Moving from 25’s to 28’s...different PSI?

[SoCalSteve]

Hi all,

Incoming bike that will accept 28’s. Bought a set of Conti GP5000 700 x 28. Do I keep the same tire pressure I run now on my GP4000’s 700 x 25 or adjust? I’m very happy with the tire pressure I run now.

As always, thank you all in advance!

Steve

[kppolich]

Drop er 5-10 and give it a go. You have some more tire on the ground now so enjoy the ride.

Tubeless?

[Blue Jays]

Inflate to recommended factory-specified pressure.
Then deflate/test until you find *your* particular handling/comfort "sweet spot" that works. :banana:

[David Tollefson]

I haven't run 25's in a long time, but for 23's I run 90/100 (f/r). For 28's that would drop to 75/82 (f/r). YMMV

[Mark McM]

A rough rule of thumb is that tire pressure is inversely proportional to tire width. Since 28mm tires are 12.5% wider than 25mm, a good starting point is to lower the pressure by 12.5%. Then adjust from there based on ride feel and preferences.

[John H.]

Josh at Silca recommends dropping 3-4% for each mm wider in tire width.
So 9-12% less than what you currently run.

[SoCalSteve]

Drop er 5-10 and give it a go. You have some more tire on the ground now so enjoy the ride.

Tubeless?

The wheel set I bought is tubeless compatible, but I decided to go old school this time around. Maybe the next set will be tubeless.

[oldpotatoe]

The wheel set I bought is tubeless compatible, but I decided to go old school this time around. Maybe the next set will be tubular.

FIFY....C’mon ya Nancy!!:)

[joshatsilca]

Josh at Silca recommends dropping 3-4% for each mm wider in tire width.
So 9-12% less than what you currently run.

Equivalent small bump stiffness of the tire can be achieved at ~2% per millimeter of width change.. so every mm larger in measured casing size you need 2% lower pressure,

Part 1 of our episode on Asymmetry starts to get into this topic and brings some context from racing: https://marginalgainspodcast.cc/asymmetry-part-1/

[Clean39T]

Equivalent small bump stiffness of the tire can be achieved at ~2% per millimeter of width change.. so every mm larger in measured casing size you need 2% lower pressure,

Part 1 of our episode on Asymmetry starts to get into this topic and brings some context from racing: https://marginalgainspodcast.cc/asymmetry-part-1/

Josh, I'm loving those episodes - keep em coming - and THANK YOU! :beer:

[Clean39T]

I'll just add that while lower pressure feels great on rough roads, on smoother roads it can feel funky if you don't spin smoothly or are sprinting out of the saddle - undamped suspension can get bouncy...

[muz]

A rough rule of thumb is that tire pressure is inversely proportional to tire width. Since 28mm tires are 12.5% wider than 25mm, a good starting point is to lower the pressure by 12.5%. Then adjust from there based on ride feel and preferences.

This just doesn't feel right. I think the pressure should be more proportional to the square of the diameter (or cross-section area). I certainly don't need to go to a 50mm tire to use half the psi, but more like 35mm.

[BobO]

I'll just add that while lower pressure feels great on rough roads, on smoother roads it can feel funky if you don't spin smoothly or are sprinting out of the saddle - undamped suspension can get bouncy...

I'll add that when I switched to 28s from 25s I found that there was a low pressure limit after which they became dramatically splashy. In this condition the tires also felt uncomfortably squirmy in corners. I now run it just a few pounds higher and it feels right. The performance seems to fall off a cliff after the low limit. On the other end I've run them as high as 100# and other than getting unforgiving in ride quality, the grip doesn't seem to change dramatically.

[HTupolev]

This just doesn't feel right. I think the pressure should be more proportional to the square of the diameter (or cross-section area). I certainly don't need to go to a 50mm tire to use half the psi, but more like 35mm.
How you scale depends on what you're trying to scale for. If you double the width of a tire, but you're still riding it on smooth pavement, it makes no sense to quarter the pressure. If you're riding a supple 1" tire on smooth pavement, and a beefy 2" tire on gnarly trails, then it might make sense to be running the 1" tire at 80PSI and the 2" tire at 20.

[muz]

How you scale depends on what you're trying to scale for. If you double the width of a tire, but you're still riding it on smooth pavement, it makes no sense to quarter the pressure. If you're riding a supple 1" tire on smooth pavement, and a beefy 2" tire on gnarly trails, then it might make sense to be running the 1" tire at 80PSI and the 2" tire at 20.

I suspect the "ideal" formula is a polynomial, but the second order may dominate for a large percentage change (say 40% to 100% increase). I feel I can reduce the pressure by half going from 25's to 35's on the road, with similar protection against pinch flats. This is a 40% increase in width, and 96% increase in volume.

