Vibrations at speed on A&C
 

This isn't aimed just at A&C owners but a general question.

I have just completed my build and am in the find and fix the bugs stage. Really I guess I am pretty lucky as the only bugs I have are cooling at long idle periods, which the cure is just an additional fan (puller) instead of just the dual pushers.

The other is vibration at speed. I seem to have 3 different vibrations.

One is at low speed (around 30-40 mph) and is felt in the steering wheel. I think I have figured this one out that I have what I believe to be excessive run out (between .004"-.007") on the machined surface that the front wheels mount to on the rotors which equates to around .080"-.100" at the road contact part of the tire.

The second transitions in above the 40 mph mark and up to at least 70 doesn't diminish, it is felt more in the chassis. I have an idea of what this is but am not sure. I have not done any measurements yet so these measurements are just a guess. My drive shaft angle is going uphill from the tail shaft of the transmission to the pinion of the rear end, this angle would be somewhere around the 10 degree mark as measured from the output shaft. This seems very excessive to me and could be the culprit as I think that this may put a heck of a bind on the rear u-joint. I also only notice this vibe when I have the drive train "loaded" and not under a deceleration condition, as soon as I let off the accelerator it goes away for the most part.

The third is at highway speed of 60 mph and up and is felt in the steering wheel as a shimmy, almost like the whole steering column wants to get out and dance a jig, this may be related to the first one I described here.

I have had it to an alignment shop and it was set up to an early Fox chassis specification and balanced the wheels off the car and then fine tuned the fronts on the car.

I have contacted Advanced Chassis in Antwerp, Ohio (an NHRA certified chassis design shop) and will be taking the car to them soon, but I would like to get some ideas from anyone that may have had some of these same issues on theirs and what they found to be the problem.

Any input would be greatly appreciated.

Thank you.
John

John,

My A&C had a little vibration when I first got it on the road. A good alignment and balance (inside and out) corrected the problem and I get absolutely no shake, vibration or shimmy.
Sometimes the tire/rim out of balance is so great you have to turn the tire to another spot on the rim. Otherwise you will be putting too much weight on the rim. I found an alignment place that does alot of roadrace cars and has a computerized alignment machine. Cost me 80 bucks but took care of all my problems.

Roscoe

John,

Just resolved a shimmy by having the wheels balanced with weights on both sides of the wheels... Tried to just put weights to the inside but could not get them to balance.. Did not want weights on outside but had no choice... Actually, it looks just fine and I wish I didn't fight it as long as I did...

Roscoe and Casaleenie,

I have already done this. The individual that originally started this kit had them balanced using the tape weights inside the wheel. I did not like this so when I had the car put on the rack to check 4-wheel alignment and do the front end alignment, I also had them balance the wheels using weights on both inner and outer rims. I later had the 2 front tires replaced as they were egg shaped, so when I did this, I had them rebalanced and also check the balance and roundness of the rears. I was for a split second concerned about the weights on the outside of the wheel but then decided that because these things are so wide that they needed inner and outer weights.

Roscoe,

When you get a chance, will you please measure how high off the ground your main frame rails at you rear lower control arm mounting point is? And, Are your rear lower control arms parallel to your 2X4 main frame rail? How tall are your rear tires as measured sitting on the ground?

Thank you for the input.

John

John,

Rear rail is 5 1/16" or 5 1/8"

Control arm goes up toward the rear at about 10 degrees or so

Rear tire is 25 1/2"

Roscoe

Roscoe,

I will check those measurements to mine and see what the difference is. Height wise we are running about the same size rear tire, mine is a 315/35-17 which equates to just over 25 1/2". My rear lower arms are also at an approx. 6-8 degree upward rake from the front, this concerns me from a performance angle as suspension twist is going to make it even more off. Since you do race yours, have you ever experienced any rear traction/handling problems? Other than wheel spin, Do you notice any swaying in the rear upon acceleration or deceleration?

