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Old 05-13-14, 08:45 PM   #51
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Originally Posted by HawkOwl View Post
Interesting chart. But I haven't seen 70Kg since the 6th grade.

Extrapolating it leads to some really big pressures on road bike tires and goes way over manufacturer's recommendations.
The advice from the authors is to go to larger tires, if the pressures are too high for 15% drop on a smaller size. I tend to agree.
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Old 05-13-14, 10:23 PM   #52
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No tires for sale here. Go to the bottom of this PDF:
http://www.biketechreview.com/tires_...sting_rev9.pdf
and note how every increment of pressure up the the 200 psig max resulted in a decrease in CRR. This is true for every tire, up to their sidewall max.
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Old 05-13-14, 10:40 PM   #53
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Vertical forces have zero affect on horizontal motion. So we're looking for horizontal forces only. <snip for brevity>
Vertical accelerations absorb energy. F=MA as you say. F over time = W.

You are ignoring that we ride pneumatic tires which deflect enough to develop a contact patch an inch or two long, depending on tire width and pressure. Chipseal rocks are much smaller than that. If the bumps are larger than chipseal, or even as large as Texas chipseal, then lower pressures are for sure faster. In particular, narrow tires run at pressures all the way down to almost pinch flatting. But for small bumps, not so. See the 1000 HZ roller data, taken with varying tire pressures:
http://www.biketechreview.com/tires_...a_BTR_rev1.pdf
At 20 mph, 1000 Hz would equal chipseal made of ~3/8" rock, pretty common. Note that there's not much change in resistance above 130 lbs. for a 23mm tire at 108 lb. load. Thus I would have been just about as fast with 130 psi instead of 140 psi. No biggie. I went with max sidewall pressure.
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Old 05-13-14, 11:10 PM   #54
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.....Chipseal rocks are much smaller than that. If the bumps are larger than chipseal, or even as large as Texas chipseal, then lower pressures are for sure faster.
Yes, we may be talking at cross purposes. It might be a question of where we live.

I'm not talking about rough pavement as in chipseal vs. smooth blacktop, I'm talking about the kinds of bumpy, patched and uneven roads we see in the northeast. It isn't the vibration or many small insults of coarse pavement that creates what I called "bump braking", it's vertical bumps in the1" + range where the strike angle is less vertical an therefore generates the braking effect.

Brandt referred to cattle guards, but here in the east we ride on things like cobblestone, and patchwork quilt roads. and thedrawbacks of excess pressure become obvious.
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Old 05-14-14, 04:41 AM   #55
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Originally Posted by Carbonfiberboy View Post
No tires for sale here. Go to the bottom of this PDF:
http://www.biketechreview.com/tires_...sting_rev9.pdf
and note how every increment of pressure up the the 200 psig max resulted in a decrease in CRR. This is true for every tire, up to their sidewall max.
I know this. Read what I wrote. Larger and less inflated tires will roll faster than smaller higher inflated tires....also, look at the curves of inflation pressure vs CRR and there is a point of diminishing returns; furthermore, most of these data are from the lab. Real world figures would show a greater a greater efficiency of say a 23mm pumped to 115 psi vs say a 28 mm pumped to 85 psi. For a larger rider on rough roads, my money would be with the 28mm assuming the same tire. Again. My comparison was from tire size to tire size and not a simplistic look at one CRR vs inflation curve.

Look, don't try to understand what I posted. Who cares. I know you like me are an old curmudgeon and you are not going to change.

The fastest and best tire/tube overall combo for many riders will probably be the Continental GP4000 Sii in 700x28 with latex tubes whenever they get to the USA. Lowest rolling resistence, reasonable flat resistence, half decent comfort, and long wearing. The Vittoria evo cx iii clincher will roll even better and be more comfy but less aerodynamic and shorter life. The Schwalb One is also a great choice in a 700x28.
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Old 05-14-14, 04:50 AM   #56
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Originally Posted by FBinNY View Post
Yes, we may be talking at cross purposes. It might be a question of where we live.

