TiHabanero

In planning a touring frame using 26" wheels I am trying to increase stand over height without going to a sloping top tube. I have done some preliminary figures and believe a 7-7.5cm BB drop will give 7.5 to 8.0cm ground clearance at the crank arm using a 170mm crank. Does anyone see any problem with this?

Andrew R Stewart 

Yes, sloping the TT up toward the seat will increase stand over

My recently done 559 wheeled bike has a 5.5cm drop and with the Panaracer 1.5" Protites has a pedal spindle center about 8.8cm off the floor. With 2" dirt tires this floor to pedal will grow.

When I designed the frame I wasn't thinking past having the pedals about the same eight off the floor then that on my sporting bikes. It worked out just about the same which worked well enough so that this aspect is not even on my radar while riding. But I do need to be mindful of front pannier drag when u turning on a narrow road... Andy

TiHabanero

The 75-80mm crank arm pedal center to ground distance includes a 40mm tire height. My race bike is around 90mm and I don't plan on racing this rig. Front panniers mounted on lowriders may be an issue, however if mounted on a front rack there should be no issue at all.

Andrew R Stewart 

Yes to this bolded part. The touring bike has low riders, 26' tires and I have used Arkel GT-18 on a Tubus low rider rack, rather large for the front. The outer bottoms drooped down and scuffed a few times. Andy

fietsbob

No habit of pedaling while in corners? ..

TiHabanero

Rarely pedal in corners such as side street corners. Large radius bends in the road is a different situation and of course will be pedaling. My friend, Pete, has 220 cranks on his 26" commuter and said that it really isn't an issue as long as he doesn't ride it like a performance bike, so I figured extra BB drop will do the same thing as a longer crank. I know, it is unconventional, but that is why I build my own.

fietsbob

Touring, I like Toe Clip pedals , so I have a high enough BB, so they didn't scrape against the pavement when starting off before I flip them over ..

Doug Fattic 

A 26 X 1.5” tire has a radius of 316mm. A custom low BB height is 260mm when using 170 cranks. Subtracting 260 from 316 leaves a drop of 56. I’ve made a travel bike for myself with a BB height of 250 (super low). Even at that extreme of a low height, I would have a drop of 66 using 26 X 1.5 tires.

BB heights were established in an era when pedals had a surrounding cage that pointed beyond the side of the foot. This meant the BB had to be raised so the cage didn’t hit the pavement when leaning and pedaling through a corner. Now with clipless pedals it is safe to build with a lower BB height since there is less pedal to scrap the ground.

Still my math and experience says your drop is too low. I can’t imagine going lower than 250mm as a BB height (I would never build a customer’s frame with a BB that low). However if you decide to stick to your plan and go even lower, get back to us and tell us if you had any problems with pedal strike.