Bishop, The trail is the distance from where the angle produced from positive caster would hit the ground in front of the contact patch of the tire back to the contact patch. Look at the wheel from the side of the car. Draw a vertical line through the center of the tire to the ground..now imagine the angle of the kingpin is leaning back with the top towards the rear of the car. The line of the kingpin will hit the ground in front of the center of the wheel/contact patch. The steeper the kingpin angle the more directional/straight ahead stability you have ..like the forks on a bike that lean back so you can take your hands off the bars and do the special tricks we all tried when we were kids. Lean the pin the other way and you have zero stability like a shopping cart that wants to drive to the drains in the parking lots...where you parked your car and got the dents.(the contact patch is now in front of the point where the kingpin angle hits the ground) your results may vary...I could be all wrong..ignore what I just wrote.
That is the way I look at it. I'd like to have a donor car here with adjustable 4 bar front, with adjustable perches, just to experiment with caster. You know, the more you start thinking about cars that don't quite feel right, other things could be the problem other than caster. I have 2 mid 50s Olds rears here, and by accident, found out that both had toe-out! One was twice what the other was. Just wonder if people check for weird stuff like that on a problem car? I did start a thread on ideas to fix those rears and got plenty of good advice...so it went pretty easy, back to dead straight.
In addition to the 'trail' dimension, I was also thinking about king pin inclination and wheel offset, affecting the return-to-center behavior.
Caster/KAI/SAI thoughts from Motorsports Chassis Newsletter Caster causes the front wheels to lean in the direction of steer. With a given spindle/upright geometry, more caster implies more trail. Caster combined with trail causes steer drop or steer dive. The front of the vehicle drops as the wheels steer away from center, if caster is equal on right and left. This tends to cause an anti-centering force at the steering wheel. It is the reason why the front wheels of a dragster at rest tend to flop to one side or the other. Caster combined with scrub radius causes the car to drop as the wheel steers forward (toes in), and lift as the wheel steers rearward (toes out). When this occurs on the right and left wheels as one steers forward and the other steers rearward, the result is steer roll. The car leans away from the direction of steer. The wheel loads also change. The car de-wedges: the inside front and outside rear gain load; the outside front and inside rear lose load. This effect can help the car turn in slow corners, especially with a spool or limited-slip differential. In excess, it can create low-speed oversteer and over-sensitivity to steering angle. In general, cars running on lower-speed tracks need more steer roll, and cars on fast ovals should have very little. The camber change associated with caster is favorable, particularly for road racing cars, which usually cannot get favorable camber on both front wheels any other way. We can have too much of this good thing, but that's extremely uncommon. Steering axis inclination (SAI) causes both front wheels to gain positive camber as they steer away from center. SAI combined with scrub radius causes steer lift. The front of the vehicle rises as the wheels steer away from center. This induces a self-centering force in the steering which seeks vehicle center rather than inertial/gravitational center. This is particularly useful in passenger cars because it reduces the car's tendency to follow road camber, and therefore reduces the need for the driver to pay close attention in casual driving on roads with varying slope. The centering force also tends to suppress steering shimmy. Hoop
Hey guys interesting thread, newbie question here, i notice a couple guys here talking about pie cutting, cutting the wishbones etc, if the caster is built into the front crossmember which is obviously welded into the chassis and non adjustable, by pie cutting, cutting etc the bones will that not cause the spring to be being pulled back at the bottom and fixed at the top causing a binding condition? is that true or am i way behind the 8 ball here? i bought a brookville frame i haven't put my chassis together yet still have to set everything up but i seem to remember when i bought it they said it has 6* caster on the front crossmember, im gonna set it level and measure the angle with my angle finder before i start welding brackets haha
^^^ Alot of times when that is done the yoke stays in the same place but the tube is brought up closer to the frame so the mounts dont hang way down. Also the front cross member is adjusted or replaced and welded in at the correct angle when the caster is adjusted.
