Bridge down in Northolt - Paddington Arm

[MtB]

34 minutes ago, Ken X said:

The forks, (why are they angled forwards and what loads do they see, particularly during braking?)

 

Disc brakes or rim?

 

 

[Ken X]

35 minutes ago, MtB said:

 

Disc brakes or rim?

 

 

Exactly,

If we got a really bright candidate we would produce a white board and pen and start throwing a few numbers and calculations around. Some of our interviews suffered from spectacular thread drift, just like this 😀

[David Mack]

3 hours ago, IanD said:

Yes, unless the spoke tension is *far* too low or the rider is *far* too heavy... 😉

 

Each spoke has typically 100kg or so of tension, which is why they go "ping".

 

Assume a wheel has 32 spokes, and to simplify things split them into 8 "top", 16 "side", and 8 "bottom".

 

With no rider on the bike, all the  spokes have 100kg of tension in them, which is what keeps the wheel rigid.

 

Even with a 400kg rider (!!!) over one wheel the top spoke tension would only increase by 25kg/spoke to 125kg/spoke (200kg "pulling up") and the bottom spoke tension would decrease by 25kg/spoke to 75kg/spoke (200kg less "pulling down").

It's more complicated than that. Because with the weight of the rider pulling down on the top of the rim (via the top spokes), the ring tends to squash a little, becoming less tall and a little wider, which is resisted by an increased tension in the side spokes. 

 

And even with the heaviest rider and lightly tensioned spokes, the bottom spokes will never go into compression, because as soon as the initial tension is removed, the spokes will go loose. They can't carry any compressive load.

[MtB]

57 minutes ago, David Mack said:

And even with the heaviest rider and lightly tensioned spokes, the bottom spokes will never go into compression,

 

Ahem, they do under really heavy braking. And then the wheel folds. DAMHIK.

 

 

[IanD]

1 hour ago, David Mack said:

It's more complicated than that. Because with the weight of the rider pulling down on the top of the rim (via the top spokes), the ring tends to squash a little, becoming less tall and a little wider, which is resisted by an increased tension in the side spokes. 

 

And even with the heaviest rider and lightly tensioned spokes, the bottom spokes will never go into compression, because as soon as the initial tension is removed, the spokes will go loose. They can't carry any compressive load.

I know all that, but the request was for a simple explanation that a non-engineer could understand... 😉

 

If you want to get into the details -- including what actually happens with a circular wheel not the square one I assumed for simplicity -- then you also need to consider the difference between traditional alloy rims and the far stiffer carbon fibre ones used on racing bikes, which also have a much deeper and more rigid cross-section which hardly flexes at all. And they often don't have wire spokes in tension at all, they have integral moulded CF ones which also work in compression and torsion. 

 

But this is the wrong forum for that... 😉

[Scholar Gypsy]

Then there is the nice problem of what happens when you stand next to a bicycle, with the pedal at the lowest position, and pull the pedal backwards. Neat video here. Not sure what this has to do with bridges but it's fun ....

https://www.scientificamerican.com/article/mathematical-impressions-the-bicycle-pulling-puzzle/ 

[David Mack]

5 hours ago, Scholar Gypsy said:

https://www.scientificamerican.com/article/mathematical-impressions-the-bicycle-pulling-puzzle/ 

Another video from the same source taking a mathematical approach to a subject which will be of interest to some on here.

 

 

[Scholar Gypsy]

On 23/03/2023 at 23:27, David Mack said:

Another video from the same source taking a mathematical approach to a subject which will be of interest to some on here.

 

 

Thanks - my great uncle Walter was a famous Oxfordshire bellringer.  Here is my favourite bellringing video... 

https://www.itv.com/news/update/2012-05-26/diamond-jubilee-pageant-bells-ring-out-in-rehearsal/

[MtB]

On 23/03/2023 at 23:27, David Mack said:

Another video from the same source taking a mathematical approach to a subject which will be of interest to some on here.

 

 

 

 

Curiously we call this "method ringing" here in Ingerland. Change ringing is something else. 

 

Ten bells is difficult, these chaps and chapesses in the video are really good! 

 

 

[ditchcrawler]

On 20/03/2023 at 11:14, IanD said:

Blimey, that was quick -- I stand corrected! 🙂

There was a picture on Facebook of a boat steaming through a few days ago before it was cleared

[MtB]

2 hours ago, ditchcrawler said:

There was a picture on Facebook of a boat steaming through a few days ago before it was cleared

 

Just as I proposed earlier then eh?!!

 

 

[ditchcrawler]

2 hours ago, MtB said:

 

Just as I proposed earlier then eh?!!

 

 

Spot on