Swing Low, Sweet Bottom Bracket

Scribblin' to illustrate bottom bracket drop. Ahh.....hope that helps!

Sometimes I get comments on the posts I do that are great questions. Questions that deserve to be answered out in the open as opposed to being "just another comment" on a post you won't look at again. (Likely)

So, today I wanted to address a great question that I got on yesterday's post and today I am posting the answer. The Question:

"G-ted, it would be great to hear your thoughts on bb (bottom bracket) drop. I know you have mentioned it before, a refresher would be good or links to your older posts on the topic would be helpful" 

Okay, so first things first. What is bottom bracket drop? This is a feature of geometry that has to do with the relationship in space between the wheel axles and the center line of the crank spindle, (which goes through the bottom bracket shell).  Bottom bracket drop can be "negative", (think BMX bikes where the bottom bracket is higher than the wheel axles) and it can be "high" or "low". The image here, by the way, has my chicken scratches showing a line through the wheel axles and a line through the bottom bracket center line. The distance between the two, (red chicken scratches) is the bottom bracket drop.

This dimension will determine where your pedals describe a circle when you pedal -or not when you coast- in relation to the axles. Why does that matter? Well, it makes a bicycle feel different. "What?!", you may exclaim, "How can a few millimeters one way or the other make any difference at all in bottom bracket drop? You're loony." Well, I would answer by saying, "How about we drop you saddle 5mm and you get back to me after a 100 miles.". Yeah...... That 5mm may mean the difference between trashed knees or a great ride. You can feel that 5mm. You can also feel a 5mm lower bottom bracket. So yes- millimeters matter. 

Twin Six's Standard Rando has a 75mm bottom bracket drop. You can feel that.

 Okay, so here is the big question: What does a lower bottom bracket do for a rider?

Whoa buddy..... Hold on a min. The first thing we need to define is What is "Lower"? We need a base line. So, I will give you one. It's subjective, and my opinion here, but hey! We gotta draw a line somewhere, right?

Before there were "gravel bikes" we had road bikes, touring bikes, and cyclo-cross bikes which were the bikes that made up the majority of rigs using a "road bike standard" drive train set up. Typically road bikes in the USA used "crit geometry". This was, besides other things, a set up that allowed a rider to really lean the bike over in a turn and keep pedaling. That required that the bottom bracket be a "ceratin height" to avoid pedal strikes in corners. Touring bikes, which were not expected to be leaned over in turns and pedaled, were set up with lower bottom bracket heights than "crit geometry" (Not always. Trek being a major offender in this category). The theory being that lower bottom bracket heights put the center of a rider's gravity lower in the bike in relation to the axles, lending a feeling of more stability, among other sundry things that lower bottom bracket does. Cyclo cross bikes, on the other hand, had to be able to clear ruts while pedaling, clear rough grounds, and barriers. Higher bottom brackets were the norm.

So, from lowest to highest we had touring bikes, road bikes, then cyclo cross bikes. My theory back in the early part of this decade was that "gravel bikes", if they were ever to be made, should have lower bottom bracket height, similar to touring bikes. Why? Well, now we can answer that earlier question.....

What does a lower bottom bracket do for a rider? (On a gravel bike) Okay, again- my theory, and it was adopted by Raleigh for the Tamland, was that a lower bottom bracket height was okay because gravel riders typically were not going to pedal corners "criterium style" and we weren't hopping barriers or pedaling in ruts. (Well......Level B Roads notwithstanding) So, the typical cyclo cross bike, especially Euro influenced ones, were running 65mm drops. That is high! Road bikes were typically 70mm. Still too high in my mind. Raleigh was advised by me to go to a 72mm drop. I really wanted a 75mm drop, but I felt that was too radical for Raleigh to accept. In fact, a friend of mine, Ben Witt, and I were bandying about with the idea of a 77-80mm drop! Of course, we were dreaming of 45mm tires and the geometry would work with a big tire. However; in 2010 that did not exist.

650B tires change what you want in a bottom bracket drop

When is a lower bottom bracket a bad thing? So, how low can you go? Well, it depends. I gave a hint above. It will be dictated somewhat by tires. Especially if you want to go with 650B tires at times, or all the time.

I tried 650B tires on the Twin Six Standard Rando, which has a 75mm bottom bracket drop. It was a great bike in most every way with those tires, but I did experience a not insignificant amount of pedal strikes. Even on regular gravel roads. That was a bit of a concern.

In my experience, a 70mm bottom bracket drop is a great compromise dimension for wheel swapping between 700c and 650B. You could probably do a couple millimeters lower. My Tamland seems to be okay at 72mm drop with 650B wheels, but 70 is a figure a lot of companies use already, so that is readily available. Just know that 75 and deeper for a 700c design is going to start to cause pedal strikes when that bike is set up with the smaller 650B wheels and 47mm-ish tires.

Higher bottom brackets really make a gravel rig squirrely  on loose descents. I hate the feeling that the bike gets at speed when descending with a higher bottom bracket. It is unnerving. Lower bottom brackets quiet that down a lot. Let's face it, when you are descending at 30mph+ on loose gravel you are just along for the ride. Having a better feel can be the difference between riding that out, or scrubbing speed and crawling down the hill. I'd rather bomb the down hills, so I like a deeper bottom bracket.

It also is great when you are on the flats on looser gravel as well. I just feel like the wheels are less likely to be skittish over the marbles and that the tires track better. Now, it isn't always great to have a lower bottom bracket. Those lower bottom brackets make it marginally tougher to stand up out of the saddle and hammer, for one thing. So, on a single speed, I would opt for the higher bottom bracket and longer cranks, but that is my opinion.

Anyway, a long winded answer, but there you go. I hope that you find my opinion helpful. Try experimenting with different bikes yourself and make up your own mind. But as for me, I swing that bottom bracket low on my sweet chariot. It's my jam. It may not be for you.