BB height: a moron's question

[Climb01742]

ok, dumb question: when measuring BB height, it's from the ground, yes? and to where on the BB? bottom of BB or center of BB? (look, i was dropped on my head as a baby...repeatedly...from a great height. ;) )

[Smiley]

center of BB , and you are forgiven

[eddief]

I don't know how height is measured, but my understanding is the definition of drop is the distance of the center of the bb measured below the plane of a line drawn horizontal through the front and rear axel.

How does one measure height?

[Smiley]

oh yes I forgot Eddie s correct its the drop to the line between the axis of the wheel hubs if its measured as a 6 , 7 or 8 cm but if given as a bigger number then its to the ground like 26..... .

[OldDog]

If to the ground, then is tire size taken into consideration? I've never heard of this being measured this way. :confused:

[Smiley]

I think there is a default for tire height being a 23 mm tire. Some manufactures do give the drop to the ground.

[Climb01742]

i'm trying to measure the BB height on my MX Leader. measuring BB drop at home for most of us would be quite hard, yes? i don't have one in front of me, but i believe on serotta's build spread sheet there is a BB height measurement. but being a moron, i could be wrong. :) i'm just trying to figure out why my MXL handles the way it.

[DfCas]

if you have the same tires front and rear.With the bike vertical,measure from the ground up to the center of the front and rear axles several times and get that height.Then measure from the ground up to the center of the BB axle.The difference is the drop.I'd like to know what it is on an MXL so let us know if you measure it.

Thanks,Dan C.

[Peter]

Wrap some thread or string around one quick release lever. Run the string to the other wheel and repeat

Now measure from the taut string to the center of your crankarm bolt. This is your BB drop.

I just tried it with some thread and my Trek. 3 minutes, 7 cm. Easy.

BB "Height" doesn't specify wheel diameter or tire height, so it is a less reliable measurement.

[Sandy]

If a question is asked in order to understand and learn and asked with sincerity, then it is never a moron's question. If someone thinks it is, perhaps he or she is the moron.

Moron Mirman

[Climb01742]

hey, the home version of measure that frame is fun! to the best of my abilities, the BB drop on my MXL is....7 cm. does that sound reasonable?

ps: thanks for everyone's help. ;)

[Smiley]

whats the trail , you can do that measurement too . Don't stop now pal .

[e-RICHIE]

don't stop now.
let us know if the bicycle disappears beneath you.
i can recommend a kryptonite lock.
e-RICHIE

:banana: :banana: :banana:
:banana: :banana: :banana:
:banana: :banana: :banana:

arranger of disorder

[Michael Katz]

How do you measure trail? Would like to compare between my road bikes.

[Climb01742]

i'm curious too--how do you measure trail? :confused:

[keno]

do we have a "hell hath no fury..." problem here?

keno

[flydhest]

Ken,

That is one of the most misquoted sayings. Originally, the phrase came from the era when shoes were far less common, and wooden shoes, or sabots, were the norm. Because cobblers (shoe makers) were mostly men, they built the shoes on a last for men's feet. As a result, they were too wide in some areas for womens feet. A woman in the northeastern French town of Arras had very bad feet and the mis-sized shoes exacerbated the condition so badly that she went to the cobbler's, broke his equipment, and set the place on fire. As she was leaving, set against a backdrop of the conflagration that was the shoe maker's shop, the terrified cobbler (I think his name was Willy) exclaimed she must be possessed by a demon from hell. His wife looking on, while rubbing her own aching feet said, no, no, "Hell hath no fury like a woman's corns."

[Kane]

Definition 1:
Use a metal straight edge and center the edge on the axis of the headtube. Mark the point at which it touches the ground. Mark another point where a vertical line from the center of a the front axel hits the ground. The distance between these two points (conventionally measured in centimeters) is the trail.
The bike should be propped up on a wall as closed to vertical as possible.

Definition 2:
Trail is the horizontal distance between a woman's legs at the most proximal point along the medial aspect of the adductors. This term is also know as the 'window'. It is not present in all women due to the fact that their thighs touch and there is no 'trail' or 'window'. Usually, this is measured with the woman in a vertical position, when she is wearing tight pants. Bike shorts are appropriate.