[Joxster]

Depends on your rims, some will cut up more than others due to poor testing and blame the tyre

[David Benson]

Bear in mind that the GP5000 is more true to size, and the 700 x 28 GP5000 is only marginally wider than a 700x25 GP4000

[marciero]

A rough rule of thumb is that tire pressure is inversely proportional to tire width. Since 28mm tires are 12.5% wider than 25mm, a good starting point is to lower the pressure by 12.5%. Then adjust from there based on ride feel and preferences.

An inverse proportionality would not imply the same amount decrease in pressure but 1/1.125 times the pressure, for about an 11.1% drop. Not sure if that was what you meant or not.

This just doesn't feel right. I think the pressure should be more proportional to the square of the diameter (or cross-section area). I certainly don't need to go to a 50mm tire to use half the psi, but more like 35mm.

It's just a rule of thumb. You can propose a different one. With yours in order to halve the pressure you would only need to multiply the width by sqrt(2) for about a 41% increase in width. 25 to 35 mm is about that.

[marciero]

Equivalent small bump stiffness of the tire can be achieved at ~2% per millimeter of width change.. so every mm larger in measured casing size you need 2% lower pressure,


Are you saying that an increase n millimeters would require .98^n times the pressure? So for example, 10mm would require about 82% the pressure for a decrease of 18%? That's the only way to make this consistent. If 10mm increase implies 20% less pressure, then someone would arrive at different final pressures depending on whether the width increase was all at once or in two or more successive increments.

[MikeD]

Good article on tire pressure https://janheine.wordpress.com/2016/03/09/tire-pressure-take-home/

The chart by Frank Berto in the article is a good starting point.

[SoCalSteve]

Bear in mind that the GP5000 is more true to size, and the 700 x 28 GP5000 is only marginally wider than a 700x25 GP4000

That’s too bad! Well, time will tell...:confused:

[dddd]

Good article on tire pressure https://janheine.wordpress.com/2016/03/09/tire-pressure-take-home/

The chart by Frank Berto in the article is a good starting point.


I would say that for the narrower tire sizes in the chart, those pressures seem a little low in terms of pinch-flat resistance.
It matters how fast that the rider is traveling over what kind of road conditions though, since hitting even small rocks is usually what causes my occasional pinch flat while running 23mm tires at 90psi for my 145lb body mass. Paceline riding blocks one's view of the road, increasing the chances of hitting small-scale debris of the sort that doesn't usually get called out by leading riders.

And I've never had much luck dropping the front tire pressure much below the rear tire's pressure, since pinch flats then seem to crop up before long.

At least for me, the lower limit of pressure is primarily defined by the occurrence of pinch-flatting.
I can get away with 90psi in a 23mm tire and 60psi in a 29mm tire (actual inflated widths), assuming that rim width somewhat grows with tire width.

[ColnagoC59]

on my 23's i run 90-95, on my 25's i run 85-90 and on my 32's 70-75
hope that's a guide.

[spinarelli]

How much pressure would it be safe to reduce if going to a wider rim, 6mm wider?
Set up is 28C vittoria Corsa G+
Shamal rim c15 is 15mm inner , 75psi
Hed Belgium+ is 21mm inner, 50psi
On the c15 I was using 75 psi but on the Belgium+ the tires ballooned to 30mm at 50-60psi. I may have to go down to a 25mm tire to avoid wheel flex rubbing on the chain stays.

[Louis]

Someone needs to ask Tom Brady what he thinks of this issue.

Robert Kraft might also have an opinion, but his involvement has been much overblown.

[OtayBW]

A lot of 'modeling' going on for something that has a lot of variables - e.g., rim width, actual tire width, road conditions, and even rider weight I would imagine, etc. Given the known approximate decrease in pressure going up size, the only way forward is to tweak it and find out what is the best pressure for your given set of conditions.

[zmalwo]

Just stop at the PSI you like :)

[SoCalSteve]

Thank you all for your input, much appreciated!

I guess what I was really after was knowing if I needed to go up or down. I do understand that there are MANY variables and I need to tweak it to my personal preference.

Again, thank you!!!

[Ken Robb]

Someone needs to ask Tom Brady what he thinks of this issue.

Robert Kraft might also have an opinion, but his involvement has been much overblown.

Too FUNNY!

[dbh]

I also think rider weight has a lot to do with it. Featherweights can get by with a lower PSI without running the risk of a pinch flat. They're also likely to have less of that squishy feeling, particularly in the rear, by dropping additional PSIs.

[pdonk]

Had my first ride on 28s on a wide rim. Had them at 85 lbs vs 100 on 25s on magic open pros. Wheels felt really sluggish accelerating. Once at speed rode well.

[Clean39T]

Had my first ride on 28s on a wide rim. Had them at 85 lbs vs 100 on 25s on magic open pros. Wheels felt really sluggish accelerating. Once at speed rode well."Felt fast" and "is fast" could be different things..

But since neither of us is racing the clock to feed our family, I think optimizing to "feel" is justified and with some iteration, I'm sure you'll get to a pressure for those wheels, with those tires, on your roads, specifically for your Vagen - and ride happily ever after..

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