How "Tight" should the spokes be on a hub motor?

[El_Steak]

I just came back from a 20km ride and my front-wheel nearly self-destructed. I didn't hit any serious bumps, just cracks in the pavement. There's a HUGE wobble in the wheel at speeds around 20km/h. Many spokes are completely loose.

I'm going to take the wheel to a local bike shop to get it fixed but was wondering if there was any special directions to follow when truing a wheel with a hub motor in it. Should the spokes by "tighter" ?

It's a 9c motor in a 700c wheel.

 

[Ypedal]

As tight as possible without breaking the rim or stripping the spokes.. :wink:

 

[johnrobholmes]

Have you ridden about 100 miles on it since new? They loosen up at first.

 

[El_Steak]

I've ridden 100km on them and they are pretty much all loose. I can turn the "nipples" with my fingers! (Did I really say that? )

I just took it to the bike shop and told the guy to fix the wheel and get the spokes tight.

I'll buy a small spoke wrench and double check them after each ride for a while.

 

[jag]

I've ridden 100km on them and they are pretty much all loose. I can turn the "nipples" with my fingers! (Did I really say that? )

I just took it to the bike shop and told the guy to fix the wheel and get the spokes tight.

I'll buy a small spoke wrench and double check them after each ride for a while.

Same thing happened to me on my 9C rear. Had to tighten spokes twice in the first couple of weeks. I tighten them by ear until all the frewheel ones have the same tone, and the other side another lower tone. I'm not that musical, but I'm guessing the high tone is in the 400-800Hz range. If I had a tuning fork I could compare...

 

[Chalo]

I just came back from a 20km ride and my front-wheel nearly self-destructed. I didn't hit any serious bumps, just cracks in the pavement. There's a HUGE wobble in the wheel at speeds around 20km/h. Many spokes are completely loose.

Don't tell me... you have "strong" 13ga or 12ga spokes in your hubmotor wheel. This is an ignorant tradition; thick spokes are strong individually, but they build weak wheels with chronic spoke loosening problems.

I'm going to take the wheel to a local bike shop to get it fixed but was wondering if there was any special directions to follow when truing a wheel with a hub motor in it. Should the spokes by "tighter" ?

Spokes laced to a normal rim should be tensioned to at least 100kgf (220lbf) per spoke. This is enough tension to stretch 14ga, 15ga or 16ga spokes of normal length to the point where they will not become slack under normal loads, and thus they will remain tight. Shorter than normal spokes such as those in a hub motor application should be _thinner_, not thicker, in order to allow the requisite amount of spoke stretch for decent reliability. Thicker spokes don't stretch enough at normal spoke tension to resist loosening in use. That's why a large majority of the most demanding riders in all disciplines use spokes no thicker than 14ga or 2.0mm.

If you have fat spokes in your wheel, consider having the bike shop replace them with straight 14ga or butted 14/15ga spokes.

Stiff rims, like deep section double walled rims, also help resist chronic spoke loosening.

Chalo

 

[liveforphysics]

A 16awg spoke made of 304 stainless steel will permanently deform (yield strength) at 63lbs of force.
A 14awg spoke made of 304 stainless steel will permanently deform (yeild strength) at 101lbs of force.

The same spokes made from 316 stainless steel:
A 16awg spoke made of 316 stainless steel will permanently deform (yield strength) at 121lbs of force.
A 14awg spoke made of 316 stainless steel will permanently deform (yield strength) at 191lbs of force.


I think the bulk of the wheel failures you guys have are from excessive spoke tension. I always run thinner gauge spokes, and I don't try to use up the available strength on static tension and internal stress on the wheel.

A proper wheel should be able to have any spoke, or any pair of spokes clipped out of it, even right next to each other, and have minimal effect on the wheel as a system. You guys that make the wheels so tight that loosing a spoke causes the wheel to shift and fold up on itself are putting so much more internal stress into the wheel as a system than your riding loads apply to the wheel.

Pretension is critical to enable spokes to smoothly transition and share stresses in the system. Pre-tension beyond 1/3-1/2 of yield strength is absurd IMO.

IMHO- Strong wheel = thin spokes with 1/3 yield strength pre-tension. Tight spoke wheel = an overly stiff wheel waiting to spring in on itself the moment stresses become unbalanced.

