Can you lock up your disc brakes?

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racerjoe

racerjoe

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Curious if anyone with 4 wheel, manual disc brakes can lock up the tires under hard braking? I have 13.75" rotors with Brembo 4 pistons up front, mustang cobra discs in the back. 15/16" bore master cylinder, and adjustable prop valve. Although, the valve is completely open and not regulating any pressure to the back brakes. Also, the brakes are manual, no hydroboost or vaccum booster. I also have a 245 tire up front and a 285 tire out back. Tires are 200 tread wear. I can't lock the tires up, but it stops really well. It is a combination of the "sticky" tires and not having some type of power assist? Or is there an underlying problem I'm not aware of?
 
I bet if you put some 70/205s on there you could lock them up all day long. My guess is you have excess traction.

On the plus side you also have controle in a panic stopping situation
 
Sounds like you built your own version of ABS.Have you measured the stopping distance at a said speed? Say 30 MPH ?
That would be my main concern. Check what a new car in similar weight stops at 30 MPH and do a test (yeah, all new cars have power brakes,but you get the Idea)I have SSBC manual 4 wheel discs on my 65 B body. It stops well and short without any lock up. I set my prop valve till the rears locked and then backed off 2 turns.Works for me.
I understand you have yours wide open. If you are pleased with the way it stops,sounds good to me.
 
I have just the KH 4-piston stuff up front and 10" drums out back. Tires are 235/60-14s and 295/50-15s , all BFG Radial T/As,or Cooper Cobras, and with no proportioning at all. I also use the 15/16s M/C, cuz it has excellent feed-back.
and yes it will lock up the fronts, to 30 mph at least, which is where my testing stopped.
This 68 Barracuda is the best stopping car I have ever owned; IMO, for a streeter, it stops fantastically, lol.
 
Darter6 said:
Sounds like you built your own version of ABS.Have you measured the stopping distance at a said speed? Say 30 MPH ?
That would be my main concern. Check what a new car in similar weight stops at 30 MPH and do a test (yeah, all new cars have power brakes,but you get the Idea)I have SSBC manual 4 wheel discs on my 65 B body. It stops well and short without any lock up. I set my prop valve till the rears locked and then backed off 2 turns.Works for me.
I understand you have yours wide open. If you are pleased with the way it stops,sounds good to me.
Click to expand...

This is pretty much the worst possible way to adjust a prop valve, and is also a piss poor assessment of braking performance.

If it won't lock the FRONTS you're losing stopping ability. If it EVER locks the rears, you're spinning out of control whether you like it or not.

There's so much more to this equation, it's not even funny. What condition are the rear shocks in? If they're blown, your car will have insane stopping power due to the weight transfer. My neon used to stop so hard it would literally make you light headed and nauseous. It also bounced down the freeway. New rear struts, and now it locks the fronts, predictably and calmly.

Riding the edge between stopping and slipping the fronts is called threshold braking. It's what ABS does, and it's what you should learn to do, because it's the maximum stopping power: there's no more grip to be had, but you're not sliding. Anything else, and you're giving up braking performance and that might be the foot or two you needed to avoid a crash.

Road condition, tire age and type (as mentioned), existing weight bias (full tank or running on fumes), brake fade, even the presence of a passenger or four, all count in the braking equation. A good hard stop without locking the rears is NOT the determination of a prop valve adjustment, nor is backing it off a couple turns to call it good.

Begin at 30 mph on a DESERTED highway that's lightly slicked with rain. You should be able to lock the fronts and have NO indication of rear lockup. Up your speed slightly, rinse, repeat. If your tires are worn or older, that's in your favor for this test. Adjust to get the rears to lock and back off for the max performance stops, but add in a generous safety factor to prevent rear lockup.

You should be able to lock the fronts without the rears ever locking. If the rears lock, you WILL spin.

Once the fronts lock without the rears, THEN you can learn threshold braking. If you ever change anything in the system, even just new pads, it's best to verify these tests again.
 