Very good stuff!!....Hoop you are dead on!.....all I can say is a king pin front end ...the spindle is aligned (which most you guys on here know) by the two bosses with the bushings and pin...the independent front ( as long as balljoint pins are aligned properly) spindle both work the same. Depending on your caster settings. As you turn ( left spindle hub pin or snout ) forward or right (dewedges) then back or left (wedges) the hub pin is forced into the ground (positive caster) which creates "jacking force"...(in which Hoop98 mentioned ) will help the car go down the road straight, or more steering "stiffeness" in the feel of the wheel. If you have power steering on a IFS you can run more caster. To help with the crown in the road some later vehicles did have some lead built in the right front so the vehicle could drive (straight down) the road with the same casters on both spindles......but Hoop98 said it best!
I have 3 degrees in my '39, the same is built into the crossmember. With today's roads and better engineering in the tires, you can get away with it and it steers better at anything less than 5 degrees. Now, if I'm building a highboy Deuce chassis car, It wouldn't have less than 5 degrees, but that is mostly for aesthetics.
Adding Trail, same source Trail causes lateral forces at the contact patch to produce a torque about the steering axis. This causes the steering to seek a gravitational/inertial center. The driver feels lateral cornering force through the steering. He also feels the lateral force that the tires must generate to make the car run straight on a laterally sloping, or cambered, road surface. It is worth noting that this is only one component of the self-centering forces the driver feels. Another is the tire's own self-aligning torque, which is present whenever the tire runs at any slip angle. This will provide some feedback of cornering force even in the total absence of trail. This effect is sometimes described as mimicking trail. The amount of tire self-aligning torque, divided by lateral force, is sometimes called pneumatic trail. Note that this is a calculated value which depends on tire properties, and not an actual steering geometry parameter. One important distinction between the forces from trail and tire self-aligning torque is that tire self-aligning torque is not a linear function of lateral force. It builds at a decreasing rate as lateral force increases, and at a point a bit short of maximum lateral force it actually begins to decrease. This means that if our car has little or no trail, the steering will start to go light a bit before the point of tire breakaway. Some argue that this is a good thing, especially for a passenger car, because it gives the driver a signal to ease up short of the point of actual loss of control. In a race car, this type of steering feel requires that the driver be accustomed to driving just a controlled increment beyond the point where the steering wheel tells him/her that the limit of adhesion has been reached. If the driver is used to having more trail, he/she will often find this very difficult. Trail also causes a small lateral movement of the front of the car with steer, in the direction of steer. We might call this steer yaw. It can rationally be argued that this improves turn-in, both by yawing the car promptly and by causing the rear wheels to develop a slip angle promptly. Scrub radius or steering offset causes longitudinal forces at the contact patch to generate a torque about the steering axis. If right and left scrub radii are equal and longitudinal forces at the right and left wheels are equal, no net torque at the steering wheel results. The driver feels the difference between the longitudinal forces at the front wheels. The driver feels one-wheel bumps, brake pulsations, and crash impacts where one wheel hits something, in direct proportion to scrub radius. A car with a lot of scrub radius is sensitive to wheel imbalance and tire and brake imperfections, has a lot of "wheel fight", and has greater tendency to injure the driver's hands in one-wheel crash impacts or curb or pothole impacts. A car with very little scrub radius is less subject to these problems, but the steering will tend to be numb and uncommunicative. A car with large scrub radius may steer more easily at parking speeds, depending on other parameters, provided the brakes are not applied. This is because the wheels can roll as they steer rather than purely scuffing. With the brakes applied and the car stationary, a car with a small scrub radius steers more easily.
EXACTLY! That's why I don't understand how people set up their cars without having damn near everything they plan to use. Although dropping from 29" tall tires to 26" tall tires barely changed the angle at all. On my Go cart it didn't have enough caster. Is was a squirrely mess! Impossible to steer straight.
Hey, Mr. Spadaro, I just got my new Streetscene and saw the article on you! NICE to see it. Gary Back to the debate... it's nice understanding (or trying to) the various aspects of suspension design, but what ARE the numbers we should be seeking? How much trail / caster? Or how do we design our stuff so that should we get it wrong the first time, we have the range / flexibility already built into our rigs to fine tune it? Forget the bones and use batwings? And... what about wheel offset? Does it have to be dead nuts 0 offset or what for straight axles? How might that also effect the trail / caster deal? Gary
I run most of my stuff at 7, but I tell people who ask no less than 5. The more caster, the more stable in a strait line. BUT the more caster, the harder it is to turn.
