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V1 Stretch’s ext. track goblin #386 SS/TC

Stretch2126
Are you running stock brakes?
edit:
never mind. I looked back in the trad to see you are. Odd it‘s shaking.
Yes. I’ve replaced the pads up front before too almost a year ago to try a different compound but no difference. In another video it’s as the wheels are doing more in and out with steering rather than a standard wheel hop like a driven tire does.
 
Stretch2126
Another piece of data, the shake comes on quicker if I stiffen up the shocks on the front. Not sure if that means it’s putting more pressure on the arms, rack, or another surrounding pice that causes a flex and rebound effect.
 
Desert Sasqwatch
Sounds like you are on the correct path. Did the addition of the two extra triangulation bars make any difference? Did it change the behavior of the wheel hop? Are both wheels hopping synchronously (both up and down at the same time) or alternately (up on one side as the other is moving down)? This will make a big difference to understand what dynamic force is causing the front of the frame to oscillate.
 
Stretch2126
Sounds like you are on the correct path. Did the addition of the two extra triangulation bars make any difference? Did it change the behavior of the wheel hop? Are both wheels hopping synchronously (both up and down at the same time) or alternately (up on one side as the other is moving down)? This will make a big difference to understand what dynamic force is causing the front of the frame to oscillate.
From the video it’s more of a side to side rattle rather than up and down like traction hop. The bars didn’t seem to make a difference for my case but it could still be a benefit for overall stiffness in racing. In the video slowed down by scrubbing through frames it looks like the arms twist forward slightly which seems to be normal, upright has slight toe out due to tie rod vertical travel but it has decent toe in static to counteract. I ordered a new oem rack to try because part of me would at least like to believe there is deflection inside the steering rack that loads and unloads in the case of hard braking events. The video shows the tie rod moving back and forth but it’s not felt in the hands as much and not in the brake pedal at all.
The rack I had is the stock rack to my knowledge which is 145xxx. Hoping new parts fixes it lol
 
Desert Sasqwatch
Hmm, that sounds like something is loose/worn with the steering rack - mounting or internally. You mentioned something about play with the mounting bolts? This should be addressed first - the rack needs to be rigidly mounted with zero play. Any movement of the rack in the mounts will translate in side-to-side uncontrolled motion (called death wobble).

Believe these are 1/2 inch diameter bolts (grade 5) in the kit and they can be tightened to up to 65 ft.lbs. if it gets everything rigid.

If the steering rack is tight and this persists, then the gears in the rack have worn and this will likely require finding another rack. Note that many GM models used this same or similar rack.
 
Stretch2126
Hmm, that sounds like something is loose/worn with the steering rack - mounting or internally. You mentioned something about play with the mounting bolts? This should be addressed first - the rack needs to be rigidly mounted with zero play. Any movement of the rack in the mounts will translate in side-to-side uncontrolled motion (called death wobble).

Believe these are 1/2 inch diameter bolts (grade 5) in the kit and they can be tightened to up to 65 ft.lbs. if it gets everything rigid.

If the steering rack is tight and this persists, then the gears in the rack have worn and this will likely require finding another rack. Note that many GM models used this same or similar rack.
Yeah it’s almost like there’s play in the mounts for the rack. It’s very small but still something. I ordered another rack last night. Gonna see later this week if it makes a difference.
 
Waterdriver
You could check if there's any play with the inner tie rods. The joint can wear and get sloppy.
52627
 
Stretch2126
No difference with the new rack and rod ends. Still shakes like crazy when you have good traction and stop hard. Guess I need to put it on a lift and look for cracks in the frame or something.
 
Desert Sasqwatch
What is the caster set to? Not enough caster may cause the tires to shake side to side - due the caster is reduced by several degrees under braking with the nose dipping hard. Are the upper and lower heim joints on the a-arms and hub knuckles good, no play, torqued correctly? Are the bearing hubs new or original donor? Trying to think of everything that could contribute to this problem - short of making more frame mods.
 
Stretch2126
What is the caster set to? Not enough caster may cause the tires to shake side to side - due the caster is reduced by several degrees under braking with the nose dipping hard. Are the upper and lower heim joints on the a-arms and hub knuckles good, no play, torqued correctly? Are the bearing hubs new or original donor? Trying to think of everything that could contribute to this problem - short of making more frame mods.
My caster is at 8 degrees. Measured using a straight edge on the upper and lower outer rod ends. Trying to balance the little angle finder on the two studs is 7.5-8. Everything is tight. I replaced my heims a while back but it didn’t make a difference then either. I need to try to get more slowmo video probably from different angles up front.
 
Stretch2126
Here’s another clip. You will have to scrub by hand to really slow it down. Looks like there’s a drastic caster change and it rattles the nose pretty good. Not much of a hop up and down but side to side.
 
G
Are your lower control arms and tie rods level with the cars loaded with normal weight, looks like they are angled up. This may be some variation of bump steer. If they are angled up to start with that will increase bump steer even more as you load the front end under braking.
 
Desert Sasqwatch
I slowed the video down and the side to side motion is clearly visible, but there is also bouncing. Did I ask if this bouncing was both tires in synchronization or opposite motion?

Watching the upper control arm, it looks like it might be twisting and allowing the upper side of the knuckle to rotate forward. Watching the shock mount 'ear' looks like it too may be flexing up and down. This may indicate the front of the frame is twisting, allowing the a-arm mounts and shock 'ears' to flex and impart the wheel hop.

The diagonal bars you added - which are similar to what DF added to the AT and Ross added to his frame - helps prevent the frame twisting, but may not be enough. When I did some frame stiffness simulations at work, based on the stock frame rails, it resulted in a lot flex with moderate lateral and vertical forces applied - singularly and in combination. This resulted in my addition of extra triangulation bars to help control the flex inherent to the frame in front of the a-pillar.

20231211_151051.jpg
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Note that the crossed bars close off the top of the 'open box' above the footwell. These bars plus the diagonal bars tying the middle frame bar at the front suspension upper mount point to the top bar at the a-pillar increased front frame twisting stiffness by more than 75%. But I had to change the brake vacuum booster to a 7 inch frome the donor 11 inch and carefully align the routing to miss everything else at the pedal box. I'm not saying this is what needs to be done to fix your problem, but I'm showing an example of what additional bracing is needed to significantly reduce a potential short coming of the frame.
 
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