Rear Caster Angle measuring and adjusting

[GTS Dean]

If there is one recurring theme here when it comes to properly measuring and setting the rear suspension - it's CASTER ANGLE. Whether it's front or rear, the Viper's is individually adjustable and I don't know of any commercial alignment machines that have the capability of measuring the rear at all.

Looking at wheels from the side of the car, caster angle is the relationship of the upper and lower ball joints from a line perpendicular to the ground. Positive angle means the lower ball joint is ahead of the upper joint. Positive caster provides straight line stability and helps the steering wheel return to straight ahead. Any imbalance will cause the car to drift toward the side with the lower angle. Roadways are built with "crown" or cross-slope to help shed rain. A common angle is 1.5 to 2.5 percent (run/rise ratio). Most alignment shops will put a few tenths more positive caster on the right side so the car will naturally drift slightly uphill, (left) against the cross slope. This is called cross-car imbalance and up to about 0.3 degrees is not considered significant.

Camber is the angle that tires tilt in or out when viewed from the front or rear. Negative camber is typical and that means the tops of the tires lean in toward each other.

Our Vipers's suspension adjusts by moving the lower wishbones inward or outward by means of eccentric cam bolts. The bolts move in horizontal slots in ears on the frame. Everything is pretty well designed around having the wishbones centered in the slots with the cams facing fairly straight up. Negative camber rolls the bolts outward together. Once camber is set, then positive caster happens by rolling the front in and the rear out by equal amounts to hold the camber reading desired.

I will be focusing specifically on the Gen 2 cars here. The factory dials in Caster of +6.0 at the front and +1.0 degrees at the rear. The Camber angles are very modest - less than -1.0 degree. This causes the inside shoulders to be loaded a little heavier than the outside shoulders. All commercial machines determine caster by turning the front wheels 20 degrees left/right and measuring the camber angle change. There's no room at the rear to do a "caster sweep" on a Viper.

Dodge's 6990 Viper Service Tools contain adapter brackets for both front and rear knuckles that rest against machined flats. Inclinometers screw directly to the adapters, wires go to a junction box, and then to the DRB-III computer. The inclinometers read the caster angle directly off the knuckles without having to do a sweep. There are quite a few Vipers running around that have the eccentric cams adjusted incorrectly after pothole or curb strikes. All the shops can do is camber and toe-in at the back.

A fellow VOA member brought his '97 by complaining of very unstable/unbalanced handling under cornering or rolling on and off bridge abutments. He was basically 2-fisting the wheel all the time to stay under control. The "before" pictures below paint an ugly picture of how it started. Then "after" pics show the final arrangement. It took 6 adjustment iterations to get both the caster and camber set to 1.0 and -0.7 degrees respectively. We took a drive afterward and he was grinning ear-to-ear at the improvement. Pay particular attention to the cam orientations and DRB readings before/after. From MORE THAN 6 DEGREES CROSS-CAR to only 0.1 degrees!

[GTS Dean]

Here are the final results:

[Steve M]

Wow on those initial adjustments :eek:

Thanks for taking the time to post this. Since DRB-IIIs are becoming hard to come by, would it be possible for someone to duplicate the mounting bracket and make it so that you could use a regular digital angle meter? Or is there something more complex going on here?

It would obviously need to be properly designed/machined, but I'm sure someone like Doug Shelby or TKO could make something provided I'm not missing something...

[GTS Dean]

This guy:
https://driveviper.com/forums/threads/24279-Prepping-my-GTS-for-the-Track?highlight=prepping+track

Gen 1's are +6.0/-6.0. Gauges are simple gravity style and have magnets to stick to the cast iron knuckles on the early gen cars. It pays to search eBay Motors from time to time for deals.

[ViperSRT]

Wow on those initial adjustments :eek:

Thanks for taking the time to post this. Since DRB-IIIs are becoming hard to come by, ...

Gee, is my DRBIII becoming valuable? I only use it to reset MILs and calibrate tire sensors. Not much else, so it is in very good condition.

[J TNT]

Wow on those initial adjustments :eek:

Thanks for taking the time to post this. Since DRB-IIIs are becoming hard to come by, would it be possible for someone to duplicate the mounting bracket and make it so that you could use a regular digital angle meter? Or is there something more complex going on here?

It would obviously need to be properly designed/machined, but I'm sure someone like Doug Shelby or TKO could make something provided I'm not missing something...

Great Point Steve !
I've heard of some making up a bracket and using an inclinometer on it to measure with good success.

[Steve M]

Gee, is my DRBIII becoming valuable? I only use it to reset MILs and calibrate tire sensors. Not much else, so it is in very good condition.

You could try putting it on Bring a Trailer?

