Rear brakes locking up (no ABS)
22 January 2019, 10:45 AM
#1
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Rear brakes locking up (no ABS)
Right, I was unsure which section to put this in so it's going in General unless a Mod moves it.
Impreza Type R so no ABS. Newly installed KW V3, AP CP9040 6pots with 330mm discs up front, standard 2 pots at the back, CL RC5+ all round. WL anti dive, bumpsteer correction, camber adjustable front top mounts, rear WL adjustable control arms. Tyres are Eagle F1 XL, 3.5mm tread. Wheel alignment was set to a generic fast road setting by a reputable place, leaving me to play around as I see fit.
I've not really had chance to play with the settings of the KWs yet so I've set them mid way on the damping and ride height to where it was sitting on my old Tein coilovers (can't remember the hub-to-arch height off the top of my head.
Wet weather braking, threshold braking, the rears lock and the car initiates a spin from 50mph down (braking in a straight line). Whilst I can control the lock, get the rears moving again and regain grip, I'd prefer it if it didn't initiate spinning as soon as they lock up.
So I'd like to increase rear grip during the braking phase and/or make it stop in a straight line.
I don't really know where to start with adjusting this, most online guides on suspension adjustment refer to adjusting for understeer/oversteer which I'm happy to do but I would like to sort this before moving onto cornering dynamics.
I think I need to either increase front ride height to increase weight on the back. I also think if I increase damping on the front it won't make much difference because the lockup (due to lack of grip/weight on the back) is happening during the later stages of braking. Is this the right train of thought? Or should I be playing round with the rear wheel alignment?
I know this is a fairly basic thing ask but I've only ever played with the chassis to adjust cornering and front braking so please be gentle on me!
Loads of thanks :-)
*ADDED* I'm going to try an MOT station to see if my brakes are operating correctly but that'll be a few days away
Impreza Type R so no ABS. Newly installed KW V3, AP CP9040 6pots with 330mm discs up front, standard 2 pots at the back, CL RC5+ all round. WL anti dive, bumpsteer correction, camber adjustable front top mounts, rear WL adjustable control arms. Tyres are Eagle F1 XL, 3.5mm tread. Wheel alignment was set to a generic fast road setting by a reputable place, leaving me to play around as I see fit.
I've not really had chance to play with the settings of the KWs yet so I've set them mid way on the damping and ride height to where it was sitting on my old Tein coilovers (can't remember the hub-to-arch height off the top of my head.
Wet weather braking, threshold braking, the rears lock and the car initiates a spin from 50mph down (braking in a straight line). Whilst I can control the lock, get the rears moving again and regain grip, I'd prefer it if it didn't initiate spinning as soon as they lock up.
So I'd like to increase rear grip during the braking phase and/or make it stop in a straight line.
I don't really know where to start with adjusting this, most online guides on suspension adjustment refer to adjusting for understeer/oversteer which I'm happy to do but I would like to sort this before moving onto cornering dynamics.
I think I need to either increase front ride height to increase weight on the back. I also think if I increase damping on the front it won't make much difference because the lockup (due to lack of grip/weight on the back) is happening during the later stages of braking. Is this the right train of thought? Or should I be playing round with the rear wheel alignment?
I know this is a fairly basic thing ask but I've only ever played with the chassis to adjust cornering and front braking so please be gentle on me!
Loads of thanks :-)
*ADDED* I'm going to try an MOT station to see if my brakes are operating correctly but that'll be a few days away
Last edited by cardinal.sin; 22 January 2019 at 01:58 PM.
23 January 2019, 12:09 PM
#2
Could it be the front calipers / discs are just too good compared to the rear ?
I say this because ive just done front and rear discs/pads and front caliper overhaul on mine and upon checking disc temps the rears are pretty darn cold compared to the front, so with your setup maybe the front just grip too well.
Do some progressive braking and feel the temps of the discs
I say this because ive just done front and rear discs/pads and front caliper overhaul on mine and upon checking disc temps the rears are pretty darn cold compared to the front, so with your setup maybe the front just grip too well.