[keno]

I had a similar experience with a pair of bike shoes. In my situation, however, as my former LBS was burning to the ground, the owner was heard to mutter "Sidi, Sidi man."

keno

[Needs Help]

No need to measure anything—unless you want to compare what it should be to what it actually is. With a few numbers from your spec sheet and some trig, you can calculate the trail. All you need to know is the radius of your wheel(you might want to measure that), the fork rake, and the head tube angle.

Here is a beginning diagram with some initial labels:

http://www.pbase.com/image/31912975.jpg


Examining only the angles, you can determine some more information:

http://www.pbase.com/image/31912977.jpg


Putting all the information in one diagram:

http://www.pbase.com/image/31916748.jpg

With the above relationships, you can calculate the trail knowing that:

a)The cosine of an angle is the adjacent side over the hypotenuse.
b)The sine of an angle is the side opposite the angle over the hypotenuse.
c)The tangent of an angle is the side opposite the angle over the side adjacent to the angle.

To start with,

(1) wheel radius = a + b

In the upper triangle, if you designate H as the head tube angle, then

cos H = rake / a
==> a = rake / cos H

In the lower triangle,

tan H = b / trail
==> b = trail * tan H

Plugging the values for a and b into equation (1) gives

wheel radius = rake / cos H + (trail * tan H)
==> trail = wheel radius - (rake/cos H)
--------------------------
tan H
Or, you can do some simplifying. Previously, it was stated

(2) tan H = b / trail

but we also know that

sin H = b / hypotenuse
==> b = sin H * hypotenuse

and

cos H = trail / hypotenuse
==> trail = cos H * hypotenuse

Plugging the values for b and trail into equation (2) gives

tan H = sin H / cos H

Using that result for tan H in the equation we found for trail:

trail = (wheel radius - rake/cos H) / tan H

gives

trail = [(wheel radius - rake/cos H) * cos H] / sin H
==>trail = (wheel radius * cos H) - rake
-----------------------------
sin H

[alembical]

Wow....Needs Help! Are you by chance a math teacher?

Alembical

Isn't the radius a standard number? I know my rake (43), Head tube angle (actually now I cant remeber if it is the 73 or 73.5), but I am at work and can't measure the radius, any help? Also on the standard sizes (58-58) I know my seat tube angle and head tube angle measure 73 and 73.5, but I can't remember which is which, anyone know?

[hooverone]

How do you measure fork rake?

What is the best way to measure seat, and head tube angle?

Consering all of this what is the best way to look at all of these numbers and say this will ride like so or this bike will fit me? For example some ppl are very concened about setback but isnt that just inrealation to seat tube angle ?

For example a bike with 71.5 seat tube anlgle would have more setback than a bike with a 73 degree seat tube angle?




Jim

[keno]

the best way to measure those angles is with a digital level, I believe Stanley is the manufacturer. They cost about $90. As I remember, it was Kevin from NYC who put me on to the device. It also can be used to measure gradient, so if you truly want to know just how steep a particular road is you can do it with ease.

keno

[Needs Help]

How do you measure fork rake?

You could tape a piece of string or dental floss in the middle of your head tube somewhere near the top of the head tube, and then tape the other end on your rim so that the string is parallel to your head tube. Then measure the perpendicular distance from the center of your hub to the string.

What is the best way to measure seat, and head tube angle?

With a digital level.

Consering all of this what is the best way to look at all of these numbers and say this will ride like so or this bike will fit me?

To put it simply--you can't.

[Needs Help]

It also can be used to measure gradient, so if you truly want to know just how steep a particular road is you can do it with ease.

Just so you know, the angle of the road is not the same as the grade(gradient?). The grade will be nearly twice the angle. More precisely,

angle = arctan(grade)

The "arctan" is tan-1 on scientific calculators and it's the inverse of the tangent. For details on how to determine the grade or the angle, see this thread:

http://forums.thepaceline.net/showthread.php?t=3561&highlight=grade

[hooverone]

This answers some of the questions about bike measurement.


http://www.parktool.com/repair_help/roadposition.shtml

I wonder where one can find that angle finder they are using.

JIm

[Needs Help]

I am at work and can't measure the radius, any help?

The rim size is not the whole story. It also depends on the tires you have on the rim since they add to the radius that is relevant for determining trail. And, since tire sizes don't easily equate to a certain additional radius, you may have to measure. According to Leonard Zinn:

"The wheel radius for a 700C wheel with a 23mm clincher on it is about 33.4cm."

