paulbi

I was looking at proper inflation. I'm new to commuting (been doing it a little over a month) and I read up on what Sheldon Brown says about inflation / tire width etc. He also links to a page in Bike Quarterly about proper inflation (here).

I'm heavy (312 pounds, which is a large part of why I'm riding to work now) and doing the math about weight distribution (I've got about 60% of me+bike is on the rear tire) so I should put my rear tire at 120 psi and the front at about 74 psi according to the Bike Quarterly article. The max recommended on my 32 kenda kwicks is 85....

So according to the Bike quarterly I'm under-inflated if I stick with 85 on the back, but according to the recommended psi from the manufacturer I'm over-inflated if I go with 120 from the bike quarterly recommendation.

Sheldon mentions that bike manufacturers usually put the max pressure at half a blow out psi. Should I try 120? I have to imagine I'll get quicker going from the 85 to 120. That's a big jump though. While I want to be faster, my biggest concern is no flats. But being under-inflated causes pinch flats....

Thoughts? Thanks

for reference on tire pressure from the bike quarterly:

dsbrantjr

Can you fit larger tires?

paulbi

I could fit slightly larger tires. But wouldn't bigger tires mean slower?

ThermionicScott 

In theory, maybe. In practice, aerodynamics (are you on a mountain bike?) and weight (sorry) are bigger factors. I commute on 42mm tires, and they don't seem to slow me down.

pavement_nyc

I would invest in some better tires. Pressure aside, those Kendas have no puncture resistance and you will get more flats than on a better tire. I would go to at least 100 on the rear tire though if you're going to keep it.

FBinNY 

No wider tires most definitely won't mean slower, and may actually offer lower rolling resistance.

Discounting tread friction on the pavement, the main source of energy loss in a rolling wheel is the flexing of the body plies. That's what makes auto tires get warmer as you drive, and bike tires are the same. For practical purposes the air inside is a perfect spring so there's no energy loss there.

The issue therefore is the extent of tire flex as it meets the pavement. That's determined by the shape of the contact patch. Since the tire itself cannot support your weight, the contact patch size is equal in area to the weight it's supported divided by the air pressure, ie for 100#s, the area of the patch will be 2 sq" at 50psi, or 1sq" at 100psi. Now that you know the area, it becomes a question of shape.

A narrower tire will always have a narrower patch, which therefore would need to be longer for any given area. That means that the flat line of the contact area will be farther in from the original circle of the tire, creating more distortion, thus losing more energy.

Look around at the automotive/truck world. The higher the axle weight, the larger the tire, and the wider the contact patch. With fuel as expensive as it is, would anybody do this if it weren't the most efficient way?

At your weight, I'd strongly suggest the widest tires that your frame can clear, then inflate them in accordance to the data provided in the BQ pressure chart (or as close as the maximum pressure allows).

JohnDThompson 

No. In fact, since rolling resistance is largely a function of sidewall deformation, at any given pressure a wider tire will tend to have less rolling resistance than a narrow tire.

N.B. at high speeds, the wider tire will be less aerodynamic than a narrow tire, but this is unlikely to be a significant factor when commuting.

Al1943

You should also be concerned with the rest of the rear wheel, not just the tire and pressure.
You need a strong wheel. What kind of wheel do you have, how many spokes, number of crossings, type of rim?

paulbi

Quote:

Originally Posted by Al1943

You should also be concerned with the rest of the rear wheel, not just the tire and pressure.
You need a strong wheel. What kind of wheel do you have, how many spokes, number of crossings, type of rim?

It's what came standard with my Insight. They call it a 32h Equation R23 Doublewall, so 32 spokes I guess.

FBinNY 

AL1943 is right that you want a strong wheel at your weight, but IMO that's a question for the future. You have a wheel which either will or won't hold up, but you might as well get best value from it until it dies. Using the right tire at the right pressure will make a difference. Developing good riding technique will also help -- don't rock the bike when climbing or sprinting, learn to lift out of the sadle and letting the bike float under you on bumpy stretches, if you bunny hop, make sure the wheel comes down squared up, etc.

If/when the time comes to rebuild the wheel, get good advice on the best rim and spoke combination for your weight and purposes.

Bill Kapaun

I'd do 2 things.
Get a better quality tire that will handle higher pressure. You can save the take off for when your weight is down or you wear out the front.
Get the spokes properly TENSIONED. At least the rear.
At your weight, that can help a 32 spoke wheel survive much longer.

Retro Grouch 

I don't put much confidence in that "rated tire pressure = 1/2 blow off pressure" statement.

