Bottom Bracket Drop
 

How does one figure it and what's acceptable?

Many thanks,

--A

Bottom bracket drop is the vertical distance from a line connecting the front and rear hub axles to the centerline of the bottom bracket shell. A larger drop makes the ride more stable; a shorter drop increases pedal clearance in cornering and uneven terrain. Thus bikes intended for long distance road riding usually have more drop, while those intended for criteriums, off-road, or fixed gear use will have less drop.

Thank you John.

Lower or higher drops also changes the chainstay length, so it should affect rear-end stiffness as well.

I think that traditionally MTBs have drops from -10mm (full susp.) to 30mm (hardtail), track bikes in the 50s (mm), cross bikes is the 60s and road bikes in the 70s.

i like that

I don't understand this business where people say a low bottom bracket creates stability. A low BB lowers the center of gravity, but that makes it easier to lean since the distance between the ground and the roll axis is shorter. I believe that a lower bottom bracket makes the bike turn easier, and with less effort - if anything, less stability although that's not the words I'd use to describe the steering feel. Keep in mind though that the overall range of high to low BB drop figures fall within a range of about 1.5 cm, which is not much.

I always assumed what they meant by stability was likelihood to fall off the bike. A balance thing. Stearing stability being an equally relevant concern as you outline. I started out on 700c bikes, and then spent about 20 years on MTBs, after which I returned to a 700c bike with a high BB. Felt like being way up in the wind like a Penny Farthing.

Anyone have a number for normal drop on a single speed city type bike. Like the couriers use?

MTB bikes have high BB's for ground clearance. I haven't measured but my guess is that the seat is roughtly equally as tall as a road bike. Time to get out the tape measure...

I thought that higher bikes where easier to balance actually. Like a penny-farthing or tall bike, or balancing a short vs long ruler at the tip of the finger. Maybe they're easier for going straight, but as you say steering and leaning is easier with a lower bike. Don't know.

PP1 there was an issue in the Bicycle Quaterly mag. outlining the different geos of some porteur/courier bikes. Don't have it but perhaps somebody can check.

Looks to me like in a touring bike with long wheelbase for heel and toe clearance and for good front load handling, the low BB may be an attempt to restore some handling response, with these more-stable bikes.

I`d heard the same thing about higher being easier to control, and like Tuz, I don`t have any real experience on any bike with enough difference in COG height to say on my own- just that I`ve heard that`s part of why it takes a while to get used to a recumbent. Road Fan`s theory sounds like it might have merit to it, but I thought of another possiblility that might explain low BBs on tourers. Maybe, even if the rider`s weight works to a better advantage when it`s up a bit higher, dead weight of cargo is easier to manupulate if it`s lower, and somehow the two ideas got mixed up or merged. Any rate, I also doubt it really makes a huge difference to raise or lower COG by an inch or so. Lower BB would have a slight aero advantage by lowering the rider`s head and shoulders shoulders, too, but that would be much more of a racer`s concern.

I guess someone should build a movable bottom bracket to test the theory.
Dont forget crank size and BB height.
You can decide on the required pedal-ground clearance for cornering, then factor in the expected crank size. Big riders with big legs need big cranks so you need to put the BB up higher to prevent pedal strike. Smaller riders use smaller cranks so you can lower the BB to give the same pedal clearance. A lower BB makes mounting the bike easier for a small person.

I've never done any experiments. The general consensus is that lower bb feels more stable. This generally means that it takes more input to get a bike to turn.

Guess I disagree with "the general consensus". To me, a low BB means easier to lean and the bike requires less input to hold into corners. High bottom bracket want to go straight. Again, when you consider the total range is only 1.5 cm (.6 inch), the difference is relatively minor.

FWIW, I like the feel of a low bb. It seems to influence more than handling: a low bb bike seems to get down the road easier. And high bb's just feel awkward to me, especially riding out of the saddle, on long climbs (5+ mi). For several years, I commuted (and trained and did recreational rides) on a bike with 10" bb height (8.5cm drop), and 180mm crank arms. Hit a pedal with it exactly once, on a steel mounting bracket (for a removable post) sticking up out of the pavement of a MUP.

SP
Bend, OR

One downside to a high CG is the amount the CG drops when you lean in to a turn. You can really feel it on a motorcycle with a high BB, like going through chicanes on a dirt bike -- switching from left to right you have to lift your body up and over a hump and it takes longer to settle in to a turn.

This.

It's rare enough to find off the shelf bikes which spec different cranks lengths for different sizes. Then when I find one I look at the geometry and find that they screwed it up and gave all the sizes the same BB drop.

