Tire pressure limited by rim?

[SkipdiverJohn]

1 1/4” is 32mm as near as damn it. Unless a heavy rider you wouldn’t go anywhere near 85 psi. I have 32mm on my road bike and rear of recumbent. I have them at 65 psi for rear and 60 psi for front.

Pneumatic tyre pressures are load-dependent, so the actual inflation pressure should depend on the weight of the rider, bike, and any cargo carried, and how that weight is distributed between the front and rear wheels.
This chart is a good starting point, and what I go by myself, with small adjustments.

 

[Ming the Merciless]

Pneumatic tyre pressures are load-dependent, so the actual inflation pressure should depend on the weight of the rider, bike, and any cargo carried, and how that weight is distributed between the front and rear wheels.
This chart is a good starting point, and what I go by myself, with small adjustments.

View attachment 603210

As you can see for 32mm to get to around 85 psi it states 70kg. That is per wheel. With a typical rear wheel load of 60%, that means rider weighs approx 116kg. Say 100kg with 16kg of panniers, tent etc. Either way it comes under “Unless you are a heavy rider” as per my post you were referring to.

 

[Ajax Bay]

The irony is that hookless rims are coming back, facilitated by the current trend for fat 28mm+ tyres on road racing bikes, which can be run at modest pressure.

Could you name (or share a link to) such a rim, please? What is ironic about such a development, btw?

 

[Ajax Bay]

Yesterday they needed inflation. The tires specify 85-95 psi.

Jim - share where you find such a specification. More likely it's the max pressure for the tyre (tire).

 

[BataviaJim]

Jim - share where you find such a specification. More likely it's the max pressure for the tyre (tire).

It's embossed on the tires. That's what I've always gone by for bikes, lawn equipment, snowblower, etc. I assumed that was the recommended, or at least acceptable, pressure. I put in a little less -- just over 80 -- to err on the safe side. Not safe enough though.

 

[rogerzilla]

Could you name (or share a link to) such a rim, please? What is ironic about such a development, btw?

It's ironic because the OP has a bike that suffers from blow-off because its technology is so old, yet that technology is again the latest thing in rims. I suspect the motive has to do with the difficulty of forming the hook in CF rims and also saving a few grams.

https://www.cyclingweekly.com/news/product-news/what-are-hookless-rims-and-do-you-need-them-468466

In bikes, what goes around comes around, and lessons are rarely learnt! The industry attracts marketeers more than engineers.

 

[Chislenko]

If the OP does have hookless rims don't you have to get tyres that are specifically for hookless rims?

I thought that was the case. Are the OP's new tyres compatible with hookless rims (if he does have hookless)


For example an article about Giant hookless rims

https://www.bikeradar.com/news/giant-hookless-approved-list/

NB. I am aware the OP doesn't have a Giant, just an example.

 

[Ajax Bay]

I don't know if the rims are hooked or not.
back from the shop . . . the tires were inflated only to 50 psi. I . . . only weigh 135 lbs. . . . should I put more than 50 in?

Seems like I can believe the bike shop that the rims were the cause. Although I fault them for not warning me when I bought the tires. They mounted them so they saw the bike and rims. They didn't say a word about inflation, and since 85-95 psi is stamped on the tires, casual riders would inflate to that pressure. So is it safe to ride if I keep it at 50 psi?

they're the original 1979 rims. And there aren't any markings of any kind on the rims. It would have been nice if the shop had said something. I don't know how anyone is supposed to know this stuff if you're just a casual rider, especially with the tires marked 85-95 psi.
I'll keep it at 50 psi to be on the safe side.

Your rims are probably normal rims with a hooked profile. Take a tyre off and check. The load on the tyres are (roughly) 40kg/30kg rear/front and with a width of 1 1/4" that's ~32mm. Look at the graph which Skippy shared and Ming pointed up: the 'right' pressures are 50/40. I add 5psi to the graph as a 'safety margin': so 55/45. The pressures given on the tyre wall are max pressures. The shop mounted them and inflated them to (surprise, surprise) 50psi: didn't need to say a 'word'.
It is 'safe' to run them at 50psi.

 

[BataviaJim]

... The pressures given on the tyre wall are max pressures. The shop mounted them and inflated them to (surprise, surprise) 50psi: didn't need to say a 'word'.

I agree that the shop didn't need to tell me, but it was the decent thing to do. Of course they got another $20 from me for a new tube after the blow-out, but they also reaped some ill will. Next time I'll go to a different shop.

It seems like most people here are aware of the pressure issue but I don't know how non-experts are supposed to know, especially when there's so much misinformation from apparently expert sources. For example, these websites:
www.sixthreezero.com:

How Much Air Should Be in My Bike Tires?​

The recommended air pressure is written on the side of your tires in embossed lettering. Simply fill your tires with a pump and check with a gauge that the pressure matches the recommended range.​

www.theproscloset.com

Conveniently, every bike tire has recommended PSI stamped into the rubber on the sidewall.​

Here are some more specific recommended starting points for your tires:​

Mountain bike tires: 25 PSI​

Gravel bike tires: 40 PSI​

Road bike tires: 90 PSI​

​

https://communitycyclingcenter.org

Decoding all those numbers: If you look closely at the side wall of the tire you’re likely to find a few sets of numbers embossed in the rubber, but what do they mean?​

Recommended minimum and maximum tire pressure: Skinny, road racing tires might range from 90-130psi.​

​

Note that they all say that the psi stamped on the tire is the recommended pressure, NOT the maximum pressure. My recent blow-out proved to me that's incorrect, but how was supposed to know that unless the shop had told me?

It is 'safe' to run them at 50psi.

Thanks, that's what I'll do.

 

[neil_merseyside]

$20 for a tube?

 

[BataviaJim]

$20 for a tube?

$21 installed with tax.

 

[neil_merseyside]

$21 installed with tax.

Installed? You'll never learn about bikes if you always use a shop.

 

[BataviaJim]

Installed? You'll never learn about bikes if you always use a shop.

I'm almost 74 years old, I don't have too much learning left in me. 👨‍🦳

 

[Chislenko]

I'm almost 74 years old, I don't have too much learning left in me. 👨‍🦳

Good shout Jim, I'm a bit like that now with technology, if I don't know it by now I don't want to know it!

 

[BataviaJim]

Good shout Jim, I'm a bit like that now with technology, if I don't know it by now I don't want to know it!

Thanks.
Actually I'm okay with technology since I'm a retired IT guy and I've kept up with it reasonably well. Mechanical things are a different story -- I wasn't adept when I was younger, and I sure haven't improved with age.