ABS unit unplug

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Teufelshund
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ABS unit unplug

Post by Teufelshund »

I put in a brake proportioning valve over the winter and no matter how I adjust it, I'm just not getting the braking in the rear that I should. To the point of being able to lay fingers on the rear rotors when I could fry eggs on the front. I've bled and bled and re-bled the brakes by foot and with a pressure pot, so I'm confident there's no air in the system. New (reman) rear calipers. I'm thinking about unplugging the ABS unit and seeing what happens. Opinions on the problem or what unplugging the ABS unit will accomplish, if anything? I'll obviously make sure the brakes are functioning before getting out on the road.
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Re: ABS unit unplug

Post by MARKP »

I don't think unplugging it would create a dangerous situation. I do think that the proportioning valve will mess with its operation, though. You might need to take it completely out of the loop.

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lcoleman
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Re: ABS unit unplug

Post by lcoleman »

So fluid flows strongly out of the rear lines when you bleed it?

I actually just deleted my ABS, installed a bigger master cylinder, and relocated my prop valve to a better location. Still haven't driven it yet, but hopefully that fixes my spongy pedal as well as any ice mode pants-filling incidents like I had at the rainy Pelli event last season.

Keep in mind that a prop valve can only reduce pressure. If you're starting with a system that is biased towards the front (either from staggered MC pistons, different caliper piston sizes, rotor diameters, or some combination of these) and adding a prop valve onto the rear lines, it's only going to make the problem worse.
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jcoatney
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Re: ABS unit unplug

Post by jcoatney »

If you don't plan on going back to ABS I would ditch the unit. This guy http://prbmachining-absdeletemanifolds.com/ makes a delete manifold and I think it drops about 15 lbs off the front of the car.
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MikeKelly
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Re: ABS unit unplug

Post by MikeKelly »

lcoleman wrote:
Keep in mind that a prop valve can only reduce pressure. If you're starting with a system that is biased towards the front (either from staggered MC pistons, different caliper piston sizes, rotor diameters, or some combination of these) and adding a prop valve onto the rear lines, it's only going to make the problem worse.
Bingo.

Off the top of my head - Mustang rear calipers (all) have 38mm (1.5") pistons and SN95 front single piston calipers are ~2.5"

If you go to the later model (and Cobra) double piston front calipers, your bias gets even worse (Detroit mostly says you don't need effective rear brakes, we might get sued over them).


Some of the (cheap or rule limited) spec racing guys put the proportioning valve in the front circuit and dork with it until they get satisfactory results.


I will warn you that increasing your rear bias can be hazardous - it can quickly produce a very unstable car if you go just a little too far.


It can and will bite you hard and fast - forewarned is forearmed... (skull and crossbones emoji needed here - it's deadly serious)


Ask Ricky Rudd - this incident got driver adjustable bias banned from Nascar (watch what it did to a pro):


https://www.youtube.com/watch?v=cYERm7bzM0s
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Teufelshund
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Re: ABS unit unplug

Post by Teufelshund »

It has a new manual master cylinder and Cobra calipers all the way around. I am planning on using ABS, so I'd like to get it fixed.
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Re: ABS unit unplug

Post by MikeKelly »

So what I'm telling you is that the 2 piston Cobra (and late model) front calipers have so much more piston area than your 38mm piston rear calipers you're not likely to ever get effective rear braking out of them with a conventional tandem MC in a heavily sprung car (=less weight transfer to front on heavy braking).

You could try more aggressive pads in the rear and less aggressive pads in the front, but that doesn't leave much opportunity for adjustment - this is where you could use the proportioning valve in the rear to dial it down once you get too much.


Jim and I were both using single piston front calipers with dual MC setups, 5/8"MC to the rear and 3/4"MC to the front to get enough pressure to make the rear calipers effective (and I am running 45mm piston Tbird rear calipers).

Jim has gone to Wilwood all the way around (4 piston rear, 6 piston front), and I think he's running 3/4 x 3/4 dual MC now.

I am still running 5/8" to the rear and 3/4 to the front.


Dual MC systems don't typically plumb nice with ABS, and require more force to get equal pressures.



