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Rim width and tire pressure charts?

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Rim width and tire pressure charts?

Old 08-01-18, 04:23 PM
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Rim width and tire pressure charts?

I was wondering how rim width should be factored into the various tire pressure verses tire volume and rider weight?
I'm at about 230 right now with 25-30 pounds of every thing else. I have Panaracer Grave kings in 28mm mounted on CR18 rims that are approximately 18mm interior width. The tire are rated at 105psi. I had the rear tire up past 110psi and the ride was harsh. I got a pump with a reliable gauge and got the pressure down to 105 psi. and the ride is much smoother. I am cringing every time I hit a obstruction or pothole, but so far so good.
The Dorky pants 15% drop charts seem to indicate 118 psi for a rear tire at my weight.
My question is how do I do I determine what rims the charts are based on and how do I gauge the extra volume added to the tire and how much pressure can be dropped? thanks, Woody
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Old 08-01-18, 04:31 PM
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I have 25mm Contis mounted on 23mm (external) rims. Mounted and inflated they measure almost 28mm. So I use the 28mm curve based on actual measured tire width.

That said, if you prefer to run 105, go for it. As long as it feels better and you don't pinch flat.
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Old 08-01-18, 05:19 PM
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My question is how do I do I determine what rims the charts are based on and how do I gauge the extra volume added to the tire and how much pressure can be dropped? thanks, Woody
The famous 15% tire drop chart is not based on rim width, only nominally on tire width. Many tire widths can fit on the same rim width. It's just a heuristic starting point anyways.
[QUOTE=bark_eater;20482963The Dorky pants 15% drop charts seem to indicate 118 psi for a rear tire at my weight.[/QUOTE]
If you use a wider tire you can drop that psi. At a certain point, a wider tire will no longer work well with the rims you have, and if you want to keep going in that direction you'd have to buy wheels with wider rims.
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Old 08-01-18, 05:21 PM
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Do what feels good, after all the chart is
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Old 08-01-18, 05:29 PM
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They are measuring out at a true 28mm. My last primary ride had 38mm Marathon tour plus tank treads so I don't have much to compare, except A cheap road bike with 28mm tires is hella faster than a hybrid with 38mm's.
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Old 08-01-18, 08:53 PM
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100 K (220 lb)

Alex TD17 (17mm) rear rim
28,, Conti GP
Best efficiency rear pressure +/- 70 psi
Best comfort without getting squirmy 60 psi

Same wheel, 42 mm Conti Crossrides, run best 60-45 psi
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Old 08-02-18, 04:52 AM
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Originally Posted by TGT1 View Post
100 K (220 lb)

Alex TD17 (17mm) rear rim
28,, Conti GP
Best efficiency rear pressure +/- 70 psi
Best comfort without getting squirmy 60 psi

Same wheel, 42 mm Conti Crossrides, run best 60-45 psi
Do you know what the actual width of your tires ended up at?
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Old 08-02-18, 05:19 AM
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Originally Posted by bark_eater View Post
I was wondering how rim width should be factored into the various tire pressure verses tire volume and rider weight?
I'm at about 230 right now with 25-30 pounds of every thing else. I have Panaracer Grave kings in 28mm mounted on CR18 rims that are approximately 18mm interior width. The tire are rated at 105psi. I had the rear tire up past 110psi and the ride was harsh. I got a pump with a reliable gauge and got the pressure down to 105 psi. and the ride is much smoother. I am cringing every time I hit a obstruction or pothole, but so far so good.
The Dorky pants 15% drop charts seem to indicate 118 psi for a rear tire at my weight.
My question is how do I do I determine what rims the charts are based on and how do I gauge the extra volume added to the tire and how much pressure can be dropped? thanks, Woody
Go tubeless; not tube = no pinch flat. I'm 260, ride on 28 mm tires at 75 psi.
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Old 08-02-18, 06:29 PM
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Old 08-02-18, 08:43 PM
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Originally Posted by bark_eater View Post


Do you know what the actual width of your tires ended up at?
1MM or 2MM wider than advertised
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Old 08-05-18, 12:06 AM
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I found a calculator intended specifically for us clydes.

at least for me, it results in about 5-10psi less than the Dorky calculator, especially on bigger tires.


