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How dumb of an idea is this?


irdave
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Ok, I'm really trying to believe Marshal, et al, over on s14 dot net about how the stock E30 m3 brakes are just fine. Mine that came on my 2002 felt like crap. It's possible the pads are severely glazed, and obviously the rotors aren't flat, see pictures. 

 

I don't have a rotary table for my baby mill, but I'm not afraid to buy one,  but is this appropriate? They seem to fit ok...

 

Primarily a street car, will do some track days. Kinda just want to be able to run 15s instead of having to use 16s...

 

And that's not a shadow, it seems to not be flat for a braking surface and the braking surface seems to not be parallel with the mounting surface.

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The trueness of the rotor is related to the hub mounting surface which one would assume is parallel to the wheel mounting surface you are measuring. You know what they say about assuming. I think cutting the rotors on the lathe should be fine as long as your lathe cuts reasonably square.

 

If I was doing that job I think I would pull the rotor as is, mount it on the vehicle and check the run out of the area was just cut. This will confirm if your setup on the lathe matches what you are trying to achieve. 

 

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1 hour ago, Son of Marty said:

Can't you just skim them on your lathe?

 

Yeah, that's how it's set up- just trying to get a sanity check.  This isn't the current brake set up on the car, so it's not too big of an issue regardless- I mean, if I kill these rotors, it's not effecting my ability to drive the car

 

And green scotch brite cleaned up the cutting surface from the tool just fine.

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You'll want very little rake on your tool for cutting grey cast iron-

you're scraping it off rather than peeling it.  You also don't

want much pressure against the rotor, as they tend to distort 

when pushed on too hard.

 

I've done that a few times for shiggles- I've NEVER had a rotor

with absolutely zero runout new out of the box.

 

But pads are what matters, as you surmise above.  The rotor's just the victim,

98% of the time.  

Altho, I had one with varying hardness once- that was one I figured

out on the lathe, as one arc through the rotor would not cut, even with carbide.

 

t

blanchard ground

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Regardless of what I say people throw rotors in lathes, face them off and no problems. But like Lorin is saying about assumptions, to me building sports cars and bikes to be used hard is about knowing what isn't causing problems. So here goes. 

Use an old rear axle to bolt rotor to same as its mounted on the car. Its best to cut both sides in one setup but your lathe isn't big enough to reach a tool around and cut the back side. What you might be able to do is center drill the hub end of axle so you can hold the other end in the chuck at the very end like on the threads to give room for the carriage on the left side of the rotor. Thats two setups and creates potential for out of parallel but if both bearing diameters run true both setups and it measures parallel when done your good. Or you might turn a length on the hub end of the axle so you can turn the axle rotor assembly around, chuck on turned hub end with your center in the threaded end. Again two setups but if its all running true

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Why not just go to a workshop that has the device for surfacing them directly on the car?
If you find somebody with the tool it's a no-brainer and pretty cheap.
Although I have a lathe myself I get this done by the Volkswagen Dealer in the next village.

E326g-Maschine.jpg

Edited by uai
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Or, maybe buy one!  There is one for sale on CL near here for $500.  I don't know what parts you would need to hold the rotor on that thing, but it is a cool looking little tool.  

 

https://seattle.craigslist.org/tac/tls/d/eatonville-ammco-brake-rotor-lathe/7372598280.html

 

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It looks like he's got a drum lathe too.

 

https://seattle.craigslist.org/tac/tls/d/eatonville-brake-drum-lathe/7372598781.html

 

1

 

 

Tom

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In re- reading- Dave, would you restate your question?

 

And, what car is this for?

 

We race the E30M20B25's on stock brakes-  they're not OVERbraked, but there's enough there.

 

So if you have a 2002 with an S14, you ought to be able to get a 15" package

that can survive just fine.

 

And if you're 5- lug (based on pic) that opens up all sorts of E36 rotor options, too.

 

Pedal feel's got a lot to do with ratio, both hydraulic and mechanical, and also with

system stiffness.  It's possible to get it really wrong- but using BMW parts, it's unusual

to be far out of the ballpark.  Pads matter so much it's not funny- when in doubt, start with

stock Textar, if you can get them- and on track days, airflow is essential.

 

hth,

t

 

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