Had a bit of a "moment" yesterday.

markbognor

markbognor

Ex. Club Member
Club Member
I feel stupid for not considering this before, I had a nagging feeling that I should really sort it out but just hadn't got round to it. No symtoms had shown their head. I will be fixing it post haste.

Yesterday afternoon while driving down the local bypass at 60-70mph a car changed lanes unexpectedly. I stood on the brakes, as might be expected in such a situation. In hindsight a bit of an overreaction as the speed differential between our two cars was not actually that great.

Unfortunately I braked to such an extent that the rear wheels locked, which sent the car into a side ways slide, I wound on some opposite lock and came off the brakes, by which time the car was at about 45 degrees to the carrageway, it then bit and sent me back the other way. I managed to gather the fish tailing together, but I did need most of both lanes, fortunately no one was on the nearside, the van I was overtaking was still a little way behind.

So all ended well, it was just a "moment".

On to considering why it happend, well firstly I could have been paying closer attention, and reacted in a more measured fashion. I could also have been wearing different shoes - the fairly new, un broken in Birkenstocks provided little in the way of feel through the brakes. I might have cosen to fit better grippier tyres, and use a tyre pressure that favours ultimate grip over fuel economy. It could cetainly also be attributed to the stiff springs bounce not being controlled by the low damping ability of the standard shocks. However I think the biggest cause for my situation was the rear brake load compensator.

K11's have a brake bias compensator built into the rear brake circuit that allows more braking force to be applied if the car is carrying a load. How does it know the car has a load? - it rides lower. The brake compensator is essentially a valve mounted on the car body with an actuating rod mounted on the suspesion, as the susspension compresses under load it opens the valve to allow greater fluid flow and braking pressure to the rear. Beacause my car has been lowered it "thinks" that it is carrying a heavy load, and allows maximum breaking force to the rear, even when there is nothing in the back to increase the force on the rear wheels, and friction between them and the road surface and stop them from locking.

So my next step forward is to investigate the possible adjustment on the actuating rod, or prehaps modification of it to take account of the lowered susspension. I'm off the road for a bit now anyway as the MOT expires today and I have no time to carry out the required work until next week (CV boot + whatever else I or indeed they find).

I know solarice mentioned a modifiaction to it in his blog, so I'll have a hunt through at some point. If anyone else has already done this, what did you do?

And if yer driving a low micra that hasn't had this issue addressed, then it might be worthwhile having a look.

Mark.
 
Bet that got your attention :eek: and not a fun thing to be involved in.

Sorry Mark i forget how low your car is :S but if its like 35mm or so you can get away with just adjusting the bias position...the haynes give a good idea of what to do...easiest way is just to check if you can get the clerance.

Mine is so low though (and probs anyone else on coilovers...a 60mm drop maybe getting close also), that the bias was bottomed out constantly...so i just made a bracket up to extend the main pivot position. (save you looking for it ;) )
 
Hey guys, My cars is very low and this doesn't sound like fun at all. I had once "moment" on motorway when some nearly got in the side of me. I was swerving all around the place with cars all around me. Really not nice.
I got rear discs and ABS, do you think thats any help? cheers
 
not sure on the ABS models.... Easiest way check behind the rear pass wheel...if theres box with a big spring/rubber boot then its the same...But the brake bias maybe setup differently (its where the brake lines come in / go out)...if its there and totally compressed, it shouldnt be :)
 
the brake bias compensator is on both abs and non-abs micras of mine, same unit.

having abs helps, at least you'll have some control under demanding conditions.

i learnt that in fwd if the cars oversteering, you point the steering where you want to go (don't over-do it or it'll snap the other way when the front grips), give it the beans, the fwd will then pull the front of the car to where ur pointing, as the car points straight then progressively straighten the wheel.

it takes some skill and nerves to learn (preferably on a wide clear tarmac or clear race track) cos ur gut instinct during oversteer is to steer towards the skid and then lift off power or even brake which is the worst thing to unsettle the car even more, and how i first crashed mine.
 
Glad your ok Mark.

Thank you very much for the info, soon my coilies will be here so Ill come under this category and I had no idea about this! First thing I'll be sorting (Y)
 
i had the opposite, my back brakes locked up after taking everything out of the back of the car. i stripped the car and it made the brakes at the back lock up now and again.

are you sure the bias doesn't apply more to the front the lower the back?
 
