I'm a 6'6" 230lb rider on a Domane with GP4000SII tires. I've been running 100psi front and rear for the longest time and recently tried 90f/95r and it felt fine. On my CX bike, I just got the 28c version of the same tires. What would be a good starting PSI for these? 70/70? 80/80?

I saw a test recently (and linked it in another thread here...somewhere) that concluded you should reduce pressure about 10psi for every size larger tire to maintain the same tension on the surface of the tire. So that would suggest about 80f/85r for the 28mm tire. I'd start there and see what you think.

 

Yup. The 10% number comes up again and again as a good starting point.

 

But more to the point, why are you running only 28mm tires on a CX bike? That high of a tire pressure on a CX course is going to cause you cornering grip problems. Better to go 32mm tubeless or tubular for CX where folks run pressures down around 20-30 psi (maybe 35).

 

I don't know why he is but CX bike doesn't necessarily mean CX riding. I use my CX bike for road and good shape gravel roads with 28mm tires quite often.

 

Here is a chart from Michelin Tires that's a good starting point.

 

Here is a chart from Michelin Tires that's a good starting point.

View attachment 323376

The pressures on that chart are wayyy too high.

 

Rim width and type of tire affect the optimal tire pressure. But terrain may be the biggest factor. If you ride where there's a lot of potholes and you're in a group where you can't see them coming you may want to increase pressure a bit to avoid pinch flats.

Here's Enve's recommendation for tubeless tires, which may seem low, but seems to work well for me. Note, not all tires of a given size measure the same.

 

Here's Enve's recommendation for tubeless tires, which may seem low, but seems to work well for me. Note, not all tires of a given size measure the same.

For my weight class anyway, yes, it does seem low on the left for 23mm. But they don't drop a lot as tires get bigger so it starts to not seem low as I work my way right into bigger tires on the chart.

Definitely looks reasonable though. I use tubes and I'm pretty sure my rims are not as wide as Enve. Factor that in and I'd probably be right about where the chart is for 23mm tires (if I still used them). 33mm is the biggest I use frequently and even with tubes I'm pretty much the same or even below the chart if I think I can get away with it which I generally can except for trails with exposed roots.

Definitely one of the better 'charts' I've seen and the only one I can recall from a tire or rim maker that didn't appear to be based on 'wisdom' from 1972.

 

I'm close to your weight (215 lbs). When I was running 25c tires, I ran tire pressure at 90 rear, 80 front. I since jumped to 28c tires. Tire pressure is now at 80 rear, 75 front. And I'm going to drop the psi another 5 f/r. Trying to find the balance between comfort and flat protection. Try the tire pressure at 80/80 and go from there.

 

Here's my method for calculating tire pressures over different sizes:

I weigh 175, and I have one bike with 23mm tires, one with 28mm tires, one with 42mm tires. From flat, I use my floor pump, and give 15 full strokes on the rear tire, and 13 strokes on the front. Works perfectly. Works out to about 90/100 on the 23's, 70/80 on the 28's, and about 35/40 on the 42's.

 

I gotta ask...what hand pump gives 85-90 psi with 15 strokes?

 

My thoughts exactly.

 

EDIT-- I just noticed that this is a somewhat old thread, with a couple new comments. (Yeah, my floor pump does maybe 5 psi per stroke, so more than 15 strokes to fill a tire.)
I'll leave these comments anyway.

...

A lot of riders post their inflation numbers with the front just 5 psi lower. The front can often be much lower.

Lower pressures on the front really help with rough road vibrations.

Too low, and there's a risk of pinch flats when the tire bottoms out and mashes the tube between the tire tread and the rim edge. But pinch flats are more likely on sharper edged hits and at higher speeds.

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The GP4000S 28mm have been reported to be actually 30-32mm. So charts showing 28mm can likely go lower pressure for GP4000S

This tire pressure calculator gives me a good starting point.
Deducting 15% for the front tire is a good ratio for a 45% front to 55% rear weight distribution. (The actual difference is about 22%: 55/45=1.22, but I don't want the front "too" low.)

Scroll down, use the "fully dressed rider + bike" section, and the "45%-55%" weight distribution.

I have 25mm tires on wide HED rims, measuring 29mm inflated.
The calculator tells me, at 170 pounds+20 bike: 65psi front and 80 psi rear for a 28mm. That's actually what I use. I've tried 70F 85R and the ride isn't any faster, but it's not as comfy on rough roads. I could likely use 60F 75R, but it's fine as is.

For you, conservatively at 230+25 bike+rider+water+tools, etc:
25mm:
102 front, 127 rear!

28mm:
86 front, 106 rear.
I'd try 100-105 rear, and 80-90 front. See how it rides. Try a test ride with a little more pressure to compare.
For 32mm, it says:
67 front, 82 rear. (even moderately wider tires have a lot more air volume! )
So if it's actually about 30mm, you might try:
90 rear, and 75 front.


I want to see a slight bulge in the tire sidewalls if I look down while riding.

 

Why so much lower for a few mm wider tire?

Tire pressures are related to the air volume inside the tire. The area of a circle is pi x (diameter / 2) ^2. The volume is area x distance around the rim, which is basically the same with different sized tires. close enough.

Assuming a circular cross section of the tire + rim:

25mm
490 sq mm cross section.

28mm
615 sq mm. That's 25% larger! Just 3mm wider.

32mm
804 sq mm. That's 64% larger! This is why the pressures can be reduced so much.