Grassroots Motorsports

Register Log in to GRM
- Community
Birthdays
- 4g63t
- broken_cynic
Recently on the Classic Motorsports forums

Subscribe get a full year for only $19.95 Sample get a free issue—no strings attached

Login to post Forums » Grassroots Motorsports » Help Solve a Brake Debate

1 2

April 8, 2009 10:02 p.m. Salanis UltraDork

Firstly, could someone remind me which issue the "Think Globally, Stop Locally" article was in?...

So a good friend of mine and I are in a debate about brakes, bedding them in, turning rotors, and switching to larger rotors. Both of us are driving '94 Miatas. Mine is a base model with locking brakes. His is an M edition with ABS. My car brakes noticeably harder than his. I'm saying that's because my car weighs less and I'm using Hawk HPS pads, while he has less sporting pads.

He thinks that if he switches pad compound, he will have to resurface the rotors to get all the old pad material off. I'm telling him that happens as part of the normal bedding-in procedure, and referring him to this article by Carroll Smith to back that up: http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

He's saying he wants to go with an upgrade to his brakes and is looking at this kit, that lets you use an 11" Corrado rotor on the front: http://www.etdracing.com/m-tuned/product_info.php?cPath=13&products_id=80&...

I'm sticking with saying that he really just needs to upgrade the pads, and that the "upgrade" will only degrade performance by using an unnecessarily heavier rotor (nothing else is beefed up, just the rotor).

Can anybody weigh in on this one way or the other?
April 8, 2009 10:07 p.m. mrdontplay Reader

Press the pedal harder
April 8, 2009 10:10 p.m. confuZion3 Dork

Didn't he just create a thread a few days ago about the 11" Corrado brakes?

I still have the rotors on my Miata that came with the car (when I got it from the last owner--not the car's original rotors!). I didn't get them turned or even sanded down when I put the Hawk HPSs on them. They work perfectly (I bedded them in according to their instructions). I may have gotten lucky, but it may not matter that much.

He just needs better pads to get better bite and better fade resistance. (But I cannot imagine how adding .50 pounds to the rotor's mass will degrade performance.) I have the same car he has (M Edition) and I can run into ABS on every corner of whatever track I'm at with HPSs on Falken Azenises.
April 8, 2009 10:21 p.m. Salanis UltraDork

confuZion3 wrote:

Didn't he just create a thread a few days ago about the 11" Corrado brakes?

He just needs better pads to get better bite and better fade resistance. (But I cannot imagine how adding .50 pounds to the rotor's mass will degrade performance.) I have the same car he has (M Edition) and I can run into ABS on every corner of whatever track I'm at with HPSs on Falken Azenises.

I don't think he did. As far as I know, he's not on this board.

Okay, maybe they won't harm handling significantly, but I don't think they're the answer to his problem. Like we;re saying, I think the answer is better pads.

I can lock the brakes on my car with Hawk HPS on Nitto NT01s. It takes work, but I can do it.
April 8, 2009 10:30 p.m. Keith UberDork

They can harm handling - brake rotors are not only unsprung, but rotating mass. I've tried a set of 12.5" solid rotors on a Miata and it was like having gyroscopes attached to the wheels. The 11" ones won't do that, but they will give him more front brake bias when he probably needs the exact opposite.

Some pad compounds don't share rotors well. When I went from carbon kevlar Porterfields to Carbotech XP8s on my Locost, the guy from Carbotech (sorry,I forget his name) specifically warned me about pad contamination from carbon kevlar pads.

The first thing I'd do in this case - which is also the easiest and cheapest, cuz that's how I roll - would be to bed his pads in again. That might bring them back.

When you say your car "brakes harder", I'm assuming that means more braking power from the same pedal pressure?
April 8, 2009 11:14 p.m. Salanis UltraDork

Keith wrote:

They can harm handling - brake rotors are not only unsprung, but rotating mass. I've tried a set of 12.5" solid rotors on a Miata and it was like having gyroscopes attached to the wheels. The 11" ones won't do that, but they will give him more front brake bias when he probably needs the exact opposite.

The first thing I'd do in this case - which is also the easiest and cheapest, cuz that's how I roll - would be to bed his pads in again. That might bring them back.

When you say your car "brakes harder", I'm assuming that means more braking power from the same pedal pressure?

I'll tell him what you said about greater front bias, when he probably does not need that. I'll also suggest he try re-bedding them.

Yes, it takes a lot more force on his car to get the same amount of braking power as on mine. I suspect a significant amount of that may be weight, since my car has no options, and his has pretty much all of them.
April 8, 2009 11:18 p.m. Appleseed Reader

I would think any tech directly from StopTech trumps any "I heard this from" knowledge. Isn't the general consensus that if you aren't overheating the brakes or inducing fade, larger rotors are pointless?
April 8, 2009 11:44 p.m. PaulY New Reader

The Think Locally, Stop Globally article is from June 2008. There was another recent brake post for a chevette I think and the author of that article posted in there.
April 9, 2009 12:57 a.m. joey48442 SuperDork

Calling angry corvair! They are talking about your article!!!

