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Eccentric bushing witness mark orientation
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av8shunmeckaneck
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PostPosted: Thu May 07, 2020 2:53 pm    Post subject: Eccentric bushing witness mark orientation Reply with quote

Looked around but didn't find any references. Has anybody seen any official reference to where the witness mark should generally be? In troubleshooting my shimmy I decided to check how the eccentric was oriented in regards to caster even though, technically, caster isn't really called out in the manual. I found mine with the witness mark directly at the front 12 o'clock viewed from the top. If the eccentric is able to adjust +/-2 camber (not sure what it actually is, just a guess for discussion) then rotating it 180 will effectively add caster. The witness mark appears to indicate where the ball joint shaft is offset toward. My only experience with actually measuring and adjusting caster is on aircraft. All of the aircraft we manufacture typically have about 3 and just a couple deg off really affects nose wheel shimmy.

BTW: Alignment and balance had previously been accomplished at a chain tire shop along with new tires after I rebuilt the front end.

Here's a pic of the witness mark, after rotation, that I'm referencing.


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germansupplyscott
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PostPosted: Thu May 07, 2020 5:12 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

The 'starting location' is 12 o'clock, or straight ahead, wheels points straight ahead.

In the photo above the notch appears to be a 6 o'clock.

SL


Last edited by germansupplyscott on Sat May 09, 2020 5:26 am; edited 1 time in total
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av8shunmeckaneck
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PostPosted: Thu May 07, 2020 9:58 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

germansupplyscott wrote:
The 'starting location' is 12 o'clock, on straight ahead, wheels points straight ahead.

In the photo above the notch appears to be a 6 o'clock.

SL


Good eye. You are correct sir! I didn't think to take a pic till after I rotated it.

Added info: Had to adjust alignment. Used a string bungee cord method.
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Al Capulco
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PostPosted: Thu May 07, 2020 10:56 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

This is the alignment chart that most Alignment Shops use. When I had my Alignment done, I don't remember the bushing being moved that far. They did a complete 4 wheel alignment for $60.00. And they adjusted and checked everything.
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av8shunmeckaneck
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PostPosted: Sun May 10, 2020 9:20 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

Al Capulco wrote:
This is the alignment chart that most Alignment Shops use. When I had my Alignment done, I don't remember the bushing being moved that far. They did a complete 4 wheel alignment for $60.00. And they adjusted and checked everything.
Image may have been reduced in size. Click image to view fullscreen.


Where is the toe measured? At the edges of the wheel or outside of the tire diameter?
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Al Capulco
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PostPosted: Sun May 10, 2020 11:33 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

From the center of the tire.
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Wildthings
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PostPosted: Wed May 20, 2020 7:36 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

delete

Last edited by Wildthings on Wed May 20, 2020 8:51 pm; edited 1 time in total
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Wildthings
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PostPosted: Wed May 20, 2020 7:37 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

Quote:

Where is the toe measured? At the edges of the wheel or outside of the tire diameter?


The toe-in specification at the rim is 1/8" +/- 1/16", with the numbers doubled when you check the toe-in at the tire tread.

Image may have been reduced in size. Click image to view fullscreen.


You have maximum caster when the mark points towards the front of the vehicle. I don't turn the mark further than 45 away from straight ahead to maintain as much caster as possible, and then try to get the camber the same on both sides even it the camber is out of spec a bit. This way it will not pull and will have maximum return to center.

If you are getting shimmy then check the function of your steering dampener.


Last edited by Wildthings on Wed May 20, 2020 8:52 pm; edited 1 time in total
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Al Capulco
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PostPosted: Wed May 20, 2020 8:35 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

The chart I posted has been around on Samba for years. I believe it is the same one that comes up when you get a computer alignment. The toe in measurement on the chart I posted is measured off the tire tread so I think it is the same as you posted. Mine says 1/4" and your's says 1/8" on the rim or double if taken of the tread.
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Wildthings
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PostPosted: Wed May 20, 2020 8:50 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

Al Capulco wrote:
The chart I posted has been around on Samba for years. I believe it is the same one that comes up when you get a computer alignment. The toe in measurement on the chart I posted is measured off the tire tread so I think it is the same as you posted. Mine says 1/4" and your's says 1/8" on the rim or double if taken of the tread.


You are correct, their degree mark () looking suspiciously like an inch mark (") fooled me. Sad
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Al Capulco
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PostPosted: Wed May 20, 2020 9:02 pm    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

Yeah, it's not a very good copy.
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66 Shorty
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PostPosted: Thu Oct 29, 2020 9:07 am    Post subject: Re: Eccentric bushing witness mark orientation Reply with quote

germansupplyscott wrote:
The 'starting location' is 12 o'clock, or straight ahead, wheels points straight ahead.

In the photo above the notch appears to be a 6 o'clock.

SL


Thanks for that! I'm about to put my front end back together and wasn't sure which direction to start!
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