because I’m a cautious descender and generally managing speed given the conditions to avoid sudden issues like that.

    • Absolutely. You’ll experience it if you have a flat on the front and don’t notice it. It’s actually more of a problem with stiff tires, because the sidewalls hold up the bike, but when they collapse, they do so suddenly. And as they bulge outward, they offer less resistance, so it’s like a negative-rate spring (which gets softer the more you compress it).

    • Matt Surch says:

      Yes, certainly something I experience, as do folks I ride with.

    • Larry says:

      It reveals itself more if you are decelerating AND turning. It’s also exacerbated by running too narrow a rim so that the tire’s cross section is more lightbulb-shaped instead of horseshoe-shaped; the narrow “neck” collapses. This is perhaps a bigger issue in the MTB world, though nice wide tires are now more available for road/gravel riding, thanks largely to Compass (so watch your rim/tire pairings).

      Jan, I appreciate your point about stiff-sidewall-collapse — it was a spooky factor in a recent MTB ride with a too-narrow rim. My experience was like the stiff-tire casing’s spring rate as it collapsed — VERY NEGATIVE!

      • When stiff tires on wide rims collapse, it’s more dramatic, because the ‘horseshoe’-shaped sidewall holds up more of the rider’s weight. When that tire starts becoming ‘lightbulb’-shaped, then the spring rate decreases rapidly as the tire deforms. So there is almost no stopping it.

        With supple tires and/or narrow rims, you always run higher pressures, and it’s mostly the air that is holding up the tire. So the change in spring rate is less as the tire flexes.

  6. John C. Wilson says:

    Here’s a cite to experimental testing of the g forces bicycles generate on braking:
    http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.466.2501&rep=rep1&type=pdf
    Testers were able to get about 0.5g without taking crazy risk. When all the weight of the bike is on the front under hard braking the weight supported is 150% of the static weight of bike and rider.

    Acceleration g’s were also measured but did not amount to much. They didn’t test kilo riders who can do 0-60kph in three seconds.

    On limiting climbs where the rider must expend effort to keep the front wheel on the ground 100% of the weight of bike and rider is on rear tire.

    As a practical matter tire inflation must be sufficient to keep the bicycle under control when the bike is used to the limit. Experience shows that the normal pressures cyclists adopt covers the extreme cases pretty well. Now that some riders are experimenting to see how low they can go, Jan’s caution about collapsing sidewalls is timely.

  7. Matt Surch says:

    I have come to the same conclusion as you, Jan, albeit from another angle. Since I grew up mountain biking, we were able to start lowering front tire pressures as front suspension came in. This gave us more traction, and became our default approach to tire inflation for all our bikes. On the road bike, at 100psi or so, this generally wasn’t an issue. For criterium racing, I was able to drop to 80psi using Compass Cyuse Pass tires (25mm), and even as low as about 75psi in the rain, for maximum grip. But at that pressure I’d run the same front and rear. It was underbiking on cyclocross bikes that really reminded me, or re-taught me, that I had to run as much front tire pressure as rear, sometimes more. When riding gravel with large rocks here and there and a fairly low, aggressive position, my front wheel would take an initial impact (which I wouldn’t see, normally) hard, then I’d unweight the rear wheel. Front flat. On steep trails (again, underbiking), I’d be braking or not, either way, more weight on the front wheel, and hit rocks harder with the front, flat. So I bumped up pressure to match the rear, and even do more sometimes, when I know the trail has really nasty stuff.

  8. I’ve always run my front tyre at about 5 psi less than the rear one (f/r = c.55/60 psi). If I neglect to pump my tyres for, say, two weeks, my front tyre always loses more pressure than the rear (f/r = c.40/50 psi). I’ve assumed that while the rear tyre may bear most of the load while riding at steady speed on a straight level road, once I begin cornering, braking & descending—esp. on rough roads—the front tyre takes more of a pounding than the rear (remembering also, that the rear wheel can be unweighted somewhat by lifting off the saddle).

