Front wheels shaking and hopping under hard braking (Now fixed!)

Torchandregdoc

Goblin Guru
Does your rack and pinion have any slop in it? Is anything worn out in the front end? If you're running a bunch of wheel offset, it can amplify worn steering components.

It also makes me curious why your alignment is changing when you are rolling the car forward and retesting.
 

Ross

Goblin Guru
True, wide tires will increase wear. My donor had a lot of wear to start with, I have racked up maybe 3000 miles.
Not much rack wear - when I move the front tires, the wheel moves. Maybe 5 thou of slop, which is pretty tight.

Alignment reading changes are from multiple reasons...
-The digital angle gauge is trying to give precision beyond it's sensor capabilities, so if I measure the exact same surface twice, I can get 2 slightly different readings.
-Placing a straight edge against the tire can have different measurements because the straight edge will be placed slightly different.
-The shop floor might have a slight dip/bump, the tires can have slight dip/bumps, the way the tire is mounted on the wheel, etc.
 

Torchandregdoc

Goblin Guru
True, wide tires will increase wear. My donor had a lot of wear to start with, I have racked up maybe 3000 miles.
Not much rack wear - when I move the front tires, the wheel moves. Maybe 5 thou of slop, which is pretty tight.

Alignment reading changes are from multiple reasons...
-The digital angle gauge is trying to give precision beyond it's sensor capabilities, so if I measure the exact same surface twice, I can get 2 slightly different readings.
-Placing a straight edge against the tire can have different measurements because the straight edge will be placed slightly different.
-The shop floor might have a slight dip/bump, the tires can have slight dip/bumps, the way the tire is mounted on the wheel, etc.
It was that donor car wear I wondered about. Mine has 177000 miles. If there's a little slop in 3 or 4 worn components, it might add up.

When it starts hopping, do you notice the steering wheel shaking farely violently? I'm sure it's hard to sort out what all is shaking in the moment, especially if you're in a hurry to get shut down and it ain't stoppin'.
 

JERMzSS

Well-Known Member
Ross my car started this when I installed the slicks but only at speeds over 70mph and HARD braking. I have no doubts my alignment is off alittle but I feel it has alot to do with the speeds and sticky tires.

I haven't been able to due away with it but unhooking the vacuum from the brake booster has allowed me to have alot more control over the shaking. It's much more manageable with non-boosted brakes imo.

Can someone try hard (not locking up but close) braking with 200 tread wear tires or lower above 70mph and see if they have the same issue. At 30 to 40 I just stop like I hit a wall with no shaking.
 

Torchandregdoc

Goblin Guru
Maybe a little wheel shaking. I had to watch the video to know... 1:45 - 2:05 in
Well, that is a great video. I wish I had a clue how to read what was going on there. That would be borderline terrifying on a road course with barriers. I'd say that's "fairly violent". I'm just about ready to buy wheels and tires soon, so I'll be watching what you figure out. Hope it's not tire width. I like to stop fast as much or more than I like to accelerate fast. This light of a car should do 100-0 really well.

I'm still wondering about slop someplace in the front end. I can't imagine a rack and pinion having much at all. I'm think as familiar as you are with your car, yiu would have noticed bad tie rod ends. Shock setting? I doubt they could be worn out yet.
 

Ross

Goblin Guru
Well next weekend I will be meeting up with a bunch of owners at the Midwest spring meet, so maybe comparing my car to others will give me a better idea. Checking the steering for wear/slop when the front end is jacked up in the air, might not be the best testing method.

Jermey, I have noticed that the brake vacuum regulator has helped reduce shaking, but it is a band-aid solution to the problem.
Right now I'm thinking I need to keep searching for the magic alignment numbers for wide tires.
 

Waterdriver

Goblin Guru
Running the R888r's at 100 tredwear I haven't experienced the tire shake I've seen in your video.
And it looks like it could be that, tire shake, like in drag racing, except occurring during braking. Tire deforming under hard braking.
Can you swap tire and wheels with someone and test?
Does the amount of shake decrease with the increase of tire pressure?
 

ctuinstra

Goblin Guru
Well next weekend I will be meeting up with a bunch of owners at the Midwest spring meet, so maybe comparing my car to others will give me a better idea. Checking the steering for wear/slop when the front end is jacked up in the air, might not be the best testing method.

Jermey, I have noticed that the brake vacuum regulator has helped reduce shaking, but it is a band-aid solution to the problem.
Right now I'm thinking I need to keep searching for the magic alignment numbers for wide tires.
Maybe Lonny will bring his toe plates and you all can do a parking lot alignment like we did last year. We are still running on that alignment.
 

Ross

Goblin Guru
This weekend we tested Brad's 600 lb/in springs & shocks on my car, and changed the toe in. Still hops the front wheels under hard braking.

I was telling Lonny that I ran out of adjustment the first time I adjusted the toe in. The steering rack was too long, so I had to adjust my front suspension out farther. He suspects I put the wrong tie rod ends on the front. A quick inspection, and I have the long steering tie rod ends on the front, and the short ones on the rear... which is backwards.

So I hope that bringing the front suspension in closer to the frame will increase the front stability, and eliminate the hopping.
 
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Fozda

Goblin Guru
This weekend we tested Brad's 600 lb/in springs & shocks on my car, and changed the toe in. Still hops the front wheels under hard braking.

I was telling Lonny that I ran out of adjustment the first time I adjusted the toe in. The steering rack was too long, so I had to adjust my front suspension out farther. He suspects I put the wrong tie rod ends on the front. A quick inspection, and I have the long steering tie rod ends on the front, and the short ones on the rear... which is backwards.

So I hope that bringing the front suspension in closer to the frame will increase the front stability, and eliminate the hopping.
Are we talking inner, outer, or both tie rod ends here? I believe we put the OEM in front and the supplied in the rear which, now that I think about it, doesn't make sense!
 

Karter2026

Goblin Guru
Are we talking inner, outer, or both tie rod ends here? I believe we put the OEM in front and the supplied in the rear which, now that I think about it, doesn't make sense!
Outer.... Stock Cobalt ones go in the rear. The ones that come with the kit go on the front
 

Ross

Goblin Guru
The tie rod end is about 1" shorter on the DF supplied ones (black) vs the Cobalt ones (white)
14854
14853


So I (mistakenly) added a 1/2" extra threads at all 6 of the swing arm's heim joints.
I should have moved the white tie rod ends to the rear of the car.
14851
 
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Desert Sasqwatch

Goblin Guru
Hmm, interesting that an extra 1/2 inch of exposed threading on the heim joints would allow the flexing of the a-arms to cause the hopping you're experiencing. Wonder if the overall length is at the 'harmonic' length for the arm tube to wall thickness ratio? Kinda like a tuning fork when the right combination is achieved and sufficient energy is applied, like hard braking. :rolleyes:
 

Ross

Goblin Guru
Hmm, interesting that an extra 1/2 inch of exposed threading on the heim joints would allow the flexing of the a-arms to cause the hopping you're experiencing. Wonder if the overall length is at the 'harmonic' length for the arm tube to wall thickness ratio? Kinda like a tuning fork when the right combination is achieved and sufficient energy is applied, like hard braking. :rolleyes:
So far, it is a hypothesis, but a likely one, as most Goblins don't have this issue, so it is something that I have done differently.
 
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