2.0/1.5/2.0 spoke stretch/length

[ktm520]

Do you go with a shorter spoke length on the ds when using the Lasers/Revs to account for the extra stretch of the thin spoke? The spoke calc I always use (dt swiss) has always been spot on when I have used 2.0 and 1.8 spokes in the past. After tensioning the rear wheel, the ds spokes are 2-4mm too long, but the bs are perfect. The front wheel is only about 50% tensioned and it is looking like the bs spokes are going to be too long as well. I double and triple checked all of my hub dimensions and measured the erd of the rims before I sized the spokes.

So, the spoke has 10mm of threaded length. That leaves worst case 6mm of thread engagement, which is 3 times the diameter of the spoke. I know its not ideal, but theoretically it should be ok. The majority of the ds spokes are only 2-3mm too long. On the other hand, I'm not sure why I'm even considering chancing it since its only 30$ in spokes and a couple more hours of my labor.

[figo]

I've not used the DT swiss spoke calc but I guess it's accurate once the correct numbers are in, I've been using Damon Rinards spocalc xls with great results.

Regarding spoke stretch, I'm sure there are some experts in this forum on metal structures and stuff, but I'd be hard pressed to believe spokes would be tensioned to the point where they would stretch by any meaningful amount. When spokes are already stretched by a fair bit I guess on a ride they'd be stretched further and probably break quite soon.

On the building materials as spokes I think it makes no sense to save, especially factoring in own time to build&fix it.

[ktm520]

Figo, the spokes actually stretch a lot more than you would think. And, the 1.5 versus 1.8 diameter is spoke is a 30% reduction in cross-sectional area, which equals 30% more deflection under the same load (tension).

I tried 4 different calculators and they all agreed with dt swiss.

[Jwiffle]

I tried 4 different calculators and they all agreed with dt swiss.

Interesting. I was going to say the dt swiss calculator was probably off.

I've always used the calculator on qbp's dealer site. Dt swiss's calc always came up with something different, but using the numbers off qbp has always worked well for me. The wheels I just built had a very different numbers for spoke length than qbp. I tried several other online calculators, as well as spocalc, and they agreed with qbp. I used the qbp numbers, and the wheels built up great.

I have read elsewhere that the dt calculator is great when using their hubs, but when you try to input the numbers off other hubs, it doesn't come out right for some reason. My experience seems to match this, as I've never used dt hubs.

[ktm520]

Ya, I've always used the dt swiss calc in the past inputing my own numbers and it has always agreed with spocalc. The main reason I don't think it is the calculators is because the low tension sides are coming out spot on.

I did a quick deflection calc, which I should have done in the first place, and based on 304 stainless, a 1.5 spoke 292mm long will elongate 1mm with a 120kgf load. For comparison sake, a 1.8 spoke would deflect .6mm under the same conditions but probably won't yield as much. Not what I was expecting, but I'm not certain of the modulus of elasticity for the material.

Considering the spokes where 1mm longer than calc to begin with, add 2mm for elongation and yielding, that puts it right in the 2-4mm "too long" range I'm seeing.

[beanbag]

Elastic modulus is the same for most steels.

[customfab]

How much tension are you trying to achieve? I've found that revolutions start to stretch at about 135 kilos and if for some reason you want a tension over that they are not an acceptable spoke to use. Now if your building wheels to a more common 100-130 kilos of tension then they are fine and the lengths have been interchangeable for me.

[ktm520]

110-115kgf

[customfab]

110-115kgf

You've got the wrong length. No way are the spokes stretching 2-3 mm at that tension.

[ktm520]

You've got the wrong length. No way are the spokes stretching 2-3 mm at that tension.

Ya, I realize that after running the calcs. Check hub dimensions again, correct. Rims are laced, can't re-check but they are in line with other members. Check spoke length, and they are cut 1mm longer than what I asked for. Also, calculator said 291 bs/290 ds, and went with 291 for both as usual. So, that account for 2mm. Looks like I'm a victim of tolerance stack up here. Live and learn.

I went ahead and finished tensioning both wheels. Ended up being the best balance job I've done in my short life of wheel building. I'm going to keep an eye on them, but considering I am not hard on wheels, I'm pretty confident there is still sufficient thread engagement for mechanical strength.

[meltingfeather]

Interesting. I was going to say the dt swiss calculator was probably off.

I've always used the calculator on qbp's dealer site. Dt swiss's calc always came up with something different, but using the numbers off qbp has always worked well for me. The wheels I just built had a very different numbers for spoke length than qbp. I tried several other online calculators, as well as spocalc, and they agreed with qbp. I used the qbp numbers, and the wheels built up great.

I have read elsewhere that the dt calculator is great when using their hubs, but when you try to input the numbers off other hubs, it doesn't come out right for some reason. My experience seems to match this, as I've never used dt hubs.

The DT Swiss spoke calculator does simple math that you can easily duplicate. It also shows you ALL of the inputs it uses, unlike most if not all of the other online spoke calculators, which use behind-the-scenes assumptions.

Any bad results coming out of it are a direct result of the inputs (user error).

Just wanted to clarify that, as it seems the remnants of this misinformation seem to still float around in nooks and crannies.

[Jwiffle]

The DT Swiss spoke calculator does simple math that you can easily duplicate. It also shows you ALL of the inputs it uses, unlike most if not all of the other online spoke calculators, which use behind-the-scenes assumptions.

Any bad results coming out of it are a direct result of the inputs (user error).

Just wanted to clarify that, as it seems the remnants of this misinformation seem to still float around in nooks and crannies.

