bb drop and fit

[the unjet]

My current bike has 7cm bb drop. If I go with 8cm drop, what other measurements would I need to change to maintain the same fit and position?

[e-RICHIE]

"drop" is not related to any position measurements.

[Wayne77]

My current bike has 7cm bb drop. If I go with 8cm drop, what other measurements would I need to change to maintain the same fit and position?

Since a cm more of bb drop would lower your saddle by the same amount, you would need to decrease your bar height by the same amount to maintain the same saddle to bar drop. You could do this by decreasing your HT length by a cm, changing to a stem with less rise (going from a -6 deg to a -10 deg for example) removing a spacer, etc. It would probably decrease your reach slightly as well. I'll let the *real* fit guru's here chime in on what other effects it would have.

Entering your numbers into BikeCAD is a fun and easy way to see what effects gemoetry changes have. Plug your frame numbers in, go to the display menu and select the various dimensions you are interested in and then you can adjust various things to see how things such as saddle to bar drop, reach, wheelbase, etc change. http://www.bikeforest.com/CAD/bikeCAD.html

[dirtdigger88]

what about bar height? If all else remains the same but the bb drops by a cm wouldn't you lower the bars by a cm- either through spacers or a reduction in HT length

jason

[e-RICHIE]

Wayne77-issimo wrote (snipped):
"Since a cm more of bb drop would lower your saddle by the same amount..."



wha?

drop and position are unrelated; your saddle height
is measured from the bb centerpoint, regardless of
that point's relationship with the hub axle.

jerk. help these guys.

[dirtdigger88]

sir richie- please pardon my lack of understanding - I get that saddle height stays the same despite what BB drop you have- but wouldn't the drop from your saddle to your bars decrease with lower BB

jason

[e-RICHIE]

sir dirtdigger88-issimo...

your prescribed position has nothing to do with
drop; drop is the difference between the height
of the bb and the height of the hub axle.

[the unjet]

Thank you all for the input. It seems to me that increasing bb drop would not effect position unless you had to change the angles of the seat tube and down tube to maintain tire clearance. So I guess my question really is- Does a 1 cm increase in bb drop reduce clearance enough so that the geometry of the "triangle" needs to be altered?

[dirtdigger88]

master- richie- I guess I am using the word drop incorrectly? I understand the "drop" of which you speak- but again with the magical bike if the bars stay at the same position relative to space and you lower the BB and the seat tube and the seat (thus increasing the drop at the BB- yes) if you did this with out moving the saddle- wouldn't the distance from the top of the saddle to the top of the bars- measured with a carpenter square- decrease?

Jason

[e-RICHIE]

look...

my saddle height is 72cm, approx.
my bars are ??? below the saddle.

if if switch to a frame with a different drop,
the saddle height is still 72cm, and the
bars remain the same distance below.

no?

[dirtdigger88]

I don't know? You are the builder- so I am not going to say that you are wrong- I am I guess- It just doesn't sound right to me? I am invisioning the BB moving closer to the ground and taking the saddle with it- but the bars remaining at the same height- It seems to me that the saddle would be lower compared to the bars- again- I am wrong I guess- I will default to you due to me lack of understanding

Jason

[e-RICHIE]

"I am invisioning the BB moving closer to the ground and taking the saddle with it- but the bars remaining at the same height..."




why would the saddle drop as the bb gets closer
to the ground, BUT NOT THE FRONT OF THE BIKE?
the bars would drop as much as the saddle does.

[mtflycaster]

For two different bikes of the same size with all personalized fit conditions relative to the bottom bracket being the same, then the saddle and bars would be 1 cm lower relative to the ground on the bike with the 8 cm drop than the bike with the 7 cm drop.

[Matt Barkley]

Wowza!

Yeah, maybe I can help with this E-Ritchie :beer:

The only real "fixed" point on any bike (including bikes with eccentric bbs) IS THE ctr of the BB, wherever that may be on 'yer bike (you should probably know this.)

