tube80z Posted September 29, 2008 Share Posted September 29, 2008 What is the perceived advantage to cutting the rotors vs shimming the hats? If you cut the rotor you now have a spot that is thinner. So under severe use that will get hotter and will expand more, which leads to runout when used hard. If you don't cut the rotor and shim the hat then you don't have this issue. At least that's what I'm getting out of this. Cary Quote Link to comment Share on other sites More sharing options...
JMortensen Posted September 29, 2008 Share Posted September 29, 2008 That's a good argument. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted September 29, 2008 Author Share Posted September 29, 2008 We shall see over time. I am particulary hard on, or have bad luck with rotors, so if there is a problem, I will find it. I do feel MUCH better about not having to thin out a brand new set of rotors unevenly on a lathe though. Not too mention they are not well balanced if you have to thin one side out more than the other. Shimming the hats from now on in my book is a MUST DO. You only need to do it once. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 21, 2008 Share Posted October 21, 2008 I'm having a similar situation with 84 300zx rotors on the early 4 bolt hubs. I have a new set of rotors on order but want to get some shims in advance to have on hand if I need them. Anyone have a good source for shims that will work in this application? Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 21, 2008 Author Share Posted October 21, 2008 McMaster.com Use stainless steel shim washers. Quote Link to comment Share on other sites More sharing options...
jasper Posted October 21, 2008 Share Posted October 21, 2008 Question? Do you cut your rotors on the car? From my expereince, it's the best, and makes sense. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 22, 2008 Author Share Posted October 22, 2008 Cutting rotors on the car is the best way. The only place I knew that did it was the BMW dealer I worked for in the Bronx. F&S motors. Not sure who does it nowadays. I think if you start with new rotors and they are off center (lateral runout) by 0.006" or more, after you install them, you should figure out if your hub, hat, or rotor is off before you cut anything. I shimmed my hats to near perfect and it's been great so far with NO cutting nothing. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 22, 2008 Author Share Posted October 22, 2008 If you cut the rotor you now have a spot that is thinner. So under severe use that will get hotter and will expand more, which leads to runout when used hard. If you don't cut the rotor and shim the hat then you don't have this issue. At least that's what I'm getting out of this. Cary I think Tub80z hit the nail on the head here! Dont' cut until you know why you have to cut them in the first place. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted August 20, 2009 Author Share Posted August 20, 2009 Austin's thread sparked me to post an update. STILL NO SHIMMY! Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 27, 2009 Share Posted October 27, 2009 Just getting around to installing my JSK rear brake kit and I remembered this thread and thought maybe I'd shim the rotors. So I throw one of the rotors on the hub and measure the runout. .035". No ♥♥♥♥. I grabbed the other rotor and tried again. .040". I'm looking at this and there is just no way the thing is that far out of square, so I did a little test. I put the strut in a vise which holds it at an angle, put the rotor on the hub, and spun it. You could see the rotor ROCKING on the hub when it hit a certain spot. Pull the rotor off and you could see two little tiny wear spots on the inside of the pilot hole. Now I know what the problem is: the pilot hole in the hat is just a hair too small. The only reason I recognized this is because I had a set of wheel spacers that had a pilot that was like .003" too small and when you'd torque the wheels they seemed fine, but when you'd drive it you'd swear that it had a tire balance issue. Took me forever to figure that one out, and I finally did because the wheel spacer started to crack. So anyway I used the old finger and sandpaper "lathe" to open up the pilot hole and checked again, and now runout is .005". At what point is the runout going to be acceptable? With .005" I would think it would be close, and I don't know how easy it will be to shim the thing since I think the size of the shim is multiplied by the time you get to the rotor edge. What do you think Dave? Is this shimmable and is it worth the hassle to shim it? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 27, 2009 Share Posted October 27, 2009 Just checked and the thickness of the McMaster Carr shims for a 5/16" bolt is .002" minimum. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 28, 2009 Share Posted October 28, 2009 Went out and bought a couple cheap feeler gauges and used a hole punch to make shims out of them (drilling DOESN'T work). I don't get what is going on. I find the low spot, shim it, and then it is LOWER afterwards. I triple checked to see if I'm reading the dial indicator the right way, and I am, so I'm just a bit stuck. I guess tomorrow I'll shim it the exact opposite of what I think I should and see what that does, but I really don't think I have the dial indicator fundamentals wrong. I'm also worried that I might be chasing the torque on the rotor. If we know that torquing the rotor warps it just a hair, and so I'm taking the bolts off, shimming, and putting them back on and then getting different readings, isn't it likely that just tightening the bolts again is the cause? Thinking hard about taking all the shims off, torquing the bolts, cutting the rotor, then reinstalling and seeing what that does on the dial indicator. I suppose the other thing I could do is take a little more off of that pilot hole, just to make sure that is not the issue. Frustrated... Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 28, 2009 Author Share Posted October 28, 2009 If your rotor is only out about 0.005 at the outer edge, you can probably live with it and not feel it. If you can't get it closer by shimming, then turn it, while mounted to the hat/hub and torqued if it bothers you. Cutting that little off, won't make a difference in the performance. It is a strange thing that shimming it makes it go the other way. Is the shim causing it to "bellville" or "cone" for lack of a better description. Try shimming the bolt, AND the two neighboring bolts on either side with half the thickness of the center shim if possible. A shim at the hat bolts, definitely mulitiplies the runout at the outer edge. I would think a 0.002 shim would fix right around 0.005 at the outer edge. Personally with 0.005 I would mount it and test it. I think it will be OK, especially in the rear. Quote Link to comment Share on other sites More sharing options...
Drax240z Posted October 28, 2009 Share Posted October 28, 2009 Jon, I'd aim for 0.003" or less runout. Searching my memory banks I seem to remember anything greater than that being noticeable under braking. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 28, 2009 Author Share Posted October 28, 2009 Yes, in my experimentation...0.003 was smooth up front. I think out back you can get a way with a little more....I suspect. Quote Link to comment Share on other sites More sharing options...
mark Posted October 28, 2009 Share Posted October 28, 2009 , Jon, are you sure you are shimming in the right direction? I had the same problem the first time I did it. I was shimming the low spot and I should have been shimming the high spot. If you are measuring from the outside face of the rotor, and shim the low spot, it makes it lower. Shim the high spot to push the rotor back. Hope that makes sense. Went out and bought a couple cheap feeler gauges and used a hole punch to make shims out of them (drilling DOESN'T work). I don't get what is going on. I find the low spot, shim it, and then it is LOWER afterwards. I triple checked to see if I'm reading the dial indicator the right way, and I am, so I'm just a bit stuck. I guess tomorrow I'll shim it the exact opposite of what I think I should and see what that does, but I really don't think I have the dial indicator fundamentals wrong. I'm also worried that I might be chasing the torque on the rotor. If we know that torquing the rotor warps it just a hair, and so I'm taking the bolts off, shimming, and putting them back on and then getting different readings, isn't it likely that just tightening the bolts again is the cause? Thinking hard about taking all the shims off, torquing the bolts, cutting the rotor, then reinstalling and seeing what that does on the dial indicator. I suppose the other thing I could do is take a little more off of that pilot hole, just to make sure that is not the issue. Frustrated... Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 28, 2009 Author Share Posted October 28, 2009 Well yes, establishing a reference point and then reading the proper direction of the deflection is critical of course. There is no right way to do it. It depends on which side of the rotor you measure from, and which side you are adding shims. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 28, 2009 Share Posted October 28, 2009 Jon, are you sure you are shimming in the right direction? I had the same problem the first time I did it. I was shimming the low spot and I should have been shimming the high spot. If you are measuring from the outside face of the rotor, and shim the low spot, it makes it lower. Shim the high spot to push the rotor back. Hope that makes sense. I'm measuring on the backside of the rotor and shimming the high spot as viewed from the outside to push it back towards the dial indicator. I had spent quite a bit of time on it yesterday/last night. I'm going to try again today, maybe a little time off will help. Quote Link to comment Share on other sites More sharing options...
mark Posted October 28, 2009 Share Posted October 28, 2009 Good luck, It took me a couple of hours to get mine right. One side was perfect right out of the box. The other took some work to get it right. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 28, 2009 Author Share Posted October 28, 2009 It's worth every hour of time it takes. Mine have been pin straight since I did the shimming. Hot or cold. Quote Link to comment Share on other sites More sharing options...
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