Rear camber question

[Plastermaster]

I was thinking of raising the rear of my Ghia just to compensate for the bit of sag that comes with the years of use. Since I only have aout 1/4 inch of clearance between tires and fender, I am concerned that I might change the camber in such a way as to cause even less clearance. Is this a legitimate concern? I would raise it an inch at the most.
Ron

[Guest]

With a later IRS car you won't have a problem. With a swing axle, you'll get a small amount of that tucked-in look until you start moving, then it all settles in. If you do this with a swing axle, install a camber conpensator to keeps things in line. Nearly all Ghias, older or newer respond very well to heavier front sway bars, after market rear sway bars or camber compensators, and Kafer Bar supports attached to the frame horns. All of this just makes for a much more fun, and safer, car.

TC

[Plastermaster]

Oh Yeah, I forgot to mention that I had an IRS. I knew the swing axle camber responded greatly to raising or lowering, but since Im have so little clearance already I thought I would check.

I have front and rear sway bars, and it reall does make a difference. I could even fell it just turning corners at low speed. The steering felt more positive. But now that I'm used to the improvement, in the words of the great BB King "the thrill is gone"

The KCB will be coming soon when I swap my engine for a new 2270. That should take care of the fun for awhile.

Ron

[Ron]

The idea of raising things a bit opens up some other issues. I read a couple books on handling and suspension mods. There is alot of inter-relateted stuff going on. After looking into it I have a few other questions:

Anyone know where the vertical center of gravity is on a stock ghia? (how high off the ground)

In dealing with cornering ability, There are a few things that happen with the car around curves. Basically the car wants to go strait. When we try to make it change direction too quickly, the car will loose traction and/or tip over. Loosing traction is not the end of the world if it is controlled. Tipping over is a real bummer. From all I have read, there is little emphisis placed on tipping over. There is alot of info on increasing traction which is the prerequesit to tipping. With a bug or a Ghia (both are pan cars and I imagine have similar CG and track) How easy is it to flip them with sticky tires and good front and rear sway bars? (sway bars help prevent roll, but also make the car less likely to loose traction)

Thanks,
Plastermaster (My computor died and I have not gotten a new password for my new one)

[gcorrado]

regarding ride height, tipping, loosing traction, and slip angles.

the short version:
lower is better for racing
you won't roll the ghia in a turn
the fastest way around a track never a skid
don't confuse a skid with a slip angle


the long version:

i don't know the vertical height of the center of gavity of the ghia, but for handling corners lower centers of gravity are always better. the lower the center of gavity the less chance there is of rolling the car - but the ghia's CG is already low enough to not worry about this. more important is the fact that the lower the center gravity the less weight is transferred from the inside wheel to the outside wheel in a turn - and therefore the faster you can go before you loose traction and start to skid.

anyhow, you're right not to worry about rolling the ghia in a curve. unless you hit something, or leave the road, you very very unlikely to roll a ghia. you will go into a skid long before that. (in fact your ghia is probably more likely to end up upside down cause it flipped over when you were going over a 100mph in a straight - the aerodynamics of these things is like a wing.)

now if you do push it too far in a curve, you will likely skid from the rear - what i mean is that the tires in the back will loose traction and you'll go into a tailspin. this is no fun, and extremely difficult for any but the most skilled drivers to recover from.

why will you skid from the rear? most of the weight in AC VWs and porsches is over the rear wheels. this means you are demanding more force there to keep the car going around the curve all together. when you run out of cornering force, it will almost certainly be in the rear, and it will slide. you have crossed the maximum slip angle for the rear tires, and so they give up - but more about this later.

now about the anti sway bars. anti sway bars reduce body roll. it helps keep the car from rolling, and it also reduces the suspension travel. this reduced travel helps keep the car feeling crisp and responsive, and also keeps the tires flat on the ground where they can do the most good.

but in order to do this, anti sway bars INCREASE the wieight transfer from inside to the outside wheel in a curve. this puts more pressure on the outside wheel (making the car easier to skid) and lifts the the inside wheel (so if you hit the gas you may not be able to put very much power to the ground). so bigger anti sway bars are not always better. if the get too big, you'll hurt your cornering ability. that said, bigger than stock sway bars are DEFINITELY a good thing. serious racers like the adjustable anti sway bars so that they can tune the suspension to the balance of body roll and weight transfer that best gets them through the corners.

