Correct tyre pressure?
 

So I have been reading through these forums, googling, etc, but still not 100% sure how to get this right. How do I get the definitive answer?

Details below.

My weight (fully dressed plus a backpack with some snacks and personal items) approx.: 85-90Kg / 187-198 Lb
Bike weight approx.: 13Kg / 29 Lb
Bike type: Hybrid
Tyres: 700x40c

The "correct" pressure is what works for you, kind of. The tire should not be rock solid, or you'll feel every bump, or vibration from a rough road, etc. Don't want it too soft either, or you risk the chance of a "snakebite" or "pinch" flat if you happen to rub tubes and hit an uneven crack in the pavement or a pothole. Tubeless tires can usually be run a bit lower than tires with tubes. To start off, maybe try running 5 lbs less than max in front, 10lbs less than max in back, and go down from there. You'll find what is best for you, your bike, where you ride. May just take a little trial and error.

Full system weight for me on my gravel bike with 40mm tires is prob 10-15lbs less than you, I run 45-50psi on the road, and 30-40psi on gravel depending on conditions. That's tubeless tho, maybe lil extra pressure if you're running tubes. The goal is to find the right pressure for you that feels comfortable and stable, while not low enough to allow for pinch flats (if you're running tubes). Start at maybe 60psi and work your way down and find what works for you.

Bike pressure calculator using rider and bike weight:

Bicycle tire pressure calculator

I use the IDGAS system. Since I mainly ride on hard roads I look for maximum tire pressure rating on side of tire and pump to that. Works for me

Thanks everyone. I was aware that there will be some sort of deviation form the "norm" based on my situation or personal preferences. But just wanted to make sure I get the correct starting point before trialling further instead of poking blind in the dark.

Appreciate everyone's responses.

With 40mm tires I would probably be 50-ish.

Thanks

My figures are pretty much exactly the same, down to the the tire size. I usually run about 50-55 psi, may deviate some 5psi en either direction depending on the expected road conditions (recommended pressure for these tires is 35-65 psi). Although lately I've been going just by feel, pinching the tires with my fingers. That actually tells me more than figures which may vary from gauge to gauge and I have no idea how precise they are.

Ahh the good old "pinching the tires with my fingers" method. Definitely on the list of my options :) used that as a kid before I even knew how to use the gauges :) Thought this time might go a bit more serious, but I agree this method has its pros.

I think that the larger 40mm tires don't pinch flat as easily as a 25mm tire. At least, they won't pinch flat on a one inch sized piece of sharp edged gravel on the road, like my 28mm did. I was going 30 mph when I hit it. Slower cruising speeds won't pinch flat on these rocks, since the tire has time to rebound. Slamming a deep pothole could still pinch flat a 40mm, so look out for those!

On 38mm tires, I run 38 psi front and 45 psi rear for rough paved roads. I'm 170 pounds, the bike around 22-23 pounds. These are smooth tread, very flexible tires, built like road tires, so they flex over bumps and still roll fast. A stiffer, more puncture resistant 40mm tire might benefit from higher pressures to lower the rolling resistance.

Some years ago, I saw a recommendation to squeeze the sidewalls with thumb and index finger, instead of just pressing down on the tread with a thumb. This gives me a better idea of the tire inflation. I've pumped up my various tires, then did the sidewall squeeze to help remember what good pressure feels like.

I've been using "Mymavic" app and go from there. Best part, is you can save all your bike profiles in the app and update as necessary .

I simply inflate to 10psi less than what's stamped on the sidewall of the tire.....then add or minus after a few kms depending on taste of the day.
The important thing is to check the tire pressure every few rides (if not before every ride)......

This^^^^ is the best starting place I know of.

Use it.

Am I missing something with this website? I just put in some generic information (190 lbs and 25mm tire) and it is saying to inflate to 176 for the "15% drop method" and inflate to 102 psi for Michelin 700c tires.

I would start w max pressure in rear, maybe burp 5-10 lbs out & run the front 10 psi lower & see how it “feels”

I have never used the sidewall pressure to pump my tires to. In fact I don't even look. I base my pressures on ride feel, need to not pinch flat and grip. With smaller tires, those last two can be a balancing act. The intent of the ride will be the deciding factor. In the days when high quality tires were (nearly) all small, that could be a fine balance and one I had to keep in mind as I rode. Now, I can ride very high quality 28c tires and never have play that balancing act. (I weight 150-155 pounds.)

35s should give the OP the same margin as I get with 28s. 40s are my 32s and have lots of leeway. I'd probably start me ride with 40s at around 42psi so I didn't get hammered. I like the suggestion above for the OP at 50 psi.

I always go 5 psi more in back than in front. The old "rule" from all the experienced racers when I raced back when we still had dinosaurs. Modern theory tell us we should have far less in front since when rolling along, we have 55-60% of our weight in back. But that doesn't reflect that split second when the car taillight comes on right in front of you, you slam on your brakes and simultaneously hit the deep pothole you could not see because of that same car. Now - all your weight is on that front wheel which is slamming into the far pavement edge. Pinch flat, bent rim, maybe blown tire time. And exactly when you full control of your bike! So when I think a pressure, I automatically 2-3 psi more in back and the same less in front.

OP, note the sidewall deflection of your tires, ie "squish" when you achieve a pressure that feels good. And do the pinch (sidewalls, thumb and index (or middle) as suggested above. I actually usually pinch just below the tread, The "pinch gauge" does have some "drift" to it and is not of digital accuracy but the portability and ease of use counts for a lot! (And can be more accurate than a pump gauge!)

Ben

It wasn't that long ago that the "norm" was simply to inflate to the max pressure rating! As FZ once said "Without deviation from the norm, progress is not possible." :)

Yes, you didn't split the weight up per wheel.

So the 176 psi suggestion would actually be 88 psi per wheel?

This is what I'm getting when I input data per wheel:

https://cimg2.ibsrv.net/gimg/bikefor...ec0dcebc58.png

Yep, if you have about the same amount of weight on each wheel. I prefer to use the second calculator on the page anyway, since I assume a 45/55 weight distribution in my bikes, and that works out nicely for me. :)

ON EDIT: Looks like you figured that out already. :D

If you look above the first calculator on the page, the one that gave you the 176 psi figure, it say, "Weight measured at wheel." It won't be an equal 50/50 split, the rear will have more weight on it than the front. You will have to account for that.

Thanks for the help!

I'm a heavier rider - 230ish - and it suggested 91 psi front and 114 psi rear on the 45/55 chart. That seems extremely high.

The Michelin calculator recommends 102 split between the two, which makes way more sense.

That sounds right to me for 25s. The rear will generally be at higher risk of pinch flats, so you don't want to underinflate it. If your frame can fit 28s, that would be much better. :)