Just my $0.02

 

[biohazardman]

I have 12gauge stainless on mine and after a couple of adjustments I no longer have problems with them getting loose. I do not tighten the motor build spokes any more than a regular wheel build. They will flex 3/16th to 1/4 inch when pressed together with the fingers. I have run off plenty of curbs and even over a sawhorse type road sign lying in the middle of a dark street @20mph with no damage to my wheels. In fact, they did not even need trued after the incident. My stock Chinese 13gauge steel spokes constantly stretched then after four adjustments were good for only few hundred miles and began to break at the thinner than 13gauge nipple. I ran the wheel with several broken spokes for some time before I replaced them all. I personally believe that many if not most of the spoke problems are caused by cheap spokes and or over-tightening. Still new spokes will stretch and may need adjusted a couple three times in the first few hundred miles after that they should seldom need attention. Bad things can happen if spokes and wheels are not properly maintained.
I have heard of many with decent 14gauge stainless spokes who swear by them as well.

 

[gwhy!]

Even spoke tension is a key factor to wheel strength or this sort of thing can happen opps.....

 

[dogman dan]

No expert here, but in 5000 miles I have not broken spokes except for the time the bike fell halfway off the cars bikerack. I keep em snug but not crazy tight, just like on a regular bike, and never rode with em loose. I keep after a brand new wheel daily for about two weeks, or 100 miles. After that, I only need to do any truing at most twice a year, like 2000 miles. For those that don't have the wheel truing skills, mount wheel, ride 10 miles, then take to a shop. Ride another 100 miles, checking for really loose spokes, and snugging the loose ones up, and then take to the shop again for a final truing

Never ride with noticably loose spokes.

 

[icecube57]

My GM has the original stock spokes with the original Deep DW rim. Ive tighten 3 spokes over the 2000 miles Ive rode my bike. The only reason why i tighten 3 spokes is because when i started using regen they loosened up. Otherwise I can say anything bad about the stock rim and spokes in my build.

 

[biohazardman]

My GM had an oval shaped aluminum foil single wall rim with some finger tight spokes and others so tight I had a hard time getting them off. So I was not so lucky but expected to toss the rim before my purchase so not a big deal. This was over a year ago. Think I broke 8 or 10 spokes before I dumped them all and put some decent stainless on the wheel. I used regen alot, loved it, till I toasted the controller one to many times then I unhooked it. My motor is still going strong although every other part of the kit has died with <3K miles.

 

[El_Steak]

Interesting to see all the different opinions on the proper spokes and tension for hub motor wheels­.

My wheel was custom-built by ebikes.ca. They took a 2806 motor from a 20" kit and relaced it in a 700c wheel using Phil Wood spokes (14G I think). The wheel itself is double-walled and has a "crystalyte" sticker on it.

 

[jag]

Interesting to see all the different opinions on the proper spokes and tension for hub motor wheels­.

My wheel was custom-built by ebikes.ca. They took a 2806 motor from a 20" kit and relaced it in a 700c wheel using Phil Wood spokes (14G I think). The wheel itself is double-walled and has a "crystalyte" sticker on it.

If the wheel needs serious trueing it is probably best to take it to a shop as you say in your first post.

In general it is a good idea to pay attention to the wheel before it gets severely loose or out of true. If some spokes pling with a lower sound than others (or in the extreme just a thud), they are loosing tension, and can be retensioned to the same tone as others.

When the whole wheel gets loose, one first starts hearing the spokes rubbing against each other between tensioning and untensioning. I seem to hear it most easily when coasting to a stop. (so hub motor and other noises are minimal). This rubbing is a sign that the whole wheel needs to be retensioned.

Sheldon Brown has more detailed info on wheelbuilding:
http://www.sheldonbrown.com/wheelbuild.html#tensioning

 

[johnrobholmes]

I have found that almost all wheels will loosen in the first hundred miles, and must be retensioned. the only exception is the wheel I built with 10ga spokes. Maybe I don't ride it hard enough to stretch the spokes at all.

 

[Chalo]

A 16awg spoke made of 304 stainless steel will permanently deform (yield strength) at 63lbs of force.
A 14awg spoke made of 304 stainless steel will permanently deform (yeild strength) at 101lbs of force.

