[iksmudge]

There is much debate on the merits of higher bb's for bmx.
Clearly where your feet and hands are are critical in deciding bike control.
So if you had larger bars (placing hands higher) would this not off set the effects of a higher bb or is there some other physics at force - leverage on the rear axle for example?
Just curious.
Ian

 

[TortugaTonta]

You are on the right track. How high the bb is off the ground isn't as important as where it is in relationship to the axles. That is why taking a frame with 30mm of drop and going from 26" wheels to 24" wheels doesn't feel much different in terms of manuals and cornering, it still has 30mm of drop. Think of it like taking a boat and loading all the weight below the waterline, it will be stable. If you load all the weight above the waterline it will want to capsize. Or you could think of it like a hammock tied really tight between two trees or really loose.

The other force at work is the differential between actual and effective chainstay length. The more drop you have the longer the actual cs length is compared to the virtual length. So if you take a plumb line through the rear axle and a plumb line through the bb you may have 15" but if you measure the actual length between the 2 points it may be 15.5".

As you start to lift the front wheel and the chainstay becomes parallel to the ground the center of gravity is pulled foreward and tries to keep the front wheel on the ground, until you move past parallel and get the bb above the axle centerline, then the center of gravity starts to move back towards the rear axle.

On a 20 where the bb is allready above the axle centerline and cs above parallel the center of gravity is allready moving back as soon as the front wheel leaves the ground.

This is most noticeable if you ride a 29" xc bike where they usually have about 60mm of drop, very difficult to manual but that is also why they tend to climb well, the front wheel stays planted on steep climbs.

 

[iksmudge]

Many thanks.
I have been lucky enough to get a sunday model c frame and forks.
So I was wondering what bars to go with - I'm thinking with the raised bb that I'll need the same ones I run on a 20" i.e. 8"+, otherwise I'll feel hunched up and way over the bike?

 

[FR4life.]

Many thanks.
I have been lucky enough to get a sunday model c frame and forks.
So I was wondering what bars to go with - I'm thinking with the raised bb that I'll need the same ones I run on a 20" i.e. 8"+, otherwise I'll feel hunched up and way over the bike?

8"+ bars are probably a good idea. I first rode my liquid with the 6" bars and when I later went on to ride 8" bars the bike became much more responsive in all aspects. 8.25"+ however is quite possibly overkill for a 24.

 

[cmc]

Many thanks.
I have been lucky enough to get a sunday model c frame and forks.
So I was wondering what bars to go with - I'm thinking with the raised bb that I'll need the same ones I run on a 20" i.e. 8"+, otherwise I'll feel hunched up and way over the bike?

i think Sunday got the message and has been putting 8"s on the complete Model C's : http://bikereviews.com/bmx-bikes/sunday/2010-sunday/sunday-model-c-bmx-bike/

2010 Sunday Model C BMX Bike Specifications
Sunday Model C 2010 BMX Bike Frameset
Frame: Full 4130 chromoly, 21.25″ TT
Fork: Full chromoly, 1pc CNC steerer tube

Sunday Model C 2010 BMX Bike Wheels
Rims: Alex 24″ DM-24, double wall, pinned, 36h
Front Hub: Sealed bearing, 3/8″, 36h
Rear Hub: Odyssey V3 Cassette, 14mm, 36h, 10t
Spokes: Steel (F/238mm and R/235mm)
Front Tire: Odyssey Aitken, 2.25″
Rear Tire: Odyssey Path, 2.10″

Sunday Model C 2010 BMX Bike Drivetrain
Sprocket: Sunday 25T 6061 CNC, adaptor included
Crankset: Chromoly 3pc tubular, 175mm
Bottom Bracket: Sealed mid w/19mm spindle
Pedals: Odyssey Twisted PC, 9/16″
Chain: KMC Z510H 1/2″ x 1/8″

Sunday Model C 2010 BMX Bike Components
Handlebar: Full chromoly 24umph style bar (28″ wide, 8″ rise, 12 deg. backsweep, 1 deg. upsweep)
Stem: Sunday Front Load
Headset: Integrated 1-1/8″
Spacer: 5mm (1pc)
Brake Lever: Odyssey Monolever, small
Brake: Odyssey Evo II
Brake Cable: Odyssey Linear Slic Kable
Grips: Odyssey Team Grips w/plug
Seat: Odyssey Aitken, pivotal
Seat Post: Pivotal
Seat Clamp: Sunday

Sunday Model C 2010 BMX Bike Details
Color(s): Flat Black, Vanilla
Retail Price (MSRP): n/a
Type: 24 Inch Dirt/Street/Park BMX