okay. now don't confuse a skid (loosing traction), which is always a bad thing thing in a race, with a slip angle, which is a normal and necessary thing. so what is a slip angle?

following your logic that the car does what to go straight, and you use the side friction of tires to force it another way. the tighter you turn the more side force you demand of the tire, the more side force you ask for the more the tire deforms, this deformation results in "slip angle."

the slip angle is defined as the angle between the direction that the rim is pointing, and the direction that the car is actually going. there can be a difference between these two because the tire is deforming to provide side forces. this all happens BEFORE you loose traction and actually start to skid.

the slip angle is a useful because it's a measure of how much the tire is giving to help in the turning process. because our cars are tail heavy, we demand more of the rear tires than the fronts. this means that the rear tires run larger slip angles. this is definition of over-steer. if the slip angles on the front tires were larger than the slip angles of the rear tires, it would be under-steer. if all tires ran the same slip angles thorugh the curve, it's called neutral-steering.

a tire looses traction and skids when you try to use it above it's maximum slip angle. when you take your ghia to far and skid from the rear, it's because you've asked the rear tires operate at a slip angle that's beyond their range.

with this concept of slip angle we can now see how to improve the cornering ability of our tail heavy cars. we want to reduced the slip angles in the rear of our cars. wider tires (particularly when mounted on wider rims) produce more force for the same slip angle. therefore, we can mount wider rear tires than in the front and get more cornering force before we break free and spin out.

this is the reason why virtually all porsches have bigger wheels in the rear than the front. now you could get more balanced steering by putting on skinny front tires - but that would just be reducing your overall cornering power. bad idea.

does that make sense?

[Plastermaster]

Thanks for the detailed reply. Good information.

The swaybars have been hard keep understood. I tend to think body roll =weight transfer, but wrongly so. As the car leans, the CG moves slightly but has minimal effect on the overall dynamics.

That being said, Body roll is considered "bad" but weight transfer is also. This is a contradiction. Where is the balance between the two?

On slip angle is it true that all else being equal, the car will understeer due to the front tires pointing into the curve?

Back to my original question, It sounds like it is hard to roll a Ghia, but I am interested in raising the rear about 3/4" just for ground clearance at my driveway, and I may be installing a typical rear exiting exhaust that will reduce clearance. Weight transfer is directly proportional to CG hight.

weight Transfer= (car weight X CG height)/ track

I have not found a formula for tipping, but I guess as long as body roll is not excessive, weight transfer from CG height will result in loss of traction, and prevent tipping over. It would be interesting to see what is happening in a tip over. I guess weight trans fer is less becausethe CG is higher...instead of weight transfering to the outside wheels laterally, the inside lifts. If the suspension were kept loose all for wheels stay on the ground and the body just leans instead.

Babble babble

Ron

[john deaver]

Is it correct to consider rolling a car over to be an extreme case of body roll? By that I mean is rolling a car more a function of excessive grip, or loose suspension? I’m sure there is no way to guarantee that a car will never roll but can we hedge our bets by limiting body roll?

Fear of rolling my nice little bug is the only thing I am concerned about at the track. (I could care less if some dude in a new miata eats me for breakfast)

Car is a 69 with drop spindles and wide low-profile tires and ¾ sway-a-ways front and rear. Its sits about 3 inches off the ground.

I have spun other cars at the track and its no big deal but the bug is my baby. I think it’s the looks of the car that have me worried. Bugs are taller than most cars (Distance from running boards to top of roof.) and even lowered it looks taller than most of the cars at the track (mostly optical illusion I think)
But…. Most of the weight is at or below the drivers waist. That’s not too bad is it?

john

[Theo]

The aircooled VW's I've watched roll were almost all due to hittin an object with a tire while sliding.

There is a way to flip one on level ground by overcorrecting a slide and then overcorrecting again and again. A 'beach ball' affect will bounce the car from side to side and then over. Often this is seen as tucking the rear tire on a swing axle. It's tough to do on accident but I'm sure it happens. Usualy the inside tire lifts and the resulting loss of traction results in the remaining tire sliding. This position is famous for setting up to hit the curb and roll. If the tire fails while skidding and the rim hits and digs in you can put dents in the roof. This is my main worry when racing and I check the tire pressures and sidewall roll to assure I'm not going to catch an edge.