The same spokes made from 316 stainless steel:
A 16awg spoke made of 316 stainless steel will permanently deform (yield strength) at 121lbs of force.
A 14awg spoke made of 316 stainless steel will permanently deform (yield strength) at 191lbs of force.

Your numbers are incorrect, because spokes are highly cold-drawn and have yield stresses much higher than you project. Even a really thin spoke can withstand tension of more than 200kgf, and a normal 14ga spoke can withstand a tension of about 400kgf. Such tensions are not used in practice because they lead to short fatigue life, difficult assembly, stripped nipples, cracked rims, etc.

I think the bulk of the wheel failures you guys have are from excessive spoke tension. I always run thinner gauge spokes, and I don't try to use up the available strength on static tension and internal stress on the wheel.

A wheel bears a load by trading off deflection at the rim over the contact patch against spoke static tension. The more tension a wheel has, the more radial load it can carry before the spokes slacken and it becomes unstable.

A proper wheel should be able to have any spoke, or any pair of spokes clipped out of it, even right next to each other, and have minimal effect on the wheel as a system.

That would make for a horribly weak and unreliable wheel, whose spokes went fully slack once per rotation. The nipples would have to be threadlocked, and the wheel as a whole would possess only a little more than the inherent strength of the unlaced rim. Wheels have been built more or less this way, traditionally with enormously heavy steel rims and thick spokes, a la Worksman Cycles.

You guys that make the wheels so tight that loosing a spoke causes the wheel to shift and fold up on itself are putting so much more internal stress into the wheel as a system than your riding loads apply to the wheel.

Pretension is critical to enable spokes to smoothly transition and share stresses in the system. Pre-tension beyond 1/3-1/2 of yield strength is absurd IMO.

That's true. But your are mistaken about the yield stresses of spokes and rims. At 100kgf, a straight 14ga spoke is running at significantly less than 1/3 of its breaking stress.

Chalo

 

[Drunkskunk]

Tight is relitive. I used to tighten my roadbike wheels by tuning fork; a 440hz "A" to tension all the spokes, then adjusted as needed to run true. That would be too tight for a hub motored bike.

Spokes are like shoe laces. too tight is as bad as too lose. And also like shoe laces, they will losen over time. More when new and being broken in.

 

[liveforphysics]

Help me understand here Chalo. 16awg is 1.31mm^2, or 0.00203in^2 of cross sectional area (645mm^2 per 1in^2).

To pre-stress this spoke at 220lbf, you are putting a load of 108,000 lbs/in^2 tensile pre-loading on the spoke.

Are we in agreement up to this point?

 

[monster]

i don't know about spoke tension but have you tried cutting spokes out of a rim with pliers? they shoot out like bullets! factory assembled wheels are under maximum tension!

 

[Chalo]

Help me understand here Chalo. 16awg is 1.31mm^2, or 0.00203in^2 of cross sectional area (645mm^2 per 1in^2).

To pre-stress this spoke at 220lbf, you are putting a load of 108,000 lbs/in^2 tensile pre-loading on the spoke.

Are we in agreement up to this point?

Let's do numbers.

A 16 gauge spoke is 1.6mm in diameter.

13ga = 2.3mm
14ga = 2.0mm
15ga = 1.8mm
17ga = 1.5mm

According to spoke manufacturers, the drawn stainless steel (almost always 304 stainless) in spokes can exceed 200,000 lbf/square inch.

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3386&detail=butted
This specifies the center section of these 14/17ga spokes as having 1500N/mm^2 strength. That's 217,600 psi. The center section is 1.5mm in diameter and has a nominal area of 1.767mm^2. So it should have a strength of 2650 newtons, which is 596 lbf. Putting 220 lbf of tension on such a spoke should be OK-- and this is the thinnest size of spoke in general use.

The 14ga ends and elbows of these butted spokes have a nominal cross-sectional area of 3.14mm^2. If we assume the lowest tensile strength specified for a Sapim straight-gauge spoke in these areas, then their strength should be 1080N/mm^2 x 3.14mm^2, or 762 lbf.

http://www.sapim.be/index.php?st=products&sub=spokes&category=3959&id=3340&detail=aero
This one claims 1600N/mm^2. That's 232,100 psi. These elliptical spokes weigh the same as the aforementioned 14/17ga round spokes, so their center section must be equivalent. So they are good for 2827N or 636 lbf.