Clydesdale ? Road/Tri Bike Tire Pressure Calculator | Engineered Insanity

Last edited by rgconner; 08-05-18 at 12:09 AM.
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Old 08-06-18, 01:10 PM
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This calculator gave me psi level greater than the dorky, but gave a significantly lower preasure when i put the same data in In a different order. That a feature, right?
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Old 08-09-18, 07:55 AM
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Personally I just use the rated tire width in pressure calculators. If you really feel the need the account for rim width then mount the tires, fill them to their max pressure, measure the tire width, and use that width to start with. Then calculate the pressure, set them to that and measure the width. Now recalculate the pressure.... You can repeat that process until it become too tedious to continue but you won't really gain anything by it. Using the rated tire width is close enough really.

Tire grip, comfort, and handling will generally improve at lower pressures (to a point) but rolling resistance will increase and resistance to pinch flats deceases. There are many tradeoffs and Berto's 15% drop pressure was set based on his assessment of where the tradeoffs are optimum or well balanced. I doubt he intended to be dogmatic about that, I suspect the assertion made above that it was intended to be a guideline, not a RULE, is close to the truth. It's a good place to start if you are willing to give up a little rolling resistance in return for other benefits but there is no reason you should not deviate from the Berto pressure if you feel the tradeoffs are optimum at a higher or lower pressure. Those assessments are better made by test rides at various pressures than by obsessing over the actual mounted and inflated tire width. My actual practice is to run at max pressure on good paved roads and to move towards or to the Berto pressure on rough roads or gravel or offroad.

If you go too low you enter pinch flat territory -- or you go tubeless as someone suggested above!
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Old 08-10-18, 09:21 PM
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Originally Posted by bark_eater View Post
This calculator gave me psi level greater than the dorky, but gave a significantly lower preasure when i put the same data in In a different order. That a feature, right?
I don't know. On my browser it refreshes after every number changes.

Dorky: 45/55% weight dirt, 255 Lbs combined, 37mm (the max dorky supports) : 51F, 62R

Insanity: 44/55%, 255 combined weight, 38mm tires: 48f 55R
37mm: 50F 61R

1mm seems to make a significant difference.
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Old 08-10-18, 10:26 PM
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Berto’s 15% rule was based on the crappy clinchers available at the time, which did degrade performance at lower pressures. It was done with actual width, not the lies printed on the tire. Rim width is both a small variable, and easily accounted for by measuring the net width. And, it is just a starting point so you don’t go crazy hard or soft for a given tire size.

There are still many crappy tires on the market, especially wider ones, but good tires don’t change rolling resistance at any useable pressure. Personally I shoot for 20% sag as a starting point and have set all my PB at this level. If you don’t get a pinch flat every now and then, you probably are over inflating.
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Old 08-11-18, 05:06 AM
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How would I adjust Berto's graph for 20% sag? I'm allergic to math but its not kryptonite if any one know the formulas involved...
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Old 08-13-18, 12:49 PM
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Originally Posted by rgconner View Post
I found a calculator intended specifically for us clydes.

at least for me, it results in about 5-10psi less than the Dorky calculator, especially on bigger tires.


Clydesdale ? Road/Tri Bike Tire Pressure Calculator | Engineered Insanity
Nice! 250lb on 23s with a 70/30 distribution (I think more realistic than a 55/45) requires 205psi in the back tire!

My bike (700x50 on the back, 700x42 on the front) is recommended to have a cush-tastic 32/52psi
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Old 08-13-18, 01:07 PM
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I'm trying to give the the gravel king's a fair chance. i've got about 75 miles on them and they feel like they roll over when I carve a tight turn. If I cant get comfortable on these I will try the the 28mm Continental gp4000 II's. I think they top out at 120psi. They are reportedly 30+ MM actual size, so I may have to deflate them to get them past my rear fender and horizontal drop outs.

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Old 08-13-18, 01:27 PM
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Originally Posted by bark_eater View Post
I'm trying to give the the gravel king's a fair chance. i've got about 75 miles on them and they feel like they roll over when I carve a tight turn. If I cant get comfortable on these I will try the the 28mm Continental gp4000 II's. I think the top out at 120psi. They are reportedly 30+ MM actual size, so I may have to deflate them to get them past my rear fender and horizontal drop outs.
I had this happen with Soma Supple Vitesse.

I had a slow leak, did not notice.
Carved a 90 degree corner and it folded under, instant lowside slide.
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