    • Larry says:

      Or you have a slow leak in your front tire. I have one on my rear tire on my MTB — ever since I loaned it to a friend who is a bit of a newbie :-/

      “Cave fenerantis!”

  9. sofauxboho says:

    Any information on how Frank Berto measured his tire drop figures? I’d love to confirm my tire drop for my specific bike / rims / tires / load.

    • It was a pretty involved process. He built a big machine that pushed down on the wheel. I don’t recall the specifics… It would be interesting to repeat the experiment, but use a glass surface that pushes on the top of the tire, so we could see the size and shape of the contact patch.

  10. Mike says:

    I believed in this myth for years, too, though I didn’t check my tires that carefully. I shoot for 20-25 psi for both tires and I don’t fuss about it if one’s off by a little bit.

    I regularly ride over to the local hardware store for some concrete pavers to finish off my backyard patio. I usually get four pavers at ~15 pounds each, two in each of the rear panniers. With this load on the rear wheel, the ride was, let’s say “squishy”. Nowadays, if I know thatI’m going to carry a heavy load, I’ll inflate the rear tire for 10 seconds or so, and all is well on the ride home; then I release the extra pressure when it’s unloaded.

  11. Derek says:

    I ride tandems a lot. My wheel loads are 45% front, 55% rear. I find I can run pressures proportional to that, and less than 2x what I use for the same tires on a solo bike. Never had a pinch flat, or handling problems unless there was a puncture. Does this make sense to you? Any further advice specific to tandems?

    • The weight distribution on a tandem depends a lot on the relative weight of rider and stoker. More than 50% of the rider’s weight is on the front wheel… It seems that you found something that works well. I’d stick with that!

      • Sam Carlson says:

        That’s so true. I will contribute a data point of my own. I have a curved-seat-tube “short wheelbase” tandem with Barlow Pass 700×38 tires. The stoker sits very close to the rear wheel. Under hard cornering with heavier stokers I have either collapsed or folded the rear tire over – or at least it’s felt like that. It’s never come off the rim, but it has gotten awfully difficult to control, even at 80psi, although I first experienced the effect at 65psi. With a light stoker, everything feels fine. Maybe Jan knows what is going on more than I do. I think it was a tire collapse. The rear wheel felt like it lost grip and slid sideways a bit, so I instinctively countersteered or at least widened the turn. The rear regained grip, I lightly turned in again to make it around the corner, and I lost grip in the rear again. I regained control by aborting the turn and braking in front. Stoker felt “tossed around” and was not pleased. No pinch flat or anything, however. Sidewalls looked fine afterwards and there was no telltale sand or grit in the corner, just nice asphalt.

      • Derek says:

        I should be more specific. I think tandems are an exception to this myth. Setting the front pressure based on static load is risky on a solo bike, but seems to work fine on a tandem. Due to the bike’s length it does not suddenly overload the front wheel during braking. Unless you have a very light stoker, the rear does need to be significantly higher to prevent the loose cornering described by Sam.

  12. Larry says:

    Just had an underinflation rear tire flex on our recumbent tandem (Rans Screamer) and the stoker was quite alarmed. It occurred to me that whatever I felt was muted by being almost atop the front tire which had proper inflation; sort of how a given angle is a smaller deviation closer to the vertex (front tire being the vertex here). A quick stop to increase rear tire pressure also reduced the stoker’s blood pressure!

    Mea culpa as the “team mechanic”. I felt the tires with my hand before our ride and they seemed OK. I’m more conservative about the front tire’s pressure as it provides lots of annoying squirmy feedback when underinflated; the steering wants to wander left and right.

    Jan, the tandem’s former owner suggested I NOT use a supple sidewall front tire because of the weight that wheel carries on this odd bike. However, the stiff-sidewall-negative-spring-rate issue now has me wondering about that. Ideally, since this tandem is SUCH a pig going uphill, I’d run the most efficient tires possible. Also, it is hard to get over programming about tougher sidewalls and casings providing flat resistance.

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