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Old 05-22-2011, 08:54 PM   #61
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Old 05-24-2011, 01:58 AM   #62
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This has always bugged me - if friction is independent of contact area, then why do wider tires grip better than narrow tires? I would think the coefficient itself would vary a bit with the contact area
For the most part, larger tires don't grip a whole lot better than smaller tires, and either increasing the load(like with weight or downforce) on the tire or using a stickier tire will make the biggest difference and is the safest way to think of things.

The grip does "vary a bit" though. As you increase the load on the tire, the coefficient of friction goes down. An example is if you have a tire that with 500 pounds resting on it, the coefficient of friction will be 1.0 and the amount of "grip" will be 500*1.0= 500 pounds of "grip" in some direction. If that same tire has 1000 pounds on it, the coefficient of friction might drop to 0.9, so the "grip" will be 1000*.9= 900 pounds of "grip" in some direction. The load was doubled but the grip was not. The more load, the more total grip you will have, but the lower the coefficient of friction of the tire will be. So tires with a larger contact patch spread out the load more and help, but the difference is probably not as big as most people expect.

There is a very good explanation of this in Carroll Smith's book, Tune to Win.

In the case of a brake pad, I don't think the coefficient of friction changes nearly as much as a tire with the change in load.
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Old 05-24-2011, 02:16 AM   #63
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For the most part, larger tires don't grip a whole lot better than smaller tires, and either increasing the load(like with weight or downforce) on the tire or using a stickier tire will make the biggest difference and is the safest way to think of things.

The grip does "vary a bit" though. As you increase the load on the tire, the coefficient of friction goes down. An example is if you have a tire that with 500 pounds resting on it, the coefficient of friction will be 1.0 and the amount of "grip" will be 500*1.0= 500 pounds of "grip" in some direction. If that same tire has 1000 pounds on it, the coefficient of friction might drop to 0.9, so the "grip" will be 1000*.9= 900 pounds of "grip" in some direction. The load was doubled but the grip was not. The more load, the more total grip you will have, but the lower the coefficient of friction of the tire will be. So tires with a larger contact patch spread out the load more and help, but the difference is probably not as big as most people expect.

There is a very good explanation of this in Carroll Smith's book, Tune to Win.

In the case of a brake pad, I don't think the coefficient of friction changes nearly as much as a tire with the change in load.
Maybe it's placebo, but I could swear that even relatively minor changes in tire width seem to have a profound effect on the traction available in the lower gears. In 1st gear, my car would squirm all over the place with 255 width rear tires. After upgrading to 275s, the rear end is planted unless I purposely abuse the clutch a bit.

Probably a dumb question, but does total load effect static friction differently than kinetic friction? And I know "vary a bit" isn't exactly a scientific term lol. I have no idea what all goes into calculating the friction coefficients, so I couldn't use a more precise term

I'll have to look that book up
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Old 05-24-2011, 02:36 AM   #64
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Maybe it's placebo, but I could swear that even relatively minor changes in tire width seem to have a profound effect on the traction available in the lower gears. In 1st gear, my car would squirm all over the place with 255 width rear tires. After upgrading to 275s, the rear end is planted unless I purposely abuse the clutch a bit.

Probably a dumb question, but does total load effect static friction differently than kinetic friction? And I know "vary a bit" isn't exactly a scientific term lol. I have no idea what all goes into calculating the friction coefficients, so I couldn't use a more precise term

I'll have to look that book up
I don't believe the load affects static and kinetic friction any differently. Definitely not a dumb question.
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Old 06-02-2011, 03:50 PM   #65
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I like this thread a whole lot better after last few responses, I didnt even wanted to check back but I am glad I did..

I always believed the reason cross drilled rotors performed better was due to cooling made possible by the actual holes in extreme conditions when racing and not due the grip of of the rotors. I guess placebo gets the better of a driver often.

And I am just as shocked of my mature response as well, I guess I got a lot of practice.. aka "Yes, dear."

A question, big brake kits are also heavier, I wonder if that plays a factor at all in actual braking as well, whatchumacall it the free/loose weight or something? or am I mixing that with wheels?

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Old 06-02-2011, 09:22 PM   #66
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Vented rotors on my other car make it seem like the pads "dig" in upon heavy braking. Cross drilled seem to be less problematic.
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Old 06-02-2011, 09:33 PM   #67
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Almost off topic here, but there are a bunch of slotted and drilled rotors on craigslist going for fairly cheap. I was wondering has anyone delt with buying rotors from 88rotors? It's almost time for me to change mine.
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Old 06-02-2011, 10:40 PM   #68
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I like this thread a whole lot better after last few responses, I didnt even wanted to check back but I am glad I did..

I always believed the reason cross drilled rotors performed better was due to cooling made possible by the actual holes in extreme conditions when racing and not due the grip of of the rotors. I guess placebo gets the better of a driver often.

