Same deal as when a few guys told me I was gonna die if I ran a torque-tube with 1/4 elliptics and no shackles. They know the "rules" but they dont know the math...
That looks better....must have been the camera angle....thought it might be some kind of Bonneville setup..BEAUTIFUL CAR... I followed your build thread.....to say you did a Great job would be an understatement.
If I understand your hypothesis correctly. It wouldn't function any differently than a wishbone. The heims couldn't rotate independently of the third pivot point. You would just end up with a expensive wishbone.
Rootie, I probably screwed up my description. I wasn't referring to using these extra rod ends with OEM wishbones, only with split bones or hairpins / 4-bars mounted directly on the axle or ICW some sort of batwing style bracket and then to the sides of the frames. I guess I was thinking that adding some flexibility where these trailing arms were mounted to the axle would help reduce some of the twist on the axles. Gary
Yes, I was referring to split bones or hairpins. Once the heims are bolted solidly to the axle, top and bottom, in a 3 point configuration they can not pivot as they are on different planes. In effect they become solid just as in the yoke end of a wishbone, split or other wise.
With transverse spring and hair pins .... You have three point suspension articulation on the front - center mount on the spring and 2 frame mounts. This is on a horizontal plane. You have three point axle fixation on each side- top bar, bottom bar, and frame mount. This is in vertical plane. You have shackles on the spring involved too. In order to twist the axle thru the rods, the frame mounts (roughly 36" back from the axle) would need to be significantly moved higher and lower from each other in the horizontal plane. Thus twisting the frame significantly at the horns and I'm talking about a hole great big bunch of twist to get the frame mounts 36" back to transfer the twist to the axle. Somebody draw a 6 foot diameter circle, put a 4" straight edge against the radius and measure how far off the radius line the straight edge is. This represents suspension travel up and down. Then again at 4-3/4" this is the standard C-C on batwing brackets. You tell me that and ill tell you how much this system is asking the axle to twist.
Frank Kurtis's solution to take the bind out of a tube axle was to allow the bracket for the linkage on one side to pivot.
Yes and here's how it's done on my avatar... The wishbone pivots on the custom 4130 yoke. Note the grease fitting in the end of the special pivot bolt which has a ball turned on the inner end for the shock link socket.
That's clever. What if you used a birdcage on both sides, or is just one side needed? I also like the gizmo 28dreyer posted, too!
It's only done on the left front on oval track cars. The right side wants to maintain caster. Also two parrallel radius rods on the right side and only one on the left were commonly seen.
Aside from the whole breakage deal, which I think in light of the arcs involved, the stiffness (or lack thereof) of the early ford chassis, and the typical suspension travel of a transverse spring traditional hot rod, is about 90% hype and hysteria, the thing that puts me off it is the point someone made on another thread (could have been Rootie? It was one of the oval track guys) of the front axle being a big anti-roll bar(with split bones/hairpins), which means the stiffer tube axle is is going to tend to push like hell at the limit of adhesion. Now THAT made sense to me...
Found it, Mac Miller. If you are worried about having a hot rod with neutral or near neutral cornering characteristics (I doubt most HAMBers give a rats ass, but I do) the V-8 60 axle with hairpins or split bones, as pretty as it is, is probably best avoided. http://www.jalopyjournal.com/forum/showthread.php?p=2983010#post2983010
If you're thinking in same terms as a rear axle in birdcages, they have a torque tube or torque arm holding the axle in position. Of course the front axle has neither and the only thing keeping a front axle from turning in 2 birdcages would be the panhard bar and that wouldn't last long
Cowgirls the axle does not twist, it appears to twist but the pivot point is the tire center point on the opposite side tire tread. The axles apparent twist rotates thru the wheel bearings not a locked point. The axle actually moves in an arc in two axis' based upon the pivot radius. One arc is the caster arc on the longitudinal (x) axis created by the radius rod length dimension. The other is the wheel base arc on the lateral (y) axis created by the tread width dimension. Under most bump of around 3" or less of travel this movement is a minimal distance and can be hardly felt at a low speed however has a noticeable effect on the tracking of the vehicle at high speed. As stated before the biggest problem with straight axle tube or beam is with the spring bind created by the spring shackle and the caster gain during bump. The slip yoke design (birdcage) you are talking about is to do nothing more than negate both the spring bind and the caster gain during operation. Rooties picture only represents an idea that allows for two different caster angles to be adjusted into a tube axle for circle track purposes.
