Why do I read so much about spokes breaking?

[smee]

I have read a lot of threads where they mentioned spokes breaking as being a problem with some e-bikes. Sometimes the bike I am buying is mentioned as one that suffers a bit from this (Freego Eagle).

The last time I rode a bike seriously was in the 1970s, and I don't remember ever breaking a spoke. Given I was much fitter and often rode at the same 25km/hr that I will be able to manage with my new e-bike, I have to assume it's basically the same input power from the hub, and that it's a fair comparison. So why are spokes a problem now?

 

[trex]

occasionally, spokes break, usually on rear hub motor wheels.
Spokes break when the wheel is out of true or the stress gets too much.
The spokes usually break at the elbow, where the steel is thinner and rubs against the metal flange of the hub causing metal fatigue.
E-bike spokes are 13 gauge, 50% stronger than 14 gauge spokes on normal bikes, but the stress put on them is much more than doubled: a rear motor wheel can produce 5 times more torque than the average human. However, a well built wheel can cope with that, this is the case for a large majority of rear hub motor wheels. They never have broken spokes, so you should not worry. However, some do. This is because it's much more difficult to keep the rear hub motor wheel trued and spokes tight all the time.
When you ride off the kerb for example, the shock to each wheel is directly proportional to the weight on it, the rear wheel takes it much harder than the front wheel. Furthermore, the front wheel is usually suspended, the shock wave is absorbed by the fork's springs. The rear wheel supports most of your weight, takes most of the stress and often has no springs. Bikes with crank motors do not suffer from this problem because the motor torque is transmitted onto the chain rather than directly onto the spokes.
The only way to avoid breaking spokes if you have a rear hub motor is to keep your motor wheel trued and your spokes even tensioned. Spokes work only under traction, if they are compressed, they break instantly. Each spoke can take as much as 300kgf before they break, I tension them at 90 to 100kgf - they never break. You don't need to buy a Park spoke tensiometer to find loosened spokes, ping your spokes with your fingers, if any one of them isn't tight, you will feel it. Invest in a £2 spoke key, that's all you need.

 

[smee]

Thanks for the long reply. Can I clarify what you mean by "out-of-true"? Do you mean a sideways wobble/warp, or an off centre hub?

I will take your advice about getting a spoke key. Although I don't intend to be riding off too many kerbs, I am heavy, so I think I need to pay attention to maintenance.

 

[trex]

yes, you are correct. The rear wheel needs to be dished to keep the rear wheel to the middle line of the bike, so there will be one side where the spokes are tighter than the other - but they should really be within 10%.
The key to maintenance is prevention. The bolts and nuts of your new bike will loose a bit of their tightness in the first week of commuting. Typically, tighten crank bolts, handlebars, rack, mudguard, chainguard, pedals and lights. Do that before you start commuting and again at the end of the first week. If you find any bolt working loose at this time, get a bit of blue loctite and threadlock any loosened bolts. Check again the bolts and nuts after the first month. That should settle the bike down for years to come.

 

[smee]

Sounds like good advice, thanks

 

[KTM Bike Industries UK]

E-bike spokes are 13 gauge, 50% stronger than 14 gauge spokes on normal bikes, .

Trex... where do you get these "facts" you state with such confidence?

This is total nonsense, there is not even such thing as eBike spokes!!!

and where have you read that 13g is 50% stronger than 14g - spokes are designed to flex and stretch to give the rim the ability to move a bit to give comfort and strength to the wheel as a whole. So a spoke being "stronger" isn't even something thats actually really a good thing.

The fact spokes break on cheap build eBikes, is because cheap normal bikes can cope with cheap spokes... cheap eBikes put lots of extra stress and weight onto cheap components and often badly built wheels with cheap rims. So the spokes break, because they are not able to cope with the work they are being asked to do. This could be due to the spoke being poor quality, the rim being poor quality, or the build being bad... or all 3.

A well built wheel can allow poor quality spokes and rims to last a bit longer. But when you're looking at any bikes, the wheels are one of the major things that can effect the feel, performance and value of a bike.

They are rotating weight so can make a big difference to how a bike feels, and if the brand has compromised on hubs, spokes or freehub bodies, this is something that people don't often spot when buying a bike. It only comes to light afterwards when spokes start snapping or hubs fail.

If you are looking at any eBike, you need to consider the value of the electrics and subtract that, and then think about what you're left with, and look at the components on that bike. These days most of the electrics from most brands are reliable and offer good performance, so the differences are more subtle, but equally important.

 

[KTM Bike Industries UK]

yes, you are correct. The rear wheel needs to be dished to keep the rear wheel to the middle line of the bike, so there will be one side where the spokes are tighter than the other - but they should really be within 10%.
.

again - totally wrong.

You use shorter spokes on the drive side, so the spokes are shorter...

they should be the same tension on both sides!!!

