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whats a spec for a "low" BB height (road)
 

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walter
Joined: 11 Jan 2004
Posts: 4391
Location: metro-motown-area

8/17/13 5:53 AM

whats a spec for a "low" BB height (road)

been riding for a long time, since the mid 80s, i'd always read that lower BBs made a bike's handling better, but never really delved into the specifics of my particular ride at any given time.

whilst i was working on a merckx and gazelle in my workshop and figured i'd do an actual comparison...both are mid-90s steel bikes...the merckx BB is a whopping 0.75" lower than the gazelle! merckx BB centerline was ~10" from the ground with aired-up tyres. also checked a steel colnago from about 2000 and carbon colnago from 2005 and both were closer to the gazelle >10.5".

FACK -- that's gotta make a big difference in handling! i'm still building up the merckx and have yet to ride it, but now i know to pay attention!

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ErikS
Joined: 19 May 2005
Posts: 8337
Location: Slowing boiling over in the steamy south, Global Warming is real

8/17/13 9:54 AM

Just don't crit that bugger. That is low.

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Sparky
Joined: 08 Dec 2003
Posts: 19068
Location: PDX

8/17/13 10:13 AM

I love low BB, never strike on 175mm cranks.

75mm of drop is my sweet spot, but 80mm is good too.

I won't ride a road bike with less than 70mm BB drop.

Height being contingent on tire size to me is a MTB CX/measurement. ;)

BTW, along with the lower BB and 'better handling' the frame won't be quite as stiff VS less BB drop.

FWIW, the Merckx EX I had way back when had 80mm drop and also 180 MM cranks. Never had a strike. And I do not always time my pedal pauses nice/nice coming out of turns and have 1/2 expected to now and again.. Keywins probably help...

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dfcas
Joined: 11 Jan 2004
Posts: 2815
Location: hillbilly heaven

8/17/13 10:41 AM

I would call a BB low when the drop is greater than 7 cm. I've had a few high BB bikes and hated them all, so if I had a choice I would like a BB drop from 75mm to 80 mm.

My Lemond Poprad has a BB drop of 74, and I think thats about the lowest cross bike out there.

let it be low.

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Sparky
Joined: 08 Dec 2003
Posts: 19068
Location: PDX

8/17/13 2:59 PM

Run a string from the front to rear axle center with the fork straight and measure from BB center to the string [perpendicular to string horizontal axis]. inches*2.54-CM, * 25.4=mm

I like to see 2.875" or more, BB center to the string.

I have played with shorter crown to axle forks to lower the BB and Steepened HTA with reasonable success FWIW. I will make a SS/Fixie out anything I can. ;) Frame/Parts should roll. ;)

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Nick Payne
Joined: 10 Jan 2004
Posts: 2625
Location: Canberra, Australia

8/17/13 3:58 PM

My Litespeed has 76mm BB drop, and I've never had any problem with pedal strike racing it with 700c wheels (170mm cranks). I also rode it for a couple of years with 650b wheels and 32mm tyres, and taking into account the difference in bead seat diameter and tyre size over 25mm 700c racing tyres, that was the equivalent of having 88mm BB drop on 700c wheels. Having the bottom bracket that low did mean that I couldn't pedal through fast corners without hitting a pedal, but it made it the most amazing descending bike I've ever ridden for going fast down winding mountain passes.

Crank width will also make a difference to how likely you are to strike a pedal. If you have a bike cranked over at 40 degrees in a corner, having a crank that is 10mm wider each side at the pedal spindle attachment has the same effect on the likelihood of striking a pedal as dropping the bottom bracket another 6˝mm with the narrower cranks.

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daddy-o
Joined: 12 Apr 2004
Posts: 3307
Location: Springfield

8/17/13 4:06 PM

I like to strike a pedal every once in a while, it keeps me awake. Kind of like "oh, these are the good old days."

Thanks for the geometry lesson. I've never studied it like that, even though I knew it was there.

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dfcas
Joined: 11 Jan 2004
Posts: 2815
Location: hillbilly heaven

8/17/13 5:27 PM

If you don't strike your pedals, the BB is too high. if you strike pedals, the BB is too low.:)

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walter
Joined: 11 Jan 2004
Posts: 4391
Location: metro-motown-area

8/17/13 9:33 PM

re-measured, got the BB drop spec using thread between axles

80mm on the merckx, 67mm on the gazelle

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Sparky
Joined: 08 Dec 2003
Posts: 19068
Location: PDX

8/17/13 9:50 PM

What is the Gazelle again??

to me 67=crit 80=double century 75=century ;)

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Nick Payne
Joined: 10 Jan 2004
Posts: 2625
Location: Canberra, Australia

8/18/13 12:12 AM

Richard Sachs uses 80mm BB drop as standard on his road frames: http://www.richardsachs.com/site/2003/12/05/an-interview-with-richard-sachs/

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Andy M-S
Joined: 11 Jan 2004
Posts: 3377
Location: Hamden (greater New Haven) CT

8/18/13 4:51 AM

The VO Rando is spec'd at 75mm drop (for my 57 cm frame size--less for smaller sizes) and I ride with 175 cranks w/o any strikes at all. Of course, the frame is designed for 28+ mm tires.

