Originally Posted by
dsaul
The amount that the bottom bracket height will change by half the difference in the tire diameter. The other half is at the top of the wheel, where it doesn't effect the bottom bracket height.
I didn't know or realize that. Thank you for clearing that up.
The real number that will effect the pedal strike potential is the bottom bracket height. Bottom bracket "drop" is a frame building spec and it stays constant regardless of tire/wheel size.
That is what I thought. All the manufacturers list BB drop, but they never say the "height." And I assume that will change depending on the specific tire you have and that number will be dependent on that correct?
My understanding is the BB drop doesn't change at all. So when folks say, "you have to be careful that your BB drops more, they are not referring to the "BB drop" number, because that can't change, but they are referring to the BB being lowered in general (along with the rest of the bike), and the BB "height" from the floor is changing. Do I understand this correctly?
So technically, what I need to know is how this all effects the Bottom Bracket "height," and without a wheel set, it would be impossible for me to know correct?
That's what I am trying to figure out. A Surly Ogre typically takes a tire size of 29 x 2.25 is a good range per Surly, but the manufacturer range is 29 x 2"-2.5"; and 27.5 x 2-5"-3" max. If I could get away with it, I would like to run 27.5" in the 2.4" range, and run Schwalbe Super Moto-x HS 439 Ebike Bicycle Tire 27.5 X 2.40 if possible, but I assume that lowers BB too much.
The distance from the bottom bracket to the ground is the thing that changes with tire/wheel size.
In the case of the Ogre, you would take half the tire diameter(730/2=365) minus the BB drop of 68 and that gives you a bottom bracket height of 297mm. For the 706mm tire that would be 353 minus the 68mm bottom bracket drop for a bottom bracket height of 285mm. You can then subtract the crank arm length(actual length from the spindle to the end of the arm) to get the distance from the crank arm to the ground.
This is what I was after. I have to soak this in for a bit. Thank you.