The qbp calc and spocalc will let you input ALL the info, as well. Yet, they have always agreed, whereas dt's usually does not. Good thing I did not go with dt's numbers on my recent build: spokes would have been 12 mm short!
So if it is user error for so many on dt's calc, they need to make it more user-friendly.

[meltingfeather]

The qbp calc and spocalc will let you input ALL the info, as well. Yet, they have always agreed, whereas dt's usually does not. Good thing I did not go with dt's numbers on my recent build: spokes would have been 12 mm short!
So if it is user error for so many on dt's calc, they need to make it more user-friendly.

Rinard's Spokecalc spreadsheet does the exact same (correct) math that the DT Swiss calc does. Interesting you should bring that up. I don't want to get into a pissing match about your experiences with the DT Swiss calc, I was just pointing out for others that it performs the correct calculation every time, and that anyone's experiences to the contrary are user error.
If you think there is something wrong with the DT Swiss calc, post up a screenshot showing the "erroneous" calc and I will tell you what you did wrong.

[figo]

meltingfeather, what about spokes stretch? Is that taken into account in these calculators?

[meltingfeather]

meltingfeather, what about spokes stretch? Is that taken into account in these calculators?

No.
The only calculator I have seen that accounts for spoke strain is the one I built. It's not really a factor in terms of selecting spoke length. It would take ~230kgf to stretch a CX-Ray (highly elastic) 1.0mm. A DT Comp at 110kgf will lengthen by 0.3mm. That's with 29er-length spokes.

[ktm520]

It would take ~230kgf to stretch a CX-Ray (highly elastic) 1.0mm. A DT Comp at 110kgf will lengthen by 0.3mm. That's with 29er-length spokes.

What are you using for modulus?

[meltingfeather]

What are you using for modulus?

2.76*10^7 psi

[ktm520]

Thanks. I was using 2.8e7, so not far off. When you say the Rays are "highly elastic", are you just referring to the small cross-sectional area or does the material they are made from have a lower modulus?

My numbers aren't matching up with yours. I'm getting 1.9mm of elongation for cx-ray at 230kgf/291mm long. I used the ellipse equation for area.

[meltingfeather]

Thanks. I was using 2.8e7, so not far off. When you say the Rays are "highly elastic", are you just referring to the small cross-sectional area or does the material they are made from have a lower modulus?

Small cross-sectional area (1.77mm^2)

My numbers aren't matching up with yours. I'm getting 1.9mm of elongation for cx-ray at 230kgf/291mm long. I used the ellipse equation for area.

Don't know... it's a pretty straightforward calculation and the that's a large difference.
Did you use the same cross-sectional area for the center and account for the fact that a CX-Ray does not have that cross-section for it's entire length?

*EDIT*
woops... I just realized that I did the quick calc using the NDS rear tension, which for the wheel I tested is 61% of DS tension.
I get 1.86mm at 230kgf, or 1.0mm of elongation at 123kgf
sorry about that.

[Mike T.]

sorry about that.

Go stand in the naughty corner.

[ktm520]

Did you use the same cross-sectional area for the center and account for the fact that a CX-Ray does not have that cross-section for it's entire length?

No, I just used the smaller section for the whole length. I thought of that after I posted. Good too see our numbers are matching up.

[Jwiffle]

Rinard's Spokecalc spreadsheet does the exact same (correct) math that the DT Swiss calc does. Interesting you should bring that up. I don't want to get into a pissing match about your experiences with the DT Swiss calc, I was just pointing out for others that it performs the correct calculation every time, and that anyone's experiences to the contrary are user error.
If you think there is something wrong with the DT Swiss calc, post up a screenshot showing the "erroneous" calc and I will tell you what you did wrong.

ok, I finally figured out why I couldn't get dt swiss calc to work. the 0 of spokes holes I thought meant number of spoke holes in the hub, but it's the diameter of the spoke hole. d'oh! qbp doesn't ask for it, and spocalc always had a value for that I never messed with. Spocalc list puts in 2.4. if I put in 2.4 it's close on the dt swiss calc (would use the same rounded length, just different tenths/hundredths), 3.0 gets it closer to spocalc and qbp.

Well, now that I've figured that out, guess I can find the dt swiss calc more useful!

[meltingfeather]

ok, I finally figured out why I couldn't get dt swiss calc to work. the 0 of spokes holes I thought meant number of spoke holes in the hub, but it's the diameter of the spoke hole. d'oh!

Ø is (engineering?) shorthand for diameter.
I will say that a diagram showing the nomenclature they use tied to the actual dimensions would be very helpful, but I doubt many people click the "Help" tab anyway.

qbp doesn't ask for it, and spocalc always had a value for that I never messed with. Spocalc list puts in 2.4. if I put in 2.4 it's close on the dt swiss calc (would use the same rounded length, just different tenths/hundredths), 3.0 gets it closer to spocalc and qbp.

a HA! so the qbp calc doesn't show you all the inputs it uses.
AFAIK spokecalc duplicates DT Swiss results & vice versa- to the 0.01mm, as shown in the screen grab I posted. My calculator does the same.
Because spokes are measured from the inside of the bend and the flange diameter is measured to the center of the spoke holes, all three calcs correct the calculated length by subtracting 1/2 the spoke hole diameter. You can verify this in the DT Swiss calc by changing the spoke hole diameter and looking at the changes in "precise length." The last term in the length calculation cell of spokecalc shows this as well. It's a simplifying assumption.

Well, now that I've figured that out, guess I can find the dt swiss calc more useful!