If I read correctly, you're saying that I should see neg camber if I jack up the car and unload the suspension, right? So If I take some pics showing the front wheels with POSITIVE camber with the car jacked up, will you believe me?

 

Not really, positive camber on droop does not absolutely proof it's gaining negative on bump but I'd still like to see the pic

The easiest way to check it is if the lower control arm cross shaft & ball joint are in line horizontalle, in that position note the upper ball joint and cross shaft. If the cross shaft is higher than the balljoint it will gain positive, if it's lower it will gain negative.

 

TT, i understand how the location of the cross shafts and balljoints will dicate if it gains positive or negative camber. Could you explain how to identify kingpin, and what it does? I remember you guys talking about kingpin when comparing c5 wheels on a c3 vs the c5 and how much tire squeel there was with the c3. I didnt unstand what it does though, or how to identify it.

 

Oh, it was said that moving the cross shaft down and backward 3/8 of an inch is better then just adding spacers to the spindle. Is there any reasoning behind that? Couldnt you just use the spacers, then make an upper control arm that moves the balljoint backward 3/8 of an inch instead of moving the whole upper control arm?? Also, how does lowering the front end affect camber change? Wouldnt it basically be like adding a spacer to the spindle, requiring a smaller spacer then would normally be needed?

 

moving the cross shaft backwards increases the caster setting. Moving the cross shaft down accomplishes the same as making the spindle longer, the camber will gain more aggresively. You can't moce the cross shaft down a whole lot without chopping up the mount though.
Caster is important for stability but also influences how the suspension reacts under bump . See it like this, if you look at the caster you draw a line though both balljoints. Extend the line to floor level, if the wheel center is behind the intersection point the wheel is "dragging" just like a caster on a shopping cart, this gives stability because the wheel drags, in a similar way as it does with the cart casters (that's why these things are probably called casters) It also influences the bump behaviour. If the caster is set so the wheel travels up and BACKWARDS (pos. caster) then when you hit a bump the wheel will bounce up and back, this is in correspondance with the obstruction you are running over. If the wheel moves FORWARD and up then the wheel wants to run into the obstruction since bumping the suspension up will force the wheel forward (this is neg. caster). This negative caster will provide a degree of anti dive but also anti-bump. This will result in a much stiffer ride. Brake leverage will also give anti dive or pro dive, depending on the caster, the setup of the cross shafts and the position of the caliper.

The kingpin angle or kingpin inclination is the angle that the line through upper & lower ball joint makes in relation to the axle pin. If you extend the line through both balljoints it will intersect with floor level. The distance between that and the tire centerline is the srub radius. The less the scrub radius the easier the steering effort, no scrub radius gives very little steering feedback and scrub radius is needed for straight line stability, just like toe is. Too much scrub radius will make it hard to turn corners. You can actually see it, with the car standing still turn the wheel and notice how the tire makes an arch that is not perfect with the steering axis (the axis through upper & lower ball joint) and it actually tries to push the car from side to side.
Here's a pic, D = scrub radius

http://autozine.kyul.net/technical_s...ng/kingpin.jpg

humm...remote viewing not allowed, click the link, then place your cursor at the address bar and press enter, this will reload the url and then the pic will show.

 

Thanks, Zwede! I've got the Gymkhana suspension, so I think the sway bars are covered. I'm gonna print out those specs and get my car set up, with new bushings and springs (f-41) in the front. The rear is up to snuff.

 

So if I am looking at this correctly, lowering the car will actually cause the car to gain positive camber even more quickly? How is our scrub radius, is that decent?

 

Our scrub radius is decent but changing the wheel offset and tire width changed the radius and if you change the overall wheel diameter (tire + wheel) it changes also. The one major drawback of using spacers gives the same problem as using wheels with more positive offset, the further you move the centerline of the wheel outboard the bigger the scrub radius. The spacer effect is somewhat comromised by the negative offset since that moves the centerline inboard. Positive offset wheels combined with spacers will give you the worst scrub radius increase...I myself have that, my ront wheels are 11"w wide with only 4" backspacing and they have a 3/8th spacer also. The scrub radius is terrible.

Lowering the car, if done by cutting the spring, will get the geometry closer to the side where it will gain negative. This is when the lines through the upper cross shaft & ball joint and the lower cross shaft and balljoint intersect on the inside of the wheel you are looking at and not the outside. This crossing point is called the instantaneous center and is the fictional center of all motion the suspension goes through, it's a virtual trailing arm of sorts. If the lines are dead parrallel the IC is non existent (it's in infinite from the suspension corner you're looking at), in that case there is no camber curve at all, it's a steady state. The pivot points will form a trapezeum. Not tht the IC is NOT a fixed point, it changes with suspension travel. It is possible for the suspension geometry to be such that the IC suddenly goes from being outboard to inboard during travel but only if it passes through the parrallel state where the IC is in point infinite.

