An interesting take on B.B. drop

13 replies · 7,910 views

cmcalpin
107 posts
#1Nov 17, 2017, 8:32 PMedited
I didn’t know which forum this would be best in but since it pertains to bikes I thought I would post it here. Always lots of discussion about bottom bracket drop. I have even heard people say they don’t like “said” bike for gravel because it does not have enough drop yadadadada. I have been on the fence about it at the mercy of advice from people more experienced than I but still haven’t fully subscribed to the hoopla. On a bike the weight of the cyclist gives it a high cg regardless of bb drop. I talked to a well known frame builder recently and he said it was hype. I won’t get into everything he said cause that’s too much typing but he definitely got me to re thinking. I stumbled across this article tonight and really found it good reading. Maybe you guys have read this and maybe not but for those who can’t decide on what their next frame or bike may be, maybe this might be of interest to you. http://davesbikeblog.squarespace.com/blog/2007/2/21/bottom-bracket-height.html

I certainly don't know everything but I am always on the quest for truth and knowledge.
0
nalax
84 posts
#2Nov 18, 2017, 6:31 PM
Thanks. Dave Moulton probably knew a thing or two about frame geometry.
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dangle
465 posts
#3Nov 20, 2017, 11:42 AM
He's right, but there are two things to add to the conversation.

1) Bottom bracket height will change with whatever tire is on there. How does one measure bottom bracket height unless the exact wheel and tire is used on every bike out there? BB drop is more universal.

2) Yes, the rider determines the center of gravity. Assuming the same crankset and pedal is used with two frames, one with a 70 mm drop and one with a 60 mm drop, it seems the one with less drop would result in a center of gravity 10 mm higher.
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chas
1,305 posts
#4Nov 20, 2017, 11:55 AM
Interesting read. Thanks for posting.
In my quiver, I have an extreme range of BB drop from 50mm to 80mm.
For me the biggest and most obvious difference is pedal strike. The 50mm drop will never have pedal strike (unless I hit something), I can ride on a 45 degree incline with no strike. The 80mm drop threw me off the road at 30mph when the pedal hit the road turning left. When your pedal hits the ground, your rear wheel is lifted off the ground and then dropped back down. Not a pretty site.

I don’t know if I can really attribute much stability to drop. The wheels are huge gyroscopes and I can easily ride with “no hands” on either bike. A mountain bike with “long, low, slack” geometry is easier to ride fast down hill, but I’m thinking that is more of a head tube angle thing (and rider position on the bike). Going down hill standing tall, sitting down, or using a dropper post makes a difference. But does moving the BB 10mm? Kinda, a little maybe….? Clearly a couple of inches (i.e. dropper post) makes a difference for hard mountain biking.

Interesting comments about its effect on the frame. Higher bottom bracket allows for shorter chain stays (and presumable a more responsive bike). One can of course make longer chain stays without dropping the bottom bracket.
Bottom bracket is just a part of the picture though. Typically a low bottom bracket (these days) is included with longer wheelbase and slacker head tube angle. To me, these make the bike feel sluggish and less responsive. Of course this is a good thing if you like bombing down hill, or take trips that are so long that you are totally wiped out at the end of the ride.
I agree with the author, that on most bikes, a 70mm drop is a good target for a responsive bike. That is my target.
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cmcalpin
107 posts
#5Nov 21, 2017, 6:44 AMedited
I always struggle with things I read. That goes both ways. Let me put it this way. A year ago I was texting a friend of mine who races occasionally. I was asking him about frame geometries etc in regards to a bike I was riding. He replied, and I quote, "Man I don't know. I just ride the damm things" That stuck with me. If your racing competitively then every little MM helps somewhat over 200 miles. But for the rest of us "Just ride!!"
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GuitarTed
25 posts
#6Nov 22, 2017, 10:36 AM▲ Top
Anything geometry related ends up becoming a story of "The Princess And The Pea" to a degree. Yes- some folks really can discern a few millimeters of change while others cannot. We are a diverse species and we all will not feel things similarly.