I'm not talking about rough pavement as in chipseal vs. smooth blacktop, I'm talking about the kinds of bumpy, patched and uneven roads we see in the northeast. It isn't the vibration or many small insults of coarse pavement that creates what I called "bump braking", it's vertical bumps in the1" + range where the strike angle is less vertical an therefore generates the braking effect.

Brandt referred to cattle guards, but here in the east we ride on things like cobblestone, and patchwork quilt roads. and thedrawbacks of excess pressure become obvious.
In the Northeast the roads are horrible especially after this winter. My relatively wide Challenge Paris-Roubaix tires not only efficiently absorb much of the vertical deflections but they also stay planted on corners much better than narrower, higher pressured tires due to enhanced contact patch size and compliance to the road.

The type of ride also matters. If someone is going on a short ride (maybe under 4 hours) at a higher speed (more than 20mph), comfort matters less and aerodynamics play a larger role in the tire selection process.
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Old 05-14-14, 05:10 AM   #57
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Anybody remember when Bicycling magazine under Ed Pavelka actually had real engineers on staff and wrote technical articles? The recommendation then was to run tires at 15% deformation, not maximum pressure. Berto rode wider tires. Nuff said.

This advice is true today as well.

Quote:
Once reserved for training, touring or Paris-Roubaix, now 25mm tyres are being used by nearly half the World Tour peloton. Increased grip and better rolling resistance come with width.
Reinventing the Wheel: The 25mm Revolution

Quote:
Wider tires roll faster than narrower ones: Riders have argued for years that narrower tires – especially on the road – roll faster and are more efficient than wider ones when in fact, the opposite is true. According to Wheel Energy, the key to reducing rolling resistance is minimizing the energy lost to casing deformation, not minimizing how much tread is in contact with the ground. All other factors being equal, wider casings exhibit less 'bulge' as a percentage of their cross-section and also have a shorter section of deflected sidewall
Bicycle tires ? puncturing the myths - BikeRadar
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Old 05-14-14, 05:30 AM   #58
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Different tires, different routes, different air pressures;

700x25 Vittoria Corsa CX. This is a race quality 320 tpi tire: 105 front, 120 psi rear
700x28 Vittoria Rubino ProTech. This is a 4 season 150 tpi training tire: 90 front, 105 rear
700x27 Challenge Parigi-Roubaix.This is a 300 tpi high performance tire for damaged roads: 90 front, 105 rear
700x32 Vittoria Voyager Hyper. This is a 120 tpi touring tire for pavement and gravel:70 front, 85 rear
700x40 Vittoria Voyager Hyper. This is a 120 tpi touring tire for pavement and gravel:55 front, 70 rear

A smaller tire requires a higher air pressure than the same tire in a larger size. A lighter tire with a higher tpi (thread count) also benefits from a higher air pressure since the light and flexible construction is less stiff.

A knowledgeable cyclist will adjust each tire to adjust for several factors, including total weight at each wheel, road type, tire size and tire construction.
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Old 05-14-14, 06:29 AM   #59
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On my Cannondale Criterium with Vuelta Corsa Lites and Michelin Krylion 23's I run 105psi front and 110 rear. I weigh 175 and ride smooth asphalt.
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Old 05-14-14, 06:38 AM   #60
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Quote:
Originally Posted by Weatherby View Post
Thin butyl tubes can be lighter than latex.

The light weight of latex is not really a factor.

Some comparison data at the end of this table

http://www.biketechreview.com/tires_...sting_rev7.pdf
Interesting lab results, it looks like I'll get a 5-15% wattage increase riding in a lab But hey, I'm still a lazy Fred who only wants to pump tires every third ride, so I'll leave my latex tubes & UL butyl's in my extra tubes drawer. On the plus side it's nice to know that a quick tube change and I'll be faster.

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Old 05-14-14, 08:00 AM   #61
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This thread is the Cognitive Redundancy Principle applied to tires - every conversation about tires is the same. It pops up in various forms and various forums quite often. Everyone has an opinion and almost everyone has "facts", some of which are published measurements done under specific circumstances but sometimes nominally applies universally.

