Omni GLH with big brakes problem

[iTurbo]

I sold my old beloved Omni GLH Turbo to my older brother, but I still take care of it (warranty haha).

He recently told me that the brakes need to be bled. He described the problem and it sounds to me like he was having a problem with braking on icy roads on the way home on the highway with premature brake lockup.

I've driven the car recently, and it has a nice firm pedal. It's just fine when it's dry, but gets a little hairy with bad weather.

The brake system consists of:

Fronts: SLH III package front brakes from an '89 Caravan. 10" plain vented disc (similar to stock), Single pin KH calipers with 60mm piston and Raybestos PG+ pads.

Rears: Vented rear discs from a Spirit R/T. Plain 11" vented rotors with Raybestos SuperStop pads.

Hydraulics: 24mm MC, braided steel line set from FWDP. Proportioning valve is from a 4-wheel disc car. DOT 4 fluid.



My gut feeling is that there is simply too much rear brake bias. I have thought of a few options to correct this:

1. Reinstall stock drum brakes. I would have to locate all the parts because I no longer have them.

2. Install smaller 10" solid rear discs. I have the parts.

3. Install even larger front brakes to induce more balance. I have the parts for this as well, from a Spirit R/T.

4. Buy two adjustable prop valves and all the work plumbing them in and dialing them in.

5. Install the cheapest non-aggressive rear brake pads ever.



What would you do? Personally I like option #3 but...maybe a very non-aggressive rear pad could be the easiest fix here? Back when I delivered pizza in the car, I was aware of the brake bias and had lots of fun initiating a drift down Foothills Blvd 'corkscrew' by just jabbing the rear brake. It was fun as hell and the car never got away from me......but now my brother owns the car and it is mostly highway driven to/from work.

[AngelesRunner]

Go with 200 or 220 mm rear drum brakes which are everywhere in the auto parts yards. Lighter and plenty of stopping power. You may find a car with a recent brake job which has almost new hardware. Or just get the backing plates and proportioning valve and buy the rest new at AutoBone.

[cordes]

There's no way around the fact that you need to go to some wimpy brakes in the rear. I had the same brake setup you describe on my black Omni and it was unreal with some DOT legal autocross tires on there when dry and warm. If it was cold or wet the rears would lock up and the back end would step out. IMO I don't think you could get enough tire on the front for the bigger brakes to keep the rears in check.

[wheming]

Just do #4 and be done with it.
minivans have that fancy valve that changes the rear brake pressure when the car is in heavy decel and the back end lifts...
This was bad on MeanMini when my brother Dan did all the brakes and brakelines when i fjrst got her. Since that valve is not available we decided on those needle valves to help the rears not lock up.
I'm sure they would work just as well in your application. Got them from Jeg's iirc. You might even get a hat!


http://www.jegs.com/i/JEGS/555/63022...cvwaAqnq8P8HAQ

[contraption22]

If available, you could try ceramic pads on the rear. They have a low CF and require more clamping force to stop.

[contraption22]

You could try ceramic pads on the rear, and shaving them down for less contact area.

[tryingbe]

I vote #2.

[Dr. Johny Dodge]

remembering how much of a difference my 200 pound buddy made sitting in the passenger side of my turismo , years ago , maybe some weight in the trunk would help

the car is light so the brake lock up is probably much like the issue I had when I put the tires with the least tread on the back - ok fine till it rained then the back would slide out

with no traction the car's simply not heavy enough to keep the wheels turning when the rear brakes are applied - same as my turismo wasn't heavy enough to press the back tires down hard enough to keep traction in the rain

just remember to strap the weight down - a guy here had a buddy die when the patio stones his father put in the back of a 5 litre mustang went into the passenger compartment when the car ran into something

[iTurbo]

I really appreciate all your input guys! It looks like I'll be working on the car the winter to fix and upgrade things. I've given my brother "carte blanche".......that might mean a simple Spearco intercooler upgrade to the stock TI (which is already fairly built), or even a 16v, but no 707 allowed!

All of the options have their plusses and minuses.....I will have some fun with the car myself before I decide, but I appreciate the input. Cordes hit the nail on the head about the car's behavior.

In the end....I would pay to see Ken Block drive the car. I mean, it would be no SF video, but still.

[tryingbe]

1. Reinstall stock drum brakes. I would have to locate all the parts because I no longer have them.

Cons, you'll have to buy the parts. Drum brakes sucks to deal with.
Pro, work like stock, probably.

2. Install smaller 10" solid rear discs. I have the parts.

Cons? There is no cons other than you'll have to spend an hour, maybe two if you put in some new bearings.
Pro, works like stock, fit like stock, bolt them up, bleed the brakes (come to think of it, does 11 inch disc and 10 inch disc uses the same caliper and pads?) and you'll done. This is the setup I have, and I never experience fishtail while braking.

