Lotus Elan

Left rear rotor off center

PostPost by: Tahoe » Fri May 17, 2013 6:51 pm

Okay, I still have a click, click, click, coming from the rear. Had the car on 4 jack stands, wheels off, and idling in 1st gear. Clicking coming from left rear and after close examination it appeared a few of the donut bolts were not fully tightened. After tightening the clicking was still there, but seemed to be coming from the outside donut. I was watching the left rear rotor rotate looking from the hub and all of sudden noticed that outside of the rotor was running true, but the inside where the bolts are was not. It is visually obvious as the rotor turns that the bolt pattern of the rotor is not true with the centerline. It is off by about .125" making rotate off center yet the outside of the rotor appears to rotate perfectly. I'm not sure if this is causing the sound as I don't see any interference, but this can't be good, as this is also affecting the bolting of the donut making it run untrue as well. The PO had replace all the rotors and he bought them from Dave Bean but that was about 3-4 years ago.
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PostPost by: AHM » Fri May 17, 2013 7:47 pm

Measure the disc for runnout and ecentricity relative to the caliper.

Check that the components are visually ok - identify if anything is bent.

The input and output shaft are not on the bisecting plane so there will be an induced 3rd order (?) rotation of the shaft.

Are the wheels done up tight?
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PostPost by: Tahoe » Fri May 17, 2013 8:20 pm

Wheels were not on the car as I had it on 4 jack stands. Run out seems fine, eccentricity on the outside of the rotor is fine, eccentricity on the inside, where the bolts are, is off by approx. .125 inches (TOO MUCH) which is only on the left rotor, the right is fine. How in the world could this have been machined incorrectly? It's so weird to see the outside be true and the inside wobble slightly. I do believe I need a rotor, but I will drive it gently the rest of the Summer, and in late Fall-early Winter I plan on putting my CV joint axles on, and at that time I'll replace the rotor. Plan on taking a look under the left side every week all Summer long to keep an eye on any problems. If I see a major issue it all comes apart.
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PostPost by: AHM » Fri May 17, 2013 10:27 pm

The rotor is only a rough old bit of iron so the inner faces may not be machined.

If the holes were machined in the wrong place it a. wouldn't fit and b. wouldn't run true.

Measuring bolts is not the same as measuring the PCD/ concentricity/ runout. What datum are you using for your measurement - the centre is in thin air! The head of the bolt is hexagonal and they may even be different sizes if they are old, the suspension is at full droop and the whole thing is rotating.... but it runs true

If you want rough look at the outer circumference of the front hubs - hand cut! but the wheel is still in the middle
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PostPost by: Tahoe » Fri May 17, 2013 11:34 pm

Concentricity is the tough one, and for that I roughly measured from the floor. Using the floor as a datum is not ideal, but the car is in the air and is fairly stable. Can't exactly get a CMM over to the house to do the job. Doing the best I could, I measured the outside of the rotor from the floor to the rotor edge using a steel rule. For the bolts on the rotor I also used the steel rule and because there was some thread to measure to I started with one bolt and set the rule as best I could. As I turned the hub it was obvious the other bolts were off from the 1st one while the outside of the rotor stayed true. I know it doesn't sound reasonable but that's what I'm seeing Since I still have a clicking noise I may need to take it all apart anyway.
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