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feature
How to choose your tyre pressure August 2018How to choose the best bike tyre pressure — balancing speed, comfort and grip
Getting the right tyre pressure is simple, right? Pump your tyres to the range recommended on the sidewall and away you go. Turns out it’s not quite that straightforward.
Your tyres do a lot of things. They grip the road so you can steer and go forward. On a road bike, they provide the only significant suspension, cushioning you from bumps and holes. They have to be tough enough not to puncture too easily, but thin and light enough to roll well. All these things are affected by pressure, as well as the design and construction of the tyre itself.
When you sit on a bike, your tyres compress. If they compress too much, they’ll writhe and squirm on the rims, making the bike harder to control, increasing rolling resistance and putting you at risk of pinch punctures. If they doesn’t compress enough, the ride will be harsh and there will be so little rubber on the road that grip will be reduced.
Somewhere in between those extremes, there must be an ideal compromise. How do you find it?
The happy medium
As well as pressure, how much your tyres compress depends on your weight, so if there’s an optimum pressure it will depend on your weight and the type of riding you do.
Engineer Frank Berto, who investigated this issue for Bicycling magazine back in the late 1980s, came up with a formula based on the weight on each tyre; he reckoned that the happy medium involved a tyre being compressed 15 percent of its height.
As a recreational and touring rider, Berto was probably more interested in comfort than speed, so this idea is controversial, because Berto recommends lower tyre pressures than most of us use.
Tyre drop is hard to measure, but Berto did a shedload of measurements, and plotted the pressure needed to give a tyre drop of 15 percent for a range of rider weights and tyre widths. Here’s a graph of his recommendations, showing the relationship between pressure and wheel load for each common road bike tyre size.
Weighty matters
There are two important things to bear in mind here. The first is that the tyre width is measured not claimed. When Berto originally did his work on tyre drop there was a big problem with tyre manufacturers mislabelling their tyres because the easiest way to claim you had the lightest 23mm tyre was to mark a 21mm tyre as a 23mm. That’s improved, but some tyres are still wrongly marked; I recently put calipers on a nominal 28mm tyre that turned out to be just 26mm wide.
The other is that wheel load is per wheel. If you weigh 72kg and your bike weighs 8kg, then your tyres carry a total of 80kg but it’s not evenly distributed. The rear wheel carries more of the load, usually between 55 and 65 percent.
To determine the right pressure, you’ll need to measure the load on each wheel. Put a bathroom scale under one wheel and enough wooden blocks, books or old magazines under the other to level the bike. Lean very lightly against a wall to steady yourself and sit in your normal position on the bike. Get someone else to read the scale for you. Repeat the process with the scale under the other wheel.
If your rear wheel is carrying 44kg and your front 36kg (a 55:45 weight distribution) and you’re running 25mm tyres, then reading from the graph tells you that you want about 90psi in the rear tyre and 70psi in the front.
That’s probably lower than you’re currently running, so think of it as a starting point from which you can tweak the pressure until you get a feel you like.
If the pressure comes out well below the minimum recommended pressure of your tyre, then you can go skinnier; if it’s well above, then use a fatter tyre if your frame will accommodate one.
Controversy
As I mentioned, this approach is controversial. Another engineer, the late Jobst Brandt, author of ‘The Bicycle Wheel’ wrote in a newsgroup posting: “What Berto did not seem to consider is that hard cornering and rough pavement require higher inflation than comfort or other considerations might demand. Banking over to a maximum lateral acceleration of about 1g is not something that works reliably with a comfortably inflated tire, nor is encountering rough pavement with breaks and patches in the surface.”
Brandt was also sceptical about Berto’s notion that front and rear tyre pressures should reflect the loads on them. He wrote: “I run my tires at the upper end of pressure because snake bites are always a threat on mountain roads. When descending with hard braking, the front wheel carries the entire bicycle, with the back wheel at lift-off. The same is true climbing while seated on steep grades where front wheel rise is close at hand.”
More recently, Bicycle Quarterly magazine did some tests that revealed there was no speed advantage in pumping tyres up very hard. It was already known that when measured on a smooth drum rolling resistance didn’t decline much beyond a certain pressure. But as editor Jan Heine discusses here Bicycle Quarterly’s real-world testing indicates that when tyre pressures get too high, there’s no further reduction in rolling resistance. Heine believes that you lose the suspension effect of the tyres and that’s enough to push the rolling resistance back up.
What about tubeless?
With no tube to pinch, tubeless tyres give you the opportunity to run lower pressures with less risk. Jobst Brandt's concerns about high-speed cornering atill apply, though.
In fact, tyre deformation in low-pressure tubeless set-ups is a major consideration because if you load a tubeless tyre sideways hard enough it can lift off the rim and burp out your air and sealant. On a mountain bike that usually just leads to an embarrassing minor spill, but on a high-speed road descent, it could easily be disastrous.
Even with tubeless tyres, then, you don't want to go to soft, and you don't want to go too hard either for the reasons discussed above.