Also, does your driveshaft go uphill from the rear of the transmission to the pinion of the rear end? If so, about how many degree?

Thanks again.

John

John,

I haven't had any problems with the rear suspension. I did however install Metco adjustable upper control arms in order to get the pinion angle right. I believe (without looking) I'm running about 6 degrees up from the rear.

Roscoe

Roscoe,

Am I understanding correctly that your drive shaft points down from the transmission?

John

I better go check it.

Roscoe

Driveshaft goes UP from the trans about 2-3 degrees

Roscoe

Roscoe,

Okay, I was told 0-3 degrees is the range that is acceptable because of the stress that the rear u-joint can take over the long run. Mine is in the neighborhood of 8-10 deg up from the transmission which could be over stressing the rear u-joint and making it do funny things.

Other than the upper arms is your rear suspension the same as A&C supplied to you. Have you added a sway bar or panhard bar? Did the new adjustable uppers use the stock mounting points on the housing and frame?

Thank you

John

Other than the uppers the rear is exactly as it came from A&C.

1" Sway bar on front only.

We had to grind a bit here and there on the Metco uppers for a proper fit but they work fine. I had to go with the adjustable uppers cause I couldn't get the pinion angle right and the driveshaft was binding.

Personally I believe that A&C used two different chassis jigs over the years and they are slightly different, especially in the rear.
The hump that goes over the axles is different on mine than my friends who got his about 3-4 years before me. My roll bar mounting brackets would not fit properly due to the distance between the hump. Apparently A&C did not allow for this with different brackets and I believe the same is true for the control arms. Now I don't know if I am right but that's how I see it.

Anyway, no problems that can't be corrected.

Roscoe

Roscoe,

I am looking at your build site right now (I have it saved as a HOT button in my favorites) and am trying to see the differences between yours and mine. I did have the problem with my roll bar mounts as you mentioned, I had to mount my rear bracket differently than what their manual said to but it is still their supplied bracket.

I noticed that you are running the 11" brake up front. Is this the Granada upgrade? Where did you get the componants from?

Where did the 1" sway bar on the front come from? I did not notice this on your build site.

John

BTW, no women or cattle have gotten excited here for quite some time.:LOL:

John,
there is 1" differece between the old chassis and ours and this is where the problem with the roll bar mounts comes in. I used the supplied mounts but had to modify them to fit.

I got the Grenada rotors from A&C but upgraded to Outlaw 4 piston calipers from the GM single piston that came with it. I've got 11" rotors in the rear, all Explorer.

Sway bar came from Willy at Rod & Custom in Florence, SC. but you can probably find one closer to where you live.

Roscoe

Roscoe: check your rear end diff case for cracks. I know you saw my pic's. I intend to talk with Joe T. tomorrow to see what he will do for me. Pete said they actually ground down the weld in the area where the crack is. they needed more room to mount the suspension brackets.
John: check yours as well.
Steve: check yours, John M: check yours. We may have a problem here with A & C suppliers of the narrowed 9" rears.
JIm

Most tire shops balance tires on a dynamic machine off the car. This works on 95% of most production cars but on a car with a sensitive suspension and no rag joint in the steering shaft this usually will not work. I would suggest you check around to see if there is a shop that does tire trueing and on the car balancing. I had this done on a Honda Passport that had a balance problem between 65 & 75 which is normal Highway speed. It rode like glass after this. Honda had paid for some real nice Michelin tires and they were better than the stock Yoko's but still had a vib.

The guy that did the tire trueing said no tire comes out of the mold perfectly round. He had the same Michelin on his truck and he had to true three of them. My new set needed three done and he balanced all the tires on the car. The OTC balance gets everything balanced together including rotors, axle shafts, bearings, etc and really makes a differance on a sensitive vehicle. If you had a Lincoln Town Car you could probably get away with not balancing tires at all and you might not feel it with all the numb suspension.

