With all the "wider is faster" hype there's a lot of data showing rolling resistance differences between different tire widths, but mostly without any correlation to aero/weight penalty.
Is there a way to calculate net difference between 25 and 28 mm tires?
Say we take GP5k TL in 700x25 and 700x28, put them on respectively optimized aero rims (to keep them both under 105% rule, something like 21 internal 28 external rim width for 25 mm tire, and 25 mm internal 32 mm external for 28 mm tire) of same depth (say 45 mm) and pump them to similar comfort level pressure (90 psi for 25, 80 psi for 28, heavier rider).
Rolling resistance gain should be just 0.1W with the same comfort level: https://www.bicyclerollingresistance.co ... comparison
What would be aero loss because of frontal area increase?
Weight penalty can be estimated around 160 grams, how many watts is that?
Ultimately, can anyone guess what would be net gain/loss in these two scenarios, on average road, 30-32 km/h average speed, rolling terrain, same comfort level for 85 kg rider, all other things being equal?
My guess is 25mm would be faster, but how much?
And then, where is all the "wider is faster" hype coming from?
Rolling resistance/aero/weight net gain
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Proper way to do tire sizing for speed is wider wheel first.
Then match your (actual mounted) tire width to rim width. Don't do the other way around. That hurt both aero [cite: https://blog.silca.cc/part-5-tire-press ... rodynamics] and handling.
You need to prioritize avoiding "rolling impedance spike". read here https://blog.silca.cc/part-4b-rolling-r ... -impedance
This rolling impedance will never show up on metal drum rolling resistance test. But it exist in real life.
Roughly speaking, for good road it means avoid going over 95psi regardless of tire width. The threshold go down further on bad road, like 70psi on roughened concrete (red line in the article). If you are deep into the too high pressure zone, you lose more power than what going up the size and pay aero penalty would cost.
Then when you are safe, in not to high pressure zone and tire not too wide for the rim. You are safe to select narrower tire (with higher pressure but not too high) for lower weight or wider tire (but still not wider than the rim) for more comfort.
'Wider tire is faster hype' comes when heavy people moved from 23c at 120psi to 25c at 90psi and is faster everywhere by avoiding rolling impedance spike.
Same goes for rider on rough road or off road where threshold for too high pressure is even lower.
Frontal area difference of tire is not the main source of aero penalty (really down the list of things to care on the bike, actually). Tire wider than rim and screw up aero profile is the main penalty, focussing only on the wheel. It is avoidable by using wide rims.
Weight to watts calculation use this http://bikecalculator.com/ try matching same speed going up the same grade with different power, you'll notice power difference.
Then match your (actual mounted) tire width to rim width. Don't do the other way around. That hurt both aero [cite: https://blog.silca.cc/part-5-tire-press ... rodynamics] and handling.
You need to prioritize avoiding "rolling impedance spike". read here https://blog.silca.cc/part-4b-rolling-r ... -impedance
This rolling impedance will never show up on metal drum rolling resistance test. But it exist in real life.
Roughly speaking, for good road it means avoid going over 95psi regardless of tire width. The threshold go down further on bad road, like 70psi on roughened concrete (red line in the article). If you are deep into the too high pressure zone, you lose more power than what going up the size and pay aero penalty would cost.
Then when you are safe, in not to high pressure zone and tire not too wide for the rim. You are safe to select narrower tire (with higher pressure but not too high) for lower weight or wider tire (but still not wider than the rim) for more comfort.
'Wider tire is faster hype' comes when heavy people moved from 23c at 120psi to 25c at 90psi and is faster everywhere by avoiding rolling impedance spike.
Same goes for rider on rough road or off road where threshold for too high pressure is even lower.
Frontal area difference of tire is not the main source of aero penalty (really down the list of things to care on the bike, actually). Tire wider than rim and screw up aero profile is the main penalty, focussing only on the wheel. It is avoidable by using wide rims.
Weight to watts calculation use this http://bikecalculator.com/ try matching same speed going up the same grade with different power, you'll notice power difference.
Last edited by Hexsense on Tue Jun 18, 2019 8:43 pm, edited 2 times in total.
There is no magic formula. Depends on the rider's style and power. Some might need a bit less vibration to give the confidence to pedal hard, some might be ok with narrower and can put power down over bumps anyway.
85kg + 7kg for bike + 2kg gear lets say 95kg riding weight should probably look at 28mm if the road quality is less than ideal, lets say as an allround solution.
90psi = 25mm is for good roads.
That means rim AND tire 28mm. Some use 25mm front and 28mm rear as a compromise for narrower rims to get around it.
For a skinnier rider like me 65+6kg I'm not even reaching 25mm on my tires, but for allround performance 25mm is pretty close.
The aero penalty is smaller than the rolling resistance penalty. What you don't see in the tests at bicyclerollingresistance is the impact of a harsh ride + high pressure on your power output. Stopping pedaling over occasional bumps will eat into any savings you make by going narrow so find out your minimum comfort width where you can pedal hard - all the time-. If the pressure lowers to 70psi to do this then up the tire width to not bottom out.
85kg + 7kg for bike + 2kg gear lets say 95kg riding weight should probably look at 28mm if the road quality is less than ideal, lets say as an allround solution.
90psi = 25mm is for good roads.
That means rim AND tire 28mm. Some use 25mm front and 28mm rear as a compromise for narrower rims to get around it.
For a skinnier rider like me 65+6kg I'm not even reaching 25mm on my tires, but for allround performance 25mm is pretty close.
The aero penalty is smaller than the rolling resistance penalty. What you don't see in the tests at bicyclerollingresistance is the impact of a harsh ride + high pressure on your power output. Stopping pedaling over occasional bumps will eat into any savings you make by going narrow so find out your minimum comfort width where you can pedal hard - all the time-. If the pressure lowers to 70psi to do this then up the tire width to not bottom out.
So, 700x25c measured 27-28mm on 21C internal 28mm external rim (something along the lines of Light Bicycle WR46C02)?
At what pressure for 95 kg riding weight? 90/95 psi for good roads, 80/85 psi for bad roads?
Last edited by igs417 on Tue Jun 18, 2019 1:18 pm, edited 1 time in total.
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CrankAddictsRich
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Wider isn't faster.... when all things are considered. It is simply more comfortable, which is what most people really care about, when they're being honest with themselves.
The pressure doesn't depend on rider weight, it depends on road quality.
A light rider is efficient at the same pressure as a heavier rider. They just don't need to go as wide to handle that pressure.
Decide what pressure you need for the road quality you intend to ride on. Smooth roads 90-95psi. Medium 75-85psi and rough roads 60psi.
For 95kg riding weight on medium rough roads you'll probably need ~28mm tires and wheels. For smooth roads I guess you could make due with a streamlined 25mm tire on the front wheel at 90psi, and a 28mm tire at 90psi in the rear.
A light rider is efficient at the same pressure as a heavier rider. They just don't need to go as wide to handle that pressure.
Decide what pressure you need for the road quality you intend to ride on. Smooth roads 90-95psi. Medium 75-85psi and rough roads 60psi.
For 95kg riding weight on medium rough roads you'll probably need ~28mm tires and wheels. For smooth roads I guess you could make due with a streamlined 25mm tire on the front wheel at 90psi, and a 28mm tire at 90psi in the rear.
I'm basically riding that combo. I run 75/80psi because we have poor quality roads around here. If my frame had more clearance I'd want to run 28mm tires at less pressure.
Why is that? Pinch flats? Rim protection over potholes?
I was under impression that heavier rider must pump up narrow tires to more psi compared to wider tires to achieve same result, hence lower psi on same road/rider for wider tires.
Around the same as the stated 95kg. I could probably ride 70/75 comfortably but I want some margin of safety for rim protection and pinch flats.
I'm also riding 25mm GP4000s. They probably measure 27mm wide and are a tall tire.
I'm also riding 25mm GP4000s. They probably measure 27mm wide and are a tall tire.
100% this. I just tried out 28mm tires for the first time two weeks ago and the comfort as a result of riding them at 70 psi is staggering. And I had no difficulties whatsoever maintaining my usual pace on laps in the park.CrankAddictsRich wrote: ↑Tue Jun 18, 2019 1:13 pmWider isn't faster.... when all things are considered. It is simply more comfortable, which is what most people really care about, when they're being honest with themselves.
But, and this is purely subjective, I'm still not used to seeing their girth up front.
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I almost agree on this.alcatraz wrote: ↑Tue Jun 18, 2019 2:14 pmThe pressure doesn't depend on rider weight, it depends on road quality.
A light rider is efficient at the same pressure as a heavier rider. They just don't need to go as wide to handle that pressure.
Decide what pressure you need for the road quality you intend to ride on. Smooth roads 90-95psi. Medium 75-85psi and rough roads 60psi.
For 95kg riding weight on medium rough roads you'll probably need ~28mm tires and wheels. For smooth roads I guess you could make due with a streamlined 25mm tire on the front wheel at 90psi, and a 28mm tire at 90psi in the rear.
Experiment with tire pressure until the road buzz goes away. (vibration = energy = not good)
If your pressure is so low you can bottom out the tire => use a tire size up and or rim width up
Possibly you can lower the pressure of your next size up tire a little bit because the casing tension of the carcass would go up if you use the same tire pressure as the narrower tire.
http://flocycling.blogspot.com/2014/11/ ... -tire.html
keep the 105% rule in mind.
I weigh 62 kgs and I can get away with vittoria corsa 25mm on 622-21 rims. @ 60 psi
Road surface is very poor in Belgium: broken pavement, concrete roads with tar seams, asphalt with large pebbles, patched up,...