Autocrossing my Ghia I purposely tried to spin a 360 with no luck. With my tires and suspension the best I could do was a 180.

Racing this weekend, I can't wait!

Theo

[Theo]

Great post,

regarding ride height, tipping, loosing traction, and slip angles.

the short version:
lower is better for racing
you won't roll the ghia in a turn
the fastest way around a track never a skid
don't confuse a skid with a slip angle


the long version:

[Plastermaster]

John, Tipping over is not extreme body roll. Imagine looking down the length of the car down the center, from the rear. Say the CG is 24" above the grouind. Now imagine a line from the CG down to the rear axle. (pretend the axle runs all the way across the car) If the weight transfer is to the left, the CG move to the left in the path of an arc. The radius is the line from the nuetral CG down to the axle. All this results in the outside wheels taking some of the weight from the inside wheels. The combined traction is decreased in a sence, because even though the total weight is the same, the ability of the tires to not slip compared to the lateral force, decreased with weight. More weight yeilds more traction, but not enough so to keep up with the lateral forces. SO, less weight on the inside means less traction, more weight on the outside means more traction on those tires but all four tires combined now have less than they did befor the weight transfer.

Tiping over is when the radius of the arc is so big it arcs over the outside wheels. Body roll prevents this somewhat by only allowing the body to lean without picking up the inside wheels, meaning less weight transfer.

If someone can clarify this, please do!

Ron

[gcorrado]

I tend to think body roll =weight transfer, but wrongly so. As the car leans, the CG moves slightly but has minimal effect on the overall dynamics.

you're right, you were wrong. when the body rolls the CG moves a tiny tiny bit. that contibution is totally negligable.

consider the case of a car with perfectly rigid suspension. it's all just welded together. this car has zero body roll (or very very close to zero). but imagine the force under the tires as you go into a curve. the car wants to lean, but there's no suspension to allow it. the forces are still there even if theirs no apparent movement. the outside wheel bears a tremdous weight, and the inside wheel is ready to lift right up.

to a very good approximation this is how trains worked (in the good old days). but based on what i said, this really stiff suspension should make things spin out, and you never here of trains spinning out. well trains don't care cause they solve the sliding problem another way - with rails!!! the huge weight transfer is no big thing. but eventually there comes a limit. the inside wheel will lift and the whole thing is going over. rolled one train.

That being said, Body roll is considered "bad" but weight transfer is also. This is a contradiction. Where is the balance between the two?

no contradiction. they can both cause problems, but in different ways for different reasons. like you say, it's all a balancing act. where is the best balance? well it's a compromise based on the demands of the track, the style of the driver, the quality of the tires, and the need for a comfortable ride.

as a rule, production cars have suspension systems that are too "soft" for serious track use. it's not a bad design, it's just that they are willing to trade some peak cornering performance for ride comfort. so, getting more cornering power out of a car often evolve stiffer springs, stiffer sway bars, and uprated shocks. of course we add nice wide and stick tires to handle the extra loads this modifications add. there are some exceptions to this, to try to solve other of specific problems (like being able to put power to the ground while in the turn). but the fastest way around the track is usually stiffer suspension and solve those problems another way (like with a limited slip differential)

On slip angle is it true that all else being equal, the car will understeer due to the front tires pointing into the curve?

most cars do understeer. they were designed to have larger slip angles in the front than the rear. but this isn't really because the front wheels do the pointing. it's kind of hard to describe without a picture, but when you think about a car going around the corner realize that all the four wheels are approximately following the arc of circumference of the circle you're turning.

I am interested in raising the rear about 3/4" just for ground clearance at my driveway, and I may be installing a typical rear exiting exhaust that will reduce clearance.

if you need the ground clearance then you need the ground clearance. yes you will have somewhat more weight transfer, and yes that will reduce peak cornering power, and yes the car would be a tiny bit easier to roll. but it's not the end of the world. if you really don't want any of that, but you still want more clearance, then the only option is to push the wheels in proportion to the lifting - this is not an easy thing to do on a ghia.

I?m sure there is no way to guarantee that a car will never roll but can we hedge our bets by limiting body roll?

stiff sway bars will make the car more likely to skid and less likely to role. but like theo said, you could skid into something and make it roll that way.

if your car were swing axle rather than IRS, it might be a different story. swing axles can do all sort of bad things jack up the center of gravity and wreak havoc.