I more-or-less agree with Nessism. Most of the frames I have built have low to extremely low bottom brackets, because I once rode a Sachs and really liked it. But to be honest, the difference between very low and very high bottom brackets, geometry for geometry, is so small that it might be imaginary -- at least in my experience. I'll continue to build low bottom brackets for most purposes, but bottom bracket height is one of the very least things I worry about.

Richard Sachs at one point advised he used 8cm of drop. That is quite a bit until one pairs that with modern pedal systems, even using 175mm cranks with most modern pedals the cost of having that much drop is pretty small.
Way back 75mm of drop was the amount often seen on an Italian road bike, these bikes often were cited as being good decending machines when coupled with the geometry of the day.

There are other elements to be sure, the effective top tube length of today is frequently less than "old school" geometry, and often places the front center a bit smaller.

A higher BB means a higher CG, like a long broomstick. A long broomstick takes longer to tip over given an initial displacement, when balancing it on one's hand. This isn't hypothesis, it's completely accepted physics, an undergraduate homework problem. A lower BB conversely tips over faster. Based on conventional physics and engineering definitions of stability, the higher BB is more stable. But, the lower BB is more controllable. The rider will have an easier time stabilizing the bike in the face of disturbances. A given tweak on the handle bars or swivel of the hips has more effect than it would on a taller bike. Again, this is not hypothesis but consistent with current science in controlling mechanical systems. The lower BB is less stable but more controllable.

But here's the hypothetical part: the rider feels more stability because it is easier to control. Apparent stability of a low-BB bike is a human factors matter, not a physics matter.

I have a bike with a high BB (Woodrup Giro, 6.5 cm) and several with lower BBs (Masi Gran Criterium 7.5 cm, Mondonico 7.5, Trek 610 7.2), and there is a significant difference in the steering responsiveness of the Woodrup compared to the others.

you may be right about rider perception, it seems to me that the same is true of trail, where a high trail bike that takes more effort to get turned is considered less stable. I'm going to do experiments with this issue, probably not with the bb height issue.

my analysis of a bike/rider mechanical system is that they aren't attached to each other so a broomstick analogy is somewhat flawed. It's very common for the rider to steer the bike under the body when the body takes much more of a straight line. I was reviewing the bicycle stability literature recently, and researchers have generally refrained from modeling the rider.

All I know about stability and height is that I once read a learned article on it, and it was all counter intuitive...

"Looks to me like in a touring bike with long wheelbase for heel and toe clearance and for good front load handling, the low BB may be an attempt to restore some handling response, with these more-stable bikes."

Another thing about touting bikes is that often the style is for a high and sometimes level top tube. So particularly with 700c or bigger wheels and towering bags of gear, you can find yourself in an embarrassing dilemma when you choose to get off...

Drop is a more elegant way of describing the geometry of frames, but on bikes one has to take wheel, even tire, size heavily into account so the 80 mm of drop on a 700c turns out to be around 10.7" from the ground, and is nothing all that out of the current ordinary.

I see your point about the rider, and I agree that it applies once the rider has started to correct the lean or other disturbance. Before he has done that, he's sitting plop on the saddle, weight on sit bones, et cetera. The rider mass is then coupled to the bike, contributing to the higher CG. So still, a lower cg will have leaned over farther than a higher CG in that first second. The rider's response time (a typical value for "most people") is between 0.7 and 1.0 seconds. Many values have been measured, but these numbers are in the middle of the range. So I'm talking about how far the bike leans in that first second.

Yes, this stuff is counterintuitive. That's kind of why I'm turning to math and physics to keep the basics straight in my mind.

Yes, I should have been talking about BB height rather than drop, especially since the only way I know to measure drop is to derive it from measurements of the wheel radius and BB height.

You're right about SOH and tourers. I'm leaving that strictly up to the individual, as to how friendly they can stand to get with their frames. I like my fits French, but not too French.

But my main point was that a low BB in a touring bike can help restore the controllability to the bike that a sport bike might have. It won't be fully effective, you can't totally negate the effect of 50 added pounds of non-personal ballast.

Can you recall what that paper was?

No idea on the paper, if it hits me...

I didn't mean to suggest there is anything wrong with the idea of drop, just so long as anyone dropping in to read this thread doesn't loose site of the fact that there are wheels and a ground level there. :)

It is amazing how maneuverable a touring bike is when you are about to hit that skunk! In general, though, I am dragging the line, so I actually want it to ride like a rail bike if I can get it. Of course if the doctor clears me for the The Great Divide, I would want a totally different bike.