[SRT_BluByU]

Can use the mopar brackets with magnetic inclinometer.. no need for a drb

[lisekplx]

Thanks for the write up. I've been having issues getting the front aligned after replacing the ball joints, bushings and stabilizer links. I welded up my own camber gauge with a digital angle finder and some skid plates, 1/2 ball bearings sandwiched between 1/4' steel plates. It got me close but I just can't get it dialed in. I have caster on both sides at around + 6* and camber -1.7* L and -1.5* R. Toe-in is around -1/8" L and R, that was done using the string method, multiple times. I must have adjusted it over 10 times, jacked it down, rolled it and checked again. It still pulls a little to the right and just doesn't feel tight. At higher speeds it feels darty. Not sure how much of that is because of my tires, they're pretty worn but it handled better before. I'm in Laredo and I don't trust the alignments shops here. Let me know if you can look at mine. I'd drive up there and obviously pay for your time.

[GTS Dean]

Are you still on stock bushings? Improper bushing preload can cause some significant dartiness.

[pokeyl]

I just picked up a DRBIII and the alinement sensors....



Gee, is my DRBIII becoming valuable? I only use it to reset MILs and calibrate tire sensors. Not much else, so it is in very good condition.

- - - Updated - - -

[GTS Dean]

Good job! Hopefully, it will have the PEP module on the back.

[lisekplx]

Are you still on stock bushings? Improper bushing preload can cause some significant dartiness.

Polyurethane bushings from energy suspension.

[GTS Dean]

What are your rear settings? Assuming you did bushings F/R, I would be sure to center the wishbone cams in their slots at each corner, check and set camber, then toe.

Your front camber and toe are very aggressive for street use and is probably generating camber thrust issues. I don't run over about -1.2 camber and just a touch of toe.

[SRT_BluByU]

not sure it matters but even the base Gen V front camber is higher than that at -1.4.. HAs and ACRs much higher...?

[usmcfieldmp]

A fellow VOA member brought his '97 by complaining of very unstable/unbalanced handling under cornering or rolling on and off bridge abutments.

This sounds like my car. If I hit a dip in the road or drop off a bridge (because TxDOT doesn't know how to make things flat, apparently), the rear end feels like it kicks out to the right.

I'm in the process of replacing all my bushings though. I'll have to keep this in mind though. Great informative post.

[SRT_BluByU]

that's sounds more likely the dynamic toe that's off.. aka bump steer. can be set in the rear of the Viper.. its a bit of a process but not too bad.

When rear caster is off the snake seems to do weird things when accelerating and braking

[TKO MOTORSPORTS TEAM]

that's sounds more likely the dynamic toe that's off.. aka bump steer. can be set in the rear of the Viper.. its a bit of a process but not too bad.

When her caster is off the naked seem to do weird things when accelerating and braking

Rear bump steer correction is real easy. Front bump-steer correction is also very easy

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[GTS Dean]

Your bump steer kits are fantastic for the serious track hounds and there is no debating the quality provided.

Even for a pretty serious enthusiast like me with a dual-purpose street/time trial car, bump steer measurement and determination of the correct adjustments required is a very time- and effort-intensive exercise. IMO, getting the baseline alignments accurate and balanced, along with balanced ride heights are by far the most important thing for a street car. If they have been smacked hard enough for frame damage, or lowered dramatically, THEN bump steer becomes an increasingly important measurement to verify and adjust for correctness.

Your mileage may vary.

[Back In Black]

Can use the mopar brackets with magnetic inclinometer.. no need for a drb Brackets are aluminum.

[SRT_BluByU]

incorrect.. they are tool steel. I own a set. You are probably thinking of the spindles on the car which are aluminum. In my opinion every Viper owner should own a set of these.. I wont let anyone attempt an alignment without them having these and understanding how to use them AND dynamic toe (what to do and what not to do). This was an expensive lesson for me.. Now finally sorted.


These are for the Gen V.. but ive been told they are the same back to Gen III
49854

[J TNT]

incorrect.. they are tool steel. I own a set. You are probably thinking of the spindles on the car which are aluminum. In my opinion every Viper owner should own a set of these.. I wont let anyone attempt an alignment without them having these and understanding how to use them AND dynamic toe (what to do and what not to do). This was an expensive lesson for me.. Now finally sorted.


These are for the Gen V.. but ive been told they are the same back to Gen III
49854

Who sells the adapters ?
Thanks !

[SRT_BluByU]

Got 'em from the dealer.. thats the part number right on them.. (you need both they are side specific). They're not cheap. But the headaches a messed up alignment can cause I see them as required tools.

[Zubik]

Can you explain or post instructions on what exactly to do with that tool?

[usmcfieldmp]

"Use these adapters, along with a magnetic Digital Angle Gauge, while performing alignment procedures on the 2013 SRT Viper (ZD)."

https://www.moparessentialtools.com/item-detail.aspx?itemid=21560003&type=tools

[J TNT]

"Use these adapters, along with a magnetic Digital Angle Gauge, while performing alignment procedures on the 2013 SRT Viper (ZD)."

https://www.moparessentialtools.com/item-detail.aspx?itemid=21560003&type=tools

If anyone has a used set for sale let me know , I'll buy them !
Thanks !

[Zubik]

I read the service manual, but it goes into connecting drbIII to it and lowering and raising car to read the dynamic caster. Hoping a quick explanation what exactly one should do with that adapter and what reading to look for. Or are you still following the fsm with lifting car off suspension and taking measurements.

