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

[IanD]

Well it's June 26th...

 

P.S. Also an example of towpath surface round here... 😉

 

Edited June 26, 2023 by IanD

[magnetman]

Is that a loose gravel top surface ? 

 

I wonder how many people if shown that would call it 'tarmacking'. 

i don't think those tiny little stones move about much.

Hopefully someone got a nice bit of firewood from the previous terribly poor quality bridge deck. It will be interesting to see the replacement.

 

 

[IanD]

Just now, magnetman said:

Is that a loose gravel top surface ? 

 

I wonder how many people if shown that would call it 'tarmacking'. 

 

That's quite an old path now (several years) so most of the loose gravel has been compacted down or disappeared into the bushes or the cut.

 

They can call it what they want to, it doesn't mean they're right... 😉

 

(according to polls, most people -- the majority -- think that "the yoof" account for the majority of crime (more than 50%), but they're wrong -- it's about 20% IIRC. Lots of other examples about what most people think being wrong...)

[magnetman]

3 minutes ago, IanD said:

 

That's quite an old path now (several years) so most of the loose gravel has been compacted down or disappeared into the bushes or the cut.

 

So its basically a road then. Yes I know it isn't suitable for Mr Tesla or Mrs Artic but it is a road surface which means rapid transit on wheeled vehicles. 

 

I think this is what the OP was getting at. 

 

 

I believe the concern raised with Mr Parry was around the principle of applying surfaces to towpaths which encourage rapid 2WV transit. I really doubt they were worried about whether the surface itself was black or a sand colour. 

 

 

 

He (Parry) has responded by saying the CRT are not tarmacking towpaths. This is using a brand name to steer away from the real subject which has nothing to do with tarmac and a lot to do with creating roads on towpaths. 

 

I don't really mind as I never have the misfortune to be on one of these paths in the first place these days but I still think it is dodgy. 

 

Inevitable I expect. 

[IanD]

1 hour ago, magnetman said:

So its basically a road then. Yes I know it isn't suitable for Mr Tesla or Mrs Artic but it is a road surface which means rapid transit on wheeled vehicles. 

 

I think this is what the OP was getting at. 

 

I believe the concern raised with Mr Parry was around the principle of applying surfaces to towpaths which encourage rapid 2WV transit. I really doubt they were worried about whether the surface itself was black or a sand colour. 

 

 

If it's wide enough for cars (and they're allowed on it), it's a road.

It it's too narrow for cars (and they're not allowed on it), it's a path.

The picture shows a (tow)path 🙂

 

https://www.englishforums.com/English/RoadWayAndPath/cgrbm/post.htm

 

Please suggest what other type of surface gives good all-year-round access for walkers, mums pushing buggies, people pushing wheelchairs -- oh yes, and sensible-speed cyclists -- while preventing use by high-speed ebikes/escooters etc.

 

P.S. The answer is not a compacted dirt surface, any dips in these turn into muddy swamps in winter.

Edited June 26, 2023 by IanD

[Tim Lewis]

On 17/03/2023 at 20:24, Tim Lewis said:

Its replacement is being craned in this week 😀

Edited June 27, 2023 by Tim Lewis

[IanD]

10 minutes ago, Tim Lewis said:

Its replacement is being craned in this week 😀

Which is what the photo I posted showed -- signs saying it should be happening starting yesterday, but no action yet...

[Scholar Gypsy]

The new bridge. Photo taken this morning by David L on the way back from SPCC tideway cruise

[IanD]

10 minutes ago, Scholar Gypsy said:

The new bridge. Photo taken this morning by David L on the way back from SPCC tideway cruise

Blimey, that went up quick -- there was no sign of life on Monday afternoon, Tuesday morning it's up!

 

Still looks blocked off for pedestrians though, maybe that'll be cleared soon -- or even by now... 😉

[Scholar Gypsy]

Just now, IanD said:

Blimey, that went up quick -- there was no sign of life on Monday afternoon, Tuesday morning it's up!

 

Still looks blocked off for pedestrians though, maybe that'll be cleared soon -- or even by now... 😉

A good example of measure three times, cut once.  It was pretty clear they were reusing the existing abutments.

[matty40s]

1 hour ago, IanD said:

 

Still looks blocked off for pedestrians though, maybe that'll be cleared soon -- or even by now... 😉

Just because theyve put the bridge back doesnt mean that it will ever actually open, the old one was closed for years.

[oboat]

On 17/03/2023 at 21:09, magnetman said:

I don't think it is treated. Surely It is hardwood.

 

I've not had the awful misfortune to be in that area for a couple of yars but One does note that it appears to already have been closed off.

 

Safety for pedestrians passing over was perhaps viewed as more important than those passing under it.

 

 

 

It has been closed off for some years pre CV19 and did look unsafe. Only needed some kids playing on it to bring it down.