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Brake servos.
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Ray White



Joined: 02 Dec 2014
Posts: 6282
Location: Derby

PostPosted: Fri Jul 23, 2021 1:41 pm    Post subject: Brake servos. Reply with quote

This may be a bit technical for the home mechanic but I have noticed that brake servos are available with a range of different pressure assistance and I wondered if anyone knows of a formula or "rule of thumb" method of establishing the optimum ratio. ?

I have seen servos with between 1.65: 1 through to 3:1 - although I expect there are many more ratios available.
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Bitumen Boy



Joined: 26 Jan 2012
Posts: 1733
Location: Above the snow line in old Monmouthshire

PostPosted: Fri Jul 23, 2021 10:27 pm    Post subject: Reply with quote

It's something I'm curious about too, Ray. Most folk that do fit servos seem to go on the experience of owners of similar cars, as far as I can make out. The optimum ratio may well depend on the preference of the driver...
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Ray White



Joined: 02 Dec 2014
Posts: 6282
Location: Derby

PostPosted: Fri Jul 23, 2021 11:39 pm    Post subject: Reply with quote

Bitumen Boy wrote:
It's something I'm curious about too, Ray. Most folk that do fit servos seem to go on the experience of owners of similar cars, as far as I can make out. The optimum ratio may well depend on the preference of the driver...


I have been doing a bit of research and it looks like the heavier the vehicle the higher the ratio; which would make sense but there is also the diameter of the master and slave cylinders to take into account.

I think I should clarify that I do understand that a servo will do nothing to improve a car's brakes; just reduce the effort required.

Here are a few examples:

https://www.scparts.co.uk/pdf/533613.pdf

According to MG guru Eric Worpe, the TC front wheel cylinder ( for example) has a piston diameter of 1" ... giving a surface area of 0.785 sq. inches. The rear wheel cylinder's piston area is 0.875 inches in diameter giving a surface area of 0.60 sq. inches. This gives an applied force ratio 57% to the front and 43% to the rear.

Clearly enlarging the surface area of the pistons is not an option. The main mechanical advantage is through the brake pedal which is 10 inches long; with the master cylinder operating lever just 2.5 inches there is a mechanical advantage of 4.

Under hard braking, foot pressure on the pedal can reach 100 ft/lb producing a force of 400 ft/lb at the master cylinder, with the 0.6 square inches of piston area reaching a pressure of 670 psi. Such a hydraulic pressure gives a braking force of 525 ft/lb at the front shoes and 400 ft/lb at the rears.

A remote servo is a popular option among older owners who want to get the most from their brakes but find they can no longer rely on leg power especially in an emergency.

Personally, I have suffered from a medial collateral ligament injury which could return unexpectedly.


Last edited by Ray White on Mon Jul 26, 2021 5:34 pm; edited 1 time in total
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lowdrag



Joined: 10 Apr 2009
Posts: 1585
Location: Le Mans

PostPosted: Sat Jul 24, 2021 7:36 am    Post subject: Reply with quote

I was told a few years back that for racing preparation they have found that with the new type of pads now available they have no need of a servo any more.
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Ray White



Joined: 02 Dec 2014
Posts: 6282
Location: Derby

PostPosted: Sat Jul 24, 2021 8:28 am    Post subject: Reply with quote

lowdrag wrote:
I was told a few years back that for racing preparation they have found that with the new type of pads now available they have no need of a servo any more.


I am not surprised that new materials have moved things along; especially in racing.

With my new braking system I have tried to keep appearances traditional by fitting "Alfin" type brake drums. TC racing enthusiast, Peter Edney (yes, the guy who had that disastrous fire) has developed special brake shoes using Mintex halo material to give improved performance without losing feel at slower "around town" speeds. These shoes are both bonded AND riveted.

I might add that the investment has been substantial.

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Ray White



Joined: 02 Dec 2014
Posts: 6282
Location: Derby

PostPosted: Wed Jul 28, 2021 7:47 pm    Post subject: Reply with quote

Sorry to bring this subject up again but I have a question that I am having trouble in getting an answer to.

I have seen an MG TD installation with the remote servo situated in the space just ahead of the rear axle. The brake master cylinder is in the footwell so the feed pipe is quite long. The supply pipe from the servo to the brake lines, however, is short ..so much closer to the rear brakes than the fronts.


My question is : does it actually matter where in the system the pressure is supplied? Usually, the servo is close to the master cylinder in the engine compartment.