Lowering the car by moving the kingpin axis up (drop spindle) does not change anything to the suspension, the only thing that will change is the scrub radius, it will get larger.

The easiest way to study this would be to have all the dimensions, put them into a suspension modeling program and have it dynamically recreate the IC and geometry for se4veral points of suspension travel, the movement of IC, the result on the camber curve and such would be easily visible. Plotting it all out by hand is a lot of work.

This is quite a complex matter to explain in text, pictures will greatly aid in the explanation but they are also static depictions of a single state. It greatly helps if you have an understanding and a feeling of 3 dimensional geometry and modeling. Some people have more feeling for this than others and if you have a real poor feeling for this then it's almost impossible to explain. It's then also very hard to imagine what a certain change does for the rest, it's all a complex mix of pivot points, angle changes and most important the changes on vertical and horizontal resultant components of the members. ... almost feels like geometry class all over again (eeuw )

 

Thank you for this explaination. I am really getting a great grasp on this stuff. What suspension modeling programs are there that would simulate the effects of different changes? Oh yeah, your upper arm pic isnt working, tag needs editing. [img]http://www.corvettefaq.com/suspdim/upperarmdimensions.jpg[/imh]

 

I edited the url, thanks for the heads up.

Performance Trends has what looks to be a nice package but I'm too cheap to buy it. If only I could find an "evaluation" copy They have a suspension analyzer & a roll center calculator (which is quite handy to compute the necessary camber curve to counteract the roll and ensure a maximum footprint for the tire)
Well, I'll be damned..there's a demo available for download.. hmm.

 

TT, your figures are all in milimeters right? Where does this 40 value belong? The one at the very top?

 

To me, it looks to be the line of the shock centerline.

I understand the concepts in general, ad it would be great to model this in all three dimensions, but perhaps there is a way to measure the outboard travel of the spindle/hub/rotor with a dial gauge? Clamp a solid bar to the rotor in the vertical plane, but what to measure against? Need a fixed, stationary, vertical reference. Measuring the actual path of a point on the bar would be the most accurate, no?

 

the basic idea they showed was getting the upper arm at a more aggressive angle, with the ball joint in a higher plane than the cross shaft, at ride height. that will ensure a negative camber gain.

i spent an entire winter modifying the spindle and just as much time on changing the length of the upper A arm. i also built alot of extra caster into the cross shafts, i have many different custom cross shafts with different camber offsets and caster settings, i don't use shims, i just change the shafts.
i have a very accurate caster camber gage and used it while checking the camber.
without the 1 1/2 inch longer spindles it was not possible for me to get a negative camber curve.
like twin turbo i don't see how it is possible.
i feel like i wasted an entire winter when i should have just left it alone

 

TT, I think you have some measurements wrong. On the pic that was just quoted two times, the distance from ball joint to ball joint is labeled to be 300 mm (assuming the arrows are a good indication), yet it should actually be much less given the scales of everything else in the picture. Yes, I realize there is some perspective, etc., but it seems way off. The ~240-250 mm I guess at from the other scales also fits in with giving the 8 degree angle on the upper A-arm that seems to be there.

In any case, that doesn't change your argument. If that is a normal ride height, then it looks fairly obvious that the tire will gain a bit of positive camber with compression. At least if the bushings don't move around too much under load.

 

Someone give me all of the information and I'll get it into a program. Preferably in the form of an absolute X,Y,Z position of each point relative to a common zero point.

 

I certainly don't claim to be a suspension expert but I think we would all agree that John Greenwood knows a few things about C3 suspension. This is what he had to say in the Vette Improvement Program series of articles he wrote for Corvette Fever in 1986.

"The basic geometry in the front of a Corvette is generally considered very good for performance. This performance can be attributed, at least in part, to the amount of negative camber, or camber gain, that develops when it goes through full suspension travel or jounce. When your car is set at proper height for your desired application, the static camber (camber the car has under normal driving) will be between (0) to 1/4 degrees negative camber for street use and up to 3/4 degrees negative camber for full race use. The camber change should give your car the maximum tire "footprint" on the road under all types of performance.

To determine proper front-end height, you must consider how the car will be used. Negative camber increases for every inch of suspension travel into jounce (this is called camber gain), in proportion to the amount you lower the front-end. As you lower the front-end you increase the amount of camber change and the center of gravity (CG) of the front-end is lowered resulting in better cornering power."

Seems like he supports Zwedes concept.

 

Hey ddecart, any chance of getting a copy of that program?? Im sure TT would like something like that

 

I noticed the same thing yesterday, either it's a type and it's supposed to be 200mm for the spindle height or there is something wrong there. I don't have this stuff handy, can't mesure but if someone can measure the ball joint to ball joint height on the spindle that would help.

I will re-measure the suspension sometime soon, the measurements in the pics were done pretty hasty back then, just to try to prove a point and illustrate it, that's why you see rounded numbers. I will try to get it as accurate as I can.

 

For about $15,000 I'm sure I could hook you up!

 

what program would that be? Catia?