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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 06-22-07, 04:24 PM   #1
Stujoe
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Tire Pressure Question

Probably simple but I am not sure (and it has a Clyde issue at the end)...

My tires (Specialized Resolution 26x2.1 knobbies) say they are 35-80psi but I can't find anything on my rims (Specialized Alex HR) that say what pressure they are good for. This is all stock stuff on my HardRock.

Do rims have a limit or are they designed for more than any tire could handle? One other thing that entered my mind...do the tubes have any pressure ratings?

I am at 50psi right now but was thinking maybe I should go up to 60 or so since I mostly ride on pavement and crushed limestone trails.

Oh, and does rider weight play a part?...as in, does my 255lbs reduce the max from 85psi to something else?
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Old 06-22-07, 04:42 PM   #2
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No worries, just use the max tire pressure.....

The rims will take it, and when you or if you switch to higher pressure slicke, no worries there either because the rims will also handle 100PSI.
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Old 06-22-07, 05:05 PM   #3
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Nope you don't have to worry about your weight blowing rims off the tire. If you air it up and it doesn't blow off you should be fine.
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Old 06-22-07, 05:09 PM   #4
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Your rim limits are for tire/tube width. Whatever pressure the tire/tube has should be fine.
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Old 06-22-07, 05:16 PM   #5
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Thank you for the info! Eases the mind for sure.
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Old 06-22-07, 05:36 PM   #6
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I weigh far more than you and I push 80PSI.
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Old 06-22-07, 05:45 PM   #7
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And you will find the riding a LOT easier going when you inflate your tires to the max indicated on the sidewall. After you get hooked on that, NO CHEATIN and putting in just a little bit more air, based on that usual man theory of "if X is good, then X + Y = better." After all, do what I say, not what I do.
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Old 06-22-07, 06:15 PM   #8
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I just got this image of Tim Allen saying 'More Power!" right before something blows up.
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Old 06-22-07, 06:22 PM   #9
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Quote:
Originally Posted by Stujoe
Probably simple but I am not sure (and it has a Clyde issue at the end)...

My tires (Specialized Resolution 26x2.1 knobbies) say they are 35-80psi but I can't find anything on my rims (Specialized Alex HR) that say what pressure they are good for. This is all stock stuff on my HardRock.

Do rims have a limit or are they designed for more than any tire could handle? One other thing that entered my mind...do the tubes have any pressure ratings?

I am at 50psi right now but was thinking maybe I should go up to 60 or so since I mostly ride on pavement and crushed limestone trails.

Oh, and does rider weight play a part?...as in, does my 255lbs reduce the max from 85psi to something else?
It's a complex system, but force on the rim is a function of both air volume and pressure, a 55mm wide tire at 65PSI, a 35mm tire at 85PSI and a 25mm tire at 105PSI all have different volumes, so may in fact exert similar forces on the rim, I don't know exactly, because I don't have the formula for figuring it out.

The force on a rim, is mostly where the rim and tire meet, your more likely to blow a tire off a rim, then damage the rim. Tubes are held in place by the tire, so the tire takes most of the force. Rider weight is a very small factor, although it also depends on riding style, a 150lb rider, riding heavy (seat firmly planted on saddle at all times) can put more force on a rear tire, then a 300lb rider, riding light (lifts seat for bumps, pot holes, crappy pavement, railway crossings, etc.).

Having said all that, there are times you might want to lower your pressure, for example, when off road, a little lower pressure can help, give the tires more grip, too low though, and you get more snake bite punctures.
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Old 06-22-07, 07:04 PM   #10
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I probably won't be doing a lot of off road which is why I wanted to look into raising the pressure. Probably not long or difficult off road at all. I will probably do 90% road and crushed limestone and 10% something else...the occasional dirt path, grass, field, ride down to the shore of a lake, etc.

I may end up with a less knobby, smaller tire eventually but I think I need to get a real good idea of what conditions I am going to ride. I eventually put thin FatBoy slicks on my MTB years ago but I don't think I want to go that far again. They were fast but pretty much eliminated any idea of off road stuff. I imagine they have more in between tires these days.

I have the knobbies at around 70psi now. I am riding the paths in the morning and will see how it goes. I want a better pressure guage before I take them up closer to the 80 limit. I do ride pretty light and use my arms and legs as shock absorbers as I go over bumpy areas.
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Old 06-22-07, 10:02 PM   #11
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Quote:
Originally Posted by Wogsterca
It's a complex system, but force on the rim is a function of both air volume and pressure, a 55mm wide tire at 65PSI, a 35mm tire at 85PSI and a 25mm tire at 105PSI all have different volumes, so may in fact exert similar forces on the rim, I don't know exactly, because I don't have the formula for figuring it out.
I'd Imagine that the force exerted by pressure on the rim in a static situation would be something along the lines of

Surface area of rim/tube contact in inches x pressure in tube = Force exerted by the rim on the bead of the tire.

Which i suppose explains why there is a great deal of risk of rolling a tire off the rim at lower pressures....

Under a dynamic load the physics gets really ugly really fast... I just tried to play with it on the back of an envelope... and gave up when I hit 6 different forces.
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Old 06-22-07, 10:03 PM   #12
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Quote:
Originally Posted by Stujoe
I probably won't be doing a lot of off road which is why I wanted to look into raising the pressure. Probably not long or difficult off road at all. I will probably do 90% road and crushed limestone and 10% something else...the occasional dirt path, grass, field, ride down to the shore of a lake, etc.

I may end up with a less knobby, smaller tire eventually but I think I need to get a real good idea of what conditions I am going to ride. I eventually put thin FatBoy slicks on my MTB years ago but I don't think I want to go that far again. They were fast but pretty much eliminated any idea of off road stuff. I imagine they have more in between tires these days.

I have the knobbies at around 70psi now. I am riding the paths in the morning and will see how it goes. I want a better pressure guage before I take them up closer to the 80 limit. I do ride pretty light and use my arms and legs as shock absorbers as I go over bumpy areas.
If you don't need the traction, slick tires are pretty awesome.
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Old 06-23-07, 08:35 AM   #13
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Quote:
Originally Posted by Halthane
I'd Imagine that the force exerted by pressure on the rim in a static situation would be something along the lines of

Surface area of rim/tube contact in inches x pressure in tube = Force exerted by the rim on the bead of the tire.

Which i suppose explains why there is a great deal of risk of rolling a tire off the rim at lower pressures....

Under a dynamic load the physics gets really ugly really fast... I just tried to play with it on the back of an envelope... and gave up when I hit 6 different forces.
Does the contact area need to be inches, I think the math would probably be easiest in mm˛ after all smaller units eliminate fractions, and inches seem to only make sense with fractions......
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Old 06-23-07, 09:08 AM   #14
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Quote:
Originally Posted by Halthane
If you don't need the traction, slick tires are pretty awesome.
No doubt. When I had the slicks on years ago, it was much faster and much more quiet.

The tires today did fine on crushed limestone and asphalt. Hard to tell how much faster as I did more limestone than I have ever done before - which tends to slow me down - but my average speed was still higher than normal. About 1 mph faster over 30 miles.
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Old 06-23-07, 09:18 AM   #15
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It is essential that a tire and rim be designed for each other. If a tire is a perfect match for a rim, you will have no problem using the "maximum" PSI stamped on the tire, and can add an extra 10 PSI to the rear tire without problems.

As a rule of thumb, any rider over 250 pounds needs to do two things:

1. Make sure his tires are the correct width and correct design for a secure fit on his rims.

2. Inflate the front tire to the maximum PSI stamped on the tire. Inflate the rear tire to the maximum PSI, or even 10 PSI beyond the maximum PSI (perfectly safe if the tire is the correct width for the rim).

Tires are designed to provide their best performance at about 15% deflection. That 15% deflection means that the rim sags slightly toward the pavement when the rider gets on the bike. That means the "best" PSI depends on the weight of the rider. With a tire that has a "maximum" PSI of 80 PSI, a 250 rider might get MORE than 15% deflection at 80 PSI. If that occurs, the rider really should move to a wider size of tire.