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I don't get the big deal about not running a booster. Mine doesn't build vacuum in the chamber at idle either, but as soon as the engine picks up a few rpm, or I blip the throttle, there it is. I don't care about boost at idle because I have a manual trans, and when stopped, I don't need any boost. My booster will hold plenty of vacuum to stop the car at least once from 60mph with the engine off. But, if I slam the throttle shut in gear, the compression braking, at 11/1, works pretty good too, lol. What some guys might not know is that you can alter the assist by adjusting the pushrod length; you don't have to run it instant-on to the max, like Grandpa's FifthAvenue.
 
jos51700 said:
This is pretty much the worst possible way to adjust a prop valve, and is also a piss poor assessment of braking performance.

If it won't lock the FRONTS you're losing stopping ability. If it EVER locks the rears, you're spinning out of control whether you like it or not.

There's so much more to this equation, it's not even funny. What condition are the rear shocks in? If they're blown, your car will have insane stopping power due to the weight transfer. My neon used to stop so hard it would literally make you light headed and nauseous. It also bounced down the freeway. New rear struts, and now it locks the fronts, predictably and calmly.

Riding the edge between stopping and slipping the fronts is called threshold braking. It's what ABS does, and it's what you should learn to do, because it's the maximum stopping power: there's no more grip to be had, but you're not sliding. Anything else, and you're giving up braking performance and that might be the foot or two you needed to avoid a crash.

Road condition, tire age and type (as mentioned), existing weight bias (full tank or running on fumes), brake fade, even the presence of a passenger or four, all count in the braking equation. A good hard stop without locking the rears is NOT the determination of a prop valve adjustment, nor is backing it off a couple turns to call it good.

Begin at 30 mph on a DESERTED highway that's lightly slicked with rain. You should be able to lock the fronts and have NO indication of rear lockup. Up your speed slightly, rinse, repeat. If your tires are worn or older, that's in your favor for this test. Adjust to get the rears to lock and back off for the max performance stops, but add in a generous safety factor to prevent rear lockup.

You should be able to lock the fronts without the rears ever locking. If the rears lock, you WILL spin.

Once the fronts lock without the rears, THEN you can learn threshold braking. If you ever change anything in the system, even just new pads, it's best to verify these tests again.
Click to expand...

My car is build for handling with loads of Hotchkis parts, and large t-bars. Worn out parts aren't even part of this discussion. After about 8 hard stops to bed the new pads in, I was starting to feel a bit light headed. I'm sure if I had a passenger, they would have gotten nauseous. I think I'll do a 30mph and 60mph stopping distance measurement and compare it to my '19 Ram. I bet the Duster is better.
 
racerjoe said:
My car is build for handling with loads of Hotchkis parts, and large t-bars. Worn out parts aren't even part of this discussion. After about 8 hard stops to bed the new pads in, I was starting to feel a bit light headed. I'm sure if I had a passenger, they would have gotten nauseous. I think I'll do a 30mph and 60mph stopping distance measurement and compare it to my '19 Ram. I bet the Duster is better.
Click to expand...

But is it the best it can be? That's the question... :poke:

Besides, brakes on pickups are always compromised. Bed loaded? Bed empty? Trailer?

That's why Chrysler had all those ride height detection and weight distribution detection schemes in the minivans, and factory rwabs in the early 90's
 
jos51700 said:
But is it the best it can be? That's the question... :poke:

Besides, brakes on pickups are always compromised. Bed loaded? Bed empty? Trailer?

That's why Chrysler had all those ride height detection and weight distribution detection schemes in the minivans, and factory rwabs in the early 90's
Click to expand...
Hell if I know if it’s the best it can be. I’ll see how it does at the Holley Moparty speed stop challenge. Hopefully I won’t be known as a cone killer.
 
Update on this topic. I put some heat in the pads by riding the brakes and I was able to lock them up. This car stops incredibly well!
 
That's a lot of brake, and a lot of tire, but you should be able to lock them up, and you want to be able to lock them up.

You shouldn't have to "build heat" to make it happen, you won't have time to in a panic stop.

