Measuring BB drop

[Greg Achtem]

Where does one measure to when checking BB drop? Centre of the BB shell or bottom?

I did a quick search on the rules of thumb for 650B conversions but I can't find the rule of thumb for bottom bracket height/drop. Does anybody know off hand? Or can you direct me to the link? Or both?

TIA

Greg

[Alistair Spence]

Greg,

you can use the spreadsheet that (I think) Anthony at Longleaf set up
a while back, to get your tire OD,
https://docs.google.com/spreadsheet/ccc?key=0AhJ3Eg2EndMUcFpsLUVBTm5XWWk3aGlEVXZfS2pvTHc#gid=0

Then just measure the height to your BB spindle center from the floor.
Subtract BB spindle center height from the radius of whatever tire
you're using, that'll give you the drop.

FWIW, I seem to relate better to BB height.My first 650b conversion
experiment had a BB height of 255mm and was a tad on the low side. I
had to be careful with pedal strike, even at moderate angles of lean
(wide MKS touring pedals though). I got used to to it of course and it
was perfectly rideable once I got attuned to it.

On the next one I bumped up the BB height to 265mm and found that to
be fairly optimal for my tastes.

Hope that helps.

Alistair Spence,
Seattle, WA.

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[Ken Freeman]

There's really no need for a spreadsheet, which may contain errors or become incomplete if it's not maintained.  With the bike upright on a flat surface, measure from the center of the front hub to the ground, and the center of the BB spindle to the ground, both measurements in millimeters.  Then subtract.  The difference is the BB drop.

--
Ken Freeman
Ann Arbor, MI USA

[Alistair Spence]

Yep,

Ken's right. I only mentioned the spreadsheet from a point of view of
getting and idea of BB height if you don't have the actual tires in
hand. Greg mentioned 650b conversion in his initial post, which got me
thinking that way.

As for the accuracy of the spreadsheet, I can only comment on the
tires that are on it that I've measured. The CdlV, Paris Moto and
Hetre numbers look right to me.

Alistair.

[Ken Freeman]

Apologies, Alistair, I didn't mean inaccuracy due to careless measuring.  I meant inaccuracy due to changes in tire pressure or batch to batch variations, or perhaps even mold wear.  And of course, tire makers might shift the design from time to time for marketing reasons, or discontinue whole products.

I can't say if any of the numbers are actually correct - I'm just considering going 650 now for the first time.  

But I think we answered teh OP, that BB height measurement should be done from the center of the spindle.  

If one really wants to be accurate, either measure both left and right and average the numbers, or take care that the bike is as balanced as possible (leaning as lightly as it can without falling over) and don't let it shift while taking measurements.  If one cares about a few millimeters - I'm not sure it would be necessary for BB height.

[Alistair Spence]

No apologies necessary at all. The inconsistencies in the parameters
you listed are real for sure and I agree with your last point. For
what we're talking about measuring here, and for it's intended
purpose, the required accuracy for the BB height measurement is pretty
slack.
I'm thinking plus or minus 2mm on the tire radius is probably good
enough to give a fair idea of what could be expected as far as pedal
clearance and general handling characteristics.

Alistair.

[Greg Achtem]

Thanks guys. I'll measure correctly (more or less) when I get home this afternoon.

Anybody remember the rule of thumb for BB height on conversions?

[Ken Freeman]

For trail calcs and such I either measure the axle height and multiply by two, measure across the tire with a tape from rim seam to valve hole, or use the tire diameter table from Tony Oliver's Touring Bikes.  I run the tape through the spokes to get as close as possible to a straight line.  When I want good trail accuracy, I add a 15% sag corrector formula to the spreadsheet.  But here I'm only sure of the rake if I measure the fork off the bike.

On Monday, October 24, 2011, Jan Heine <hei...@earthlink.net> wrote:
> I find it difficult to measure the radius of a tire...
>
> An easier way of measuring BB drop is to wrap a string (best is sewing thread) around the front and rear quick release.
>
> Then you can measure the BB drop directly at the crank bolt. Don't forget to deduct 1/2 of the diameter of the quick releases.
>
> You should be able to get within 1 mm with this technique, and all you need is the thread and a ruler.
>
> Jan Heine
> Editor
> Bicycle Quarterly
> 2116 Western Ave.
> Seattle WA 98121
> http://www.bikequarterly.com
>
> Follow our blog at http://janheine.wordpress.com/

[Joseph Hogg]

Hi Jan,

What if you skip the thread and measure the distance from the center of the axle to the ground and from the center of the bottom bracket to the ground, then subtract?Well, OK, you could measure from the centers of the front and rear axle, average them. Differences in tire inflation should cancel in the subtraction since they influence the height of the bottom bracket in the more-or-less same degree. Lots of nitpicky fun here.

Best,

Joe

On Mon, Oct 24, 2011 at 7:29 AM, Jan Heine <hei...@earthlink.net> wrote:

I find it difficult to measure the radius of a tire...

