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How do you determine BB drop?

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How do you determine BB drop?

Old 12-08-19, 08:14 PM
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bikingman
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Thoughts and Questions on BB Drop

I've been playing around with bicycle design lately, and I'm thinking about all the adjustable angles and lengths that need to blend into something called a bike frame.

But right now, I'm thinking about BB drop - or the vertical distance between the center of the BB spindle and the center of the rear axle. Below are thoughts on what I've learned and a few follow up questions. Please do respond, but I don't expect to everything.

I've read this article that suggests a shallow BB increases the ability of the rider to lift up the front wheel/bunny hop, and suggests a deeper BB drop creates a 'sitting in' the bike feel. There's also this great discussion that suggests a shallow BB drop will increase the rigidity of the frame (through shorter tubes), while a deeper BB drop will increase comfort (through compliance). This article also tries to debunk the claim that a higher BB will raise the rider's center of balance and therefore cornering stability, pointing to lean and ground conditions instead. However, I do believe the good author could have addressed how a high positioned rider compared to a low positioned rider may feel more 'on top' and potentially more playful while riding. There's this one that discusses a growing interest in deeper BB drops on cross bikes as clearance isn't much of a problem during cross racing, toe clips aren't a problem (mostly), and jumping off/on may be easier while racing over barriers. Finally, there's this post on the triathlon forum that suggests deeper BB drop might be more aero. However, I would suggest aerodynamics is more of a function of fit and rider flexibility than BB drop. Do other's have ideas on how changes to BB drop will impact ride quality?

Secondly, I'm wondering how frame designers set the BB drop to a bike given a customer's preference? What indicators do you use to define the BB drop? I would think the following are important:
  1. Use case - is the customer going to corner tightly (crit racing, xc mtbing) - points to shallow BB drop
  2. Crank arm length - are long or short crank arm being installed - long arms point to shallow BB drop, and short deeper (at least is available)
  3. Customer preference - does the customer want to sit 'in' the bicycle or does the customer want to bounce around on top. - latter points to shallow, the former, deep.
Another angle to this question might be, are BB drops informed by customer preference or designer preference? Thinking about a designers 'style' here.

Also, what's the deal with BB drop + BB height = ~ radius of 26" wheel? -> see paragraph 3 here. Anything?

I understand this might be a little long. Please comment with thoughts on any part. I don't expect anyone to respond to everything here.

Thanks in advance!

Last edited by bikingman; 12-08-19 at 08:16 PM. Reason: title needed to be changed by the time I finished typing.
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Old 12-08-19, 08:57 PM
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Andrew R Stewart 
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My first reaction was don't limit the BB drop relative to only the rear axle height/tire radius. Terry type bikes and suspension come to mind...

Certainly BB drop is a factor in how a bike feels but perhaps is less understood then other design factors. The older I've gotten the more I prefer a larger drop then when I started building/riding. I attribute some of this to spending more time on the saddle then when I was younger.

# 3- Simple example of coincidental dimension math. Why would a wheel or a BB car what the other is? The touring bike I made a few years ago uses 559 wheels and the BB drop I wanted for various reasons, none were about the wheel diameter though.

Generally I feel that pedal strike is the biggest driver of BB height. How much potential is acceptable? Andy
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Old 12-08-19, 09:26 PM
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Most frames fall into a fairly tight range of about 6.8 to 7.5 (road frames.) As Andy says a pedal strike is a big worry driving that characteristic. My preference is towards a little more drop than some use but lug angles can be a factor if you get too far out of the norm.
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Old 12-08-19, 09:29 PM
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Ah yes, I was going to add a note about how BB drop can be dynamic with suspension. Thanks for pointing that out. Can't seem to place Terry types. And thanks for providing insight on #3 , I was confused on why the author highlighted that measure.

Agreed that strike potential is what stands out to me as being among the most important factors.