I read the linked article. If bottom bracket drop doesn't matter, then neither does a dropper post. Let me explain: A lower bottom bracket affects how high your saddle is from the ground, if only by a few millimeters, this is true. Using a longer crank than a shorter one will also affect this. So, by having a lower bottom bracket versus a higher one, the mass of the rider is lowered a tiny bit. This can be felt by some riders. Some won't be able to tell. But the effect is similar to a dropper post, or you can see this by simply lowering your saddle 5mm on a bike not equipped with a dropper. The body of the rider is lowered a tiny bit. However you accomplish the lowering of the rider, the overall effect is the same.

Loose conditions riding and cornering are enhanced by a lower bottom bracket/lower center of gravity. And remember, not everyone leans over with the bike into a corner. In fact, coming from a mountain bike background, this isn't the best way to corner on loose surfaces anyway. Lowering your body mass, keeping it vertical, and leaning the bike makes for better loose conditions cornering, but that's another subject.....

This tiny variance in bottom bracket height in itself does not make or break a bicycle. We tend to look at one aspect of a bicycle's geometry and ascribe too much weight to it as that specific aspect is really working in a system. The whole bicycle has to be reviewed as it relates to bottom bracket height, or as has been mentioned, tire volume.

So, as Dave Moulton says, lower bottom brackets are typically mated to longer chain stays, slacker angles, and the overall effect is one of more comfort and stability. Also, keep in mind that this is for classic frame building techniques and the parameters that enforces upon geometry. Today, we have more options which open up geometries previously impossible to achieve. Namely, carbon fiber as a frame material has allowed for this.

Take the 3T Exploro, and other bicycles which have followed the Open UP design cue of a dropped drive side chain stay. This has allowed bottom bracket drop and short chain stays to coexist. A thing Moulton points to as being not compatible.

So, anyway, bottom bracket drop has to be contemplated as part of the whole bicycle, and for some folks, it does matter.
4
cmcalpin
107 posts
#7Nov 23, 2017, 7:58 AM
Ted, I never thought of it the way you put it but that makes perfect sense!! Have a great Thanksgiving everyone!
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chas
1,305 posts
#8Nov 29, 2017, 10:35 AM
GuitarTed wrote:Anything geometry related ends up becoming a story of "The Princess And The Pea" to a degree. Yes- some folks really can discern a few millimeters of change while others cannot. We are a diverse species and we all will not feel things similarly.
...

So, anyway, bottom bracket drop has to be contemplated as part of the whole bicycle, and for some folks, it does matter.

Geometry is an interesting subject. I loved the work you did with swapping out forks (although I don't remember the conclusion, other than we are pretty adaptable once we get used to a geometry).

Have you thought of testing some of these headsets with adjustable angles? Could be an interesting way to make an older bike a little more slacker...

https://www.canecreek.com/products/headsets/angleset

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Flydyl
39 posts
#9Apr 20, 2020, 1:59 PM
As I look to buy a gravel bike, I found geometrygeeks.bike very helpful. But I was alarmed to see that all of the bikes I'm considering have a BB Drop of 70mm! And the bike that I was most excited about--the Marin Gestalt x10 or x11--has a bb drop of 80mm! Given that I ride a lot of single track on my "gravel" bike, this is a deal-breaker. I'd be pedal-striking left and right, with a much greater chance of crashing to boot. I know that big tires (and maybe shorter crankarms) would help, but I really fear the new normal in bb drops.
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ljsmith
762 posts
#10Apr 20, 2020, 5:56 PMedited
Flydyl wrote:As I look to buy a gravel bike, I found geometrygeeks.bike very helpful. But I was alarmed to see that all of the bikes I'm considering have a BB Drop of 70mm! And the bike that I was most excited about--the Marin Gestalt x10 or x11--has a bb drop of 80mm! Given that I ride a lot of single track on my "gravel" bike, this is a deal-breaker. I'd be pedal-striking left and right, with a much greater chance of crashing to boot. I know that big tires (and maybe shorter crankarms) would help, but I really fear the new normal in bb drops.