We explored the subject of tubeless tyre pressures in a lot more detail in this article.
Choices
Where does this leave you and me then? I think there are five take-homes:
If you ride in a leisurely manner — a short commute, gentle pootling around the lanes — then you can afford to run quite low pressures for comfort.
At the other extreme, don’t bother over-inflating your tyres for races and time trials. Unless the road surface is glass-smooth you won’t get any advantage, and let’s face it where are you going to find a road like that in the UK?
In between, you should tailor your tyre pressure to your riding style and roads. Ride in flat country and on smooth roads? Go for the lower end of the range between the Berto 15% drop figure and the range marked on your tyres.
In the hills I’d follow Jobst Brandt’s advice for equal pressures front and rear if you like to descend quickly. I love to go downhill fast (it makes up for the fact you need time-lapse photography to observe me climbing) but a front wheel impact puncture at 50mph is high on my list of things I’m not keen to try, along with BASE jumping and being visited in hospital by Boris Johnson.
If you're not a demon descender, you can run a softer front tyre for comfort.
if you're running tubeless tyres, you have more leeway to experiment with low pressures, but you should probably still treat the Berto recommendation as a lower bound, or at least experiment carefully with pressures lower than the Berto graph recommends.
For many of these situations, going up in tyre size is also a good idea. Our Mat Brett explored the reasons for fatter tyres in a Trendspotting piece. It’s a convincing case.
Gauges
To set your tyre pressure right you’ll need a pressure gauge. Track pumps usually have one built in, but they’re often not very accurate, especially if the pump is a bit old and has been kicked around the workshop floor.
A standalone gauge, properly looked after, is a better alternative. If you can find a sturdy, metal-bodied analogue gauge, grab it, but digital gauges like these are more convenient.
Topeak D2 Smart Head Digital Pressure Gauge — £29.99
This well-regarded digital gauge measures in 1psi increments and automatically adapts to Schrader and presta valves. It reads up to 250psi so you can even use it on your mountain bike’s shocks too.
SKS Airchecker — £21.99
Accurate enough for workshop duties, yet compact enought to slip into the smallest of seat packs or jersey pockets, the SKS Air Checker is a very tidy digital tyre pressure gauge.
John has been writing about bikes and cycling for over 30 years since discovering that people were mug enough to pay him for it rather than expecting him to do an honest day's work.
He was heavily involved in the mountain bike boom of the late 1980s as a racer, team manager and race promoter, and that led to writing for Mountain Biking UK magazine shortly after its inception. He got the gig by phoning up the editor and telling him the magazine was rubbish and he could do better. Rather than telling him to get lost, MBUK editor Tym Manley called John’s bluff and the rest is history.
Since then he has worked on MTB Pro magazine and was editor of Maximum Mountain Bike and Australian Mountain Bike magazines, before switching to the web in 2000 to work for CyclingNews.com. Along with road.cc founder Tony Farrelly, John was on the launch team for BikeRadar.com and subsequently became editor in chief of Future Publishing’s group of cycling magazines and websites, including Cycling Plus, MBUK, What Mountain Bike and Procycling.
John has also written for Cyclist magazine, edited the BikeMagic website and was founding editor of TotalWomensCycling.com before handing over to someone far more representative of the site's main audience.
He joined road.cc in 2013. He lives in Cambridge where the lack of hills is more than made up for by the headwinds.
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73 comments
Tyre inflation is nothing like home roasting of coffee. Firstly, it used to be very common for homes to do their own roasting of coffee (often just using the oven though there were simple hand-cranked roasters as well). It wasn't until instant coffee (invented late 19th century) became popular (Nescafe - 1938) that people decided that home roasting was too much effort and that decent coffee wasn't worth the occasional house fire (coffee roasting creates lots of flammable chaff from the outside of the bean).
Secondly, hand grinding is usually cheaper than buying a 'decent' electric burr grinder, so will produce better results for people who shirk away from forking out £800 on a decent machine. I used to get a more consistent grind with my handheld Porlex grinder than with the rubbish electric grinder that I used to use (upgraded it to a low-end Baratza).
The problem with inconsistent grinding is that you end up with a big range of particle sizes - from tiny 'fines' through to larger 'boulders'. When brewing, you end up with over-extracting the fines and under-extracting the boulders and thus end up with a less than optimum brew.
Thirdly, how can you ensure the optimum brewing time if you keep varying the amount of coffee that you use? Weighing is the only accurate way to determine the amount of coffee that you want to use. You wouldn't bake a cake without weighing the ingredients, would you?
Fourthly, unless you live near a decent coffee roaster/shop, the only way to guarantee the freshness of your coffee is to roast it yourself. Green (unroasted) coffee beans will keep fresh for about 18 months or so without any noticeable degradation. Once roasted, the beans will keep fresh for about two weeks - it's worth giving roasted beans a couple of days to de-gas before brewing so that you don't get too much CO2 coming off and causing too much bloom in your brewing receptacle. Once you grind the beans, they degrade MUCH faster - maybe an hour later and they'll start to taste stale.
