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Old 11-09-2020, 08:22 AM   #1
wkvasnicka
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Default Shimmy when braking

My 31Cabriolet is a body off restored car with all running gear appearing like new. However, when braking at high speed I have a front end shimmy. Any help in identifying the problem would be appreciated. Thanks
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Old 11-09-2020, 08:28 AM   #2
BRENT in 10-uh-C
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Default Re: Shimmy when braking

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My 31Cabriolet is a body off restored car with all running gear appearing like new. However, when braking at high speed I have a front end shimmy. Any help in identifying the problem would be appreciated. Thanks
Could be from several things. First, check the Steering Gear (Pitman) Arm bolt to make sure it is properly tightened onto the Sector shaft. Another very likely area is where the Front Radius Rod (Wishbone) attaches to the Clutch Housing. If this area has any looseness, it will allow the front end to move during braking.
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Old 11-09-2020, 08:29 AM   #3
1931 flamingo
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Default Re: Shimmy when braking

Make sure the brake backing plates are not loose.
Paul in CT
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Old 11-09-2020, 08:27 PM   #4
Purdy Swoft
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Default Re: Shimmy when braking

Check the toe in . the front wheels need to be toed in 1/16 .
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Old 11-09-2020, 08:53 PM   #5
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Default Re: Shimmy when braking

Worn or loose spring shackles, loose wheel bearings. The list is long!
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Old 11-09-2020, 09:06 PM   #6
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Default Re: Shimmy when braking

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I agree with Purdue check your front end alignment but first do the visual and check your front end parts
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Old 11-09-2020, 10:21 PM   #7
Herb Concord Ca
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Default Re: Shimmy when braking

Ditto check your toe-in, that is what finally solved my low speed shimmy when brakes were applied.
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Old 11-10-2020, 08:13 AM   #8
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Default Re: Shimmy when braking

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Ditto check your toe-in, that is what finally solved my low speed shimmy when brakes were applied.
If you don't mind me asking, exactly what about the incorrect toe-in alignment causes shimmy only when braking?
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Old 11-10-2020, 09:15 AM   #9
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Default Re: Shimmy when braking

Shimmy when braking. I would check the brake system first. Look at the drums, out of round?

I am told by my modern car mechanic that he would check that first. Along with balance, toe in, and front wheel bearings.

Given the age of these cars it also could be a combination of many different things. Just because it looks good doesn’t mean it was done right.

Enjoy.

Last edited by WHN; 11-10-2020 at 09:37 AM.
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Old 11-10-2020, 08:05 PM   #10
Herb Concord Ca
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Default Re: Shimmy when braking

Brent, this all started when I did a complete brake job, new cast iron drums, composite bonded linings (drums and linings from Randy Gross) in 2017. Had the front shoes off 3 times, found one brake shoe pivot hole, slight undersize, still had problem. Adjusted toe-in twice, set to about 1/8". Installed perch washers, no change. Finally increased toe-in some more this spring (did not have a gauge), just turned tie rod 1/2 turn, just measured the increase in tie rod length. Don't remember how much it increased! I know this sounds hookie but this is where I'm at today. Also tried adjusting front brakes a few times along the way.
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Old 11-11-2020, 04:44 PM   #11
BRENT in 10-uh-C
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Default Re: Shimmy when braking

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Brent, this all started when I did a complete brake job, new cast iron drums, composite bonded linings (drums and linings from Randy Gross) in 2017. Had the front shoes off 3 times, found one brake shoe pivot hole, slight undersize, still had problem. Adjusted toe-in twice, set to about 1/8". Installed perch washers, no change. Finally increased toe-in some more this spring (did not have a gauge), just turned tie rod 1/2 turn, just measured the increase in tie rod length. Don't remember how much it increased! I know this sounds hookie but this is where I'm at today. Also tried adjusting front brakes a few times along the way.

Ok, ...well lets go just a tad further. Front end 'shimmy' is the tandem alignment of the wheels wobbling rapidly while the steering wheel is remaining stationary. I personally cannot think of something that would cause the wheels to 'shimmy' only when the brakes are applied, ...unless the entire front end (axle, front radius rod, etc.) is being allowed to move in a vibrating manner. This typically happens when the connecting point (-a ball) that anchors the front radius rod (wishbone) to the Clutch Housing is unsecure/loose. Because the suspension is allowed to move both laterally and vertically in the socket, this transfers the motion through the drag link to the steering wheel causing it to want to wobble.

Now we also need to make sure we do not confuse brake chattering or harmonics with front end 'shimmy'. A loose brake housing plate, or defective brake shoes, etc. can also cause severe chattering/harmonics which can be transferred into the front suspension, however this is not 'shimmying'.
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Old 11-11-2020, 07:44 PM   #12
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Default Re: Shimmy when braking

Did you have the drums turned to true them up and have the shoes arched to match the drums? Did you check to see if the shoes are installed so that there is 0.010 to 0.015 clearance at the top and zero at the bottom using one of the tools available?



Of course follow the other's advice and check everything.
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