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+--- Thread: Query about gearing... (/showthread.php?tid=4913)
RE: Query about gearing... - Parazine - 18-09-2020
Howard Wright Wrote: It always confuses me when I’m told friction is a function of perpendicular force not area of friction material. Theoretically 1” brake shoes should perform the same as 1 1/4” shoes. But that doesn’t seem to be the case in practice.
It always confuses my students as it did me as a student in the 'seventies.The formula for simple friction force, f = mu x N, where mu is the coefficient of friction and N is the normal (i.e. right angle) force. You can get into more complicated formulae but.....
A 1" wide shoe will produce the same friction force as a 1 1/4" BUT the force has to be the same, that's the catch! (And mu has to be the same, which is a different argument)
The real benefit is that a 1 1/4" shoe dissipates heat better so less prone to fade although, subjectively, they do seem to work better and defy the mechanics lesson.....
RE: Query about gearing... - Renaud - 18-09-2020
With the same force and twice the surface the friction is the same because the pressure is halved. In my opinion that is!
RE: Query about gearing... - Bob Culver - 18-09-2020
Hi Howard
According to the not high tech book The Sports Car by Campbell, rolling resitance, due mainly some cahracteristic of the tyres, increases vaguely as the square of speed, and directly with weight. For unspecified probably crossply tyres at 35 psi and an unspecified, vehicle rolling resistance per 1000 lb given as about 2 1/2 hp at 50 mph, 7hp at 100 mph.. Not clear if it includes loss thru the gearbox and diff.
The air resistance of a modern with the same cross section is only about half that of a Seven. Moderns are heavy cf cars of the late 60s so the two losses may roughly match at 50 mph, but on a Seven the wind resistance dominates
Wind hp is given as Cd x area x vel cubed (mph) all divided by 146,600
From the same Autocar article Cd Ford Pop 1948 .493, VW Beetle .48, Consul mk1 .455, BMC 1100 .453, Jowett .39, Porsche 356 coupe .333, Citroen DS .326. Open topped sports cars worse than coupe equivalent.
RE: Query about gearing... - Malcolm Parker - 18-09-2020
Duncan, I have a standard manifold fitted with a Zenith 26VA jetted 75/57 which you are welcome to try. Years ago it was fitted to an engine in a special and it worked well.
I would be happy to nip down to Boroughbridge with it.
RE: Query about gearing... - Tony Press - 18-09-2020
(18-09-2020, 10:23 AM)Parazine Wrote:My late uncle, chief engineer at a large engineering company. always claimed that friction couldn't be independentHoward Wright Wrote: It always confuses me when I’m told friction is a function of perpendicular force not area of friction material. Theoretically 1” brake shoes should perform the same as 1 1/4” shoes. But that doesn’t seem to be the case in practice.
It always confuses my students as it did me as a student in the 'seventies.The formula for simple friction force, f = mu x N, where mu is the coefficient of friction and N is the normal (i.e. right angle) force. You can get into more complicated formulae but.....
A 1" wide shoe will produce the same friction force as a 1 1/4" BUT the force has to be the same, that's the catch! (And mu has to be the same, which is a different argument)
The real benefit is that a 1 1/4" shoe dissipates heat better so less prone to fade although, subjectively, they do seem to work better and defy the mechanics lesson.....
of the area of contact, citing the difficulty in separating interleaved telephone books.
But if I remember this is in fact due to the normal force being multiplied
RE: Query about gearing... - Duncan Grimmond - 19-09-2020
Malcolm, that would be very helpful. I'm planning to whip the engine out for further investigation early next week....
RE: Query about gearing... - Malcolm Parker - 19-09-2020
Steve and myself are available Monday 21st if you would like us to call in before you whip the engine out. If I bring the manifold and carb and Steve brings a known distributor it might do the trick.
RE: Query about gearing... - Dave Wortley - 19-09-2020
Malcolm and Steve. Austin Seven Friends indeed!
RE: Query about gearing... - Steve Jones - 19-09-2020
(19-09-2020, 12:56 PM)Dave Wortley Wrote: Malcolm and Steve. Austin Seven Friends indeed!
Yes we are, Dave, but we should confess that we need to meet up to conclude a bit of 'business' so a trip to Duncan's provides a perfect opportunity and, hopefully, help for a friend as well.
Steve
RE: Query about gearing... - Duncan Grimmond - 20-09-2020
I'm looking forward to tomorrow in hope I won't have to pull the motor...