Mountain Bike Reviews Forum banner
1 - 20 of 20 Posts

·
Out spokin'
In cog? Neato!
Joined
·
12,556 Posts
(The title of this thread is supposed to be "Let's kill the notion that bikes with high BBs have high centers of gravity." My edit didn't seem to fix it on the main page.)

Largely, the discussions over the years on this board regarding long cranks motivate me to address this subject specifically.

First a little background for those unfamiliar with my own quirks and the quirky bikes I ride. I'm 6'3" with a 36" inseam. In 1992 I purchased my first custom mountain bike designed around long cranks -- 202mm cranks to be exact. Currently I have four bikes with much-longer than standard cranks: two with 202mm cranks and two with 195mm cranks. Most of my other sleds employ 180mm cranks, which feel quite short to me.

When a frame is designed to employ longer-than-standard cranks, more than just bottom bracket height needs to be addressed. (Disclaimer: the aspects that follow are generalities. I'm not a framebuilder.) Since "knee over pedal spindle" (KOPS) still needs to be considered, the seat angle should steepen. This farther-forward seat position in turn produces the need to lengthen the top tube, which increases the front-center measurement (distance between BB and front axle). Ideally, chainstay length will diminish as much as possible in order to keep the rear tire's traction point as tucked up under the rider's butt as possible and shorten wheelbase. Because of the long front-center, it's common to steepen the head angle slightly in order to keep the bike's handling as nimble as possible.

Now on to BB height per se...

First of all, saddle height is no further from the ground on a bike with long cranks than it is on a bike with short cranks. This is because a rider will set his seat height the same distance from the pedal regardless of the crank's rotational center. In fact, if a rider is installing long cranks on a bike that previously had shorter cranks, (s)he'll actually need to lower his/her saddle in order to achieve proper leg extension when the crank is in the 6 o'clock position.

So if we're talking about riding while seated, no center of gravity penalty is incurred via the use of long cranks.

Okay, so what about riding long cranks while standing? The common train of thought goes something like this: The rider's center of gravity will be higher because (s)he generally descends with cranks horizontal and the BB is higher on a frame designed for long cranks.

But just where is the center of gravity? Is it at the BB? Or is it higher, closer to the rider's center of mass?

Personally, on a bike with a high BB, I just bend my legs more whenever I'm descending out of the saddle. And we all bend our legs while descending out of the saddle, right? What's another 5mm? Or even another inch?

BB height and it's affect on handling may ultimately be an argument that boils down to personal preference. But in my nearly 14 years of experience of riding off-road bikes with various BB heights, I've found no direct correlation between BB height and handling. I've had low BB bikes that didn't handle as well as high BB bikes and high BB bikes that didn't handle as well as low BB bikes.

Other aspects of frame design have a far greater effect on overall handling than center of gravity, and as I pointed out above, I question whether BB height even has a noticeable effect on center of gravity. This contention, if correct, undermines the whole high-BB/bad handling argument.

My point isn't to say one's better than another -- low or high. I just think the notion that a bike with a high BB won't handle well because "the center of gravity is too high" is a common misconception. I'd hate to see anyone lose out on the potential benefits of long cranks simply because they feared they'd wind up with a bike that handled poorly due to a marginally higher BB.

Personally, I don't believe it's so.

Discuss.

--Sparty

P.S. Happy Thanksgiving, ya'll. :)
 

·
NedwannaB
Joined
·
12,771 Posts
I think you've just about covered it!

Sparticus said:
My point isn't to say one's better than another -- low or high. I just think the notion that a bike with a high BB won't handle well because "the center of gravity is too high" is a common misconception. I'd hate to see anyone lose out on the potential benefits of long cranks simply because they feared they'd wind up with a bike that handled poorly due to a marginally higher BB.

Personally, I don't believe it's so.

Discuss.

--Sparty

P.S. Happy Thanksgiving, ya'll. :)
NObody will dare challenge your theory, O' Crankmastah! :rolleyes:

But then/than (shiiiiggeeee??) again, @ 5'-6" using 175's, what do I know? :D

Happy Turkey Day to you too Sparty, enjoy.
 

·
Premium Member
Joined
·
48,238 Posts
JMac47 said:
...But then/than (shiiiiggeeee??) again, @ 5'-6" using 175's, what do I know? :D ...
If you still do not know the difference between "then" and "than", then you know nothing at all.
 

·
Premium Member
Joined
·
48,238 Posts
I agree with yee, Sparty. I also find whacking my pedals on the ground/rocks to be a huge detriment to the handling.
 

·
NedwannaB
Joined
·
12,771 Posts
Hhmmmh

shiggy said:
If you still do not know the difference between "then" and "than", then you know nothing at all.
Just a subtle jab to your earlier post, geessch, what's with the ridecule. Hope you got out dicipling in the dirt, I did. That I do know!
 

·
Premium Member
Joined
·
48,238 Posts
JMac47 said:
Just a subtle jab to your earlier post, geessch, what's with the ridecule. Hope you got out dicipling in the dirt, I did. That I do know!
Just a jab back about an older post that was not on this board and has been played out.
 

·
Registered
Joined
·
266 Posts
i would ( horror ) tend to agree with grant on this. there is no question to me that BB height affects a bike's "feel" - generally to the negative as you go higher. as to the physics i do not know, but to me, once you get adequate ground clearance for the cranks you like and the terrain you ride in you want the BB as low as you can get. to the original poster, if it doesn't matter like you say why not put it up at 15 inches or so and get all the clearance you could ever want ? the answer is the bike would ride like crap. i would offer that, for you, the negative of the higher BB is offset by your preference for longer cranks - but even you will remain with the lowest that works, given your preferences.
 