 

[trex]

they feel tighter when you ping the shorter ones with your fingers, the spoke tension should be within 90kgf to 100kgs.

The 300kgf figure is the force you need to break a spoke when it's being pulled.

Sapim publish their datasheet for all their 14G in PDF.
example:
sapim super spoke: middle section 1.4mm
tension strength = 2900N/mm2 * pi * 0.7^2 * /9.98m/sec2 = 629kgf
the 13 gauge stainless steel spokes on Chinese e-bike have 2.1mm diameter. The steel does not have as much high breaking tension strength, but even at 2000N/square mm, the breaking force is:
2000 * pi / 9.98 = 629kgf
just as strong as the Sapim super 14G spoke in the worst case scenario, reduce tensile strength at the elbow. Of course, inferior Chinese stainless steel can do much better than 2000N/mm square. In fact, Chinese type 316 stainless steel has the same tensile strength (290 MPa), 939kpf is the breaking force.
Col talks a lot about cheap Chinese spokes - ask all the suppliers on this forum how many spokes they have seen broken a year - my guess less than 50 each?

 

[RobF]

Disc brakes torture the spokes even more.

A cheap rear hub bike with disc brakes and a heavy rider is a very poor combination.

Freegos are decent bikes, but were I the OP, I would look for a front hub or crank drive.

 

[Deleted member 4366]

again - totally wrong.

You use shorter spokes on the drive side, so the spokes are shorter...

they should be the same tension on both sides!!!

I used to think that too, but then someone pointed out an online spoke tension calculator, which showed that the difference in spoke tension between each side increases substantially with the amount of dish. I confirmed that with my own calculations and then by actually feeling the spokes. Changing the length of the spokes doesn't stop the change in tension.

a 5mm offset gives a tension ratio of 151%
a 10% offset gives a tension ratio of 236%

 

[KTM Bike Industries UK]

they feel tighter when you ping the shorter ones with your fingers, the spoke tension should be within 90kgf to 100kgs.

The 300kgf figure is the force you need to break a spoke when it's being pulled.

Sapim publish their datasheet for all their 14G in PDF.
example:
sampim super spoke: middle section 1.4mm
tension strength = 2900N/mm2 * pi * 0.7^2 * /9.98m/sec2 = 629kgf
the 13 gauge stainless steel spokes on Chinese e-bike have 2.1mm diameter. The steel does not have as much high breaking tension strength, but even at 2000N/square mm, the breaking force is:
2000 * pi / 9.98 = 629kgf
just as strong as the Sapim super 14G spoke

ok...

but where on that datasheet does it suggest you should have different tension on the drive and non drive sides of a rear wheel? You do realise that if the non drive was tighter it would just pull the rim across to balance out the tension... and therefore take the wheel out of line as it would be over dished....thats how wheel building works.

and

you can't make sweeping statements about all spokes on all bikes from China. Thats simply going to cause confusion to people who read your posts and don't understand things.

you've plucked some numbers from the internet there and not really understood them, or the reasons why spokes break.

Cheap wheels, tend to suffer from breaking spokes for a range of reasons, and its to do with compromises made in all aspects of the wheels production to get them to the price point thats being demanded by the brand. The lack of flex in the spoke and poor wheel builds and badly finished holes in the hubs are all factors as to why the spokes break at the head.

All we're saying is you're perhaps best leaving offering advice about things like this to people who actually know, and not just repeating things you've read on-line.

 

[amigafan2003]

I've always built my own wheels and used 14 double butted spokes, including a MAC front motor running 2kw and I've never snapped a spoke.

Unless the bike is involved in an accident (derailleur going into the rear wheel for example) then 99% of all spoke failures are due to the wheel builder. The other 1% are due to manufacturing defects.

I've relaced ebike wheels and resused the cheap Chinese spokes they came with and those wheels have gone onto never break another spoke and wear out their rims first.

ok...

but where on that datasheet does it suggest you should have different tension on the drive and non drive sides of a rear wheel? You do realise that if the non drive was tighter it would just pull the rim across to balance out the tension... and therefore take the wheel out of line as it would be over dished....thats how wheel building works.

I have to chip in here and state you're wrong. If the angle of spokes on both sides were the same then yes, the spoke tension would be the same. However, the angle on the drive side is less and hence the spoke tension needs to be higher to stop the non-drive side pulling the wheel over.

The difference can be quite large - I built an 11 speed 150mm wheel last week and the drive side spokes were 30% tighter than the non drive side.

Incidentally, if you ever need to true a rear wheel you should only ever work on the non drive side as the high tension on the drive side risks rounding a nipple if you adjust that side and you risk screweing up the radial trueness. If you don't round a nipple if you do try to adjust the drive side then the spokes weren't tight enough when the wheel was built initially ;-)

Source:- Cytech level 3 MTB trained.

