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

Ross

Goblin Guru
Justin Reed & George (that is his forum name) but he goes by Brad, both are hard core racers, and run 600 lb/in springs up front.
Brad has QA shocks, but a bit longer than the DF supplied ones.
 

Ross

Goblin Guru
I think it is fixed now. Does this look like your Goblin under real hard braking?
70mph to 0 hard stop:

Much better than before. Barely has any shaking/hoping just before the tires lock up.
Finally swapped the tie rod ends, and gave it a new 4 wheel alignment.
I might add more brake boost now that it is working correctly... I can run out of pedal!
Getting ready for Saturday's autocross with Reaction Motorsports.

This is my settings for the street, not a racing alignment. All set with the weight of 2 people in the car.
I'm playing with various settings trying to find the best mix for wide tires.
Rear: 1 degrees camber, 1/8" toe in. Bottom of the aluminum swing arm is horizontal.
Front: 1 degree camber, 1/8" toe in, 6.25 degrees caster,. Lower A arm is horizontal.
 
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Desert Sasqwatch

Goblin Guru
Ross, did you shorten the a-arms? Change the steering settings? Change springs? Give us a good explanation of what was done and what you think finally fixed the problem. May come in handy down the road for a fellow builder. :D
 

Ross

Goblin Guru
I put the correct tie rod ends on my car. The shorter tie rod ends go in the front.
This allowed me to shorten up the front A arms about 1/2" shorter.
My bottom A arm only has 1 thread left all 3 of the heim joint ends, the top A arm was used for the alignment.
The bottom A arms are tight to the body.
The wheel alignment angles are basically the same as before the change.

Springs are back to the same as before (125 lbs / inch) which is a very light spring, but our Goblin has a light front end.
Now that I can brake hard, I might want to get heavier springs up front... they dive a fair bit under braking.
 

Ross

Goblin Guru
This year I put Toyo Proxes R1R on the front, so they are new.
Those tires are only 275 wide, which barely fit on my rims, but they are 200 tread wear, so they are soft rubber.
The rear is getting new 315 tires tomorrow.
 
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Torchandregdoc

Goblin Guru
This year I put Toyo Proxes R1R on the front, so they are new.
Those tires are only 275 wide, which barely fit on my rims, but they are 100 tread wear, so they are soft rubber.
The rear is getting new 315 tires tomorrow.
The r1r is what I'm looking at. I think tire and wheel selection has been the hardest part of our build. I'm looking at 275/40r17 17x9.5+22. I'm just concerned that the control arms on the Goblin can't handle the torque generated by the massive meats on your car. Im curious if the reduction in width is partly responsible for the reduced hop.
 

Ross

Goblin Guru
My Goblin hopped with the Cobalt tires, and they were 215 mm tires.

I am running the 275/40r17 R1R in the video. It works well, and I see a few other builds have that tire too.
 

Torchandregdoc

Goblin Guru
That's cool,well.....not cool, but I'm glad it wasn't tires. Looks like you probably have it figured out. How fast were you going in the above video? Looks like it stopped fairly effortlessly. A little shaking, but nothing violent. Thanks for the tire info.
 

Lonny

Administrator
Staff member
The motion ratio of our front suspension is .65" shock travel to every one inch of suspension travel.
125lbs. x .65 = 81.25lbs of effective spring rate.

An Ariel Atom has an effective spring rate of 283lbs in the front, which is the equivalent of a Goblin using 435lb springs.

QA1 says that for a streetcar the springs should typically be compressed 25% of the free length when supporting the weight of the vehicle.
5.5" x 25%= 1.375"
300lbs front corner weight /.65 motion ratio = 462lbs at the shock.
462/1.375" =336 lbs
I think this means you should have a minimum of 336 lb spring rate for a streetcar and for a performance setup it should be more.
https://www.qa1.net/tech-center/spring-rate-tech
 

JBINTX

Goblin Guru
This year I put Toyo Proxes R1R on the front, so they are new.
Those tires are only 275 wide, which barely fit on my rims, but they are 100 tread wear, so they are soft rubber.
The rear is getting new 315 tires tomorrow.
Do the R1R's come in 100 tread wear too? Mine are 200.
 

Jlarson524

Active Member
The tie rod end is about 1" shorter on the DF supplied ones (black) vs the Cobalt ones (white)
View attachment 14854 View attachment 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.
View attachment 14851
So I am just seeing this now. I don't believe I swapped out the Tie rod ends at all so I am still running the long ones on the front and short on the back. I remember hearing my alignment guy state something about they should be shorter in the front and longer in the back. I am assuming its highly recommended to swap them?
 

ctuinstra

Goblin Guru
Springs are back to the same as before (125 lbs / inch) which is a very light spring, but our Goblin has a light front end.
Now that I can brake hard, I might want to get heavier springs up front... they dive a fair bit under braking.
I was thinking about this the other day and forgot to post. It seemed as if your front was diving a heck of a lot. And if you have light springs, that would make sense. When it dives that hard, the tie rods will pull the rear of the tires in causing a heck of a lot of toe out during a hard stop. I could see the tires fighting the excessive toe out and causing this. While bringing the wheels and tires closer in will help control them more, it might be a 1-2 punch to also go with a stiffer spring and really get it under control.
 

Ross

Goblin Guru
I bought those light weight springs for the rear of the car, but swapped them front/rear as an experiment. Now that the brakes work without wheel hop, I do need stiffer front springs. I went to summit and bought these QA1 10HT225/475 - QA1 High Travel Coilover Springs
 

ctuinstra

Goblin Guru
I bought those light weight springs for the rear of the car, but swapped them front/rear as an experiment. Now that the brakes work without wheel hop, I do need stiffer front springs. I went to summit and bought these QA1 10HT225/475 - QA1 High Travel Coilover Springs
Progressive. Interesting!
 
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