And, a 90 pound rider might get 15% deflection on that SAME tire at just 35 PSI. One of the reasons many women complain about a harsh and painful ride is that some GUY has inflated her tires to the correct PSI for a 250 pound guy. And, with her weight, that high PSI insures her tires will ride like they are made of iron.
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Old 06-23-07, 12:18 PM   #16
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My Hardrock is an ENTIRELY different beast when I run the tires at 80 as opposed to 50 or 60. Entirely wonderful! Seriously, I couldn't believe the difference when I took a suggestion and cranked 'em up to the max. It was.. glorious. Magically my speed averaged jumped several miles per hour, and I wasn't pushing as hard all the time. I'd recommend giving 80 a shot, and see what you think.

Same thing with my roadie, the tires are rated to 110psi. I can actually feel a difference when they dip below 90, it's crazy.
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Old 06-23-07, 01:00 PM   #17
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Quote:
Originally Posted by Wogsterca
Does the contact area need to be inches, I think the math would probably be easiest in mm˛ after all smaller units eliminate fractions, and inches seem to only make sense with fractions......
You'd have to convert the pressure from PSI to metric units to use mm˛ and if you just let the numbers collapse into long decimals say 4-5 places you'd end up in the same place i.e. no fractions.

I found a book at a bookstore here in town called Bicycle Science by david gordon wilson. Its really a textbook more than anything else, but it explores some of the physics in pretty serious detail. They really are amazingly complicated. I'll try to play with drawing up some of the force diagrams and post them, just because they really are unimaginably complicated. To really get viable equations you'd have to simplify some things, and I don't understand enough trigonometry/calculus/dynamics to do that. Jobst Brandt's The Bicycle Wheel is also reputed to cover these things really well. I don't own a copy yet, but I look forward to it.
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Old 06-23-07, 01:35 PM   #18
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Quote:
Originally Posted by Halthane
You'd have to convert the pressure from PSI to metric units to use mm˛ and if you just let the numbers collapse into long decimals say 4-5 places you'd end up in the same place i.e. no fractions.

I found a book at a bookstore here in town called Bicycle Science by david gordon wilson. Its really a textbook more than anything else, but it explores some of the physics in pretty serious detail. They really are amazingly complicated. I'll try to play with drawing up some of the force diagrams and post them, just because they really are unimaginably complicated. To really get viable equations you'd have to simplify some things, and I don't understand enough trigonometry/calculus/dynamics to do that. Jobst Brandt's The Bicycle Wheel is also reputed to cover these things really well. I don't own a copy yet, but I look forward to it.
Given that rim diameters are provided in mm, and that mm rules are available, and it's simple to get mm˛,
rim diameter from Sheldon brown a 26" wheel is 559mm or 22.007874 inches given a distance of about 5mm or 0.19685 inches, converting PSI to millibars is easy visit www.onlineconversion.com.... Your gauge probably has both on it anyway.

So we work with numbers like 559 x 5 x 2 5590mm˛ or roughly 8.66451 square inches. I think you actually would need square inches rather then inches, to work it right though.

Actually speaking of metric, my Brother-in-law just bought a new motorcycle (Kawasaki Ninja 650R) , the speedometer is km/h only, this is Canada, which has had metric speeds for about 20 years now, don't know if cars still have both or not, any car I would get (mine just died) is going to be an older one I can buy for cash.
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Old 06-23-07, 05:33 PM   #19
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Quote:
Originally Posted by Stujoe
Probably simple but I am not sure (and it has a Clyde issue at the end)...

My tires (Specialized Resolution 26x2.1 knobbies) say they are 35-80psi but I can't find anything on my rims (Specialized Alex HR) that say what pressure they are good for. This is all stock stuff on my HardRock.

Do rims have a limit or are they designed for more than any tire could handle? One other thing that entered my mind...do the tubes have any pressure ratings?

I am at 50psi right now but was thinking maybe I should go up to 60 or so since I mostly ride on pavement and crushed limestone trails.

Oh, and does rider weight play a part?...as in, does my 255lbs reduce the max from 85psi to something else?
See: http://sheldonbrown.com/pressure

However, you should REALLY ditch the knobbies, they are totally unsuited for the use you describe.

If you replace them with smooth tread tires, you will be astonished by how much more pleasant the ride becomes.

Knobbies are only good for mud, sand and the like, soft surfaces where the knobs dig in. They are very slow on hard surfaces, and give very bad handling/cornering performance.

Sheldon "Smoooth" Brown
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Old 06-23-07, 05:53 PM   #20
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Thanks for the link!

