custom 650b frame: bottom bracket drop?

[Rio]

guys, I need some help. I'm going for a custom build 650b frame but do not have enough knowledge about bottom bracket drop. Doing a search on the web I found that 650b frame builders use bottom bracket drops from 32 to 50 mm. What is the effect on handling?

Frame dimensions I do know:
Seat tube length: 430 mm
Head tube angle: 70,5
Seat tube angle: 73
Effective TT length: 585 mm
Chainstay length: 425 mm
Headtube length: 110 mm

Frame will be fitted with a rigid Kona P2 fork (I think it's 440 AC?)

Any other comments/advice on geometry is very much welcome!

 

[benjy]

maby ask a few frame building company's what they recommend and try to look at maby similar frames to what you want or what its intended for to see there hights, i havnt a clue realy but from bitd when riding bmx i thought higher made the front wheel go skywards easyer

 

[Raging_Bulls]

BB drop would be how much lower the centre of the BB is compared to the wheels' axles. The ideal value is hard to determine because it depends on what you expect from the bike.

As for the effect on handling, a higher BB will move the CG upward, making the bike more stable in terms of side-to-side movement. However it'll be more likely to flip over forward or backward.
BB height is only one of the deciding factors of course. Head angle, rake etc also have a major influence on how a bike behaves.



Start by measuring your favourite rigid bike's BB height. If you know the exact diameter of your new 650b wheels with the tyres you'll use, it should be easy enough to calculate the desired drop. The formula is easy : wheel radius minus preferred BB height equals drop

As for knowing the diameter of the wheels, don't just trust the general "650b = 27.5 inch diameter" guideline. Tyre size has a tremendous effect.
Swapping my Schwalbe 26x1.95" City Jets for a pair of 26x2.35" Big apples will push my 26" wheels' diameter from 26.2" to a full 27", effectively moving my BB up by more than 10mm and drastically altering the bike's stability and handling characteristics.


My 26" Scott is a perfectly stable yet nimble bike, at least for me. It has an axle height of 33.2cm and a BB height of 31cm, which means that as a 26" bike it has a 22mm BB drop.
If I use that generalization I prefer to avoid, a 650b has a 3/4 inch (19mm) taller radius than a 26" wheel. Taking my scott as an example, the drop would become 41 mm (22mm+19mm), which is bang in the middle of the values you mentioned.


EDIT : 425mm chainstay on a 650b? I hope you are either building an e-stay, a road bike or plan on using a single front chainwheel, because that's going to be a tight squeeze if it's an MTB.

 

[Rio]

Thank for that! I thought of going for 40 mm, but was mostly wondering what the effect was of going for more or less drop. I don't think I understand that a higher center of gravity makes the bike more stable in side to side movement.

I think what you experienced with the larger tires is stability from a larger radius of the tires itself, not the raised bottom bracket. That's what I thought after feeling this after going to wider tires, but maybee I'm wrong about that.

Riding with bikes with different bottom bracket hight I mostly felt a difference with wheelying and bunny hops and stuff like that. I just don't know anymore what bikes I liked and didn't like unfortunately...

I never thought about spacing issues to be honoust. I'm building a mtb with 2x8 gearing, having a 24/36 tooth setup up front. I thought this wouldn't be a problem, as there are 26 inch frames around with 425 mm chainstays that work with normal gearing. Well, that's what I heared... Maybee check this! Thanks for the insight ;-)

do you think this setup could work? It's only temporary, as the bike gets sliding dropouts and I plan to mount a Rohloff in the future. But of course it should work now too