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Tyre pressure

Old 04-24-18, 10:24 PM
  #1  
tungsten
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Tyre pressure

So, according to GCN's math I should, for my weight, be using pressures of 75psi front 80psi rear for 28c which testing around the block feels real soft.

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Old 04-24-18, 11:31 PM
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dabac
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And your question is?

Tire widths and pressures is one of the bigger revelations in bicycles over the last years.
The old theory was that thin+hard=fast.
Many riders got this imprinted in them as ”how things are”.
Then roll-out tests, powermeters, windtunnel work etc disproved the old theory. There’s good evidence that (a little) wider and softer is faster.
But old habits die hard. If you are set to think of the snappy, rattly ride of 23s begging for mercy as fast and racy, then a plush 28 will feel slower.
The difference isn’t huge though. You need a fair number of well documented rides to reliably see the difference.
And of course, reality is the always the final judge.
If recommended pressure gives you snakebite, then you gotta have more.
If you don’t trust or enjoy the ride, then what’s the point of being faster?
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Old 04-25-18, 04:46 AM
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I think it depends on your use case.

Soft = no road buzz = float over all the little bumps in the road instead of fight through them = faster.

But (I think) harder means a more direct transmission of power (less energy is lost to deflection), so you get better acceleration?

So if you can sacrifice acceleration to gain overall speed and comfort, lower the pressure.

But if not being able to respond means you get blown off the back of a break, then you might have to sacrifice comfort to avoid getting dropped.
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Old 04-25-18, 06:29 AM
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guidelines are guidelines because theyre instructive, not prescriptive.

go with whatever makes sense for your frame, your tire, your tube, your road, and your ass
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Old 04-25-18, 06:38 AM
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I had never seen the GCN video before but realized that Simon's recommendations were spot on to what I use for both 23 and 25 mm. Obviously, I must have seen the same guidance somewhere else since I'm too darned dense to figure it out on my own.

The numbers work for me; I've been using them for the last several years.
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Old 04-25-18, 08:29 AM
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Does anyone use the Berto guidelines? Using that chart, I'm getting something like 75psi front / 92psi rear for 23's and 65psi front / 74psi rear for 25's. That seems awfully low, but then again, the trend these days is lower and lower, so I guess it's possible.

Last edited by cthenn; 04-25-18 at 08:35 AM.
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Old 04-25-18, 11:18 AM
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tungsten
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Originally Posted by dabac View Post
And your question is?
lol...is this how it's supposed to feel?

Given lower back issues and the relative discomfort felt with 90/100psi on my last ride I intend to fully embrace this theory of lower tyre pressures!

At least my tyres done good....... https://www.bicyclerollingresistance...-4000s-ii-2014

Last edited by tungsten; 04-25-18 at 02:02 PM.
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Old 04-26-18, 08:50 AM
  #8  
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I'm just starting out... That being said, I tested my bike before I bought it and thought it felt great (coming from a MTB), then I checked the tire pressure last night... It was under 15! Ok so now I'm at 95 front and rear and holy s**t the thing moves out now like I'm not even trying. Anyway, I still don't know what the correct pressure is but hard seems fast to me...
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Old 04-26-18, 09:08 AM
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After years riding 90-100psi, I tried reducing to 85/75 last night, and it was just as fast or faster, based on several Strava segments, while being noticeably more comfortable. I think I'm gonna stick with this for a while.
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Old 04-27-18, 10:06 AM
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I've settled on 80/110 on 25mm conti gp4k and I'm 150lbs. I went as low as 60 and felt that I was begging for a pinch flat. 110 on both feels ok, but up front it makes things a little rough for me. Rear being hard feels much faster and my carbon post and saddle took out the difference in buzz.
You just have to do a bit of trial and error. Make sure to bring an extra tube and pump for when you get too low and pinch flat!
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Old 04-27-18, 10:59 AM
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At 170 pounds:

26mm (actual width)
80-85 psi front, 95-100 psi rear.
I've only had one pinch flat in 4 years of riding this setup. That was an egg sized, sharp edged rock that I hit at about 30 mph. I'd probably still pinch flat at 100+ psi.

