Originally Posted by
Iride01
Not quite certain I agree. You might drop the pressure 4 or 5 PSI for a rim that is 4 mm wider internally, but the same road forces are still going into the rim/wheel at that lower pressure.
But maybe you are correct. I'm not certain 5 PSI would be that noticeable a change in ride comfort. And if you are claiming the PSI should be dropped much lower then you could have just dropped the pressure for the tire on the original rim.
On Silca's tire pressure calculator (
SILCA Professional Tire Pressure Calculator) I selected the following parameters:
Total System Weight: 220 lbs.
Surface Condition: New Pavement
Wheel Diameter: 700C/29"
Average Speed: Recreational
Measured Tire Width: 25 mm
Tire Type: Mid-Range casing butyl tube
Weight Distribution: 48/52 (road bikes)
The calculator returns: 94.5 PSI rear, 92.5 PSI front.
Keeping all parameters the same except increasing Measured Tire Width to 26 mm results in 89 PSI rear, 87 PSI front.
Keeping all parameters the same except increasing Measured Tire Width to 27 mm results in 84 PSI rear, 82 PSI front.
If I recall correctly, going from 17 mm IW to 19 mm IW increased the measured width of a slightly used GP5K (having a nominal width of 25 mm) from 25 mm to 26 mm.
For comparison, BRR testing 25 mm wide tubeless tires (which have stiffer sidewalls) shows a 1.7 mm increase in tire width going from 18C to 22C wheels.
The Rim Width Test | Bicycle Rolling Resistance
I can feel a 5 PSI difference; whether that significantly affects comfort is a more difficult question.
So, if one is increasing internal wheel width by 4 mm as you originally posited, Silca would suggest a 10 PSI decrease, while BRR would suggest a 10-12 PSI decrease.
Originally Posted by
Iride01
I must have read too many Technical FAQs recently; I cannot access that site anymore.