Rubbish rear Brakes (part 2)

[gaz_moose]

rtbcomp,,,, I guess I forgot that the slave pistons have very little travel. cheers for putting me straight again, sometime I can over think things and get in a muddle.

mike,,, I assume it has ABS as my last one did and this one is a better spec and newer. although I cant see a pump in the engine bay as its fairly crowded.(I think the abs only works the front wheels anyway)


I realised since fitting my suspension that actually moves (instead of the seized old stuff) that I have not seen how far it compresses with a car on the back, so I put my old Vauxhall Astra on the back and noted that the suspension went down about 3cm, I slid underneath the truck and found that the actuator arm on the load compensator valve had not moved at all from the 'fully off' position.

I whipped the car off the back and jacked the rear wheels off the ground then used a block of wood to jam the brake pedal down, found I could rotate all of them by hand with some force.

I then wedged the load compensator to 'fully on' and found that the wheels were locked solid.

so I guess the suspension doesn't compress enough to actually operate the load compensator with a 1200kg car on the back.

 

[mike 109444]

Could be, reading your other post that the allen bolt on the load compensator could be for setting it up. Your headach will be getting it set with out being on a rolling road. Also might be that there is not enough adjustment and you may have to create a (adjustable?) bracket that can operate on it at the correct loading.

 

[knighty]

not sure if it was this post or the other one... but you said you braked to wind up the self adjusters

I've never done them like that, it's far better to wind them up manually... wind it up a bit try drum on, drum off wind up a bit more drum back on etc.. you want them to be pretty tight, so they're only just not binding

handy to put the drum on and press the break a few times too, senters the shoes so the drum goes on and off easier

 

[wookie]

I realised since fitting my suspension that actually moves (instead of the seized old stuff) that I have not seen how far it compresses with a car on the back, so I put my old Vauxhall Astra on the back and noted that the suspension went down about 3cm, I slid underneath the truck and found that the actuator arm on the load compensator valve had not moved at all from the 'fully off' position.

I whipped the car off the back and jacked the rear wheels off the ground then used a block of wood to jam the brake pedal down, found I could rotate all of them by hand with some force.

I then wedged the load compensator to 'fully on' and found that the wheels were locked solid.

so I guess the suspension doesn't compress enough to actually operate the load compensator with a 1200kg car on the back.

This sounds like you're getting somewhere now, for a quick an' dirty test I'd wedge the load compensator "fully on" and then very carefully go for a little test drive with nothing on the back. When you brake you should be able to lock up the rears easily, if that is the case then you need to mod the linkage to the load compensator so you can adjust it and then with the help of your local mot station you should be able to adjust it to get the required force when both loaded and empty.

 

[gaz_moose]

I tend to set the shoes as you say knighty so they are just kissing the drums, then I was under the impression that they 'self adjusted' when reversing, so I tried some fast reverse/hard braking shenanigans down my street.

ive since found out the load compensator is off a VW T4 so the allen bolt is not actually an adjuster, seems they would have adjustable drop links between the lower suspension arms and the compensator valve bar thingy. think I will just remake the entire load compensator assembly as the linkage bar is like an antiroll bar but make of 10mm solid bar so pretty bendy.

wedging the compensator to 'fully on' will help me to bed the rear shoes into the drums. so I will give that a go tomorrow and then readjust the shoes back out. hopefully this will improve the handbrake.

 

[knighty]

load adjusted will have no effect on handbrake ? - adjuster is hydraulic and handbrake is cable operated ?

if you pull the handbrake up without pressing the button in, so the ratchet clicks as you lift it, how many times does it click for full on ? - 4 is normally considered the right setup (and then how many more clicks if you pull it really hard?)

self adjusters should work in forwards or reverse thing is they're normally pretty rubbish anyway

when you said you adjusted adjusters so shoes just kiss the drums... is that putting them on/off or spinning the wheel ? - set them so they catch when you spin the wheel


it's a pity you're so far away, a lot of times with things like this a fresh pair of eyes makes the world of difference
(also wouldn't have to keep asking you so many questions :-o )

 

[gaz_moose]

id got the cables the wrong way round in my original diagram, the cable to the handbrake lever is 11" away from the pivot point and the pushrod the then goes to the rear cables is 7" away from the pivot point.

if Im correct that give me a mechanical advantage of (11"-7")/11" = 0.36 so a 36% advantage (I think that's the right way to calculate it)

knighty

the footbrake and the handbrake are both rubbish I think it was just a coincidence they scored about the same on the testing machine.

I set them up by taking the drum 'on and off' and adjusting the shoes out till I can feel a decent amount of resistance when turning the drum. then I would pop the outer wheel bearing/hub nut back on and operate the handbrake/footbrake to centre the shoes, then check I can still turn the drum by hand.

I was trying to set them up so that the actuator arm on the actual brake shoe( where the pip on the end of the cable locates in the shoe) was fully retracted / back with the handbrake off.

these shoes were in my old knackered drums for about 150 miles so im kinda confident that they just need a bit of bedding into these new drums, then I will whip the drums off and readjust them up snug.

 

[wookie]

id got the cables the wrong way round in my original diagram, the cable to the handbrake lever is 11" away from the pivot point and the pushrod the then goes to the rear cables is 7" away from the pivot point.

if Im correct that give me a mechanical advantage of (11"-7")/11" = 0.36 so a 36% advantage (I think that's the right way to calculate it)

If I remember my physics from school correctly it's...

actuator length to pivot / resistive length to pivot

so 11"/7" gives 1.571 which means you gain just over 50% more force than you input.

 

[Crater Creator]

If I remember my physics from school correctly it's...

actuator length to pivot / resistive length to pivot

so 11"/7" gives 1.571 which means you gain just over 50% more force than you input.

Yep - ^^^ what Wookie said.

 

[gaz_moose]

I re-adjusted the shoes into the drums and took it for a drive and found that I could do a rather impressive handbrake skid down the road. doing an emergency stop also left some skid marks, so I popped it back into the garage to check it on the brake roller machine for me and they said 40 to 50KG braking force per wheel again for both foot and handbrake.

knowing that I could lock the wheels on both the foot hand handbrake I found this hard to swallow so popped it to a different place for a 'pre-mot check' and it pulled 100 on the footbrake and 120KG on the handbrake. I booked it in with them and now have an MOT (((YIPPIEE))).

not too sure what the first garage were playing at giving me low readings on purpose, caused me loads of stress.

cheers for the help guys!!..helped me retain some of my sanity.

 

[normspanners]

Most empty vans give about 250 - 300 kg before locking on decently maintained rollers ( a lower lock if the grit on rollers is not so good )
seized load sensing valve/ linkage is a very common reason for low rear brake effort
some vans like fiat ducatos have a fixed pressure limiting valve that just reduces brake pressure on some systems I have amate with amotor home that has exactly this issue the front brakes are red hot and the rear are barely warm on long descent it is crazy because a motor home is ALLWAYS close to the weight limit even empty and the rears should be more effective than the fronts in that case
for instance a truck strongest brakes are the rears because of the load that should be the same for a motorhome we are going to look at piping out the limiting valves shortly.