Slotted/drilled myth?

[Legendary]

For stock tires/suspension - replacing the rear drums with discs does virtually nothing to improve stopping power of the car. Mod the suspension to limit dive, or add more traction - and the variables change.

Seeing that my suspension is modded to fight brake dive I don't feel so bad about your statement!

 

[Michael Yount]

...and remember, limiting dive through spring rate is different than building 'anti-dive' in through revised suspension geometry/design.

 

[93 teal terror]

The leg bone's connected to the hip bone -- rotor's connected to the hub's connected to the wheel's connected to the tire. The tire locks very shortly after the rotor does - due only to the flex in the tire's sidewall.

If what you say is true then bigger brakes indeed increase stopping power. The idea being to try to limit brake lockup. So you use more surface area (ie. bigger rotors, calipers) to decrease the psi applied to the pads/rotors.
Example:
Lets say the lockup on brakes is 20psi (just a guess). thats on a brake setup with a brake contact area of 20in squared. So your stopping power would be 400lbs. Now lets say you have bigger rotors/pads with the same lockup capacity. Now the only differece is that you have 30in squared of contact area. this allows you 600lbs of force before the brakes lock up. Sure every brake setup locks up, its just how much force can you apply before the brakes do lock up. Otherwise we'd all be running around with honda brakes and the grippiest tire we could find. But like I said I have no proof if the brakes lock up before the tires, but it generally makes more sense that way, and explains bigger brakes, calipers, etc. that we've all seen before.

 

[Daggar]

If what you say is true then bigger brakes indeed increase stopping power. The idea being to try to limit brake lockup. So you use more surface area (ie. bigger rotors, calipers) to decrease the psi applied to the pads/rotors.
Example:
Lets say the lockup on brakes is 20psi (just a guess). thats on a brake setup with a brake contact area of 20in squared. So your stopping power would be 400lbs. Now lets say you have bigger rotors/pads with the same lockup capacity. Now the only differece is that you have 30in squared of contact area. this allows you 600lbs of force before the brakes lock up. Sure every brake setup locks up, its just how much force can you apply before the brakes do lock up. Otherwise we'd all be running around with honda brakes and the grippiest tire we could find. But like I said I have no proof if the brakes lock up before the tires, but it generally makes more sense that way, and explains bigger brakes, calipers, etc. that we've all seen before.

But wouldn't it also make sense that with larger brakes and a larger surface area (rotor, drum, whatever) that it would be easier to control that threshold between being "locked up" and supplying maximum stopping power without skidding the tire across the surface? That can account for allot of the performance increases in braking distances, I would think. From what I've read from varous places here and there; maximum braking occurs BEFORE the tire locks up and starts skidding (result of wheel hop, harder sub-surface layers of tire, stretch limit on sidewall, etc). Isn't that correct?

 

[HighMileage]

used cross drilled and slotted ... F1, NASCAR and the rest alike all use them. Them guys know their stuff and there is a reason that they use them.

I heard the exact opposite about professionals.
Anyone else?


Seems to me that drilling came to lessen the weight of larger rotors, to aid acceleration and braking. Slotting came as another good way to make money, without a better practical reason. Both weaken the rotor. If such rotors get to be under enough racing stress, they will fail. I imagine that swiss-cheesed parts will fail before scratched-up parts?

 

[93 teal terror]

But wouldn't it also make sense that with larger brakes and a larger surface area (rotor, drum, whatever) that it would be easier to control that threshold between being "locked up" and supplying maximum stopping power without skidding the tire across the surface? That can account for allot of the performance increases in braking distances, I would think. From what I've read from varous places here and there; maximum braking occurs BEFORE the tire locks up and starts skidding (result of wheel hop, harder sub-surface layers of tire, stretch limit on sidewall, etc). Isn't that correct?

I agree, it would seem that this would allow better control between the limit and skidding. The more I think about it, it would seem that there needs to be a good mixture between the tire and the brakes. Obviously you can't have an extremely thin, bald tire or your stopping distance will be controlled by the tires. At the same time you can't have tiny brakes or they will be the cause of lockup.

 

[93 teal terror]

Couldn't have good brakes that work so well that you wouldn't have to lock up the tires? Then the tires wouldn't too much make a difference in braking. With my rear disc & 73mm front set up I can brake hard and not lock up at all. And it brakes alot better than when I had the stock brakes. That's without changing tires.

I think thats the general idea. Tire COULD be an issure at some point, but for most practical purposes, its the brakes that lock up not the tires.

 

[93 teal terror]

In that case, stickier tires will stop you shorter - IF - big if - your brakes will hold the stickier tire on it's higher adhesion limit. Most of the foxes don't have abs -- but, see what you've got already before you changing things to be certain you're not gonna be disappointed with the result.

I think thats the key is IF...which I think most braking systems, definatly not stock fox brakes aren't capable of.

 

[Michael Yount]

You're zero-ing in on it. All that stops the car is the friction interface between tire and pavement. IF - there's the test - the brakes on the car will get the most out of the tire (same as saying the brakes will hold it on the limit of sliding for the entire stop), then all bigger brakes will do is lessen the force needed to be applied at the pedal. They won't stop you any quicker under those conditions. They will disapate heat more quickly, setting you up to be effective on the next stop.

Whether that allows for better modulation depends on the circumstances. On the street - probably it allows for more control. On the track (at least my experience road racing) - I want to be able to apply pretty hard pressure on the pedal. Adrenaline gets pumping pretty good, and having to 'cub bear' the pedal doesn't work as well for me. What I don't want to happen on the track is to accidentally lock them because I applied a bit too much pressure. It's more of an issue on the track because you're breaking so hard most of the time. I prefer more force requirement on the track, not less.

