Hi,
My Girlfriends Ford Ka failed its MOT on an inefficient nearside rear handbrake and footbrake.
I removed the drum and the shoes were shot (the friction material had split from the steel.) I replaced the shoes and thought the problem must be sorted.
After several failures / restests I finally figured out that a spring on the automatic adjusters had stretched and was stopping the adjuster working correctly. I manually adjusted the shoes and the brake passed (hand and foot) with flying colours.
The original problem was the shoes - I just created a new one by not recognising the inpact of the damaged spring.
So my quesion is, how could this happen? Surely the hydraulic cylinder would take up the 'adjustment' on applying the footbrake? The brakes are diagonally connected so surely there was sufficient pressure in the cylinder or the front brake would not have worked correctly?
The handbrake I think I can explain. There was such a lot of travel on the handbrake lever attached to the shoe, it was actually hitting the centre of the hub (where the bearings are held.)
Thanks.
|
When was it last serviced?,this should have been noticed much earlier.rear brakes have a valve to prevent rear wheels locking though I can only think it worked due other side working,from what you said it implies the adjuster has not been used/or refitted without adjustment-meaning it fell off Unless I read this wrong??
--
Steve
|
|
I seem to remember that one of my previous cars had an automatic adjustment on the rear shoes that worked when you reversed. So the technique for adjusting the rear shoes was to put them on and then reverse several times, braking each time to a stop. I can't remember if you had to use the handbrake to do this, but I don't think so.
|
Keith,
(to be said in a cynical tone) "do automatic adjusters ever work?"
"not in the age of cars I own!"
Coming back to your original query, could it be that the pedal angle and leverage obtained is set to be at a maximum when the shoes are adjusted correctly? When incorrect adjustment has to be taken out before shoes hit drum the pedal will be further down its travel before it is effective; maybe servo action is not as good?
Don't know the answer, just thinking aloud, interesting query though
|
|
|
|
rear brakes have a valve to prevent rear wheels locking
The brake limiter valve does not prevent rear wheels locking,that is what ABS does.It ensures that braking force is split at about 70% to the front to reduce the risk of oversteer,also making the brakes more efficient as front brakes have more work to do.
As for automatic adjusters,I do not know why they bother fitting them,they rarely work properly and manual adjusters would make things a lot easier,maybe why discs are becoming more common.
|
>>The brake limiter valve does not prevent rear wheels locking
It *is* designed to make sure that the rear wheels don't lock, as per fairly lax requirements in British Construction and Use regs**, and much tougher European law. The usual method of demonstrating compliance is for a manufacturer to compile adhesion utilisation curves, with rate of braking along the x -axis of the graph.
The C&U regs just say that the rear wheels shall not lock, allowing manufacturers to fit simple valves which severely restrict the rear brake force, the tougher European laws demand that the rear brakes do not lock, and over a range of rate of braking are above a threshold level off efficiency, which forces manufacturers to fit more complex valving on cars with small wheelbases, forward weight bias, and high c of g.
How the rules change with the introduction of compulsory ABS will be interesting - I hope that they demand that the rear brakes should not lock even with ABS disabled, but since when have rules and regs been sensible!
Locking of the rear wheels is extremely dangerous, while locking of the fronts is comparitively benign.
Number_Cruncher
|
Think steptoes theory is interesting.
The self adjusters are actually quite good (as long as the system is in good condition lol) but I really cant understand this.
Even with the pressure limiting valve, surely the force is not dependent on taking up the adjustment???
|
>>surely the force is not dependent on taking up the adjustment???
when the pedal is depressed the adjuster should take up any play,more often than not they dont work.
so what happens is the shoe eventually does not contact the drum,the master cylinder piston/s react or travel only as far the pads will travel to disc meaning the front discs work before the rear-unless the rears are adjusted correctly,in which case all brakes will work fine.Adjuster is usually at fault/tell tale of rear drums needing adjustment-is pull handbrake on with foot on pedal the pedal will go down slightly
--
Steve
|
Could it be that with a badly mis-adjusted rear brake one of the pistons in the tandem master cylinder is bottoming out?, and the pedal feel is all from the remaining circuit?
Number_Cruncher
|
|
|
|
Locking of the rear wheels is extremely dangerous, while locking of the fronts is comparitively benign.
Have to disagree with that,last time my front wheels locked in an emergency situation I ended up bouncing off a Mondeo,not at all benign.I would sooner deal with a rear wheel skid than a front one.I can also make my rear wheels lock if I brake hard enough
|
"Could be that the pedal feel is from the one circut in the master cylinder maybe?" I've no idea how the master cylinder really works. The brakes didnt really feel any different with the rear n/s not working, would have thought if the front o/s was also not working that the brakes would be terrible?
Wonder if the master cylinder can send pressure to the front o/s without needing pressure in the rear n/s??
Seems that the thread is getting a bit distracted/off topic with a debate on brake force distribution etc.
|
Keith,
I think that it might be possible, depending upon what type of pressure reducing valve is fitted, or indeed if a differential pressure valve is fitted (although I haven't seen one for years). The pressure reducing valve *could* have shut off the flow to the rear brake before the shoes had travelled far enough to touch the drum.
I think that because the locking of a front wheel on a bike or motorbike is such a catastrophic event, sometimes, people don't realise that the reverse is true for motor cars.
In a car, if you lock your front wheels, you continue in a straight line. Possibly not the best course of action, but definitely not the worst. If you lock the rear wheels, the car is *unstable in yaw, and you could go anywhere. I have mentioned on this forum before about my brief experience with an overly powerful rear brake during an emergency, and can confidently say that such a configuration is not at all safe.
While I am sure that Sierraman holds his views sincerely, he is, alas, misled.
Number_Cruncher
* Even rally drivers, who exploit this yaw instability to improve the turn-in response of the car do so by taking a hand off the steering wheel and pulling the handbrake rather than by altering the bias of the brakes.
|
|
|
|
|
|
|
