Why Do the same size tires say they can hold different amounts of air (psi), and does that matter? [closed]

Question

Sadly, my wonderful electra cruiser got stolen on tuesday. I am a college student with no other transportation, i need a new bike RIGHT away, the busses in my town are currently on strike and not running, so im looking for a cruiser around 120$. Im 5'7 300 pounds (yikes!)

Before, i kept getting 4 flats a week. I already determined the cause in my previous question, it was that, in short, i need a higher air pressure. The tires i had (that were stolen with the bike) were 26 x 2.125 and read on the side wall that they can hold 45-65 PSI of air.

Walmart has cheap cruisers. So today i went to walmart and saw:

• Most cruisers there had 26 x 1.25 tires, much thinner than mine. Only a few had 26 x 2.125..... (Which is what i thought was standard for cruisers)

• They all said on the sidewall the "tire can only hold 40 PSI of air"

Why is it that, a tire the exact same size as mine, would hold much less air than mine? And if i bought one and kept the air at 40 psi as it says, wouldnt it feel just like my old bike did at 40 psi (harder to ride, meaning more difficult to pedal) and have the flat issue all over again?? Huffy cruisers usually match the tires to the bike... So it would not be ideal to simply buy new tires.

There is another bike on craigslist im looking at. Its a genesis cruiser, big bike, with 29" tires. The reviews all say its ideal for bigger people... However yet again, the tires are only 1.25" wide... (I thought heavier people need wider tires?) And again, they only hold 40 psi of air (Ugh!)

In conclusion, i am asking the following:

1. Why would the same size tire have different air limitations?

(The bike(s) im looking to buy, the tires say "fill tire to 40 psi" when normally, it would say "40 PSI - 65 PSI". All the tires say this no matter the size)

2. since tires usually say "40-60 psi" rather than just one number, could these tires say just one number (40 psi) because thats a good median, meaning i could get away with 50-60 psi?

3. If i can not afford new tires in addition to a nee bike, would you reccomend i stay away from these bikes with tires that can only hold 40 psi?

4. Say i were to purchase the genesis, which has 29"x1.25" tires that say they can only hold 40 PSI.

   • I heard bigger people need wider tires (like 2.1" and up) and more air pressure. Would the fact that these are bigger, 29" tires make up for the fact that they are thinner and hold less air in terms of an easy ride?

   • Could i change the 29" x 1.25 tires for 29" 2.125 tires with out changin the rims?

5. Any suggestions for where i can purchase an inexpensive beach cruiser style bike that doesnt have this tire issue?

Answer 1

The pressure you can put in a tyre depends on the tyre and the rim.

The tyre bead (the band that stops it popping off the rim) varies from tyre to tyre, and some will not withstand as much pressure as others. An expensive tyre with a kevlar bead should be fine up to 120psi. The volume of the tyre is a factor too, but even a quality large volume MTB tyre would be capable of holding 80+psi. The tyre threadcount is also potentially a factor - this is basically a measure of how durable and robust the tyre is.

The wheel rim is obviously also important, as it needs to be capable of withstanding the pressure too. Weaker / cheap rims will not withstand the same pressure as a more expensive rim.

I'm guessing the bikes you're looking at have tyres only designed to hold a lower pressure due to the bead and lower threadcount.

Answer 2

Ok there are a number of questions in there.

1) 29" vs 26": that's not a question of which would hold your weight better or worse. 29" rolls a bit better, especially over rough ground, so if that's important to you, then go for 29". A bike with a very small frame for a short person, will work better with smaller wheels - i.e. 26" - but at 5'7" I reckon you'll be fine on either.

2) Wider tires, e.g. 2.5" vs 1.25", with the same amount of air pressure will make the ride firmer, and will hold more weight. E.g. if you put 40psi in a 2.5" tire, and 40psi in a 1.25" tire, then the wider one will feel harder when you're riding it, and will make it less likely that you squash the tire right down when you hit a bump - so you'll get fewer pinch flats.

Note that there is a maximum width tire that you can fit on your bike before it will start rubbing on the sides of your frame. This will not be printed anywhere so you have to figure it out by trying different sizes.

However, if you put more air pressure in the narrow tire then it will support you just as well. In that example you'd have to put double the pressure in the 1.25" tire because it's half as wide as the 2.5". So why don't you just put more air in the thinner tire? See below....

3) Why do different tires have different "limits" on them. That's a quality question. The pressure recommendation is usually written like this: "25 psi - 50 psi". What this means is that if the pressure is less than 25 psi, you'll almost certainly get pinch flats, and if the pressure is greater than 50 psi, then you risk blowing the tire off of the rim. Better quality tires have higher numbers at the high end: e.g. 25 psi - 70 psi. Some tires go as high as 140 psi, especially very narrow racing tires.

So given your concern about supporting your weight and not getting pinch flats, I would recommend:

1. Stay with the 26" wheels if that is what your bike has now.
2. Get the widest tires that will fit in your frame easily (e.g. 2.5").
3. Go for a good quality tire, not the cheapest.

Good luck!