Do some progressive braking and feel the temps of the discs
23 January 2019, 07:37 PM
#3
I suspect you need to look at the front/ rear bias after the front brake change. I expect piston area is different resulting in different brake effort on front for given line pressure and the oe brake bias valve is not designed for this, or has been removed?
Could perhaps look at adjustable bias valves or changing rear pads to one's with lower friction coeffs across the used temperature range?
Could perhaps look at adjustable bias valves or changing rear pads to one's with lower friction coeffs across the used temperature range?
23 January 2019, 11:14 PM
#4
Last time I had this issue was with EBC pads that had glazed during low speed use...first four to five hard braking attempts the rears locked, then eventually the fronts cleaned themselves up and got up to temperature and only until then did the fronts actually start doing any proper work.
Then it be ok, until the car was used for slow commutes again and it was back to square one. Swapped fronts for Ferrodos and problem fixed! The main clue was the front discs surfaces gained a dull grey appearance when glazed, and were nice and shiny when ‘cleaned’ and working.
I know these are DC5 pads and different front calipers but maybe checking with other people who use them and under what driving style and see how they behave, it could well be their cars have 4 channel ABS which can mask issues like this (until the fuse is pulled
).
MOT station shouldn’t really be using the two wheel rollers, and even if they do the LSDs (rear and centre) will screw up the readings. It’ll pass as the limits are so pathetically low you literally have to a serious imbalance on the same axle or no brakes to fail.
Then it be ok, until the car was used for slow commutes again and it was back to square one. Swapped fronts for Ferrodos and problem fixed! The main clue was the front discs surfaces gained a dull grey appearance when glazed, and were nice and shiny when ‘cleaned’ and working.
I know these are DC5 pads and different front calipers but maybe checking with other people who use them and under what driving style and see how they behave, it could well be their cars have 4 channel ABS which can mask issues like this (until the fuse is pulled
).MOT station shouldn’t really be using the two wheel rollers, and even if they do the LSDs (rear and centre) will screw up the readings. It’ll pass as the limits are so pathetically low you literally have to a serious imbalance on the same axle or no brakes to fail.
Last edited by ALi-B; 23 January 2019 at 11:24 PM.
24 January 2019, 11:04 AM
#5
As ALi-B said, the problem sounds like it's with the front brakes not the rear, or as tjmatt said, there is too much rear bias.
The front brakes should do most of the work, and should lock up first. I would check the pads and discs at the front. If they don't show any signs of a problem such as glazing, maybe it's the bias.
6-pot AP sound pretty monstrous, and the anti-dive should make the car more front braking biased, as far as I know.
With my car I had the opposite problem, that the rear 1 pots did nothing at all. I could touch the discs and tell that the rears were not being used. My solution was to put EBC orange on the back (I was very lucky and got given EBC blues as part of a trial, so I had them all round in the past). This helped. When the front blues wore out I put Axxis Ultimates on the front, which I believe are less effective than blue stuff. There was no loss of braking, as the rears still don't lock up before the fronts, but the principle is that the rear brakes start to work hard before the fronts lock up.
In my case I had rebuilt the front Subaru 4 pots and the rear 1 pots with new seals etc.
If the bias is favouring too much rear braking, you won't get the benefit of your monster front brakes.
The front brakes should do most of the work, and should lock up first. I would check the pads and discs at the front. If they don't show any signs of a problem such as glazing, maybe it's the bias.
6-pot AP sound pretty monstrous, and the anti-dive should make the car more front braking biased, as far as I know.
With my car I had the opposite problem, that the rear 1 pots did nothing at all. I could touch the discs and tell that the rears were not being used. My solution was to put EBC orange on the back (I was very lucky and got given EBC blues as part of a trial, so I had them all round in the past). This helped. When the front blues wore out I put Axxis Ultimates on the front, which I believe are less effective than blue stuff. There was no loss of braking, as the rears still don't lock up before the fronts, but the principle is that the rear brakes start to work hard before the fronts lock up.
In my case I had rebuilt the front Subaru 4 pots and the rear 1 pots with new seals etc.
If the bias is favouring too much rear braking, you won't get the benefit of your monster front brakes.
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