There is more to it than offset of the wheel. You can't specify one offset for any Ford axle setup, because there are differences in the wheel mounting surface....and that is one thing dictates offset choice. In a perfect world the imaginary line of the king pin should intersect the ground under the center of the width of the tread. This changes with tire height change. When you switch to Buick finned drums, that pushes the tires out, and a more drastic move happens with disc brake kits. Wrong offsets can induce death wobble, just like too much caster can. Not the cause, but they can induce it. There are so many aspects of the changes we make as we build these cars, and many things are ignored. How many guys here bother to check the specs for toe-out-at-turns, after dropping your steering arms, or swapping to aftermarket bolted ones? It is listed in the old Motors manuals, with specs, and how to check it.
So level carb pad, 7* caster, 3* on the output shaft/ differential, left sock on before the right... It might all work, some way are just better than others.
Radial tires ? I was reading on the trifive site that the 5-7 chebbies like a bit more caster with radials and power steering. Otherwise they have a tendancy to wander and don't return well. Yeah, this is OT and for an independant front, ---but still.
Great info on Caster. Never did take into account "Trail" or should I say never thought about it. Makes very good sense.
Caster Trail and Offset.. Dick was the first to ever mention the caster trail (that I have seen) on hamb a couple years ago on a death wobble thread. That small patch of ground is pretty important in making a front end correct. The tire "wants" to track this line as you go to positive caster. But how does the wheel offset change where the tire wants to track to? In other words, if the offset was wrong and then the center of the wheel/tire was too far out, which way would that tire turn if the tie rod end fell off? Food for thought, IMO. I would imagine it has to turn outwards. So, if that is true, wrong offsets are putting a load on the front end steering linkages and components such as the tie rod. "Tudor" had a thread going about death wobble, and he noticed that his repro tie rod would flex as he pushed/pulled the tires together and apart, while he was under under the car. Then a salt racer chimed in about wobble/vs/tie rod strength: He has a historic race car that he won't change, and the caster was in the high teens. His wobble was so intense that he showed pics of tire marks on the headlight buckets. He had to go to a 1-1/4" thick wall tie rod tube to stop the wobble. The tie rod (and other components) gets more forces applied as you go higher with caster. It also gets more stress from wrong offsets. so, do you really want to always choose 7 as a start point?
I don't want to keep loading up this thread with verbose posts. Here is a link to a good explanation used by people modeling front end geometry. Why it's hard to give a simple explanation This is just one corner.
Ok, if we are running radial tires on a 32 to 48 Ford beam axle (original or replica), should we have more caster / the same / less caster than if we are running bias ply tires???
That is not really too clear on where the centerline of the tread is, compared to the king pin centerline? I can't tell. My entire obsession with front ends and wobble goes back over 40 years ago when my dream of having a hotrod as a young kid, was smashed by having death wobble. In my case, the car was fine until the seller delivered it and I put mags on the front. I did not get 100 feet past the driveway and hit a bump in the road from a cross culvert, and all hell broke loose. My car was a 50s built mopar full-fendered roadster with a later mopar wavy tube axle. Twin leaf. that's why I bring up the offset/caster/wobble discussion. As the front wheel hits a bump or hole with a wrong offset, that wheel tries to steer outwards, and you feel it in the steering wheel. But it also instantly forces the tie rod towards the other front wheel. If you have a lot of caster, that opposite wheel wants to stay straight, which causes the tie rod to bow a bit, until the force is great enough, to finally start to steer that wheel. Before it does steer, you are now in a toe out condition. When that wheel starts to steer, the bow in the tie rod releases and changes the toe again. A true low speed death wobble is a chain reaction. Something starts it, and then it feeds off of itself. My feeling is that the tie rod bowing or other flexing of steering arms, etc, actually get both front wheels fighting each other and trying to steer out of sync. Increasing caster makes it worse IMO. When that opposite wheel finally does start to steer, the first wheel is now trying to turn back straight again, due to it's caster. I believe this is why some have said they fixed their low speed death wobble with a toe change, yet others have not been able to do the same. Could be their offsets and caster were no where near the same as that other car had.
