Back wheel to avoid broken spokes?

[ColinJ]

I have been descending Yorkshire hills at speeds often exceeding 50 mph for nearly 30 years, and my weight was often > 108 kgs (17 stone). The only spokes that I ever broke were in a very cheap and nasty old wheel with an unknown history, and a Mavic Aksium rear wheel which had been damaged by a mate's rear mech getting too friendly with the drive side spokes. (They definitely did snap rather than unhook on the Aksium but a post-mortem clearly showed deep scores across the spokes from the earlier damage.)

My way of avoiding wrecked wheels (and a wrecked ColinJ!) is to watch the road ahead of me. I do my best to steer round problems or occasionally bunnyhop over them. It works for me ...

 

[Profpointy]

Plenty has been written about spokes breaking here. I suggest you search for:

"metal fatigue"
"Double butted"
"Stress cycles"
"potholes" in relation to broken spokes.
"Stress relieving"


Potholes have NOTHING to do with your broken spokes, the lack of stress relieving, everything.

Could you explain a bit why potholes have nothing to do with it? Seems counter intuitive that an extra heavy biff won't ever damage or weaken a spoke.

 

[Yellow Saddle]

Could you explain a bit why potholes have nothing to do with it? Seems counter intuitive that an extra heavy biff won't ever damage or weaken a spoke.

The reason being that any load on a spoked wheel only reduces tension in the spokes. Loads are taken up between the hub and road - the hub pushes down on the road and the road pushes up towards the hub, via the spokes. Only the bottom two or three spokes take up that load and they do that by losing some tension. Note I didn't say they go out of tension, they only lose some tension.
If they each have say 1000N of tension (a typical value) and three spokes are in the load-affected zone, your weight of 80kg divided by two (for two wheels) puts a 40kg load on those three spokes. 40Kg = 400N. In other words, the three spokes with a total tension of 3 000N only lose 400N between the three of them.

All spokes outside the load affected zone (essentially the area above the contact patch) remain at exactly the same tension throughout the wheel's cycle. Nothing you can do on a bike can increase the overall tension on the spokes. Braking via a disc brake increases tension in every other spoke slightly (less than 3%) and decreases tension in every other spoke by a similar 3%, keeping overall tension the same.

If a BMX rider jumps off a ramp, only the bottom three or four spokes lose tension and none increase in tension.

 

[alicat]

Nonsense

What charm school did you go to, @Yellow Saddle?

 

[Profpointy]

The reason being that any load on a spoked wheel only reduces tension in the spokes. Loads are taken up between the hub and road - the hub pushes down on the road and the road pushes up towards the hub, via the spokes. Only the bottom two or three spokes take up that load and they do that by losing some tension. Note I didn't say they go out of tension, they only lose some tension.
If they each have say 1000N of tension (a typical value) and three spokes are in the load-affected zone, your weight of 80kg divided by two (for two wheels) puts a 40kg load on those three spokes. 40Kg = 400N. In other words, the three spokes with a total tension of 3 000N only lose 400N between the three of them.

All spokes outside the load affected zone (essentially the area above the contact patch) remain at exactly the same tension throughout the wheel's cycle. Nothing you can do on a bike can increase the overall tension on the spokes. Braking via a disc brake increases tension in every other spoke slightly (less than 3%) and decreases tension in every other spoke by a similar 3%, keeping overall tension the same.

If a BMX rider jumps off a ramp, only the bottom three or four spokes lose tension and none increase in tension.

Well OK (I think) but in an earlier post someone mentioned fatigue hence cyclical loads being the issue. If you biff a pothole aren't the cyclical loads considerabky greater hence worse ?

 

[PK99]

What charm school did you go to, @Yellow Saddle?

The Jobst Brandt school of wheel building and internet diplomacy.

 

[PK99]

The reason being that any load on a spoked wheel only reduces tension in the spokes. Loads are taken up between the hub and road - the hub pushes down on the road and the road pushes up towards the hub, via the spokes. Only the bottom two or three spokes take up that load and they do that by losing some tension. Note I didn't say they go out of tension, they only lose some tension.
.

