What's the effect of a low BB?

Jeff Potter said:

I thought that classic road bikes had low BB's to improve stability,
handling, descending.

Then I recently read that someone thinks that physics means that a low
BB makes a bike easier to flick back'n'forth. I'd never heard that a
low BB adds to 'flickiness.'

So what really does a low BB do?

I recall that bikes only went away from BB for two reasons: lawsuits
due to pedal-scrape and worse crit cornering due to pedal scrape.

Well, for small-shop builders whose customers use clipless pedals
aren't those two reasons now immaterial?

I would think that any rider---even a crit rider who no longer worries
about pedal-scrape due to cliplessness---would appreciate the added
stability that SUPPOSEDLY (traditionally) comes from a low BB. Anyone
really know the score?

And what is a low BB anyway? My roadbikes seem to be about 10.5". How
low can ya go?

As always, firsthand, enlightened experience and science are
appreciated over conjecture.

Thanks, JP
outyourbackdoor.com

Low bottom brackets:
1. increase the length of the chain stays for the same wheel base
2. make the chain "go uphill" more from front to rear. This makes the front derailer adjustment more critical and chain rub is more prevalent. This effect is "magnified" when using smaller chain rings (44/34/24 instead of 52/42/30 ranges)
3. lower the seat position relative to the ground, but it doesn't improve stability
4. decrease ground clearance for chain rings and pedal clearance
5. Adds downtube length ... and all other things held the same (which they almost never are)... decreases bottom bracket lateral stiffness.

These are my observations based on 3 similar bicycles, that have different bottom bracket heights (Ti Cycles, Litespeed, Serotta Ti) that I ride regularly.

Jeff Potter said:

I thought that classic road bikes had low BB's to improve stability,
handling, descending.

Then I recently read that someone thinks that physics means that a low
BB makes a bike easier to flick back'n'forth. I'd never heard that a
low BB adds to 'flickiness.'

So what really does a low BB do?

Makes it easy to put down a foot at traffic lights without dismounting.

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In article <[email hidden]>,

Jeff Potter said:

I thought that classic road bikes had low BB's to improve stability,
handling, descending.

Then I recently read that someone thinks that physics means that a low
BB makes a bike easier to flick back'n'forth. I'd never heard that a
low BB adds to 'flickiness.'

So what really does a low BB do?

_ I don't really know and I can't find anybody else that does
either. Some frame builders (ie. GP at rivendell) think this is
very important, but he can't tell you why either. I think a lot
of this is mistaking the cause for the effect. It's isn't any one
thing, but the combination of a lot of things together affect the
sublties of bike handling.

_ In the old days when you were much more limited in frame angles
because you needed new lugs for every angle change, a low BB meant
long rear chainstays which meant a long wheelbase. I'm pretty
sure a long wheelbase helps all of the above, and having raced
a few crits with a long wheelbase bike, I KNOW it is less than
ideal for that kind of riding.

_ With modern welded construction you could drop the BB on
a crit bike, but that would make it HEAVIER by probably at
least 10 grams. No self respecting racer would ever accept
that. And I'm not so sure it would really affect handling
that much, I think it's the implications of a lower BB to
the rest of the frame that really make the difference.

_ Booker C. Bense

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Jeff Potter said:

I thought that classic road bikes had low BB's to improve stability,
handling, descending.

Then I recently read that someone thinks that physics means that a low
BB makes a bike easier to flick back'n'forth. I'd never heard that a
low BB adds to 'flickiness.'

So what really does a low BB do?

Not much, because the difference between a "low" one and a "high" one is usually
less than 1cm. In fact, it's usually about half that. This is just one more
trivial thing for witless bike writers and geeks to fawn over.

There's a much bigger difference among mountain bike frames. Rock strikes,
etc., are more common on the lower ones.

Matt O.

Jeff Potter said:

As always, firsthand, enlightened experience and science are
appreciated over conjecture.

Heh. Then politics is right out.

Lowering the BB will make a bike "flicky" because it
reduces the torque needed to push your feet - and anything
standing on them - sideways.

Reducing the bike to a lever with the fulcrum at the
bottom, the handle at the top, and the mass being levered
at the BB in-between, by lowering the BB you're moving the
mass closer to the fulcrum, and increasing the leverage.

The result is that it takes less force on the handlebars
to flick the bike from side to side.

--Blair
"Now, on the matter of why nobody
should vote for plutocrats..."

quote='Blair P. Houghton'Jeff Potter said:

As always, firsthand, enlightened experience and science are
appreciated over conjecture.

Heh. Then politics is right out.

Lowering the BB will make a bike "flicky" because it
reduces the torque needed to push your feet - and anything
standing on them - sideways.

Reducing the bike to a lever with the fulcrum at the
bottom, the handle at the top, and the mass being levered
at the BB in-between, by lowering the BB you're moving the
mass closer to the fulcrum, and increasing the leverage.

The result is that it takes less force on the handlebars
to flick the bike from side to side.

--Blair
"Now, on the matter of why nobody
should vote for plutocrats..."[/QUOTE]
Blair's answer is dead on in terms of how bb drop affects handling.

Booker C. Bense wrote
"low BB meant long rear chainstays which meant a long wheelbase"
Not necessarily quite true - depends how long the chainstays are in the first place. Picture a bike with no bb drop at all. Then when you start to drop it the horizontal distance those same length chainstays cover is actually shorter than when you started, and wheelbase is decreased. If you want to keep the same length wheelbase the chainstays actually need to increase in length. There is unfortunately a lack of consistency amongst framebuilders exactly how chainstays are measured; some measure the direct distance from centre of bb to centre of dropout, some measure the effective horizontal distance. There are arguments as to which is most relevant.

One of the challenges of designing a 29" wheeled mountain bike is keeping the wheelbase short enough for quick handling, but having enough clearance for the wheel against the seat tube. Doing this with a low bb (and even moreso with a slack seat tube as is my wont) becomes difficult because of the abovementioned effect.

Sam

I duly note that none of the replies in either this thread or one that
Sheldon and Jobst posted to ever mentioned direct riding experience. I
specifically asked how bikes rode with various BB heights, not about
physics.

Thus everything that everyone has posted has been basically
irrelevant.

Who cares about the physics of a lever? The relevant question is: What
does the lever do in the whole system of a bike?

Sheldon and others mentioned things like stable highwheelers and
'tippy' lowracer recumbents (they're not tippy when under way).

Jobst said BB height within the normal range is "immaterial" and not
to fool with it.

Someone else posted that BB's used to be low because seatposts were
shorter. DOUBTFUL chain of design!

So let's back up: I believe that bikes, even including top racing
bikes, had lower BB's back in, say, the 1950's. Why? Did anyone try
out high BB's back then?

If they were more 'flickable' then that must've been a good thing.

I note that Grant Petersen of Riv/Bridge has often pushed for lower
BB's, saying that they're only high to avoid lawsuits and to please
crit riders from before the era of clipless pedals. He made a "long
low" Riv model. Apparently the low BB improved handling in some
fashion, according to some folks.

Anyway, I'd like to hear from people who've ridden very similar road
bikes with low, med, high BB's, in a variety of conditions then see
what they say.

--JP
....The All New OutYourBackdoor.com!

Jeff Potter said:

...
Sheldon and others mentioned things like stable highwheelers and
'tippy' lowracer recumbents (they're not tippy when under way)....

A properly designed lowracer is easy to balance at low speeds. If one
does fall over, it hurts much less than doing so on an upright!

--
Tom Sherman

Jeff Potter said:

Someone else posted that BB's used to be low because seatposts were
shorter. DOUBTFUL chain of design!

Quite doubtful.

Quoted message said:

So let's back up: I believe that bikes, even including top racing
bikes, had lower BB's back in, say, the 1950's. Why? Did anyone try
out high BB's back then?

Did they? Well, they also had longer wheelbases, and with the limitations
of lugs, maybe there was a correlation -- intentional or not.

My old track bike, as most track bikes do, has a very high bottom bracket.

Quoted message said:

I note that Grant Petersen of Riv/Bridge has often pushed for lower
BB's, saying that they're only high to avoid lawsuits and to please crit
riders from before the era of clipless pedals. He made a "long low" Riv
model. Apparently the low BB improved handling in some fashion,
according to some folks.

This reasoning is very doubtful. If you haul a bike over enough to scrape
a pedal, then you are not going to be pedaling. If your inside pedal is
down, well, that's your own fault.

Quoted message said:

Anyway, I'd like to hear from people who've ridden very similar road
bikes with low, med, high BB's, in a variety of conditions then see what
they say.

My previous frame had a very low bottom bracket for a modern bike, and my
new one has a much higher one. I'd say there was a 2 cm difference
between them. I was puzzled at first, since the seattube and top tube
were the same lengths, and same angles, but the new frame had a much
longer head tube (so that the old fork would not fit w/o getting a new
headset with a much lower stack height).

