Bottom bracket drop

Just curious about bb drop. What is a realistic limit? I see a lot of frames with 6.8-7.0 cm of drop
but some advocate a drop of 8cm. I know that great a drop increases the chances of scraping a pedal
and isn't something you would want on a crit bike, but just how much does an 8cm drop enhance
handling? Is the lower centre of gravity very noticeable?

I am thinking of getting an 8cm drop on my next frame and am looking for justification for
changing it.

thanks

 

>Just curious about bb drop.

good question. I am waiting to hear the answers also. B

(remove clothes to reply)

 

"Geoffrey Stone" <[email protected]> wrote in message
news:[email protected]...
> Just curious about bb drop. What is a realistic limit? I see a lot of frames with 6.8-7.0 cm of
> drop but some advocate a drop of 8cm. I know that great a drop increases the chances of scraping a
> pedal and isn't something you would want on a crit bike, but just how much does an 8cm drop
> enhance handling? Is the lower centre of gravity very noticeable?
>
> I am thinking of getting an 8cm drop on my next frame and am looking for justification for
> changing it.

Having designed and built some custom frames and having consulted and advised on a large number of
custom built frames ( Waterford, etc) over the years here's my advice to you:

Tell the builder how you want this bike to handle, how you'll equip it, how you ride, how long and
how hard and where. Show him your position either in person or by photo with measurements from your
present bike. Tell him what you like and don't like about your present bike and /or others you have
riden a lot. Tell him if you plan on racing and what your event is ( road.crit/tri) and whether or
not you'll be adding camping gear and such.

Then let the man do his job.

I have seen, in an extreme case, a specification that was literally unbuildable because too many
dimensions were specified and the figure has to close!!

BTW I would personally build a crit bike with a 6 or 6.5 BB depending on the rider but not an 8 in
any event. YMMV.
--
Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971

 

"A Muzi" <[email protected]> wrote in message news:[email protected]...
>> BTW I would personally build a crit bike with a 6 or 6.5 BB depending on
> the rider but not an 8 in any event. YMMV.

Is that "any event" meaning a racing event? Or for any bike used for anything?

Waterford seems to spec a drop of 8 on their less racy designs. "Conservative" Rivendell uses
something like 7.5 cm on their road frames.

I was bemused by an article I recently read in a Rivendell publication. It seemed like Grant was
saying that a high bb can actually give the rider a sense of too much stability.

Making me more confused is that my Bianchi which feels very stable and neutral, feels more stable
than an 80's Gios which has a higher BB.

I do realize that "drop" is a means to even the playing field for comparison purposes between
frames. BB height is dependent on tire profile, and hence the tires used plays some roll (P.I.) in
the perception of bike handling related to BB height.

 

> "A Muzi" <[email protected]> wrote in message news:[email protected]...
> >> BTW I would personally build a crit bike with a 6 or 6.5 BB depending
on
> > the rider but not an 8 in any event. YMMV.

"Gary Jacobson" <[email protected]> wrote in message
news:[email protected]...
> Is that "any event" meaning a racing event? Or for any bike used for anything?
>
> Waterford seems to spec a drop of 8 on their less racy designs. "Conservative" Rivendell uses
> something like 7.5 cm on their road frames.
>
> I was bemused by an article I recently read in a Rivendell publication. It seemed like Grant was
> saying that a high bb can actually give the rider a sense of too much stability.
>
> Making me more confused is that my Bianchi which feels very stable and neutral, feels more stable
> than an 80's Gios which has a higher BB.
>
> I do realize that "drop" is a means to even the playing field for
comparison
> purposes between frames. BB height is dependent on tire profile, and hence the tires used plays
> some roll (P.I.) in the perception of bike handling related to BB height.

Thanks for addressing the bogus "BB height" about which I wasted a stack of letters to magazines for
a few years.

I meant that I would not seriously consider a criterium bike with a drop that low. IMHO a criterium
bike should be capable of being pedalled in the corners!

In search of "stability" you need to look at a host of interrelated factors besides the height of
the bike off the ground. It is folly to isolate one item and obsess over it. Consult your
framebuilder and talk about the entire frame and its fit to you!

--
Andrew Muzi http://www.yellowjersey.org Open every day since 1 April 1971

 

I recall that Independent Fabrications also uses an 8.0 cm bb drop. I had my frame built 7.5 and I'm
very happy with it for my non-racing applications.

"Gary Jacobson" <[email protected]> wrote in message
news:[email protected]...
>
> "A Muzi" <[email protected]> wrote in message news:vdak58836men[email protected]...
> >> BTW I would personally build a crit bike with a 6 or 6.5 BB depending
on
> > the rider but not an 8 in any event. YMMV.
>
> Is that "any event" meaning a racing event? Or for any bike used for anything?
>
> Waterford seems to spec a drop of 8 on their less racy designs. "Conservative" Rivendell uses
> something like 7.5 cm on their road frames.
>
> I was bemused by an article I recently read in a Rivendell publication. It seemed like Grant was
> saying that a high bb can actually give the rider a sense of too much stability.
>
> Making me more confused is that my Bianchi which feels very stable and neutral, feels more stable
> than an 80's Gios which has a higher BB.
>
> I do realize that "drop" is a means to even the playing field for
comparison
> purposes between frames. BB height is dependent on tire profile, and hence the tires used plays
> some roll (P.I.) in the perception of bike handling related to BB height.

