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euro911sc's Avatar
 
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Brake Rotor Hat Design Q's

So I am designing a hat for my new used calipers. I'll be using 11.75" x 1.25" rotors which are common and inexpensive



However, during design I have come up with a few questions. The primary being:

Why do the hats and rotors have scallops?

like almost all of these on the Wilwood site: Wilwood High-Performance Disc Brakes - Rotor Hats

Scalloped hats are the norm it seems... though several pelicans have skipped that step:





Weight savings? Reduction of thermal conduction? Air flow?

Seems to me that we want that area sealed up so the air runs through the center and out the vanes radially since most if not all of us have ducted air to the rotor's center.

though some pros obviously went the other way



Now it sure would be cheaper to not scallop and I really think that the savings in weight, while on a rotating area, is minimal. So that leaves a reduction in thermal conductivity... That is transfer of heat from the rotor through the hat to the hub and wheel bearing. The less contact area between the hat and rotor the less conduction... but there again I'm not sure that the area reduction is significant enough to make a difference...


So please chime in and lets see if we can get some real answers! Hell maybe its just for looks...

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Michael
'82 Euro SC 'Track Rat' 22/29 Hollows, 22/22 Tarrets, Full ERPB F/R, Rennline Tri Brace, Glass bumpers, Pro 2000's, 5 pts, blah blah blah
'13 Cayenne GTS
Old 04-02-2013, 11:36 AM
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bump... anyone?
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Michael
'82 Euro SC 'Track Rat' 22/29 Hollows, 22/22 Tarrets, Full ERPB F/R, Rennline Tri Brace, Glass bumpers, Pro 2000's, 5 pts, blah blah blah
'13 Cayenne GTS
Old 04-15-2013, 06:17 AM
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I'm pretty sure the scallops are for weight savings. Pretty important area to remove weight from.

You can easily enough close off the openings this leaves with light aluminum which has little strength. That's what I do with my stock type 911 front hubs - make an annulus with tin snips, punch holes in it to fit around the bolt heads, and pop rivet it in place. To reuse, drill the rivets. No precision needed.

As to the tube frame race car, it is not using center out through the middle of the rotor cooling. It is using clamshell cooling, so that air never gets into the center. It appears that center out cooling is more efficient, which is why we all use it these days. But explains why those guys didn't worry about the holes.

I don't know about the thermal conduction part. Seems like it isn't a bad thing to keep the heat from the hub and bearings, but as long as they stay within their operating range and the capacity of the grease involved, does it matter?

Yes, the MOI reduction here is less than that at the outer edge, but it is still a reduction, and in unsprung weight, etc. The tube frame guys thought it worthwhile to punch those holes even closer to the axis. Though perhaps that was to allow heated air from the rotor vane area another escape path, since it would be moving through the middle of the rotor backwards from what we all do.
Old 04-16-2013, 06:23 PM
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That is a 917/30.

How thick is the hat you are making? Front or rear? Which caliper and rotor will you use?
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Old 04-16-2013, 07:24 PM
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I'll be using a 964 front on the front of my SC. With some milling of non-essential material I can widen the mouth to accept an 11.75" x 1.25" rotor. This seems to be a common spec size for some NASCAR/ARCA series and can be found from several manufacturers in different materials and prices... most importantly they are fairly inexpensive compared to some of the perfect fit rotors I found in European catalogs only... that would be $160 vs. $420 EACH!

A lot of people will just run the 285mm x 24.5mm stock Carrera rotor. Cheap and easy! But I hear that they really just don't have the mass to take it thermally on a track car. Moving to a 28mm rotor seems to be the ticket with no machining of caliper, but those are actually quite expensive ($400+ each!) The bigger 1.25"/31.75mm rotors are only $120-70 each depending on your desires.

The hat thickness is still under debate. However it looks like I'll have 0.25" at each mounting area (inner hub and outer rotor) with a thicker middle section to handle the change in position of the two areas. I am now just trying to figure out how far I can move the rotor out from the ball joint and remain centered on the caliper. This will take some test fitting that I have yet to perform.

See my other thread that I kinda hijacked here:
964 calipers over 28mm rotors
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Michael
'82 Euro SC 'Track Rat' 22/29 Hollows, 22/22 Tarrets, Full ERPB F/R, Rennline Tri Brace, Glass bumpers, Pro 2000's, 5 pts, blah blah blah
'13 Cayenne GTS
Old 04-18-2013, 11:46 AM
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I think the answer to the "scallops let air escape" issue on custom milled hats is to leave about 1mm or so of the base stock unmilled in this area. Plenty to block off the air flow.
Old 04-18-2013, 01:02 PM
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Yeah I was thinking about that. Easy to do once its in the machine. Looks like I'll have a few hours of CAD work ahead of me this weekend I found some nice hardware on AP racing's site. I had planned on shoulder screws, but they are a bit weighty...
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'82 Euro SC 'Track Rat' 22/29 Hollows, 22/22 Tarrets, Full ERPB F/R, Rennline Tri Brace, Glass bumpers, Pro 2000's, 5 pts, blah blah blah
'13 Cayenne GTS
Old 04-18-2013, 02:06 PM
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I can see the alure of shoulder bolts if you want to let something slide. As noted elsewhere, the Porsche stock setup (leaving out turbos) is just 8.8 threaded bolts, and it doesn't fail. I wonder how many of us generate so much heat braking that the more complicated sliding systems are warranted.
Old 04-18-2013, 03:03 PM
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Yeah I have no idea on the heat thing. I do know that the wheels get hot enough for the weights to slide off if they are not taped I have the typical ducted air from the front air dam into the center of the rotor for cooling. Should be enough.