And I am just as shocked of my mature response as well, I guess I got a lot of practice.. aka "Yes, dear."

A question, big brake kits are also heavier, I wonder if that plays a factor at all in actual braking as well, whatchumacall it the free/loose weight or something? or am I mixing that with wheels?
They are heavier, but the brake torque that's generated will be greater due to the larger diameter, but more importantly, the increased mass allows for significant increases in heat capacity and therefore fade resistance. The increased mass is sprung weight and will have a detrimental effect on acceleration, though not to the same extent as wheels and tires (since brakes obviously have a smaller radius than a wheel and tire)
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Old 06-02-2011, 11:34 PM   #69
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They are heavier, but the brake torque that's generated will be greater due to the larger diameter, but more importantly, the increased mass allows for significant increases in heat capacity and therefore fade resistance. The increased mass is sprung weight and will have a detrimental effect on acceleration, though not to the same extent as wheels and tires (since brakes obviously have a smaller radius than a wheel and tire)
Does this also mean then cross drilling even though helps with cooling also, causes less heat capacity and fade due to decreased mass then? Wow I have great respect for brake designs after reading this thread, and cooling techniques such as ducts, hoses and brake dust collectors after this and might apply some of it.

Most don't realize that we can all go pretty fast but in a race slowing down quickly to take that turn often means not getting passed and passing.

Very informative thread, kinda belongs in tracking forum.

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Old 07-12-2011, 09:16 AM   #70
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The F1 brakes are the best of course. But, cross drilled and slotted gives better performance for street cars. Its makes sense to me that F1 brakes arent drilled and slotted because of the heat and constant heat to cool would cause warping or maybe crack...A solid mass would seem to hold together best at the F1 rates... Any ideas

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Old 07-12-2011, 10:15 AM   #71
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Use what the pros use. Carbon fibre composite plain brake rotors used on F1 cars. No slots or holes. Slots and holes are just for looks mostly.

They were only useful to slot or drill rotors in the 50s when abestos pads were still used and the pads would glaze over fast so the drills and slots were used to shave new material off the pads. If you want a brake upgrade increase your surface area aka rotor size and pads, not take away surface area but cutting holes in it.

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Old 07-12-2011, 10:58 AM   #72
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Running cross-drilled Zimmermans for over a year, absolutely no cracks, no problems.

Only reason I'd go back to blanks is if I was tracking the car, but for spirited daily driving with some light auto-x, the cross-drilled Zimmermans have held up incredibly well
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Old 07-12-2011, 02:32 PM   #73
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The F1 brakes are the best of course. But, cross drilled and slotted gives better performance for street cars. Its makes sense to me that F1 brakes arent drilled and slotted because of the heat and constant heat to cool would cause warping or maybe crack...A solid mass would seem to hold together best at the F1 rates... Any ideas

Negative. They do not. Also, rotors dont actually warp from heat. Warping is due to uneven brake deposits on the rotor.
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Old 07-12-2011, 02:38 PM   #74
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Does this also mean then cross drilling even though helps with cooling also, causes less heat capacity and fade due to decreased mass then?
No. Cross-drilled actually get hotter.
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Old 07-12-2011, 06:58 PM   #75
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Negative. They do not. Also, rotors dont actually warp from heat. Warping is due to uneven brake deposits on the rotor.
not entirely true. rotors can and do warp from heat. but many times people think their street rotors are warped when infact it's just pad deposits on the rotor

*and yes i've read the StopTech page

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Old 07-12-2011, 11:31 PM   #76
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Cross drilled rotors are absolutely derived from a performance background, the pad materials used on the racing cars of around 1950's (I think) were not as developed as the pads we all have access to now, one of the problems was that when operating at high temperatures these pads often exhibited out-gassing, the gasses created from the pads was enough to cause an ever so slight layer of 'boundary lubrication' between the pad and disc, that is, not enough to separate them, but enough to reduce some of the asperities of each part contacting, the holes gave somewhere for these gasses to escape, some of the early pads also had low co-efficient's of friction, the multiple edges gave the brakes a bit more bite, with increased pad wear and dusting.

As for cooling, I am unsure, I would think that would probably have to be looked at on a case by case basis, however I do not think cooling gains from cross drilling would normally be anything special, I have never seem any numbers on this.

Anyway, modern pads are much better in regards to gas production and co-efficient of friction, Cross drilling is more of a cosmetic thing than it used to be, though it's benefits do remain, and yes the hole is a stress raiser, however a good quality floating rotor should still give many miles of trouble free service, like everything it's a trade off, less service life - more performance. I have found the standard floating rotors to be of decent quality, I have had them red hot (literally) on numerous occasions, while they do overheat too quickly, they have decent power before this happens.
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Old 07-12-2011, 11:44 PM   #77
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Someone who bought aftermarket drilled rotors from ebay said its the worst from all the break dust being collected.
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