The new frame handles a lot better, and is better for climbing. But there
are a lot of factors for that beyond bottom bracket height.

--

David L. Johnson

__o | If all economists were laid end to end, they would not reach a
_`\(,_ | conclusion. -- George Bernard Shaw
(_)/ (_) |

Jeff Potter said:

I duly note that none of the replies in either this thread or one that
Sheldon and Jobst posted to ever mentioned direct riding experience. I
specifically asked how bikes rode with various BB heights, not about
physics.

Thus everything that everyone has posted has been basically
irrelevant.

Who cares about the physics of a lever? The relevant question is: What
does the lever do in the whole system of a bike?

Sheldon and others mentioned things like stable highwheelers and
'tippy' lowracer recumbents (they're not tippy when under way).

Jobst said BB height within the normal range is "immaterial" and not
to fool with it.

Someone else posted that BB's used to be low because seatposts were
shorter. DOUBTFUL chain of design!

So let's back up: I believe that bikes, even including top racing
bikes, had lower BB's back in, say, the 1950's. Why? Did anyone try
out high BB's back then?

If they were more 'flickable' then that must've been a good thing.

I note that Grant Petersen of Riv/Bridge has often pushed for lower
BB's, saying that they're only high to avoid lawsuits and to please
crit riders from before the era of clipless pedals. He made a "long
low" Riv model. Apparently the low BB improved handling in some
fashion, according to some folks.

Anyway, I'd like to hear from people who've ridden very similar road
bikes with low, med, high BB's, in a variety of conditions then see
what they say.

--JP
....The All New OutYourBackdoor.com!

Actually you got half of what you asked for:
"As always, firsthand, enlightened experience and science are
appreciated over conjecture."

Since you asked for science you got the physics.

Since you didn't define flickyness, it appears we have the lattitude to be our own lexicographers.

It appears Blair has a narrower definition of "flickyness" than I. I think of flickiness as the ability to move the bike side to side about the roll axis rather than merely doing so via a leveraging of the handlebars. My broader interpretation factors in flicking about the roll axis by all means, including counter-steer, and adjustment of center of mass in a turning/leaning mode.

So an explanation for a more general case is that the center of gravity is nearer the roll axis resulting in a lower polar moment of inertia so that the back can be oscilated at a higher frequency.

I have a low BB on my folder, and transient movements are faster than any other bike. And if leveraged at the handlebars, the bike could move back and forth with less effort (assuming I didn't snap the 30"' or so tall stem) than a high bb bike. With 44gi in high/29 in low, I don't have a lot of need for leveraging the bike.

The bike does handle well-better than any other of my other dozen plus bikes.

Swapping out pedals recently, I found groundstrike became an issue when I deviated from the original pedals on this short crank arm bike.

Over many years I've probably dropped the height of me and the bike's center
of gravity by about 3 cm by using longer cranks, lower seat height, and
shorter pedal to spindle height and 75mm bb drop which translates to 26.5 cm
bb height. I'd have to agree with the nonchalant types and say that even
that change isn't too noticeable.

"Jeff Potter" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

I duly note that none of the replies in either this thread or one that
Sheldon and Jobst posted to ever mentioned direct riding experience. I
specifically asked how bikes rode with various BB heights, not about
physics.

Thus everything that everyone has posted has been basically
irrelevant.

Who cares about the physics of a lever? The relevant question is: What
does the lever do in the whole system of a bike?

Sheldon and others mentioned things like stable highwheelers and
'tippy' lowracer recumbents (they're not tippy when under way).

Jobst said BB height within the normal range is "immaterial" and not
to fool with it.

Someone else posted that BB's used to be low because seatposts were
shorter. DOUBTFUL chain of design!

So let's back up: I believe that bikes, even including top racing
bikes, had lower BB's back in, say, the 1950's. Why? Did anyone try
out high BB's back then?

If they were more 'flickable' then that must've been a good thing.

I note that Grant Petersen of Riv/Bridge has often pushed for lower
BB's, saying that they're only high to avoid lawsuits and to please
crit riders from before the era of clipless pedals. He made a "long
low" Riv model. Apparently the low BB improved handling in some
fashion, according to some folks.

Anyway, I'd like to hear from people who've ridden very similar road
bikes with low, med, high BB's, in a variety of conditions then see
what they say.

--JP
...The All New OutYourBackdoor.com!