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Air over hydraulic brakes ?


R Cubed

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If you are thinking what i think you are thinking (err???), then it would be difficult to fit a straight air system to a GMC as the actuators work in a different way. Air systems have the actuator mounted where it can operate a cam shaft or push rod to apply the brakes. Most hydraulic systems have the brake cylinders mounted on the back plate directly acting on the shoes (some Bedfords had remote hydraulic cylinders on the rear axle operating a push rod).

 

Because of the pressures involved, hydraulic actuators are much more compact than air actuators. Air over hydraulic systems are used (I think) because the mechanical set up of the brakes on the back plate is very simple. Unfortunately the rest of the system is not! Air hydraulic systems create higher pressures at the shoe than a vacuum/hydraulic systems using smaller components. A vacuum resevoir is generally larger than an equivalent air receiver therefore takes up more space under a vehicle.

 

All modern air systems aire split systems, some older vehicles systems ie Matador and Ward LaFrance (yes Mike i know yours is a twin) are single cicuit systems. Just to confuse things, AEC used air, air/hydraulic and vacuum/hydraulic systems on Matadors!

 

Routemaster brakes are fiendishly complex (power pressure hydraulic) but have great pedal feedback and excellent stopping power for the time. Older air systems do lack a bit of feel compared to hydraulic systems, but modern ones have great feel.

 

Hope this info is of help.

Edited by Grasshopper
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Is your question is related to a possible GMC engine conversion?

 

In which case you are thinking about the possible merit of using air / hydraulic rather than vacuum / hydraulic?

 

The window wipers will go better :D

 

:cool2::cool2::cool2::cool2::cool2::cool2:

 

maybe !!!!!!!!!

 

I am trying to understand how this type of system worked I thought that the foot pedal was on air and the only hydraulic bit was from the air over hydraulic master/slave cylinder unit to the wheel cylinders but it seems that it is just like the vac/hydraulic set up on the GMC but with air pressure rather than vac !!!

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The Stalwart is air over hydraulic - when the system works right it can stand the beast on it's nose!! There is a hydraulic feed from the pedals to the air packs which opens a valve in the pack and applies the air pressure from that point on. Circuits are split - hence twin master cylinders that cover the front and rear wheels on side and the centre wheel on the other side to give a balanced braking in the event one system fails (sore point right now). It's a complex system compared to plain air or hydraulic. If you want - I have a scan of the schematic here somewhere?

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And Foden used Hydraulic over haydraulic on the Foden FG8/80 amongst others. A non servo system used brake fluid from a foot pedal, acting directly on some of the wheel cylinders, but two of the three axles had this hydraulic brake fluid acting on a servo were the additional power was from the engine oil pressure feed system, and the Engines lubrication pump output was therefore proportinally fed to these wheel cylinders(if I remember correctly....)

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:cool2::cool2::cool2::cool2::cool2::cool2:

 

maybe !!!!!!!!!

 

I am trying to understand how this type of system worked I thought that the foot pedal was on air and the only hydraulic bit was from the air over hydraulic master/slave cylinder unit to the wheel cylinders but it seems that it is just like the vac/hydraulic set up on the GMC but with air pressure rather than vac !!!

 

The system you describe is how a standard air/hydraulic ssytem works. There are many other weird and wonderful variants as others have described!

 

I think a 3.5 tonne Rio is what you are really after, it has air assisted hydraulic brakes (like a jimmy but with air) and a diesel engine, plus the advantage of a synchro box. The cab is soft top too...:D

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One important point to consider, R3. If you wish to put an air / hydraulic servo on, you will need one with sufficient capacity (in terms of volume of fluid pumped) to operate all six wheel cylinders. Any modernish truck over 7.5t or having more than two axles is not likely to have an air / hydraulic system.

 

You will most likely find that these modern servos (e.g. from a 7.5t truck or even Bedford MJ) will only have sufficient capacity to operate four cylinders, and then only two from each of the two circuits.

 

Finding one with enough capacity will not be easy!

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One important point to consider, R3. If you wish to put an air / hydraulic servo on, you will need one with sufficient capacity (in terms of volume of fluid pumped) to operate all six wheel cylinders. Any modernish truck over 7.5t or having more than two axles is not likely to have an air / hydraulic system.

 

You will most likely find that these modern servos (e.g. from a 7.5t truck or even Bedford MJ) will only have sufficient capacity to operate four cylinders, and then only two from each of the two circuits.

 

Finding one with enough capacity will not be easy!

 

Transplant a Stollys - 6 wheels and 14 tons capacity. I suspect a GMC would stop rather quickly then!!! :D

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Transplant a Stollys - 6 wheels and 14 tons capacity. I suspect a GMC would stop rather quickly then!!! :D

 

Stollys are on disc brakes though and Jimmys are on drums. Disc pads tend to run closer to the surface and need a lot less fluid movement to apply pressure. Disc calipers also tend to require higher fluid pressure as the braking friction surface is a lot less than drums.

