Tire Pressure Revisited

[kokopellioh]

I checked the tire pressure for the first time this past weekend, and found it at what I thought were unbelievable levels. Now I'm not so sure.

Read the book and sticker, which said 29 front and 29~36 rear depending on weight. Me, I'm 225, plus riding gear and backpack of stuff for work.

Found the front around 36 and the rear around 42. Backed them off some for my Sunday ride, and tonight after the tires got stone cold, backed them down to 29/36. What I’ll be looking for tomorrow is whether29 in the front cures some squirrilyness I found on the trips in the afternoon around 50 mph.

I read some old forum notes, and now am concerned: Is 29 psi too low for the front? Is there anything seriously wrong with 29 front and 36 rear?

[John Hopkins]

If that's you in the photo I reckon you could go to 32 on the front, and then look closely at the tire to see if there are any age cracks...any higher than 32 on the front will help cause the squirrely feeling..

John.

[oconeedan]

Tire pressure is an individual preference. Generally, the higher the pressure, the better it handles, up to a point. Also, the higher the pressure, the better the fuel mileage you'll get. And, the higher the pressure, the harsher the bumps will feel.
That said, never go above the tire manufacturer's max pressure on the side of the tire. It is a good idea to follow the parameters of the sticker on your bike, or manufacturer's recommendation.
Always measure pressure when the tire is cold.
Dan

[rjdaines]

AZ is having some big temp swings right now so checking cold could be an ambient temp if 70 or 90, not sure how much of a difference that will make. My Aero is set for 30 front and back.

[adlowe]

You got stock tires? What are they?

Generally, you should run with the cold pressure that gets you a 10% increase in PSI when the tires get hot. Good luck finding a reasonable way to determine that!
o A rule of thumb that I've never tested: Each 10°change in ambient temperature = 1 lb change in PSI. If you're playing around to find that 10% "sweet spot" then please see what you get with the 10°swings.

My tires are Metzeler ME880s and I run at the high end of Metzeler's recommendations -- about 40 PSI front and 42 PSI rear.

[gat803]

The tire pressure that anyone runs depends on many factors, and can/should change depending on how the bike is used, the load it carries, the surface it's used on, the ride desired, the brand of tires, etc.

There should be no "one size fits all" air pressure recommendation, only a starting point to tune from.

[Scrapdog2grand]

as adlowe stated - I was taught 1 psi for every 11 deg (or about 10 deg), increase in tire temp.
tire size and type comes into play, but this seams to be pretty accurate for all tires.
so you can use this to adjust them hot and be in the ball park, if you keep your infared temp gun with you

I found that 29 was a little to low for me on the front and I'm only 180, I'm up to 31-32 cold.

[kokopellioh]

Wow, thanks guys!

1. Yep that's me. If I'm up at the crack of dawn again tomorrow, I'll sneak over for some free air at Quick Trip and fill the front back up to 32 ... maybe get some gas too, who knows. Think I'll leave the rear at 36.

Speaking of which:
Front: Maxxis 120/90-17
Rear: Shinko 150/80-15
Not sure either of the brands or good bad or indifferent, or what direction to go if I change. Rear is so new it has the little spikey things still showing all around. Front is well loved, but still legal. Would rather it not be an April expense, and I have no plans to leave city streets until at least April 29th. Maybe not even after that.

2. I dug into the historical weather for last night at 10:30 PM (Weather Underground), and it was around 70 degrees or so. old is relative. Come July one would need a air conditioned garage to get any where near cold.

[kokopellioh]

Originally Posted by gat803

The tire pressure that anyone runs depends on many factors, and can/should change depending on how the bike is used, the load it carries, the surface it's used on, the ride desired, the brand of tires, etc.

There should be no "one size fits all" air pressure recommendation, only a starting point to tune from.

For me the starting point is:

Commuting on city streets at less than 50 mph for the next month or so, then 70/30 commuting / freeway.

I'm 225 before gear and backpack with work stuff.

NEED to get real comfortable with low-speed work for rider safety course scheduled for April 29th so I don't look like a total NOOB during the course.

[Earthling789]

Originally Posted by kokopellioh

NEED to get real comfortable with low-speed work for rider safety course scheduled for April 29th so I don't look like a total NOOB during the course.

No problems being a Noob, and the course is designed with Noobs in mind
Of course, I do agree, if you're not worrying about "how to work a clutch", you'll be more attentive to learn some good info an advice about "riding" ...


WARNING - SCIENCE LESSON

Now, as for the comments about measuring tire pressure "cold", that simply means "tires at same temp as ambient air"... in other words.. tires that have been stationary for several hours, and allowed to cool (internally and externally) to the same relative temperature as the air temp.

This is why it is a good idea to check air "before" you ride.

Yes, others are correct, each 10-deg change in air-temp will cause tires to gain or lose 1-psi... so measure your tires in the morning at 40-deg... and they are perfect... but then check them again in the afternoon (temps in the 90s), and they will show 5-psi higher....

Tires also heat up as they roll due to friction generated heat... that whole molecular excitation issue, lol... so after you've ridden a few miles, tires will show up to 5-psi higher pressure than they would compared to "cold", because they are now "hot".

Several companies have duplicated and published an experiment where they put different size tires under load (real-wold conditions), with different starting pressures in each tire... measured in 5-min intervals, each tire, regardless of tire size or starting pressure, gains ONE PSI for each five minute interval, and stabilizes internal pressure after 20-minutes (on average)... thus giving the 4-5-psi gain "rule of thumb" for "hot" tires.

SO, knowing this, you can safely gauge proper pressure after you've ridden a bit.... Need 32-psi "cold"? Then tires should measure 36-37-psi when "hot".

This is why some people can become obsessed with tire pressure... air temps, sunshine (radiant heat), and driving on tires will change the pressure... which is why it is something you have to play with and judge for yourself what works best for you, your tires, load, and driving style.