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Brian Nystrom
Joined: 26 Jan 2004
Posts: 5096
Location: Nashua, NH

8/18/13 6:50 AM

I've never paid much attention to BB height/drop...

...on road bikes; I just assumed that the designer knew what he was doing.

My current bikes have drops of 70 (Look 585) and 67 (Cannondale Supersix). The Supersix is the best handling bike I've ever ridden. My old Vortex had a drop of ~74, but it didn't handle as well as the Cannondale.

I'm not convinced that BB height/drop really makes much difference except in cases where extra height is desirable for cornering clearance (crit' bikes) or obstacle clearance ('cross and MTB). There are too many other factors that affect a bike's handling for BB height/drop to make a huge difference on it's own. I think it's a pretty fair assumption that the BB height/drop is selected as part of an overall design philosophy for the frame in question. I strongly suspect that if you look at frames closely, you'll find that you'll see that bikes with similar BB dimensions also share other characteristics, because they're designed with similar uses in mind. For example, crit' bikes have higher BBs, plus typically steeper ST/HT angles, shorter chainstays and less fork rake. These characteristics go together to create a bike that's suited for the intended purpose.

The comment that a lower BB height makes a frame less stiff makes no sense at all with carbon frames or even with Al and Ti frames that have highly shaped tubing. Perhaps there is some degree of validity to it with steel frames where the slightly longer seat and down tubes would add a bit of flex.

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ErikS
Joined: 19 May 2005
Posts: 8337
Location: Slowing boiling over in the steamy south, Global Warming is real

8/18/13 8:42 AM

It affects the CG of the bike and rider. A slightly lower CG, all other things being equal will be more stable.

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Steve B.
Joined: 19 Jan 2004
Posts: 769
Location: Long Island, NY

8/18/13 2:54 PM

"Run a string from the front to rear axle center with the fork straight and measure from BB center to the string [perpendicular to string horizontal axis]. inches*2.54-CM, * 25.4=mm "

Or, measure axle center from ground, measure b-bracket center from ground. Difference is drop.

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Brian Nystrom
Joined: 26 Jan 2004
Posts: 5096
Location: Nashua, NH

8/18/13 8:05 PM

Lowering CG

How much difference do you really think a lower CG is going to make? Considering that my butt is perched 103 cm off the ground, I find it really hard to believe that lowering it a centimeter or two is going to make a significant difference in handling. I don't notice any difference in handling when I wear a hydration pack vs. water bottles and that probably lowers the CG more than a centimeter or two.

Head tube angle, fork rake, chainstay length and frame/fork stiffness are all much more important to handling than BB height. My Supersix is a perfect example of that; the handling is impeccable, despite having a relatively high bottom bracket.

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daddy-o
Joined: 12 Apr 2004
Posts: 3307
Location: Springfield

8/19/13 11:04 AM

I was going to write a similar post yesterday but one accepting the significance is real. --Then the tablet keyboard just got in the way of editing.

I wanted to write about "lowering the BB 1cm compared to lowering your shoulders 3 cm in turns"

What about other CG factors?
--Water bottles (Brian)
--Weighting the outside pedal in turns -- Seriously, it works for me but I can't think of any reason why, it even seems like it would raise my CG because I've slightly lowered the pressure on the saddle, or "raised my pelvis." (My conclusion is somehow it's a sense of control that has nothing to do with weight, like an ice skate.)

I'll give credit to the million miles of posters' experience. You are an analytical bunch, and you make sure nothing is different than the BB height / drop when you compare. If you say a centimeter makes a difference in a top tube, why won't one on a BB? The huge difference between 72' and 73' degrees, the unridablility (to me) of a 71' frame, and the snap of a 74', 2 cm on a wheelbase. And rake and trail, forgetaboutit.

Hey! I hear Ben Serotta has some free time these days. There's probably 50k worth of Serottas between all of us (you.) He owes us!

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daddy-o
Joined: 12 Apr 2004
Posts: 3307
Location: Springfield

8/19/13 11:40 AM

Dave Moulton on the subject

http://davesbikeblog.squarespace.com/blog/2007/2/21/bottom-bracket-height.html

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sandiway
Joined: 15 Dec 2003
Posts: 4902
Location: back in Tucson

8/20/13 2:37 PM

perception

Sitting a bit lower on a bike changes visual perception a bit, especially on descents. I notice immediately if I'm on a bike with a higher setup. I agree with everyone that physics-wise it's a minuscule change.

Sandiway

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Brian Nystrom
Joined: 26 Jan 2004
Posts: 5096
Location: Nashua, NH

8/20/13 8:07 PM

Weighting the outside pedal...

...does NOT lower one's center of gravity at all, since your body is still in the same position. It does lower the center of pressure on the frame, which changes the way it flexes and that can affect handling.

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