Tandem MC:

Image

Dual MC (w/ balance bar):

Image




P.S. - In my experience Mustang rears are notoriously slow to bed in with new pads, and once they do bed in they're far more effective than they were when the new pads went on. So if you just put new pads on proceed with great caution, because they might bite you...
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Re: ABS unit unplug

Post by lcoleman »

MikeKelly wrote:Some of the (cheap or rule limited) spec racing guys put the proportioning valve in the front circuit and dork with it until they get satisfactory results.
This is a cheap/easy solution, but I believe it will increase pedal effort (or the distance you have to push the pedal, or both, not sure) to produce the same pressure in the front and therefore the same stopping performance. Just noting this for posterity. I'm cheap/easy, so if my ABS delete doesn't work out like I want I'll probably just swap the bias valve to the front brake circuit. I already have a master cylinder with larger pistons, which increases pedal effort but decreases the necessary pedal distance, so that should work OK....in theory.

Running pads with high initial bite in the rear and crappy pads up front is the normal go-to without mucking with the hydraulic system, at least for autocross. It helps, but it may not help enough.

Using ABS and changing the brake bias to suit may or may not be possible. The ABS system may not work as intended if you start changing the relative pressure going to each axle. YMMV, try at your own risk.
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MikeKelly
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Re: ABS unit unplug

Post by MikeKelly »

lcoleman wrote:
This is a cheap/easy solution, but I believe it will increase pedal effort (or the distance you have to push the pedal, or both, not sure) to produce the same pressure in the front and therefore the same stopping performance. Just noting this for posterity. I'm cheap/easy, so if my ABS delete doesn't work out like I want I'll probably just swap the bias valve to the front brake circuit. I already have a master cylinder with larger pistons, which increases pedal effort but decreases the necessary pedal distance, so that should work OK....in theory.

Running pads with high initial bite in the rear and crappy pads up front is the normal go-to without mucking with the hydraulic system, at least for autocross. It helps, but it may not help enough.

Using ABS and changing the brake bias to suit may or may not be possible. The ABS system may not work as intended if you start changing the relative pressure going to each axle. YMMV, try at your own risk.

I haven't heard any complaints (at all) from the spec racer types about pedal pressure going up.

I have heard a lot of technical disapproval from engineering types due to "tip-in" issues using a proportioning valve on the front.

I would say maybe, try it, test it, and watch closely for any weirdness - specifically weirdness in sudden pressure curve transitions (tip-in) that cause one end or the other to act up.


heh, I hope no one thought Jim and I both spent $275 on dual MC systems so we could polish them with diapers and talk about how cool they were.

It was the only thing we could come up with to get our brake bias under positive control...


Pedal effort is nothing to sneeze at (dual MC w/ balance bar).

And a 5/8" MC is pushing the ragged edge of (barely) moving enough fluid (single stroke) to take up rotor run out (x2 38mm or 45mm single piston calipers).

That gets worse (rotor runout, piston pushback - requiring "fullish" stroke of MC) with 8.8" axle flex on solid axle autocross / rr cars - I need a little less due to IRS (a little less flexible):

https://www.youtube.com/watch?v=eNDjVobCnGk
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90civichb
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Re: ABS unit unplug

Post by 90civichb »

That dual MC with balance bar is fancy.

Brian, most things I read about a bias valve on an ABS car is usually to avoid it. I am not sure how an ABS system works so I really can't talk theory of operation but, people seem to have issues with the operation of ABS while running a bias valve. I think going full retard on the manual MC with a balance bar is the best option before getting crazy on a whole pedal box assembly.

I ran crap big box store pads up front on my stock 1.6L brake setup and went to a larger 1.8L rotor with more aggressive pad (Hawk DTC-30) in the rear on the Miata. This helped tremendously with bias.

Another point on pedal effort with a manual MC. I was reading up on this the other day and many people (atleast with a Miata) will move the pivot point on the pedal, 10mm or so, to help with pedal effort. Keeping the pinion 90* to the MC but the point of leverage moved upwards.
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Re: ABS unit unplug

Post by jcox07 »

Upgrade your entire system to the 2010 gt500 system to include master cylinder and brake booster and that should fix the problems.
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Re: ABS unit unplug

Post by thrdeye »

It's times like this that I am thankful for Street category.
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Re: ABS unit unplug

Post by MikeKelly »

90civichb wrote:
Another point on pedal effort with a manual MC. I was reading up on this the other day and many people (at least with a Miata) will move the pivot point on the pedal, 10mm or so, to help with pedal effort. Keeping the pinion 90* to the MC but the point of leverage moved upwards.

Just another data point offered - your typical detroit power brake pedal is going to have ~3:1 leverage ratio.

You're going to need 6:1 leverage for (typical detroit) manual brakes, minimum (I presume Brian has already done this mod).


I am closer to 6.5:1, but you quickly run out of vertical room and pedal travel.
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