·
Registered
Joined
·
153 Posts
That makes some wheels turn

shiggy said:
Here is an interesting piece (older) about BB height on the Rivendell site:
http://www.rivendellbicycles.com/html/rr_stilstep.html

The saddle height and weight idea makes perfect sense. I've been riding Cannondale Beast of the East frames since 92 with 13" BB and in size an 18 only measures as a 16 because of the BB. The weight of the seat is higher. Aside from the feeble revelation, there is a difference on tight technical trails, flickable, sharper cornering but still stable at speed. Probably not the hot ticket out west but I'm spoiled by it. Chainstay length can make huge difference too
 

·
Out spokin'
In cog? Neato!
Joined
·
12,556 Posts
Discussion Starter · #11 ·
mon t said:
...i would offer that, for you, the negative of the higher BB is offset by your preference for longer cranks - but even you will remain with the lowest that works, given your preferences.
You'd be right. I'm not advocating anybody use a two foot high BB when 13" or 14" will do (although I do happen to own a bike with a two foot high BB. :))

--Sparty
(Original poster)
 

·
Premium Member
Joined
·
48,238 Posts
Sparticus said:
You'd be right. I'm not advocating anybody use a two foot high BB when 13" or 14" will do (although I do happen to own a bike with a two foot high BB. :))

--Sparty
(Original poster)
Said bike's stability is affected more by the short wheelbase to BB height ratio than the high BB itself.
 

·
Out spokin'
In cog? Neato!
Joined
·
12,556 Posts
Discussion Starter · #13 ·
shiggy said:
Said bike's stability is affected more by the short wheelbase to BB height ratio than the high BB itself.
I hope nobody construes this to mean the two-foot BB bike is unstable. In fact it is surprisingly stable. However it is by no means flickable.

Perhaps what I've said immediately above ties into what Grant was saying in the article shiggy cited: "What I think: A high bottom bracket increases a bike's stability to the point where it becomes harder to control. A bike with a low bottom bracket feels squirrelly ... if you're used to the stiffness or stability that comes from a high bottom bracket, but is a cinch to get used to..."

--Sparty
 

·
Premium Member
Joined
·
48,238 Posts
Sparticus said:
I hope nobody construes this to mean the two-foot BB bike is unstable. In fact it is surprisingly stable. However it is by no means flickable.

Perhaps what I've said immediately above ties into what Grant was saying in the article shiggy cited: "What I think: A high bottom bracket increases a bike's stability to the point where it becomes harder to control. A bike with a low bottom bracket feels squirrelly ... if you're used to the stiffness or stability that comes from a high bottom bracket, but is a cinch to get used to..."

--Sparty
Your "SkyBike" is amazingly stable...until you lift the front wheel. Then your weight is well behind the rear axle. Actually my preferred dismount method on the bike.
 

·
Recovering couch patato
Joined
·
14,017 Posts
Sparty,
I've been lusting for truly long cranks since I ride bikes, but never got myself to pull the trigger on a set. I own 2 sets of 185's though, and hope to soon build a bit with one of those again for the first in years.
How do you rate the difference long cranks make on you hop-ability? As when your behind leaves the set, cranks are level, your centre of gravity IS higher. To me it seems great to be able to use some knee angle to take up hits on DH's, and to brace for hops. When out of the seat, I always felt that I with my 39"+ inseam got kicked in th butt by my seat more than people with shorter legs and thus relatively longer cranks.

Anyone ever manage to obtain these cranks? http://www.kocmo.de/kurbeln.php4

Happy trails,

J
 

·
Look at the time!
Joined
·
1,139 Posts
I can definitely feel the difference on road bikes. My workbike (messenger) has a frame that is designed for a fixed drivetrain, so the BB is about 20mm higher than on a regular roadbike (~285 against ~265), and that difference was just about the first thing i noticed when i tested it.
Really love the trackbike-like pedal-to-ground clearance though, so getting used to it didn't take long.


On a mountainbike, where i'm out of the saddle on descents, i only notice BB height when trying to put a foot down. I don't think i would even notice a 20mm change.
 

·
hands up who wants to die
Joined
·
1,405 Posts
Thankfully I don't have major bike-fit issues. I am 6' with a 33.25 inseam and have no problem with 175mm cranks.

I can very definitely feel the difference in cornering ability between high and low BBs; I used to own a tall bike and got rid of it. Sure weight, head angle and wheelbase all contribute; but just because there are contributing factors doesn't make the BB height irrelevant.

I really enjoy descending technical trails at speed, and spec my bikes accordingly. I also lower the seatpost to descend when the trail gets steep and tech.

-rob in NY
 

·
Registered
Joined
·
218 Posts
Sparticus said:
You'd be right. I'm not advocating anybody use a two foot high BB when 13" or 14" will do (although I do happen to own a bike with a two foot high BB. :))

--Sparty
(Original poster)
I am interested to see this two foot high BB, got any pics?
 

·
NedwannaB
Joined
·
12,771 Posts
Now you've got me thinkin'...

...back to original topic:

JMac47 said:
But @ 5'-6" using 175's, what do I know? :D
Hey Sparty,

What are your thoughts on me running this length crank arms @ 2:1 gearing, a 29.5 inseam (granted that's more of a standover thang), and that I run 170's on the road bike. After reading alot of posts, compared to taller riders using the 175's, seems maybe I should be running abit shorter on the SS?? It's a 16" frame, not exactly sure of the bb hieght, I guess it would vary given the different diam/circ. of tire in use at the time of course. Do you vary your arm length based on different gear combos? Or just for the different bikes?

Appreciate your input, if any. Thanks in advance.

JMac
 
1 - 20 of 20 Posts
Top