 

[KTM Bike Industries UK]

Col talks a lot about cheap Chinese spokes - ask all the suppliers on this forum how many spokes they have seen broken a year - my guess less than 50 each?

you've added this, since I replied.

I don't think I've every said anything that specific... i've been saying that the reason the spokes fail is due to a number of things, wheel build, rim quality, spoke quality etc etc. As I have already said, as a long standing wheel builder I know its possible to build a good wheel from lower quality components, just as its possible to build a bad wheel from good components.

I have stated that the straight guage spokes that are used, and seem to be the ones that fail most in discussions on here, are best replaced with double butted, and rebuilt by someone who knows what they are doing. This will solve the problem. Its not an eBike problem, its a wheel build problem.

 

[trex]

ok...

but where on that datasheet does it suggest you should have different tension on the drive and non drive sides of a rear wheel?

I didn't. All spokes must be evenly tensioned on each side. Wheels done by machine often put 10% less tension on the non-drive side. I did not recommend that either. I recommended that if you don't have a spoke tensiometer, you can ping the spokes with your fingers to find loosened spokes, that is the point where you need a £2 spoke key. Do them up before they break. And don't be alarmed if one side feels tighter because of dishing.
it's true that I usually pull figures out of the internet, but surely 'quantitative' is better than 'qualitative' like 'cheap' or 'poor'

 

[los monty]

Trex... where do you get these "facts" you state with such confidence?

This is total nonsense, there is not even such thing as eBike spokes!!!

and where have you read that 13g is 50% stronger than 14g - spokes are designed to flex and stretch to give the rim the ability to move a bit to give comfort and strength to the wheel as a whole. So a spoke being "stronger" isn't even something thats actually really a good thing.

The fact spokes break on cheap build eBikes, is because cheap normal bikes can cope with cheap spokes... cheap eBikes put lots of extra stress and weight onto cheap components and often badly built wheels with cheap rims. So the spokes break, because they are not able to cope with the work they are being asked to do. This could be due to the spoke being poor quality, the rim being poor quality, or the build being bad... or all 3.

A well built wheel can allow poor quality spokes and rims to last a bit longer. But when you're looking at any bikes, the wheels are one of the major things that can effect the feel, performance and value of a bike.

They are rotating weight so can make a big difference to how a bike feels, and if the brand has compromised on hubs, spokes or freehub bodies, this is something that people don't often spot when buying a bike. It only comes to light afterwards when spokes start snapping or hubs fail.

If you are looking at any eBike, you need to consider the value of the electrics and subtract that, and then think about what you're left with, and look at the components on that bike. These days most of the electrics from most brands are reliable and offer good performance, so the differences are more subtle, but equally important.

Another harsh criticism of so called cheap bikes.

Personally I have never spent more than £500 for a new ebike and the cheapest was £250 no broken spokes yet and enough left in the bank to repair them if it happens.

 

[smee]

Disc brakes torture the spokes even more.

A cheap rear hub bike with disc brakes and a heavy rider is a very poor combination.

Freegos are decent bikes, but were I the OP, I would look for a front hub or crank drive.

I am just putting my toe in the water, so to speak, with getting back into riding a bike, and crank drives all seem way more expensive around here (don't think I saw any front wheel drives).

Anyway, I have already ordered it, and it arrives tomorrow!!!

My question was more about knowing what is happening, and also what to do to help with the problem, if there was one.

 

[trex]

smee, should have joined the forum before ordering!
the site has a searchable database of all the models you can buy from sellers on the forum
http://www.pedelecs.co.uk/buy/find-an-electric-bike/
and post about what you want to buy - that will help you get the best bike for your budget.

 

[KTM Bike Industries UK]

Another harsh criticism of so called cheap bikes.
.

sorry but it wasn't intended to be a criticism of "cheap bikes" and cheap isn't something that can be defined anyway because its relative to the available budget of the customer.

one of the brands that seems to have had a major problem with this is BH, and they are neither "cheap" or Chinese. But they have specced' "cheap" wheels / spokes on their bikes clearly, which is why there is a problem.

 

[flecc]

cheap isn't something that can be defined anyway

Certainly can't be, in many ways our silliest word. Here's just two of the OED definitions of cheap:

Offering good value.

Of low value.

Clearly a word with bipolar disorder!

 

[smee]

smee, should have joined the forum before ordering!
the site has a searchable database of all the models you can buy from sellers on the forum
http://www.pedelecs.co.uk/buy/find-an-electric-bike/
and post about what you want to buy - that will help you get the best bike for your budget.

Except I am in Australia, and I want to buy locally for support/warranty etc. Not all brands are available locally. As it happens, for a rear hub drive, the Freego Eagle that I ordered, is one that is found by that search.