He stated he did a lot of performance cars like Vettes, 300Z's, BMW's and MB cars.

I got tired of having to do OTC balance after a couple of tire rotations and traded the Passport(actually a Rodeo with badges). I talked to the Honda mechanic and he stated that when they redesigned the car in 1998 there was a lot of complaignts. I think that Honda/Isuzu(sp?) built the suspension too sensitive. This was the first time they put rack and pinion on the Passport/Rodeo and I think it just transfered too much thru the wheel. I guess they corrected it on later models by using rubber mounts etc.

Just do not trust most local tire shops to be able to get a perfect balance on their spin balance machine. Another note is the trueing guy said by shaving the high spots off the tire it would actuall last longer because the tire would roll smoother and not beat itself to death.

This should be a one time deal on a Cobra as you cannot rotate tires anyway due to differant sizes.

PNJSNAK,
I will inspect mine and see if any cracks exist, but just to make sure of the location you are talking about, this is the area of the tube where the coilover mounting point is welded in?
Also, when I was putting my axle together I noticed that the axles wouldn't go in real smooth. I dismissed this at the time as I figured it was because everything was brand spankin' new and would be a little tight.It almost looked like the axle housing as supplied by A&C was not welded together straight so I took it to a frame shop for the alignment and they hooked up to all four wheels and didn't mention anything about rear axle mis- alignment. Did any of you notice this in your build?

aumoore,
I have contacted another shop over in Antwerp, Ohio that is a specialist in designing/building complete chassis systems and they are NHRA/IHRA certified. I will be taking mine to them for their input. I will try to remember to let you all know what the outcome is but it will probably be a couple of weeks yet.
I like the thought you posted and mentioned that to the shop that did my alignment and they kind of brushed it off however, they did spin the front wheels on the car and made a few adjustments but all in all they didn't do it right because I noticed a severe vibration in the windshield frame when he was spinning it. I mentioned it to him and he said that he was spinning it to about equal about a 90-100mph speed and that is well above what it would be driven at:LOL:. I will eventually get this thing straigtened out, it's just a matter of time.

Thank you for the input.

Keep the ideas rolling in.

John

Jim,
I'll check it but the last time I was under there everything seemed ok. A friend with an A&C had a problem last year when the weld cracked on the axle tube. Joe replaced with a new Currie unit.
Roscoe

I checked my rear housing for cracks last night and I didn't notice anything and no leaks were found. The weld seams on mine are between the two vertical "ears" that the lower control arm bolts to and they don't appear to have ever been ground smooth.

While I had it up in the air, I swapped out the transmission x-member that had been custom made by the individual I purchased the original box-o-kit car-parts from, I noticed that I had this after investigating Roscoe's web site and in one of your pictures I saw a "C" channel style flat x-member. For some reason this guy(or someone) decided it was necessary to have a x-member with a 2.5" drop in it. Well, after swapping this with the correct one that I still had in my box-o-parts, it raised the tailshaft of my transmission quite a bit which made my driveshaft almost straight back into the pinion. Now I may not have to flatten the tires on my friends trailer to get it on and off due to catching the transmission pan on the back of his trailer. I also removed my driveshaft from Denny's (like yours Roscoe) and inspected the rear u-joint bearings. I removed the bearing caps to inspect and noticed what I perceived to be the smell of burning oil/grease. I re-installed the caps as there was still plenty of grease in the caps and spun them by hand several times to redistribute the grease through the bearings.

I also went through all the drivetrain bolts while I was under there and insured everything was tight.

After all that, down it went and to the highway ----- VIOLA ----- almost all of the vibrations are gone. I still have a slight front end vibration which I still have not corrected the runout issue on the front rotors and will look at the tire truing idea and see if I can find someone to do this.

Thank you all for listening and giving your inputs.

John.:D

John,
Glad to hear things are working out. I seem to recall that I made a little notch in my x member to lower it a bit.

Roscoe