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 Post subject: Rotors, Rotors, Rotors
PostPosted: April 8, 2017, 11:45 am 
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Joined: September 21, 2013, 5:44 pm
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dose any one have any reference material suggestions / ideas about choosing rotors .. in finding a balance between the theory of the rotor mass acting as heat sink/dissipation for kinetic to thermal energy and the idea that the less rotating mass the better.. there obvious trade offs however in my build I do have the flexibility to choose a hand full of different options, diameters vs weights.
what would be and optimal sizing, and would some heat build up be beneficial in getting the pads to bite better ? or could one use a pad with more initial bite to compensate for an slightly over sized rotor with no heat build up ? on the street how much heat build up would I even have?
And how rotational mass dose it take to see a difference in a street driven car 1 or 5 pld?? or oz. ??

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PostPosted: April 9, 2017, 8:10 am 
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Reducing unsprung weight is not important when the trade off is not enough brakes when you need them. Track cars need way more brake capability than street use, don't ask how I know. G;-)

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PostPosted: April 9, 2017, 1:45 pm 
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Quote:
I do have the flexibility to choose a hand full of different options


I would suggest for us amateurs, this is more a matter of trial and error than engineering. There is a LOT of variables to brakes (don't forget about pad size, leverage, clamping force etc etc etc). I would just start with around a 10" rotor and a set of good pads and go from there. That will cover any street use and light track days while fitting under 15" rims. When you want to do more serious track work, some race spec pads with the same 10" rotors will be a good amount of brakes in a sub-2000lb car.


For reference I daily a 09 Yaris. 2300lbs with ~10" front rotors and drum rears. It stops very well, rotors do not overheat/warp, and B-spec guys race the snot out of them on stock sized rotors with no warpage. I also autoX it and daily drive on NT01 tires, with some Hawk HPS pads.... braking is not an issue. The only downside to the HPS pads is they do require a tiny bit of heat and the Yaris is easy enough on brakes to not always retain that heat when just driving around town. Even cold they have roughly the same braking as the OE pads, but they do grip a LOT harder once warm. It's mildly annoying because your pedal feel changes during harder stops on the street. Then cools down and does the same thing for the next time. Still, I won't be going back to OE :mrgreen:

Cheers.

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PostPosted: April 9, 2017, 2:01 pm 
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Joined: April 23, 2006, 8:26 pm
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Many Locost builders figure that since their car is lighter than the car the brakes came from, they'll be more than enough on-track. What they forget is that heat is proportional to weight, but also to speed - squared. The Locost will likely go faster than the donor car and get up to speed faster as well. The result is that you can't cut corners with brake discs and may in fact need larger ones.

That said, if it's just a street cruiser than none of the above matters and just about any brakes will be fine.

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Last edited by KB58 on April 9, 2017, 7:08 pm, edited 1 time in total.

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PostPosted: April 9, 2017, 3:18 pm 
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That is true but its still going to be half the energy of a normal sized car. Miata sized rotors are more than enough for any Locost of an appropriate weight - they work just fine on Spec Miatas and very few Locosts will ever go any faster.

Pad selection is going to be more critical. I originally put el-cheapo Advance Auto Silver pads on my Locost and the brakes were awful. After about 1,000 miles the cross hatching on the rotors was untouched. The car was so light that it couldn't get enough heat into the pads to actually get them to work so it was like two pieces of glass sliding on one another.

My Sprite has even smaller brakes and I've never had an issue with the front brakes beyond completely baking a set of street pads on the track. The 7" rear drums are the issue due to heat and the currently available linings. Going to rear disks is frowned upon in vintage racing.

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PostPosted: April 9, 2017, 4:13 pm 
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Location: Kamloops, BC, Canada
I built my car using stock brakes from a Toyota Corolla from about 1983. Solid front discs with single piston calipers and drum rears. I'm using generic parts store pads, nothing fancy. For autocross they worked great, no fade or overheating, even on slicks. Now that I've got the car registered and I drive it on the street, I've noticed that if I'm pushing on a windy road I can make the brakes start to go away. That involves some pretty irresponsible driving on roads without any other traffic though, and I run Toyo R888's for street tires.
Kristian

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PostPosted: April 9, 2017, 7:22 pm 
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a.moore wrote:
That is true but its still going to be half the energy of a normal sized car. Miata sized rotors are more than enough for any Locost of an appropriate weight - they work just fine on Spec Miatas and very few Locosts will ever go any faster.

