650B, min BB height, other geometry concerns...

AngryScientist · #1 03-22-2018, 07:33 AM

So, i built a 650B bike, a conversion on an older Spec Allez frame.

currently running 650x38 tires on it, and dont LOVE them. would like to try some 650x34 tires.

650x38 is about the same overall diameter as 700x23, which the bike was designed for.

going to 650x34 will lower the bb height by a bit, how low is too low...?

p nut · #2 03-22-2018, 07:36 AM

So we’re talking 2mm difference? Doubt that’d cause any issues.

David Tollefson · #3 03-22-2018, 07:57 AM

700c bead seat diameter is 622mm. So half of that, plus the tire size (abouts) gives 334mm for the axle height on a 23mm tire.

650b bead seat is 584mm. Yields 330mm on a 38mm tire. So you've already dropped 4 mm, and you want to drop another 4. While 8mm isn't trivial to drop the BB, unless it was already designed low, you're probably okay, but don't try to pedal through corners. Or over speed bumps.

AngryScientist · #4 03-22-2018, 08:23 AM

thanks David. 6mm or so is basically what i was calculating as well. it's a bit difficult to predict, as tire height is not necessarily an easy number to put yur finger on without having the tires mounted on the rims in question.

sounds like i should be OK if i'm mindful of the slightly lower BB

Kontact · #5 03-22-2018, 09:37 AM

Quote:

Originally Posted by David Tollefson

700c bead seat diameter is 622mm. So half of that, plus the tire size (abouts) gives 334mm for the axle height on a 23mm tire.

650b bead seat is 584mm. Yields 330mm on a 38mm tire. So you've already dropped 4 mm, and you want to drop another 4. While 8mm isn't trivial to drop the BB, unless it was already designed low, you're probably okay, but don't try to pedal through corners. Or over speed bumps.

Those are diameter numbers. The BB is only going to change height by radius. 38-34 / 2 = 2mm additional BB drop.

David Tollefson · #6 03-22-2018, 10:05 AM

Quote:

Originally Posted by Kontact

Those are diameter numbers. The BB is only going to change height by radius. 38-34 / 2 = 2mm additional BB drop.

But the tire is essentially a round object at the rim radius -- thus, tire diameter plus rim radius.

Kontact · #7 03-22-2018, 10:22 AM

Quote:

Originally Posted by David Tollefson

But the tire is essentially a round object at the rim radius -- thus, tire diameter plus rim radius.

Ah - gotcha.

AngryScientist · #8 03-22-2018, 10:29 AM

i just checked, the axle height is about 2mm lower on my 650bx38 wheel/tire than an 700c open pro with a 23c tire on it, so quite close. if i go down 4 more mm with the 34c tire, should be OK. stay tuned!

CMiller · #9 03-22-2018, 12:01 PM

I've never liked tires smaller than 38mm in 650b. You know the feeling of putting small 1.5 inch tires on 26" bikes then it feels kind of squirrelly, comes up to speed fast but doesn't roll as smooth. I hope this makes sense, not very scientific haha

CMiller · #10 03-22-2018, 12:02 PM

What is the bottom bracket drop on the Allez?

Mark McM · #11 03-22-2018, 12:33 PM

Quote:

Originally Posted by Kontact

Those are diameter numbers. The BB is only going to change height by radius. 38-34 / 2 = 2mm additional BB drop.

BB height changes by the radius of the wheel, but by the diameter of the tire. (Remember - only half the wheel diameter is below the axle, but nearly the entire tire width diameter is below the rim.*) So a change from 38mm to 34mm tire is closer to a change of 4mm in BB height.

*Since the tire casing cross-section isn't a complete circle, the actual height of the tire is about 80-90% of the tire width, but using the tire width is close enough for BB height calculations.

AngryScientist · #12 03-22-2018, 12:44 PM

Quote:

Originally Posted by CMiller

What is the bottom bracket drop on the Allez?

good question. i'll have to measure at some point...

p nut · #13 03-22-2018, 02:14 PM

Quote:

Originally Posted by Mark McM

BB height changes by the radius of the wheel, but by the diameter of the tire. (Remember - only half the wheel diameter is below the axle, but nearly the entire tire width diameter is below the rim.*) So a change from 38mm to 34mm tire is closer to a change of 4mm in BB height.

*Since the tire casing cross-section isn't a complete circle, the actual height of the tire is about 80-90% of the tire width, but using the tire width is close enough for BB height calculations.

Also consider sag. 34mm tire will most likely be run at a higher psi than 38mm (assuming). It should narrow that gap a little bit more.

ravdg316 · #14 03-22-2018, 02:24 PM

Why don't you like it, Angry? Considering doing the same type of conversion

Quote:

Originally Posted by AngryScientist

So, i built a 650B bike, a conversion on an older Spec Allez frame.

currently running 650x38 tires on it, and dont LOVE them. would like to try some 650x34 tires.

650x38 is about the same overall diameter as 700x23, which the bike was designed for.

going to 650x34 will lower the bb height by a bit, how low is too low...?

palincss · #15 03-22-2018, 04:33 PM

Quote:

Originally Posted by AngryScientist

So, i built a 650B bike, a conversion on an older Spec Allez frame.

currently running 650x38 tires on it, and dont LOVE them. would like to try some 650x34 tires.

650x38 is about the same overall diameter as 700x23, which the bike was designed for.

going to 650x34 will lower the bb height by a bit, how low is too low...?

How long are your cranks?