 

Isn't home modification of the braking system going to be frowned upon by the authorities, VOSA and your insurance to name a few?

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Stormin - I guess if you are just replacing an automotive application power booster like a GMC Hydrovac with another automotive application booster like an air over hydraulic booster from a truck of comparable brake fluid displacement, then that might be seen as quite acceptable -

 

If you were to convert from single line to dual line, or start altering actuators then your point could be very important.

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I can't really see the point of altering the brakes on a jimmy, the one I used to own and others I have driven had better brakes than a lot of modern HGV's I've driven which obviously had full air brakes. Properly maintained and set up GMC brakes will lock all wheels without using excess pedal pressure and yes before you ask I have used them fully laden.

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Errr.........a Stolly Airpak only works on 3 wheels, remember there are two. Unless you mean use parts from a Mk1, then you have the other problem, they use mineral fluid. Best not go there.

 

I'm sure a person as versatile as the Cubed one would have no issues in finding homes for the two airpacks, twin master cylinders, sensing tank and regulator plus routing all the hydraulic lines, as well as the low and high pressure airlines.. :D:D:D

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AEC used air, air/hydraulic and vacuum/hydraulic systems on Matadors.

 

AEC never used Vacuum over Hydraulic on Matadors. Some RAF Matadors had a an Exhauster fitted between timing gears and the fuel injection pump. A Vacuum tank was mounted under the fuel tank, but the vacuum was used only for trailers fitted with vacuum brakes, being actuated by an air over Vacuum proportional (reaction) valve. The vacuum was not connecfted with the Matadors own air brakes.

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I stand corrected Mike. I knew that AEC did have a few different systems on the Matadors but couldn't remember the exact ones. I know they had a few differences ie axle casings, tractor/rzeppa jopints as well. Too many variables but as there was a war on i suppose they used whatever was to hand at times!

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I stand corrected Mike. I knew that AEC did have a few different systems on the Matadors but couldn't remember the exact ones. I know they had a few differences ie axle casings, tractor/rzeppa jopints as well. Too many variables but as there was a war on i suppose they used whatever was to hand at times!

As for the Air over Hydraulic, AEC used two variants, Westinhouse Compressor and Westinghouse Air over Hydraulic Servo, and Clayton DeWandre compressor/ servo set up. I am not aware clayton compressor was used with Westinghouse Servo or Vice-Versa.

 

There were at least four different specifications for wartime full air pressure braked Matadors and another post war system using Clayton Dewandre gear....add to this Warner Electric Trailer brake gear on some versions and a fifth air cylinder under rear cross member to pull on cable brakes and you approach lottery number combinations!

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One important point to consider, R3. If you wish to put an air / hydraulic servo on, you will need one with sufficient capacity (in terms of volume of fluid pumped) to operate all six wheel cylinders. Any modernish truck over 7.5t or having more than two axles is not likely to have an air / hydraulic system.

 

You will most likely find that these modern servos (e.g. from a 7.5t truck or even Bedford MJ) will only have sufficient capacity to operate four cylinders, and then only two from each of the two circuits.

 

Finding one with enough capacity will not be easy!

 

Yes this has sprug to mind I was thinking of calculating the volume of all the master / slave cylinders and then doing the same for all the wheel cylinders and also measuring the fluid pressures at the wheel cylinders so I know where to start !!! :nut: I think....

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Stollys are on disc brakes though and Jimmys are on drums. Disc pads tend to run closer to the surface and need a lot less fluid movement to apply pressure. Disc callipers also tend to require higher fluid pressure as the braking friction surface is a lot less than drums.

 

Isn't home modification of the braking system going to be frowned upon by the authorities, VOSA and your insurance to name a few?

 

Arr but disc calliper pistons are also a bigger diameter than a wheel cylinder so although it might not actually move as far there is more fluid entering it to move it in the first place, if you see what I mean, the volume might be roughly the same comparing it to a smaller dia wheel cylinder moving further, or not, I would have to work it out.

So with discs needing a higher pressure the brakes might be very good or even too good.

Edited by R Cubed
Added some finer words
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I can't really see the point of altering the brakes on a jimmy, the one I used to own and others I have driven had better brakes than a lot of modern HGV's I've driven which obviously had full air brakes. Properly maintained and set up GMC brakes will lock all wheels without using excess pedal pressure and yes before you ask I have used them fully laden.

 

This is a very interesting point, which I have heard before, now I have rebuilt my brakes completely from the maser cylinder to the wheel cylinders including all the flexible hoses the steel lines in the chassis have been replaced with copper nickel, all brake shoes, and hydrovac seals. Now although they are much much better than they were when I bought it 14 years ago I would still say you have to push the brake pedal quite hard to get it to stop quick !!!

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