[SRT_BluByU]

These set rear Caster; you can read the FSM for instructions. In a nut shell these hook to the ball joint and spindle with the hooks and mate to the machined surface of the spindle. from there you set the mag inclinometer on the pad and use that to get the correct setting by adjusting the rear lower control arm camber bolts. Gen V recommended setting is .7; but best to look at service manual for your year.

Setting the dynamic toe, which moves the suspension through the compression and rebound range (+2", zero, and -2") , involves plotting on the graph and making sure the plotted arc is within the range and is similar to the ideal arc referenced.My understanding is this shouldn't be touched and is set from the factory - unless the car has been in an accident of some type or someone has accidentally adjusted the inboard toe camber bolts (frame side) which they shouldn't.... but sometimes happens.. This procedure is an iterative process that folks seemed to keep close to the chest.. after having to have it done a handful of times I was eventually forced to learn it myself chasing down another problem.. The Gen V service manual has the procedure and I image all the gen's are pretty similar.. some say there are quicker ways but I finally like the results we've gotten and the snake is slithering smoothly again.

random thought - Maybe a nice club perk would be if they got a set to loan out (with deposit of course) to members in need that could mail them back when done. I've found most of the local dealers don't have the tools and the ones that do don't always know they have them, or how to use them, or maybe want to use them... it takes a special tech who has the OCD for for the job - including patience and experience. I've been fortunate to have some wonderful alignment help lately!

[Back In Black]

incorrect.. they are tool steel. I own a set. You are probably thinking of the spindles on the car which are aluminum. In my opinion every Viper owner should own a set of these.. I wont let anyone attempt an alignment without them having these and understanding how to use them AND dynamic toe (what to do and what not to do). This was an expensive lesson for me.. Now finally sorted.


These are for the Gen V.. but ive been told they are the same back to Gen III


I stand corrected. Mopar labeled are steel. My Millers are aluminum.

[GTS Dean]

A small strip of steel gauge metal glued or screwed to the aluminum bracket will allow the use of a magnetic inclinometer.

[Zubik]

So I got a pair of the adapter, in aluminum. That's not a big deal, as I'll bond a piece of machined flat stock. My question is about the magnetic inclometer/degree measurer. The one I got is digital and needs to be zeroed to something. How does one go about zeroing it out to a good reference or should I get an old school analog one?

[TKO MOTORSPORTS TEAM]

So I got a pair of the adapter, in aluminum. That's not a big deal, as I'll bond a piece of machined flat stock. My question is about the magnetic inclometer/degree measurer. The one I got is digital and needs to be zeroed to something. How does one go about zeroing it out to a good reference or should I get an old school analog one?

use a bubble level if meter does not have built in zero to world. Level bubble level and put meter on it and zero. This will get you really really close to zero plenty good for what your doing. You also always want the least amount of caster possible. If spec is 1-2 always shoot for the lower number.

[GTS Dean]

Many electronic levels have a built-in calibration routine. It will usually require a couple of button presses and turning the gauge around to point the opposite direction while it takes an average. If that's not included, use the above recommendation.

[TKO MOTORSPORTS TEAM]

use a bubble level if meter does not have built in zero to world. Level bubble level and put meter on it and zero. This will get you really really close to zero plenty good for what your doing. You also always want the least amount of caster possible. If spec is 1-2 always shoot for the lower number.

You will probably figure this out but just in case. If your meter is oriented horizontal when you zero it on the bubble level it will be zero " horizontal" . When you check caster it will go "90" ( should be some "positive" number no matter what) so you will have to do the simple math to figure out your positive caster number. It probably sounds a little confusing so best way to figure out how this all works is to just do it, I think you will see how the meter and caster works pretty quick.

[GTS Dean]

I was checking on some alignment recommendations I had made to an owner a little while back. I had a lightbulb moment I thought I would share for making reasonable rear adjustments without rear inclinometer adapters and gauges. Most shops can calculate the rear axle thrust angle and wheelbase difference L/R. These are indirect indicators of caster imbalance, or toe that isn't set exactly off the vehicle centerline.

Begin up front with the caster/camber/toe in-spec and equal on both sides at the front. I still think it's a good idea to baseline your car by centering both cams on a corner, then rolling the bolts out in time to set camber. With camber held constant, adjust caster by rolling one bolt outward and the other wishbone bolt inward by equal amounts.

Next set rear camber as above, then adjust toe to zero and measure wheelbase difference and thrust angle. If one side has a longer wheelbase than the other - it means that side has less positive caster (lower ball joint further behind). To shorten wheelbase while holding camber fixed, you push the long side wishbone rear adjuster bolt inward and the front outward by equal amounts. Then, reset to zero toe and check readings again.

The Gen 1 and Gen 2 cars have 96.2" design wheelbases. I'm pretty sure the G3-G5 went several inches longer - maybe 4"? With the ideal wheelbase measured relative to correct and balanced front front spindles, the rears should come in without much trouble. Since you never want negative rear caster on G2-5 cars, I would want the wheelbase just a tad on the short side to be sure. This does not give you an actual angle, but could keep you from having squirrely behavior at the back of the car. Of course, this all assumes that there is no frame damage or twist - or else you'll be endlessly chasing your tail.