I have felt this twice in about a week under hard cornering and applying the brakes. Will need to sort it out too if someone can come up with a perfect solution
 
NeX said:
i had the opposite, my back brakes locked up after taking everything out of the back of the car. i stripped the car and it made the brakes at the back lock up now and again.

are you sure the bias doesn't apply more to the front the lower the back?
Click to expand...

during braking the cars weight transfers to the front inherently making the front tyres grip more than the rears. the bias compensator is configured to provide the required bias for the micras original weight distribution & ride height.

taking weight out the back shifts the weight bias of the car forward. without moving the brake bias forward by either readjusting the compensator or upgrading the front brakes, there'll be too much rear braking force for the given weight pushing the rear wheels down therefore under heavy braking & turning the rear tyres could lose grip more easily.

in marks case he lowered and stiffened his suspension without readjusting the bias compensator so even if the weight bias is the same the mechanical compensator is tricked into thinking there's more weight acting down on the rear suspension/wheel/tyres therefore will provide more rear bias therefore again there's too much rear braking for the given weight bias making the rear tyres lose grip.

in either case the brake compensator arm needs to be repositioned at a lower setting on the trailing arm to reduce rear braking
 
hi i was going to fit some 30mm lowering springs on my super s im guessing i will have to sort this little problem out as well?????
 
pollyp said:
during braking the cars weight transfers to the front inherently making the front tyres grip more than the rears. the bias compensator is configured to provide the required bias for the micras original weight distribution & ride height.

taking weight out the back shifts the weight bias of the car forward. without moving the brake bias forward by either readjusting the compensator or upgrading the front brakes, there'll be too much rear braking force for the given weight pushing the rear wheels down therefore under heavy braking & turning the rear tyres could lose grip more easily.

in marks case he lowered and stiffened his suspension without readjusting the bias compensator so even if the weight bias is the same the mechanical compensator is tricked into thinking there's more weight acting down on the rear suspension/wheel/tyres therefore will provide more rear bias therefore again there's too much rear braking for the given weight bias making the rear tyres lose grip.

in either case the brake compensator arm needs to be repositioned at a lower setting on the trailing arm to reduce rear braking
Click to expand...

interesting read thanks for that!

its also interesting that it seems like it should be a big problem, but i have had plenty of micras, some stripped some lowered some both, only one as ever locked at the back, and that was once going round a corner down hill whilst braking and only for a split second. so i suppose its not a massive issue, the cabriolet is on coilovers and no problems there, though i don't do much braking ;)

something to think about tho, i might get under the car and start messing with it!
 
i just read though it and people saying it happens mainly when they brake while cornering. Well. Dont get me wrong, but you really shouldnt be doing that. am I right?
 
NeX said:
interesting read thanks for that!

its also interesting that it seems like it should be a big problem, but i have had plenty of micras, some stripped some lowered some both, only one as ever locked at the back, and that was once going round a corner down hill whilst braking and only for a split second. so i suppose its not a massive issue, the cabriolet is on coilovers and no problems there, though i don't do much braking ;)

something to think about tho, i might get under the car and start messing with it!
Click to expand...

tyres also have an influence too cos end of the day that is what translates ur braking force down to the tarmac into retardation. different makes have various levels of grip under various loads and various conditions.
cold harder compound road tyres = less grip

you mention you were goin downhill on a gradient. that also changes the weight bias forward abit too = less weight on rear tyres

and then another biggie that mckenzie mentions too is driving/braking style.
when braking hard as i said the weight goes to the front wheels.

now if ur braking hard in a straight line, sure the rear wheels/tyres will have less weight on em but there's just enough to maintain straight line stability and the driver won't notice the rear unsettling much.

But if we brake hard AND turn, depending on the tyre co-efficient friction, the fronts may have enough grip to both brake and turn or if the tyre's overloaded it'll just lock and/or skid leading to understeer.
meanwhile at the back, the rear wheel/tyres have to:
-follow where the front wheels are going and
-hold the back of the car steady against the sideways centrifugal force induced during turning
-brake
all above BUT with a big reduction in downward weight/force.

most likely the rears would then lock cos the rear tyre cant support the car against the sideways force, breaks traction, rear grip between tyre & road plummets, the rear braking force would then be greater than the available traction and so the wheels lockup making the situation even worse.

i think i read that the brake compensator operates in two modes. under normal medium braking the unit sends most braking pressure to the fronts first then bleeds the rest to the rears depending on the rear loading (ie. heavier/lower the rear = more rear bias). but under sudden heavy braking like emergency braking the jump in pressure initially restricts pressure to the rear brakes so almost all braking pressure goes to the fronts first (sudden front brake bias) then bleeds to the rears to prevent the rears locking up first.

conclusion: always brake hard in a straight line cos trail braking or mid-corner braking just upsets the cars handling. if ya absolutely need to avoid/swerve round an obstical try cadence braking on non-abs models to allow the tyres to regain some traction.
 
good point at the end there, probably the reason my cab has never locked up even though its slammed on coilovers is because of the ABS.

as to why i was braking on a corner, i wouldn't normally but i came round a corner at about 20 mph to meet a 4X4 on my side of the road, it was a tiny panic braking

but it sounds like its not really a big problem to worry about
 

Similar threads

A
A bit of info please.
Replies
3
Views
889
Abaaba
A
superls
a little bit of what you fancy?
Replies
11
Views
2K
mckenziecz
mckenziecz
M
Car had mot yesterday, and my driving test date
Replies
8
Views
1K
Daisy
D
C
Scary moment..................
Replies
6
Views
130
CMF_domedeli
C
B
strange moment
Replies
11
Views
904
R05_Micra
R05_Micra
Back
Top