Car is a 69 with drop spindles and wide low-profile tires and ¾ sway-a-ways front and rear. Its sits about 3 inches off the ground.

really, don't worry about rolling especially if you're autocrossing. without hitting something or loosing a tire i think it would be impossible to roll your car at the speeds you'll see in solo II

[gcorrado]

John, Tipping over is not extreme body roll.

that's true. in a rigid car you could end up on your roof without any measurable body roll before the wheels pick up.

still it's true that most things which combat body roll reduce the tendancy of a car to tip over, and increase it's tendency to spin at the limit.

something bad always happens a the limit... it's just a question of what.

[john deaver]

Ron, I think I understand what you mean.

Tipping over would only be possible if the effective CG existed outside the area of the car. (exacerbated by the wheel becoming a stationary object by digging in/hitting a curb )

So, how do we keep the CG in?
Shorten the radius?..physically move the stationary CG closer to the axil by lowering

so limiting body roll should make things better. certainly the trend is to limit body roll. I have seen some high doller prototype GT cars at our local road course (Barber Motorsports Park in Birmingham Alabama) take some tight corners at stupid fast speeds just as flat as they are in the paddock.

Those guys certainly know something. Not that a 35 year old economy car is a multi million dollar race car but? the principals should be the same. Newton didn?t make any exceptions for these guys when he wrote the three laws.

So I?m hearing low and stiff
Surly however there is a limit

If absolutely no body roll is the best then we would all be replacing our shocks with steel rods when we went to the track??But we don?t so body roll must serve a positive purpose (besides not making the car so rigid that we all shake the fillings out of our teeth.)
What is that positive purpose?

Thanks

John

BTW I love theoretical discussions, they give me theories with which to base decisions on in the absence of experience.

[Plastermaster]

I don't think there is any positive purpose for body roll, except to make the little ones go WEEEEE! Body roll is just the price you pay for keeping those fillings in your teeth. I'm just theory on this too aside from doing some mods on my car, but I think the best solution is the low center of gravity. Next solution is stiff suspension, at the expence of a bumpy ride, and lastly is sway bars, which alot of us rely on as an easy way out of the other solutions. Problem with sway bars is they take an otherwise indepndant 4 wheel suspension, and transfewr the bumps and loads to the other side of the car. Sway bars should be used to fine tune the understeer/oversteeer bias. For us who just want a sporty ride on the street, they do cut down on body roll without making the spring rate too stiff, so IMO they are a nice cheap stabilizer.

Ron

[gcorrado]

If absolutely no body roll is the best then we would all be replacing our shocks with steel rods when we went to the track??But we don?t so body roll must serve a positive purpose.

okay, now there are two seperate points here.

(1) why have suspension?
(2) why permit body roll?

answer to (1): the tires are our only interface with the road. stop, go, turn - it's only thorugh the tires that we can do any of this. the performance purpose of suspension is to keep the tires on the road. if we made it all rigid every bump, ripple, or shift in weight would cause us to loose some of our contact with the road and degrade our control, our grip, our stopping power, and our ability to accelerate.

so sure, we could eliminate body roll by eliminating suspension, but the car would handle FAR WORSE, so there's no point.

now, like we talked about before there is a certain stiffness of suspension that maximizes road holding, and for most chassis designs this is somewhat stiffer than what people find comfortable on the street. when ride comfort becomes a factor, we make suspension a bit softer than it otherwise needs to be.

this means that when we take street cars to the track, we want them stiffer than stock.


answer to (2): as far as i know body roll in and of itself is not a good thing, nor is is really a bad thing. you do want the wheels to be close to vertical at all times (again so we can keep those tires working), and in many common chassis designs that means limiting body roll (this is particularly true of our VWs). so in many practical case we try to keep body roll smallish - but we are forced to tolerate some body roll in ALL PASSIVE SUSPENSION systems. there is just no way around it, so we live with some.

ACTIVE suspension is a whole different thing. there a computer measures the forces the car is under and can actually dynamically alter suspension parameters to meet road conditions. for example when you go into a turn, the suspension on the outside wheel gets stiffer and the inside wheel actually gets softer. this can keep the chassis far more level and is the ONLY WAY to completely eleminate body roll and still have suspension at all. now they don't allow active suspension at the races. why? because if you could afford active suspension you'd win, and if you couldn't you'd loose, and that get's pretty boring.