29mm (actual)
65-70 psi front, 80-85 psi rear. I rarely go higher than 65F/80R. It's very smooth and still fast.

~~~

I go with about 15% less pressure on the front tire (That's a 45% front, 55% rear weight distribution)

~~~~~~~~~~~~~~~~~~~~~~~~~
Inflation Efficiency
I posted this in another recent tire thread. The tire testing with a wheel and a roller doesn't take into account energy lost from the rider + bike bouncing on rough roads. It would be difficult to test, I suppose, needing rough and smooth roads with the same grade, wind, etc. And the effects are fairly small compared to the rider's wind resistance.

For example, it's noticeably harder to climb on rough surfaced roads, compared to fresh pavement. A tire and pressure that absorbs the bumps would help.

Jan Heine (I know, I know...polarizing) says pressure isn't critical. He is talking about larger tire volumes most of the time.

from the link:
The reason is simple: Higher pressure decreases the energy required to flex the tire. Less energy is lost due to internal deformation (hysteresis). But higher pressure increases the losses due to the vibrations of bike and rider. More energy goes to suspension losses. The two effects cancel each other. Whether you pump up your supple tires super-hard or ride them squishy-soft, they have the same resistance.

Last edited by rm -rf; 04-27-18 at 08:40 PM.
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Old 04-27-18, 11:25 AM
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GCN says that at 185 lbs and with 25mm tires I should be running 96 psi rear and 90 psi front. Seems low to me. I generally run with 110-120 and 95-100.
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Old 04-27-18, 12:36 PM
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Isn't 120 overkill for 25mm?
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Old 04-27-18, 01:23 PM
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Originally Posted by evan326 View Post
Isn't 120 overkill for 25mm?
That is the question. I've been to several PSI calculators on the internet and they generally show 101-120 for my weight. So, I'm not really sure.
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Old 04-27-18, 01:45 PM
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I just go by how the bike feels on a combination of smooth and rough pavement, including on fast curves. At 160 lbs I'm satisfied with 90 psi front and rear. At the moment the front is a 700x23 Schwalbe One V-Guard, rear is a 700x25 Continental Ultra Sport 2.

With any new set of tires, or different combo of tires, I'll tote a gauge for the first couple of rides to adjust pressure as needed. After I find a comfortable pressure I just stick with it and leave the gauge at home.

Higher pressure feels quicker on smooth pavement but we don't have much of that sort of luxury. At higher pressure on rough pavement the bike feels jittery, chattery, and I can feel brief moments of the rear wheel losing contact while standing during climbs on chipseal and rough pavement. I finish rides with a sore neck and back and feel like I've been wrasslin' gators. Lower pressure might have higher resistance for lab purposes, but doesn't feel slower on real roads.

When I had a 700x25 Conti on the front I'd ride it at around 85 psi, but the 700x25 was too wide for that narrow rim and felt squirmy at any pressure on fast curves. Just a mismatch between tire and rim, not a pressure issue.
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Old 04-27-18, 02:57 PM
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I just use the middle calculator found at the below link, but use the front tire reading shown for 45/55 weight ratio, and inflate the rear to what it indicates after switched to the 40/60 F/R ratio.

