grizzly907la

I have 700x45c tires on my bike. The tire says max PIS is 85, however I have blown out inner tubes at that pressure. Anyone have an idea of what the real PSI for that size of tire? Thanks!

Litespud

That's the max pressure - the pressure over which you're at risk of compromising the tire or possibly rupturing it or blowing it off the rim - the actual pressure to be used will depend on weight (you plus the bike). The heavier you are, the more pressure you'll need in the tires to ensure you don't pinch-flat if you roll over some obstacle or the edge of a pot hole, for example. At the same time, you don't want the tires so hard that they'll rattle your fillings out. For example, the max PSI for my tires is (IIRC) 147 PSI. However, I run them at 90 front, 95 rear, which is about ideal for me. There are plenty of on-line calculators that will spit out a recommended PSI based on tire width, rider weight and riding conditions.

veganbikes

One can blow out inner tubes from not seating the tire properly or using the wrong sized tube or having something in the tire. It is possible you pump is way off and you are putting more pressure in the tires than the gauge is telling you but that is less likely.

If your max PSI is 85 you might try running it a bit lower. Most quality tires will list minimum and maximum pressure. Those that don't will require a bit of experimentation but usually you have 15-30 PSI range on many tires but sometimes more.

If you decide to go tubeless you can generally go very low in pressure as you aren't going to pinch the tube but sometimes going too low can be bad without foam inserts.

Iride01 

The real PSI? What is that?

What you saw on the tire is likely the max recommended PSI. Certainly on a tire that big you don't need that much. The max PSI might be the real PSI to use if the bike, rider, bottles and other stuff add up to 500 pounds.

I'm all in favor finding out what pressure between max and min work for you. Too low and you'll have a lot of flats. Too high and you might be bouncing around so much you'll actually have less speed and for certain a more uncomfortable ride. Just maintain a certain PSI for a dozen or so rides and note your data and how your legs felt. Then try another PSI for a dozen rides. Repeat till you know.

There are calculators online that tell you what they want you to use, but even the ones that seem to take into account road surface still give me a lower pressure that my data and legs tell me I'm slower at and feel more drained of energy. So I'm about 10 PSI more than any of them. But I also don't run near as wide a tire as you.

79pmooney

Consider the rim too, 45c is a big tire. Many manufacturers never dreamed their rims would be used by a tire that big.

Tires exert a side force on the rim that is proportional to both the tire pressure and the tire size. Say the rim was intended for 28c tires at a max of 120 psi. If indeed I am correct, 28c at 120 psi is the equivalent to 45c and 75 psi. Pumping the 45c tire to 85 psi would be like pumping that 28c to 137 psi. (Wear ear plugs and stand back.)

I've seen a few blown rims. The sidewall comes off in long, jagged shards. They can saw well into CF forks and stays if you are moving and do an ugly slice on your calf. Also sounds like a gunshot. (I blew on in my garage. Thankfully, I was inside.)

Edit: I rode a 37c tire in front yesterday on pavement. Front tire. 60 psi was perfect. That would be 49 psi for 47c. (I weigh 150 lbs.)

HillRider

I'm really surprised the sidewall recommended maximum pressure on a 700-45 tire is that high. I have 700-32 Vittoria tires on one of my bikes and the sidewall listed maximum pressure is 70 psi.

MNebiker

My rule of thumb was to use no more than 75% of max as an initial pressure, then refine it from there.
I have 700-35 on 622x19 rims. Tire shows min 50 psi, max 85. I run 60. Seems to work well for traction and wear.

DiabloScott

85 PSI is pretty high for that size tire. BUT
True blow-outs are much more likely to be user error than a problem with pressure rating on the tire.
A good fit should probably allow twice the rated pressure before it blows off the rim.
If your tire blew out while you were pumping it up, it was probably a bead seat tube pinch, or it's on a rim that's too narrow.
If it blew out while riding it could've been a bead pinch or a tire casing failure (would've been obvious) or maybe a hot descent with bad braking technique.

davidad

This does not go over 37mm but you get the idea. Bicycle tire pressure calculator (dorkypantsr.us)