I would talk to someone who knows brake pads (like a Brembo or a Wilwood rep) and have them recommend a pad for you. It's a bit like getting the correct cam or torque converter.

You may need to play around with different master cylinder bores and/or pedal ratios to optimize your setup.
 
Just thought of this... if you didn't break in your new pads properly, they can still feel kind of slick, and could potentially prevent lockup.

Maybe by riding your brakes, you completed the break-in, and now they are functioning as they should.
 
Ask yourself "when is braking least effective?" The answer to that is "In a slide". So with that in mind, it sounds like you have a pretty good setup now.

Just for experiment's sake, get out and try some hard braking on wet pavement. Not at speeds to lose control of course, but to see IF it will lock the brakes, front, rear, or both and if one or the other locks first or both.
 
racerjoe said:
Curious if anyone with 4 wheel, manual disc brakes can lock up the tires under hard braking? I have 13.75" rotors with Brembo 4 pistons up front, mustang cobra discs in the back. 15/16" bore master cylinder, and adjustable prop valve. Although, the valve is completely open and not regulating any pressure to the back brakes. Also, the brakes are manual, no hydroboost or vaccum booster. I also have a 245 tire up front and a 285 tire out back. Tires are 200 tread wear. I can't lock the tires up, but it stops really well. It is a combination of the "sticky" tires and not having some type of power assist? Or is there an underlying problem I'm not aware of?
Click to expand...

I cant either, 205 70 15 on front , 295 60 d15 on rear , 15/16 master, 4 wheel disc, does stop pretty well tho.
 
jos51700 said:
This is pretty much the worst possible way to adjust a prop valve, and is also a piss poor assessment of braking performance.

If it won't lock the FRONTS you're losing stopping ability. If it EVER locks the rears, you're spinning out of control whether you like it or not.

There's so much more to this equation, it's not even funny. What condition are the rear shocks in? If they're blown, your car will have insane stopping power due to the weight transfer. My neon used to stop so hard it would literally make you light headed and nauseous. It also bounced down the freeway. New rear struts, and now it locks the fronts, predictably and calmly.

Riding the edge between stopping and slipping the fronts is called threshold braking. It's what ABS does, and it's what you should learn to do, because it's the maximum stopping power: there's no more grip to be had, but you're not sliding. Anything else, and you're giving up braking performance and that might be the foot or two you needed to avoid a crash.

Road condition, tire age and type (as mentioned), existing weight bias (full tank or running on fumes), brake fade, even the presence of a passenger or four, all count in the braking equation. A good hard stop without locking the rears is NOT the determination of a prop valve adjustment, nor is backing it off a couple turns to call it good.

Begin at 30 mph on a DESERTED highway that's lightly slicked with rain. You should be able to lock the fronts and have NO indication of rear lockup. Up your speed slightly, rinse, repeat. If your tires are worn or older, that's in your favor for this test. Adjust to get the rears to lock and back off for the max performance stops, but add in a generous safety factor to prevent rear lockup.

You should be able to lock the fronts without the rears ever locking. If the rears lock, you WILL spin.

Once the fronts lock without the rears, THEN you can learn threshold braking. If you ever change anything in the system, even just new pads, it's best to verify these tests again.
Click to expand...

this is *** backwards from what a brand new 68 forrm S barracuda would do from the factory , in a panic stop , front discs would not lock up , the back drums would !
 
flingdingo said:
Just thought of this... if you didn't break in your new pads properly, they can still feel kind of slick, and could potentially prevent lockup.

Maybe by riding your brakes, you completed the break-in, and now they are functioning as they should.
Click to expand...

I think this was the problem. For the record, I had just installed the big brakes on the front and had practically zero miles on them. Now I have some regular driving along with riding them to bed them in.
 
bob mCcarty said:
this is *** backwards from what a brand new 68 forrm S barracuda would do from the factory , in a panic stop , front discs would not lock up , the back drums would !
Click to expand...

Agreed 100%, but that ain't right
 
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