Slant Six Forum :: Tire lock-up when braking (theory)

Since there has been a couple braking threads go by, I wanted to ask opinions on whether it's better to set the brake system up so the rear wheels lock first or the front wheels lock first, and why.

my Dart is set up so that the front and rear lock up at pretty close to the same time, with the front locking up slightly before the rear (usually). I've never had difficulty keeping the car straight in a panic stop.
My Satellite is all stock, and the rear brakes lock up hard long before the fronts, and in a real panic stop it's near impossible to maintain my lane. It's given me a couple scary stops.

Did a search with the following terms:
'front rear brake bias'
Then just randomly pick a few (about 5 so far)
They all say front should lock first.

If the fronts lock, you'll go straight.

Quote:

If the fronts lock, you'll go straight.

If you're on a straight road with neutral crown, you'll go more-or-less straight. If you're on a curved road or one with other-than-neutral crown, your travel path when the fronts lock will go tangent to the curve. That'll do a nice job of putting you into a ditch, wall, vehicle in the next lane, or whatever else happens to be in your path.

Locked front brakes = you've lost your primary braking power and your ability to steer.

Locked rear brakes = you've lost your secondary braking power only.

Which one sounds less dangerous to you?

It's much easier/safer to modulate the brakes and recover steering than going backwards off the road.

Quote:

If the fronts lock, you'll go straight.

So you've never been in a snap spin in the rain (on a curve) when the rears have locked up............... It went faster than I could counter steer..

It's much easier/safer to modulate the brakes and recover steering than going backwards off the road.

The majority of your braking is up front. Say 70% F / 30% R (varies, but you get the idea)

Quote:

Correct.

Quote:

If you have any unused braking, you want it to be in the lower %

Correct.

Quote:

So the fronts should lock first to get the most out of your brakes.

Wrong. Read what you just wrote! If your fronts lock, the higher % of your braking is going unused.

Quote:

(that's assuming you modulate them and unlock them)

The majority of your braking is up front. Say 70% F / 30% R (varies, but you get the idea)

If you have any unused braking, you want it to be in the lower %... So the fronts should lock first to get the most out of your brakes......... (that's assuming you modulate them and unlock them)

Author:	72polara [ Sun May 13, 2007 8:54 pm ]
Post subject:	Tire lock-up when braking (theory)
Since there has been a couple braking threads go by, I wanted to ask opinions on whether it's better to set the brake system up so the rear wheels lock first or the front wheels lock first, and why. Naturally, it's probably best that all 4 wheels lock at precicely the same instant, but that doesn't really seem acheivable. My theory is that it is best to have the front brakes lock ever so slightly before the rear brakes. Justification being that this creates an understeer situation, which is more predictable for most of us, and you keep going in whatever direction you were going in when the skid occured. Pumping the brakes (on our non-anti-lock cars) will result in a predictable, straight stop. When the rear brakes lock up first, an oversteer situation is effectively created, and the rear end of the car tends to want to pass the front end. Thus, when this happens, you may no longer be going the same direction as when the skid began. You will be required a significant amount of steering input to maintain your lane while panic stopping. My experience seems to be consistent with this - my Dart is set up so that the front and rear lock up at pretty close to the same time, with the front locking up slightly before the rear (usually). I've never had difficulty keeping the car straight in a panic stop. My Satellite is all stock, and the rear brakes lock up hard long before the fronts, and in a real panic stop it's near impossible to maintain my lane. It's given me a couple scary stops. Is my cause-and-effect thinking along the right lines?

Author:	SlantSixDan [ Sun May 13, 2007 9:06 pm ]
Post subject:	Re: Tire lock-up when braking (theory)
Quote: Since there has been a couple braking threads go by, I wanted to ask opinions on whether it's better to set the brake system up so the rear wheels lock first or the front wheels lock first, and why. Generally it's held that if you cannot get all four wheels to lock simultaneously, the rears ought to lock just slightly before the fronts. That way, you can still steer. Once the fronts lose traction, your steering wheel input no longer determines where the car's headed. Quote: my Dart is set up so that the front and rear lock up at pretty close to the same time, with the front locking up slightly before the rear (usually). I've never had difficulty keeping the car straight in a panic stop. My Satellite is all stock, and the rear brakes lock up hard long before the fronts, and in a real panic stop it's near impossible to maintain my lane. It's given me a couple scary stops. OK, but you haven't got comparable cases there to judge front-locks-first vs. rear-locks-first. You've got the Dart on which the fronts lock just slightly before the rears, and the Satellite on which the rears lock long before the fronts. Go put a set of Raybestos WC37696 (13/16" bore vs. stock 15/16") rear wheel cylinders in the Satellite and then either the problem will be all the way solved, or it'll be so significantly reduced that you can spend some time safely comparing the effects of front-locks-slightly-early vs. rear-locks-slightly-early.