Cool topic.
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Old 12-08-19, 09:32 PM
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Originally Posted by Nessism View Post
Most frames fall into a fairly tight range of about 6.8 to 7.5 (road frames.) As Andy says a pedal strike is a big worry driving that characteristic. My preference is towards a little more drop than some use but lug angles can be a factor if you get too far out of the norm.
This range is good to note. I wish I could adjust my road bike downstairs and observe the difference. Also good to note that lug angles can play a role outside the norm.
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Old 12-08-19, 09:39 PM
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Richard Sachs has said, about his CX frames, that he uses a pretty standard road drop (of BB). That the taller tire profiles provide the added pedal clearance that CX might need. Andy
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Old 12-08-19, 09:45 PM
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Originally Posted by Andrew R Stewart View Post
Richard Sachs has said, about his CX frames, that he uses a pretty standard road drop (of BB). That the taller tire profiles provide the added pedal clearance that CX might need. Andy
hm, would be interesting to get his take on the idea that a deeper BB drop lowers the seat, making it easier to jump over the frame. Here, sounds like there's room to note frame rigidity, esp if using canti brakes.
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Old 12-08-19, 10:36 PM
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I think any frame stiffness differences (between more or less Bb drop) is a minor amount. Likely less then a few PSI in one's tires or a half turn of the spoke nipples change in tension. Now how the higher CG might effect the perception of the rider is another aspect. Andy
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Old 12-08-19, 11:10 PM
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Richard Sachs is the outlier in this category since he uses 8.0 cm drop for most of his frames. There are a few others with that much drop but it's clearly not the norm.
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Old 12-09-19, 07:27 AM
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If I'm remembering this right, RS has stated on several occasions that he set the BB drop on his jig to 80mm back in around 1980 and hasn't moved it since.
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Old 12-09-19, 07:32 AM
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my most recent frame has 85mm drop, and I haven't noticed any untoward effects. Lots of people riding around on 650b bikes with lower bb than that and it doesn't bother them. I haven't scraped a pedal in a long time, but that's probably because I'm not pedaling through the corners as much.
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Old 12-09-19, 07:44 AM
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Originally Posted by Andrew R Stewart View Post
I think any frame stiffness differences (between more or less Bb drop) is a minor amount. Likely less then a few PSI in one's tires or a half turn of the spoke nipples change in tension. Now how the higher CG might effect the perception of the rider is another aspect. Andy
Changing the spoke tension shouldn't affect the way the bike rides unless it's so low that they're actually going slack (which would be considered a failure). The stiffness of a spoke is the same whatever tension it's already at.

Tyre pressures of course will make a big difference. I agree with you that it's unlikely that the small amount more flex from a lower BB drop would be noticeable.
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Old 12-09-19, 06:42 PM
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The touring frame I built this year has a BB drop of 9cm. I did it for two reasons, stand over height and handlebar to saddle offset. The extra 1-1.5cm in drop affords higher tire profile without sacrificing stand over clearance. It also allows the handle bar to be more on par with the saddle height. I like the aesthetics of it and I can use a lower rise stem, again, more pleasing to the eye. Not a fan of sloping top tubes, plus using lugs that were designed for a level top tube.

Riding the bike feels just like any other bike. Cornering is just fine, have yet to dip a pedal, however I will say if this were not a loaded touring bike, then the BB would be higher as pedaling deep into a turn is more common when playing racer boy. The body does not "feel" like it is "in" the bike, just feels like a regular bike. Not sure what that feeling is and how it can be achieved when sitting atop a bicycle with the wheels beneath your body anyway. It isn't a Dan Gurney motorcycle where the rider really does sit between the wheels!

I believe BB drop or conversely BB height is much ado about nothing when it comes to the handling of the bike, however I believe it should be used to address saddle to bar offset and stand over clearance. One little caveat that needs to be mentioned. I did have a mountain bike with a lower than usual BB height and I had a lot of rock strikes with it. Obviously rocks are not an issue out on the tarmac, but for mountain bikes it is something that does need to be taken into consideration.
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Old 12-10-19, 11:34 AM
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I don't think about BB drop at all. BB drop is a driven dimension. It's the measurement you refer to when setting up your frame jig.

IMO a frame should be designed using BB height. That means you design around the wheels, tires, and crank length the bike plans to use. For road bikes, I aim for 260mm BB height with 170mm cranks and adjust the BB height up or down if the cranks are longer or shorter. For all-mountain hardtails (130-150mm travel fork), I like a BB height with the fork at sag at around 300mm. Shorter travel forks have less affect on BB height through their range of travel than do the big forks. For a XC fork (80-120mm travel) I'd think about lowering the BB height by 5-10mm.