3. Install even larger front brakes to induce more balance. I have the parts for this as well, from a Spirit R/T.

Cons, you'll have to make the parts fit. GLH strut doesn't fit the R/T knuckle without modification, then you'll have to align the car, more expensive than #2 option.
Pros, stops better? Maybe. I bet you'll still have the problem as there is no weight in the back of the Omni.

4. Buy two adjustable prop valves and all the work plumbing them in and dialing them in.

Cons, unknown part, unknown fit, unknown performance, and you'll have to spend money. In theory, it should work good.
Pros, adjust, adjust, and adjust somemore! Wait, I think this is a con as well, since you'll never finish adjusting.

5. Install the cheapest non-aggressive rear brake pads ever.

Cons, next person who is not you that changed the pads will bring the problem right back, as if it solves the issue in the first place. Dealing with unknown here.
Pros, cheap? Not cheaper than using the parts you already got.


Which option is the cheapest? #2, as you have the parts and there is nothing to buy.

Which option is most likely solve your issue? #2, as this is how cars came from factory and many people with Omni/Charger have the minivan front brakes and 10 inch rear disc with no report of fishtailing while braking.

Which option takes the shortest time? #2, no parts to order, no parts to wait for, no parts to go to the parts store for, install the parts in an hour or two and you're done. No dialing/adjusting afterward.

Am I bias to #2? Yes, I know it works.

[ajakeski]

10 & 11 inch rear disc brakes use the same caliper and pad. The hub is the same too.
All that is different are the backing plate and rotor.
The parking brake parts are interchangable as well.

[Force Fed Mopar]

Solid rear rotors and be done.

[cordes]

What kind of tires do you solid rear rotor guys use?

[supercrackerbox]

I've got 205/50/16 Firehawk Wide Ovals on my Charger. They've been discontinued for many years now, however.

[tryingbe]

What kind of tires do you solid rear rotor guys use?

Had Kumho Ecsta Supra 712 195/50R15, Nitto Neo Gen 205/50R15, Dunlop Direzza DZ101 205/50R15, and currently running Hercules RAPTIS WR1 205/55R16.

Experience no fishtailing with any. Understeer on the other hand...

[GLHNSLHT2]

I say go with the 10" rears as well. Then play with pad choice to fine tune in the balance if need be.

[cordes]

Had Kumho Ecsta Supra 712 195/50R15, Nitto Neo Gen 205/50R15, Dunlop Direzza DZ101 205/50R15, and currently running Hercules RAPTIS WR1 205/55R16.

Experience no fishtailing with any. Understeer on the other hand...

Even on wet roads when it's 50-60* or so out?

[AngelesRunner]

The equation for friction is F=un where friction is a product of u (the coefficient of friction) and n (the force perpendicular to the plane of the motion). The amount of force generated is not dependent on the area of contact between the two rubbing surfaces. Your suggestion to use lower coefficient of friction pads will reduce braking force but shaving them down will just make them wear out faster. Sorry, physics.

Bigger brakes effectively work by applying the force of friction further from the center of rotation. You generate more torque by using a longer moment arm. Think, longer breaker bar. Bigger brakes also give you more area for heat dissipation and thus, less fade.

The other side of brakes is that all of the weight moves with the wheel. Bigger rotors and drums have to be accelerated to go faster and bigger rotors, drums AND bigger hardware (adapters, calipers, pads) drums and rotors gets bumped by the road irregularities and will move! The motion upward by heavier brake hardware has to be stopped from impacting the vehicle body through the bump stops by heftier springs or by further travel than stock hardware when the spring rate is not increased. The oscillation has to be damped out by the shock absorber, heating the oil by pumping it through holes.

I would suggest the 200 mm rear drums from the standpoint of lower unsprung weight and a larger diameter wheel cylinder if more force is required. I would go to the 220 mm rear drums if more braking force was required and again, a bigger wheel cylinder if more force is required. Remember that both of these options can lock up the rear wheels against any stock tires.

I would go to lighter unventilated rear disks, if needed, for the original reason that all cars went to front disks, I have to stop or slow over and over and I need the heat dissipation and thereby fade resistance of disks. I.E. autocross style driving. Said by someone who experienced front drum fade on both a 67 Camaro 327 and a 67 Corvair.

I have wrenched cars for >45 years, and been a biomedical engineer for 38 years.

Horizon x2, Spirit/Acclaim x5, Shadow, 1st gen turbo Voyager, 2nd gen Caravan 2.5, wife's Escort ZX2

[Force Fed Mopar]

I ran 205's on my GLHS w/ solid rears, worked great. Then ran 225's up front, still no issue.

[TheCanadian007]

+1 to solid rear discs.

From what I recall, the calipers are different on the 11" vented rear brakes vs the 10" solid discs. Rear pads are the same for both.