Author:	emsvitil [ Sun May 13, 2007 9:45 pm ]
Post subject:
I like the fronts slightly first............ If the fronts lock, you'll go straight. If the rear lock you can end up going straight, going left, going right..... it's pretty unpredictable...... In either case, you're supposed to modulate the pedal unlock the locked brakes... Plus if you're capable of modulating the pedal, you'll get the most out of your brakes if the fronts lock first because the majority of your braking is up front. (with the rears locking first you're not get the most out of your fronts....)

Author:	dakight [ Mon May 14, 2007 5:41 am ]
Post subject:
I had brakes locking up on the front; don't want to do that again! There was absolutely no directional control which was really scary on a crowded freeway.

Author:	440_Magnum [ Mon May 14, 2007 6:22 am ]
Post subject:
First, if EITHER end locks up "significantly" before the other, the system is broken and should be fixed. Now that that obvious bit of trivia is out of the way... if one end has to lock up before the others, it had better be the rears. If the fronts lock first, ALL control is removed because not only have you lost braking effectiveness, you can't steer either. Yes, its technically "understeer" but IMO even mild understeer is far worse than equally mild oversteer under braking. You can drive out of an oversteercondition, but when a car is understeering there's nothing you can do whatsoever except give up a little bit of control in hopes of recovering as a secondary effect. And whats worse, its a razor's edge between understeer and snap oversteer that occurs when a driver cranks in more steering input because of understeer, then the cause of the understeer disappears (ie, you lift off the brakes) the front wheels bite, and the car swaps ends before you can say "Whiskey Tango Foxtrot???"

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Author:	emsvitil [ Mon May 14, 2007 12:20 pm ]
Post subject:	Front first....... (so sayth the internet)
Did a search with the following terms: 'front rear brake bias' Then just randomly pick a few (about 5 so far) They all say front should lock first.

Author:	SlantSixDan [ Mon May 14, 2007 2:00 pm ]
Post subject:	Re: Front first....... (so sayth the internet)
Quote: Did a search with the following terms: 'front rear brake bias' Then just randomly pick a few (about 5 so far) They all say front should lock first. I, uh, don't really think a Google popularity contest is a particularly scientific way of answering a question.

Author:	SlantSixDan [ Mon May 14, 2007 2:09 pm ]
Post subject:
Quote: If the fronts lock, you'll go straight. If you're on a straight road with neutral crown, you'll go more-or-less straight. If you're on a curved road or one with other-than-neutral crown, your travel path when the fronts lock will go tangent to the curve. That'll do a nice job of putting you into a ditch, wall, vehicle in the next lane, or whatever else happens to be in your path. Locked front brakes = you've lost your primary braking power and your ability to steer. Locked rear brakes = you've lost your secondary braking power only. Which one sounds less dangerous to you?

Author:	emsvitil [ Mon May 14, 2007 2:22 pm ]
Post subject:
Quote: Quote: If the fronts lock, you'll go straight. If you're on a straight road with neutral crown, you'll go more-or-less straight. If you're on a curved road or one with other-than-neutral crown, your travel path when the fronts lock will go tangent to the curve. That'll do a nice job of putting you into a ditch, wall, vehicle in the next lane, or whatever else happens to be in your path. Locked front brakes = you've lost your primary braking power and your ability to steer. Locked rear brakes = you've lost your secondary braking power only. Which one sounds less dangerous to you? So you've never been in a snap spin in the rain (on a curve) when the rears have locked up............... It went faster than I could counter steer.. It's much easier/safer to modulate the brakes and recover steering than going backwards off the road.

