Front wheel spoke gauge - twin disc build

mdt-son

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Hi

I am building a new front wheel for my Mk3 featuring almost straight spokes. I will be fitting twin calipers to custom made sliders.
Spokes will be single butted and my initial choice is 7/9 gauge (or nearest metric size). Is this a sensible selection when using twin stoppers?

-Knut
 
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What do you mean by straight spokes, their angle to the rim or their angle at the hub holes. A straight angle to the rim will not cope regardless of gauge. Use rubber lined pliers on the hub nearly straight spokes to stop them turning until the tension builds and the spoke no longer want to turn with the nipple.

What material are you having the spokes made from.
 
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Dan1950

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We used to call it "straight laced wheel".

While they are as strong radially as any other pattern they will not cope with torsional loads. They worked fine on "choppers" with no front brake but not when a brake is present. The more angle on the spokes (the more times they cross other spokes", the more torsional strength.
 

mdt-son

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By straight spokes I mean the spokes themselves are straight! Or almost - there is a slight bend towards the head. Spoke orientation (whether radial, 2-cross or 3-cross lacing pattern) is a different subject, so please forget the discussion assuming a radial pattern - the pattern is 2-cross lacing.
My question was about spoke gauge and not spoke pattern. Spoke material is stainless steel.

Cheers,
Knut
 

concours

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First time I'd EVER seen spokes pulled out from trauma. Be sure it's correct.
7A79D83E-77B7-4B59-A031-152B0CCAFE9E.jpeg
B116E4D8-3649-4716-BDDE-D112AC9E3224.png
 
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With stainless I would be going the largest single diameter gauge you can get the nipples for, which when I last looked was 8ga. If possible I would redrill the rim for 3 over, using a larger diameter nipple than the current holes allows you to do this within limits, a chromed rim risks the chrome but alloy and stainless rims will retain their looks.
 

mdt-son

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First time I'd EVER seen spokes pulled out from trauma. Be sure it's correct.
What are you trying to show here? An assertion of badly sized spokes? Obviously they were not. Most spoke heads have survived meaning the hub failed. Looking at the deformed fork and frame, hub failure did not cause the damage, and neither did the 4LS brake.
I gather spokes were pulled out of the injured person! It just shows motorcyclists are at danger in case of a collision. But this has no bearing to my question.

-Knut
 

concours

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What are you trying to show here? An assertion of badly sized spokes? Obviously they were not. Most spoke heads have survived meaning the hub failed. Looking at the deformed fork and frame, hub failure did not cause the damage, and neither did the 4LS brake.
I gather spokes were pulled out of the injured person! It just shows motorcyclists are at danger in case of a collision. But this has no bearing to my question.

-Knut
I was attempting to articulate that this CUSTOM BUILT wheel, failed during a crash, AND, (most importantly) I had never seen this type of broken flange/pulled spoke EVER in my 50 years of abusing/repairing motorcycles. I found that curious. That is all, nothing to see here. A very small contribution to what I thought slightly relevant. I'm sorry to have replied to your question.
Folding my tent here. :cool:
07AFA04B-90C3-489C-8601-1E333B769705.jpeg
 
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mdt-son

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With stainless I would be going the largest single diameter gauge you can get the nipples for, which when I last looked was 8ga. If possible I would redrill the rim for 3 over, using a larger diameter nipple than the current holes allows you to do this within limits, a chromed rim risks the chrome but alloy and stainless rims will retain their looks.
Thank you for your views, kommando. May I ask why you would up gauges of stainless steel spokes? I believe a high tensile steel wire is used (e.g., Sandvik 18/8) which is then further cold-worked, raising the tensile strength to about 1000 MPa or more. Are you worried about fatigue strength?

What do you mean by the prase "I would redrill the rim for 3 over". I plan to use 0.3" nipple (barrel) heads.

-Knut
 

mdt-son

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I was attempting to articulate that this CUSTOM BUILT wheel, failed during a crash, AND, (most importantly) I had never seen this type of broken flange/pulled spoke EVER in my 50 years of abusing/repairing motorcycles. I found that curious.
Maybe it is. A flanged alloy hub will give at some point, but a crash scenario is not a guiding factor for designing a hub or selecting a hub - loading at the spokes is.

Norton's single disc front hub is flanged. I am avoiding this design by using a hub in which straight spokes are anchored in solid metal, aiming at providing a near tensile load only in the spokes.
Matchless used the same design principle starting in 1954.

-Knut
 

concours

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Maybe it is. A flanged alloy hub will give at some point, but a crash scenario is not a guiding factor for designing a hub or selecting a hub - loading at the spokes is.

Norton's single disc front hub is flanged. I am avoiding this design by using a hub in which straight spokes are anchored in solid metal, aiming at providing a near tensile load only in the spokes.
Matchless used the same design principle starting in 1954.

-Knut
:cool: I am aware. You have a good plan. It should work well.
 
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tensile strength to about 1000 MPa
Compare that to a good zinc plated high tensile wire of 1370 MPa and then reconsider your gauge.

"I would redrill the rim for 3 over"

3 over against your plan for 2 over will increase the resistance to braking forces. If your rim is already drilled for 2 over then as long as the current holes are .250 then you can drill them for your .300 nipples and change the angle to suit 3 over. If your rim is currently undrilled then you just need longer spokes with a slightly different angle at the hub and the rim drilled to suit.

There are online calculators for all this info, I used them to spec out a CB750 cotton reel hub with 3 over spokes with an increased gauge for a project.
 
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If you look at the rear BSA crinkle hub you will see straight spokes, Triumph fronts before the flanged TLS. Straight spokes are less prone to losing tension without the bend at the hub to unbend, especially with stainless as your spoke maker cannot bend them to the same rules as steel so the radius of the bend is greater and there are different limits to how close they can get the bend to the head. You are more reliant on the size of the holes in the hub and the spoke head, get that wrong and the pics shown will happen without the need for an accident.
 

gortnipper

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Hi

I am building a new front wheel for my Mk3 featuring almost straight spokes. I will be fitting twin calipers to custom made sliders.
Spokes will be single butted and my initial choice is 7/9 gauge (or nearest metric size). Is this a sensible selection when using twin stoppers?

-Knut
Maybe send an email to Buchanan's and ask them?
 

mdt-son

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Compare that to a good zinc plated high tensile wire of 1370 MPa and then reconsider your gauge.

"I would redrill the rim for 3 over"

3 over against your plan for 2 over will increase the resistance to braking forces. If your rim is already drilled for 2 over then as long as the current holes are .250 then you can drill them for your .300 nipples and change the angle to suit 3 over. If your rim is currently undrilled then you just need longer spokes with a slightly different angle at the hub and the rim drilled to suit.

Thank you for your views! I will consider this.

As for tensile strength (I guess the figure you quoted is for tensile strength), thinner wire tend to give higher strength values due to a higher degree of cold working. 1370 MPa will be encountered for very thin wires only. I have seen values of 1750 MPa and beyond for certain high performance bicycle spokes. For plain motorcycle spokes, I think values in the order of 1100 MPa is realistic.
Today, stainless spokes keep pace. As delivered wire material of SS AISI304 will have an UTS of about 750 MPa. Contrary to bicycle spokes, wire material for motorcycle spokes is cold worked in the manufacturing process, which raises tensile strength to about 900 MPa. AISI304 is a vanilla quality however. If you ask for quality spokes made of Sandvik 18/8, tensile strength of the finished spokes will be in the region of 1200 MPa. This quality is employed by Haan Wheels and others.


Maybe send an email to Buchanan's and ask them?
Good idea - I will. Not sure if they are able to advise on performance of the assembled wheel (they tend to deliver what the customer asks for). I will check out, and also ask for the material spec of the spokes they fabricate.

-Knut
 

mdt-son

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Is it a known thing that fitting an extra disc causes an issue with the spokes?
It depends on the configuration. If the wheel was designed for one brake disc and caliper only, it's more than likely that adding a second disc and caliper (which exerts the same braking force as the first one) will cause an issue when braking hard. A braking force acts tangent to brake disc at the center of the braking pad, which creates a braking torque (Mt), which in turn is transmitted to the rim and tire.

Consider this simplified equation: Assume all spokes have the same angle (alpha) in the central plane towards the rim tangent line. Disregard the out of plane angle.
Let T be a component of the spoke tensile force acting tangent to the rim bead of radius R. Then,
Mt = n * T * R
Substituting the spoke tensile force (S), we derive at
Mt = n * S * R * cos(alpha)

Now, if Mt is doubled, tensile force in the spokes will also double. A manufacturer would have to size for the worst case, meaning thicker spokes are needed. Assuming the same spoke material is used, and safety factors remaining constant, the new thickness d2 = d1 * sqrt(2) = 1.4 * d1 (rounded).

The formula also shows that decreasing the angle (alpha) by (delta), by selecting a 3 crossing vs. 2 crossing pattern, increases the tensile force in the spokes for a given torque. For example, (alpha) = 55 degrees and (delta) = 10 degrees (resulting in a 45 degree slope towards the tangent line) would increase S by 1/( cos(delta)-tan(alpha)*sin(delta) ) = 1.357 meaning spoke stress increases almost 36%. Thus, slanted spokes means thicker spokes also, unfortunately. For this reason, hubs have increased in diameter in recent years, allowing spokes to be kept short.

Adding a second disc in a responsible way inevitably makes the wheel heavier! Namely, by 40% of the initial spoke weight (assuming constant slope or angle). In addition nipples will be a little heavier. We neglect the weight loss at the hub and rim by drilling larges bores for spokes and nipples. The additional brake disc adds to this.

- Knut
 
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