You should measure everything from the center of the bb. Or another way of saying this is in relation to your bb as with you h/bars and stem. If you measure your bar position from the ground - well measure the bb position from the ground too. (If it is different on one bike to the nbext - then so should 'yer h'bars by doing the groud measurement thing as some do)

So, to recap - repeating a bit of what E-Ritchie has already expressed. Know your saddle height. (Mine is 75.5cm from ctr of the bb to the top of the saddle following parrellel to the seat tube) If you know you bar reach and drop form where your saddle is (mine is in a diagonal 57cm and my drop from a right angle level from the saddle is 9cm) then all that can stay constant. Because it is all in realtion to the BB.

If you have one bike with a bb drop of 7 and you are moving to a bike with a bb drop of 8 - you just keep all the measurements in relation to the bb the same. Same saddle height. Same bar reach. Same bar drop. Hope this helps. Cheers :beer: - Matt

[vandeda]

why would the saddle drop as the bb gets closer
to the ground, BUT NOT THE FRONT OF THE BIKE?
the bars would drop as much as the saddle does.

e-Richie,

I think Jason and I had the same thought in mind ... if all else stayed the same, and you dropped the BB, then wouldn't the handlebar need to come down also. There the answer would be sure. But, you had in mind, that the front of the bike would be modified to accomodate the drop in the BB.

I think the answer to the original question would be depends on the geometry of the 2 bikes. Knowing that will tell you what you will need to modify to get the same fit. Beyond that, only giving the difference in BB height isn't telling us much .... yes/no?

If you have one bike with a bb drop of 7 and you are moving to a bike with a bb drop of 8 - you just keep all the measurements in relation to the bb the same. Same saddle height. Same bar reach. Same bar drop. Hope this helps.

And having read this a bit later .... Matt, and this therefore leaves the option for different handlebar/stem combination so that your bar reach is the same. Say your bar to ground position stayed the same, but your bb to ground position became less, then your bar would need to be lowered in order to maintain your bar to bb measurement. Therefore, other changes may be necessary to keep the same fit.

Dan

[Dave]

It's true that saddle height, relative to the BB does not change, regardless of drop and the head tube NORMALLY is reduced in height, relative to the ground by the same amount as the seat tube (on a bike with more BB drop), so nothing should change with regard to the stem setup. This assumes a horizontal TT.

What probably has some people confused, is that they are thinking of a bike with more BB drop, but a fixed or specific head tube length. This would define a frame with a sloping TT.

Perhaps a more accurate answer is if the BB drop is 1cm more, and the head tube length is also 1cm shorter, then there would be no change to the stem/spacer setup.

If you want to get real picky, the BB drop is a vertical measurement and the head tube/seat tube vertical changes are the proportional to the sine of their respective angles, which is about .5mm less than 1cm.

[David Kirk]

You all talk'in diff stuff.

eR is right ......your position is your position regardless of the BB drop. Drop doesn't affect fit.

Digger is right that bb drop affects the frame in funny ways.

Two frames that are identical with the exception of bb drop will have different head tube lengths ( more drop gives a shorter head tube). I feel that this is where the confusion comes from.

One thing that I think would be great is for folks to stop looking at head tube length as an indicator of size or fit. Head tube length is nearly meaningless and can only be used as a indicator of bar position IF all other numbers on the bike are IDENTICAL. Head tube length is affected by bb drop, fork span, head angle, fork rake, top tube diameter, headset lower stack...etc.

When setting up a bike the handlebar height should referenced to the seat and not to the head tube. I know it's inconvienient but there you go.

Does that make sense?

Dave

I just reread this and it has a bit of a pissy tone to me. Not at all intentional. I need dinner.

[Blastinbob]

Example: my old legend had a 15cm and 15mm of spacers and a 7cm drop, my kirk fits the same with 15.5cm ht, no spacers same stem and a 8cm drop. :confused: :confused: Sure am glad I build exhaust systems and not bikes ! Dave.....go eat !!