Fifthly, NEVER use boiling water.
Wait a minute, which forum is this one again?
Reducing pressure GRADUALLY say by 5psi every few days in order to see how far you can go isn't going to destroy you tyres/tubes/rims etc... You'll know when to back off.
Yes. You'll know. When you suffer a flat / dented rim / loose traction. what you are proposing is using your bike/health in a to-the-point-of-failure (i.e. destructive) test.
What's next - arguing against torquing brake cable bolts correctly - just keep backing it off until the cable slips under emergency braking, then after you've healed up and the insurers have settled, you'll know to make it *that little bit tighter*?
I acknowledge that some people simply do not like being 'told how to do something', but will reject vociferously their leading others down a path of trial-until-damage- or injury-causing error. Just do the very simple weighing / maths, and be confident of the result, FFS.
Like I said letting a few psi out between one ride and another is NOT going to dent your rims destroy your bike etc. You seem to be making a big fuss out of nothing.
I agree about the maths and I'm a big fan of the 15% drop method (as a starting point) but experimenting is the key. Every tyre is different. E.g. at the same pressure a Vittoria CX will ride completely differently than say Marathon Plus (which will need much lower pressure due to stiffer casing).
+1 for lower pressures - always find my alloy commuter most comfortable and still fast at 70 front 80 rear - on 23mm tyres!
Agree with KiwiMike, how low can you go? After several weeks off the bike, l joined last Sunday's clubrun without inflating the tyres and enjoyed a bash in the Essex lanes, from time to time noticing how pleasantly the bike was floating over the rude surfaces. When I finally got the pump out, I found the tyre pressures were in the mid-40s psi. Oh dear! That's not supposed to work! (Michelin Optimum Pro 25s on Velocity A23s, measured width around 27 mm)
This is probably the most important article Road.CC will publish this year.
I've been a fan of the Bicycle Quarterly 15% tire drop chart for years. I even have a copy stuck to my workshop wall for quick reference. What I have found over the last few years as tire carcass technology has improved, is that you can go even lower especially if you're using tubeless. I regularly go 10 to 15 PSI lower than what the chart says and find that my times are just as fast if not faster and I'm a lot more comfortable - Plus the brakes work a hell of a lot better when you've got a bigger rounder footprint and more rubber in contact with the road at any given fraction of a second.
Part of this I think is down to the fact that tubeless is a different game to clinch plus inner tube, the other part of it is the advances in material technology that have seen things like Schwalbe's Microskin and other technologies come into tire construction. These rewrite the rulebook regarding what was considered an appropriate TPI threadcount.
The bottom line is, don't be afraid to experiment and go lower than you ever thought possible. Your arse and your Strava times may well thank you.
Really? I sincerely hope not!
I agree with BBB - you don't need long-winded sciency guff to establish tyre pressures that suit the individual. If his perfectly concise sentence isn't enough then try Michelin's simple chart.
Adjust in 10 psi steps above or below as you see fit (the above is a starting point, not a Rule).
This makes good sense to me, and ties in well with what a friend and former pro triathlete told me a while back - weight in kg divided by 10 gives you the pressure in bar you should run your tyres at (23mm). Has worked well enough for me over the years.
There was an article in a bike magazine I read a couple of years ago that quoted a Specialized team mechanic saying pretty much that - he stated the weight of the rider + bike in kg.
For me, that's about 110 psi, which concurs with the chart above for 700x23c tyres - which, it turns out, is pretty much what I'd been running them at for years by feel alone, according to the gauge on my pump.
I keep meaning to check them with a proper pressure gauge to see what they're actually at.
I've never felt the ride on my bikes was overly harsh as a result of those pressures - apart from my hack bike, which has a monstrously rigid frame anyway (hi-tensile steel pile of crap - long story!).
Seriously?
Just like we don't need long-winded sciency guff to explain why the world isn't flat, the sun doesn't rotate around us, and the universe wasn't created in a few days.
Here's to the people who are curious enough to care about and understand such things, for they are the ones who make the world a better place for the rest of us. Hence medicine, electricity, and a few other things.
So, road.cc, thanks for the article. It's interesting, useful and I'm pleased you published it.
Yep.
If you removed those blinkers you would find that I'm not against science or technical articles, quite the reverse. I am more than comfortable with the idea that some people like to have such detailed investigation; I simply pointed out that the vast majority of people can set tyre pressures using a simple rule of thumb and/or chart. It doesn't have to be complicated.
Why is that such a big problem for you?
this article will prove to be pretty important indeed if it stops people using that guff. I am 80kg, so according to that chart, I should pump my 32mm tyres to 87psi. At that pressure there will be no give at all, every bump in the road will be smashing my wrists, while the chart in the article has 80kg+10kg bike, split 45% to the front tyre, 55% rear gives 65psi front and 75psi rear for my 28mm tyres. Smaller size, lower pressure.
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