Question guys, how much does different rear tire sizes affect gearing?

[Sierra Thumper]

I'm going from a 120/90/18 rear knobby to a 120/100/18.....the sizing seems to be off between the 2 brands because the 120/100 is substantially taller and wider than the 120/90. How much will this affect my gearing? I'm going down a tooth on the front sprocket for better dirt performance, I don't want to mount up a tire thats going to take me back to stock gearing. If its going to make too big of a diff I'll be exchanging it for another 120/90.
Any feedback is appreciated

[kubiak]

Originally Posted by Sierra Thumper

I'm going from a 120/90/18 rear knobby to a 120/100/18.....the sizing seems to be off between the 2 brands because the 120/100 is substantially taller and wider than the 120/90. How much will this affect my gearing? I'm going down a tooth on the front sprocket for better dirt performance, I don't want to mount up a tire thats going to take me back to stock gearing. If its going to make too big of a diff I'll be exchanging it for another 120/90.
Any feedback is appreciated

one size difference like that, i would guess by dropping 1 tooth on the cs should still drop the gearing. just by feel i think it would be around 1 to 2 rear sprocket teeth worth, so 1 cs drop is equal to about 3.5 rear teeth. this is just a guess on my part by experience on my bikes though.

[victor441]

It will affect it by the ratio between the tire diameters which should be available on the manufactures web site....for example I went from a 150/60-17 to a 150/70-17 on my bike, diameters are 625mm and 656mm....the change in gearing is 656/625 - 1 = 1.05 - 1 = 0.05 or 5% higher gearing with the bigger tire. A typical front sprocket change from say 15 to 14 teeth would drop gearing about 7% so the tire change would undo most of that (15/14 -1 = 7%) in my case

[Tosh Togo]

Originally Posted by victor441

It will affect it by the ratio between the tire diameters which should be available on the manufactures web site....for example I went from a 150/60-17 to a 150/70-17 on my bike, diameters are 625mm and 656mm....the change in gearing is 656/625 - 1 = 1.05 - 1 = 0.05 or 5% higher gearing with the bigger tire. A typical front sprocket change from say 15 to 14 teeth would drop gearing about 7% so the tire change would undo most of that (15/14 -1 = 7%) in my case

Ummmm not quite. That'll get you close, but the measurement that you need to use in order to understand the change in overall gearing is the circumference, and the easiest/best & only way to do that is to measure each one. Using nominal tire sizes (ie: 150/70 vs a 150/60) in order to calculate real-world differing tires sizes assumes a lot from the tire manufacturers... that all of their tires are built to exactly those proportions.

They're not.

To think of it another way, the tire is the last gear in your drivetrain, and its' affect on gearing varies with circumference. A tape measure and some simple arithmetic will give you your answer.

[victor441]

Originally Posted by Tosh Togo

Ummmm not quite. That'll get you close, but the measurement that you need to use in order to understand the change in overall gearing is the circumference, and the easiest/best & only way to do that is to measure each one. Using nominal tire sizes (ie: 150/70 vs a 150/60) in order to calculate real-world differing tires sizes assumes a lot from the tire manufacturers... that all of their tires are built to exactly those proportions.

They're not.

To think of it another way, the tire is the last gear in your drivetrain, and its' affect on gearing varies with circumference. A tape measure and some simple arithmetic will give you your answer.

Very few people would need the precise difference in gearing to three or four places ....reread what the OP asked a bit more carefully ....he asked how the tire size change compared to a countershaft sprocket change and whether it would counteract it, close is good enough in this case pal.

[elsalvadorklr]

Originally Posted by Tosh Togo

Ummmm not quite. That'll get you close, but the measurement that you need to use in order to understand the change in overall gearing is the circumference, and the easiest/best & only way to do that is to measure each one. Using nominal tire sizes (ie: 150/70 vs a 150/60) in order to calculate real-world differing tires sizes assumes a lot from the tire manufacturers... that all of their tires are built to exactly those proportions.

They're not.

To think of it another way, the tire is the last gear in your drivetrain, and its' affect on gearing varies with circumference. A tape measure and some simple arithmetic will give you your answer.

I agree 100percent...

although not wanting to get too detailed the only way to to really know is to measure THE ROLLING DISTANCE OF 1 circumference specifically the rear wheel...

when dialing in a bicycle speedo for example road racers roll the bike and measure the distance traveled and then digit this into the speedo...that is the ony TRUE way of knowing what speed and or rpms you will be riding at certain speeds to the t.

sooo if you feel inclined sierra do this after buying your new tire...

if not just know that not all size tires are created equal so some guessing and trial and error is involved...

,my maxxis desert for example had a better topspeed than my michelin t63 even though the michelin was a 130 and the maxxis a 110...what that means is the maxxis has a taller profike aka bigger circumference aka more topspeed

the inverse is true...a smaler profile tire will give you quicker acceleration.

I think like kubiak that at most you are looking at 2-3 teeth difference in the rear sprocket if you thing gearing wise will be the difference you feel.

so you are looking at possibly offsetting your front sprocket combo when NEW if you have a tore that is noticeable bigger out back...

does that make pseudo sense? jajaja

in my mind however I would not bother

stick a knobby out back for dirt...and a 14 tooth for the xrl upfront and be happy that the bike will handle much better that way...with the slight loss of of topspeed

christian

[elsalvadorklr]

sierra just for shits and giggles measure the rolling distance of your current tire(even though its used) then when you get your new tire do the same...compare them

lastly if you want opinions on tires and their sizes check any of the online seller reviews, users usually point out stuff like:

"these tires sizes run really small" or something like "get the smallest size as they are heavy and or big" etc...that way you can order the size that will suit you the best

only real world user experience will tell you this not simply assuming that a 120 will be bigger than another brands 110...it will be if they are the same brand and model however sometimes very impercebtable

good luck!