[e-RICHIE]

1) you "gots" a position; it's your 3 contact points on the bicycle.
2) on a frame with a 7cm drop, the contact triangle is "correct".
3) increase the frame drop on that frame to 8cm, and the entire
triangle position gets 1 cm closer to the ground.


6a) how can this be misinterpreted?!



10w) i ate!

[vandeda]

Dave.....go eat !!

"Eat poppa eat ... no one likes a skinny Santa"
name that Christmas movie

Dan

[vandeda]

1) you "gots" a position; it's your 3 contact points on the bicycle.
2) on a frame with a 7cm drop, the contact triangle is "correct".
3) increase the frame drop on that frame to 8cm, and the entire
triangle position gets 1 cm closer to the ground.


does this say that the frame w/8cm drop will also have 1cm greater standover height?

Dan

[Dave]

With today's sloping top tubes and integrated headsets, the total length of the head tube WITH THE HEADSET is important when trying to predict, in advance, a decent looking spacer and stem angle combination.

The vast majority of production frames have a BB drop within a few millimeters of 70mm, so most often, head tube length (with the headset) is a good indicator of a frame's vertical size. You just have to be smart enough to look at the geometry and realize that if the BB is 1cm lower, like on a Serotta, the head tube length can be 1cm less and not affect the stem setup.

You also have to be smart enough to add the appropriate headset stack height to the the head tube length. A lot of folks look at a frame using a conventional threadless headset and compare the head tube length directly to one with an integrated headset. Integrated heasets only have a stack height of 5-15mm and conventional headsets are 25-30mm, so this can lead to a big mistake.

For me, I know that I need 150mm of total length (including headset and spacers) to get the bar height I want with a now-common 84 degree stem, IF the BB drop is about 7cm. If I insisted on an 80 degree stem, I'd need about 8mm more length.

[csb]

barbie has a bike with adjustable bb drop,
you just pull the string @ said bottom and it emits
a little scream about a big lug and his contact points

[Dave]

If the BB drop is 1cm greater, the frame would have a 1cm lower standover height.

[dirtdigger88]

Ok- ritchie- I understand the triangle doesn't change based on the location of the BB- I was going on the assumption that the bb drop increased but the head tube stayed the same (creating a sloping geo) - anyway I propably cant explain it but I get it- we were talking about two different issues

Sorry I wont even hint that I doubt you again :D

jason

[e-RICHIE]

dave is right, dan...


the cg moves downwards, but the frame drop
dimension doesn't alter the rider's position.

at least it never has before today!

[vandeda]

If the BB drop is 1cm greater, the frame would have a 1cm lower standover height.

Doh ... sorry, my bad. By "greater" I meant "additional". So more room between your top tube & your crotch.

Dan

[e-RICHIE]

Ok- ritchie- I understand the triangle doesn't change based on the location of the BB- I was going on the assumption that the bb drop increased but the head tube stayed the same (creating a sloping geo) - anyway I propably cant explain it but I get it- we were talking about two different issues



jason-issimo...
why would you assume that everything
changes except the head tube?
curious-in-chester.
e-RICHIE

[Wayne77]

Wayne77-issimo wrote (snipped):
"Since a cm more of bb drop would lower your saddle by the same amount..."



wha?

drop and position are unrelated; your saddle height
is measured from the bb centerpoint, regardless of
that point's relationship with the hub axle.

jerk. help these guys.

I'm obviously in no position to debate with the likes of e-Richie, but someone needs to throw me an e-Bone here.. My thinking is that if the BB is lowered, the distance to the bottom of the pedal stroke increases as well, so I would lower my saddle a corresponding amount no?

I'm open-minded, someone set a wayward soul straight...

Maybe we are mixing terms, I am thinking drop as in Bottom Bracket drop.

[dirtdigger88]

My current bike has 7cm bb drop. If I go with 8cm drop, what other measurements would I need to change to maintain the same fit and position?