Have you ever ridden singletrack on a gravel bike? I have, and honestly the type of single track that you can actually ride a gravel bike on, a low bottom bracket is not going to hinder you. Its not like you can hammer a gravel bike through monster rock gardens or massive roots where pedal strike is a big concern. By the way, 70mm is not a "new normal" for gravel bikes. Dedicated gravel bikes are relatively new, and pretty much came with bb drops around 70mm from the very start. It was only when people were riding cyclocross bikes on gravel that the bb heights were higher and even then most cross bikes have about a 65mm bb drop. That extra 5mm isn't going to do a whole lot for you if you have pedal strikes with the 70mm bb drop.. Really it sounds like you should be looking at drop bar mountain bikes.
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Flydyl
39 posts
#11Apr 21, 2020, 8:55 AM
Thanks for your thoughts, LJSMITH! I currently ride an old cyclocross bike in low-key singletrack. Pedal strikes are never an issue at all, even when getting through rocks, roots, etc. No clue what the bb-drop is on that old bike (2004 Giant TCX), but I'm guessing it's pretty high.

I have a proper hardtail for "real" singletrack, but I love to take my CX bike on a variety of routes that include mellow singletrack. So I'm really scared that the 70mm bb-drop is going to cause problems.

The one bike I demo'd, a 2016 Cannondale Slate, gave me all kinds of pedal strikes! It was really fun in singletrack, except for the constant banging. And I'm guessing that the other bikes I'm looking at will be even worse.

Of course, I'll demo before buying, but I was just wondering if anyone else could speak to this issue of gravel bikes, bb-drop, and pedal strikes. Cheers, Dylan
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Slim
300 posts
#12Apr 22, 2020, 5:26 PMedited
I disagree and agree with the linked article on Dave’s bike blog.
As he points out, the center of Gravity is not significantly changed by lowering the bottombracket.

However, lowering the bottom bracket does improve both stability and cornering.

Imagine the bike leaned over a certain degree. Your weight on the pedal creates a moment arm, trying to tip the bike over even further . The longer this arm (taller BB), the higher this moment. In that way, lower bottombracket heights do make a bike more stable.

The other is changing directions, from turning left to turning right (slalom). The bigger moment arm resists this switch more. This is very noticeable on a mtb, even with a fairly small change in BB height.

The final issue is traction. When cornering in a situation with poor traction, it is easier to adjust downward pressure (=traction) on the tire, while still leaning the bike over, if your foot is closer to the tire.

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chas
1,305 posts
#13Apr 22, 2020, 5:27 PMedited
I understand your concern. I don't think there are realistically any negatives to a high BB, but pedal strike on a low BB has thrown me off the bike more than once. I road a Diverge with a 75mm drop and pedal strike was an issue with me. Then they went and lowered the BB even more!

My bb is 66mm, and I haven't been able to hit the pedal in aggressive riding. But keep in mind, if a typical road bike has 70mm drop on 23mm tires, you can go a lot lower on 40mm tires.

If you need higher than 70mm, you need a CX bike. Long/low/slack is in fashion these days. While it is great for a lot of people, it isn't for everyone.
2
Slim
300 posts
#14Apr 22, 2020, 5:29 PMedited
Flydyl wrote:Thanks for your thoughts, LJSMITH! I currently ride an old cyclocross bike in low-key singletrack. Pedal strikes are never an issue at all, even when getting through rocks, roots, etc. No clue what the bb-drop is on that old bike (2004 Giant TCX), but I'm guessing it's pretty high.
..., but I love to take my CX bike on a variety of routes that include mellow singletrack. So I'm really scared that the 70mm bb-drop is going to cause problems.
... but I was just wondering if anyone else could speak to this issue of gravel bikes, bb-drop, and pedal strikes. Cheers, Dylan

well the easy answer Dylan, is measure/look up the BB drop and crank length of your current bike, since you say you are ok with that. Let’s say it’s 60mm drop and 175mm cranks. I assume your old CX bike has skinny tires, maybe 35mm?
If the new bike you are looking at has 70mm drop, then you know you will be ok as long as you run tires 15mm wider(10mm+ some extra to account for deeper sag on the softer tire), or 10mm wider tires and 170mm cranks.
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