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Originally Posted by Jeff Neese
(Post 21457074)
And when you put different tires on that old VW bug, everything changes. It is literally impossible to know (or "calculate") the optimum pressure for a tire, without knowing exactly what tire you're talking about. That shouldn't be hard to understand.
Now, I modified a truck, and went from 235 75/15s to 31" 1250 15s, and then, I had to adjust the amount of air in order to get the proper contact patch. That is the same as you have to do when you go with wider bicycle tire, which is exactly what the tire pressure calculators do. Regardless, psi will still be determined by the weight the tire needs to carry. Yes, the tires construction comes into play, mainly the width when we are talking about bicycle tires, you still determine the pressure by the weight that tire has to carry. |
Silca has a pressure calculator that I prefer to the DorkyPantsr calculator. However, it has the annoying 'feature' that you have to add someone's email address to get access to the full calculator.
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As long as your tire will allow it fill it to 50 and see from there. If the tire doesn't support 50 it's probably a crappy tire though there are some exceptions.
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Originally Posted by Jeff Neese
(Post 21456627)
Maybe we are. My point is illustrated with this example:
My wife's bike has Kenda Small Block 8s, which are rated for 50-85 PSI. My bike has Maxxis Ikons, rated 30-65 PSI. So when we are going to spend the day on smooth hardpack and want to roll fast, I fill her tires up to 85 PSI, and mine to 65 PSI. Body weight is not a factor - we use what's on the tire. When we want to ride rougher or looser terrain, we reduce hers to about 55 and mine down to about 40, and that gives us each the best results with the tires we're each riding. Still within the manufacturers specifications, but different because they are different tires. However, pumping it up to the max pressure is almost never a good strategy. You are most likely way over-inflated. Harsh ride, less traction, with no significant increase in speed (maybe even decrease). What size are your Ikons? Even with the 2.0 size, you would need to be really heavy to warrant 65 psi. The max pressure listed on the tire is never what you should use to determine the best pressure. It is only useful for determining if that tire can HANDLE the best pressure for you. And the best pressure is going to be highly dependent on your weight. In fact the optimum pressure will vary roughly proportionally with your weight. For example, If you (plus bike) are 200 lbs, and the best pressure for you on a particular tire for you is 60 psi, then for someone 150 lbs the best pressure on that tire with be ~45 psi (in that ballpark, at least). Go to any tire pressure calculator, and the two things they all ask for is tire size and rider weight. BOTH make a huge difference. Max/Min pressure is totally unnecessary for the calculation. Think psi limits like the stated size of a box of cereal. There are a lot of factors that go into how much cereal you eat, but the size of the box should not be one of them. The size of the box just tells you whether there is going to be enough, or if you should get a bigger box. That's why asking what the best tire pressure is, without knowing exactly what tire we're talking about, is pointless. It depends on the tire |
Originally Posted by Kapusta
(Post 21457212)
...
Go to any tire pressure calculator, and the two things they all ask for is tire size and rider weight. BOTH make a huge difference. Max/Min pressure is totally unnecessary for the calculation. ... The best advice for the OP is to simply read what it says on the tire, and experiment within those upper and lower limits to achieve the best performance for the conditions that he rides. Every tire and rim combination will behave different. |
Originally Posted by 79pmooney
(Post 21456681)
I don't like that at all! Runs counter to the age old advice to new racers. And yes, we laugh because we now know that some of that advice wasn't the fastest. But ... the huge but ... the race veterans and longtime coaches knew that staying upright; not crashing was key to good training and fitness and hence the ability to produce real speed when needed.
A 190 pound rider slamming into a pothole with all his weight on the front wheel (see my post last page) and only 73 psi in a skinny 25c tire? That's not going to end well. In fact, that might cause conditioning issues months later. Being at 85% of one's best because of a bad crash two months ago is a loss of 15% power. Real. 10% more rolling resistance in the front tire because you had nearly the same pressure up front as the back - well, rolling resistance is rarely more that about 20% of one's total resistance. The front is say half the total rolling resistance. So 1/2 X 20% X 10% increase for the additional pressure = 1% change in the total resistance, Yes, measurable but compared to a crash from loss of control because the front tire blew out or the rim collapsed, peanuts. Those pretty charts are going to get people hurt. We hit potholes, If we ride long enough, we hit them without doing the proper un-weighting of the wheels and we may hit one one day while braking hard. Front tire pressure could make a very real difference. (If you do choose to ride the low front tire pressures, wear a helmet and shave your legs. Both make the ER experience a lot better and speed recovery. Ben And BTW that is 190 lbs for the rider AND bike. |
Originally Posted by Jeff Neese
(Post 21457279)
Which would be fine in an imaginary world where all tires have the same tread pattern, tire compound, sidewall stiffness, and a whole host of other factors including the rim width. In that imaginary world, sure - you can just use the tire size and rider weight because those are the only two variables. In the real world however, different tires behave differently, even with the same pressure and rider weight. There's no "calculator" that can tell you what tire pressure to use, without knowing the specific tire (and rim) that we're talking about. 40 PSI on one tire/rim combination might give you equivalent performance as 55 PSI on a different tire and rim. That's all I'm trying to say, and I think it's sort of obvious.