I know my current tires are way overkill for my use. I am more than likely going to replace them. There are so many different options out there, though. lol

I was thinking something smoother down the middle with some grip on the sides might work. Something like the Specialized Crossroads. The link above doesn't like that type much though. I am a little afraid to go with something like the Specialized Nimbus which doesn't have much tread at all because I am not sure how it would handle on the limestone and packed dirt. I have a No Fall rule I like to obey.
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Old 06-23-07, 08:44 PM   #21
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Quote:
Originally Posted by Stujoe
Thanks for the link!

I know my current tires are way overkill for my use. I am more than likely going to replace them. There are so many different options out there, though. lol

I was thinking something smoother down the middle with some grip on the sides might work. Something like the Specialized Crossroads. The link above doesn't like that type much though. I am a little afraid to go with something like the Specialized Nimbus which doesn't have much tread at all because I am not sure how it would handle on the limestone and packed dirt. I have a No Fall rule I like to obey.
I don't know that I'd say "overkill" they're just totally mismatched to the surfaces you ride on.

Something like the Nimbus would be an excellent choice. The ones that are smooth in the middle and knobby on the side are snake oil, a generally bad design. Their handling changes suddenly as you lean over into a sharp turn and the knobs begin to meet the surface. The knobs have much worse roadholding on hard surfaces than smooth tires give, because the knobs bend over in unpredictable ways.

Are you in the habit of doing tight high-speed cornering on the limestone trails you ride?

Knobby tread is BAD for traction on hard surfaces. Smooth tires are the SAFEST as well as the most comfortable and easy rolling.

Knobbies are ONLY useful in mud, soft sand and similar surfaces.

Sheldon "Bald Is Beautiful" Brown
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Old 06-23-07, 09:55 PM   #22
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Quote:
Originally Posted by Wogsterca
Given that rim diameters are provided in mm, and that mm rules are available, and it's simple to get mm˛,
rim diameter from Sheldon brown a 26" wheel is 559mm or 22.007874 inches given a distance of about 5mm or 0.19685 inches, converting PSI to millibars is easy visit www.onlineconversion.com.... Your gauge probably has both on it anyway.

So we work with numbers like 559 x 5 x 2 5590mm˛ or roughly 8.66451 square inches. I think you actually would need square inches rather then inches, to work it right though.

Actually speaking of metric, my Brother-in-law just bought a new motorcycle (Kawasaki Ninja 650R) , the speedometer is km/h only, this is Canada, which has had metric speeds for about 20 years now, don't know if cars still have both or not, any car I would get (mine just died) is going to be an older one I can buy for cash.
Good point, hadn't thought about rim sizes being stated in metric... and yeah you would need square inches since its a surface area issue. Don't forget Pi when you calculate rim circumference. I like metric, just not used to it.
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Old 06-23-07, 09:56 PM   #23
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+1 to the nimbus I love mine.
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Old 06-23-07, 10:55 PM   #24
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I know personally that the slick tires look downright scary for limestone - but trust me - 100% safe! I've ridden about 150 miles on limestone in the past several weeks on my 700x32 Bontrager Race Lites (which are SLICKS) and had no issues. In fact, this morning I rode about 10 miles on a very wet brand-new limestone trail that was something akin to riding on sand, I'd assume. I experienced nary a issue, thankfully, even when coming upon a unexpected mud section! I thought for sure I was dead, but nope, I lived .

I've ridden my slicks on washboard gravel, gravel, limestone, mud, and pavement without any problems. However, I HIGHLY recommend against mud at all possible! In fact, I've even seen full-on carbon road bikes on one of those limestone trails, and I assume they lived. There's a group of roadies that rides a portion of it on a regular basis.

Another to check out is the Continental Town and Country. I've heard nothing but raves about it, it's supposedly a great tire. The Nimbus looks quite good as well.

Good luck! Let me know how it goes, I'll use you for a guinea pig for my Hardrock tire upgrade
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Old 06-24-07, 09:35 AM   #25
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It is sounding more and more like I am needlessly being a scaredy cat about the slicker tires. lol It is probably especially silly considering I used to have 1.25'' slicks on my old MTB 15 years ago and never wiped out and only had one flat in thousands of miles.

I am going to try the Nimbus.
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