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PostPosted: Wed Mar 19, 2014 10:20 pm 
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Posts: 194
Geoff wrote:
+1. With a high-volume tire, there is no need for that kind of pressure, even for a larger rider.
In re-reading the OP post #1 - racing and sprinting for 110kg bike and rider - at min 130psi unless on bad roads - then go tubulars and still go pretty high pressure.

Image

http://www.vittoria.com/tech/recom-tyre-pressure


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PostPosted: Thu Mar 20, 2014 2:42 am 
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I'm a big guy like OP, I've ran Veloflex Corsa 25c, Vittoria Corsa CX 25c, and Schwalbe Ultremo 25c at all sort of pressures on my Pacenti Sl23 rims. I keep coming back to 110 psi front and 120 psi in the back. Any harder and it shakes my bones. Any softer and I start to feel the tire roll in sharp turns and the front tire is squishy when out of the saddle climbing.


I tried the Vittoria app and the recommended pressures are horrible for my wife and I (too hard).

Zoro wrote:
The guys that are in the saddle all day weigh around 150lbs. Total with bike - say 165lbs. They tend to race with ~115psi.
So 1.43#/PSI That is neglecting the casing flex (worse for light guys).


The problem with your 1.43#/psi ratio is that it only addresses part of the problem. Tires will decrease in rolling resistance as pressure is increased, however as pressure increases so does forward resistance (resistance to overcoming bumps and cracks). When plotted on a graph the optimum point/pressure will be the intersection of these two lines. Also note forward resistance is a dependent on course roughness.


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Posted: Thu Mar 20, 2014 2:42 am 


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PostPosted: Sun Apr 06, 2014 6:39 am 
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bombertodd wrote:
... Tires will decrease in rolling resistance as pressure is increased, however as pressure increases so does forward resistance (resistance to overcoming bumps and cracks). When plotted on a graph the optimum point/pressure will be the intersection of these two lines.
Show me.
Maybe your roads and our roads are different. I know my kid request higher pressure all the time. So I give it to him. Does that make me a bad parent?


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PostPosted: Sun Apr 06, 2014 9:05 am 
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Last edited by Causidicus on Wed Jun 18, 2014 12:34 am, edited 1 time in total.

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PostPosted: Sun Apr 06, 2014 4:43 pm 
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Zoro wrote:
bombertodd wrote:
... Tires will decrease in rolling resistance as pressure is increased, however as pressure increases so does forward resistance (resistance to overcoming bumps and cracks). When plotted on a graph the optimum point/pressure will be the intersection of these two lines.
Show me.
Maybe your roads and our roads are different. I know my kid request higher pressure all the time. So I give it to him. Does that make me a bad parent?



Show you what? A graph? :noidea:


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PostPosted: Sun Apr 20, 2014 9:56 pm 
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bombertodd wrote:
Zoro wrote:
bombertodd wrote:
... Tires will decrease in rolling resistance as pressure is increased, however as pressure increases so does forward resistance (resistance to overcoming bumps and cracks). When plotted on a graph the optimum point/pressure will be the intersection of these two lines.
Show me.
Maybe your roads and our roads are different. I know my kid request higher pressure all the time. So I give it to him. Does that make me a bad parent?



Show you what? A graph? :noidea:

Please. A graph would be great. Or some test data.
At what pressure will rolling resistance increase on a nice smooth road with a 320tpi tyre with a 115kg load?

I am that (too). I do fine around 130 PSI with no bones rattling, but that also depends on rims, spokes, frame stem handlebars....


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PostPosted: Mon Apr 21, 2014 2:42 am 
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At 253 pounds (115kg) 130 psi in 23c clinchers sounds alright if you're riding smooth roads. I'd recommend going to bigger tire altogether. Something like 25c front/rear of 25c front and 27c rear (if your frame allows). You could figure this with a roll down test if you're really interested. You should see an improvement as tire pressure increase until a point, then it should decrease.


Here is a great article from Tom Anhalt:
http://www.slowtwitch.com/Tech/What_s_i ... _1034.html

At 253 pounds the Crr curve will be higher than what Tom has. I don't know what Tom weighs but I'm assuming he is closer to the average cyclists weight.


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PostPosted: Mon Apr 28, 2014 6:59 am 
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bombertodd wrote:
...Here is a great article from Tom Anhalt:
http://www.slowtwitch.com/Tech/What_s_i ... _1034.html
I had not seen that article. Thank you. It explains a lot of the jargon I hear and I believe is being re-quoted.

"Very little data is commonly available that shows this effect. Mainly out of curiosity about whether or not this effect was measurable,"

"since there were so many things I didn't record, the best we can do
with these data is to calculate CdA usng a rough guess at my total
mass and assuming different values of Crr and air density"


There is way too much calculation and theory based on so little data. I understand the theory. I understand the math.

I think we are overstating the over pressure problem. 1st hand - on smooth Kreitler rollers with a power tap higher pressure decreased watts are required for the same speed - at least up to 150psi. A tubular mounting will be out of round and create a bounce more than the rollers do - or most roads. Some roads are near as smooth as rollers.

I got a cheap 120TPI clincher tyre last week (free). It is way way stiffer than my existing tyres. I would not want that tyre flexing at all and as such will put more air in it than my more supple tyres (actually - I won't bother riding it at all). The discussion should not be about psi until you can compare similar tyre suppleness on similar roads. Otherwise its quite difficult to get any useful information.


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PostPosted: Mon Apr 28, 2014 7:44 am 
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Last edited by Causidicus on Wed Jun 18, 2014 12:44 am, edited 1 time in total.

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 Post subject: Re:
PostPosted: Tue Apr 29, 2014 3:24 am 
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No need to quote above

The 15-16 year old kids in Belgium are mostly running over 100psi. While it is a completely valid topic to discuss if that is good or not, I know first hand of the 5 riders I knew of (not all our team) all were running closer to 120psi. They were on clinchers and I believe ran higher than if on tubulars. The kids doing PR were running around 80psi - tubulars.


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PostPosted: Tue Apr 29, 2014 4:08 am 
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5 local level riders "running close to 120 PSI" is your comeback, after demanding emperical eveidence from others?


It's little wonder people continue to deck it through corners in local racing. Motor sports laughs at our lack of tyre pressure knowledge.


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PostPosted: Tue Apr 29, 2014 5:17 am 
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MarkTwain wrote:
5 local level riders "running close to 120 PSI" is your comeback, after demanding emperical eveidence from others?
I gave my evidence long ago with video and links.
Local level riders?
Where did I say that? The winners were International, as were the owners of the tyres, including the winners 2 times, I knew the tyre pressure for. Belgium was just the venue. PR = Paris Roubaix (France) the other venue. I don't have pictures with pressure gauges.
What level would you like?

So what pressure do you think they are actually using in clinchers at the top level clinchers ever race? Not what is best, but what they actually pump them to.

Here is a picture of a 25mm, silk, no sidewall coating, record tread FMB Francois made for me. What pressure do you think he suggests a 63kg rider should use on smooth roads?

Image

In the *what should be done* category - these can be run lower psi as the cost of flex is so low. But most tyres have a more severe penalty for casing flex - so the pressure should be higher to minimize it.

To the OP @115kg - 130psi - I don't think its an issue.


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