Spoke Tension

[Frank21144]

I have just over 1,000 miles on my Aventon Level. Last week a spoke next to the valve stem broke off at the rear hub while I was checking tire pressure. So I ordered replacement spokes and a spoke tension gage. I discovered that the outside spokes were very tight. The gage goes from 0 to 50. They were near 50. The inside spokes were mostly near 40. I suppose these ebike spokes need to be routinely checked, because the electric hubs exert much more force on the spokes compared to a regular bike. I could not find what the spoke tension should be for the electric hub. I adjusted them to 35 - 40 on the gage. Hope that is ok.

 

[scrambler]

You should ask the manufacturer what the spoke tension should be.
If the rim is centered on the HUB, the tension would be the same on both side, but if the rim is off center (asymmetric lacing), then the tension would be different.

 

[harryS]

They use or should be using different length spokes on opposite sides of the wheel to accomodate moving the rim off center from the hub, necessary when there are gears. This allows the tension to be kept similar.

I've built a number of hub wheels. I should have bought a spoke gauge for them, but did it by feel. I can still tell when one side is way too tight. Then I loosen everything, and retighten..

 

[spokewrench]

Normally, spokes break near the nipple or at the bend. I had the heads pop off three ebike spokes. The heads were a little too small, and they went too deeply into the holes in the hub, which allowed the elbows to flex, which eventually snapped the heads.

When I assemble a bike, I'll take a test spin, then adjust the spokes, then take another spin. Usually, the handling feels better. When your weight is on the bike and the spokes come under tension (pointing up), looser spokes will let the rim move sideways toward the tighter ones. That can make handling less precise, and the metal fatigue will eventually break spokes.

I prefer the kind of wrench pictured. With my fingers out of the way, I can better see which way to tighten or loosen. One opening will grip three corners of a nipple, which I prefer. The 4" handle makes it easy to see how far I'm turning a nipple. The wrench is also a dandy hammer to tap spokes so I can hear the tension. (On my new Aventon I had to remove the reflectors to let all the spokes vibrate.)

Starting at the valve, I'll go around tightening dead spokes because it's the loose ones that allow movement. Pinching pairs together is another way to find loose ones. I loosen any whose high-pitched "pink" shows they are much tighter than the rest. The second time around, I adjust for roughly the same pitch.

Then I rest my hand against the fork or chain stay with the end of a finger lightly against the rim. I turn the wheel to see if my finger drags evenly all the way around. A six-inch sector of my new Aventon front wheel was slightly out of true, but it seemed to be less than 1mm, so I let it go. To fix it, I would have marked the sector with tape, taken half a turn off the spokes pulling the sector out of line, and tightened the opposite spokes half a turn. Then I would have adjusted for similar pitches and checked again for runout.

That rim is offset, with spokes on one side 3mm longer than those on the other.

 

[spokewrench]

I'd never used a tension gauge, so I looked at videos by two pros. Both showed how gauges worked, and both said you don't need one. The said they may use a gauge after they adjust by sound, to check. They said within 20% is fine.

Gauges did not appear until about 1960, after more than a century of bicycling. One would have been devised sooner if anyone had seen a need. It reminds me of the torque wrench, a simple tool where a pointer shows how far the handle flexes, yet none was devised until the 1930s, perhaps a century after steel machine screws became common. By 1930, cars and other machines were built on assembly lines. A new employee might not know how to torque a fastener by feel, and many were being screwed into sheet steel or soft alloys. On an assembly line, an employee could do a lot of damage by over- or undertightening. A torque wrench could provide a number for him to learn to torque by feel.

About 1960, the market for 15 mph utility bikes was drying up. People were buying 30 mph "racers"with thin spokes and light rims. I think thinner spokes would stretch elastically more than thicker ones and therefore needed a preload to keep the rim from moving laterally. At the same time, overtightening could damage light hubs, spokes, and rims. A mechanic who tensioned a lightweight wheel too tightly or loosely could cause trouble for a lot customers and perhaps damage the manufacturer's reputation. I think the tension gauge was devised for a mechanic to document that his tension was within the manufacturer's numbers.

HarryS noted that said that when there are gears, the rear rim must be offset from the hub. The rim of my Abound isn't offset because the hub is 2" from the chain stay on one side due to the cassette and 2" from the other side due to the disk brake. My front rim has spokes on one side 3mm shorter than the other. The contact patch must be centered on the steering axis, especially when the brake is applied, but the hub is offset 12mm to make room for the disk and caliper. Laterally, the nipples are 18mm from the disk-side flange and 42 from the other. If a balance in the lateral component of spoke tensions held the rim in place, tension on the shorter spokes would have to be twice that on the longer ones. Dinging, that would be like the difference between C and F sharp.