Salanis, could your friends brakes possibly just need to be bled? I know weight is important, but I can't imagine you car weighs much more than 100 lbs less than his, and that's less than some of my passengers, and I don't notice much of a difference with them in the car.

Joey
April 9, 2009 1:16 a.m. Salanis UltraDork

joey48442 wrote:

Salanis, could your friends brakes possibly just need to be bled? I know weight is important, but I can't imagine you car weighs much more than 100 lbs less than his, and that's less than some of my passengers, and I don't notice much of a difference with them in the car.

Dunno. I'm pretty sure he's good about bleeding his brakes. Pretty sure he's bled them more recently than I did mine.

I'm not sure how much more his car weighs or not...

Power windows, locks, A/C, power steering, ABS, leather, speakers, bigger wheels/tires... I think the actual brake pad compound is the biggest difference between our cars for braking.
April 9, 2009 2:46 a.m. Johnboyjjb New Reader

The other thing that could be a possibility is vacuum. If it is a vacuum based power booster for the brakes a minor leak or differences in vacuum sources can create differences in pedal effort and feel.
Also, most rotors won't need resurfaced unless they are warped or scored. As stated above, changing friction materials can also cause problems.
April 9, 2009 8:33 a.m. Keith UberDork

It's harder to get a good bleed on an ABS Miata than a "locking brake" car - love that term. Make sure all of the slider pins on the calipers are lubricated as well.

But if it's simply "bite" you're looking at, I'd be spending all my time on the pads. First bedding, then possibly replacing with something better.
April 9, 2009 8:55 a.m. AngryCorvair Dork
Sorry for being so late to the party.

Are we talking about subjective evaluation (it "feels" like your car stops shorter) or objective (your stopping distance is actually shorter)?

If you want apples to apples, both cars have to be on the same tires. Or, as my buddy (and Stop-Tech consultant) James Walker Jr loves to point out:

The brakes don't stop the car! The tires stop the car!

If your buddy's tires are at 0.8 mu (friction at tire/road interface), all the ABS in the world won't make that car decelerate harder than 0.8g. And if your tires are 0.9 mu on the same surface and your brake system is pretty close to balanced at 0.9g, your threshold braking will crush his ABS. Physics FTW.

FWIW, on an ABS-equipped car, if the ABS is cycling on all four corners, then we know the following:
- The driver is pushing the pedal hard enough.
- The system is converting driver input force into retarding force at the contact patches sufficiently to overcome the available grip of the tires.
- The vehicle is tire limited!
here's a lazy-afternoon test for you take both cars to the same deserted location for testing. take a floor jack and a set of stands. take two cones and some "sidewalk chalk" with you. set up a braking point cone. same driver for both cars, probably the driver of the non-ABS car since he's less likely to flat-spot his tires. Run the non-ABS car first. choose a speed and run each test at the same speed. make several stops, same initial speed and same braking point each time. use the second cone to mark the position of the front tire on each stop, and use the sidewalk chalk to make a mark and note on the surface for each stop, so you can see how distance varies as you get used to the car's behavior on this surface at this speed. then swap your wheels and tires onto the ABS car, and repeat the procedure. One difference: On the ABS car, stomp the berkeley out of the pedal, to make sure all four corners are cycling. If only one corner is cycling, you're not getting maximum effort out of the other corners.
April 9, 2009 9:11 a.m. John Brown MegaDork

First things first.

Before any testing you need to get things to a repeatable state. Flush and bleed the fluid from both cars remove the wheels and make sure all the hardware is clean and lubed and caliper brackets are cleaned. make sure the vacuum line to the booster is clean internally and the valve is operating properly. Vacuum test the booster to ensure no leakage exists.

After reassembly build a small amount of heat in the cars by travelling a circuit that includes right and left turns and many braking points that do not require you to stop completely. Run the cars together for about a 20 minute drive. After you recheck the fluids get to stopping. Use an area that allows you to stop from 55 mph without risk of killing kids and affecting traffic, a closed GM plant works locally ;) mark a point where you will start braking at 55mph and using a waterbased marking paint have your helper mark where the front tire stops. Mark a line and compare stopping distances through a series of ten stops.
April 9, 2009 10:22 a.m. Keith UberDork

Both you guys missed this tidbit, which is why I asked it:

Salanis wrote:

Keith wrote:

When you say your car "brakes harder", I'm assuming that means more braking power from the same pedal pressure?

Yes, it takes a lot more force on his car to get the same amount of braking power as on mine.

The complaint is not about ultimate stopping distance. It's about stopping force relative to pedal pressure, and the tires aren't involved there unless you're near their lockup threshold.
April 9, 2009 10:53 a.m. walterj Dork

I'm saying that's because my car weighs less and I'm using Hawk HPS pads, while he has less sporting pads.

That is the answer right there. Weight makes a huge difference - combined with a "better" pad the difference will be night and day. Is your car also lowered? Dropping the CG make a another huge difference.

