Which is correct on a beam axle hot rod, ACTUAL castor or EFFECTIVE caster? Actual caster is measured at zero chassis rake, while effective caster is actual caster (in degrees) minus chassis rake (in degrees). In other words, if a hot rod has 6 degrees actual positive caster when level, it would have only 3 degrees effective positive caster if it had a chassis rake of 3 degrees. So, in order to get a desirable 6 degrees of positive effective caster on a car with 3 degrees of chassis rake I'd have to set the actual caster at 9 degrees. Am I thinking straight...? While all responses are welcomed I'd prefer responses from qualified chassis experts, not the opinions of keyboard wizards. Thanks.
If you are building a real hotrod as in one that we would have built when we were kids and like about every hot rod on the road prior to the revolution it would be actual. Prior to rubber rake or lowering or both. If you are building a new millennium hot rod that what is correct is after it is all set up the say it is going to be when it is power parked at the good guys. Back in the day a lot of hot rods handled a little goofy because of the changes made to the way that they set. Mostly everyone learned to live with it and the really good guys actually learned to compensate for the changes.
The car does not know what rake it has, as it goes down the road. Caster is caster.....It is the angle of the king pin vs. the surface it is driving on. Relative to the ground, going uphill, or level, or dowhill roads.
Okay. So if that's true any change in the chassis rake would affect the caster angle, yes? If caster was set at 6 degrees positive with the chassis level, and then the car was raked 3 degrees forward, the caster would have to be adjusted by adding 3 degrees positive to compensate for the change, yes? So regardless of where it started and what changes were made, it should end up at 6 degrees positive at the end of the day, yes?
If you want to be absolutely correct, I would build the car and before you take it on its first test drive, set the caster.. I would bet most guys set caster at the frame build up stage and not worry about it.
That's what I will be doing also. My car was built many years ago and has undergone many changes since then by various owners, some skilled and some not so much. Apparently those who worked on the car before me never bothered to check the caster when they installed the front end the way they did. The radius rod pivots are on the frame, not on brackets below the frame. This always looked a bit hinky to me, but the car drives okay so I didn't worry too much about it. But now I plan to change the front radius rod pivot points as a part of my own update and decided to check the caster to see what effect my changes would have. Surprised is hardly the word for my reaction when I found the caster to be just 1.5 degrees positive the way it sits right now. (Yes, I checked it several times to be sure I was right. I was right.) So, in preparing to make the necessary changes I want to be sure how caster changes are calculated and what the final result of my planned changes will be. Thus my original question.
I set the caster when building then check again when finished, I like 6-7 degrees with 1/8" toe in, my cars have always driven nicely.
This is CORRECT If you change the rake, the caster will change. Caster is always checked After the car is done and sitting on the ground the way it is driven, that is the only true measurement of it. By the Way CASTOR is a brownish, unctuous substance with a strong, penetrating odor, secreted by certain glands in the groin of the beaver, used in medicine and perfumery. CASTER, as it applies here, is the angle that the kingpin makes with the vertical. Automobiles are usually designed with the upper end of the kingpin inclined rearward (positive caster) for improved directional stability
also, picking an "arbitrary" caster setting, without any further info on engine weight, wheel offset deviation(which messes with correct scrub radius), as well as the "ratio" of the steering box, etc...can lead to a poorly set up car.....just one simple example would be a very heavy motor, some deep dish wheels with wider tires on front, and a very fast ratio steering box. The car would be a nightmare to steer into a tight paved parking spot if it ran heavy caster. If you get an old Motors manual that covers antique car specs, you will find that "most" cars ran very low caster on I beam front ends. Like one to maybe three deegrees max. Ford was the only one that ran way more, and the repair-specs on typical Ford V8 I-beam cars were "4.5 to 9" degrees. Think about where the old hot rod wives tale of "7 degrees" comes from. The halfway spec would be 6.75 degrees ! That's where the 7 comes from in my opinion. Dick Spadaro was always suggesting to stay at the low end. If someone wants to learn about front end problems and proper set up, you need to read every "front end" post he ever did on hamb. .
Thanks for the info guys... We set our roadster about 3 degrees and I'm thinking that may not be enough? Car handled like crap I thought - wander back and forth going down the road. I'm sure the bias tires didn't help. We just replaced the cheapo Vega style steering box I bought when we built the car. About 4 years and that turd was wore plum out..... The mystery box was probably an off shore thing - it had no markings of any kind on the outside. Picked up a new and better quality box at Turlock. Makes a big difference. Should have know better to begin with. We have split A bones running a flathead. I'm still considering changing the caster to around 7 degrees as suggested.
Well, you said it wandered a lot with the old box, but got a lot better with a new box. Think about that before going to a big change in positive caster.... If it came to my shop, the first thing I would check for, is if the box(worm gear) is centered when the car wheels are dead straight? Some people don't know that a steering box worm gear has a "designed tight spot" at true dead center. Any wheel position besides dead center, the box is supposed to have some freeplay, so it can return to center after making a turn. The way to check to see if your steering link is properly set to that center; car not on jacks, and slowly check for ZERO freeplay when the wheels are straight. Now, if it really has ZERO play at center, move the steering wheel 1/4 turn from center, and you should now have a tiny bit of free play befor the pitman arm starts to move the linkage. Test 1/4 turn in both directions to make sure. But, If you did have some play when wheels are straight ahead, you need to find out where the tight spot is. Most times when a box is intalled, you'd count the total turns of the box, then go half way back, then set the steering link adjustment to have both wheels dead straight. Sometimes, if the box is not a good repro, or a used one, you can't "feel" a tight spot. In that case, you slowly turn "in" the slotted adjusting screw at the back side of the sector shaft(pitman arm shaft). As you turn it in, keep sweeping the steering wheel back and forth to find the tight spot. (tires off the ground). A correctly adjusted box, set to perfect center with wheels dead straight, with quality gears, you set the screw (to feel it touch the shaft) when the steering wheel is turned 1/4 turn from true center. Then as you go back to center, you should barely feel a bit of drag on the steering wheel as you get back to dead center. Lets say your steering link was adjusted too long or too short. The box "tight spot" would actually be not at dead center as you go straight on the highway. That would mean you definately have some freeplay at dead straight, so the car will tend to wander. Do this stuff before changing caster. IMO
Let's say everything is correct when the car was set up with a level chassis at ride height . Now throw on just a rubber rake and The caster is changed, the driveline angle is changed. If you're one of those 3* Trans down guys then that's off too. The visual angle formed between the valve covers and firewall gets wonky for the guys who are in the 3* down 4* chassis rake crowd. It's like 7*
Lots of rods running around with the wrong caster. Back in the day.......you split your bones, then bought the aluminum side plates to mount your radius rods and never gave it a thought. Until it wandered all over the road.
31Yicky. I can't see how the driveline would be affected. That caster angle yes but the drive angles remain the same ;trans to rear axle. If I am not right, assuming only a rubber rake please take me to school.
Ok Imagine a level chassis. The engine is set at 3* Trans down The pinion is set parallel Both in relation to level ground. Got that picture? Now drop the axle, throw on some big and littles. The frames isn't level anymore- usually a good hot rod rubber rake is 4*, nose down ass up- measured in relation to level ground. The Trans angle moved because its attached to the frame that moved. The Trans is 1* up in relation to level grind now. Where's the pinion? That's why it's important to build them at ride height if those details are important to you.
31Vicky, You are correct with that scenario. For some reason I was thinking the car was built and setting on the ground, then the rubber rake was added. I didn't read into the original post that the guy was building the car......just that he was asking about caster angle
I used to do alignments at Sears always with car sitting just as it drove in. I've never seen any one jack the frame up level to do the alignment. When they install the alignment machine they take great pains to get it level.
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