The best advice for the OP is to simply read what it says on the tire, and experiment within those upper and lower limits to achieve the best performance for the conditions that he rides. Every tire and rim combination will behave different. However, they are far, FAR outweighed by the two 800 lb gorillas in the room: tire volume and rider weight. Tire volume and weight are what determine whether those other factors are moving you around the 25-30 psi range vs the 100-110 psi range. An in the real world, the Dorky Pants calculator gets in the ballpark what a lot of people who experiment a lot end up with. Good enough as a starting point, and way more useful than looking at max/min pressures. |
Originally Posted by PoorInRichfield
(Post 21457107)
Silca has a pressure calculator that I prefer to the DorkyPantsr calculator. However, it has the annoying 'feature' that you have to add someone's email address to get access to the full calculator.
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Tire pressue
Originally Posted by bran1986
(Post 21447256)
Am I missing something with this website? I just put in some generic information (190 lbs and 25mm tire) and it is saying to inflate to 176 for the "15% drop method" and inflate to 102 psi for Michelin 700c tires.
Interestingly, for cyclocross racing, I always did 25 psi front/ 28 psi rear with my Vittoria clinchers. Never flatted. Not once. Lower pressure is better. |
Originally Posted by Kapusta
(Post 21457291)
If you are THAT worried about being impervious to pinch flatting, you should be on a bigger tire.
And BTW that is 190 lbs for the rider AND bike. Now, if I was running tubulars and a sponsor was paying for my tires, I would have far less issue with low front tire pressures - because the tire will never come off the rim if properly glued, even if the rim or tire or both are trashed and I will not have to try to negotiate the road surface with a bare aluminum or carbon fiber rim (or get the tire stuck in the fork or stays). BTDT. All three. Ben |
Originally Posted by ridinginjeans
(Post 21457750)
That website gives weird figures. I do 90 psi for my 23mm road tires. Never a pinch flat. And I will do around 60 psi for the 28 mm tires I just bought. I weigh around 162 lbs.
Interestingly, for cyclocross racing, I always did 25 psi front/ 28 psi rear with my Vittoria clinchers. Never flatted. Not once. Lower pressure is better. |
Originally Posted by 79pmooney
(Post 21457782)
Thanks. I missed the rider and bike. Still, I would NEVER ride a 25c at 73 psi and I weigh 155 pounds so 180 total. I'd be riding a nominal 100 (98 front, 103 rear) at least unless it was wet or icy and I needed to get home. (Like i screwed up and went out on the wrong tires.) If I had to go out and ride dayh in and day out on 73 psi in front, I'd insist my tires were 32cs. Yes, a bigger tire, But the example I was objecting to was 25c, Now, I bet it you ran the same calculator for 32s, it would come up with something like 45 psi and I would be coming back with the exact same argument, only now I'd insist on, oh, 65 psi in front for peace of mind.
Now, if I was running tubulars and a sponsor was paying for my tires, I would have far less issue with low front tire pressures - because the tire will never come off the rim if properly glued, even if the rim or tire or both are trashed and I will not have to try to negotiate the road surface with a bare aluminum or carbon fiber rim (or get the tire stuck in the fork or stays). BTDT. All three. Ben |
Originally Posted by phughes
(Post 21459173)
As I posted above, the calculator you are commenting about, gives inflation recommendations PER WHEEL, not total, so you have to figure the weight on each wheel individually and input that number, not the total weight of the bike and rider. Look above the calculator, it tells you that.
Ben |
Originally Posted by 79pmooney
(Post 21459535)
Yeah. And when you are slamming on your brakes because of the car in front of you, you have all your weight on the front wheel. That pressure for 45% of your weight won't save your butt in those conditions. Those old-timers told me 5 psi less that the rear just so I wouldn't crash (and maybe lose most of a racing season. No, no calculator will ever give you that pressure, I will always be "wrong". And that 5 psi less than the rear has saved my butt more than once. I'll keep doing it. And I won't keep my mouth shut because I care.
Ben |
Originally Posted by 79pmooney
(Post 21459535)
Yeah. And when you are slamming on your brakes because of the car in front of you, you have all your weight on the front wheel. That pressure for 45% of your weight won't save your butt in those conditions. Those old-timers told me 5 psi less that the rear just so I wouldn't crash (and maybe lose most of a racing season. No, no calculator will ever give you that pressure, I will always be "wrong". And that 5 psi less than the rear has saved my butt more than once. I'll keep doing it. And I won't keep my mouth shut because I care.
Ben Also, those calculators are a ballpark for average riding. If slamming your wheels into potholes and other square edges hits with the finesse of a ham fisted gorilla is how you roll.... you should add more air... and get bigger tires. |
Another decent tire pressure calculator from SRAM:
https://axs.sram.com/tirepressureguide This one gives me pressures almost 10 PSI lower than the Silca calculator I posted earlier. |
Originally Posted by PoorInRichfield
(Post 21474635)
Another decent tire pressure calculator from SRAM:
https://axs.sram.com/tirepressureguide This one gives me pressures almost 10 PSI lower than the Silca calculator I posted earlier. |
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