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Need help finding correct spoke length

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Hi guys,

 

I have a set of 17'' excel rimsicon1.png and ts200r hubs that I am trying to lace together. I need help finding the correct spoke length. This is a rare bike so i will need to order custom spokes from Buchanan.

 

I've tried to loosely assemble the rim with my old spokes and mark the length down, but the results are unprecise.

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Most bikes use two or three different spoke lengths (usually shorter on the sprocket side on the rear and rotor side for the front).  If your old spokes have multiple lengths keep in mind that the new spokes will also require the same.

Edited by 1987CR250R

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In the bicycle world, that offset is referred to as the "dish" of the wheel.  Rear wheels are commonly dished, until disc brakes became common on bicycles, fronts rarely were.  Brandt's  formula assumes the wheel is centered in the frame or fork.

 

The "effective rim diameter", "D", needs to be determined within one millimeter.  This dimension is the diameter to which the fully tensioned spoke will extend.  Usually, this should be about where the bottom of the screw slot in a spoke nipple sits when seated in the rim dimple.  The spoke will stretch about .5-.8 mm under tension.  Lacing patterns are described by the number of times any single spoke crosses other spokes, i.e. "cross 3" or "4 cross".  Generally, the more crosses, the greater the torsional stiffness of the wheel.  The fewer the crosses, the greater the radial stiffness.

 

"S" and "s" are used to determine the diameter of the hub flange where the flange uses spokes bent at right angles, or "hook" spokes. This is used to determine the effective hub diameter, "d".  This is done by measuring from outside to outside of two opposite holes on the flange and then subtracting "s" to arrive at "d".   A lot of modern MX bikes use "nail" spokes, spokes that are straight or nearly so at the hub end, and in that case, "d" can be established by measuring the diameter at the point where the spoke head meets the shaft of the spoke when seated in the hub.

 

X = number of spokes

N = number of crosses in the pattern

S = spoke hole diameter

s = S/2

D = effective rim diameter

d = effective hub flange diameter

w = spacing of the flange from the center of the hub (center measured including all axle spacers. 

R = D/2

r = d/2

T = 720 x 3/N

A = r x sin[ T ]

B = R - (r x cos[ T ] )

C = w/2

L = spoke length

 

L = the Square Root of  (A² + B² + C² - s)  if using hook spokes. 

L = the Square Root of (A² + B² + C²) if using nail spokes.

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Thank you! this is very precious info! I came up with 167mm for the front spokes and 172/168mm for the rear spokes. At 99$ per set, i hope i didn't make any mistakes!

 

About the rear offset, Sometimes i read that the longer spokes go in the inner holes, And sometimes i read the longer spokes go in the sprocket side. Who is right? It would make sense to me that the longer ones go in the sprocket side...

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You should be able to tell by examining the flange on the hub.  It's entirely possible that your wheel will need as many as 4 different spoke lengths if the holes in the hub flange are staggered, as they are on YZ hubs, and also offset.  Take your "d" measurement for both the inner and outer hole set on both sides.

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