Author:	SlantSixDan [ Mon May 14, 2007 2:36 pm ]
Post subject:
Quote: It's much easier/safer to modulate the brakes and recover steering than going backwards off the road. I would argue it's much easier/safer to fix and configure the brake system so it works correctly But, failing that, yes, I would absolutely argue from experience that it's safer to have the rears lock slightly before the fronts. OK, you had a wet-road slide that happened before you could countersteer. But if you didn't have time to countersteer, you certainly didn't have time to modulate the brakes and get the steering back. If the fronts had locked, it wouldn't have mattered how quickly or accurately you'd countersteered...you wouldn't even have had a chance to fix it.

Author:	440_Magnum [ Mon May 14, 2007 3:25 pm ]
Post subject:
Quote: Quote: Quote: If the fronts lock, you'll go straight. If you're on a straight road with neutral crown, you'll go more-or-less straight. If you're on a curved road or one with other-than-neutral crown, your travel path when the fronts lock will go tangent to the curve. That'll do a nice job of putting you into a ditch, wall, vehicle in the next lane, or whatever else happens to be in your path. Locked front brakes = you've lost your primary braking power and your ability to steer. Locked rear brakes = you've lost your secondary braking power only. Which one sounds less dangerous to you? So you've never been in a snap spin in the rain (on a curve) when the rears have locked up............... It went faster than I could counter steer.. It's much easier/safer to modulate the brakes and recover steering than going backwards off the road. Look, ALL this discussion is PREDICATED on the fact that neither the front nor rear lock up way before the other. IOW, if the rears lock then the fronts are SO close to also locking that you're not leaving any un-used brake ability on the table. UNDER THOSE CONDITIONS, if you're on a curve and you need so much braking that you lock either end, then you over-drove the corner. Even in that case, I'd still rather be able to steer (rears locked) than not (fronts locked). Yes, you have to steer out of the spin while also modulating the brakes, but at least you are more than a passenger. As for the google search where all the hits say that the fronts should lock first... somehow the safety panzies of today have developed an absolute feral terror of oversteer and think all cars should be built to push like dumptrucks. I guess its a symptom of growing up with nothing but underpowered front-drive cars. Getting a rear-drive car loose used to be fun and educational, because you learned how to control oversteer on power, and how to control understeer on un-powered turn-in.

Author:	emsvitil [ Mon May 14, 2007 11:12 pm ]
Post subject:
The majority of your braking is up front. Say 70% F / 30% R (varies, but you get the idea) If you have any unused braking, you want it to be in the lower %... So the fronts should lock first to get the most out of your brakes......... (that's assuming you modulate them and unlock them)

Author:	Charrlie_S [ Tue May 15, 2007 4:37 am ]
Post subject:
9 inch drums, forever. Then you don't have to worry about locking up either end.

Author:	SlantSixDan [ Tue May 15, 2007 9:01 am ]
Post subject:
Quote: The majority of your braking is up front. Say 70% F / 30% R (varies, but you get the idea) Quote: Correct. Quote: If you have any unused braking, you want it to be in the lower % Correct. Quote: So the fronts should lock first to get the most out of your brakes. Wrong. Read what you just wrote! If your fronts lock, the higher % of your braking is going unused. Quote: (that's assuming you modulate them and unlock them) Again, if you don't have time to correct a rear skid, you don't have time to modulate and unlock the front brakes.

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Author:	440_Magnum [ Tue May 15, 2007 10:32 am ]
Post subject:
Quote: The majority of your braking is up front. Say 70% F / 30% R (varies, but you get the idea) If you have any unused braking, you want it to be in the lower %... So the fronts should lock first to get the most out of your brakes......... (that's assuming you modulate them and unlock them) Go back to what I said before: there should never be more than a few % difference between when the fronts lock and the rears lock. So either way, you only leave a tiny percentage of brake unused. The question is CONTROL. Put some numbers to it: let's say your rears lock first when the fronts are at 95% of their capacity, so you can threshold brake to 100% rear brake capacity and 95% front brake capacity. The unused braking potential left on the table is 5% of 70%, or .05*.7, or .035, which is 3.5%. Conversely, let's say the fronts lock when the rears are at 95% capacity, then the unused braking potential left on the table is 5% of 30%, or .015, or 1.5%. 1.5% unused with NO control left, versus 3.5% unused WITH STEERING CONTROL. I'll take the latter any time, 3.5% vs 1.5% is a trivial, trivial difference!