Classic cars forum & vehicle restoration.

oldtimer · Guest

No stop ,all you budding Maths/Science geniuses,who have already filled 3 blackboards attacking this "knotty" problem !
My query is purely hypothetical,has no practical application that I can think of,and should not be grounds for a raid on my place by the Health and Safety gang or any other jobsworthy with nothing better to do.
Now,with the disclaimer in place,........". A nos moutons " as GREENEY might say.
Consider two otherwise identical hydraulic systems,the only difference being that one is connected throughout with, for the purpose of this scenario, 1/4 inch pipe and the other with 1/2 inch.
What, if any,is the effect on pedal pressure,for example,would one appear to have an apparent servo effect when compared to the other?
If you all have more pressing problems I will understand.

oldtimer

peter scott · Joined: 18 Dec 2007 Posts: 7219 Location: Edinburgh

Hi John,

When you press the pedal a certain volume of fluid needs to pass through the pipe to move the piston at the far end. To understand the effect of pipe diameter it's probably best to consider a more extreme example.

Let's assume the smaller pipe is 1/16" rather than 1/4". We know that the volume of fluid that is needed to move the piston is the same in both cases but in our 1/16" dia pipe the resistance to flow will be higher than in the 1/2"
pipe. The fluid will probably travel at a faster speed but it will take a longer time for the required volume to pass along the pipe and the person pressing the pedal will notice that it just isn't going down as fast as it should.

The effect of the smaller pipe is therefor to slow down the process of applying the brakes. There won't be any mechanical advantage of one system over the other. The mechanical advantage is purely determined by the relative diameters of the master and slave pistons.

HTH

Peter
_________________
https://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

oldtimer · Guest

Hello Peter,
My hero,I knew you would very likely be in the forefront of those ready to explain to the Slightly Bewildered,of which group I am President for Life,in a way even I can understand.
Sadly,for me, the groves of academe are more widely known as THE COOK AND BARKER and the incumbents are invariably wrestling with weightier problems e.g.whether to have a pint of Worthington or a lager.
My thanks,again, for help present and past.I am now heading for" the groves of academe"to impress the inmates with my recently acquired knowledge!
Regards
John (oldtimer)

pigtin · Joined: 23 Nov 2007 Posts: 1879 Location: Herne Bay

Without the use of maths or science (I'm a total duffer at maths.)

With this sort of problem I tend to think of current flow in an electrical system:
However heavy the wire, it will only have to carry the current drawn from it. If the wire is too thin there will be a current drop, restricting the power available to the recipient. The available supply must always be sufficient.

Having said that: with a hydraulic system there would be extra fluid stored in larger pipes and although it is not compressible it might be more difficult to bleed and the chances of an air or moisture problem might be greater. But I have no expertise in hydraulics and may be wrong about this.

Turning the whole thing on its head: do you know that back in the 60s some bright spark came up with the idea of pneumatic transistors? They could run a control logic system with compressed air. The company I worked for, at the time, was actually considering using it. Confused

_________________
Due to the onset of my mid eighties I'm no longer sprightly and rarely seen in my Austin special. I have written a book though. https://amzn.eu/d/7rwRRqL

peter scott · Joined: 18 Dec 2007 Posts: 7219 Location: Edinburgh

dalbuie · Joined: 24 Dec 2007 Posts: 408 Location: Gullane

In the similar way I think most front brake cylinders are a wider bore than the rear cylinders giving more braking power to the front.

oldtimer · Guest

Thank you Pigtin,
It is great to find a kindred spirit who was no good at Maths/ Science but who can use some lateral thinking and apply it outside a field he is comfortable with.In my case I felt I was more comfortable with languages until the French master wrote, on a school report, "Trying,very!"
oldtimer

pigtin · Joined: 23 Nov 2007 Posts: 1879 Location: Herne Bay

Had it not been for the invention of the pocket calculator my employers would undoubtably have 'found me out' and I wouldn't have ended up running technical services. Embarassed

_________________
Due to the onset of my mid eighties I'm no longer sprightly and rarely seen in my Austin special. I have written a book though. https://amzn.eu/d/7rwRRqL

62rebel · Posted: Fri Oct 24, 2008 11:14 pm Post subject:

from Naval service; doubling the size of a given pipe Quadruples the volume it carries. at least that's what i was taught before the mast. fire mains correspondingly are large diameter nearer the source and get smaller nearer the outlet to maintain pressure/volume. of course i did drink quite heavily in those days.... and facts could go horribly awry. as could the treadle beam at the mill.

oldtimer · Guest

Hello 62 rebel,
Thank you for your input.I never cease to be amazed at the knowledge available on the forum. It seems that one can ask about the most obscure matter and someone will be able to help and,more surprisingly,can be bothered to do so!
oldtimer

Jim Walker · Posted: Tue Oct 28, 2008 6:50 pm Post subject:

No one has mentioned that a larger pipe would have a greater internal area and would therefore be more likely to burst unless the wall thickness was increased. By what factor?

Jim.

peter scott · Joined: 18 Dec 2007 Posts: 7219 Location: Edinburgh

Jim, I'm a bit puzzled by the increased likelyhood of fracture. Surely the pressure exerted against the pipe walls is unchanged by pipe diameter. The pressure in all cases is simply the force applied to the master cylinder piston divided by the master cylinder piston area.

Is the tendancy for a pipe wall to fracture not directly related to the pressure?

Peter
_________________
https://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

dalbuie · Joined: 24 Dec 2007 Posts: 408 Location: Gullane

Wall thickness certainly increases as pipe diameter increases - don't know the maths - but I would imaging all the correct pipes purchased have been through the correct stress testing.

Jim Walker · Posted: Wed Oct 29, 2008 12:34 am Post subject:

You are quite right about the pressure Peter. But the larger pipe has a greater internal area on which that pressure works, thus exerting a greater radial force. For an identical increasing pressure the larger pipe will burst first if both have identical wall thickness. Compare the pressure a cycle tyre will withstand compared with a car tyre of greater section and with thicker walls.

Jim.

peter scott · Joined: 18 Dec 2007 Posts: 7219 Location: Edinburgh

Hi Jim and Dalbuie,

Thanks for puting me straight on that. Having not paid any attention to accepted wisdom I just felt that wall thickness alone would determine the pressure limit. But I see that in general the limit pressure is directly proportional to wall thickness and yield strength but also inversely proportional to pipe diameter. (source: http://internet.ktu.lt/lt/mokslas/zurnalai/mechanika/mech_54/Bareisa454.pdf)

So in order to achieve the same pressure rating it looks like you need to maintain the same ratio of wall thickness to pipe diameter.

Peter
_________________
https://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon