I was thinking, with the inclined roll axis on a third gen, wouldn't it be best to set the car with as little rake as possible for handling purposes? And how would weight distribution come into play? I've seen 25.5-26" thrown out as good height recommendations for the front, what about the back?

 

Rake in a chassis really does not have hardly anything to do withweight transfeer balance. The efffects are extremely minimal. What rake actually is doing is sitting the suspension geometry in different positions which generally a nose raked car will drasticaally lower the front rc (roll center)- which drops faster in lowering when in proportion to rear lowering. As you know, thinks can be done to alter rc heights regardless of chassis heights or thus chassis rake.

We use rake in NASCAR in circletrack racing to sit the rear of the car in more of a compressed flat position at corner steadystate to help keep the weight on the inside rear tire. The chassis static state is raked towards the LF just slightly- Ill give chassis heights I would use:
LF 4 1/8" RF 4 5/8"
LR 4 3/8" RR 5 1/4"

edit- I should also note that rake works alot better in banked turn compression. I could explain this so easy if you were standing in front of me and I use my legs and body as a visual of how the car differs in corner entrance set.

 

Too much rake moves weight to the front of the car. If you look at an old gasser car, they have the front end high in the air to level the car out and compensate for the high rear end with taller tires. With the car more level or higher in the front, it's easier to transfer weight to the rear when launching instead of having to raise it up to level first.

For road course/autocross, a 50/50 weight ratio would be preferred. All the hard braking and cornering already throws a lot of weight forward. having the front end lower than the back will compound this.

In the "old days" cars were raked mainly to stuff bigger tires under the fenders. With more modern suspension and tire/rim choices, this doesn't need to be done any more.

 

We both know those old gasers were lifted at least ONE FOOT into the air in the nose to try and level off if not raise the front end a little higher than the rear due to the massive slick diameters. However, what they finally realised was killing them was lack of aero at the tall end of the track. What gains they were getting from launch to mid track- were equally going away from mid track to the finsish line.
They actually would have had the same results just lifting the nose and rear equally 6", and would have had a little bit better aero at the tall end.

On a 3rd gen, you could rake the car all you want but the braking weight transfer of the car can be easily overcome by a shorter tqarm pulling the chassis weight downward more aft.

You can rake a car all you want with the nose 3" lower than the rear in fender gaps- BUT you alter the roll axis and the car can and will brake & cornewr better than a car with non altered roll centers that has been lowered with 1/2" fender gaps front & rear-why? becasue your factory suspension geometry just put the front rc into the dirt and the rear is much higher in proprtion. You brake and enter a corner and the car's attitude will cnater and lift the inside rear tire regardless of chassis rake.

Again, chassis rake has very little effect on handling. It has to do with using what byproduct of suspension geometry is left over when points can not be altered under rule books- at this point you use rake to manipulate the geometry in your favor. It is a bandaid fix- you do not ahve a rulebook governing you so don;t worry about it. Put a Jegs panhard adjuster on it and use some front extended ball joints- don't worry about body rake, worry about setting your static suspension geometry to its optimum= and that is the best body attitude.

 

Dean,

I have a rear rake of about 0.5" to 0.75" depending on the side. If I get the front and rear EQUAL (by say cutting the rear springs 1/4 coil at a time), how would this affect the front and rear roll centers? Already, my front RC is higher because of the 0.75" extended ball joints, and my rear is MUCH lower because of the UE PHB relocater brackets (lowered to the max setting). Car handles ridiculously amazing the way it is now, but lowering the rear would lower center of gravity too so that would be better for handling in general but I just want to know how it would affect roll centers? Cutting the springs will stiffen them up a bit which is good anyway because there is SLIGHTLY too much rear roll IMO. From what I understand, LESS rear rake will transfer MORE weight to the rear which would necessitate stiffer rear springs, correct? But by cutting the rear springs they become stiffer so does that make up for it? Thanks in advance.

 

A 1/2" to 3/4" rake is trivial in load transfer- why?- you need to look at the overall center of gravity of the car. Lowering the rear only of the chassis by a 1/2" will only lower the center of gravity of the car siting on the wheels by 1/4"...it is trivial, albeit, something in the right directions is always a gain even if its 1/10th of a second in lap times so to speak.

Weight moves on the roll axis, thus roll centers front and rear in relation to each axle cg is most important in lateral roll- again, so to speak. There is no real front and rear axle cg's, there is one central cg that transmit over both rc's in leverage over the axles.

With that said, you need to try and visualize the roll axis at lets say (this is so hard to try and type, I could show you in two seconds- I just have a hard time telling you and not showing you)here's my attenpt at a ficticious example:

take a front rc of 6" off the grond, and a rear rc of 10" off the ground and a cg height of 18" at 42% distance along the roll axis from the front rc towards the rear. Lets say it is a tad loose.

you then lower the CG by dropping the rear of the car 1 full inch. This drops the cg to 17.5" but still at 42% along the axis, and the rear rc is now down in ratio half the chassis height lowering compared to prior settings= which now equals a 9.5" rear rc with the rear panhard adjustment height untouched in the same setting.

What just happened?

Well the first setting yeilded a roll couple of the front leverage at 12" and a rear 8",...and the second? you have a roll couple of 11.5" front/ and still the 8" rear. The front got laterally stiffer in roll, and the rear remained the same in too much roll. Since the front more dominates any rear canter becasue of the heavier front engine weight in bias, less roll up front means less likely the inside rear will canter upward in roll.

you did the right choice- but understand its not the rear that changes, its the front......... I'll give you time to re-read this about 20 times, I know, I just confused the hell out of you. (smiles)

 

Just re-measured the fender heights and got the following:

Driver front: 27"
pass front: 26.5"
driver rear: 28.3"
pass rear: 28"

It appears that I can drop it about 1.25" in the rear while still having a very slight rake on the driver's side and a 0.25" rake on the passenger side. This would drop the rear CG 0.75" and make the front lateral roll stiffer, and keep the rear inside tire down more and lift less durig cornering.?

 

It will not drop "rear cg" (no such thing as I stated above), it will drop the chassis cg by about 5/8". The chassis cg is above the roll axis at somewhere near 42% of the wheelbase length back from the front RC. It brings this cg point down "vertically" closer in leverage reducing it sightly in motion towards trhe front RC in canter (lateral and longitudinal migration of vertical cg- this is what you either can imagine, or you can't). If you can see the imaginary picture I am drawing then you will get it- most people get lost becasue they do not understand exactly how to visualize this movement of imaginary points in space.

The rear "rc' will drop 5/8" (half the chassis drop) so the overall product is the front rc stay put, the rear RC comes down 5/8", and the CG of the car comes vertically downward 5/8" [aproximate] towards the roll axis at aprox 42% longitudinal mark.
. _____________________X
. _____________________^
{front} I...........25%........cg..50%..............75%..............I {rear}
______tire ___ __________________________________ tire

 

This is very crude, but I hope you understand it. The orange would represent in essence the front of the chassis (front rc lateral plane) and the red would be the rear. blue is the cg. purple is the roll axis. green would represent chassis canter front RF chassis to LR chassis in rake attitide in corner set.

Car is coming at you. Left two is top/static, bottom dynamic chassis roll before. Right is after the changes

Note also when I used the term rake attitude- that it was not ment to be compared with the mention of "chassis rake" in terms of the topic of this post. it was merely attitude discription purpose, not weight transfer description= it is what I meant as "canter"

 

Okay. So from what I am seeing, after the changes (I assume you exaggerated the height differences to give a clearer portrayal of how the changes will affect the car) when the car is static, the roll center is closer to the CG which makes the car roll less (blue and purple dots closer together), but what is the orange dot?

When the car is in roll, the front rolls ALOT less than before, and the rear rolls a bit less, but the front rolls ALOT less in comparison to the rear. I am trying to understand the green line, and after the changes it becomes more "level" because it is tied to the rear roll and the front roll, and because both roll less but the front and rear are now rolling more equally, this somehow affects the rear inner tire not lifting as much as before.. That's the part I don't understand still? I understand why that's important for handling, especially in a rear wheel drive car you want the rear tires planted as much as possible to have maximum grip in the turns, but how does that green line affect this? I don't know what "canter" is..

 

Yes it was exaggerated. Nothing is ever exact when put into drawings. Exaggerations have to be shown otherwise the changes would be hard to notice, or draw.

The ornage dot is the front roll center. The orange square is the front RC roll plane (lateral and vertical of the chassis). the red dot is the rear rc, etc. The blue dot represents the cg -obviously it stays around the center of the chassis weight. Note the chassis is drawn as just a retangle for ease of understanding its center. The purple line is the roll axis. Not that both lower roll sketches show the rc's both front and rear in migration from the upper static drawings

 

Give me a few minutes- Ill do more drawings

 

I exagerated it more and put the rear rc dead center of roll pivot of the cg. It shows how the chassis rolls around the rc on each the front and the rear axles (front suspension lateral plane & rear suspension lateral plane). Note they are different and the front of the car will dip more thus the diagohal canter of the chasiss is what lifts the rear as it migrates sideways in downward rotation.

You can do this with a shoebox. Put a rod through the shoebox with the rear center exit and the front a lower than center exit. Hold the shoebox level off the ground 1/2" evenly all around- then simply rotate the shoeboax on the rod axis )ie-roll axis) until the front right corner touches the ground. Note how much the top RF of the box moves to the rigth in roll. The rear down not move as much as the front, nor does it go up in thr air as much as the opposite front corner goes down. It is not in equal proprtions front to rear in lift and dive.- This is the chassis canter.