Dick, if the axle doesn't twist, not even a smidgen, then everyone who spews paragraphs about pseudo physics is ,,, well dare I say it?
31Vicky Now think about it, for the axle to twist it has to have full constraint, where is it fixed? The tire it just rests on the ground by gravity, the opposite radius rod has a rotating end so how is that fixed? The only way it reaches a constraint is when the operating range is exceeded or the shackle of the supporting spring binds up and the spring opposes the action. A good example of this is swivel perches, install these and the spring bind is eliminated. The spring travels in its plane and the axle travels in its plane a smoother ride is obtained the spring no longer twists and there is no action on the axle.. Now remember this question was raised about a tube axle application not a beam axle because the operational properties are somewhat different because of design.
Plain ol' up and down suspension travel is not where the problems arise. When you turn the wheel left and the body leans right, that's called body roll, chassis roll if you like, and that's where the problems start. Let's look at 31Vicky's previous example, where the radius rods are 36" long and 36" apart. The axle stays parallel with the ground (if we ignore load transfer and resulting tire squish), if we turn a corner and the body rolls 4 degrees, the front ends of the radius rods don't move vertically, but the rear ends do. The inboard rises 1.25" while the outboard drops 1.25" (assuming the roll axis is centred between the radius rods, and at the same vertical height.) This requires the axle to twist 4 degrees between the perches (front radius rod mount), or if that's not possible, the radius rods must bend in opposite directions along their length. I didn't just pluck the 1.25" / 4 degree numbers out of the air, I plotted the setup in 2D cad, just for sport. Wanna ignore everything I say? Please feel free, that's your choice. But if you're going to design your own chassis, please do the homework first. Don't rely on the "hasn't broken yet" experience of others. May I suggest reading the books pictured below for starters. Coincidentally, ElPolacko has three of those four in his library also.
The axle is constrained between the two points where the wishbones mount to it. Forget jounce and rebound for a minute and just consider roll. In roll, the axle stays parallel to the ground while one side of the frame moves down and the other side moves up. One radius rod (split bone or hairpin) moves up and the other moves down at their rear mounting points. This action has no choice but to twist the axle or to bend something as it tries to twist it. With tie rod ends or heim joints at the rear, and the standard wishbone attachment at the axle, there's little to no compliance (flex at the mounting points like you get with rubber bushed mounts) so near 100% of the swing movement of the radius rods gets transferred to the axle. The only essential difference between a beam and a tube in this setup is the much higher torsional stiffness of the tube. Higher torsional stiffness of the axle translates directly to higher forces developed at the weakest point in the system, which will be the attachment of the radius rod to the axle. Any point on the axle outboard of the attachment points is unaffected by these torsional forces. In other words, the axle isn't trying to twist from spindle to spindle but rather just across its length between the wishbone mounting points.
Take note here, I'm a "no" in the "its gonna break and we're all gonna die" column, but a yes in the "its gonna wanna plough like a oshkosh" column... Just want to make sure thats clear...
NOPE NOPE NOPE this is going to be one of those goes on forever posts. Halfdozen, Metlmuncher, the axle doesnt twist at all, it rotates around the central axis of the radius rods. Halfdozen I think your " 4*" that means the chassis moves a deviation of 4* off level due to the roll initiated. The same would happen with a 4 bar or hairpins. As the chassis rolls the roll side radius rod goes down the opposite side goes up equal amount. If anything happens the wheel base decreases as the radius of the axle travel rotates rearward based upon the length of the radius rods . As long as the mount ends of the radius rods have the ability to rotate there will be no twist. That is the advantage of using tie rod ends or high angle ball ends rather than urethane bushings. As soon as they stop and the rotation continues the only place to twist is the axle but if you are that far into it then you are in worse trouble than you need to be.
One of several very good explanations on this topic but all though there is no doubt all of these forces are in play, no one has mentioned vehicle weight or how it is driven. Probably the very reason why on a lighter T-Bucket or other rods it is not an issue after many years of use. However try this (if it were feasible) on a heavier vehicle and auto cross it every weekend and these forces would undoubtedly tear itself apart...
Back from must see TV. Yes I see that, but I just figured the spring / shackles and shocks would hold it in place. Duh. I'm such a dummy. Gary
The Jalopy Journal