I'm probably like most people in the respect that I shoot for 7 degrees while setting up the car and I usually find it has increased a little as the car settles from driving. My 27 is at about 9- 10 and it drives so well I can't see any reason to change it. Don
A look at geometric angles below. I absolutely agree "death wobble" is a harmonic that is excited by a certain circumstance. Much like troops marching in cadence on a bridge. When we change any part of the harmonic it moves somewhere else, which may or may not fix it. Richening up fuel mixtures "fixes" a weak spark etc, or moves/masks the symptom.
I don't know about everybody else But I start by purchasing the tires I want. This gives me a starting point. They don't have to be new tires they can be old runarounds that are the heights you want. Then assemble the front end and rear end with springs on them. Single springs so you don't have to weight up the chasis to get the frame at ride height. Grab your frame or pieces for it including the cross members, then put the cross members on the springs and loosely tighten the shackle bolts. If the cross members are already in the frame then hope the front one is at the right angle. At this point the frame and axle relation is going to be pretty close to ride height as you set it. So take some extra time here and make sure everything is where you want it and square to each other. Get the empty body on it and see how it looks to your eye. Now here is the tricky part. Install the front wishbones to the frame so there is no bind in the spring pocket, making sure the spring and the top of the cross member are dead on the same plane. You can use hardwood wood spacer blocks to take up the gap from not having all the springs in. Then you check the Caster. Hoping for 6-8*. Any less and it'll want to hunt a little more, and more than this and it'll steer heavy but man it'll track straight as an arrow due to the trailing effect. Look at the front of an early Dragster and it'll all become really clear. If the front cross member isn't in the right plane then cut it loose and put it where it needs to be. It's alot of work, but this step will eliminate alot of the straight axle chasis woes down the road. This is a step many miss when building from scratch and it'll make a car handle horrible. It starts at the plane of the springs in the cross members in relation to the perches or mounting points. when these are out of whack/misaliged you get bind as it is always trying to correct itself. Then the bushings wear prematurely and the ride is alot stiffer than it needs to be. A good example is the first time i heavily dropped a model A using stock stuff. It hunted all over and had the death wobble about 35 until you drove through it. Caster angle was darn near zero. I had to heat and bend the bones a little to get the 7*. I should have cut the cross member out and started there. 7* is a lucky number so I keep using it. When you are done with this, then set the toe at 1/8" to the front and it'll ride like a Cadillac. Good Luck and don't forget to radius the bottom of the springs on each leaf and apply liberal grease. It's Physics but it's also simple stuff. It takes time to get it right but when you can take your hands off the wheel at 80 and it goes straight and true it's a great feeling. Not that I do this I'm just sayin' So my opinion is 7* after all that un-needed visual thought. But more if you are building a FED........ Tim
Well, I will wait to see how my car behaves on the highway with low caster. That is my concern at this point. It is a full fendered 32, and despite the heavy olds engine, those fenders will scoop a lot of air under the car. If it lifts, the caster will slightly increase, but the weight bias goes rearwards, and with a quick ratio box, it may be a handful at 70mph. But, I don't want a car that is a bear to parallel park. Decisions
Take the time to fix it now, after all it's a '32. They don't grow on trees. I'm not the safety police either but it'd be a shame to wad that up or hate driving it because it handles poorly. As for parking, well it's a Hot Rod. None of them turn particularly well with small fronts and big rears and a manual old style box. Tim
Well, I like to be on the heavy side of caster on an I beam set up. I build for 12 degrees, and have run as much as 14 degrees. Effortless and arrow straight at high speed (100+). Never had a problem with axle tramp (the slang term is "death wobble"). Low speed steering was never a problem for me on a lightweight Hotrod (I wouldn't put as much in on a heavy car). Just stab the gas and move the ass end around if it's bothering you that much...
As of right now, it does not need to be "fixed". I purposely set the caster to where it is. I also milled a angled shim on top of the spring to compensate for my caster change. Even trying it at 4, it still felt like a stone, steering on the shop floor. So that is why I am going to experiment with this particular build. My box is very quick ratio. My motor is a very heavy early Olds. I also have a 40 coupe that has a bone stock (never touched)chassis/steering, except for another heavy early Olds motor. That car steers like a dream while parking, and a "quick" check of the factory caster, shows around 4 to 5. That stock 40 box is a much slower ratio than what is in my 32.
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