12 replies to this topic

[Joe555]

1987 City E    8.4 discs and servo.

Has anyone fitted a Mini Spars MS72 rear brake limiter, is it just for single line brakes. If not does it fit on the rear sub frame and still use the FAM7821 that is fitted in the engine bay. Also What is the correct wheel cylinder to use. 

 

Joe

[KernowCooper]

November 1985 they introduced a new master cylinder with a stepped bore and a the same front rear split, as your is one of the split line braking line models you wont benefit on a roar car by fitting a MS72, its designed for a car where you have different ride height and maybe bigger wheels and you have upset the brake bias setup of the standard system and possibly getting lockup on the rear, you then fit the valve and adjust you own pressure to the rear brakes.

 

I don't think you will need one on your 1987 City E Joe

[Joe555]

Thanks for the reply.  My car has a servo, not standard on this car, the front brakes are standard but I am not sure about the rear. The brakes work well unless I need to do an emergency stop (I haven had to yet but you never know).  I have only tested it on a deserted road and if I brake very hard the rear wheels will lock and it will start to spin. 

[Spitz]

Perhaps if you simply change your rear wheel cylinders ( larger bore? ), they will not lock up as easy.

[Cerberus]

If you stamp on the pedal hard enough, won't they always lock up due to the fact that there's no ABS?

 

Isn't this where cadence braking would be better used rather than limiting the power of the brakes?

 

 

Just thinking out loud.

[Spitz]

Yes...without ABS they will lock up.

I think the issue is, you don't want the rears to lock up too soon/easily.  If they do, they rear of the car will very easily pass the front of the car.
If the front lock up first....you will lose steering capability, but should not go into a spin.
Threshold breaking would be ideal of course ( with properly set up non ABS brakes ).....but in a panic/emergency/OMG situation, the majority will just try to put the foot throught the floor.

[KernowCooper]

When you stamp on the brakes that hard you transfer the weight to the front and the rears go very light and even with a std pressure valve the rears can lock up, thats the issue with the mini lack of weight over the rear.

[tiger99]

To reduce the tendency to lock the rear wheels, you need SMALLER cylinder diameter, not larger as stated earlier. I find it somewhat depressing that people whe have not understood the very first thing that you need to know about the principles of hydraulics bother to post such comments.

 

By the way, it is far better to achieve balanced braking by reducing the rear wheel cylinder diameter than by using a pressure limiter, because the limiter can't set the maximum pressure differently on a dry or wet road, and has to be set for the wet road case, limiting dry braking performance unnecessarily, whereas by doing it with smaller cylinders, the front/rear ratio remains correct under all conditions, so if the fronts correctly lock up first on a dry road they will also do so in the wet.

 

And don't bother with so-called "proportioning" valves of US origin. A true adjustable proportioning valve is an impossibility in a hydrostatic system, and only viable in a pumped system like certain Citroens. What they are selling as a proportioning valve is in fact a soft limiting valve.

[Ethel]

This could be as much to do with your suspension. If cones have gone hard, lacking travel, or the dampers are too stiff, the rear wheels will lift off the road more readily. 

 

An inertia valve will come closer to doing the stuff Tiger mentions.

[Joe555]

I haven’t ruled out the suspension or anything else, any ideas are welcome. I haven’t checked the size of the wheel cylinders yet but I am guessing they will be ¾. The wheel cylinders do seem to be the best place to start though, there are five sizes on the Mini Spares web site. Should I go for the smallest ½ part number GWC1126

[tiger99]

An inertia valve is indeed a useful thing, but it still can't distinguish between the deceleration obtainable on a wet road as opposed to dry. Often in engineering, no solution is perfect, but some are better than others. I think that one of my ancient cars did use an inertia valve, fairly successfully.

 

There also used to be load-sensing PRVs, and one of my ancient and decrepit cars (was it the Avenger estate?) had one. That again solves half the problem, because it gives you more pressure to the rear brakes when you have a load in the back.

 

An ABS system, despite all the potential safety problems that they cause when they go wrong, as they do, would be almost ideal, but to use ABS on the front wheels it is essential to have zero or negative scrub radius, otherwise extremely dangerous instability results. However, a simple ABS system for the rear wheels only would do the job.

 

Or, the Citroen solution, with full load-sensitive, true proportioning valves front and rear, but possible only in a pumped system. The front to rear brake ratio is always correct, regardless of load, and set so that the fronts lock just before the rears, a legal requirement. You get close to as much braking as is theoretically possible, on all wheels, wet or dry. Very complex to fit in a Mini, and would you be able to get hydraulic seals in the correct material (to be compatible with LHM fluid) to fit Mini cylinders and calipers? If you were doing that,you would also be wanting to fit the Citroen suspension spheres and cylinders, which, if possible, would give far more comfort than any of the half-baked coil spring conversions, while retaining full suspension travel by always running at constant ride height regardless of load.

 

The sort of upgrade to the Mini that engineers like me dream about, knowing that it is probably impossible.....

 

Someone will prove me wrong, of course.

[Ethel]

An inertia valve will distinguish the deceleration obtained, so at least the driver will  be left with both ends reacting concurrent the same force he feels through the seat of his pants. It'd also be an easily feasible modification to a Mini. There are still plenty of load sensing valves about on light commercials, of course some sort of live axle makes installing one easier, hence the inertia valve on Mini commercials? 

[Spitz]

Take it easy there tiger.......that's why I put in ( ) a "?" as I wasn't sure and expect/hope for correction if wrong