I then said that the HT length would have to change (I assume it would be shorter?) or one could run one less spacer (assuming there are more than one spacer used) to get the same triangle - again I think I just misunderstood what you were saying and we are arguing the same side of a coin.

jason

[dirtdigger88]

Wayne- you and I are thinking alike- I guess I am just not able to explain what I mean- Hope you have better luck

Jason

[e-RICHIE]

I'm obviously in no position to debate with the likes of e-Richie, but someone needs to throw me an e-Bone here.. My thinking is that if the BB is lowered, the distance to the bottom of the pedal stroke increases as well, so I would lower my saddle a corresponding amount no?

I'm open-minded, someone set a wayward soul straight...

Maybe we are mixing terms, I am thinking drop as in Bottom Bracket drop.


dohhhhhhhhhhhhhh.
if the bb is LOWERED, the entire bicycle and rider is
LOWERED too; if the frame drop dimension is altered,
your saddle height remains xxcms and the relationship
of the bars to that saddle height is also a constant that
remains unchanged.
at least, THIS USED TO BE TRUE!!!

[Wayne77]

1) you "gots" a position; it's your 3 contact points on the bicycle.
2) on a frame with a 7cm drop, the contact triangle is "correct".
3) increase the frame drop on that frame to 8cm, and the entire
triangle position gets 1 cm closer to the ground.


6a) how can this be misinterpreted?!



10w) i ate!

Okay, after reading the thread I am enlightened now. I was coming at this from the perspective of lowering the bb drop, keeping HT height constant. I can see how this isn't the right approach and I will forsake my wicked ways. But lets say I have two frames or frame designs I'm considering, one with 7cm of drop and one with 8cm of drop, but both with the same HT length (Or maybe I want to minimize HT length for whatever stupid aesthetic reasons) Clearly the frame with the 8cm of drop vs 7cm (with a saddle height from bb of 77.1cm) would present less saddle to bar drop. This would allow me to either stay with 1cm less of saddle to bar drop, remove a 1cm spacer, or go with a stem with less rise. Again, all of this assumes same fork span, same headset upper and lower stack height, same HT lenght, same number of spacers, etc..)

[Dave]

Aren't you both thinking, of a frame with a fixed head tube length, but a lower BB? Then you'd obviously need less spacers or stem angle, but you have also defined a sloping TT bike. You can't have a bike with a given c-c size, lower BB and a horizontal TT without a shorter head tube. When the seat tube and head tube both become shorter then there is no change to the fit.

A lot of folks get confused with sloping TT bikes, but the proper way to compare a sloping TT bike is by c-c frame size, starting with a HORIZONTAL line from the HT/TT centerline intersection point, to the centerline of the seat tube. The length of the seat tube centerline, from the center of the BB to the ST/TT centerline intersection point is the true c-c frame size. The TT slope and actual length of the seat tube do not change the frame size or it's fit.

[Wayne77]

THIS USED TO BE TRUE!!!

And still holds true today! Halelluja! :D Unless, Heaven forbid, HT length were to stay the same between the two designs :rolleyes:

[e-RICHIE]

Quote:
Originally Posted by e-RICHIE
1) you "gots" a position; it's your 3 contact points on the bicycle.
2) on a frame with a 7cm drop, the contact triangle is "correct".
3) increase the frame drop on that frame to 8cm, and the entire
triangle position gets 1 cm closer to the ground.


6a) how can this be misinterpreted?!



10w) i ate!