If that were true, there should be an industry testing protocol published somewhere. I've never been able to find anybody who had a source for that. Also, half of the tire/rim interface is the rim. The tire manufacturer doesn't know what rim you're using and differently constructed rims would surely affect tire blow off pressure. If such a protocol existed, it would have to list the rim used for testing the tires. Also, if you're using rim brakes, the brakeing surface of your rim will eventually erode away and either crack or cause the tire to blow off at some point.

Empirically, my experience doesn't support "label pressure = 1/2 blow off pressure" either. It's certainly true that oftentimes you can get away with exceeding the manufacturer's listed maximum tire pressure. I can tell you for sure that isn't true all the time. Some years ago Santana tandems had a spate of tires blowing off rims at listed tire pressures.

Bottom line, I might moderately exceed the manufacturer's maximum pressure on a personal bike provided I was confident of the condition of my rim. I'd NEVER do it on a customer bike even if they asked.

cyccommute 

I think this idea comes from standard engineering practices of over-engineering pressure vessels. No engineer with his salt would design a pressure vessel...a bike's tire is a pressure vessel...to hold just enough pressure for the task at hand. They always build the vessel to withstand more pressure than the pressure required for the process.

That said, you are correct in not putting much confidence in the above statement. There are too many variables for that to be a reliable statement. Given the variety of rims/tire combinations, pressure gauge variance, bicycle loads, etc., it would be next to impossible to predict how a tire will react at twice the listed pressure. Like you, I'll go a little over the rated tire but not too much. On some brands...which I no longer use...I won't even go to the rated pressure. I've had touring tires that blew off the rim at less than the rated pressure...4 times in one afternoon That's two blowouts per wheel on two rims that were different brands. It wasn't the rims.

400trix

If you follow enough links from St. Sheldon's article on tire pressure, you will eventually find a document from Frank Berto detailing the typical tire pressure test, which indeed amounts to 1/2 the blow off pressure, assuming you use the same rim and mount your tire correctly. Both of which are big assumptions. That said, a 300# rider is going to have problems finding a tire that will properly support his weight. Maybe Marathon Plus/Supreme? I really don't know. It would be worthwhile to research what the best rim/tire combination would be for your use case.

davidad

That is a defacto industry standard. He should be able to go 50% higher than the sidewall pressure and be safe.

cyccommute 

Read what RetroGrouch has to say. Where is the standard published? The tires I referenced were rated to 90 psi. They blew off at that pressure. I've had others of the same brand that did the same. They blew off while standing on an unloaded bike and in the back of a pickup on a summer's day. Neither were over pressured.

Bill Kapaun

In a previous life, I was a marine Boilermaker and pressure tested boilers at 1.5X operating pressure.
Thing is, that was only done ONCE every 5 YEARS. It was also done hydrostatically, so you didn't have catastrophic ruptures if something did "let go".
How many submarines did we lose in WWII because the captains "pushed the limit"? Just because some got away with it.......

pierce

somewhere on Specialized site, I saw where they said most their bikes are designed for maximum 200 lb rider.

Retro Grouch 

Defacto industry standard?

pierce

they DO that each time you hit a serious bump, or take some G force on a turn.

Airburst

How do you get 50% greater pressure hitting a bump? Wouldn't that require you to decrease the total volume of the tyre by about a third?

JohnDThompson 

Rim design also influences the maximum pressure a tire can withstand. E.g. a hookless rim like the Weinmann A-126/A-129 ("concave") models requires tires be run a lower pressure lest the bead lift out of the rim channel.

FBinNY 

Yes, you'd only get that kind of pressure change with a non compressible fill like water. Otherwise the pressure increase is proportional to the decrease in volume. Since a bump would collapse the tire all the way to the rim before reducing volume by more than a few percentage points, we can consider any pressure change from bumps to be negligeable.

FBinNY 

That depends on what the tire was designed around. These days most HP tires are designed around hook edge rims, but that wasn't always the case. 30+ years ago IRC introduced a 27x1-1/4 90psi tire which had no problem at that pressure on the typical straight sided (Endrich) rims of the rea, including the Super Champions, and the Weinmann concave. There were many such HP tires out there, and they all worked fine.

What's changed, is that these days most HP tires are designed around the hooked rim, and have beads incapable of holding the tire without the assist of the hooked edge. Also modern rims lack the shoulders which properly locate the tire by the bottom of the bad, instead using the hook to locate the tire by the outside of the bead.

davidad

Think maybe the tire manufactures are covering their asses to prevent lawsuits for the idiots that would go over the blow off pressures?
Here's a site that will make it easier to figure at your weight. https://docs.google.com/spreadsheet/...hl=en_US#gid=0