Vibration...

[aarvig]

Thought I would run this by the brain trust.  I finally got my carb issues worked out so I decided to put a few miles on the Cobra today just to give it a test.  I noticed that at 40 mph I get a high frequency low intensity vibration that seems to be coming from the rear that I can feel in the seats AND the steering wheel.  I didn't push it any harder as I didn't want to risk things getting worse and breaking with no registration on the car it (it is insured).  Anyway, wondering if anyone has any ideas for places to start to work this out. 
The tires have been sitting for 5 years (mostly without the weight of the car on them), the driveshaft was made by Denny's driveshaft, the car HASN'T been aligned. I also hadn't fully tightened the coil spring seats together. 
Anyways, let me know your thoughts.  THANKS!!

[Greg K]

Driveshaft alignment maybe?

[aarvig]

I was thinking about driveline angles too but I was pretty particular about that when I set up the car...but I will say that since then the car has gained a lot of weight which maybe affected the pinion angle.  Maybe I should check the wheel balance first and see if that does it.  If not, the driveline angles are the only thing it could be, correct?

[s_reynolds]

Do the simple stuff first. Wheel balance and alignment.

[aarvig]

Do the simple stuff first. Wheel balance and alignment.

Yep, I have it scheduled at a speed shop for August 10.  Man, everyone around here is operating on a 2 week schedule right now.  Crazy.  They are going to set wheel alignment and pinion angle for me.  Off to paint in October!

[aarvig]

So after speaking to Denny at Denny's driveshaft he is saying with a short stubby driveshaft like the one I have there should be almost no driveline angle.  He says it should be straight as an arrow.  Does this wash with everyone's experience?  My engine/trans angle is about 3.5 degrees so there is no way it would be straight. 

[s_reynolds]

I had a Denny's driveshaft in my Hurricane and there was a slight angle, but don't know how many degrees. As I recall the shaft was about 12" long.

[Greg K]

Aaron,
Do you recall the other angles you measured?
When he says straight, the rear end would be pointing down to the rear 3.5 degrees and would have to be at a certain height for the driveshaft to be straight. Better to have engine and rear parallel and driveshaft angle in relation to the engine and rear less than 3 degrees.

So after speaking to Denny at Denny's driveshaft he is saying with a short stubby driveshaft like the one I have there should be almost no driveline angle.  He says it should be straight as an arrow.  Does this wash with everyone's experience?  My engine/trans angle is about 3.5 degrees so there is no way it would be straight.

[aarvig]

I made them parallel.  The engine and trans is 3.5 degrees and the rear is 3.5 degrees, I made them parallel.  I'd have to recheck the driveshaft angle to be sure.  Maybe I should reduce the rear angle to 2.6-2.7 and give it a test drive. 
Also, is it possible the wheels went out of balance?  They were mounted 5 years ago and for the most part have been stored in the air (car on jack stands).  With the high frequency vibration at 35-40 though I don't think this is balance since the frequency is greater then the wheel rotation speed.

[Greg K]

Some new measurements might help while it's on the ground, just not as accessible now with tub and body in place unless you have access to a drive on lift.

[aarvig]

Update:
I wasn't able to get an angle gauge on it yet as I don't have a drive on but there is a side to side and up and down angle-but they are slight.
I did drive it again last night and noticed that the vibration starts at around 22 to 25 miles an hour and goes away if I push in the clutch so it is definitely driveline.

[Dan]

Hi Aaron,

Agreed that having the wheels balanced and an alignment done would be the best place to start.

Here's another random thought....I had a vibration that I ultimately traced back to the driver's side header flange contacting the front of the foot-box. At certain RPMs the engine vibration would transfer from the header into the foot-box, up the steering shaft, steering wheel, etc.  I ended up cutting a channel in the front of the foot-box eliminating the interference. I didn't feel it in the seat, so likely not the same thing you're experiencing, but thought it would be worth mentioning.  Make sure you have sufficient clearance between your headers and the foot-boxes.  My headers needed a lot of tweaking during the build, so I suspect the issue may have been somewhat unique to my car. 

While we're on the topic of exhaust, the build manual for my Gen I car (HM1066) called for the side pipes to be hard-bolted to the frame.  MISTAKE!  The vibration of the engine ended up busting up the headers over time, including ripping the mounting tab off the side one of the side pipes.  I ultimately had a local shop fabricate a mounting system that allowed the side-pipe to move, effectively taking the stress out of the system.  I don't recall what the current set-up looks like for the cars coming out of Carroll, IA, but if they don't come with a mounting system that allows the side-pipe to move you'll want to find a different way to do it.  I know the FFR vendor community offers something similar to what I ultimately had done locally.

[aarvig]

Thanks for the comments guys.  I had the suspension and alignment as well as the drive shaft angles set up today on HM-2016.  The driveline angles were dead on...thankfully.  Alignment was WAY off.  Turns out the vibration was coming from the parking brake that was hanging up one wheel.  After everything was set up the car handled COMPLETELY different.  I could not believe what a difference it made. 

Dan...I'm with you on hard bolting the sidepipes.  It doesn't seem logical.  Who did the exhaust hanger work for you?

[Dan]

Tim's Custom Exhaust in Coon Rapids:  2650 Coon Rapids Blvd.  763-422-6720.  I don't like the aesthetics of their solution, but it did the job.  The photo below isn't the exact set-up I'm using, but close in design.  I'd think if you took the photo to Tim's they'd be able to replicate.  You could probably fabricate it yourself and have someone do the welding if need be. The rubber hanger is the key, and they're available at any auto parts store.