An easier way of measuring BB drop is to wrap a string (best is sewing thread) around the front and rear quick release.

Then you can measure the BB drop directly at the crank bolt. Don't forget to deduct 1/2 of the diameter of the quick releases.

You should be able to get within 1 mm with this technique, and all you need is the thread and a ruler.

Jan Heine
Editor
Bicycle Quarterly
2116 Western Ave.
Seattle WA 98121
http://www.bikequarterly.com

Follow our blog at http://janheine.wordpress.com/

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[Joseph Hogg]

Hi Jan,

I like the idea of suspending the bike and letting gravity remove the inclined plane distortion. Let's see, a 24 inch level would allow you to fix a level floor for a short distance and a plumb line would solve the question of whether the bike was perfectly vertical. On the other hand, leaning the bike against a wall and measuring axle and bottom bracket distance to the ground in the same plane using a straight edge might work.

Back to the sewing thread, measuring the radius of the axle need not be direct. Take the circumference, using the thread, and divide its length by 2 x pi.

I agree that a +/- 5 mm error makes the measurement worthless.

Cheers,

Joe

On Mon, Oct 24, 2011 at 9:07 AM, Jan Heine <hei...@earthlink.net> wrote:

At 8:12 AM -0700 10/24/11, Joseph Hogg wrote:

Hi Jan,

What if you skip the thread and measure the distance from the center of the axle to the ground and from the center of the bottom bracket to the ground, then subtract?Well, OK, you could measure from the centers of the front and rear axle, average them. Differences in tire inflation should cancel in the subtraction since they influence the height of the bottom bracket in the more-or-less same degree. Lots of nitpicky fun here.

You idea would work if you could measure the distance from the axle to the ground...

The problem is that if the bike is inclined, then you don't get a good measurement from either the BB or the axle end to the ground, because they aren't in the centerline of the bike. And of course, if you lean the bike against a wall, it has to be inclined, otherwise it falls over! (The same reason applies to measuring the diameter of a wheel, unless you have giant calipers.)

BB drop is meaningless if you have +/- 5 mm errors in your measurements...

If you suspended the bike from the ceiling somehow, you could get good measurements, assuming that the drivetrain on one side wouldn't make the bike lopsided, and that your floor is perfectly level.

Similar problems occur when measuring front-center and wheelbase: The fork must be aligned perfectly, otherwise, you get different measurements on the right and the left side! (In fact, at Bicycle Quarterly, we measure both sides, and if they agree, we know we got it correct.)

Jan Heine
Editor
Bicycle Quarterly

[Alex Wetmore]

It is overkill for the average home mechanic, but I built this fixture for making these measurements:

http://alexandchristine.smugmug.com/Bicycles/Framebuilding/Ivy-T/14349199_B9NN6b#1173440703_e5cU2

 

It works when the bike is fully assembled, but the wheels are removed too:

http://alexandchristine.smugmug.com/Bicycles/Framebuilding/MeasuringJig/12052225_mPM7m2#855568969_HXfkx

 

It can also act as a rack building fixture (it holds the deck of the rack level while the rack is brazed to the stays):

http://alexandchristine.smugmug.com/Bicycles/Racks/Michaels-Demountable-Rack/11515022_9Bhp28#810444156_gehG8

 

alex

 

---

From: 65...@googlegroups.com [65...@googlegroups.com] on behalf of Joseph Hogg [josep...@gmail.com]
Sent: Monday, October 24, 2011 9:42 AM
To: Jan Heine
Cc: Ken Freeman; Alistair Spence; Greg Achtem; 650b
Subject: Re: [650B] Measuring BB drop

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[Jim G]

I can report that converting from 700x28 Paselas to 650B Col de la Vie
wheels/tires results in a BB drop of about 1cm.

http://www.flickr.com/photos/jimg/5587780623/in/set-72157618420547360

-Jim G

[Joseph Hogg]

Alex,

Very nice. Overkill, but versatile.

Joe

[Ken Freeman]

That's what I suggested, sans the front-back averaging.  If you can measure elevation to teh BB axis, you can measure elevation to the hub axis.

[Ken Freeman]

that's not drop, that's a reduction in BB height.  BB drop is the difference in height between the wheel center and the BB center.  Changing a wheel or tire cannot change drop, but it will change BB height.

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[jim g]

What Ken said. I meant "results in a drop in BB height of around
1cm". Coffee hasn't kicked in yet.

-Jim G

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[Ken Freeman]

I just resolve all these alignment and balancing issues by leaning the bike against a wall with a chock (any small piece of metal or wood, such as a spare teaspoon or a pad of yellow sticky notes) at the front wheel.  I move the rear wheel in (towards the wall) until it is just about to topple, then move the rear wheel out about two millimeters.  Then I remove the wheel chock and align the front wheel, replacing the wheel chock.  Then go measure.  It really isn't that hard to wield a measuring tape without error and without knocking the bike over.  If you are concerned about the angular differential between left and right BB ends, consider that if the angle is 1 degree, the height difference across a 150 mm crank width calculates to about 2.5 mm.  I also think the bike is stable with less than a degree.  So after you measure the BB axis height at the end of the BB, subtract 1.25 mm.  That will reduce the error to probably less than 0.25 mm, and requires no special fixturing other than making sure the bike does not fall over or roll out of position (floors are rarely flat or level).

