Brake Rotor Machining?

Indy · Post by **Indy** » Wed Feb 14, 2007 4:37 pm

I've been thinking about ways to reduce unsprung weight. A scalloped rotor comes to mind. I can get a regular rotor for $30, or I can pay $100 for basically the same rotor that's been scalloped.

So keeping this in mind, I've found out that I'll likely have on-and-off access to a 3-axis CNC mill on weekends this summer.

So my next concern is how the rotor will react to being machined. Obviously it would need to be properly fixtured etc and maybe balanced afterwards. But does anyone know what kind of heat treating or post processing a rotor goes through after it's cast? I've been searching for quite some time, and I can't really find any details on that kind of thing. I'm doing good to even find what type of cast iron the manu's use.

So, anyone have any insights?
I'm probably going to call a couple of brake manu. tech lines and get some opinions there too.

Thanks!
Nate

Boscolingus · Post by **Boscolingus** » Wed Feb 14, 2007 5:16 pm

dude, its what? A couple pounds? Who cares - not worth making a career out of

Indy · Post by **Indy** » Wed Feb 14, 2007 5:25 pm

Boscolingus wrote:dude, its what? A couple pounds? Who cares - not worth making a career out of

Going to school to be an Aero Engineer...I AM making a career out of a couple of pounds.

To be exact I'm looking at going from 9.1 to about 5.6-5.8lbs.

stimpy · Post by **stimpy** » Wed Feb 14, 2007 7:16 pm

What do you mean by scalloped?

Indy · Post by **Indy** » Wed Feb 14, 2007 8:07 pm

It looks like this:

Modeled after a Wilwood 11.75" x 0.810" rotor.

Boscolingus · Post by **Boscolingus** » Wed Feb 14, 2007 8:34 pm

I am by no means a rocket scientist - or any scientist for that matter, but wouldnt that just wear the shit out of the pads? Interesting idea though

donk_316 · Post by **donk_316** » Wed Feb 14, 2007 9:00 pm

Alot of bikes come with this style now and offer retrofit wave rotors too. Check into some of those manufacturers such as Gaffer and so on.

stimpy · Post by **stimpy** » Wed Feb 14, 2007 9:24 pm

That's sexy as hell.

Post by **eHoward** » Thu Feb 15, 2007 12:26 am

that looks like a brake man rotor

Kohburn · Post by **Kohburn** » Thu Feb 15, 2007 7:54 am

dunno how much machining you've done - but in my experience a CNC with sharp cutters will only make the rotor warm to hot to the touch. compared to actual road use where people stomp on them till they glow and then leave the hot pad pressed in one spot for 40 seconds once they stop.

Pyrthian · Post by **Pyrthian** » Thu Feb 15, 2007 9:13 am

Boscolingus wrote:I am by no means a rocket scientist - or any scientist for that matter, but wouldnt that just wear the shit out of the pads? Interesting idea though

not anymore than slotted or drilled.

I would think finding the amount of braking required, and shrinking the rotor to that point would be better. being, that small diameter increases make for quite a material increase - and material increase is weight increase. but, I am also scared of the pulsing effect of the variable diameter - but DAMN that looks cool.

Kohburn · Post by **Kohburn** » Thu Feb 15, 2007 9:17 am

here is a scalloped toyota rotor.

as long as you were careful in the design of the scalops there shouldn't be any pulsing

Kohburn · Post by **Kohburn** » Thu Feb 15, 2007 9:22 am

i think you will see shorter pad life though due to higher surface pressures. (less surface area for same caliper pressure = higher surface pressure)

see here wilwood scalloped rotor and pad

Pyrthian · Post by **Pyrthian** » Thu Feb 15, 2007 9:28 am

Kohburn wrote: as long as you were careful in the design of the scalops there shouldn't be any pulsing

yes - I see now - if the "scallops" are the same size as the pad - the clamping force will always be the same

p8ntman442 · Post by **p8ntman442** » Thu Feb 15, 2007 12:26 pm

you also reduce the heated gas build up like with solid rotors. This will actually increase pad and rotor life, as your getting more out of your brakes while using them less. Oh, and FUCK PAD LIFE, who gives a shit about spending an hour or two changing pads. This is for PERFORMANCE.

Boscolingus · Post by **Boscolingus** » Thu Feb 15, 2007 12:32 pm

you mention performance, but how come I have never seen these on any IRL, NASCAR, or any other prfessional racing curcuit?

Kohburn · Post by **Kohburn** » Thu Feb 15, 2007 12:43 pm

they reduce gas build up no more that slotted rotors but have ~30% less surface area - this will reduce pad life.

but like you said the reason to di it isn't for pad life - its to lose rotating mass.

i'd rather put the effort into lighter rims. but to be honest i'm not serious enough driver that scalloped rotors would improve my track times at all.

cactus bastard · Post by **cactus bastard** » Thu Feb 15, 2007 2:37 pm

Why don't you use aluminum bells / hats, and then just use an off the shelf wilwood rotor? They aren't very expensive, plus you'd drop some more unsprung weight, and raise the cool factor even more. You can use your CNC to machine yourself some nice caliper brackets instead. (you can even make me some too)

Indy · Post by **Indy** » Thu Feb 15, 2007 2:42 pm

I occasionally machine aluminum and steel tooling and fixtures on a POS manual mill where I work. The CNC that would be available has a pretty good coolant system. I don't think I'll have to worry about heat damage, I was more concerned about imparting other stresses to the rotor by machining, and was curious of rotors went through any stress-relieving processes after they've been machined by the manu.

The new rotors would yield a rotational MOI of around 66lb-in^2. Unmachined rotors would yield an MOI of around 100lb-in^2. I'd say these numbers are + or - around 10lb-in^2, since I don't have drawings of vanes profiles. That's a pretty good decrease for a 28% reduction in surface area and about 35% reduction in mass.

But really what I'm going after is a reduction in raw unsprung weight. I'm looking into this because the calipers on the way are a little bit heavier than I would've liked (5.2lbs each, ouch). But, they DO use a 4.75" long pad, which I'm hoping will help with the lower SA of the rotor.

Will probably knows what this is for.

Indy · Post by **Indy** » Thu Feb 15, 2007 3:01 pm

cactus bastard wrote:Why don't you use aluminum bells / hats, and then just use an off the shelf wilwood rotor? They aren't very expensive, plus you'd drop some more unsprung weight, and raise the cool factor even more. You can use your CNC to machine yourself some nice caliper brackets instead. (you can even make me some too)

Already going to use Al hats. The brake caliper bracket is fabbed steel and should weigh around .65lb.