Basically, I prefer the bike to be as low as possible without compromising pedal clearance too much. You also get improved stand over clearance, you can use shorter head tubes, your center of mass is lower, and in theory it's easier to quickly change directions when rocking the bike from side to side.
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Old 12-10-19, 12:30 PM
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For me, "BB drop" is just a number that the framebuilder needs to design the frame. The numbers that matter are BB height (over the pavement). This is governed by both frame drop and tire choice. BB height directly affects pedal clearance. Pedal clearance is also directly affected by crank length, Q-factor and pedal width and depth.

Needed BB height can be governed by those crank and pedal choices and riding style. Now, if you coast all your turns, what I outlined above doesn't matter. I used to race as a skinny, tallish pure climber. I rode a very high BB criterium bike. Yes, I paid a small additional penalty for more wind resistance. I also got a very welcome benefit of being able to pedal deeper into corners and sooner coming out, saving my legs a little hard acceleration every time.

That bike was steep, short and high. "Playful" is putting it gently! It took me a month to get comfortable on it every spring. Tiny touches on the handlebars did a lot. But once I was dialed in and fully trusted it, it cornered like it was on rails.

BB heights: That racing bike was a full 11". I consider 10 3/4" to be a good 175 crank Bb height for a guy who doesn't want to scrape pedals often. 10 5/8" I consider classic "race". 10 1/5" mountain descent "race" for strong riders that don't mind coasting corners and doing a little more work coming out of them. (No bigger than 170 cranks and expect to scrape easily.) 10 3/8" is very low. Short cranks, scraped pedals. I call those bikes "slinkies".

So, if you don't mind coasting corners (or the occasional strike) you can optimize for best "feel" but if you like to pedal those corners, go for the BB height you need then work out what drop that corresponds to. My custom Mooney and geared TiCycles are both around 10 3/4" with sewups or 23-25c tires. My TiCycles fix gear is 10 7/8" with the same tires. All run 175 cranks. (Knees rule and they don't like changes.) Recently my Mooney has been running huge tires for gravel. Pedals are a long ways off the road with 35c tires! But it has also been running fix gear with 28cs. Sweet! Yes I can strike but for a non-racing road fix gear, really nice!

Ben
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Old 12-10-19, 03:05 PM
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this is a framebuilder forum, we can talk about bb drop all we want. On a road bike, it's a pretty arbitrary decision. Since platform pedals went away for most of us, it's pretty arbitrary. I'm sure I can lean over a lot further with my current setup than I could with my 11" bb height crit bike from the '70s with campagnolo platforms on it.

I would be inclined to worry more about bb height on a mtb, but it doesn't seem like there is any safe bb height on the trails around here. I'm sure there is some height that is the difference between a lot of rock strikes and a reasonable number of rock strikes
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Old 12-11-19, 02:41 AM
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Originally Posted by unterhausen View Post
I would be inclined to worry more about bb height on a mtb, but it doesn't seem like there is any safe bb height on the trails around here. I'm sure there is some height that is the difference between a lot of rock strikes and a reasonable number of rock strikes
Thing is a 2cm difference in BB drop is considered a lot in frame design terms but doesn't feel like a lot from the point of view of random rocks on the trail.

I built a long low slack 29er hardtail and went with a 70mm drop. This is more than most bikes out there but there's plenty of clearance because people run such big tyres these days. Also the guy it was for rides without a dropper post.
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Old 12-12-19, 01:57 AM
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Drop is what I use to set my fixture. BB height is what I design around - based on crank length, wheel size, tires (smallest), pedal type, terrain (most extreme) and rider type.



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Old 12-21-19, 02:04 PM
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Originally Posted by bikingman View Post
hm, would be interesting to get his take on the idea that a deeper BB drop lowers the seat, making it easier to jump over the frame. Here, sounds like there's room to note frame rigidity, esp if using canti brakes.
at one point he wrote 8cm of drop, period. I think that is low by standards of 40+ years ago but - today even with a trend to longer cranks 172.5, 175's the pedal cross sections provide way more cornering angle than the ubiquitous Campagnolo Old Record quill pedal.
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