Dean My 86 GP has a high speed vibration. about 82 it becomes noticeable and about 87 it maxes out and stays that bad up to 130, i havent had it any faster. This has been here thru 3 sets of tires and 2 sets of rims and a performance alignment. And thru a ring and pinion change. Bilstiens all around and for awhile QA-1s (junk) on the back. No vibration thru the steering wheel It has been suggested drive shaft (factory) and exhaust hitting the frame. Any ideas on what it could be or the best way to diagnose it.

driveshaft is out of balance

that does sound like the most likely culprit,unless a rear wheel is bend, but id go with drivesaft

Why would a driveshaft ruin tires?

its more likely that if something bent the driveshaft, it bent the control arm as well causing the rear end to be out of alignment

A performance alignment shop aligned it and they had monitors, or whatever they are called, on all 4 wheels.

The vibration is not running the tires, esp since you dont feel it until over 80 and how often do you run that fast with a garage queen.
When we rebuilt it we went with the old tires for awhile, then replaced them. Then got new tires with the new rims.

Hey Tom, Call me next time, I think you have my #, If not the PM me and I'll get it to you and Chris again.
Anyways, It is most likely a U-joint cap that was crushed or egg shape from being over tightened. When was the last time they were replaced? Call me this weekend and I might have some time to rub by and check them out for you.

You can give me a test drive, I'll bring my helmet

Dean

Thanks Dean
I have to jack it up soon to change the governor in the trans. The front U joint felt good, the back was notchy so I replaced it. I guess the new one is worth checking.

Helmet not required, I keep the shiny side up.

It could just be the drive shaft is reaching critical design speed- it could just be an inherent defect that is not fixable.

I had this problem on my "Vetuck". The original driveshaft I had on the truck from the factory was believe it or not wrapped in carbon fiber. It is 6 feet long aluminum and they wrapped it in CF to damper vibrations for a higher critical speed.
What happened to it? well after about 15 years of use, the CF wrap decided to unbond itself from the aluminum and slipped up the shaft 3" rendering it unrepairable.
Being over 10years old, the factory dicontinue manufacturing a replacement so I went the aftermarket route and had "The Driveline" make me a trick steel shaft with 1350 yokes. One year later- two broken tailshaft housing- and it being pulled out and rebalanced 3 times to still read perfect balance from them and two other shops since I doubted them, I went to ACPT for a $1400 custom CF driveshaft that has a toal weight of 7.8 lbs rather than the 26 lb steel one. The front yoke weight was just too much in steel being that length- it did not matter the balnce, it was the weight. The steel one worked good right up to 76mph and then went violent to 85. The vibration stayed buy cycled in intencity at varying speeds above that.

My CF one has propelled me COMPLETELY SMOOTH all the way to 152mph so far through Diamond Bar late one night comming home from Toyota Speedway. I looked at Nick and said "you wanna run her up" I got a Yes, so I yelled puit your helmet on kid. The thought of getting caught with helmets on at that speed and both of us pulling out our NASCAR license's- we still laugh and wonder if a COP would have cut me a break with our credentials and it being 1am with no one around and helmets on.

Not bad for an areodynamic brick.

I hope not, I dont want to spent $1400 on a DS.

You wouldn't. Yours, I promise you, is no where near 6 feet long. That was my major issue on the longbed truck.

You might have to go aluminum though. Check the U-joint caps first for bad spots in rotation.

Vetruck
did you ever get my PM?
if so i have another question for you regarding suspension geometry

I thought I had sent you back a response. I guess not. Sorry, my computer glicthes sometimes and goes error after I type things.

ANyways, about calculating roll centers- I do not own a roll center program or software on my computer. I have never done the measurements nor have I run any given to me for a 3rd gen.

Even on the Supertruck I setup, I initially looked at where they were only for reference because I had never driven one (I cant fit in ANY of the 7 different trucks I have had an opportunity to drive, Most of these guys are tiny and my shoulders do not fit under the seat helmet supports nor the pedal room) and I was trying to gather as much knowledge on what I was dealing with when I was first learning circletrack setups- compared to what my driver (him also new) was trying to tell me what it was doing on the track. Alot of times what I saw and what he said were completely opposite. I had to teach him what to tell me as I was also learning.

Anyways, I basically set things by feel. Trail and error til it feel right to me, not much math other than stringing the chassis with basic formulas on sqaureness, etc. and then adjusting through data aqusition in practice runs. lots of #'s in my head I do not even sometimes keep charts after I do it for long enough. I am constantly calculating chassis heights, panhard split tire temps, tire sizes, tire pressures. If I make any cross weight changes or turns into the swaybars I re-measure frame heights to maintain always the roll centers and any differences of tire diameters added or subtracted to that down to the 1/16".
What you are asking me on spring rates I have said many times on here is a crap shoot. All cars are different in weights and componants as well as condition of the chassis and condition of those componants. best answer I can give you is to go read the post I made on "The Ultimate 3rdgen Suspension"

Dean