FWIW and IIRC, Sheldon Brown wrote an article arguing that it was better to have slightly higher pressure in the front, not the rear. I don't recall his reasoning but it made a little bit of sense. Not enough sense for me to deviate from just running the same pressure in each though. His analysis included the point that the "obvious" reason for running more pressure in the rear (i.e. more load) really didn't apply.

Also FWIW, I have 32, 28, 25, and high-tpi 23mm clinchers on various bikes. I like them all run high. It doesn't seem to make much difference in ride quality if I drop the pressures a bit; the bumps are what they are. But I believe I think I can imagine that they roll better and corner more precisely when higher, at least most of the time on our roads, and during the other times it doesn't much matter.

Though the 32, 28, and 25's are all on equivalent rims I can definitely feel the weight difference between them. The 23's are on rims that should be (but I didn't measure) lighter, so that could be one factor in how much lighter they feel. It has been argued that weight make no difference for constant-speed riding, that it makes miniscule difference for climbing, and it makes twice that difference for acceleration. All true. YMMV but I never run at a "constant" speed. Hills, turns, traffic, intersections, wind, road conditions are always affecting my speed so it goes up and down. In the long run the extra speed gained (or time saved) from a tiny acceleration increase doesn't mean doodlysquat except that it feels faster to me, so it matters to me.

I've recently begun riding a bike with high-tpi sew-ups on light rims with 2.0-1.7-2.0mm spokes. I really like them, so much so that I'm going to build a set as alternates to the 23mm clinchers.

Bottom line: Run what you like. Experiment if you wish. The differences can be subtle. It's okay.
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Old 05-14-14, 08:01 AM   #62
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Yes, we may be talking at cross purposes. It might be a question of where we live.

I'm not talking about rough pavement as in chipseal vs. smooth blacktop, I'm talking about the kinds of bumpy, patched and uneven roads we see in the northeast. It isn't the vibration or many small insults of coarse pavement that creates what I called "bump braking", it's vertical bumps in the1" + range where the strike angle is less vertical an therefore generates the braking effect.

Brandt referred to cattle guards, but here in the east we ride on things like cobblestone, and patchwork quilt roads. and thedrawbacks of excess pressure become obvious.
Like totally. On some Seattle roads, I wish for Fat Bike tires. The differences in road maintenance practices are interesting. Out here, one might think that the asphalt companies have an in with state government. They have a tendency to repave with chipseal way before it's necessary or even a good idea. However their influence doesn't seem to extend to city government. They never fix potholes or repave. In the Czech Republic, they simply don't have potholes, ever. They patch everything, and never repave so the roads are nice, but lumpy. And yes, HP tires aren't much fun on cobblestones.
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Old 05-14-14, 09:05 AM   #63
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Originally Posted by jimmuller View Post
This thread is the Cognitive Redundancy Principle applied to tires - every conversation about tires is the same. It pops up in various forms and various forums quite often. Everyone has an opinion and almost everyone has "facts", some of which are published measurements done under specific circumstances but sometimes nominally applies universally.
Studies!! Dont go against the studies!!

Too many other options and variables if you want a performance change....tire psi is only one player on that field.
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Old 05-14-14, 11:05 AM   #64
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Studies!! Dont go against the studies!!
I've never ridden studies, but would probably use them only when there is snow or ice on the road.
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Old 05-14-14, 11:18 AM   #65
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I've never ridden studies, but would probably use them only when there is snow or ice on the road.
Massively effective around here.
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Old 05-14-14, 01:26 PM   #66
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I'm 6'1" 235 and I run Gator Hardshells 700x28s, same specs as yours. 120 psi in rear, 95 psi in front. Usually, I re-fill them once a week. By re-fill time they're down to about 110/90.
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Old 05-15-14, 12:14 AM   #67
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Remember to divide your weight between the front and rear wheel loads ... do you weigh over 300lbs?
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Because your weight is not distributed evenly on the bike ... more load on the rear tire is typical
Went right over that. The pressure is per tire. Yeah, I think I fit

Although, in my case, I think I'd be more 50/50.

