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

[Ross]

During hard braking, my front wheels will start shaking left and right, hopping up and down, and I have to reduce braking to stop it.
I have had this issue with my Goblin from day 1, but I'm not sure if it is only happening to my car.
I am trying to identify what I have done wrong, and fix it.

You can see the issue at 1:45-2:20 in this video. I replaced the bent part with a stronger 3/8" metal part, but the shaking persists.
I had worn out wheel bearings, so I replaced all 4 of them with Moog bearings - problem persists.
I put on wide wheels with spacers, with wide tires. At first I was thinking this was the issue, but putting on the Cobalt wheels and tires still does hopping.
I put on 125lb spring in the front, but replacing them with 300lb springs doesn't fix the hopping.
I am running the shocks at halfway thru their compression setting. Maybe I should increase the dampening? Need to test that.
Is my steering rack out of spec?

Anybody else have this issue?

 

[George]

What tire pressure are you running. If the pressure is too high it will make tire hop that way. The other issue may be how much the toe changes when you brake had.
The bump steer is very bad on this car as it comes.
On mine I have no Ackernman or bump steer. With my race tires on or street tires never have that issue at all.

Brad

 

[Ross]

I was running 20 psi but have kept lowering it. Currently at 14 psi, and it has shaking at all these tire pressures.
If it is caused by toe changes when the nose of the car dives down, then other Goblin owners will have this too.
I see some race cars run toe out on the front, toe in on the rear. I currently have 1/16" - 1/8" of toe in both front and rear.

A google search suggests:
- tires worn incorrectly (my tires don't have waves in them, and it happens with new tires)
- brake rotor warped (it does it with new brake rotors)
- brake calipers/pads sticking - maybe?

 

[George]

I can bring a shocks mounted with 600# springs to Branson if You want to experiment.

Brad

 

[George]

If my memory is correct you run drag radials.
Try 30 psi and see if it changed.
Brad

 

[JBINTX]

You mentioned your steering rack. Is there any slop in it - at all?

 

[Ross]

I put toyo proxes r1r on the front this year. I will try them at 30 psi.

I jacked up the front to check the rack for play. It seems tight. Maybe a few thou play... I'd guess 5 thou.

 

[Rttoys]

Was the tie rod end nut not set? I know this is not overall cause, but it stood out to me and fixed later in the video.

I’m almost thinking you have too much traction up front and/or may need to proportion braking more to the rear. I’m still analyzing it video though. Even wondering if steering rack braces might be needed.

Just tossing some things out there. There’s a lot going on here.

 

[Ross]

Was the tie rod end nut not set? I know this is not overall cause, but it stood out to me and fixed later in the video.

I’m almost thinking you have too much traction up front and/or may need to proportion braking more to the rear. I’m still analyzing it video though. Even wondering if steering rack braces might be needed.

Just tossing some things out there. There’s a lot going on here.

Thanks for looking, and analyzing. I was thinking too much traction too, but stock Cobalt tires and wheels also shake under hard braking.

After I bent the front suspension, we were aligning the front wheels between racing rounds. It was a very quick alignment, and that steering rack nut was left loose for a round or two. It took us multiple rounds to get the alignment close.

 

[Rttoys]

I kind of figured something like that. It just stood out to me, but you said it has done it since day one, so it’ll have to be deeper. I’m wondering if others that auto cross these have had the same problem. Like they say, if you want to find the weak points, just race it.

 

[Ross]

I had the front shocks at click 12 of 18. I raised the dampening up to 16 of 18, and took it for a drive. It seems to be a little better, but still can get into shaking while braking.

Dale E sent me an email that mentions the guide pins that allow the brake calipers to slide left right, can cause shaking if they are not properly lubricated and moving smoothly. I will have to check that too. They were new, but are they without lubrication?

 

[George]

On my shock settings I have never gone above 5 from full soft

Brad

 

[ctuinstra]

That reminds me a lot of what a grocery cart wheel does when they are bent. Usually the wheels are bent in from being pushed sideways, this throws off the camber and/or camber. You may have one wheel with one camber and caster angle and the other with a different angle and since the both act as casters, they will oscillate.

Also as the car dives, it will change many of the adjustments, even the caster angle.

I would re-check all of your alignment in the front.

 

[ctuinstra]

We do not have ANY shake like that during hard braking. The turn to our house is at the bottom of a hill on a highway and I like to see how late I can brake before turning in and really mashed the brakes and not the slightest shake or movement.

 

[Ross]

Good to know. I think my alignment is the most likely reason, as I have a hard time with it. If I align it, then roll it 3 feet forward in the shop, then recheck the alignment, I get all new alignment numbers. Drives me crazy. Never seems like it is done right.

 

[G-Quest]

Ross, with those big meats you are running, have you looked into the resulting "scrub radius" of your front end set up? I've read that getting too far from 0 can affect steering under braking and acceleration.

 

[Rttoys]

Talking to a few buddies; Unless there’s loose components, between alignment, Spring pressure and dampening, you should be able to fix it. Not sure what alignment specs you are using though. Hell. I’m be going this too here shortly.

 

[Ross]

Ross, with those big meats you are running, have you looked into the resulting "scrub radius" of your front end set up? I've read that getting too far from 0 can affect steering under braking and acceleration.

Thanks for the tip.
Double wishbone suspension usually has a positive scrub radius. I bet I am way too positive with my spacers and tires. I should be able to adjust this with the Goblin's front suspension.

Talking to a few buddies; Unless there’s loose components, between alignment, Spring pressure and dampening, you should be able to fix it. Not sure what alignment specs you are using though. Hell. I’m be going this too here shortly.

Yeah, that's the issue, need alignment specs that work. The recommended Goblin alignment specs don't work well for really wide tires... the -1.5 degrees of camber doesn't allow wide tires to plant the outer edge, so I am at -.75 degrees. The 9 degrees caster angle was also too aggressive for my tires too. I think I set it to 6 degrees.

 

[Rttoys]

Yeah, that's the issue, need alignment specs that work. The recommended Goblin alignment specs don't work well for really wide tires... the -1.5 degrees of camber doesn't allow wide tires to plant the outer edge, so I am at -.75 degrees. The 9 degrees caster angle was also too aggressive for my tires too. I think I set it to 6 degrees.

I was going to mention in your video, you can see a “hot line” on the center of the rf tire, when it smokes on your braking. When your boy went off, the hot line was center to inner of the rf tire. It’s almost like the outer part of the tire isn’t even on the ground.

 

[ccannx]

I had a conversation with someone this past weekend about the Qa1’s and he told me while testing the they found them to be immovable over 6 clicks from soft and it’s possible they had an accident from running them at 6 . Luckily it’s not to hard to pull your shocks and remove the springs to see if one of them is blown. If not try running them 1-4 clicks from full soft or run the ones George is offering.