In my book I use the following example:

Quote:
Car A: 2500 lbs and goes 100 mph.
Car B: 1600 lbs and goes 130 mph.

Question: which one ends up with hotter brakes?

Answer: Car B does because of the extra heat due to the extra 30 mph in spite of being lighter.


Of course Locosts are really draggy so it's less clear whether larger brakes are really needed, or if just letting off the gas will slow it fast enough on its own. That said, if the Locost uses a dead-stock Miata engine, yeah, not much to worry about.

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PostPosted: April 10, 2017, 1:26 am 
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For context, in this builder's log he was thinking of installing late model Mustang Cobra disks. I thought he could get by with the base model disks. I just think brakes sized for a high performance 4000 lb. car is more than he is going to need.

There are many brake kit options for Pinto / Mustang II spindles, I think having an array of choices is a good thing. It wasn't clear if he already had the modern Mustang spindles, but I don't think there are good reasons to use them. I think he had the calipers though...

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PostPosted: April 10, 2017, 10:27 am 
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@JNS2SEVEN

If you're looking to educate yourself, two currently available books are:

1) "High Performance Brake Systems: Design; Selection; and Installation" by James Walker, Jr.

2) "Brake Handbook" by Fred Puhn

You can often find one or both at a local bookstore like a Barnes and Noble, but both are available online from several sources. I don't think either get into theory as much as you seem to want, however. Once you get into buying aftermarket, high-performance brakes, you are getting into BIG-$$$ territory. If you can get higher performance, OEM parts like a Mustang Cobra brake upgrade purchased from a junk yard or private party, things get much more economical.

Cheers,

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PostPosted: April 10, 2017, 7:01 pm 
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I found solid rotor on a number of british sports cars, including the base model At-om at one point. The upgrade package was for vented.


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PostPosted: April 11, 2017, 12:28 am 
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thank you all for the input yes originally I had for what ever reason settled on the mustang cobra brakes a while ago (years) and had purchased the calipers ..the exact reason escapes me now .. however i believe it was due to the large aftermarket support and flexibility in rotor and pad choice should I ever decide to track the car..after my OP I thought a lot about what choices I have within the Ford parts bin and possible use of the mustang ii spindle set up ..heres my train of thought and out come on the situation

1. yes cobra front rotors are very heavy and very much over kill..the weight alone on a mustang cobra rotors comes in at a supposed 19.5 plds..thats all rotating mass wicth i heard some consider to be in the order of 1:6 or even 1:7 worth of static weight, so for every 1 pld of rotating mass theres a theoretical 6 or 7 plds of weight added to the unsprung weight. that dose not even include the hub ( 66 mustang v8 drum brake minus the drum) , I weighed the one I was planning on using with the mustang ii spindles at 5.59 plds...
however the larger diameter offers better leverage to stop the car..

2. so I went and re looked at rotors
mustang cobra 12.99" with hub 25.45 plds
base mustang 10.88" with hub 21.95 plds
granada 11" rotor hub combo 21.65 plds
stock mustang ii 9.25" with 5 bolt 13.95 plds
but... something else caught my eye..
willwood 12.19" 2 piece rotor and a low profile hat I found, should come in at 20.29 plds
so I did order them up to go with the cobra calipers i already have.
side note apparently the PBR cobra caliper save a few plds vs. the stock gt calipers..

3. so from my understanding the longer the leverage force the better the braking power , because the pad can only clamp so much or have so much clamping area per given rotor area (diameter). I would think the larger rotor you can run for the weight the better. in my case a 12.19 dia rotor vs. 10.88 or 11" dia rotor.
so now I have to deal with running around on rotors that may never get hot during daily driving how ever I do plan on compensating for that with a pad that has a very high initial bite cold and for any high spirited driving I would think the rotors will warm to a lower but optimal temp for the pads..

as they say hind sight is 20/20 hopefully this set up will pan out .. if not i can always fall back to smaller rotors and calipers..
any thoughts ?

yes thank you for the suggestion on fed puhns book I have it on the way..

btw any suggestions for a pad that dosnt mind being cold and has a high bite ... im all ears.. ive been looking at hawk hps / ebc red / dba 500 / even thinking of ford factory for the cobra..

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