(Incidentally, 'Car Builder Solutions' don't see a problem - although they did question the length of the vacuum line).
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badhuis



Joined: 20 Aug 2008
Posts: 1390
Location: Netherlands

PostPosted: Thu Jul 29, 2021 10:04 am    Post subject: Reply with quote

Don't think it matters as long as the pipes and hoses are air and fluid tight. The Sunbeam Imp Sport had a servo mounted near the engine in the back of the car.
Years ago for a client I mounted a servo up in the LH footwell of his MG TF.
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alastairq



Joined: 14 Oct 2016
Posts: 1950
Location: East Yorkshire

PostPosted: Thu Jul 29, 2021 10:21 am    Post subject: Reply with quote

Quote:
My question is : does it actually matter where in the system the pressure is supplied? Usually, the servo is close to the master cylinder in the engine compartment.


Skoda 105/120/130/136 Estelle & RApids.
All had brake servos up front where the master cylinder is located.
Yet the engine is at the rear!
The Vacuum hose[a largish bore, possibly near 10 millies]....covered in green thread....would be 10 or 12 feet long, from engine manifold down underneath the car, and up into the front luggage compartment. All one piece.
Brakes on these Skodas really are amongst the best, too. [From personal experience]
_________________
Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
----------------------------------------------
Ford Pop chassis, Ashley 1172 bodyshell, in pieces.
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Ray White



Joined: 02 Dec 2014
Posts: 6282
Location: Derby

PostPosted: Thu Jul 29, 2021 11:36 am    Post subject: Reply with quote

alastairq wrote:
Quote:
My question is : does it actually matter where in the system the pressure is supplied? Usually, the servo is close to the master cylinder in the engine compartment.


Skoda 105/120/130/136 Estelle & RApids.
All had brake servos up front where the master cylinder is located.
Yet the engine is at the rear!
The Vacuum hose[a largish bore, possibly near 10 millies]....covered in green thread....would be 10 or 12 feet long, from engine manifold down underneath the car, and up into the front luggage compartment. All one piece.
Brakes on these Skodas really are amongst the best, too. [From personal experience]


Well that answers that question. Thank you Alastair; I would not have known.

My original point was concerning the point at which the hydraulic pressure connects to the brake lines. As I said, I can't see a problem with the point of entry being at the back.

It is an interesting task and as you know, I like to get get creative sometimes!

I am trying to decide if I can use the existing inlet manifold take off point (that I will need for the boost/vacuum gauge) with some kind of three way union? .... or should I tap a new hole at the opposite end of the manifold and have a dedicated brake vacuum fitting - maybe like the one I have seen which has a non return valve built in.?
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alastairq



Joined: 14 Oct 2016
Posts: 1950
Location: East Yorkshire

PostPosted: Thu Jul 29, 2021 2:38 pm    Post subject: Reply with quote

As an idea, why not take a gander at what servo'd cars [of old, easier to see?] use for the vacuum pipe bores? I suspect a bore size to suit a gauge may be a trifle small? Most servo lines I have seen are quite large when compared to a vac take off for a distributor, for example?
_________________
Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
----------------------------------------------
Ford Pop chassis, Ashley 1172 bodyshell, in pieces.
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Ray White



Joined: 02 Dec 2014
Posts: 6282
Location: Derby

PostPosted: Thu Jul 29, 2021 3:33 pm    Post subject: Reply with quote

Yes, the vacuum hose for the servo is rather bulky so I will opt for a separate take off. There is a flat on the opposite end of the inlet manifold - a mirror image - that can be drilled and tapped. In fact a BSP adapter is included with the Kit.

I have now bitten the bullet and opted for a British built version of the original Lockheed 1.9:1 servo; as opposed to the cheap and cheerful Chinese version. The price difference is massive but I have come unstuck buying Chinese crap off eBay before.

Correction.... I have cancelled the order. The supplier claimed in their advert that their servos were newly made in the U.K. I had hoped for a better quality product and expected it to be expensive.

However, I noticed the same thing advertised on eBay made in China.

It's the difference between £52 and £287. FOR EXACTLY THE SAME THING! Mad
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Richard H



Joined: 03 Apr 2009
Posts: 2148
Location: Lincolnshire, UK

PostPosted: Wed Aug 04, 2021 7:26 pm    Post subject: Reply with quote

I don’t know how you got on with the servo you bought Ray, but be very wary of the Chinese ones. I have encountered several people who have bought them. They are total rubbish and several don’t work out of the box.
The British made ones look outwardly identical, but are, in comparison, very good.
I sell a lot through my company and although they aren’t cheap, we’ve only had one back under warranty to my knowledge.
It’s the old chestnut of buy cheap-buy twice with these things! I wouldn’t touch the Chinese ones with a barge pole.

As for the optimum ratio, good servos should have a boost ratio of 2:1.
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