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Old 10-13-2013, 10:53 PM   #1
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Default Clunk from the rear on takeoff

Like the title says, a dull clunk as I let off the clutch.
I noticed it about a week or so ago.

I had time to get under the car and check it out today.
Turns out the pass side upper control arm slipped forward on both bushings, and it was contacting the brackets.

Here:



Close up of the bushings:



and



The driver's side looks normal:



Has this sort of thing ever happened to you guys?
Pretty interesting.

Anyway... I will just make/install delrin bushings for the upper control arm inboard mounts. Turns out the lathe is available on Wednesday.
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Clunk from the rear on takeoff-gu1c.jpg   Clunk from the rear on takeoff-dd7g.jpg   Clunk from the rear on takeoff-yu6o.jpg   Clunk from the rear on takeoff-vezv.jpg  
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Old 10-14-2013, 03:28 AM   #2
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You say it happened on acceleration? Weird, I'd expect it to want to rotate the other way.

I've never seen it happen, but if all your bushings have as many cracks in them as the ones in the photo, I'd think about doing a total replacement rather than just those four.

--Ian
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Old 10-14-2013, 03:45 AM   #3
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Yikes. that's a first for me too.

cordus has a point: I think its time for a major bushing overhaul on this car
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Old 10-14-2013, 04:24 AM   #4
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Its not about rotating, the wheel pulls the hub forward which pulls the arm forward which pushes the subframe forward which pushes the body forward..

Its the diff case which wants to rotate backwards.

Dann
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Old 10-15-2013, 12:06 AM   #5
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I've had this happen. I swapped the upper arms left to right and it happened again.
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Old 10-17-2013, 03:57 PM   #6
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Problem solved.

I just went ahead and converted those bushings to Delrin, as well.
But, with one slight modification:

In a conventional bushing, the steel sleeve is trapped inside the bracket by the bolt, and it is stationary. The bonded rubber then flexes about that sleeve to facilitate control arm movement.
When you use poly or Delrin bushings, the sleeve is again held stationary by the bolt, and the bushing material then rotates around the sleeve.
Now, if you look at this pic, you'll see that there is a small gap on either side of the bushing. The steel sleeve protrudes from both sides by about 1.75 mm. That ensures the control arm does not rub on the bracket as it rotates.



If I were to follow that recipe, the control arm would move laterally and contact the bracket, making the whole job a moot effort.
On the other hand, cutting the Delrin exactly as wide as the bracket opening would mean binding.

I made the bushings so that there was a 1.75 mm gap on either end.
Then, we carefully made Delrin spacers that were 1.60 mm thick.



The idea was to create a "bearing" with the spacer, so the bushing would not bind.
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Old 10-17-2013, 03:58 PM   #7
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Here is the spacer:



The spacer surface was left rough intentionally, just like a vinyl record.
I though it would retain the grease better that way.

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Clunk from the rear on takeoff-jsop.jpg   Clunk from the rear on takeoff-v205.jpg  
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Old 10-17-2013, 04:05 PM   #8
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And, here is the spacer in place



This is what it looks like installed.
You can clearly see the spacers at both ends.



And, there's still a very tiny gap left on both ends.
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Clunk from the rear on takeoff-qhq8.jpg   Clunk from the rear on takeoff-cfa4.jpg  
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Old 10-17-2013, 04:10 PM   #9
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I love your threads because they always have extensive pictures of the problem AND solution

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Old 10-17-2013, 04:12 PM   #10
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The whole thing.



All three bolts are torqued to 50 pounds.

Control arm movement is smooth as butter.
I test drove the car, and it actually feels very good.

Next time I have a couple free days, I will tackle the front.

Oh, we made the bushings in two pieces, just like the rest of the bushings in the rear.




Details are here:
https://www.miataturbo.net/suspensio...rection-74732/
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Old 10-17-2013, 04:14 PM   #11
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Quote:
Originally Posted by 18psi View Post
I love your threads because they always have extensive pictures of the problem AND solution

Thanks

I used black Delrin this time. It's not like anybody will be able to see what color the bushings are, anyway. Exact same material, different colors.
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Old 10-17-2013, 04:20 PM   #12
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The bushings you replaced were the stock rubber ones?
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Old 10-17-2013, 04:23 PM   #13
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Quote:
Originally Posted by Dunning Kruger Affect View Post
The bushings you replaced were the stock rubber ones?
Yes. Really tired, softened, cracked stock rubber ones.
You can see them in the first post.
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