On Mon, Oct 24, 2011 at 12:07 PM, Jan Heine <hei...@earthlink.net> wrote:

At 8:12 AM -0700 10/24/11, Joseph Hogg wrote:

Hi Jan,

What if you skip the thread and measure the distance from the center of the axle to the ground and from the center of the bottom bracket to the ground, then subtract?Well, OK, you could measure from the centers of the front and rear axle, average them. Differences in tire inflation should cancel in the subtraction since they influence the height of the bottom bracket in the more-or-less same degree. Lots of nitpicky fun here.

You idea would work if you could measure the distance from the axle to the ground...

The problem is that if the bike is inclined, then you don't get a good measurement from either the BB or the axle end to the ground, because they aren't in the centerline of the bike. And of course, if you lean the bike against a wall, it has to be inclined, otherwise it falls over! (The same reason applies to measuring the diameter of a wheel, unless you have giant calipers.)

BB drop is meaningless if you have +/- 5 mm errors in your measurements...

If you suspended the bike from the ceiling somehow, you could get good measurements, assuming that the drivetrain on one side wouldn't make the bike lopsided, and that your floor is perfectly level.

Similar problems occur when measuring front-center and wheelbase: The fork must be aligned perfectly, otherwise, you get different measurements on the right and the left side! (In fact, at Bicycle Quarterly, we measure both sides, and if they agree, we know we got it correct.)

Jan Heine
Editor
Bicycle Quarterly

http://www.bikequarterly.com

Follow our blog at http://janheine.wordpress.com/

--

[Ken Freeman]

Stop by, Jim, I got coffee ready.

[Greg Achtem]

Okay, I got out my trusty wooden "yard" stick, some string, some Hetre shod wheels, a notebook and a pencil.

Using my calibrated eye I got about 60 mm for BB drop and a height of 24 cm (9.5 in) to the bottom of the shell - 26.5 cm (10 3/8 in) to the centre with Hetres. These are all eyballed to an empty bb shell.

What do you all think? This seems okay for a conversion to me, but am I way off base?

[Alistair Spence]

In my experience 265mm BB height is a very good number. I design/make
my own bikes to this number, it works well for me.

Alistair.

[Nick Payne]

Well, the BB drop on the 650b Bleriot frame I have is, according to the specs, 67mm ( http://www.cyclofiend.com/rbw/geometry.html), so a drop of 60mm will give you a bit more cornering clearance than that.

Nick

[Greg Achtem]

Thanks all for the tips and info. I'm going to start this project, albeit slowly. It looks like I'll be able to compare rides between this UJB (a 1981 Apollo Gran Tour, Tange Champion #2, 57, 57 c-c) and my Kogswell P/R Mk. II. Will it plane?

Hopefully one day it will look half as good as Somervillian's Shogun conversion.

[Nick Bull]

I took a somewhat different approach to this because I was converting
a 700C bike to 650B. I figure that for most bikes built around 700C,
that the BB height is supposed to be acceptable for standard road
700Cx23 tires (and maybe even 700Cx20).

So I compared the OD of a 700Cx23 tire with the OD of a 650Bx38 and
realized that it would only drop my pedal about 2mm or 3mm closer to
the ground. This was confirmed by ex-post measurements from the pedal
to the floor. So if the BB height is acceptable for the standard road
tire, then it seems hard to believe that dropping by 2mm or 3mm makes
it that much less acceptable. So far I think I may have struck my
pedal once or twice in the 4000 miles I've ridden on the two bikes
that I've converted. That's probably no more frequently than when I
was riding 700x32's. I probably do not corner aggressively, not am I
trying to pedal aggressively through corners.

Nick

On Oct 24, 8:33 pm, Greg Achtem <greg.ach...@gmail.com> wrote:
> Thanks all for the tips and info. I'm going to start this project, albeit
> slowly. It looks like I'll be able to compare rides between this UJB (a 1981
> Apollo Gran Tour, Tange Champion #2, 57, 57 c-c) and my Kogswell P/R Mk. II.
> Will it plane?
>
> Hopefully one day it will look half as good as Somervillian's Shogun

> conversion<http://www.flickr.com/photos/7516215@N03/sets/72157625413101060/>
> .

>
> On Mon, Oct 24, 2011 at 6:20 PM, Nick Payne <nick.pa...@internode.on.net>wrote:
>
>
>
>
>
>
>
> >  Well, the BB drop on the 650b Bleriot frame I have is, according to the

> > specs, 67mm (http://www.cyclofiend.com/rbw/geometry.html), so a drop of