As for what Fox brakes will do or not - I think earlier I said you should go out and actually test what your brakes will do and won't do. It's the only way to know what they're capable of.

Legendary said - "Then the tires wouldn't too much make a difference in braking." Incorrect - tires make ALL the difference if you have enough brake to use them. Again - it's the friction interface between the tire and the road surface that stops the car.

Teal said - "but for most practical purposes, its the brakes that lock up not the tires." Again - you can't have one without the other because they're connected. If you lock the rotor, you'll slide the tire. If you don't slide the tire, then the rotor's turning too. The dynamic condition of one matches the other.

 

[Legendary]

...and remember, limiting dive through spring rate is different than building 'anti-dive' in through revised suspension geometry/design.

Can you explain this a little. I'm interested.

 

[Michael Yount]

Yikes - complex; antidive and antisquat are suspension characteristics that define how much of the weight transfer and torque reactions of the body when accelerating (antisquat) or when braking (antidive) are absorbed by the springs vs. how much is handled by torque reactions in the suspension control arms. It's largely a function of the center of gravity of the car, and the pickup points of the suspension control arms. Do a google search to get some more detailed perspectives -- here's a reference that does a pretty good job of explaining it. If you've ever seen a late model BMW under heavy braking you can watch a lot of anti-dive built into the suspension geometry at work. Rather than the front end dropping way down and the back end riding sky-high, the whole car just sort of squats down and stops. Without so much of the weight transferring to the front tires, the rear tires (and brakes) are able to handle more of the stopping load. Which means you have effectively more rubber friction patches trying to slow the car.

http://www.circletrack.com/techarticles/80727/

 

[astronut1885]

I agree with just about everything Mike said. I've run slotted/drilled rotors on my last two stangs, including on my current 5 lug/ SN95 disc stang. The only things I noticed that were an inprovement were the look of the brakes, and slightly less fade. The rear disc conversion really didn't help much IMO, which supports Mike's weight transfer idea. I think more came from the addition of 17x8s with sticky tires and SN95 front brakes that are 1" bigger with bigger calipers. My car has reduced front weight, and progressive rate lowering springs, but my nose will still dip a bit under braking. This means more downforce is on the front wheels, putting more stress on them under braking than the rears.
The one thing people seem to be off on, unless I am reading it wrong, is that bigger brakes will make a car stop faster. This is true to an extent. Mike explains it perfectly, although very complicated. Tires are the final word in braking. Imagine driving on ice. You slam the brakes, and I don't care if you're on the biggest, baddest Baers out there, you're going to slide. That's because the tires have no grip. On asphalt, it's the same concept. Bigger brakes don't mean diddly once the tire's traction limit is reached. 14" brakes might take more pressure on the pedal to lock them up over say 16" brakes, but once you do the result is still a flat spotted tire, a skid mark, and a bad 60-0. Upgrading brakes beyond tire capacity is like throwing 3" exhaust on a stock motor. It might look cool, but it ain't helping jack.
Bigger brakes do provide two advantages though. Because they are bigger, and the caliper sits further out from the hub, they have more leverage, or braking torque. This will slow the wheel down to it's lockup faster. If you have really good tires, then you can push to this limit and reduce braking time, but once you pass the limit, again, you slide and have bad stop times. The other advantage is a gain in heat dissipation. This reduces fade, which makes the brakes more efficient under repeated load. Overall, though, tires are the key. No traction= sliding with locked wheels. That's how I crashed my first stang, so belive me, I've learned first hand.

 

[Legendary]

Well I now have new tires with the brake job so I feel better all around!

So what I gathered is that the slotted/drill rotors are mostly for looks when it comes to a street car. They wouldn't benefit unless the brakes reached extreme racing temps that can't accomplished on the street.

 

[Michael Yount]

My summary would be - slotted may or may not help given todays rotors/pads. Drilled can cause stress risers which lead to cracks - avoid if possible. For one emergency stop if your brakes will hold your tires on the edge of lockup for the whole stop, bigger brakes won't stop you shorter. On the street, one emergency stop is about as bad as it ever gets - brakes have plenty of time to cool between hard stops. For any newer car with abs, if the brakes will activate the abs (the would or they wouldn't have it) bigger brakes won't stop you shorter. For a Fox body - many of which have upgraded wheels/tires - test what you have to see whether or not an upgrade is gonna help you in your situation for the way you use your car. Unless you just like the way they look - then go for the bigger brakes.

 

[notchman90]

i had slotted on my talon and they last for about 7 months and they warped. were brembos off ebay. didnt notice better braking

 

[blk92stangg]

I think the greatest benefit on upgrading to high zoot rotors and calipers and lines are the increased feel you have at 9/10ths breaking. The better the brakes you have, the better you can feel your way around and up to the limits of your tires adhesion.

 

[Chythar]

After a nasty encounter with brake fade that scared the crap out of me I upgraded my '94's rotors & pads. I wanted both the performance and the looks, so I went with slotted/DIMPLE-drilled (cross-drilling that only goes about halfway through the rotor), and a set of Hawk Racing pads. The company I bought the rotors from claimed that the dimple-drilling eliminated the cracking issue, but would not dissipate heat quite as well. I also flushed my brake fluid at the same time, so I can't completely attribute my findings to just the rotors & pads.

I found that my stopping power at slow speeds actually decreased slightly. It wasn't by much, and I've adjusted to it by now. The major difference is during sustained braking. I used to have to press the pedal harder to keep the braking force constant - not anymore. Braking feels very linear now. I've had to make several emergency stops here in Southern California, and the brakes have handled it perfectly every time. I slow down so fast that I have to let off the brakes some to keep the guy behind ME from smacking my bumper.

Definitely an improvement over stock. I'm very happy with the setup.