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Clydesdales/Athenas (200+ lb / 91+ kg) Looking to lose that spare tire? Ideal weight 200+? Frustrated being a large cyclist in a sport geared for the ultra-light? Learn about the bikes and parts that can take the abuse of a heavier cyclist, how to keep your body going while losing the weight, and get support from others who've been successful.

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Old 05-21-14, 04:36 PM   #1
WVU_Engineer
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Tire Pressure, should tire deform when i sit down?

Part of why I bought a comfort bike and not a hybrid was due to the wider tire being able to handle more weight and give a smoother ride. I pump my tires up to max pressure which is 65 psi but I notice that my rear tire is still deformed when I sit down, is this normal? Should I try going to 70 psi or will it go boom?
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Old 05-21-14, 04:40 PM   #2
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Dude! We're Clydesdales, not "Tinkerbelles" -- of course it's gonna deform
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Old 05-21-14, 05:22 PM   #3
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Google bicycle tire pressure calculator, pick one.

Target is for tire to deform 15%...
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Old 05-21-14, 05:33 PM   #4
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How much have you ridden? You getting any pinch-flats? If you are, more pressure. And in case you don't know, you can tell a pinch-flat because the tube looks like it was bitten by a rattle snake. It'll have two holes, actually created by the tire flexing out (with the tube stretching with it), then the rim coming down and pinching the tube between the rim and the ground.

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Old 05-21-14, 06:05 PM   #5
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Absolutely normal. Like PhotoJoe said, as long as you're not getting pinch flats you're OK.
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Old 05-21-14, 07:02 PM   #6
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Cool, I knew it would deform it just seemed like a lot but it is hard to judge looking down between my legs and hopefully the neighbors don't think I'm checking myself out.
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Old 05-21-14, 09:20 PM   #7
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I hate to sound snide but tires aren't magic. They have to deform as they are loaded.

Go outside and look at a few car tires. They all have the characteristic bulge at the bottom. Bike tires work the same way, and will have the same characteristic bulge.
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Old 05-21-14, 10:56 PM   #8
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Quote:
Originally Posted by WVU_Engineer View Post
Part of why I bought a comfort bike and not a hybrid was due to the wider tire being able to handle more weight and give a smoother ride. I pump my tires up to max pressure which is 65 psi but I notice that my rear tire is still deformed when I sit down, is this normal? Should I try going to 70 psi or will it go boom?
What is the max inflation pressure for your tires? It's stamped on the sidewall of the tire. Don't go over that, but as a clyde you should probably be right at that number, and check it before each ride (or at least daily)
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Old 05-21-14, 11:07 PM   #9
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Go outside and look at a few car tires. They all have the characteristic bulge at the bottom.
Someone beat me to it. The deformity is normal.
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Old 05-22-14, 06:46 AM   #10
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The only tires that don't deform are solid rubber *OR* those on the 1931 W-30. Neither would be my choice to ride on...
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Old 05-22-14, 02:45 PM   #11
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Like I said above, of course I knew there would be deformation but I should have written "more than expected".

What I was getting at was my tires have a recommended pressure of 45-65 psi, I ran at 70 psi a couple days ago and it made a noticeable difference and was curious if any other big guys did this.

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Old 05-22-14, 04:34 PM   #12
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There's usually very generous safety margins built into tire pressure ratings. After all they have to account for inaccurate gauges and pressure climb from braking heat and/or hot pavements.

So fee free to exceed rated pressures by 25% or so (usually) if it improves performance. Finding ideal pressure is about experimentation, and different people of the same weight, and using identical tires, won't always agree on what works best, which is OK. What's best for you is what you think is best for you, so feel free to exceed rated pressure, but be aware that heat can cause further increase.

