addressing 1G handling problems

[UCSLugRacerX]

One of my primary reasons for gravitating towards the 1G DSM, is the 50/50 power split from the center differential. However, there seems to be more built-in understeer than desired. To try combat the understeer characteristics, most people here raise spring rates, add stiff shocks, replace rear anti-roll bars with a thick/stiff ones, add stiff rear strut tower bars, and try to stuff as much tire as possible under the fenders at all costs—adding offset wheel spacers and rolling fenders. Although, most of this perhaps would lead to instable oversteer, (if they haven’t already added too much front negative camber, or front toe-out) and perhaps adding to unpredictable power-on oversteer. (Especially with the guys spooling up huge 20G’s, on a 2.4ltr.) Also, considering the turbo lag on big turbos, wouldn’t the non-linear HP curve upset steady state turning while accelerating and turning, and perhaps induce unpredictable and instable power-on oversteer? Also, wouldn’t this decrease the predictable transition period between steady-state turning tand oversteer?
My question which I’ve raised before, was that given the inherent tendency to understeer, and the power split, what can we do to increase the capacity for steady state turning? (Meaning, the car will not be understeering, nor oversteering an abhorrent amount, until tire traction limits are reached, or by extreme tire slip angles.) Also, could we do this without stuffing 275mm wide Hoosier’s under the fenders? (Nonetheless hats off to Greg Coiller.)
From reading several threads, DSM members here modify their suspension systems in a “follow the leader” fashion, without really looking at what happens overall to the car’s handling characteristics. Also taking into account the large amount of power some your 4G63t motors are putting down, having more HP will actually AMPLIFY suspension problems. (Thus I think this is a problem to be dealt with.)
What makes a 3,000 lbs car with a power split of 50/50 heavily understeer? Is it the overworked two front tires (both steering and accelerating or braking), which cause it to arrive at its limit all too soon--leaving the traction circle? Is it the “somewhat” long wheelbase, which delays the chassis turning response time? Does anyone have data on a 35/65 Cusco center diff on a 1G? (Or data on the cusco diff on a car with a better f/r weight distribution that is closer to 50/50?)
I understand that this subject was covered in the frequently asked questions, and DG-FNR exclaimed: First, we have to all understand one basic fact about DSMs - they understeer under power. NOTHING you can EVER do is EVER going to fix that.

Well, that is a bunch of BS and dramatically pessimistic. A 1G AWD will understeer AND oversteer, as it heavily depends on the conditions, whether it’s low speed, high speed turning, the steer angle, tire slip angles, power distribution, toe, camber, weight transfer and MANY other factors.


If this sounds over the top/crazy technical—it’s really not. And I don’t mean to sound like an arrogant prick, if I ever have in my posts. I’ve just got questions that hasn’t seemed to be answered.

 

[bigfoot]

Much of the power on understeer is due to the open diff in the front. A Quaife helps alot. Get as much weight to the back as you can, although there isn't much you can do here other than the battery. I'd like to hear from someone who has run the Cu$co center diff.

 

[NDgsx]

First look at the front tires contact patch.
The first thing to do is reduce deflection camber change up front. The rear a-arm mount is very soft, which not only allows the tire to gain positive camber when cornering hard, it also causes a reduction in the caster angle (pushing inwards therby moving the balljoint pivot point backwards relative to the upper strut mount). You probably know that a greater caster angle will cause an increase in dynamic negative camber as the wheel is turned.
The easiest solution to this is to replace the rear bushing with a polyurathane unit. I prefer the whiteline eccentric ones. They move the pivot point outwards and increase caster.

The next thing too worry about is the upper strut mounts. Under cornering the balljoint pivot acts as a fulcrum for the hub, trying to pull the top of the hub outwards. This has the effect of pulling the outer strut mount away from the center of the car. The only commercialy available STB that I like is greddy. It has a solid base that wraps around the whole tower and allows you too press against the shell on the lip that comes up around the strut. Turn it in as far as possible. If you want to get really serious have a custom solid unit made that triangulates to the firewall.

Then go with spring rates that are tuned for your desired natural frequency and speed, don't use spring rates to affect the handling balance of the car, or to reduce body roll. That's what sway bars are there for, overly stiff spring rates will get you into a lot of trouble when cornering at the limit when you hit a bump. Trust me on that, I crushed a 91 shell because of that

Set your static negative camber to -1.5 degrees or 2 if you don't drive on the street that often. Now find an open parking lot and bring your pyrometer. If the outside of your tire is hotter than the middle you need to get a larger front swaybar to reduce body roll in turns to help keep the contact patch flat. Keep in mind that you will have to increase your rear sway bar diameter to keep the same handling balance. By increasing the diameter of both sway bars all you will be doing is increasing the roll stiffness of the suspension. YOU WILL NOT BE INCREASING WEIGHT TRANSFER. That is determined by centrifugal force, center of gravity height, and track width ONLY.
You have reached the practical limit of the front roll stiffness when you start lifting the front inside tire, back off on the front sway bar stiffness.
Whiteline offers a 22mm front bar for the TEL and GVR4. The suspension techniques front bar goes in addition to the stock front bar and has a different lever arm length, predicting it's actual effect will be somewhat more dificult.

No onto the rear suspension.

