mstateglfr 

I just posted this elsewhere and immediately realized the sage wisdom on this forum would probably help a lot so I am breaking the rules and posting twice. Forgive me mods, for I have sinned.



My gravel bike has 70mm. A frame I am considering is 77mm. If I build a frame, I had figured I would just repeat the geometry of my current gravel bike so it would be 70mm.

More BB drop means you sit more 'in' the bike than 'on top' of the bike. And the stack height is effectively higher the lower the bottom bracket goes, so less spacers are needed to get the bars where you want. And there is more control with a lower BB drop which is also a good thing for gravel riding.

So all that seems like a plus. The only potential downside I can think of is pedal strike and I simply dont turn hard enough on gravel for that to be an issue.

Am I missing something and there actually is a big downside to 75+mm of drop?

unterhausen

I think 75mm is fine for a gravel bike. I have definitely hit my pedals on things on our local gravel just descending in a straight line. Never upset my bike though.

Andrew R Stewart 

Drop can be about pedal strike, fit (bar/seat heights) and stability. But that last one can be different then you might think. I'm no engineer but here's a situation that taller is more stable. Try balancing a tall/long stick on your hand (it was a base ball bat when we were kids). Don't let it fall. Now do the same for a short stick and see how much harder it is to keep balanced. The taller item starts to fall over at a slower rate and therefore is easier to react to and correct for.

I would follow Richard Sachs's advice and not change the drop from your standard road amount. The tire growth will raise the BB somewhat any way. Andy

Cynikal 

I was just reading a post from Richard about how he has never built a bike without an 80mm BB drop. That was the only measurement on his jig that didn't move.

Cyclist0108

This is true.

The principle of the unicycle.

unterhausen

bicycles stay up because the rider steers the bicycle under their center of gravity, on average. If the rider can't do this, they are going down*. I doubt this is enhanced by raising the rider. Most people prefer the handling afforded by lower bb's, all else being equal.

The inverted pendulum is not really an example of stability. It's unstable in all positions and all lengths. It just shows that a shorter pendulum needs a higher bandwidth control system to remain upright. Conversely, bicycles are a stable system (self-righting) under normal operating conditions.

*just a simple thought experiment, if you are leaned at a given angle and want to get the center of contact of the wheels under your center of gravity, a higher bb will mean you have to move the wheels further on the ground in order to do this. So it's more difficult to control.

Andrew R Stewart 

Note that I ended my example with "The taller item starts to fall over at a slower rate and therefore is easier to react to and correct for." I was not directly saying that a higher BB made a bike more stable, just that these things are not always as straightforward as one might think at first thought.

Note I also finished my post with Richard Sachs's advise. Andy.

dsaul

This. Especially that last point about the distance between the contact patch and the CG. With a lower bb riders will feel as if they are "in" the bike as opposed to being perched atop the bike.

To answer the OP - aside from pedal strike concerns, there is no downside to a lower bottom bracket.

Nessism

It seems the huge majority of production frames fall into a tight window from about 68 to low 70's. Sach's 80 is a clear outlier. Even still, we are just talking about 12mm or so. Not exactly a huge difference.
Personally, I typically use about 75mm drop for my road frames. Seems like a nice workable height.

unterhausen

production frames have relatively high bottom brackets because their customers don't know any better and the company lawyers are afraid of pedal strike. They also copy each other's geometry. I thought I saw that some companies are breaking out of this though.

75mm is a good drop, that's what I usually use.

Andrew R Stewart 

Most of my sport bikes use a 75mm drop too. My two recent touring builds did use 55mm drops but with the 26x1.5 tires that equates with about a 75 drop for a 700c and 25ish tires. Andy

duanedr 

Andrew touches on an important point. Drop is relative to wheel/tire size. Sachs I think builds only with 700c wheels and tires within a 23-35mm range. I haven't ever seen any of his bikes outside of that description - no BMX bikes or 29er hard tails.

I tend to think about it from a crank clearance perspective. I start with 85mm of clearance and then go from there depending on type of bike, riding style, etc.

fietsbob

Now if you were building a urban commuter bike , a lower BB makes putting a foot down at a stoplight easier..

But you are thinking of something for uneven ground...






...

63rickert

It is not all the same and you won't know until you try.

My '73 Cinelli had 82mm drop. I can't measure that accurately but that's what André Cinelli told me and my measurement confirmed it. The '67 Falcon must have had over 90 - measuring ground to center of BB axle was about 9-7/8" with a Clement 50. Lowest BB I am aware of on a road bike would be one Ron Boi built for himself with 101mm of drop. His report on that was he went through corners faster than anyone with no effort. Drafting anyone was easier. Getting a draft off him was suddenly harder. No downside. I was only 15 and 16 years old when I rode the Falcon and still recall it rolled really well. The Cinelli was a dream and very hard to say how much of that was due to drop.

Significantly lowering center of gravity does not make a bike less stable.

Experiment. If you have the option experiment. Predicting how you will like it is not going to connect in any way to reality.

Andrew R Stewart 

Anecdotal stories like the Ron Boi low BB bike being this or that without more geometry data is of little real information. No mention of weight placement (front or rear centers), steering angle or trail and the seat/bars relative position WRT his previous bike. Like has been said before- taking one geometry element and trying to associate it with some greater result can be a fool's errand. Andy

63rickert

I could agree with all that. So try it a slightly different way. The range of BB drop in current production bike is minute. Even full custom bikes currently produced stick to a small range. The range in bikes that have been built and have ridden well is large. Or larger at least. This alone is reason enough to try something different.

Or. Lowering the bottom bracket a full inch (current norm to the RRB mentioned) will lower center of gravity. Vehicles with low center of gravity tend to corner better. Will this apply to bicycles? Gee, it might be interesting to find out.

Or. Aero is everything when you want to go fast. Riders are willing to do crazy stuff, spend all their money on wind tunnel tests, wear Darth Vader helmets. Do you think sitting an inch closer to ground might be aero? Do you think maybe when drafting another rider being an inch lower might tuck you into that rider's slipstream a little tighter? Do you think if it's a rider you ride with all the time it might be possible to notice a change, and have that change be real even with no numerical data?

The gist of previous text was experimentation is good. If the issue at hand is BB drop and you have no data, then make some data. There is enough anecdote to justify an experiment. There is no good reason to shut down discussion because I didn't hand you a blueprint and a wind tunnel report.

Andrew R Stewart 

I never said to not try a different BB drop then common. I never said to stop this discussion.

I did bring up real aspects of why one frame might ride differently then another but that taking only one element and, without knowing the rest of the design, claiming this or that is not a good way to figure this stuff out.

One problem with this kind of discussion is the lack of real scientific experimentation. Double blind testing is pretty hard when dealing with bikes (and I've read of quite a few attempts at this on bikes) so the human element pretty much always creeps into the picture. Once that happens all bets are off because the human effort/performance is so dependent of psychology and that is hard to divorce from our testing.

So I say do test and keep every other geometry aspect the same (as much as it's possible) and decide what you like. But be careful in claiming that a single change does this or that for others. Andy