Okay, after reading the thread I am enlightened now. I was coming at this from the perspective of lowering the bb drop, keeping HT height constant. I can see how this isn't the right approach and I will forsake my wicked ways. But lets say I have two frames or frame designs I'm considering, one with 7cm of drop and one with 8cm of drop, but both with the same HT length (Or maybe I want to minimize HT length for whatever stupid aesthetic reasons) Clearly the frame with the 8cm of drop vs 7cm (with a saddle height from bb of 77.1cm) would present less saddle to bb drop. This would allow me to either stay with 1cm less of saddle to bar drop, remove a 1cm spacer, or go with a stem with less rise. Again, all of this assumes same fork span, same headset upper and lower stack height, same HT lenght, same number of spacers, etc..)


i guess the only logical reply from me would be this:
if you're building another frame just like the one you have, fit, and
like, and all you're altering is the cg (frame drop), then all dimensions
travel with the change; it's the exact same main triangle, except one
frame is lower to the ground than the other.

[e-RICHIE]

And still holds true today! Halelluja! Unless, Heaven forbid, HT length were to stay the same between the two designs


i'm fired.
see you down the road!

e-RICHIE

ps

:rolleyes: :rolleyes: :rolleyes:
:p :p :p
:D :D :D

arrange disorder

[Wayne77]

Aren't you both thinking, of a frame with a fixed head tube length, but a lower BB? Then you'd obviously need less spacers or stem angle, but you have also defined a sloping TT bike. You can't have a bike with a given c-c size, lower BB and a horizontal TT without a shorter head tube. When the seat tube and head tube both become shorter then there is no change to the fit.

A lot of folks get confused with sloping TT bikes, but the proper way to compare a sloping TT bike is by c-c frame size, starting with a HORIZONTAL line from the HT/TT centerline intersection point, to the centerline of the seat tube. The length of the seat tube centerline, from the center of the BB to the ST/TT centerline intersection point is the true c-c frame size. The TT slope and actual length of the seat tube do not change the frame size or it's fit.

That's exactly the angle (pun intended) I was approaching this from. As you know from previous threads I have been playing around with different aspects of frame design. More bb drop allows me to either run a shorter HT (Keeping my saddle to bar drop exactly the same) Or, if I kept the HT length constant I could eliminate a 1cm spacer under the stem. This is the way it seems to work in both Serotta's draft model and in BikeCAD. (I'm not picky about a degree or two of TT slope)

See what happens when you forum experts give a little knowledge to the newbie..Next thing you know the little smarta** punk is running around mixing it up with the likes of e-Richie and spouting off all sorts of nonsense! :D

[Wayne77]

i'm fired.
see you down the road!

e-RICHIE

ps

:rolleyes: :rolleyes: :rolleyes:
:p :p :p
:D :D :D

arrange disorder

:beer:

[David Kirk]

Who's on 1st?

Dave

[lnomalley]

if i lower the shocks on my car, will i still be able to fit in it?

there are two apartments that have an identical layout, one is on the 7th floor and one is on the 8th, which one is bigger?

my entire world is falling apart ;)

[Kirk Pacenti]

.

[Climb01742]

richie, in a previous thread i asked, "who would ever argue with you?" i now know. :rolleyes:

[Climb01742]

when i start talking about frame geometry, we're all in deep do-do but...

it seems (and this is the misunderstanding, i think) that a rider's three points of contact are fixed in space. but really, they are relative. this fit triangle is relative. it moves as the center of the BB moves. the shape and size of the triangle is fixed, but its position in space is relative. drop the BB 1 cm and, in space, the fit triangle drops 1 cm, yet its size and shape remains the same.

that's how i'm reading all this. do i have my dunce cap on or not? :rolleyes:

[e-RICHIE]

a time is a mind too terrible.
it is terrible to time a mind.
waste time and mind terrible.

i think i got it...

a mind is a terrible thing to waste.

:D :D :D
:p :p :p
:cool: :cool: :cool:

arrange disorder

[sspielman]

Wasn't it Dan Quayle who quoted a slogan wrong once(!)....and speaking before the UNCF botched up their slogan....and opened his speech "It is a terrible thing to lose one's mind"......

[Ray]

I've somehow managed to get on a Zepplin kick over the past couple of weeks (don't know why - I never liked 'em in high school when they were huge). For some reason, while reading this thread, I can't seem to get "Communication Breakdown" outta my mind. I just can't.

