Suspension/Alignment and front-end noise

[RobHook]

Well, after about a year of getting a really annoying creeaaak sound from the front end when I took off, I finally found the culprit. Turns out when I took my car to have the alignment done, the shop (a 5-star dealer in Orlando) failed to tighten ANY of the suspension bolts back to spec. Several of them were only a little more than hand-tightened. The specs call for between 75 and 155 ft-lbs on the various fasteners and these felt like they were in the neighborhood of 40, if that.

Having been under there, I have a few notes to post for posterity:

1. Special body height tools are necesessary when setting the torque. You could probably get away with not having them if you just need to re-torque but looking at the suspension, you can see why the tools are important. At the very least the pre-load on the bushings would be wrong without the tools and you'd shorten the useful life. I guess it's the short A-arms that increase all the relative angles in the suspension's geometry. Just a theory.

2. The only nuts I wasn't able to tighten exactly to spec were the ones on the lower control arm bolts. These are the ones with the caster/camber adjustment cam attached to the other end of the bolts. It's impossible to get a torque wrench (at least mine) into the space between the end of the nuts and a protruding part of the frame. How does anyone else do this? Is there some trick I didn't figure out? I thought about marking the cams, loosening them a few degrees and using trial and error to get the right torque and right angle at the same time but that could take hours. Plus I didn't think of it until I was cleaning up the garage. Is there a special tool for this or does someone make a super-thin-headed torque wrench? I'm familiar with various scientific and engineering torque gages but even the compact ones would never fit this application.

3. This isn't a job for the faint-hearted. Torquing these things (especially without a lift) is a real pain. Even if you have a lift, you have to figure out a way to raise the wheels to the right height to get to "design height" and "curb height" while the car is up on the lift. I was able to use hydraulic jacks on the frame to change the height while the car sat on ramps.

4. When you go to set any alignment with the Viper, make sure the car has a full tank of gas. I was surprised by how much ballast was required to bring it to the right height with about a quarter tank of gas. I almost ran out of room in the trunk.

The Viper suspension is definitely set up like a racecar but it's not rocket science either. Anyone who's done even a few racecar setups will be familiar with the Viper. On the other hand, the way you set caster and camber (four cams that have to be adjusted both to the right setting for camber and then to the right relationship with each other for caster) would make for a lot of trial-and-error when setting the alignment. I certainly would never let NTB touch it.

--Rob

 

[Ulysses]

It's impossible to get a torque wrench (at least mine) into the space between the end of the nuts and a protruding part of the frame. How does anyone else do this? Is there some trick I didn't figure out?

possibly with crow's feet like this:

or this:

 

[GTS Dean]

Put a 3/4" or 19mm combination on it, then pull real hard. When the veins on your temples start to bulge, say "click."

 

[RobHook]

I thought about that. A crow's foot would work but it would change the torque reading because it introduces a lever effect.

 

[Ulysses]

How about this:

Since torque is directly proportional to the lever arm length, make some measurements and adjust apropriately.

Example:
Length of torque wrench is 24".
Distance from pivot point of torque wrench to new pivot point with crow's foot on is 1".
New lever arm length is 25".
That is 24/25 = 96 %.
Adjust Torque on wrench 4% less than target Torque reading.
So if target torque without crow's foot is 100, new target with crow's foot is 96.

Thoughts?

Nawww, probably wouldn't work, since the torque wrench is still sensing at it's pivot point. Never mind.

 

[Ulysses]

Might have to buy something like this:
Torque Wrench Body, Adj. Click-type, U.S., 40-200 in. lb. Range.
Interchangeable heads permit ratcheting, fixed, or open end torquing capability with adjustable torque wrench bodies.

Torque Head, Metric, Box End, 17 mm, 12 pt.

 

[Steamroller]

Ulysses... Give yourself some credit! What you are saying is actually correct.

The torque wrench measurements are in fact with respect to the square drive with no "arm extensions (like a crow's foot)". If you extend the moment arm (that is, the distance from the bolt center to where the load is applied by your hand) beyond the square drive (as the crow's foot would do), then the moment (torque) seen by the bolt would be greater than what the torque wrench was set at (as you indicated in your post). The moment induced on the beam is maximized at the bolt end and is zero where the load is applied (that is, where your hand is). In other words, the moment varies along the beam. Thus, you can have a particular moment generated by a force (your hand) somewhere along the beam and end up with a greater moment at the bolt (assuming the square drive is not directly on top of the bolt as the torque wrench manufacturer intended).

 

[Ulysses]

Steamroller,

I was thinking the same thing, but had my doubts since the click point/ sense point is still at the center of the ratchet head. Im not sure if that sense point is still sensing whatever point you set the wrench, therefore less than your new target torque.

 

[dblankenbaker]

Put a 3/4" or 19mm combination on it, then pull real hard. When the veins on your temples start to bulge, say "click."

As strange as it sounds, that method's very accurate.

 

[RobHook]

Actually I think the tools that Ulysses found would work perfectly. Where'd you find those?

As for calculating the change in torque when moving the fulcrum, I don't think it's that simple. I just wrote two pages of explanations and formulas trying to figure it out but deleted it when I realized it was just too complicated to worry about. Ulysses is right, it's not a simple 1:1 ratio. Basically the torque wrench is dealing with a lever 1/2 the size of the bolt that is then partially offset by the bolt contact points that are opposite the lever's effort. Since it's only partial, the effect doesn't cancel itself out but is modified by the relative angle around the center of the bolt. It's interesting...this means that torque wrenches get less accurate as the size of the bolt changes. Anyway, I think I'll just buy the tool.

--Rob

 

[Ulysses]

Saw that torque wrench at snap-on on-line. I'll probably pick one up too. Definitely will come in handy.