Scratching my head...

Kim Haun

Supporter
Whenever I find myself scratching my head, I go to this forum and the incredible wealth of info that you all share so graciously never fails to enlighten. I've been doing some fine tuning on my RCR GT40 and have come across an issue with brake rotor run-out. The last time I drove the car I had a bit of vibration at the steering wheel when braking. I put a dial indicator on the rotors and found 3 of the 4 out of spec....over 3 thousandths. I turned all the rotors and solved the run-out on all but the front left. After turning the rotor I torqued the hub down with a steel spacer in place of the knock off adapter and the run-out is within spec. However, when I installed and then torqued down the adapter I got an 8 thousandths run-out at the rotor. Suspecting the knock-off adapter, I called Bob Lacey at Vintage Wheels, told him what I was finding and he graciously volunteered to send me a new adapter free of charge, even though the adapter I had was made back in 2006.(The RCR kit I purchased was originally bought by someone who did not build the car over a 10+ year period.) I received the new adapter, torque it down and I'm still having the same out of spec run-out reading. All the parts have been inspected for anything that could be causing this issue, rust, paint etc. I even marked the hub and adapter and rotated them 180 degrees to each other and still had the same readings. The front hubs are brand new and I don't have the issue on the right side. And I have to assume the new adapter is fine. Has anyone had a similar issue and found a way to correct it? Or perhaps someone can suggest what I'm missing here. I have attached a photo of the assembly with the steel spacer torqued on in place of the adapter. Assembled like this there is only 2 thousandths run-out...installing the new adapter, it goes to 8. WTF!

Thanks for your advice.

Kim

IMG_1063.jpg
 

Keith

Lifetime Supporter
Also, make sure that each bolt connecting the rotor to the “hat section” is torqued the same, each of the drive pins that connect the “hat section” to the hub are torqued the same, and all the surfaces are clean and smooth to the touch before you bolt them together.
All of the parts are made with some manufacturing tolerance and you may have to “clock” them to get the best end result.
Keith
 

Kim Haun

Supporter
Try rotating the hub in relation to the adaptor by 90 degrees ????? then check????
Thanks Doc, I’ve rotated the hub to adapter 2 bolt holes which amounts to approximately 130° (because of the five bolt pattern) and reinstalled and have the same results. I’ll try rotating the adapter just one bolt hole and try that. Not hopeful however.
 

Randy V

Admin
Lifetime Supporter
Might also consider installing the rotor as a “floating rotor” although there are definitely forces at work here.
Have you removed the rotor and just checked the runout of the mounting surface alone?
 

Kim Haun

Supporter
Also, make sure that each bolt connecting the rotor to the “hat section” is torqued the same, each of the drive pins that connect the “hat section” to the hub are torqued the same, and all the surfaces are clean and smooth to the touch before you bolt them together.
All of the parts are made with some manufacturing tolerance and you may have to “clock” them to get the best end result.
Keith
Thanks Keith, all the drive pins are torqued equally. I will re-check the bolts that connect the rotor to the hat. All surfaces are clean and straight to the touch and eye. I have also torqued the rotor hat down to the hub without the steel Spacer plate and the results are the same as with the plate, only 2 thousands run-out.
 

Kim Haun

Supporter
Might also consider installing the rotor as a “floating rotor” although there are definitely forces at work here.
Have you removed the rotor and just checked the runout of the mounting surface alone?
Hey Randy, Yes I checked tthe run-out of the hub and it is within tolerance as well. I’m going to “blue” the bottom of the new adapter and scrub it on a flat surface with a piece of emery cloth taped to it and see if there are any high or low spots on the back face of the new adapter.
 

Kim Haun

Supporter
OK, I found the culprit. Never assume "New" means good. Here is what I found after "blueing" the back face of the new adapter and sanding it with 600 grit sandpaper on a super flat surface. As you can see from the photo there is a low spot around 25% of the outside of the adapter. I checked it with a straight edge and feeler gauge and it is at least .006 low on the outside of the mounting face. Now the question is could the adapter have been warped due to tightening on a defective rotor hat? I would think the adapter would be robust enough that that is very unlikely. I'll be re-checking the rotor hat run-out but it checked out fine before. Update to follow.

Kim


IMG_1067.jpg
 

Howard Jones

Supporter
Swap rotors side to side and see what happens. Don't change anything else and use the same setup on both sides. One of the rotors may take a different "set" when assembled on the hat/hub.
 

Kim Haun

Supporter
Swap rotors side to side and see what happens. Don't change anything else and use the same setup on both sides. One of the rotors may take a different "set" when assembled on the hat/hub.
Thanks Howard! It’s been a week or so since I did my first testing and now that I remember, I did torque down one of the other old adapters to this assembly and it was within spec. So it would seem I was sent a defective replacement adapter.

Kim
 
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