I had designed in a slight elongation. Not a full floating setup. My goal with the shoulder screws was to have a nice sold core that would be in shear at the joint of the two parts. I was warned against using full threads. However, considering the stock setup with 5 full thread bolts holding the rotor to the hub, the shoulder screws are probably way over kill. I remember the 1st time I changed rotors on the car... I stripped every screw trying to make sure I had the right torque from the little spec book... I think there is a typo
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Michael
'82 Euro SC 'Track Rat' 22/29 Hollows, 22/22 Tarrets, Full ERPB F/R, Rennline Tri Brace, Glass bumpers, Pro 2000's, 5 pts, blah blah blah
'13 Cayenne GTS
Old 04-19-2013, 06:00 AM
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Here is what I was looking into:

http://109.228.5.127/Info.aspx?InfoID=92&ProductID=7
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Michael
'82 Euro SC 'Track Rat' 22/29 Hollows, 22/22 Tarrets, Full ERPB F/R, Rennline Tri Brace, Glass bumpers, Pro 2000's, 5 pts, blah blah blah
'13 Cayenne GTS
Old 04-19-2013, 06:44 AM
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Don't go crazy on the bolt selection. Most applications i've seen for grassroots type of performance brakes we're making are 12.9 socket head screws or the NAS, AN airframe bolts i've mentioned. The spendy AP stuff seems like overkill.

You must have had a typo on the stock screws because it's pretty hard to wreck a 8.8 class M8 screw when using only 18 ft-lb!

Last night I took apart my Wilwood setup to test a 11.75 x 1.25 rotor under the VCI calipers. Gonna need to counterbore/relieve the adapter to shift the caliper outboard and fit over the rotor properly. But the AN airframe bolts (partial thread with shimming washers to get the proper grip length) and jet nuts look good after a couple seasons of use.

https://www.pegasusautoracing.com/productselection.asp?Product=AN5
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Old 04-19-2013, 09:08 AM
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Michael

The use of shoulder bolts in your reference certainly looks confidence inspiring. But I think it is way overkill. My understanding of the strength of a bolted joint is that the bolt pre-load provides clamping force, and the friction of the clamped faces is what keeps the joint from moving. That's what I have been told about CV joints, and flywheels. So it isn't the shear strength of the fasteners, it is the clamping force.

Now with the shoulder bolts, it is the shear strength I guess, because you can't get all that much clamping force with the reduced size threaded end. Which would make them ideal for rotors which are designed to slide around with thermal effects, so to speak.

I've never bothered with a torque wrench bolting on stock rotors. Never broken one, never had one get loose, and never seen evidence of fretting from any back and forth movement, and some of the bolts have been reused quite a number of times by now. I don't think I have seen any evidence of the threads making an impression on the parts, and that wouldn't be hard to do if loose on the aluminum hub. So, at least with a stock setup, this may be an area where one ought not to overthink things.
Old 04-19-2013, 10:43 PM
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Brake bolts

Spend a few bucks and get the NAS... They are already drilled for safety wire...and most important consistent (read inspected). They aren't that expensive.

If you are going to make your own aluminum hats, use a 2xxx series alloy. Every thing else is near butter at temperature. Yes, people use 6061 T6 all the time...but knowing MY luck I'll have the brakes hot enough . I honestly believe steel is a good way to go too. Just drill more holes and/or sculpt .

t
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Old 04-28-2013, 05:08 PM
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I once asked a guy at a vintage race, who fielded a well prepared (and lightened) 356, and had pro driver Milt Minter piloting it, how he figured out where and how large and how many holles to drill/mill/make in the rear brake rotor hubs (or, for that matter, in any of the other Swiss cheesed parts of that car.

His answer: drill a bunch and run it. If it holds up, drill some more. Repeat until the part breaks. then make a new one with one (or a few) less holes.

I haven't been too keen on taking that approach.
Old 04-28-2013, 09:50 PM
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I'm no metallurgist by any means. All I know is what i've used. We've used 6061 hats on our MotoDelta cars since 2007 and no problems with breakage. Maybe the material is flexing enough that it doesn't break? Total guess/speculation on my part.

I was going to have a set of rear brake rotors drilled in the hub section. I think the key is to make the hole reasonably big but not too much. Space them apart a good bit and contour the holes to reduce stress risers. John at Costa Mesa R&D does it. Keep in mind though the car in this case is a little Fiat 850

Rotor_Lightening-Hall-Effect_Sensor.wmv - YouTube
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Old 04-29-2013, 02:48 PM
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I'll probably use 6061. Easy to find and machine. The 50 series is probably better, but its sometimes a pain. I have the parts drawn, just need to get them made. Though, I was considering having a prototype made in plexi so I can make sure it all fits before spending the big $$
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Last edited by euro911sc; 05-07-2013 at 12:37 PM..
Old 05-07-2013, 11:33 AM
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Hadn't considered the 2 series materials... will have to check Thanks!
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Michael
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'13 Cayenne GTS
Old 05-07-2013, 12:38 PM
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Quote:
Originally Posted by euro911sc View Post
Hadn't considered the 2 series materials... will have to check Thanks!
Let us know what you find. Quick search at industrial supply and 2 series cost goes WAY up.
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Old 05-07-2013, 03:04 PM
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I made some rotor hats back in college. These were for carbon/carbon brakes so very high temps. I believe I used 2024T4 aluminum because it had superior props at elevated temps. See if you can find the curves from Mil Handbook 5 and compare to 6061 and 7075.

Old 05-07-2013, 03:18 PM
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