Keeping the Snake STRAIGHT

[Aevus]

Ever since I have the Viper, I keep working on it to improve handling and stability. The weakness of that car, IMO.

Already made pretty much all the usual modifications: good tires (Kuhmo ACR's), solid sway bars w/TKO links, TKO bushings, coilovers with stiffer springs, Wavetrac diff... And I'm on my 3rd alignement setting.

I'm satisfied with the front-end stability. Made few errors first (too much camber/not enough caster, too much toe-in, tire pressure too low for street, etc...) but now it feels very good.

On the other hand, I'm still stucked with the rear-end problem: under hard acceleration, the car struggle to keep a straight line. Especially felt on a roll, not a standing start.


And I'm not sure what causes it, what is the culprit for that behavior.
Don't get me wrong, it's not that bad, it's a RWD without traction control (gen3) and a lot of torque, I don't expect miracle. But I'd sure like to fix that issue the best I could.
Can't imagine it could be a tire or diff problem. Maybe the BC coilovers or springs? Doubt it, but possible. So what's left?, alignement?


The alignements I tried so far:

Camber F -2.6 :: R -1.7 :: Caster F 6.6 :: Toe-in Total :: F 0,20deg :: R 0,34deg
(1st - basically ACR specs)

Camber F -2.1 :: R -0.8 :: Caster 7.3 :: Toe-in Total :: F 0,38deg :: R 0,44deg
(2nd - less camber to get more caster, and a lot of toe-in)

Camber F -1.6 :: R -0.6 :: Caster 7.8 :: Toe-in Total :: F 0,08deg :: R 0,38deg
(3rd - even less camber for even more caster, lot less toe-in front)


The 3rd one feels the best for street, especially the front-end. I'm basically maxed out for the caster while having decent camber. Having a lot of toe-in on the front was an error but I wanted to try anyway. Just a tiny bit of front toe-in seems perfect.

Now, back on the rear-end issue, my educated-guess would be to try more camber and/or more rear toe-in to keep that snake straight and stable under hard acceleration.

Any comments or tips?

[GTS Dean]

I like your approach. I don't have info to share on the Gen3+ cars specifically, but I have observed that my G2 does not benefit from rear camber over about 2 degrees. For general street/highway driving, I like camber about 0.9. It does like quite a bit of rear toe-in.

[Bob Woodhouse]

Geez, I keep following you around on this board Dean, you have a lot of experience, some here as well so lets both offer up what we know. Here is something for Aevus to ponder. Rear Camber and or Castor have little to do with keeping the car going straight if both sides are set equal. Toe In does. Imagine as you accelerate that rear tire weights can shift slightly, heavier on one side or the other from road surface changes or steering input. The heavier weighted wheel becomes dominant and the car will track where that tire is pointed. So a lot of toe in, yeah you follow the thought, the car is gonna wag its tail. Nice for auto crossing though. Now if you take rear toe down to say zero, not a good idea, leave a little, there is some flex in our suspension and under hard brakes it will tend to toe out, we dont want that, the car will feel like the ass wants to pass you. And Aevus, if all you do is drive your car to church on Sunday, then your #3 align spec is fine. If you wanna rock and roll, go back to #1 and lose some of the rear toe, some, just some, you decide.

[Aevus]

Dean and Bob, thanks for the inputs.

0.9-2.0 rear camber range seems about right.
Will try next somewhere between 0.9-1.2.

Problem I have is I don't do the alignment myself so I explore too many variables at once (which is not ideal to find answers) but I think Bob is right, rear camber probably have no impact or very little on how the car keep straight under hard acceleration from a roll, even though the contact patch is changed because of the angle.

So what's left is rear toe-in. I may try 0,52 deg total or so (translates to 1/8'' each side .. 0.25'' total) on the next alignment, which might look like:

Camber F -1.5 :: R -0.9 :: Caster 8.2 :: Toe-in Total :: F 0,08deg :: R 0,52deg

We'll see...

------------

Side note about tire pressure. Did few tests yesterday and I monitored the warm temp (not track, just highway) of the tires and raising it from about 33 psi to 35 psi helped keeping the car straight and stable under acceleration.
Since I changed the springs for stiffer ones (12/14kg spring rate), installed solid sway bars and all TKO parts, the car responds much more to variations in tire pressure, which is fun.
But it also tells me that I may compensate for too-soft springs... with tire pressure. And I don't want that.

Maybe it's time to order a set of Penske double with Hyperco springs...

[Aevus]

About caster (is it castor or caster??) I do like the relatively high 7.8 setting but it comes with a price: the steering is heavier at low speed. In town or for parking, it feels like an old BMW ;-)

But the high-speed stability totally worth it, at least for me. The car feels planted like a train, especially on high speed corners. It also feels very tight, at any speed.

