BB Drop or Height Impact to Ride

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maxima
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Joined: Fri Oct 17, 2008 9:37 am

by maxima

Dear Fellow Cyclist, I would like to seek view form folks who have custome made frames with BB drop of 80mm or more? After Knee surgery, been having on/off knee pains and this is only resolve after moving from 172.5mm cranks to a 165mm cranks. I ride a 54 to 55cm frameset and wonder with a shorter crank, I would like to lower the BB (increase BB drop from my current SL7/Aethos of 72mm to 80mm) with a custom build frameset? I wonder which builder have better ride similar to that of Aethos of SL7. whats the advantage of lowered BB that provide a lot more stability? probably better aerodynamics?

I'm looking at Festka vs Argonaut? who makes a better carbon frame?

Any real life experience appreciated.

fizzaz
Posts: 185
Joined: Wed Nov 07, 2018 5:02 pm

by fizzaz

An eccentric bb might work for this case as well, but I'm not sure you would notice the difference in crank length difference with a crank change. After all, you'll still be connected to the bb.

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Hexsense
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by Hexsense

There are gravel bikes with 80mm bb drop.
Maybe, ask how they ride with road tires?

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pdlpsher1
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by pdlpsher1

I'm a huge proponent of a large BB drop. All of my bikes have large BB drops (two bikes with 80mm drop). The main advantages are 1) increased stability and handling, and 2) decreased standover height to make getting on/off the bike easier. There are many reasons why today's mainstream bikes have too little BB drops...1) tire sizes are only getting bigger, and bigger tires raises the BB from the ground, 2) crank lengths are getting shorter, which lessens the chance of pedal strikes, 3) increased popularity of mixed-surface riding which makes a bike with a low standover height easier to get on and off on loose surfaces, and 4) pedal strike is a possibility, and bike manufacturers don't want to get into legal trouble for making a bike that could cause a crash. For the most of us (mere mortals who don't regularly race) this isn't a problem, but we often end up living with a bike, one with a full race geometry.

My road bike is a custom bike with a 80mm drop. I live on a hill and there are only two ways down to start every ride, one hill I hit a 50mph terminal velocity in a full tuck (55mph+ with a tailwind) and the bike tracks straight as an arrow. The faster I go the more stable the bike is, is the feeling that I get. On the other downhill I hit a terminal velocity of 40mph+. My gravel bike is the Diverge, also with a 80mm drop. Additionally, I'm using 650B wheels which further reduces the standover height. Both of these bikes have 170mm cranks. If I were using 165mm cranks I would go with a 85mm drop or lower. On gravel riding I do get more rock strikes, and that's just something you have to keep in mind when riding in a rock garden with babyheads.

maxima
Posts: 393
Joined: Fri Oct 17, 2008 9:37 am

by maxima

Thanks, I'm checking with a few custom builders on a 82-85mm BB drop...who ever able to make it work.

satanas
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by satanas

You'll most likely get a range of responses about this. As with most things there's personal preference involved, plus some people will notice and care while others won't; same old, same old.

Personally, I like ~80mm drop, since (IMHO) it improves handling on descents versus the typical 68-70mm, and I use small pedals, don't race crits and rarely pedal around corners. 28-622 tyres typically have a radius of ~343mm, so with 80mm drop this gives a BBH of 263mm, which is pretty much spot on for me with 172.5mm cranks; YMMV. (FWIW, with 260mm BBH I get pedal strike a bit more often than I'd like, but still only occasionally.)

With shorter cranks than 172.5mm and/or bigger diameter tyres than 700x28 I wouldn't hesitate about getting >80mm drop, however the important thing is the resulting BB height. If you ride fixed, race crits, or ride off-road in rock infested areas a higher BB might be your friend...

maxima
Posts: 393
Joined: Fri Oct 17, 2008 9:37 am

by maxima

Thanks for thr Input. Been cycling 33+ years, retired from racing and just enjoy long scenic rides, don't pedal thru corner anymore as riding is just an enjoyment not a competition - gilding thru corners at high speed is my new love :thumbup: and definitely I will benefit from a lowered BB :beerchug:

I figure since I ride 54-55cm frame, 72mm BB drop and use to ride with 172.5mm cranks w/o issue. With the downsize to 165mm cranks due to knee surgery, it should not be an issue with lowered BB height (BB drop to 80-85mm).

satanas wrote:
Thu Dec 01, 2022 11:06 pm
You'll most likely get a range of responses about this. As with most things there's personal preference involved, plus some people will notice and care while others won't; same old, same old.

Personally, I like ~80mm drop, since (IMHO) it improves handling on descents versus the typical 68-70mm, and I use small pedals, don't race crits and rarely pedal around corners. 28-622 tyres typically have a radius of ~343mm, so with 80mm drop this gives a BBH of 263mm, which is pretty much spot on for me with 172.5mm cranks; YMMV. (FWIW, with 260mm BBH I get pedal strike a bit more often than I'd like, but still only occasionally.)

With shorter cranks than 172.5mm and/or bigger diameter tyres than 700x28 I wouldn't hesitate about getting >80mm drop, however the important thing is the resulting BB height. If you ride fixed, race crits, or ride off-road in rock infested areas a higher BB might be your friend...

PeytonM
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Joined: Wed Apr 24, 2019 10:59 am

by PeytonM

Recently purchased a bike with 66mm bb drop. I was hesitant coming from a bike with 68mm drop and 25c tires to the new bike with 28c. All the reading I had done pointed to low to mid 70's as a preferred bb drop.

But needed a bike quickly so went ahead.

Turned out to be a non issue for me.

youngs_modulus
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by youngs_modulus

satanas wrote:
Thu Dec 01, 2022 11:06 pm
You'll most likely get a range of responses about this. As with most things there's personal preference involved, plus some people will notice and care while others won't; same old, same old.
This is a sage observation. Many people like to imagine that there's a direct causal relationship between changes in geometry and perceptible changes in handling, but it's often untrue.

Sure: in general, a steeper head angle and more fork rake reduce trail, which makes a frame feel a little more eager to turn. But small changes have small effects, even though confirmation bias convinces many otherwise.

To be clear, I think everyone should ride the bikes that make them happy, and if a low BB does it for you, that's great. I have a soft spot for belt-drive bikes; we all get to like what we like.

But let's think about what more BB drop does from a physical perspective. It basically just lowers your center of mass infinitesimally. How little?
satanas wrote:
Thu Dec 01, 2022 11:06 pm
Personally, I like ~80mm drop, since (IMHO) it improves handling on descents versus the typical 68-70mm
Again, I'm not criticizing anyone's taste here. But a ≈10mm drop in BB height only lowers your center of mass by the same amount (at most). That's just not much at all. In fact, you could get a very similar drop in CoM just by lowering your stem and saddle by maybe 1.5cm. That wouldn't be a bad way to test whether a custom frame with a lower BB would handle the way you hope it would. Pedaling would be weird because of the seat height change, but you could do back-to-back descents to compare the high-speed handling traits. Make like Mythbusters and experiment!

There's a lot of mythology around bikes that doesn't make much sense in terms of physics. For example, one could easily argue that if you want more stability, you should go for a very high BB. If "stability" is defined as resistance to changing direction," then a higher CoM is more stable than a lower one. That's why it's radically easier to balance a hammer in your hand when you have the handle in your palm. Putting the CG down low (hammer head in hand) reduces the hammer's polar moment of inertia in two axes, which makes it less stable.

There are conditions in which a lower CG makes things more stable, but those situations usually don't apply to two-wheeled vehicle dynamics. (Max braking is one exception).

So let's look at motorcycles: have you noticed that GP bikes have a much lower CoM than, say a Goldwing tourer with a passenger and a high rear hard-sided pannier? What about those Paris-Dakar motorcycles? They're taaaaall.

If lowering your CoM improved stability in two-wheel vehicle dynamics, don't you think that the Goldwing riders would want low-slung bikes and GP racers would tend to prefer the faster-handling of a taller bike?

And when's the last time you saw someone win your local crit on a tall bike? (I kid, but it's true that tall bikes are much harder to maneuver quickly than regular-height bikes).

Fighter aircraft are designed this way; it's called "relaxed stability." Being less stable yields faster handling because the vehicle has less resistance to changing direction.

If you have a thing for bikes with 80mm of BB drop and you want to buy a custom frame with that spec, more power to you—go out and get what makes you happy. My only points here are:

- I don't think it will make much of a difference to stability either way; and

- If a more stable bike is what you're seeking, you might consider looking at geometry with more trail, whether via less fork rake, a slacker head tube, or both. But definitely get 80mm of BB drop at the same time if that delights you.

One last thing to keep in mind: buyers of custom frames are a terrible data set for questions like "did your extra-low BB help handling?" Your fellow customers have all plunked down $3000-$10,000 on an artisan's counter, waited for months and then built up what's often a life-long dream bike. They are invested in every sense of the word.

I'm not remotely suggesting that they're lying. But they may love their bike and it's handling so much that they get a little carried away and say, "OMG, my low BB makes the bike handle SO well" when what they really mean is "I love how my bike handles."

And of course, very few people will admit to themselves—let alone you—that the slightly unusual geometry they chose handles very nicely, but not radically better than what they were riding before.

It's subconscious, but no one wants to say "it's nice, but not very different than my old bike after spending lots of money to get exactly what they wanted. When I was little, every time I got new shoes I would run around the mall in tight circles and marvel at how much more "grip" my new shoes had than the old ones did. In reality, they're about the same. But as an excited five-year-old, I KNEW I could tell the difference.

I saw a much more sophisticated version of this effect with my own eyes when I worked at a Seven dealership in Madison, WI around the turn of the century. We sold a lot of custom Sevens to doctors, lawyers and dentists—no shade to those professions—and that first ride produced a lot of magical thinking.

But so what? I never corrected them; they weren't even wrong. They were just excited about their new bike. And raining on that parade seems absurd and mean.

Ride what you like, but also consider being open to your framebuilder's thoughts on how to get the handling you want.
Last edited by youngs_modulus on Fri Dec 02, 2022 5:52 am, edited 1 time in total.

youngs_modulus
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by youngs_modulus

[Double post. Oops.]

satanas
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by satanas

Please note that I didn't make any comment about more BB drop increasing stability, but rather said that I feel it improves handling on descents; FWIW I definitely prefer <=265mm BB height to 270mm or more. A lower BB also makes it slightly easier to put a foot down at lights without getting off the saddle, especially obvious compared with a MTB with ~300mm BBH.

Re framebuilders: IME, they're just as likely as anyone else to have fixed ideas about how things should be, and to resist any changes to their pet designs. Some builders basically make stock frames in different colours with perhaps a few minor changes, while others like to experiment and/or tailor things more. It's crucially important to find a builder who's on the same wavelength as you are. There's a quote in Waltworks' FAQ thus:

"First, drop me an email or call me to introduce yourself and tell me what you’re looking for. I will not accept a deposit from you (for any item I make) without this introduction, because it may be that what you want and what I can build don’t match up well, or that one of us decides the other one is crazy. Better to know that before money changes hands."

Pato
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by Pato

10mm difference in BB height won't make much for stability while all other geometry measures stay the same. My 2 main road bikes have basically the same geo except one has 8mm more BB drop than the other. They both share the same HT angle, rear center, 2mm difference in fork offset which is neglitible, same cranks, same bb, same pedals, same handlebars. Both mounted with identical position in a jig, only difference is wheels but both use 47/50mm deep wheels with the same tires spoke tension and very well maintained.
I recently rode them both back to back and there's no real difference whatsover in stability. They both have their characteristics but can't say one is more stable than the other.
Some years ago I performed similar tests in MTB and the most noticeable difference between a 10mm BB drop was only in rock strikes while pedaling.
There's many other factors you can tweak in bike geo that will contribute a lot more to stability than the BB height.

In my own tests the factors that contributed the most stability in a bike geometry are:
- Longer rear center (also better shifting especiallyt with 12 sp)
- Slacker HT angle
- Shorter fork rake

Excluding bike geometry, well balanced wheels with the right spoke tension will have way more impact on your feeeling of bike stability than the BB height.

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Mr.Gib
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by Mr.Gib

I'm mostly with youngs_modulus on this one. It's a complicated recipe. I had two bikes with identical geometry. One was very stable and one was not. (Same wheels, drivetrain, etc. The difference was likely due to frame stiffness.) And older Colnagos that were not stiff but had lots of trail were fabulously stable. Certainly, the intuition that lower is better may not always hold, but what about longer and lower? If stability is your goal I would look in that direction. And don't forget to make it stiff with a decent trail number.
wheelsONfire wrote: When we ride disc brakes the whole deal of braking is just like a leaving a fart. It happens and then it's over. Nothing planned and nothing to get nervous for.

Hexsense
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by Hexsense

Does stability derive from CG to the ground level or to hub height? There are some self centering force and dynamic interaction when tilting/turning going on at the center of the wheel hub, thus hub height is one of the reference point I assume? Atleast that seems to be one of the reason bigger wheel is more stable than smaller wheel because center of the hub go up higher than the smaller wheel and the diff between CG height to hub height is reduced.

I think I maybe wrong. But if we consider CG to hub height, a very short rider with low CG may detect the change in stability if their CG is raised or lowered by a cm.

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Karvalo
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by Karvalo

Hexsense wrote:
Fri Dec 02, 2022 6:58 pm
Does stability derive from CG to the ground level or to hub height? There are some self centering force and dynamic interaction when tilting/turning going on at the center of the wheel hub, thus hub height is one of the reference point I assume? Atleast that seems to be one of the reason bigger wheel is more stable than smaller wheel because center of the hub go up higher than the smaller wheel and the diff between CG height to hub height is reduced.

I think I maybe wrong. But if we consider CG to hub height, a very short rider with low CG may detect the change in stability if their CG is raised or lowered by a cm.
CG to ground would be the relevant measurement because the tyre contact patch is what the CG is rotating around when you lean.

Re bigger wheels being more stable... are they? No one struggles to stay upright on a Brompton, right?

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