TireInflation Calculator
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Old 04-27-18, 03:27 PM
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Originally Posted by bruce19 View Post
That is the question. I've been to several PSI calculators on the internet and they generally show 101-120 for my weight. So, I'm not really sure.
I'm just over 200lbs and I never ran 25's that high, even when I was running clinchers and tubes. I'd run a rear 23 up to about 120, but would drop it to maybe 100 max on 25s. Running tubeless now, and I run 25s at 85-90. My rule is, I generally figure out where they start to feel "squishy" and add about 10lbs to that.
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Old 04-27-18, 03:45 PM
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Originally Posted by pesty View Post
I'm just over 200lbs and I never ran 25's that high, even when I was running clinchers and tubes. I'd run a rear 23 up to about 120, but would drop it to maybe 100 max on 25s. Running tubeless now, and I run 25s at 85-90. My rule is, I generally figure out where they start to feel "squishy" and add about 10lbs to that.
It depends on what your goals are.. maximized comfort on an absolute basis, or maximized comfort without decrease in resulting speed, or increased rolling resistance.
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Old 04-27-18, 03:49 PM
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Originally Posted by Sy Reene View Post
It depends on what your goals are.. maximized comfort on an absolute basis, or maximized comfort without decrease in resulting speed, or increased rolling resistance.
Oh, I agree, but I've found that a pair of tubeless 25c Pro Ones at 85/90 (F/R) for me are every bit as fast and more comfortable as the 23c Open Corsa CXs I used to run at 110/120. Bonus, I have to stop about 1% as much as I did with the Open Corsas to deal with punctures.
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Old 04-27-18, 04:50 PM
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It depends on the tire as well.

Supple, high thread count tires with thin casing can be very sensitive to small changes in pressure. Too high and they can be harsh and loose grip. Too low and they can get sluggish.

A tire with a thick, hard casing is often less sensitive to pressure. It won't change behavior much over a broader range of pressures.

Lighter riders will be more sensitive to pressure changes as well.

One test is braking hard into a turn. Doing so plants the front tire, increasing traction for the turn. Racers practice late braking. It is a game of nerves but when you get it right a turn can be very fast. Having the right pressure is critical and when pressure is wrong, boy you will know it.


-Tim-
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Old 04-27-18, 05:03 PM
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Originally Posted by TimothyH View Post
It depends on the tire as well.
One test is braking hard into a turn. Doing so plants the front tire, increasing traction for the turn. Racers practice late braking. It is a game of nerves but when you get it right a turn can be very fast. Having the right pressure is critical and when pressure is wrong, boy you will know it.
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I suggest everyone on BF perform these tests over the weekend and report back on Monday. If we don't hear from you... get well soon!
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Old 04-27-18, 07:29 PM
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Originally Posted by evan326 View Post
Isn't 120 overkill for 25mm?
I have 130 psi max 25 tires but even 110 is pretty darn hard when you hit a hole in the asphalt, which is why I go ~95.
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Old 04-29-18, 02:55 PM
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These recommendations from Zipp somewhat different for one of their 28c offerings...

<121 lbs(<55 kg)70 (4.8.6)75 (5.2)

121-143 lbs(55-65 kg)72 (5)80 (5.5)

143-165 lbs(65-75 kg)80 (5.5)85 (5.9)

165-187 lbs(75-85 kg)85 (5.9)95 (6.6)

187-209 lbs(85-95 kg)90 (6.2)100 (6.9)

>209 lbs(>95 kg)100 (6.9)100 (6.9)
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Old 04-29-18, 05:30 PM
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go ride

Originally Posted by tungsten View Post
So, according to GCN's math I should, for my weight, be using pressures of 75psi front 80psi rear for 28c which testing around the block feels real soft.

https://www.youtube.com/watch?v=4sDX54zNmxY&t=220s
waste of time - go ride, find out what's best for you on the road, not hear. no disrespect.
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Old 04-29-18, 05:59 PM
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Use GCN as a starting point, but experiment on your own. How the tires ride, handle, and feel depends on the roads, your riding style, the frame, and the wheels, so, as the saying goes, you mileage may vary. That said, Here's where I ended up:

Front: 95
Rear: 105

I'm riding a 2017 Roubaix with Roval CLX 32 wheels, so the setup is more forgiving than some frames. But trust your own judgement and inflate to what feels right for you. Do not slavishly follow "conventional widsom" since there is no such thing when it comes to something that has so many variables.
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