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.

[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]

Originally Posted by kppolich

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]

Originally Posted by SoCalSteve

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]

Originally Posted by 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.

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]

Originally Posted by joshatsilca

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!

[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]

Originally Posted by 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.

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]

Originally Posted by 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...

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]

Originally Posted by muz

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]

Originally Posted by HTupolev

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.