I heard no such difference when I tapped spokes the first time around, and I tuned them the same. A deck of cards can be used as a feeler gauge with increments of about 0.25mm. My rim is offset by 1 card, or about 0.25mm. That proves HarryS is right. With ebike spokes at least, tensions are similar even when a rim is offset. Length is what counts.

 

[herringbur]

Having never used a tension gauge before, I watched videos featuring two experts. Both demonstrated gauge operation and concluded that one was not necessary. They mentioned that after adjusting by sound, they might check with a gauge. Within 20%, they stated, is OK. dordle

 

[Ben J]

You can't judge the tension just from the number reading on the gauge. You need the decoder table that uses the material and the gauge of the spokes to turn it into tension.

 

[BikerG]

I just pluck the spokes to see if they sound the same.

 

[PCeBiker]

You can't judge the tension just from the number reading on the gauge. You need the decoder table that uses the material and the gauge of the spokes to turn it into tension.

My spoke tension meter came with conversion charts.

I've got 12 gage round steel spokes on the rear and 13 gage on the front, so ~27 front and ~30½ on the rear.

The ZTTO is nice because it has a large adjustable spring, so you can recalibrate the scale if spring sags.
(You would need an accurate tension gage to calibrate it though)

https://a.aliexpress.com/_mr0pA3e

 

[PCeBiker]

I've built a number of hub wheels. I should have bought a spoke gauge for them, but did it by feel.

I thought that I'd try out my new tension gage and tried to set all my rear spokes to 31.

I can still tell when one side is way too tight.

I could tell that one side was too tight because my tire hit the frame and the wheel wouldn't rotate.

Then I loosen everything, and retighten..

Then I loosened everything and started over.

I did eventually get my rim straightened out and even managed to get all my spoke tensions in range, but my rim has a quick warp to it right where the two halves are welded together opposite the valve stem hole.
I couldn't straighten that out without using a hammer.

 

[spokewrench]

You can't judge the tension just from the number reading on the gauge. You need the decoder table that uses the material and the gauge of the spokes to turn it into tension.

A tension gauge doesn't tell how much tension is on a spoke after the gauge is removed. It measures how much force it takes to make a spoke follow a slightly longer path by bending it slightly. A table takes into account the stiffness of the spoke against stretching and bending, but the length of the spoke and the flexibility of the rim would also affect the reading.

One of the pros who made a video said adjusting the spokes on that wheel was easy because it had a rigid mountain-bike rim. Probably the spokes were relatively stiff, too. I think preload (spoke tension) is less critical where there is less elastic movement in a system, and I think that is likely to be the case with an ebike.

 

[PCeBiker]

,..Probably the spokes were relatively stiff, too. I think preload (spoke tension) is less critical where there is less elastic movement in a system, and I think that is likely to be the case with an ebike.

That makes sense to me.
I've got a rear hub motor with short 12 gage spokes.

When I flick them, they make more of a thud than the ring I hear from my longer 13 gage front spokes.

 

[spokewrench]

I thought that I'd try out my new tension gage and tried to set all my rear spokes to 31.



I could tell that one side was too tight because my tire hit the frame and the wheel wouldn't rotate.



Then I loosened everything and started over.

I like my wrench with the 4" handle because I can easily turn nipples a certain amount. I've moved rims by trial and error, measuring to see how far I'd moved one, but adjustment can be calculated.

Say your spokes are 15 cm long and on the side that rubs the offset between flange and nipple is 2 cm. 15^2 - 2^2 is 221, whose square root is 14.86 cm. That's the radial distance. Suppose you want to get the tire 1 cm from the chain stay. That entails increasing the offset to 3 cm. The sum of the square of 3 and the square of 14.86 is 229.8, whose square root is 15.16. 15.16-15 is .16. You want to lengthen the near side spokes 1.6mm.

A spoke has 56 threads per inch: call it a pitch of 0.5mm. Loosening three turns would be 1.5mm: close enough. You could calculate how much to shorten the far-side spokes, but it seems easier to loosen all the near-side ones 3 turns, then take up the slack on the far side, turning them the same, which probably won't be 3 turns. Then you measure how close the rim is to being right.