On the same page I got that quote, he also detailed his theory on crank length which I thought was interesting:

Inside Triathalon
http://www.insidetri.com/train/bike/articles/1730.0.html
This report filed September 9, 2003
When 180 just isn't long enough
Dear Lennard;
I am about 6 foot 4, 220 pounds. For the last 20 years I have been riding a custom frame built by Ron Stout in Salt Lake City. I hope to replace this Columbus SP frame in the next year or so but for now I hope to start with a new crank. I currently have an old 180mm Campy Super Record crank (42X53) with a (12X23) in back. I am now fifty so I don't climb like in my racing days of 20 years ago when I was 40 pounds lighter.

My frame is 64cm. I would like to replace the crank to get a 39 chainring up front, I just can't climb on the 42 like I did 20 years ago. While I don't have a problem with 180mm cranks on the flats I was wondering about your thoughts regarding longer crank arms for climbing.

I still spend a lot of time climbing the canyons outside of Salt Lake and Park City. Even when I raced I was not one to spin at 185 or 190 rpm. Today when I climb I often stand on a 42 X 14 to climb 7 percent grades. Others think I am crazy, and say that I won't have any knees left soon but, it works just fine for me and I have been doing it for 25 years with no problems of any kind.

Since I am going to replace the crank I was wondering what your thoughts were for a rider of my size. I will say that I still love to go fast but, with the new (11X23) clusters I should still be able to get moving just as well as on my (12X23) gears even with longer slower spinning cranks. I still ride 50mph going down after all the climbing. With a 53-12 I start to spin out at 35mph on 180mm cranks.
--Larry

Dear Larry,
Well, I would certainly go up in crank length, if I were you. With your riding style in particular as well as with your height, it would be a decided advantage when climbing.

Three of us who are all the same height (6 foot 5 with 38-inch inseam) just did a crank length test on the steepest mile section of Flagstaff mountain here in Boulder. The cranks were 180mm, 186mm, 200mm and 202mm, and we monitored average and peak speed, power, torque and heart rate with a Power Tap. All of us were faster with each increase in crank length, with the exception of the 200mm being faster than the 202mm (and hence faster than all of the other cranks), but it was not a standard crank. It was a Rotor cam-type crank that always advances the top crank so that it is 15 degrees past the top when the other crank is at bottom dead center. All of the bikes were set for identical riding positions relative to the bottom bracket center. All four bikes were set at the same weight with the same tires and wheels, but the Rotor bike was set up to be one pound heavier than the others, since a Rotor crank is a pound heavier than a standard crank. I will not be publishing the test in VeloNews because, even though the results were interesting, the differences were smaller than the margin of error of the measuring equipment. It would require a lot more testing to flesh this out, and I don't know if I have it in me. It was a lot of work to set this one up as it is.

If you figure 21 percent of your inseam for crank length, that is a good rule of thumb, I think. That is generally what I use with my tall frame customers (I go a bit shorter on MTB cranks, usually), and they almost universally rave about the cranks.

Of course, you need to take into consideration your bottom bracket height, if you are going to install it on that bike, even temporarily. I have used as long as 202s without problems on a bike with a 27.2cm BB height, which might compare to that of the Stout. You need to be careful pedaling around corners during the learning curve. In general, though, I like to increase the bottom bracket height in lockstep with the increase in crank length.
--Lennard

[keno]

I can't tell you where the angle finder on the Park Tool site is from, but the digital level I have is made by Macklanburg-Duncan and is called SmartTool Builder's Angle Finder. It can read degrees (seat tube, head tube), slope=grade=gradient (hill steepness, which, as needshelp points out, is approximately 2 x degrees), and pitch (roof measurements and irrelevant to bikers, other than BMX, I guess). Amazon sells them.

keno

[flydhest]

keno,

I don't know if you understood NeedsHelp's math, but to let you know, the arctan is NOT the change in Noah's skin tone from the sun.

I don't know if you know much about maths.

[keno]

wouldn't that be the arcangel pitch? cf http://www.arcangel.org/

keno

[Needs Help]

It can read degrees (seat tube, head tube), slope=grade=gradient (hill steepness, which, as needshelp points out, is approximately 2 x degrees), and pitch (roof measurements and irrelevant to bikers, other than BMX, I guess).

Ahhh...I see. That's a smart angle finder.