 

On 28 May 2003 10:23:31 -0700 [email protected] (Geoffrey Stone) wrote:

>Just curious about bb drop. What is a realistic limit? I see a lot of frames with 6.8-7.0 cm of
>drop but some advocate a drop of 8cm. I know that great a drop increases the chances of scraping a
>pedal and isn't something you would want on a crit bike, but just how much does an 8cm drop enhance
>handling? Is the lower centre of gravity very noticeable?
>
>I am thinking of getting an 8cm drop on my next frame and am looking for justification for
>changing it.

On single track vehicles (bicycles and motorcycles,) there is no real handling advantage to lowering
the center of mass, since the bike leans in the corners, keeping its CM directly "above" the wheel
line at all times.

There may be a VERY small advantage to lowering the CM because this makes it somewhat quicker to
lean into a curve, but this is countered by the fact that you have to be more careful of
dragging a pedal.

There is probably a slight aerodynamic advantage to lowering the CM, just to reduce the frontal
area. This is much more important in motorcycles.

A BB drop of 8cm is a LOT! 5.8 to 6.5cm is a more normal range, but you also need to keep your crank
length in mind.

In general, I think BB height gets played with most often to allow one to build extremely small
frames; if you move the BB upward this moves the seat upward which allows the top tube to move up
which allows the head tube to be greater than the minimum length. This allows for smaller framesets.
Sloped top tubes have largely eliminated this incentive, however.

In my opinion, there is really just one good reason for wanting an extremely low BB, and that is
because it allows the larger sizes of a diamond frame to be more symmetrical (top to bottom, about
an axis from the rear axle to the middle of the head tube) and hence stiffer, by virtue of
shortening the seat stays. OTOH, I doubt if this effect is significant.

-
-----------------------------------------------
Jim Adney [email protected] Madison, WI 53711 USA
-----------------------------------------------

 

> (Geoffrey Stone) wrote:
>
>> Just curious about bb drop. What is a realistic limit? I see a lot of frames with 6.8-7.0 cm of
>> drop but some advocate a drop of 8cm.

I'd say "as low as you can get", but I am a tourer. As another one said, Rivendell suggests a higher
bottom bracket means one would need to do too much left to right compensation to maintain balance
(that's what they mean by "too stable"). However, I don't think it makes a significant difference.

That being said, there are a few difference:

- A very low BB might scratch in corners, a problem if you are an aggressive cyclist doing
fast tight turns and the like. I don't recommend that over here because of sand, potholes,
steel plates...

- With toe clips, pedals might scratch on the ground if you don't have your feet in the toe clips.

- A low bottom bracket means it's easy to stop at an intersection, keep the butt on the saddle and
put one foot on the ground. With a high bottom bracket, one needs to dismount, which I find less
comfortable for a long red light, and a slight problem with a leather saddle in the rain.

Guess what, I like a low BB.

Regards,

Michel Gagnon

 

"Gary Jacobson" <[email protected]> wrote in message
news:<[email protected]>...
> "A Muzi" <[email protected]> wrote in message news:[email protected]...
> >> BTW I would personally build a crit bike with a 6 or 6.5 BB depending on
> > the rider but not an 8 in any event. YMMV.
>
> Is that "any event" meaning a racing event? Or for any bike used for anything?
>
> Waterford seems to spec a drop of 8 on their less racy designs. "Conservative" Rivendell uses
> something like 7.5 cm on their road frames.
>
> I was bemused by an article I recently read in a Rivendell publication. It seemed like Grant was
> saying that a high bb can actually give the rider a sense of too much stability.
>
> Making me more confused is that my Bianchi which feels very stable and neutral, feels more stable
> than an 80's Gios which has a higher BB.
>
> I do realize that "drop" is a means to even the playing field for comparison purposes between
> frames. BB height is dependent on tire profile, and hence the tires used plays some roll (P.I.) in
> the perception of bike handling related to BB height.

Rivendell used to spec their road frames with an 8.0 drop, although now that they're all custom, I'm
sure that is in play with each frame. From what I understand, Serotta has gone to an 8cm drop for a
number of their bikes, as has Waterford, IF, and others. I have a Rivendell Road frame with a drop
of 8 cm and I run 23mm tires on it. With clipless pedals, I've scraped a pedals exactly once on this
bike and it was on a reverse camber turn with a big old lump in the tarmac that I hit. With clips
and straps, I've had a few good pedal strikes on the road on my Lemond cross bike with a drop of 7.5
cm. And all sorts of strikes riding single track with this bike. But for a non-criterium road bike
that'll generally be used with clipless pedals, I don't think there's any problem with 8cm at all.
That said, I can't feel a difference between my bikes with 7.5 and 8 cm bb drop, but the one with a
drop of 6.5 feels very strange.

Regarding what Grant said in that article, bikes with a higher bb can feel more stable going
straight down the road, but they're not as easy or seamless when you lean them over into a turn.
Conversely, a very low bb may make a bike feel a bit jittery on a straightaway (at least until you
get used to it, which takes maybe a few minutes), but they're much easier to lean into corners and
the transition between left and right turns is much smoother. My experience is similar, but, again,
I can't tell the difference between 7.5 and 8 cm with similar geometry and similar wheel/tire
combinations.

-Ray Sachs

 

"Michel Gagnon" <[email protected]> wrote in message news:<ACdDa.6502
> - A low bottom bracket means it's easy to stop at an intersection, keep the butt on the saddle and
> put one foot on the ground. With a high bottom bracket, one needs to dismount, which I find less
> comfortable for a long red light, and a slight problem with a leather saddle in the rain.

wait.. you unclip/dismount at lights?

go fixed gear!