So phase 2 of 'stop breaking axle parts' is ask people how the launch looks and try to figure our why I get the tire shudder from 15-45ft off the starting line. The default is to change tire pressure or shocks, but I think I have done a light scan of both on my setup and yielded no results. There's a drag race suspension FB page I asked on and got a lot of varied, but good comments. Typical increase tire pressure & shock settings, but a few good other checks. Actually had a few stick guys comment that the DR2 works well for them which was great to see I just don't get a 'toss a bias on it' comment. Also, similar weight and power auto combos so something to start with.
As a refresher I have 275 Hoosier DR2 radials on a 10" wheel and have tried 17-21.5 psi in the tire and 7-12 clicks from full tight on the shocks. My shake/shudder problem seemed to turn on with more power and faster 60ft times. 1.75+ not an issue, 1.68 light shudder, 1.57 heavier shudder.
My issue is that the car is heavy, makes very abrupt power with the blower & stick, and neither are kind to the suspension. I think my IC/geometry is OK, I have about 110% and have video showing the car lifting and applying mechanical advantage to the tire like a radial chassis should. I know part of my problem is the chassis roll was excessive and I've dealt with that. It's possible the geometry is going out of whack when it's twisting or topping out the shock travel on the drivers side so that needed to be addressed.
I've came to 3 conclusions/answers outside of tire pressure & shock adjustments
1. Chassis: I need to get rid of my poly LCA bushings and go solid hiem. A few people have noted even poly is too soft at this power and weight with a stick. There are also comments that I really need bracing on the chassis at this weight and power. Something like UMI UCA to LCA tie in brackets, weld in the LCA braces, stuff like that. If you watch my launch videos carefully it looks like the axle is moving front to back in the chassis on a launch. That really only happens when the tire shudders but it's something that could be influencing it.
Trailing arm mount braces are designed to reinforce the rear upper and lower front control arm mounts on your 78-88
www.umiperformance.com
2. Shock travel: I need to check where the shock travel is topping out and and see if it I hit top of stroke and that causes my shudder. I might need to move up/down the pickup point holes. I know I could have topped my suspension travel out when the shocks were loose, I tightened the shocks and the problem went away, but then I added power and now I'm shuddering. I might be topping the travel of the shock out, but it's slow and not as abrupt so I don't notice it in the video
3. Tire: Sometimes drag radials get in this odd dead zone. They are fine with low power, and fine with high power, but don't do well in this middle zone, I might be in that dead zone. A stiffer wall tire, or tube can help stiffen the sidewall up. The tire might just also be 'dead'. The sidewalls could be worn out over ~50 passes and 18 months.
Outside of that, playing with front travel extend rate or travel limiters or adding instant center adjustment position holes on the chassis side for the UCA is something to do, but that's phase 3 or 4 if that doesn't work.
I also don't need to be afraid of going much tighter on the shocks, 2-3 clicks from full tight on the rebound might be fine! I was 7+ from full tight. Also, tire pressure, 21 seems high, but I shouldn't feel hesitant to add pressure at 23 or more.
So I will for sure do 1 & 2 over this winter, and then head back to the track. Then I will play with shocks and tire pressure again. Beyond that, putting new tires on & addressing travel limit or instant center is a mid summer next year thing if 1 & 2 don't succeed.