BTW- my way of optimizing pressure, is to raise by degrees (5psi or so) over time, and riding for a while before raising again. When I find a level where the drawbacks -- skittish handling, harsh ride, etc. -- become noticeable, I back off to the last pressure that rode nice.
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Old 05-22-14, 07:26 PM   #13
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Quote:
Originally Posted by TrojanHorse View Post
What is the max inflation pressure for your tires? It's stamped on the sidewall of the tire. Don't go over that, but as a clyde you should probably be right at that number, and check it before each ride (or at least daily)
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Old 05-22-14, 08:59 PM   #14
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Quote:
Originally Posted by FBinNY View Post
There's usually very generous safety margins built into tire pressure ratings. After all they have to account for inaccurate gauges and pressure climb from braking heat and/or hot pavements.

So fee free to exceed rated pressures by 25% or so (usually) if it improves performance. Finding ideal pressure is about experimentation, and different people of the same weight, and using identical tires, won't always agree on what works best, which is OK. What's best for you is what you think is best for you, so feel free to exceed rated pressure, but be aware that heat can cause further increase.

BTW- my way of optimizing pressure, is to raise by degrees (5psi or so) over time, and riding for a while before raising again. When I find a level where the drawbacks -- skittish handling, harsh ride, etc. -- become noticeable, I back off to the last pressure that rode nice.
I was wondering the same thing as the op. Thanks for this info; good to know!
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Old 05-22-14, 10:02 PM   #15
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Quote:
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Go outside and look at a few car tires. They all have the characteristic bulge at the bottom. Bike tires work the same way, and will have the same characteristic bulge.
Well, that's not true. It depends on sidewall height, construction, rim bead width, and width ratio between tire and wheel.

For example:

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Old 05-22-14, 11:46 PM   #16
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on a road riding setup you are ideally looking for about a 15% drop... different tires will be slightly different but it's a good average... to get this you need to figure out the height of the tire from ground to rim... then take 15% of that number, when you sit on the bike (in your proper riding position) that is the height the rim edge should sit... this requires a friend and in reality I don't know any normal rider who's done this.

on a MTB out on the trail I tend to go with a more generic of "enough" I always started around 35psi (29er so larger air volume allows lower pressure than similar sized tire 26er)... if I felt the tire was rolling out from under me or I ever felt it hit the rim (or got a snake bite) i'd add a bit of pressure, on the other end I'd let out a bit of pressure as I'd ride to get the pressure low enough to what felt right... this is far easier to do and more important on a rigid bike.

here is a helpful calculator, but its only a limited size and for 700c tires.
Bicycle tire pressure calculator

in my experience I would say that most larger (250+ lbs) road clyds are prob running too little pressure based on the 15% drop rule of thumb and that most skinny riders are running too high of pressure
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Old 05-23-14, 04:43 AM   #17
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Well, that's not true. It depends on sidewall height, construction, rim bead width, and width ratio between tire and wheel.

For example:

True though for normal properly fitted and pressurized tires. Those things are ridiculous.
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Old 05-23-14, 08:02 AM   #18
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True though for normal properly fitted and pressurized tires. Those things are ridiculous.
That's still incorrect. Many stock offerings on wide, lo-pro rubber exhibit no bulging.

I also think that, rather than being ridiculous, the above pictured wheels are super stylish, and while impractical (in terms of damage risk and street ride quality, anyway), definitely make a statement about wheels and tires, specifically that the boundaries are probably way beyond what you think you know.

Stock BMW M3 '14:

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Old 05-23-14, 08:39 AM   #19
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I think if someone put a heavy enough load in those low pro cars, (like a clyde on a road bike) the tires would bulge. "Think" being the key word.
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Old 05-23-14, 09:08 AM   #20
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I think if someone put a heavy enough load in those low pro cars, (like a clyde on a road bike) the tires would bulge. "Think" being the key word.
There is a little factor known as suspension...
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Old 05-23-14, 09:11 AM   #21
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There is a little factor known as suspension...
And you don't think the suspension transfers the weight to the tire?

Of course those tires are bulging, it's just not as noticeable as it is on other tires.
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Old 05-23-14, 09:20 AM   #22
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I suspect the OP really isn't concerned with the tires on his car bulging.
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