Now let's do the numbers for a 13/14ga single-butted spoke designed for strength.

http://www.sapim.be/index.php?st=products&sub=spokes&category=3960&id=3387&detail=butted
1400N/mm^2 in the center section, but with a 3.14mm^2 cross-sectional area there. 4.15mm^2 in the elbows. Center strength: 4398N or 989 lbf. Elbow strength, again assuming the lowest published strength for this manufacturer: 4487N or 1009 lbf.

DT Swiss does not publish tensile stress for their spokes, but experience suggests that they are at least the equal of Sapim spokes in this regard. They are less easily bent when lacing, which indicates higher yield stress.

As I said, I consider 100kgf a good minimum tension for the tighter side of a spoked wheel. But if the rim is sturdy, I use up to 140kgf per spoke without issues. Thus I can make wheels that are reliable and stable under very heavy loads, because the inward deflection of the rim does not exceed the spokes' elastic takeup, and the spokes do not go slack in use.

Chalo

 

[liveforphysics]

Thank you for the great response Chalo.

I think spoke manufactures seem to list ultimate tensile stregnth rather than yeild strength. If these are yeild stregnth numbers they are listing, then they need to share some of this technology with the areospace fastener and racing comunity. To get yeild strength numbers to match these, I have to use bolts made from L19 alloy, cold forged in a vacuum starting from a billet shaped to encourage maximum grain alignment during forging, and they ship to you in tubes of a silicone based oil. Once clean, they start to oxidize in minutes if you don't get an oil film back over them.

Are spokes designed around ultimate strength, and intended to deform as part of normal function?

 

[Chalo]

I think spoke manufactures seem to list ultimate tensile stregnth rather than yeild strength.

I have wondered this myself, and I have no conclusive answer to that. But yield stress is what counts with spokes. Even tiny amounts of lengthwise yield would completely mess up a wheel, because its tension is so interdependent throughout.

If these are yeild stregnth numbers they are listing, then they need to share some of this technology with the areospace fastener and racing comunity. To get yeild strength numbers to match these, I have to use bolts made from L19 alloy, cold forged in a vacuum starting from a billet shaped to encourage maximum grain alignment during forging, and they ship to you in tubes of a silicone based oil. Once clean, they start to oxidize in minutes if you don't get an oil film back over them.

I would be not at all surprised to find that such measures are necessary to combine that kind of yield strength with a rising post-yield strain/stress curve and enough elongation to do the job, with enough consistency and uniformity to be depended upon for life-or-death applications. One of the luxuries of bicycle spokes is that they are allowed to break when they want.

Are spokes designed around ultimate strength, and intended to deform as part of normal function?

Spokes normally yield at the elbow while being tensioned and bedded in. If a spoke yielded along its length, it simply wouldn't work because the wheel would not true.

Chalo

 

[recumbent]

The spokes on my 26" rear wheel were tight when i got it new last year, but started to make a "clicking" noise after a week, i tightened them all up 1/4 turn and evened the tune, which was high pitched and managed to get 8,000 km (5,000 mi) with no issues.

This was a 28 spoked wheel with 2mm spokes and the rider is 190lbs with 14 lbs of batteries in the panniers. Lots of grocery trips and gravel roads inbetween.

My theory therefore is: thin 14ga, high quality, tight spokes will yield the strongest wheel.

I now have a 32 spoker with internal gears (Sram I-9 hub) and will test this wheel for another two seasons and i'll really know for sure.

 

[amberwolf]

Spokes normally yield at the elbow while being tensioned and bedded in. If a spoke yielded along its length, it simply wouldn't work because the wheel would not true.

That also makes sense since the only place I've ever had a spoke break was right there (of course, that is the most stressed point on it, so it *should* be where it breaks).

 

[Drunkskunk]

Spokes normally yield at the elbow while being tensioned and bedded in. If a spoke yielded along its length, it simply wouldn't work because the wheel would not true.

That also makes sense since the only place I've ever had a spoke break was right there (of course, that is the most stressed point on it, so it *should* be where it breaks).

I've had spokes break at the base of the thread. Thats where I learned the importance of rolled threads over cut threads.