What are good numbers for scrub, bump steer, caster, camber, and fore/aft with 20 inches of travel? Bump and scrub you want to go to zero, caster you want to keep positive, camber only matters at ride height... 80 overall width, 30 1/2 arm from the center of the car.
I dont know the numbers to what you are looking for, but I do know that camber matters at more than just ride height. If it didnt matter, then nobody would put the money into A-arms because I beams would be just fine if camber at ride height was the only time it mattered.
"A man with a watch knows what time it is; with two watches, he is never sure" Joseph E. Shigley
The last thing you want is to keep the castor positive, run that anywhere from -6 to -10 degress (these are just ballpark figures, I have no idea what you are building), there is no set number really for bump steer, just try to eliminate it as much as possible, that has to do with the tie rod placement not the A-arm geometry. Camber loss and gain is really application specific, but try to keep that to a minimum as well, if anything make it go to negative camber at droop and at bump for more tire clearance. Scrub radius, I am not sure but I want to say somewhere between 1" and 2". hope that doesn't confuse you more.
"I have no idea what you are building), " A-arm front end with 20 inches of travel.
"that has to do with the tie rod placement not the A-arm geometry" It is dependent on all three: upper arm, lower arm and tie rod. At least the program we have shows all three to be dependent.
"if anything make it go to negative camber at droop and at bump for more tire clearance" Clearance for what?
I was hoping one of the fab shops would crunch a couple numbers on more eloborate software (more than ours anyways) so we know what is possible to achieve. Or someone has seen numbers and knows what is possible.
You won't find any hard and fast numbers for those parameters. It is an interpretive thing and it seems that everyone has an opinion but few can actually support their opinions with facts or theory. I think the attitude is more "we know it works because we have used it for years."
SAE convention defines caster to be positive when the lower ball joint is ahead of the upper ball joint from a side elevation (top of the kingpin leaning back). SAE convention also defines camber as positive in and negative out. There seems to be a lot of confusion on the sign for many people so you are wise to clarify it to truly understand their intentions.
The idea of using a short/long a-arm design to build in camber change with wheel travel is in an effort to maximize the contact area between the tire and the ground in all cases. It is often desirable increase positive camber in bump travel to counter act the effects of body roll in corners. However, to increase positive camber in bump travel you will also realize increased positive camber in droop travel. The compromise becomes deciding on an appropriate amount.
Bump steer is basically caused by toe change as the suspension travels up or down from ride height. Bump steer should be eliminated if at all possible. If bump steer can not be eliminated, any toe change that is realized should be toe in as toe in is a much more stable condition than toe out. The effects of bump steer are directly effected by the location of the pitman arm on the spindle, the length of the tie rod, and the location of the center tie rod mount (rack, slider, centerlink, etc.). The location of these points relative to the suspension pivot points is critical.
Scrub radius is somewhat arbitrary. Increased positive scrub radius (Centerline of the tire outboard of the point where the kingpin axis meets the ground) adds feedback to the driver through the steering. A small scrub radius may be worthwhile but I would reduce it to zero if possible. The scrub radius can only be effected by the kingpin inclination angle, the size and backset of the wheel, the tire diameter, and the configuration of the hub. Unless you are designing your own kingpin and spindle, the options for you to reduce scrub radius are limited. Work with what you have available to minimize it.
The common answer for caster is lots of positive caster makes the vehicle go straight. Well, it tends to increase the self steering effect of the vehicle or the tendency to return to center. Reducing caster makes it easier to steer the vehicle while also reducing the self steering effect. Figure out what your goals are and decide on the right amount for your application. Production cars use everything from 1-2 degrees negative caster all the way up to 6-8 degrees positive or more for some applications.
The main thing to keep in mind about suspension design is that every aspect is a compromise to some extent. If you are building the car from scratch you have more flexibility but if you are trying to build a suspension for an existing car/truck you have to work around what you have. Do your homework and figure out what you want from the vehicle. Then prioritize your goals and work from there. If you need more input or you want my opinion, send me a PM or email.
"The only source of knowledge is experience." - Albert Einstein
The smith books....that is pretty much road race stuff. Not big wheel travel numbers. With unequal A-arms and 4 inches of wheel travel all that stuff is relatively easy to make go away. We can move all the pick-ups and lengths around with our program and see all the changes. There isn't a configuration that will make it "perfect". We are just looking for numbers to shoot for. This is blank paper.
How much scrub will "bind" the travel. Positive caster change due to bump through a hard pitch into a corner sounds good, but as long as it stays positive (even it goes less positive in bump) isn't a bad thing. I thought negative caster was the top of the tire leaning in to the car but I think the previous post said it the other way so the above is positive leans in at the top.
Just to get straight on the conventions. Positive caster: the top of the kng pin is behind the bottom. You say ball joints, which is the "axis" so it would be the same as the king pin analogy. And positive is what you want to make the car track straight? Alot of positive caster makes it difficult to turn in? You say production cars have negative, my ford van feels that way, that seems like it would be unsafe for grandma.
Negative camber: the top of the tire leans into the car. Or the contact patch is farther from the car than the top of the tire. Negative is good for better traction in the turns because as the tire rolls over the contact patch stays flat on the ground.
