Spoke tension and buckling

[Ben C]

On 2007-10-12, jim beam <[email protected]> wrote:
> Ben C wrote:
[...]
>> I imagine aero rims would make the biggest difference in stiffness. MA-2
>> and Open Pro are roughly the same in cross-section I think.
>
> ma2 is about as shallow as you can get for a normal brake track width.
> open pro is nearly twice as deep.

Sounds like that's the significant difference then.

http://www.astounding.org.uk/ian/wheel/sectprops.html

Ian's model used 25x10mm as the box section, Open Pro is 18.4x19.6mm.
That is about twice as deep, and "oversquare".

 

[Peter Cole]

Ben C wrote:
> On 2007-10-11, jim beam <[email protected]> wrote:
> [...]
>> bottom line, you want spoke tension as it /does/ brace the rim. and it
>> prevents spoke nipples unscrewing. but you don't want to crank it up to
>> the limit. i see it as the intersection of two lines on a graph, one
>> positive slope, the other negative [spoke load capacity vs. rim].
>
> So on that graph, spoke load capacity is load-until-slack, and rim load
> capacity is load-until-yield. So I think that intersection is the same
> one I was talking about.
>
>> intersection is ideal spoke tension, not reducing capacity of the rim
>> simply to favor the spokes.
>
> Yes, although it would be interesting to be able to estimate what the
> ideal tension is for different rim/spoke count combinations. Needs an
> FEA really I think.
>
> And of course this isn't the only factor-- the ideal tension as given
> by that intersection might be too high and lead to spoke-hole fatigue.
> Maybe that's a "badly designed rim"-- spoke bed too crummy for available
> rim strength.
>
> I imagine aero rims would make the biggest difference in stiffness. MA-2
> and Open Pro are roughly the same in cross-section I think. What's the
> difference between them anyway (apart from anodizing, machined brake
> track, the way they join up the hoop)? Do they cook the Open Pro more to
> make it stiffer?

As you've seen, a wheel is basically a 2 spring system, where the spring
constants add. The spoke "spring" drops out when the spoke goes slack,
causing the aggregate spring constant to drop suddenly at that point by
a factor of about 3. The stiffness of the rim "spring" varies inversely
with the cube of the length, so that spring constant goes down rapidly
as spokes become unloaded. Putting the spokes closer together and/or
increasing the initial spoke tension is much more effective than
increasing rim stiffness in preventing flat spotting.

 

[Ben C]

On 2007-10-12, jim beam <[email protected]> wrote:
[...]
> ma2 is about as shallow as you can get for a normal brake track width.
> open pro is nearly twice as deep.

I'm wondering now how they got it to be only 10 or 15g heavier. Perhaps
the walls are almost half the thickness.

 

[jim beam]

Ben C wrote:
> On 2007-10-12, jim beam <[email protected]> wrote:
> [...]
>> ma2 is about as shallow as you can get for a normal brake track width.
>> open pro is nearly twice as deep.
>
> I'm wondering now how they got it to be only 10 or 15g heavier. Perhaps
> the walls are almost half the thickness.

mine's about 25g heavier fwiw, but that would be my guess, yes - i've
not sectioned a ma2 so can't post a pic.

 

[jim beam]

Peter Cole wrote:
> Ben C wrote:
>> On 2007-10-11, jim beam <[email protected]> wrote:
>> [...]
>>> bottom line, you want spoke tension as it /does/ brace the rim. and
>>> it prevents spoke nipples unscrewing. but you don't want to crank it
>>> up to the limit. i see it as the intersection of two lines on a
>>> graph, one positive slope, the other negative [spoke load capacity
>>> vs. rim].
>>
>> So on that graph, spoke load capacity is load-until-slack, and rim load
>> capacity is load-until-yield. So I think that intersection is the same
>> one I was talking about.
>>
>>> intersection is ideal spoke tension, not reducing capacity of the rim
>>> simply to favor the spokes.
>>
>> Yes, although it would be interesting to be able to estimate what the
>> ideal tension is for different rim/spoke count combinations. Needs an
>> FEA really I think.
>>
>> And of course this isn't the only factor-- the ideal tension as given
>> by that intersection might be too high and lead to spoke-hole fatigue.
>> Maybe that's a "badly designed rim"-- spoke bed too crummy for available
>> rim strength.
>>
>> I imagine aero rims would make the biggest difference in stiffness. MA-2
>> and Open Pro are roughly the same in cross-section I think. What's the
>> difference between them anyway (apart from anodizing, machined brake
>> track, the way they join up the hoop)? Do they cook the Open Pro more to
>> make it stiffer?
>
> As you've seen, a wheel is basically a 2 spring system, where the spring
> constants add. The spoke "spring" drops out when the spoke goes slack,
> causing the aggregate spring constant to drop suddenly at that point by
> a factor of about 3. The stiffness of the rim "spring" varies inversely
> with the cube of the length, so that spring constant goes down rapidly
> as spokes become unloaded. Putting the spokes closer together and/or
> increasing the initial spoke tension is much more effective than
> increasing rim stiffness in preventing flat spotting.

increasing rim stiffness is achieved by bracing with spokes. however,
cranking spoke tension up so that the rim approaches yield decreases the
load the "spring" can take before plastic deformation.

 

[Ben C]

On 2007-10-13, jim beam <[email protected]> wrote:
> Ben C wrote:
>> On 2007-10-12, jim beam <[email protected]> wrote:
>> [...]
>>> ma2 is about as shallow as you can get for a normal brake track width.
>>> open pro is nearly twice as deep.
>>
>> I'm wondering now how they got it to be only 10 or 15g heavier. Perhaps
>> the walls are almost half the thickness.
>
> mine's about 25g heavier fwiw, but that would be my guess, yes - i've
> not sectioned a ma2 so can't post a pic.

This page has cross-sections and information about various Mavic
products apparently from 1996:

http://www.bikepro.com/products/rims/mavicroad.html

The Open 4 looks similar to today's Open Pro. The rim they make now that
looks most like the MA-2 in cross-section is the Reflex Tubular.

 

[jim beam]

Ben C wrote:
> On 2007-10-13, jim beam <[email protected]> wrote:
>> Ben C wrote:
>>> On 2007-10-12, jim beam <[email protected]> wrote:
>>> [...]
>>>> ma2 is about as shallow as you can get for a normal brake track width.
>>>> open pro is nearly twice as deep.
>>> I'm wondering now how they got it to be only 10 or 15g heavier. Perhaps
>>> the walls are almost half the thickness.
>> mine's about 25g heavier fwiw, but that would be my guess, yes - i've
>> not sectioned a ma2 so can't post a pic.
>
> This page has cross-sections and information about various Mavic
> products apparently from 1996:
>
> http://www.bikepro.com/products/rims/mavicroad.html

"The MA2 uses two-piece, double wall stainless steel eyelets"

that's not quite true. the "socket" is plated steel that rusts. the
"rivet" is the only stainless part.


>
> The Open 4 looks similar to today's Open Pro. The rim they make now that
> looks most like the MA-2 in cross-section is the Reflex Tubular.