Probably another story for the track, lower-speed corners entry and such, but then again you can't get that much caster while keeping -2.5 ish camber... So that means: one alignment for track and one alignment for the street.

Bob, yes for the track my 1st alignment was the best, but I'd be curious to try a little variation on the theme:
Camber F -2.4 :: R -1.6 :: Caster F 7.0 :: Toe-in Total :: F 0,08deg :: R 0,32deg

[Aevus]

At this moment I have 672lbs front and 784lbs rear spring rates.

Ratio of about 1.17 Rear/Front, that may be too low. I didnt want to have ACR-like ratio (2.11 on the 2008 ACR) because I don't have any aero... but now I think I may go for 950-1,000lbs on the rear while keeping front spring rate pretty similar as now.

That being said, I'm not sure the ratio is relevant since the Competition Coupe ratio is 1.36 (1,100lbs front, 1,500lbs rear)

it's a pain to change the springs so I want to be sure I won't regret it

[GTS Dean]

I spent a small fortune on alignment going from street to track and back, never being real happy with the finished product. Then I geared up to do it myself. Not as convenient, but always satisfying.

Just a tip based on much experience: Do not let the alignment tech turn the adjuster cams with weight on the wheels! Inevitably, they will get something cocked and end up bending the cam stops and ears, or opening up the "D" hole in the removable cam eccentric. Then, they won't be in time with each other and it just gets harder to align. Tell them they need to lift the tires clear of the ramps to adjust and make sure not to loosen them too much before rotating.

[Aevus]

Thank you Dean, I just passed the message to my alignment tech.

[13COBRA]

How hard are you driving this on the street to need anything besides the factory alignment?

[Aevus]

How hard are you driving this on the street to need anything besides the factory alignment?

It's really about how it feels. No need to drive it hard at all, in fact. Toe-in and Caster can be felt in normal street driving mode.

Lots of camber on the other hand is pretty much useless for a street-only car. That's why I am choosing to sacrifice camber for caster.

[SRT_BluByU]

With the Gen V at least, you need special ken moore Viper specific tools to set the rear caster.. These are needed if you change ANY settings on the rear. If you dont have these, and the Gen III is sim to Gen V, that could cause some issues. Bob also hit what i believe to be the biggest nail on the head. Toe. Again speaking to my knowledge of Gen V's, dynamic toe needs to be set.. the rear travel toe is measured along the arc of the wheel travel (through 2" up and 2" droop). There is a curve you need to plot the results on and makee sure you are within the limit arcs. If this has been moved incorrectly it will do all the bad voodoo things you speaking of. I'm just not sure if Gen III is same as Gen V so I hope someone with more experience in that will chime in.

The process for gen v as outlined in the service manual took a half day or more.. Viper alignments are more complex than the average minivan and knowledge from folks who know is invaluable. Woodhouse, Roanoke, Viper exchange, etc..

[Aevus]

Tomorrow they'll do my 4th alignment:

Camber F -1.5 :: R -1.1 :: Front Caster 8.4 :: Toe-in Total :: F 0,08deg :: R 0,52deg

Raised the rear camber only because I have the rear oversized ACR's 355 rubbing and I don't want to change the coilover's height...

so I will try even more caster and more toe-in but just on the rear wheels.

Then next, I will install TKO's rear shock mount with spherical bearings... and finally a set of Penske double with 750lbs/950lbs Hyperco springs. We'll see what upgrade helps the most keeping the car straight.

[13COBRA]

Help me understand a little. What is your car? A GTS, TA, ACR?

[Aevus]

2005 srt10

[13COBRA]

2005 srt10

Why not just use the factory Gen III alignment?

[Aevus]

Why not just use the factory Gen III alignment?

That's what I had when I bought the car. So far every new alignment gets better than the previous one. I sure don't think the factory gen 3 alignement is the absolute benchmark, and keep in mind that my car is modified: lowered, 295/355 tires, solid sway bars, 700/800lbs springs, all dressed with TKO's parts, Wavetrac, etc... Doesnt feel at all like a stock SRT10

[Aevus]

Got the control arm bushing kit from TKO installed and it sure helped keeping the car straight, it's more precise and predictable. I'm still on that alignement though:
Camber F -1.6 :: R -0.6 :: Caster 7.8 :: Toe-in Total :: F 0,08deg :: R 0,38deg

So far it's the best handling and steering feel I had. Few other modifications are coming and a 4th alignement that will be similar to that one.

[Aevus]

Problex fixed.

Finally it's a combination of TKO's parts (Bump-steer correction kit + Control arms bushings) and new alignment.