The hub does not push down on the road and the spokes do not push up. If the spokes are in tension the lower spokes are always pulling down on the hub and up on the rim contact point, if they lose tension they neither push nor pull.

 

[Yellow Saddle]

The hub does not push down on the road and the spokes do not push up. If the spokes are in tension the lower spokes are always pulling down on the hub and up on the rim contact point, if they lose tension they neither push nor pull.

Maybe you should do a free force diagram and take a good look at it before you write things down.

 

[Yellow Saddle]

Well OK (I think) but in an earlier post someone mentioned fatigue hence cyclical loads being the issue. If you biff a pothole aren't the cyclical loads considerabky greater hence worse ?

I've explained the pothole phenomena here many times. The last time was here.

https://www.cyclechat.net/threads/t...r-wheels-consumable-parts.216133/post-4739618

Yes, a pothole cycle is considerably worse but remember, it can only cycle the spoke to zero tension (an un-stressed state) and not beyond. Further, hitting a pothole is a rare occurrence compared to the 50 000 stress cycles from riding just 100 kms.

 

[PK99]

Maybe you should do a free force diagram and take a good look at it before you write things down.

I would suggest you do the very same thing!

if I had access to a pc I'd post a simple sketch.

 

[Tim Hall]

@Yellow Saddle And those incantations they chant "holes thirty two ... three cross", could anything be more blatant?

Time to light the pyre I think.

Holes thirty two? What are you, some kind of deviant? Holes thirty six, three cross, 531 and don't you forget it.

 

[Profpointy]

I've explained the pothole phenomena here many times. The last time was here.

https://www.cyclechat.net/threads/t...r-wheels-consumable-parts.216133/post-4739618

Yes, a pothole cycle is considerably worse but remember, it can only cycle the spoke to zero tension (an un-stressed state) and not beyond. Further, hitting a pothole is a rare occurrence compared to the 50 000 stress cycles from riding just 100 kms.

Does the extra tension in the top spoke not matter? That can go up and up until something gives, though it may be thay something else will give first of course

 

[Yellow Saddle]

Does the extra tension in the top spoke not matter? That can go up and up until something gives, though it may be thay something else will give first of course

The upper spokes don't receive extra tension. The only spoke where tension change are those in the load affected zone. The hub does not hang from the top spokes but "stand" on the bottom spokes. I think if you search here for "stand" in my posts you should come across a full explanation. I wish there was a FAQ on here.
Think of it like this: when you look at a fully loaded car's wheels, the tyres are flattened at the bottom (load affected zone) but not stretched at the top. For them to be stretched at the top, they have to be attached to some form of sky hook (counter force) which isn't there.
However, if you sit on a wheel that's off the bike, you ovalise it. If you sit on a bike, you effectively sit on the hub and only squash the part between hub and road.

 

[adscrim]

The upper spokes don't receive extra tension. The only spoke where tension change are those in the load affected zone. The hub does not hang from the top spokes but "stand" on the bottom spokes. I think if you search here for "stand" in my posts you should come across a full explanation. I wish there was a FAQ on here.
Think of it like this: when you look at a fully loaded car's wheels, the tyres are flattened at the bottom (load affected zone) but not stretched at the top. For them to be stretched at the top, they have to be attached to some form of sky hook (counter force) which isn't there.
However, if you sit on a wheel that's off the bike, you ovalise it. If you sit on a bike, you effectively sit on the hub and only squash the part between hub and road.

How does the hub stand on the bottom spokes if the nipples are not attached to the rim? Wouldn't the nipples be constantly pushing into the rim tape and tube of a clincher tyre set-up?

 

[Yellow Saddle]

How does the hub stand on the bottom spokes if the nipples are not attached to the rim? Wouldn't the nipples be constantly pushing into the rim tape and tube of a clincher tyre set-up?

Because they never go out of tension. They have say, 100 units of tension to start with, and your load takes away 30. 70 remains. The nipples only lift the moment tension and load equals.

Think of a man and a child each hanging on a similar rope. The child's rope is not slack just because the child is lighter.