Club 80-90 Forums

Is anyone conversant with fluid dynamics?

On my Merc I have two shot wheel brake cylinders, according to my vin and axle numbers Mercedes say that the cylinder should be 22.2mm bore. Now comes the problem - the ones that are on there are 23.81mm bore and aren't listed for my van!
If I were to source the correct fitment, mounting bolt spacing and machined seat, what effect would the change of bore size have on the braking performance - if any? To my thinking decreasing the bore would increase the travel of the piston for the same pressure/travel on the brake pedal therefore increase the braking effort on the shoes. Is this correct or it's the reverse, or is the overall pressure in the system when braking the overriding criteria?
The flow of electricity is easy to understand compared to fluids!
Any observations/ideas bouncing would be most welcome.

Oldiebut goodie wrote:To my thinking decreasing the bore would increase the travel of the piston for the same pressure/travel on the brake pedal therefore increase the braking effort on the shoes.

Sounds 'bout right to me.

A certain amount of fluid goes down the pipe. It has to go somewhere, and it's not compressible. If the space it's trying to get into is smaller diameter, then the result is more displacement of the piston.

Or, to put it in terms you'd easily relate to... A short wide pint pot holds the same amount of beer as a tall thin pint pot. Right...?

Put too much braking effort rearwards, and you'll get all sorts of unwanted fun'n'games if it means the back wheels start locking before the front.

Just bouncing ideas, as suggested. I'm no fluids expert!

Given that the pads have to move the same distance, this will be achieved with less pedal travel, since less fluid will be needed to move the piston the same distance. So with slightly less pedal travel I'd guess you'll have to press harder, and the pedal will feel slightly deader. But will the difference be noticeable? Going from 23.81 to 22.2 is only a 13% reduction in area.

More bouncing. Apart from pad travel, the force on the back of the pad is the key thing. To achieve the same [pressure x area] you'll have to increase the pressure in the system by 13% - ie press the pedal 13% harder.

That is what I am worrying about, messing with 5 tonnes of vans brakes is not to be trifled with.
I have just had confirmation from a Merc dealer in Germany that the correct one should be the 22.2mm one! What to do? The difference isn't too great as some models have 30mm bore cylinders but I would rather get it right. I am beginning to wonder if someone in the past has put different backplates on to the original axle and uprated? the cylinders. The axle number ties up with the chassis number.
The bus used to stop on a sixpence as the saying goes but I am renewing all the linings and found one cylinder is damaged beyond repair.

Actually, it's not even fluid dynamics, cos the fluid ain't errrrr........ dynamic. It's all about static pressure.

Reducing the diameter of the cylinders fron 23.81 to 22.2 will reduce braking effect on that axle (for a given push on the pedal, which will provide a given fluid pressure) by the ratio of the new area to the old area of the piston ie 22.2 squared/23.81squared which is, give or take 87%, thus reducing the braking effect by 13%. For this push on the pedal, the front brakes will perform just like they used to, but the rears will brake 13% less, effectively pushing the brake bias to the front.

The change won't increase the travel on the wheel cylinders. That is limited by the brake shoes and drum. It would reduce the travel of the foot pedal due to reduced volume in the cylinders.

Must go, supper's ready.

CJH wrote:More bouncing. Apart from pad travel, the force on the back of the pad is the key thing. To achieve the same [pressure x area] you'll have to increase the pressure in the system by 13% - ie press the pedal 13% harder.

The pressure to stop the bus was minimal - far easier to stop than my VW! (helped by a dirty great vacuum reservoir).
Maybe I am worrying too much!
I have to get it right for financial reasons also because if I have to replace both cylinders instead of one correct one, at £90+ each + delivery from Germany it bears thinking about.
E D I T:
Maybe I should just put what Merc say should be on there and get my MoT garage to check the braking out on the rollers.

If you really want to have your brain explode

http://www.engineeringinspiration.co.uk ... .html#bfvr" onclick="window.open(this.href);return false;

Having done yet more reading it may not be particularly relevant if I have a combined metering, proportioning and pressure differential valve which I should have as the braking system is not the normal diagonal split circuit but split into front and rear circuits.

"Metering Valve
As a result of their design, rear drum brake shoes must move a greater distance to apply as compared to
disc brake pads. If the same pressure were applied to both the front disc and rear drum brakes, at the
same time, the front discs would “catch” much sooner than the rears and cause the vehicle to be thrown
forward. Metering valves are therefore used to compensate for this condition by blocking fluid pressure to
the front disc brakes until the rear shoes have had time to make contact with the drums.
• As the brakes are first applied, fluid pressure rises above a calibrated value (approximately 25 psi
- figure 12) which closes the metering valve stem and blocks the fluid pressure from reaching the
front disc brakes. However, fluid pressure is still applied to the rear brakes, which move the
shoes out to contact the drums.
• Once the shoes begin to contact the drums, the pressure in the rear brake system starts to rise
dramatically. After the pressure reaches a second calibrated value (about 100 psi - figure 13) the
metering valve opens and begins to apply the front disc brakes.
• As the brakes are released and the system pressure again drops below 25 psi, the valve stem
reopens to allow fluid to return to the master cylinder (fig. 14). Notice also in figures 12 through
14 that there is a difference between the valve stem and the valve itself. Each is operated by a
separate spring and has a separate function.
Note: The two pressure points (25 psi & 100 psi in this example) are calibrated based on the size
and weight of a particular vehicle. Metering valves are not universally interchangeable even
though they may appear to be identical."

Interesting stuff this.

Oldiebut goodie wrote:Having done yet more reading it may not be particularly relevant if I have a combined metering, proportioning and pressure differential valve which I should have as the braking system is not the normal diagonal split circuit but split into front and rear circuits.

It's still going to have an effect. The pressure valve merely restricts the hydraulic pressure in the back suspension. But the braking effort a certain pressure generates is what's going to vary by changing the cylinders.

Given that yours "should" have the cylinders you're looking to put on it, I suspect that all you'll be doing is rectifying a change a previous owner (probably unwittingly) made, so no ill effect. Unless yours doesn't have the same master and/or proportioning valve that it "should", and they suit the cylinders it's got...

Oh no, don't even think of master cylinders at around £300 or so!

Increasing the the slave bore size gives more force but more travel on the master cylinder.

Increasing the wheel cylinder bore size will 'Increase' the pressure the cylinders pistons apply to the shoes at any given pedal pressure. However, at the same time a slight increase in pedal travel will be required before the shoes contact the drums.
The difference is probably only small and I wouldn't imagine you would notice TBH.
However, technically the ballance ratio between front and rear will have changed very slightly....again not that you would notice.
Martin

Well, it's all done with new pads, shoe linings and the new smaller wheel cylinders and there is no noticeable difference in the braking. I just have to get all the friction surfaces bedded in then I can try a couple of emergency stops. Thanks for all the thoughts on this.