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Have your say - Analyze my tire inflation...

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Have your say - Analyze my tire inflation...

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Old 03-13-18, 02:05 AM
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Abu Mahendra
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Have your say - Analyze my tire inflation...

What does the contact patch tell you? Is my rear tire (Schwalbe Marathon Supreme 42-406) over-inflated, under-inflated, or just right. Surface is mostly smooth bitumen asphalt. Don't over-think it. Over, under or just right.
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Old 03-13-18, 06:13 AM
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Absolutely no idea from that photo. How heavy are you? What size is the tire? How about a ruler in the picture to let us know how long and wide the contact patch really is.
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Old 03-13-18, 06:16 AM
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Apart from a severely under inflated or flat tire, there is nothing to know about pressure from looking at photo's of the tire.

Photos of the tread and wear are not how correct pressure is determined.



-Tim-

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Old 03-13-18, 06:36 AM
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my weight is already factored in. if i told you i weigh 77kg, how would that be of use to you? Look at the width of the contact patch--the dusty, white area in the middle--relative to the width of the tire (42mm, given earlier).

Originally Posted by HillRider View Post
Absolutely no idea from that photo. How heavy are you? What size is the tire? How about a ruler in the picture to let us know how long and wide the contact patch really is.

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Old 03-13-18, 06:37 AM
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no? really? you derive no information from the relative width of the contact patch? no one here mentioned the tread or wear. Look at the width of the contact patch relative to the width of the tire.

Originally Posted by TimothyH View Post
Apart from a severely under inflated or flat tire, there is nothing to know about pressure from looking at photo's of the tire.

Photos of the tread and wear are not how correct pressure is determined.



-Tim-
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Old 03-13-18, 07:01 AM
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A contact patch is measured by both width and length and is an area, not a linear measurement so width alone means nothing. If a tire is inflated to 100 psi and the load on it is 100 pounds, the contact patch will be 1 square inch. So the weight and inflation pressure determine the contact patch area.

In the absence of better information I also think it's pretty badly underinflated.
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Old 03-13-18, 08:03 AM
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the percentage that the tire drops under his weight can be calculated by the width of the contact patch, knowing the tire size. Some geometry and I get

percentage deflection is 1/2(1- sqrt(1-P^2/S^2) ) where P is patch width and S is tire size.

Since the percentage deflection is a fairly common way to decide what the desired inflation is, I say that OP is right. It's only a ballpark of course and would change according to intended use and personal predilection.
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Old 03-13-18, 08:17 AM
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Originally Posted by Abu Mahendra View Post
Don't over-think it.
You already have.

EDIT: I retract this statement. You have definitely not put an over-abundance of thought into this.

Last edited by Kapusta; 03-13-18 at 09:48 AM.
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Old 03-13-18, 08:33 AM
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It doesn't look bad to me. I think it's a lot easier to tell when a tire has been overinflated than otherwise, though.
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Old 03-13-18, 08:34 AM
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Originally Posted by Abu Mahendra View Post
What does the contact patch tell you?[/IMG]
Nothing. Rear tires all square off over time regardless of how they're inflated.
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Old 03-13-18, 08:40 AM
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These aren't like car tires, your car tire is much wider so that system works.
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Old 03-13-18, 08:46 AM
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What do your stats and body tell you? I don't care what anyone recommends. I don't care what tire pressure calculators say. I don't care how the tire is displaying wear when considering the pressure I ride.

If you track stats/metrics for each of your rides... keep your tire pressure adjusted to what appears to give you the compromise of comfort, speed and handling you want.

Keep you pressure consistent for a half dozen or so rides and learn what that does for you. Then raise or lower the PSI ten percent or so and see what you think about the stats and comfort for the next half dozen rides. repeat till happy.
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Old 03-13-18, 09:32 AM
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Originally Posted by Abu Mahendra View Post
no? really? you derive no information from the relative width of the contact patch?
Unless you have only ridden in a straight line since the last time the tire was washed, there is no way to know what the width of the contact patch is based on that photo.

All you can tell is that the actual contact patch is most likely as wide as, or narrower than, the strip of dirt on it.

Also, what do you consider “just right” inflation? My experience with those tires is that you have a choice of harsh, slow, or some combination of the two.

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Old 03-13-18, 09:48 AM
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Back in the olden days I worried a lot about if I was riding fast enough and I assumed that higher pressure = faster speed. I topped up my tires to 120 psi before every single ride.

At some point comfort started to become more important to me. On my road bike I lowered the air pressure to 90 psi front/100 psi rear. I still topped them up before every ride. The ride quality improved and I didn't feel like I had lost any speed.

Today I ride a recumbent with wider, lower pressure rated tires. Also, I've gotten lazy. I start my tires at about 75 psi and just ride. After 2 or 3 weeks they bleed down to about 50 psi and only then do I top them back up again. If I noticed a big difference in either performance or comfort with that 33% pressure drop, I assume that I'd change my practice. I'm not noticing any significant difference either way and I like the care free feeling of not having to pump my tires all the time.

That's what happens whey you really stop overthinking air pressure.
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Old 03-13-18, 10:01 AM
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You need to have bike and rider on a glass table , photographed from beneath, to see the contact patch, rather than just the soil on the tire edge.
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Old 03-13-18, 10:30 AM
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You like it?
Perfect
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Old 03-13-18, 10:48 AM
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Originally Posted by Abu Mahendra View Post
no? really? you derive no information from the relative width of the contact patch? no one here mentioned the tread or wear. Look at the width of the contact patch relative to the width of the tire.
I would not say that I derive nothing but it really doesn't speak at all to how the tire rides with the rider on it. One minute on the bike will tell infinitely more.

Suppleness, grip approaching the limits of adhesion in both wet and dry, predictability, nervousness, harshness on rough surfaces - these are very difficult to test objectively and change with pressure. A photo of the tread can't tell us these things and the reason why rolling resistance tests are of limited value.

I will grant that your photo shows even tire wear and you asked about pressure specifically. If even tire wear as shown in the photo is the only measure of correct pressure then your pressure looks good to perfect. That's all we can tell.

You might want to clean the tire, wipe it several times with a wet rag around the circumference and then post another photo. Maybe we will be able to see more.

I would simply ask how you feel it handles.


-Tim-

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Old 03-13-18, 11:59 AM
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Originally Posted by kapusta View Post
you already have.

Edit: I retract this statement. You have definitely not put an over-abundance of thought into this.
+1

And yet the question is way over-thought. What a paradox.
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Old 03-13-18, 02:05 PM
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It looks like your derailleur cable is sticking too far out to me.

John
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Old 03-13-18, 03:21 PM
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Originally Posted by Kapusta View Post
Unless you have only ridden in a straight line since the last time the tire was washed, there is no way to know what the width of the contact patch is based on that photo.

All you can tell is that the actual contact patch is most likely as wide as, or narrower than, the strip of dirt on it.

Also, what do you consider “just right” inflation? My experience with those tires is that you have a choice of harsh, slow, or some combination of the two.
He should stop with it on top of a ruler and have someone read off the measurement, to get the literal answer to "What does the contact patch tell you?"
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Old 03-13-18, 03:42 PM
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Originally Posted by evan326 View Post
These aren't like car tires, your car tire is much wider so that system works.
It works with car tires because car tires have square profiles. Bike tires have round profiles. On a car tire, you are supposed to the area between the corners, and the wear will show if you are doing that. Bike tires don't have corners, so the wear or dirt on your tread doesn't tell you anything.
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Old 03-16-18, 09:22 AM
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dunno, FWIW here are some of mine











haha this one probably won't help you




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