And these pressures handle the curbs and pot holes on the ride without tire damage?
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Old 05-15-14, 07:11 AM   #68
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28s won't handle curbs or potholes - ride around 'em. MHO
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Old 05-15-14, 07:16 AM   #69
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Whatever the max rating is on the tire. No rationale for doing so - seems to work - until I get a flat.
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Old 05-15-14, 07:46 AM   #70
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28s won't handle curbs or potholes - ride around 'em. MHO
What he said. Road-bike wheels aren't intended to go thump like that.

A month ago in a traffic situation and with sun in my eyes so I didn't see it I hit a pothole hard with the rear wheel on my soft-riding UO8 with 28mm Pasela TG's running their max pressure. It did not flat the tire. It did however put a flat spot in the rim, an old but perfectly functional MA2. I don't mean a slightly nicked and spread brake-surface rim sidewalls, but a real flat spot in the entire rim. Perhaps a bigger tire and rim would have survived. But it was a road bike, not a beach cruiser.
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Old 05-15-14, 03:54 PM   #71
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Lower pressures with latex tubes is faster than higher pressures with butyl tubes and more flat resistent.
I have used both butyl rubber and latex tubes at different pressures and saw no difference in riding/ speed. So I use butyl due to cost. My TT bike has tubulars so I haven't tried butyl tubed tubulars.
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Old 05-15-14, 09:45 PM   #72
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There certainly is a relationship between tire pressure, speed, and comfort. The relationship between pressure and speed is direct and between pressure and comfort is indirect. I bought a recumbent racing trike that came with Schwalbe Ultremo ZX 406 tires. The rated pressure is 85 to 160 PSI. If you run them at the highest pressure, you might as well be running solid rubber tires. The ride is incredibly harsh. The ride is faster though. I dumped them almost immediately for a set of Tioga PowerBlock tires rated at 35 to 80 psi. Much more comfortable at the rated pressure but slower. I don't like running Schwalbe tires much below the maximum pressure because of bad experience with premature sidewall failure. The sidewalls flex more at the lower end of the rated pressure range. It may mean a shorter life for the tire.
I don't think it's that simple. Generally the rule is the higher the pressure the lower the rolling resistance. Which should increase speed, but rolling resistance may not actually mean anything. Take the road I was one a couple of days ago... It's been beat up all winter, the surface is cracked and beat up, there are potholes and divots everywhere. A higher pressure means you feel more of those bumps and cracks, your jarred by them, which means you need to go slower to retain control. A lower pressure means the tires absorb these bumps and your not needing to throttle back to retain control. Tire width and construction are also important, wider tires do not need as much pressure to hold up the load, this is why your cars tires are 35PSI, even though the load weight is over a ton. Some tires have stiffer sidewalls then others, so even at lower pressures they have less deflection. I would also expect a clincher tire and a tubular tire to have different pressures under the same conditions.

The chart posted above is a good starting point, an individual rider should try both 5 and 10PSI higher and lower to see what works best for them.
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Old 05-16-14, 03:35 PM   #73
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28s won't handle curbs or potholes - ride around 'em. MHO

Oh how I wish I could! But, not only has winter done in our roads, which being chip coat or thin asphalt weren't that great to begin with, sometimes there is no choice. I've taken curbs on my old 700x23 tires with no apparent damage. Of course, I'm probably not like you folks who ride speedily.
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Old 05-16-14, 05:03 PM   #74
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Oh how I wish I could! But, not only has winter done in our roads, which being chip coat or thin asphalt weren't that great to begin with, sometimes there is no choice. I've taken curbs on my old 700x23 tires with no apparent damage. Of course, I'm probably not like you folks who ride speedily.
What kind of curb? The full curb or partial curb/ramp for barrier free?
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Old 05-16-14, 11:21 PM   #75
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What kind of curb? The full curb or partial curb/ramp for barrier free?
Regular curb. Not a ramp.

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