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:

https://www.miataturbo.net/attachmen...ine=1381715593

Close up of the bushings:

https://www.miataturbo.net/attachmen...ine=1381715593

and

https://www.miataturbo.net/attachmen...ine=1381715593

The driver's side looks normal:

https://www.miataturbo.net/attachmen...ine=1381715593

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.

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

Yikes. that's a first for me too.

cordus has a point: I think its time for a major bushing overhaul on this car

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

I've had this happen. I swapped the upper arms left to right and it happened again.

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.

https://www.miataturbo.net/attachmen...ine=1381715593

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.

http://img849.imageshack.us/img849/4566/14xg.jpg

The idea was to create a "bearing" with the spacer, so the bushing would not bind.

Here is the spacer:

https://www.miataturbo.net/attachmen...ine=1382036475

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

https://www.miataturbo.net/attachmen...ine=1382036475

And, here is the spacer in place

https://www.miataturbo.net/attachmen...ine=1382036724

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

https://www.miataturbo.net/attachmen...ine=1382036724

And, there's still a very tiny gap left on both ends.

I love your threads because they always have extensive pictures of the problem AND solution

:)

The whole thing.

https://www.miataturbo.net/attachmen...ine=1382037124

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.

https://www.miataturbo.net/attachmen...ine=1378082110


Details are here:
https://www.miataturbo.net/suspensio...rection-74732/

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.

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.