I thought scrub was the amount the contact patch moved in and out from the car as the suspension went from full droop to full bump. I have heard of steering scrub (not sure of the correct term), the tire patch moves as the steering goes from lock to lock.
1. No bump steer.
2. No scrub
3. Negative camber
4. Poitive caster
I have seen one of the CORR trucks sitting still go full lock and the camber appeared to increase negative by a huge amount.
"With unequal A-arms and 4 inches of wheel travel all that stuff is relatively easy to make go away."
If road racing were that easy to design for, people like Eric Broadly (genius designer for Lola at one time) wouldn't be in such high demand. You have to temper their short wheel travel with things like they measure bumpsteer with dial indicators and work to control it within .010" total variation or less over the full motion range. For 20" of wheel travel the equivelent tolerence for bumpsteer would be .05" I'll bet few long travel rigs get it under .125" and even fewer care. It just isn't as necessary in the dirt to control the motion that tightly, so why waste the time.
Road racers seek to control ALL of the variables this tightly, or tighter if possible. To win on pavement takes far, far greater control of the suspension dynamics than winning in the dirt does.
Design your camber curve to keep the outside tire flat on the ground for the bump range of travel during chassis roll. Then whatever static camber you dial into the suspension to deal with tire squirm will be the angle the tire is always at.
"Teach you all I know and you're still stupid"
-- Howdy Lee
Scrub radius is the lateral distance measured from a front elevation between the center line of the tire at the ground and the point where the kingpin line of action intersects the ground. It is considered positive if the centerline of the tire is outboard of the kingpin line of action. Scrub is effected by the kingpin inclination angle, the wheel size and backset, the tire size, and the configuration of the hub. Draw a picture.
What you are thinking of is the change in wheel track as the suspension cycles through it's travel. There is no way you will eliminate that with 20+ inches of wheel travel.
"The only source of knowledge is experience." - Albert Einstein
Mike has been plugging away at different geometries and "side" scrub is less than an inch, bump steer is a wobbly( changes all thru the travel) .32 deg on bump and 1.11 deg on droop. Caster change is 1.27 deg on bump and .33 on droop. Camber goes pretty big from 0 at ride height to -7.745 on bump and -2 on droop. Front to rear is 1.8 on bump and 1.29 on droop.
Just a short comment on this thread....nice posts! Lots of good feedback with references.
One thing I would like to add (remember my background with this) is that when I design a vehicle suspension, I try to design for theoetical neutral handling, and then allow adjustment to compensate for the driver preferences. One hint- when you design for "perfection" in geometry you will only please yourself. An advantage to designing for minimum scrub, track change, etc. is more constant loading, which leads to a better balanced drive, and less fatigue for the driver. A seemingly small change from 4 degrees of caster to 0 caster (only an example folks!) will lower measured effort by some 35 percent, depending on the geometry of the layout. Couple this reduction with the effort expended over a long race, such as the 1000, and you have the difference between a driver that is too fatigued and has too slow a reaction time to miss that boulder in the way, and finishing.
Bob, my new best friend, (along with Brad) I remember your posts well (sucking up here). All good technical suspension/chassis stuff. On the bump steer, 1.107 deg. toe in equals 0.5 in of total toe change with a 26 in. tire, ride height to full droop. We think we can do better. ( What is a correct/better term for droop?) Can you help us on a program that would be a little more iterative? Preferences: bump steer is 0, side scrub to 0, minimize camber change but not positive, caster is starting at around 8 degrees so either way is ok. I am thinking maybe 4 degrees caster (using your example) would be better to help on turn in. Would turn in be 35 percent easier, the front end track 35% better, feedback thru the wheel be 35% stronger? A stronger feedback would be a bad thing, especially without power steering.
Clean sheet. This isn't a truggy, it's a lightweight race car (like the Ferrari off-roader).
Just a short answer this time due to time constraints- full upwards travel is termed "jounce travel", full extension (droop) is "rebound travel". Be REALLY careful with that 4 degree number-that was simply an example-NOT a recommendation. The 35 percent number is the reduction in actual torque applied at a 16" steeling wheel with 37" tires in a suspension design I happened to be working on (for a race car project a customer is paying me for). It was NOT intended to be anything other than representitive of the effort reduction available with a properly designed system. There are other factors involved that must be weighed against effort-all of which have been mentioned by Brad and others. For example-with that reduction in effort, "feel" (or kickback) is also decreased- this is why you must balance everything to obtain what you desire. You need to watch not just the extremes of travel, but also the rates of change (how fast the changes are made-ideally, the "charted motion" should be a uniform, straight line, not a curve, indicating a variable rate of change).
I will do some investigating tonite and tomorrow on a potential program you could run on a pc. Basically, if you understand trig, you can write your own program for the basic geometry. The only thing you will not be able to account for easily (in a simple program like that) would be the stresses and flex that occurs in the real world (one of the reasons the stuff I use is so darn expensive).
You might want to check this program out. Although primarilly set up for circle track cars, the underlying formulas are the same. The difference between circle track design and off road design is understanding the results. Garbage in, garbage out.
A nice feature of this program are the photos that show you how to measure the vehicle to configure the interative inputs for the results. One point however, I have NOT used this program extensively, merely tested it. I have no part of or interest in this or the sale of such a program. You buy it, you own it....