BB Drop or Height Impact to Ride

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Hexsense
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by Hexsense

Karvalo wrote:
Fri Dec 02, 2022 10:04 pm
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?
I start to doubt what I heard and thought of it...
I agree on CG to ground would be the relevant value when cornering as tire is where bike rotate around.

For the bigger is more stable part.
Part of it is trail value that naturally come with bigger wheelset. But that can be compensate by fork rake, head angle etc.
Part of it is angle of attack on the same size obstracle (rock/road irregularity). Same size of rock or road crack are less effective at changing wheel direction when hitting the wheel with larger diameter AND higher hub height (which change vector of impact force vs center of centrifugal of the wheel.
Part of it, which I now question, Is the center of the hub height and the interaction when the wind catch the wheel. The biggest part of "wind rotate my front wheel" force is around the hub height, where the rim is furthest forward from the hub. By changing the height of that force, is it like a knee-height kick vs low sweep kick?

youngs_modulus
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by youngs_modulus

satanas wrote:
Fri Dec 02, 2022 1:57 pm
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;
You get to define "better" however you like, but you're keeping it vague enough that your claim—"lower BBs handle better in a non-specific way than higher BBs"—has become unfalsifiable. But then again, this has always been an entierely subjective exercise, so that's not actually a problem. It does mean, however, that no one can ever be either right or wrong about your claim, including you.
satanas wrote:
Fri Dec 02, 2022 1:57 pm
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.
This is a great point. My tastes run towards framebuilders like Rob English, but then again, he's also a degreed mechanical engineer—of course I like him! But yes, a huge number of framebuilders are unable to articulate why their designs work, even if their designs work fabulously well. And of course there are exceptions; English, Scot Nicol (Ibis) and Tom Ritchey are prominent examples—and the last two, AFAIK, don't have engineering degrees. And I was lucky enough once to meet Pino Moroni, who I don't believe had an engineering degree either.

A non-engineer framebuilder isn't remotely a problem—joining skill is pretty much independent from book learning. But you're right that framebuilders as a group are not repositories of objective knowledge about how geometry affects handling. To be clear, there is real value in their being repositories of subjective knowledge. Some framebuilders are more tuned into the science involved; others are more about the vibe. I'd argue there's a place for both.

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youngs_modulus
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by youngs_modulus

Karvalo wrote:
Fri Dec 02, 2022 10:04 pm
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?
Karvalo nailed this on both fronts, IMHO. But, hexsense, you're asking good questions and I would hate to discourage that.

Karvalo
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by Karvalo

Hexsense wrote:
Fri Dec 02, 2022 10:52 pm
Part of it, which I now question, Is the center of the hub height and the interaction when the wind catch the wheel. The biggest part of "wind rotate my front wheel" force is around the hub height, where the rim is furthest forward from the hub. By changing the height of that force, is it like a knee-height kick vs low sweep kick?
Whether that's the case or not I would think that by far the dominant effects are that a bigger wheel has a) more surface area and b) a longer lever arm between the wheel axis and the centre of pressure. Both of these are going to lead to more torque trying to turn a bigger wheel. In addition to more side force trying to lean the bike, obviously.

And I don't think that's the case anyway. Your 'kick' examples are ones where you take someone's feet from under them. Have you ever ridden in such windy conditions that the grip of the tyres is overwhelmed and the wheels are literally blown out from under you? I haven't, and I'm never going to. What actually happens is the wind rotates the bike/rider system around the contact patch. The same force hitting higher up means more torque and more lean.

TobinHatesYou
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by TobinHatesYou

Re: santanas

Stoopidtall and Stoopidtaller have BB heights of approximately ∞mm and ∞+1mm. They are very stable bikes in the same way that a tennis racquet is more stable balanced in the palm of your hand when the center of mass is higher (racquet head up vs handle up.)

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pdlpsher1
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by pdlpsher1

Lol. I hadn't seen the Stoopidtall videos before.

But seriously, can the Stoodpidtall take a corner at speed? The increased in stability of a lowered BB is for cornering. Matej Mohoric won Milano-Sanremo using a dropper post. It's the same concept as a lowered BB.

The OP asked for feedback from those who have low BB bikes. But how many here who responded with comments/opionions are actually riding bikes with lowered BBs?

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by TobinHatesYou

I thought Mohoric used the dropper primarily to get more aero.

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ultimobici
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by ultimobici

Karvalo wrote:
Fri Dec 02, 2022 10:04 pm
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?
Try taking your hands off the bars!

Karvalo
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by Karvalo

TobinHatesYou wrote:
Wed Dec 07, 2022 9:01 am
Re: santanas

Stoopidtall and Stoopidtaller have BB heights of approximately ∞mm and ∞+1mm. They are very stable bikes in the same way that a tennis racquet is more stable balanced in the palm of your hand when the center of mass is higher (racquet head up vs handle up.)
The one thing I wonder about here is static vs dynamic stability. So yes, a very tall bike is harder to start leaning over.... but once it starts leaning / falling then it's also harder to get it to stop, right? Yep I take a certain radius at a certain speed the CoM has to end up at a certain angle - but in the taller bike this is point is further away from where the CoM started and it has to move faster in order to get there - so what happens when you try and aggressively slot it straight into the lean angle you want and stop it there?

I wonder if that's easier to do on a lower CG bike, and that's what we perceive as 'stability'.

TobinHatesYou
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by TobinHatesYou

Karvalo wrote:
Wed Dec 07, 2022 12:23 pm
TobinHatesYou wrote:
Wed Dec 07, 2022 9:01 am
Re: santanas

Stoopidtall and Stoopidtaller have BB heights of approximately ∞mm and ∞+1mm. They are very stable bikes in the same way that a tennis racquet is more stable balanced in the palm of your hand when the center of mass is higher (racquet head up vs handle up.)
The one thing I wonder about here is static vs dynamic stability. So yes, a very tall bike is harder to start leaning over.... but once it starts leaning / falling then it's also harder to get it to stop, right? Yep I take a certain radius at a certain speed the CoM has to end up at a certain angle - but in the taller bike this is point is further away from where the CoM started and it has to move faster in order to get there - so what happens when you try and aggressively slot it straight into the lean angle you want and stop it there?

I wonder if that's easier to do on a lower CG bike, and that's what we perceive as 'stability'.

Haha, it was mostly tongue in cheek, but yes this is very much a situation where the system will want to keep doing what it’s already doing. Trying to right yourself out of a lean means your body will have to travel a greater distance…it’ll realistically happen quite slowly and you’ll end up flying off the edge of the road or whatever.

MilesG813
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by MilesG813

TobinHatesYou wrote:
Wed Dec 07, 2022 10:04 am
I thought Mohoric used the dropper primarily to get more aero.
"Dropper posts are almost exclusively seen on mountain bikes and allow the rider to drop the saddle height at the flick of a switch, making it easier to lower the centre of gravity or shift position on steep or technical terrain, improving the ability to manoeuvre the bike."
https://www.bikeradar.com/features/pro- ... 0the%20end.
Bike Radar's article on Mohoric's use and the legality of the dropper post.
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justkeepedaling
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by justkeepedaling

People who haven't used dropper posts should not comment here. The purpose is to give MTB rider clearance to move around when on rough terrain. The lower CG of the post itself is negligible to actually moving the body side to side or lower like when going over a berm.

In road use, it was 100% so that he could get lower and more aero since they banned supertuck

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