There is absolutely NO POINT to running a rear "strut" bar. We use shocks in the rear of the car. Their mounts are not laterally loaded. The rear cornering loads go through the four mounts in the floorpan of the car. To reduce and sheer loads the only thing you can do is triangulate rear mounting points of your roll cage to the floor pan. That's how the X in the back of my car started. Unfortunely the attachment points were NOT put where I specified (they go to the shock mounts, not the area of the floor where the rear subframe mounts) so I never had it finished.

The rear control arm bushings are a harder durometer that the front units, but you can still benefit by replacing them. The one problem in the back is that the suspension does not pivot exactly on the same plane as the bolts. Thereby using poly bushings you will have some binding and a increase in friction in the rear suspension. Ideally replace them with spherical bearings.
Then there are the wonderful trailing arm bushings, these leftovers from the GVR4 4ws setup cause passive toe change during conrnering. Taboo makes a kit to weld these bushings solid. More preferably Mike at DSS also has a kit that eliminates the rubber while using a rod end to replace the mount on the subframe.

Use the rear swaybar to help make more weight transfer happen at the the rear of the car. You don't need to run much dynamic camber in the rear as it gains a considerable amount under compression.
It also does another nifty thing under compression. I haven't measured it's real effect, but the rear toes in under compression due to the semi trailing arm nature of our rear suspension. There isn't a damn thing you can do about this, sorry you're stuck with it. It's the biggest inherit problem with the 1G awd suspension.

Other things you can do to reduce understeer are:
*increase front track width relative to rear track width, this will reduce weight transfer at that end of the car
*stager wheel widths front to rear. A tires potential grip depends on wheel width, typically peak grip occurs with a wheel that is at about 75% of the tires rated width. This occurs because for the same cornering load the wider wheel will support the tire better and let it run at a lower slip angle. I recomend this as more of a last resort though, as it reduces potential overall grip
*stop turning

I've spent countless hours thinking about this stuff. I've come to a great conclusion. I'm going to buy and MR. Seriously that's my solution

 

[UCSLugRacerX]

Much of the power on understeer is due to the open diff in the front. A Quaife helps alot. Get as much weight to the back as you can, although there isn't much you can do here other than the battery. I'd like to hear from someone who has run the Cu$co center diff.

Could I then draw a connection, that since a front LSD accomodates for wheelspin a greater amount than the stock 1G conventional diff, that would increase overall front tire traction (in steady-state turning) and reduce the tendancy for the steering tires to step outside the traction circle?

How about using a Quaife front LSD to efficiently grip the steering tires,
a Cusco center LSD to split the power 35front/65rear,
and a rear Kazz clutch type LSD to stick, but easily unstick the rear wheels, during weight transfer shifts attempting to power-on-oversteer?

IF this would solve the LSD's problems, what now about spring rate, ride height, shock settings, suspension travel, camber, caster, toe, wheelbase problems, weight transfer, roll center and so fourth, in increasing the steady-state turning capacity, at more steer angle?

 

[UCSLugRacerX]

Thank you very much NDgsx. I am printing out your reply to study in further detail.
Now, what about static weight distribution, and F/C/R, differentials?


P.S.: What did you do to fit 245's tires? Rolled fenders? What's your offset? Are you using spacers? Thanks!

 

[NDgsx]

Static distribution you're pretty stuck on. The engine is in front of the axle, it and the tranny make up the most weight in the car. Most weight reduction you can do easily is in the engine bay. It will never get you to 50/50 though.
You've got the right idea about the diffs. I'd try it with the quaife/cusco first before buying the kaaz rear.

 

[LONEWOLF01]

This thread has been very helpful. i was wondering on what to do with my rear suspension i would defiently look this over. do you have links to all these parts you mention

 

[zephyr_8]

What parts are you looking for?

 

[LONEWOLF01]

What parts are you looking for?

CUSCO REAR ,kazz clutch lsd.quaife front lsd

 

[zephyr_8]

CUSCO REAR ,kazz clutch lsd.quaife front lsd

If I remember correctly, Mach V has Kaaz, I think Shep carries quaife, also I think TRE carries quaife. I don't think they ever made a cusco rear, you probably are looking for a cusco center. It mentions above, Cusco 35/65 center, Kaaz rear, Quaife front. Look for the Cusco under Evo 3 Tarmac Center if you use a search engine. There is a link somewhere on this site, I just don't remember where it is.

There is a lot of "I think"'s in this, I will look for you tomorrow, but that is my best guess right now.

 

[Turbo Shogun]

Last time I checked (1 year ago or so) Alamo Motorsports carried Cusco Center diffs.

Although I've heard rumors of them going bad. I know someone on this forum runs it in SM, but I'm not sure. I've kind of put this kind of thing on the backburner.

My car will oversteer on an autocross if you drive her aggressive enough, but that's partly my fault. Trailbraking isn't always your best friend. I am planning on buying a Quaife up front. That'll help me pull out of the occasional slide.

 

[JrCRXHF]

I've spent countless hours thinking about this stuff. I've come to a great conclusion. I'm going to buy and MR. Seriously that's my solution

i am with you on that one. in about 2 months i will go by the mr but i was going to autocross the dsm just to be different but i think my CRX might be a better choice after reading all this.

 

[LONEWOLF01]

nothing beats light weight plus it does'nt hurt to be fwd. i didn't like fwd racecars but they do have their advantages on some course with limiting laps. look at the world speed challenge those hondas rock the street tuner class