I am going to use 4'' dropped drill I-beam front axle w/ adjustable spring perches and a reversed eye front spring........I need to lean back the axle and run allot of king pin inclination..... My question is......What position is the best position to mount the spring? Because of the adjustable perches I can mount the spring level or semi level in relation to the ground and adjust the axles kingpin inclination via the hairpins.....or I can mount the spring @ the same inclination as the kingpins or close to it ?......I am wondering or over thinking which way will perform better. The spring mounted semi level w/ the axle leaned back real hard or the spring mounted @ the same angle as the axle leaned real hard...I can picture it working both ways......But keep coming back to mounting the spring w/ just a little lean back and not @ the same inclination as the axle........Thinking that it will work allot better and not bind...But wonder about the forces happening while in use......What do you think if I described it well enough, I never built anything like this ?......Thanks, Littleman
You want to make sure the spring will travel in a smooth arc into the pocket of the crossmember. Mount it up without the spring and go through the travel and see if mounting the crossmember at less angle than the kingpin angle would make the spring bind, or rock forward or backwards as it travels upwards. I guess in other words, the outer ends of the spring should ideally move upwards to the same spot as the center mount, without any forward or rearward movement. Hope this makes sense....
I would think that if you use hairpin wishbones you would want to have the spring somewhat match the axle angle. The push on the spring would match the arc of the wishbone motion. If you were using a 4 bar mount the spring vertical as the push would be straight up and down.
I always think about the pivot point which is the bone mounts... And if it pivots there I would think the spring needs to be mounted flat so the axle can freely move up and down without binding!
My mind says to have the crossmember at the correct placement and angle to make everything work in an invisible line that could be viewed at the side of the vehicle. I'm reading into this question as being intended to have a transverse front spring. I see anything not being at the correct angle straight line to have problems built in that would go against the term of "complimentary arcs" of suspension movement. (________. works ok. / (________. doesn't look good to me. Obviously not al parts involved are shown as what I vision. I haven't learned how to show that. Hope this helps. If arc is correct to spring placement and spring angle originates and points to the right spot, it will work. If not, slow destruction, if not fast failure.
Ok lets sort this out, Lets get the terminology first. You wish to lay the axle back at an angle, this is not king pin inclination it is a caster inclination. Angling the axle back or forward is measured in degree rating from vertical on the longitudinal axis (wheel base). King pin inclination is the angling of the spindle rotation axis inward or outward vertically on the lateral axis (track) at the spindle boss and is usually a fixed angle based upon axle design. It seems that you want to remove the spring bind from your build and bring the vertical action of the axle closer to the action of your spring. This you did already by purchasing the adjustable perches. This allows the perch head to rotate as the axle swings thru its travel arch and eliminate spring twist. In this case you can install the cross member parallel to the axle lateral center line with zero inclination. The caster can be adjusted in the hair pin adjustment and the adjustable perch rotation will counter the spring bind as the axle bumps or drops. Next is the rationale to use excessive caster or (layback) on your build, there is no reason to exceed about 7* caster on any I beam axle and actually going to a lesser degree angle (4*) with todays tires promotes somewhat easier slow speed turning effort and decreases the potential for caster shimmy (shopping cart shake) from excessive caster. You have all the parts now its just fab time.
Thanks for the info Dick, good info.............I used the word inclination for its definition of to bend or lean, I used the king pin bore centerline to help visualize what I was talking about........This will not be a 7 deg grocery getter setup.....I am hoping to exceed over 150 mph...So that is why I am placing extra thought into this. I was just thinking eventhou I have the adjustable spring perches that their might be a better position to mount the top of the spring to work more efficient w/ allot of axle lean...Thanks for taking the time everyone, Dave
Dave...Dick is absolutely right about the caster thing, 7 degrees positive. Excessive caster will aggravate any condition in the steering linkage and cause a shimmy. Some guys call it a 'death wobble', it is not to be taken lightly.
If you were going to build a short RED or fed and use a I-beam axle and spring how much castor would you run then ?..I run 7 degrees in my Model A Pickup and have gone 139 mph and change but its a street vehicle that I ended up placing a dampner on the front end once I strapped in some real power.....The new dragcar I am thinking about running 18-22 degrees since this thing is like a RED but real short..I was just wondering about spring placement, if one way would be better than another...maybe I should rethink my axle castor as well ?.......Its always good to ask questions !.....Littleman
Back to answer your post on caster angle. Determining the correct caster angle becomes a math problem somewhat determined on the weight of the car and the wheel base, when you see excessive caster used in long wheel base dragsters or extended fork choppers this is due to the very little weight that is actually on the tire as the center of gravity is nearer the rear. More caster is required to get the vehicle to track since the small tire and light load produce a very small tread pattern caster trail at the tire.. Since you are going to be making a short wheel base car the center of gravity is closer to the front wheels so the front end will have more weight and need less caster to track.
I am with the "match the spring angle to the arc of the suspension travel rather than the caster angle" camp
Spring should be mounted square to a line drawn through the spring perch and the tie rod pivot on the back of the bones or hair pins. This will result in the least amount of possible bind. The spring doesn't see the caster angle.
That's interesting info Dick. I always thought the excessive caster in a FED was related to the speed and straight line stability. But from what you are saying it has more to do with wheelbase and therefore weight distribution. Pete
repeat of my comment on your other thread slow the steering down/longer arm w/holes to adjust later sitting up front for 1st time? you will oversteer.... just 'aim' it for a spot down track donot over correct anyone here with experience ???
This is typically what I do........and more than likely will do in this case, just thought I would throw the question out their as far as spring mounting location.........Thanks for all the responses......I look @ it this way..The only dumb question is the one you do not ask......Littleman
If this one is going to be rear engined, gasserjohn is right, the steering may have to be slowed down from what a person would do on a front engined car. When Garlits first started building a rear engine car (after his FED blew in half on him) he was having fits trying to get it to go down the track straight. After a lot of experimenting he found out his steering was too quick and once he slowed it way down the car ran great and the rest is history. As for the inclination thing, since common practice is to dial in more on a drag only car, and given the speeds you are going to run, 7 degrees is not enough IMO. I would think something in the 12-15 degree range might track better, but that might be something you have to adjust up or down as you fine tune the car at the track. To get back to your initial question, even with adjustable perches any bind you can remove from the front suspension travel will contribute to better handling, so I would try to make all the components swing in the same arc, if possible. Don
I agree with Don on the amount of caster at 7 not being enough for this build. I'd think about 10 deg caster is required for a car with that level of speed..... my last roadster was about that caster setting and I really liked the road feel at 80 and above speeds,which usually I do not like beam axle cars at any over the speed limit....
Lots of fast cars with straight axles, look at funny cars as an example. I dont see caster related to speed, the faster you go, the more caster you need. True, I'd want more than 2 degrees, but you dont need more than 7 or 8 degrees. If the car drives straight at 100, it will in theory drive straight at 180. Aerodynamics aside. Look at late model cars with ind front ends, like a new corvette. 1 to 1 1/2 degrees and it goes straight at very high speeds. The tire doesnt know whats behind it, axle or control arms. Pay attention to toe, it does affect stability at speed as much caster. My old 33 coupe would go thru the lights at the drag strip hands lightly on the wheel at 130, dead straight. Like someone said, you might want to slow the steering down, most racers over correct their car. Good luck.
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