 

Lets compare a 3rd gen in 1 lateral g corner to an indycar. The cars are coming at you and are turning to your right (their left). Note the Indy car has a cg lower than the front rc. These are only the front rc's just to ease the diagram for understanding of tire loading on the fronts only in comparison to eachother.

There is alot more going on here that changes the lateral position of the car- but I am not going to get intot he complexity of how that happens- I really can't in text or charts without taking an eternity to atempt. It has to do with lateral g's and tire flex and polar weight fore and aft in directon of travel. This stuff is so hard to explain- I read what I write and I even confuse myself when I reread it. It does not translate correctly in thought.

edit- I actually have the Indy car wrong. it moves BOTH left and right for variances so in essence it washes eachother and stays in about vertical center in the front. My drawing of it is wrong on the lower protion in centerline. It should be about centered still.

 

What I am trying to do is show the load forces of the chassis on the tires. Take into account that the tire distort with load. Also take into account the Indy car hass massive downforce with the lateral force that the 3rd gen down not have, and it does a negative roll axis which to two conter balnce eachother. It damn near loads the inside tires with equal weight of the outside tires- its why they corner so well in grip and pull over 3 g's.

It does not matter the rake of the chassis in weight, it maters where the chassis cg is period and how it acts upon the instant center and the roll axis.

When drag racers learned more about the instant center of the cars, they brought the chassis back down and simply changed the suspension geometry to get the weight transfering over the insant center to load the rear wheels- a practice they follow in modern day- that's why you don;t see the look of the old gasses in new drag race cars.

 

You can now see how if you get your roll centers up and your cg down where they are closer together, you eliminate the massive positive roll and can reduce sway bar size. A reduction in swaybar size gives you more mechanical grip on each wheel independantly.

 

By that logic, if you lower the CG ALOT and raise roll centers ALOT, you can then install softer springs and smaller sway bars and achieve the same roll? For me, softer springs and smaller sway bars are interesting because of the potholes and crappy roads. Although this would increase bounciness and nose dive and have other negative effects..

 

You do not want to go soft on the springs- think about it, the springs hold up the chassis weight, the swaybars do not. Swaybars not only deal with roll weight, they also support the oppsite wheel on that axle in bump when hitting a pothole or such. Without the transfer support in bump load, one independant spring rate (especially a softer one) would render too much bump travel.

If you are reducing your positive roll rate , and thus, you are reducing the size of swaybar needed, you are making the vehicle distribute load more evenly onto all 4 wheels more evenly all the time.

You also do not need to lift the rc's and equally the roll axis, you can easily lower the cg a little with the proper weight distribution and addition. If you recall several of my posts in the past about both polar weight (weight on the extremeties of the car away from cg compared to moving weight closer to the cg), as well as reducing positive roll rates by adding things low such as the heavy Dynomat I added into my chassis- this dropped the CG and produced a little less positive roll rate by simply counter acting the lateral weight above the roll axis with more weight below the roll axis.

The polar weight closer to the cg that acts upon the roll axis, not just in transition of steering rotation, but in actual chassis roll weight movement as well- ah yes grasshopper- is wht makes for an agile car. You can have the same overall weeight and an identical cg, but one car can have less polar weight and the other have more- the less polar weight vehicle will move the chassis weight about stopping it and reversing it quicker making the chassis alot more agile.

The more you understand all you have learned over the years, the more you understand why my little V6 was so deadly agile. It was not only the lightweight motor, it was the placement of weight, the reduction of polar and rotational mass, and the reduction of unsprung weight. The car was really one of a kind. It was completely different in dynamics than any other 3rd gen. It can be copied somewhat with the same priciples in building a V8 car, but you will have to strip alot to equal the full creature comforts I had while achieving that agility.

 

Also- keep in mind that when you decrease positive roll rates, you do not decrease swaybar size that much-Why? because you are increasing lateral grip and now you are exerting less roll leverage, but at a greater lateral grip which will still thrown the chassis to 3* roll- only this time at 3* roll @ 1.0g rather than 3* roll at 0.92g.

If you cornered at the same speed before and after then you obviously will have less roll in the "after" setup with less positive roll weight. You will never achieve a neutral roll rate with stock suspension mount poists on a 3rd gen so don't worry about ever NOT running a front swaybar.

 

I want to add to anyone reading all of this that if you ARE in fact understanding all of this, you are doing better than 99.9% of people in this world. Most people live a lifetime never understanding this stuff- and those that claim they do actually are sometimses their worst enemy becasue they often have some of it wrong. I really should write a book on 3rd gen suspensions just like Tune -To-win and such, but I have pretty much donw that explaining alot of things here on TGO over the years for free. I am one of the few in this world that grasp this so natually that I can teach it even to seasoned racers that walk away learning things they thought they knew.