Also you mentioned he has ABS: The older ABS systems, for whatever reason, often make the pedal feel spongy. My suspicion is that they use a larger bore MC to compensate for the extra fluid volume due to the ABS pump and plumbing which in turn requires more force at the pedal... maybe they compensate with a larger booster too. Anytime the volume goes up so does the force necessary - and you lose feel. If you have never driven a car with manual, unboosted brakes and a well balanced setup... you would be amazed at what the pedal can tell you.
April 9, 2009 11:09 a.m. walterj Dork

OH, and as far as getting the old material off the rotor for a compund swap... if bedding doesn't do it, 5 minutes and some emory cloth will.
April 9, 2009 11:11 a.m. Keith UberDork

Miatas use the same master diameter for ABS and "locking" cars, same booster as well I believe. Just FYI. But they are hard to bleed.
April 9, 2009 11:17 a.m. walterj Dork

Keith wrote:

Miatas use the same master diameter for ABS and "locking" cars, same booster as well I believe. Just FYI. But they are hard to bleed.

I was just speculating as to why all old ABS feels spongy. All things equal, the volume still goes up due to extra plumbing and pump so I would think it would still change the pedal feel somewhat but again... just thinking out loud on my part.
April 9, 2009 12:09 p.m. Keith UberDork

Understood, I just figured I'd throw a bit of trivia in there in case any Miata owners were wondering.

The 2001+ Sport brakes on the Miata do use a 15/16" master instead of a 7/8" for those who want to play with interchange. I have one on my stripy car.
April 9, 2009 12:24 p.m. AngryCorvair Dork

walterj wrote:

Keith wrote:

Miatas use the same master diameter for ABS and "locking" cars, same booster as well I believe. Just FYI. But they are hard to bleed.

I was just speculating as to why all old ABS feels spongy. All things equal, the volume still goes up due to extra plumbing and pump so I would think it would still change the pedal feel somewhat but again... just thinking out loud on my part.

ABS valves, pumps, etc, are fixed volume devices, and once those volumes are filled properly, they act no differently than a longer brake line. If you're talking about how in 2009 a 1986 ABS system feels spongy, I'd suggest a fluid flush and fill.
April 9, 2009 12:35 p.m. jgp1843 HalfDork

You also mention that your buddy's car has bigger wheels and tires than your car - which is more rotational mass. All the more reason to swap wheels and tires for an apples - to - apples comparison.
April 9, 2009 12:47 p.m. Salanis UltraDork

AngryCorvair wrote:

here's a lazy-afternoon test for you take both cars to the same deserted location for testing. take a floor jack and a set of stands. take two cones and some "sidewalk chalk" with you. set up a braking point cone. same driver for both cars, probably the driver of the non-ABS car since he's less likely to flat-spot his tires. Run the non-ABS car first. choose a speed and run each test at the same speed. make several stops, same initial speed and same braking point each time. use the second cone to mark the position of the front tire on each stop, and use the sidewalk chalk to make a mark and note on the surface for each stop, so you can see how distance varies as you get used to the car's behavior on this surface at this speed. then swap your wheels and tires onto the ABS car, and repeat the procedure. One difference: On the ABS car, stomp the berkeley out of the pedal, to make sure all four corners are cycling. If only one corner is cycling, you're not getting maximum effort out of the other corners.

Thanks for the suggestion. I'll pass that on to him as something we can try before he goes off and buys a probably unnecessary upgrade.

We've subjectively compared our cars before, and it was quite interesting. We noticed my car seemed to brake harder. We both agreed that my car felt like it had about 10hp more, too. The weirdest thing that bugged me about his car was the power steering. I tried doing some slaloms in it, and it felt like there was a notch or something in the middle of the steering where it took a lot more force to move through. The result was awkward transitions when changing direction.
April 9, 2009 2:45 p.m. walterj Dork

AngryCorvair wrote:

walterj wrote:

Keith wrote:

Miatas use the same master diameter for ABS and "locking" cars, same booster as well I believe. Just FYI. But they are hard to bleed.

I was just speculating as to why all old ABS feels spongy. All things equal, the volume still goes up due to extra plumbing and pump so I would think it would still change the pedal feel somewhat but again... just thinking out loud on my part.

ABS valves, pumps, etc, are fixed volume devices, and once those volumes are filled properly, they act no differently than a longer brake line. If you're talking about how in 2009 a 1986 ABS system feels spongy, I'd suggest a fluid flush and fill.

My race car with Tilton masters and no booster is exactly like a rock so my expectations are skewed... most of the "other" cars I drive are track cars, and they are usually fresh with not too badly aged rubber or SS lines. A non-ABS car always seems to have a harder pedal (stock for stock) in my not too overly large, non-scientific sampling. Maybe the pump has some give or maybe my addled brain is just predisposed to thinking that.
April 9, 2009 3:01 p.m. sachilles Reader

Encourage him to buy new pads like you have. Say you'll buy them from him if they don't work at 80% of his purchase price. If he doesn't like them, you get new pads for a discount.

1 2

You'll need to log in to post.