-Ray

[Too Tall]

Gen.11:1
"And the whole earth was of one language, and of one speech."

[dirtdigger88]

if i lower the shocks on my car, will i still be able to fit in it?

there are two apartments that have an identical layout, one is on the 7th floor and one is on the 8th, which one is bigger?

my entire world is falling apart ;)

See I think the Communication Breakdown is being taken a bit too far now ;) I mis-read the original post. I took it as to ask If I lower my BB by 1 cm what other measurementon the bike would change- As in would my HT change in size- would my seat tube be longer- I did not take it as what would I change in my fit- I totally understand the whole fit triangle being relative to space- just look at my two bikes and you will see that- seat height and reach to bars and drop from saddle to bars is the exact same on both- but my Legend has a bb that is 1 cm lower- Lastly, e-ritchie (and the others) I was not arguing that e-ritchie was wrong- not in the least- I was simply trying to understand what he was saying- I am sorry for not being a lemming and following my leaders of the cliff- I tend to question what I don't understand in an attemt TO understand- thanks for your time-ritchie-

Jason

[e-RICHIE]

ahoy, jason -
this bud's for you!

[Kirk Pacenti]

.

[dirtdigger88]

thanks richie and Kirk- off to work now :D

jason

[Climb01742]

richie, do the authorities in conn. know what goes on in your basement?

[Climb01742]

wow, richie, i'm impressed. how many builders have groupies?

[e-RICHIE]

seven does

[Kirk Pacenti]

.

[e-RICHIE]

...don't make me explain that one.

[Jeff Weir]

did anyone notice that the girls in e-richies basement are handcuffed?

[Climb01742]

that's how richie gets girls.

[slowgoing]

We want an invitation, not an explanation.

[Sandy]

Sometimes with an invitation, you later have to give an explanation.

Still explaining,

Sandy

[hooverone]

=Head tube length is nearly meaningless and can only be used as a indicator of bar position IF all other numbers on the bike are IDENTICAL. Head tube length is affected by bb drop, fork span, head angle, fork rake, top tube diameter, headset lower stack...etc.

When setting up a bike the handlebar height should referenced to the seat and not to the head tube. I know it's inconvienient but there you go.
.

Can you help explain: if you are looking at purchasing bike xyz what do you need to look at if you are reviewing the geometry specs of bike xyz without actually seeing the bike in person? Seat tube length, and top tube length but if I am not going to be concerned with head tube length then what do I need to consider to confirm the bars will be at the correct height?


Jim

[Wayne77]

See I think the Communication Breakdown is being taken a bit too far now ;) I mis-read the original post. I took it as to ask If I lower my BB by 1 cm what other measurementon the bike would change- As in would my HT change in size- would my seat tube be longer- I did not take it as what would I change in my fit- I totally understand the whole fit triangle being relative to space- just look at my two bikes and you will see that- seat height and reach to bars and drop from saddle to bars is the exact same on both- but my Legend has a bb that is 1 cm lower- Lastly, e-ritchie (and the others) I was not arguing that e-ritchie was wrong- not in the least- I was simply trying to understand what he was saying- I am sorry for not being a lemming and following my leaders of the cliff- I tend to question what I don't understand in an attemt TO understand- thanks for your time-ritchie-

Jason

Dirtdigger, you and I are on the same wavelength. It's the rest of the world that is out of sync :D

Actually, after rereading the thread, I think we were all in perfect agreement regarding the fit triangle that stays the same. -Which is exactly why if the bb was lowered the original poster would either have to decrease HT hieght (assuming the frame was still in design mode), live with less saddle to bar drop, reduce stack height, etc. Anyway, it was good for the wise ones here to hammer down the point of critical fit triangle, it makes it easier for me to visualize what would have to change on a frame if one changes various ciritical measurements. Thanks for sharing the knowledge everyone! It's great to have a forum where one can interact with such experienced and well versed (if not a little morally suspect :p ) cycling experts. :beer: