Dumb singlespeed(sort of) question. Tire circumference/diameter.

Dave Taylor

Dave Taylor

Rex kwan Do
This is probably a @UtahJoe or @Patrick question but the #1 SS raceday question is probably “what gear should I run?”. Ok, cool, now let my self mind fug you a bit…tires(I assume bike tires too) have an aspect ratio. On a car this is generally the middle # in the size(195/50-15, 50 is the aspect ratio). This basically tells you that the sidewall is 50% of the tread width. So on a tire where the tread is 195mm the sidewall height is very close to 97.5mm. Sure, this changes a bit with rim width, tire pressure and distributed weight but it’s a general way to measure tires. So if that same tire increases in width to 250mm now the sidewall is 125mm. So a 50 aspect ratio tire looks much different on a 395/50-15 tire vs say a 235/50-22 tire.
Ok, where am I going with this? The aspect ratio kinda doesn’t matter but it does. Basically, all things equal at Mohican 100 in 2020 I ran a 34-17 gear and a maxxis 29x2.1 aspen tire. Let’s say I run the same gear with an Aspen 29x2.4 how much is the gear/inches changing? I don’t have the old 2.1 anymore and theoretically I would need to mount each tire, draw a white line on the ground and tire, roll one complete revolution and then measure. Can one of you smart guys do this “on paper”? Essentially what I am saying is the 34-17 will probably be a slightly harder gear to pedal on the bigger tire.
 
I would think the increased rolling resistance makes more of a difference.
 
Dave Taylor said:
aspect ratio kinda doesn’t matter but it does.
Click to expand...
It's doesn't, it's means nothing....mount tire, measure its height mounted/filled as it would be rolling.... Circumference =pi * D..... That's the distance it will cover in a revolution.... Mount the other tire and do the same and compare results using excel and multi colored graphs. The only accurate number on the tire is the diameter of the rim it's going on... Companies fudge all the others... Only way to know is to measure.

Now once you have all this info... Remember that it doesn't mean anything and that rolling resistance will be far more important
 
There isn't enough resolution in the gears to make up for the slight change of tire diameter anyway. One tooth change in either direction on the rear cog would be the same as changing tire diameter by almost 2".
 
UtahJoe said:
It's doesn't, it's means nothing....mount tire, measure its height mounted/filled as it would be rolling.... Circumference =pi * D..... That's the distance it will cover in a revolution.... Mount the other tire and do the same and compare results using excel and multi colored graphs. The only accurate number on the tire is the diameter of the rim it's going on... Companies fudge all the others... Only way to know is to measure.

Now once you have all this info... Remember that it doesn't mean anything and that rolling resistance will be far more important
Click to expand...
Technically the line on floor should be more accurate I think as you have your weight on the bike compressing like it normally would. According to that calculator(which looks off on some tires) a 34-17 @ 90 rpm will give you 14.8mph on a 29x2.1 and 15.5mph on a 29x2.3. That’s a big difference!
 
Love the geekyness of this, but this is true:

Matt_ said:
There isn't enough resolution in the gears to make up for the slight change of tire diameter anyway. One tooth change in either direction on the rear cog would be the same as changing tire diameter by almost 2".
Click to expand...

From Sheldon Brown gear calc:

29x2.1 @ 90 rpm
34x16: 15.7
34x17: 14.8
34x18: 14.0

29x2.3 @ 90rpm
34x16: 16.5
34x17: 15.5
34x18: 14.6

I think tires will always be "run what you brung", I don't see you swapping out tires because of your gearing. This SAYS that

34x16 on a 2.1 is close to 34x17 on a 2.3, but we all know it FEELS different
 
Look at you all talking theory -

don't forget the reduced tire slip of the 2.3,
perceived headwind
change of pressure as the day/tire warms
muscle fatigue, vs cardio -
unsprung weight of the larger tire
should fill with nitrogen - not the mostly nitrogen we typically use.

carry on.

ps -
is the goal to be more "sure" of your equipment?
cause the answer is to find out empirically.
 
Pearl said:
Love the geekyness of this, but this is true:



From Sheldon Brown gear calc:

29x2.1 @ 90 rpm
34x16: 15.7
34x17: 14.8
34x18: 14.0

29x2.3 @ 90rpm
34x16: 16.5
34x17: 15.5
34x18: 14.6

I think tires will always be "run what you brung", I don't see you swapping out tires because of your gearing. This SAYS that

34x16 on a 2.1 is close to 34x17 on a 2.3, but we all know it FEELS different
Click to expand...
Right. I did the 2.2 vs 2.6. comparison and it was similar. Basically running bigger tires I would go tooth larger on the rear.Now I will have to do the rolling test to see actual circumference of the two and see how close his calculator is. Like I said, I ran a 34/18 at 101 with 2.25s, 34/19 with 2.4s and this year a 34/21 with 2.4s. The 21 actually felt like I had plenty of tension on the pedals for the amount of road I was on. I remember doing 101 my first year on 2.25 with 34/21 and felt like I spun out everywhere.
 
Patrick said:
Look at you all talking theory -

don't forget the reduced tire slip of the 2.3,
perceived headwind
change of pressure as the day/tire warms
muscle fatigue, vs cardio -
unsprung weight of the larger tire
should fill with nitrogen - not the mostly nitrogen we typically use.

carry on.

ps -
is the goal to be more "sure" of your equipment?
cause the answer is to find out empirically.
Click to expand...
Honestly, the goal is more to be on the right gear as I know it but that right gear changes as the tire size changes. Want a real mind F and I will throw in 27.5 or 26er or 29+ tire gearing.
 
Dave Taylor said:
Honestly, the goal is more to be on the right gear as I know it but that right gear changes as the tire size changes. Want a real mind F and I will throw in 27.5 or 26er or 29+ tire gearing.
Click to expand...

you are working below the resolution of 1 tooth - per @Matt_ and confirmed by @Pearl's data

It would now become a feel thing - which i why you should go out an ride both.

I agree that different size wheels would require some calcs to make the feel at the pedals close.
as an extreme example, look at the folding bikes with the small wheels - to make up the diff, they have large chainrings.

Happy Birthday!
And i did not mention Pi, gear-inches, rotational inertia.....
 
Dave Taylor said:
Right. I did the 2.2 vs 2.6. comparison and it was similar. Basically running bigger tires I would go tooth larger on the rear.Now I will have to do the rolling test to see actual circumference of the two and see how close his calculator is. Like I said, I ran a 34/18 at 101 with 2.25s, 34/19 with 2.4s and this year a 34/21 with 2.4s. The 21 actually felt like I had plenty of tension on the pedals for the amount of road I was on. I remember doing 101 my first year on 2.25 with 34/21 and felt like I spun out everywhere.
Click to expand...
Happy Birthday 🎂 Go ride your bike!
 
Patrick said:
Look at you all talking theory -

don't forget the reduced tire slip of the 2.3,
perceived headwind
change of pressure as the day/tire warms
muscle fatigue, vs cardio -
unsprung weight of the larger tire
should fill with nitrogen - not the mostly nitrogen we typically use.

carry on.

ps -
is the goal to be more "sure" of your equipment?
cause the answer is to find out empirically.
Click to expand...
Thx. .7mph at 90rpm seems like sort of a big deal no?
 
Dave Taylor said:
Thx. .7mph at 90rpm seems like sort of a big deal no?
Click to expand...

moving up or down a tooth makes a larger difference than that - so it now has to be feel,
or logic out if you want to spin more or grind more for the same speed...

Or are you going to pick a tire, based on some theoretical number?
There are too many variables...

that part is beyond me - ask @Mitch - i heard he used to ride SS.....😉
 
I do agree 1 mph is a big difference

gotta test it where you get the rolling resistance as well as puncture protection and handling difference. Need to get really nerdy and do some 2-3 minute climbs at X power, and 2-3 loops of a 5-6 minute loop to see what the difference in speed is...
 
I’m coming in late, and didn’t read much. And I’ll probably regret even positing here. But.
Even if there is a 1mph difference, it’s not free. You’re still having to put out more watts to get that 1mph, no?
 
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