Nearly spun out again to avoid a car - new brakes needed?

Blackhawk47

Blackhawk47

Blackhawk47

Soldato
Joined
17 Sep 2007
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11,117
Location
West Yorkshire / Market Bosworth
This one is probably best off on the mini forum but I shall post here.

Coming home from work on this road and nearly had an accident when some guy was reversing from his drive way with vehicles parked either side and I was coming down the hill and he pulls out straight into the middle of the road and stops so I slam on the brakes only to have (or at least I think what happened) the rear brakes lock up the rear wheels so the back end flew out left then right, and left again as I tried to correct it and I really didn't want to put into a parked car/ditch/tree/wall...pulled over at the end of the road to check everything while the other guy just carried on.

Now the thing is I meet with a meet yesterday who knows his motor tech and was blown away that I had AP Racing disc brakes at front but after checking my Mini's documents it actually does have the AP Racing disc brakes at the front but drums at the back and this is now the second time I have nearly spun out with the other time being when I skidded last december in bad snow and ice and to brake hard.

Any advice? I presume in the long run the rear disc brakes would pay for themselves - though the car is fine at the moment, i just cant do an emergency stop in it properly and it has no ABS.

:( :o
 
Drums are fine on the rear and are more than capable of locking up the rear wheels - some drums can be damned powerful! Adding discs in this instance would probably only exacerbate the issue, the problem is seemingly with brake balance.

Odd scenario though. Front tires better than the rear? Did the fronts lock up as well? Before or after the rears?

The off-the-cuff response would be to stick a brake bias valve, or load sensing valve, in if it doesn't have one, wind it in a little so the rears aren't so aggressive. Do Minis come with any kind of compensator/LSV/bias valve?
 
I would imagine the rear is doing exactly the opposite of locking up, and the bias is so forward (due to the far superior brakes), that the slightest bit of momentum either side of straight on when braking is causing the back end to move around a lot, so when trail braking etc, you'll get a lot of snap oversteer. I'm just guessing though
 
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No need for rear discs at all, what you need to do is sort your brake balance properly. Sounds like your brakes are bias too much to the back. The mini should have a shut off valve that stops the rear from being able to put on too much braking force, the question is whether it is functioning.

The reason why you would only feel it under hard braking is because the harder you brake, the more weight transfer there is and then this means the ideal braking force distribution moves more and more towards a front bias.
 
Lashout_UK said:
The off-the-cuff response would be to stick a brake bias valve, or load sensing valve, in if it doesn't have one, wind it in a little so the rears aren't so aggressive. Do Minis come with any kind of compensator/LSV/bias valve?
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Yeah they come with a simple one which just stops the braking force going above xN. I remember my lecturer going on about the classic mini having this when studying braking systems. :p

Edit - Yep thats exactly what you want there, can setup the correct brake bias that way.
 
Minis are pretty light right, I've seen vids of them doing stoppies before. Maybe your immense front brakes are just causing a lot of rear uplift hence instability/locking.
 
The problem is pretty simple, back end instability caused by the front decelerating far faster than the back. Whoever fitted uber front brakes without doing anything to the back is a moron.

The back end was sliding left and right because it wanted to overtake the front end. Brakes locking up wasn't the problem.
 
Check and adjust the drums on the back, they go out of adjustment quicker and easier than some people think in some cases. I had some horrible back end movement when mine were out of adjustment and the rear wanted to overtake the front. Skidded off the road in the snow due to complete lack of grip and then the back end just coming away from me. Luckily missed... everything!
 
adjustment issue or special pads on the front not responding well from cold

see if you can adjust the bias or see if the front calipers need a service
 
Dolph said:
The problem is pretty simple, back end instability caused by the front decelerating far faster than the back. Whoever fitted uber front brakes without doing anything to the back is a moron.

The back end was sliding left and right because it wanted to overtake the front end. Brakes locking up wasn't the problem.
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Very highly doubt that. Rear end instability is from too much braking from the back. Think of tyres having a finite grip in any direction at one time, so when you are braking hard you have less lateral grip available from the tyres. So what is happening here is due to all the available grip being used for slowing down the car, so the tyres then have very little grip available for any lateral forces. This then causes the rear of the car to want to swap ends with the front.

Whereas when you have a too forwards bias the rears will still have plenty of grip available (due to less braking force being applied) for any lateral forces, hense not want to swap ends with the front of the car.
 
saitrix said:
Very highly doubt that. Rear end instability is from too much braking from the back. Think of tyres having a finite grip in any direction at one time, so when you are braking hard you have less lateral grip available from the tyres. So what is happening here is due to all the available grip being used for slowing down the car, so the tyres then have very little grip available for any lateral forces. This then causes the rear of the car to want to swap ends with the front.

Whereas when you have a too forwards bias the rears will still have plenty of grip available (due to less braking force being applied) for any lateral forces, hense not want to swap ends with the front of the car.
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It isn't necessarily the braking force that breaks traction, it can be the weight shift, especially if the car is front heavy to start with. This can also make the brakes lock up but it isn't necessarily because they are overbraked at the back.
 
Dolph said:
It isn't necessarily the braking force that breaks traction, it can be the weight shift, especially if the car is front heavy to start with. This can also make the brakes lock up but it isn't necessarily because they are overbraked at the back.
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Well yes as you get weight transfer you need an increasing front brake bias. This is only because the weight transfer reduces the load on the rear wheels. Max grip from a tyre is purely the tyres weight times by the coefficient of friction. So it still is the braking which is doing it as all the weight transfer is doing is reducing the maximum grip on the rear tyres.

Plus it shouldn't matter about the car being front heavy as the braking system should have been designed to give a more front bias.
 
If someone has fitted bigger calipers to the front without increasing the master cylinder size then braking effort will be reduced at the front, regardless of bigger calipers/discs.
 
On a classic mini the pressure limiting valve for the rear is quite prone to seize which gives exactly the symptoms the OP has seen. Since they are such short cars and so light at the back, they can swap ends very quickly indeed.

The front calipers used on the later 12" wheeled cars can also partially seize which makes the car pull to one side quite violently under heavy braking.

amigafan2003 said:
If someone has fitted bigger calipers to the front without increasing the master cylinder size then braking effort will be reduced at the front, regardless of bigger calipers/discs.
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No it won't, how do you work this out? If you have a larger piston area in the callipers then the pedal stroke will be increased, but pedal effort will be reduced.
 
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