Bushings makes the car tighter and more precise, the bump-steer kit helps the front-end stability and that new alignement helps high-speed stability and hard accelerations:

Camber F -1.45 :: R -0.95 :: Caster 8.8 :: Toe-in Total :: F 0,10deg :: R 0,50deg

I am very satisfied with the final result. By far the best handling on a Viper I've experienced.
Because of the caster, the steering feels quite heavy at low-speed but otherwise it's great, planted like a train and it feels more like a modern car (in a positive way).

It's the very first time I feel that car's handling exceeds the engine potential. Pretty common in Porsche's world but not so much in Viper's ...

(so, i'm ready for the next step: race engine. lol)

[Aevus]

Doc-22-sept-2021-13-45.jpg

My alignement at the moment.

Correction: it's not 0,05 front wheel toe-in but 0,05 toe OUT as you can see. Neutral (zero toe) or very slight toe-in probably works well too. That tiny toe-out helps to get a better more responsive turn in, especially with such high 8.8 caster.

Probably can be tweaked further more.. maybe more/less rear camber... maybe a little less rear toe-in... But I'm very satisfied as it is right now. Not even sure I'd want much higher camber for the track, at the cost of reducing the caster... On a small technical track, yes, probably. But a track with high speed turns? Nope.

[13COBRA]

You definitely want negative toe in the front and a little positive in the rear.

[Aevus]

You definitely want negative toe in the front and a little positive in the rear.

I tried everything from (Front and total) +0,10 to -0.38 deg and it seems to work better around neutral (in or out, 0,10 deg maximum) and quite a lot of rear toe-in (0,40-0,50 total)

On the other hand, I was fighting the bump steer issue (see other thread: TKO bump steer correction kit) and I had that kit installed when I did my last alignment.
The TKO kit clearly is THE solution to the bad behavior of the car at 50-60mph+, but the alignment is felt at any speed.


Not sure I'd want to go back to slight toe-in since I don't feel any drawback from that slight toe-out. In fact, I'd be curious to try more toe-out, just to see if it would feel more responsive (without being unstable)

[Aevus]

On the front of the car, toe out decreases straight line stability. When a wheel is disturbed in a straight line with toe out, pulling the wheel rearward of the steering axis, it increases the toe out and effectively turns the inside wheel more outward, pulling the car into yaw. This decreases straight line stability as it makes the car develop a twitchy nature which can be uncomfortable at high speeds.

However, running toe out makes for faster turn in on the front end.

https://suspensionsecrets.co.uk/toe/


Oh, I just noticed that the base set-ups recommended on Suspension secrets is exactly that: toe-out front and toe-in rear.

https://suspensionsecrets.co.uk/btcc-touring-cars/

(for the rear wheel drive configuration)


They recommend 0-4mm total, which translates to roughly 0.16 in. or 0.35 degree.
I probably wouldnt dare going as far as 0.35 deg total toe out on the front but maybe 0.20-0.25.

[13COBRA]

I tried everything from (Front and total) +0,10 to -0.38 deg and it seems to work better around neutral (in or out, 0,10 deg maximum) and quite a lot of rear toe-in (0,40-0,50 total)

On the other hand, I was fighting the bump steer issue (see other thread: TKO bump steer correction kit) and I had that kit installed when I did my last alignment.
The TKO kit clearly is THE solution to the bad behavior of the car at 50-60mph+, but the alignment is felt at any speed.


Not sure I'd want to go back to slight toe-in since I don't feel any drawback from that slight toe-out. In fact, I'd be curious to try more toe-out, just to see if it would feel more responsive (without being unstable)

Negative toe is toe-in.

[TKO MOTORSPORTS TEAM]

https://suspensionsecrets.co.uk/toe/


Oh, I just noticed that the base set-ups recommended on Suspension secrets is exactly that: toe-out front and toe-in rear.

https://suspensionsecrets.co.uk/btcc-touring-cars/

(for the rear wheel drive configuration)


They recommend 0-4mm total, which translates to roughly 0.16 in. or 0.35 degree.
I probably wouldnt dare going as far as 0.35 deg total toe out on the front but maybe 0.20-0.25.

Toe in works better in front since the viper is a heavy front engine car and has a problem with excessive tire wear especially gen 5 . The toe out rule was based off rear engine light weight cars and it does work for sure. When it comes to front heavy front engine cars or cars with other problems sometimes you throw out the book and find what works best.

[Aevus]

thanks TKO for the input. I assume that is for optimal track times on a dedicated gen 5 track car, but what about a gen 3 that would be used mostly on the street, without consideration about track time but just how the car feels and responds?