Speaking strictly about bones, cowl steering and a parallel drag link, it is impossible to "engineer" out the flaws in this design. You can minimize the bump steer by limiting suspension travel but you can not "engineer" away the flaw in geometry. The axle, due to it being fixed to the bone, will swing in the same arc as the bone. You can't change that. The drag link (when mounted parallel to the bone) IS going to swing on its own separate arc ... to be engineered correctly, the bone/axle and drag link need to SHARE the same arc (or be as close as possible) and that is impossible when the drag link is parallel. In order to "share the same arc" the rear mount of the bone and the drag link end of the pitman arm need to be as close to each other as possible, basically almost sharing the same pivot point. To share the same pivot point, unless you put some whacky bend in your drag link (and end up with a "L" shaped bar), you can't possibly have the bone and the drag link parallel to each other. And that's it for me folks ... have a great evening and don't forget to tip your waitress.
There is a reason why they are not called The Friendly Suggestions of Physics. It's not dogma. It's the law.
...And it only shares the same arc when you are going dead-straight down the road, The instant that you turn, the steering link arc changes, and you get bump steer, in a corner. I feel like we can talk about this strict geometric fact as much as we want, and there will still be people that believe that it is just "our opinion", and is still somehow "open to debate". Some folks simply won't accept that sometimes facts are facts. Physics does not care one bit for anyone's opinions.
I wish I could give more likes! One way to eliminate bump steer is to use a steel mono leaf spring. Total spring rate of 1000 lbs/in eleminates movement and therefore bump steer.
I guess I should have been more specific but since this is a hot rod forum, I wasn't thinking about or referring to race cars of the era.
Shouldn't do it; but all these discussions of the swing of the wishbone, swing of the axle, swing of the kingpin center; make me curious why the nobody picks up that it is the swing of the front drag link rod end connection on the steering arm and the rear pivot of the wishbone as one arc and the swing of the drag link is the second of the two arcs that are in conflict here. Second thing; I'm thinking it would have been easier to spend a couple days switching out the cowl steer for cross steer at the get go and have some road time; cause I don't think a Henry axle and some other wizardry to arms and links will get this going.
I too wondered if swapping would have taken less time. The OP attempted to atone for his sin but says the cowl steering must stay for now. I’m thinking the mods aren’t making much of an improvement. It appears as if the perception is there are a lot of people that feel CS is great. I see it mostly on RRs so not the first mistake.
Isn't that what I said in post #63 ?? ... " The axle, due to it being fixed to the bone, will swing in the same arc as the bone. The drag link (when mounted parallel to the bone) IS going to swing on its own separate arc".
Based on this pic I believe you have two options ... replace the front shocks with solid bars or extend your pitman arm by about 12" (Ok, that was a lame attempt at a joke ). Speaking very generally here, the BEST location for the box is on top of the frame with the pitman arm hanging down to where the mounting point of the bone and the drag link end of the pitman arm are almost sharing the same space. Since that is not really an option this far along in your build (I think you explained earlier that you had no intention of changing things drastically at this point), I think the only thing you can do at this point is drive it ... you can then decide whether or not the bump steer is something you can live with. There are plenty of cars out there with cowl steering and apparently plenty of owners who claim bump steer is either not an issue or such a small issue it isn't really an issue. Put it together, install some stiff shocks/another leaf and find out if it's tolerable.
Ley's recap fellas. The goal is to get this where I can drive it the rest of the year so I can then make changes during the winter. Yes, I know the CS is never going to perform as well as the other options. To change it now would mean major surgery and further delays in this 10 year project. I believe I can get it to where (as Gimpy said) it is minimized for the remainder of the driving season. The existing axle is junk and the dropped Model A is on its way back to me which will fix some of the flexing/tracking issues and the 10 degrees of caster . I have raised the steering arm three inches and shortened the pitman accordingly keeping the two pieces (radius rod and drag link) parallel. Now it steers easier. I do understand that the radius rod and the drag link being in this posture will never fix the issue but, it does help make it more tolerable given the amount of travel in the front axle. Still not right I understand but the goal is to be able to drive this the remainder of the driving season. That will give me a good handle on what changes have to be made in the off season.
Raising the steering arm and shortening the pitman arm has just canceled each other. I am very glad you are going to sort this out after summer.. happy cruising. JW
Pull the front spring and shocks, mark the top of the steering wheel and match that to a similar mark on the dash and raise/lower the axle, making note of what the steering wheel does in relation to the mark on the dash. Turn the wheels to each stop and repeat. It's going to move, but only you can decide if you're comfortable with how much it moves. That's your bump steer.
Sorry, I wasn't referring to or disagreeing with your post; but was just putting in my two cents about which arcs are being discussed. My thoughts are arcs A & C are the ones that matter vs. arcs B & C. Being there is a difference in length between A and B and also a difference in swing being A is farther over center in relationship to the vertical travel of the axle. But then again I may be rusty; been 58 years since 10th grade geometry; on the other hand I have put together a lot of side steer cars using a variety of boxes that all worked. Never tried cowl steer; learned of it's ills long ago helping another kid converting an old drag car to street use.
Exactly! I was trying to decide whether or not to say just that. The wishbones, axle, spindles, brakes, wheels, tyres, etc. all form a single ***embly which rotates together as a unit about the rear wishbone pivots. On that ***embly there is only one point which is relevant to bump steer: the drag link front pivot on the steering arm.
Agreed. I often read the comment "as long as the drag link is parallel to the wishbone, you're good!" It's a mistake to apply the "4 bar" geometry rules to a split wishbone set up. Here's a sketch to show that the drag link must move fore and aft when the axle/wishbone ***embly is moving up and down.
The only issue with all these diagrams is that they show the amount of suspension travel of an off road vehicle, most of these Hot Rods have about an inch both ways and i am sure thats how the Rolling Bones guys get around it, i have seen many of their cars in person. On my RPU i have a setup that has no bump steer and we have rough roads and i often watch my left front shock when on a **** road and observe very little travel(1/2''-3/4''). But on cowl steer with the drag link running down hill like the one in this thread (and many others) you have to have an issue with some bump steer...but the real question is just how much!. JW
Some of you must have been extras in Mean Girls or Heathers, you sure ***** and like to shame others that don't follow your lead.
the ratio of the box comes in to play also. if a guy is running an off road shroeder box with super fast steering on the street, he'll notice a lot more wheel play when bump steer happens compared to someone running a box with a more streetable ratio. i don't think anybody runs cowl steering to be cool or for the look, it becomes a necessity when larger motors and starters and exhaust come in to play, try mounting a steering box without having to use half a dozen u-joints. there's no denying bumpsteer, it's real, but there are ways to minimize it's effect
OK folks, here's your chance at fame and notoriety. Design an automatic length compensating drag link.
I'd submit, though, that bump steer becomes actually negligible if the rear drag link pivot is as close as possible to the wishbone pivot. Transposing the drag link arc fore and aft will result in bump steer determined by the distance transposed and the cosine of the swing angle of the wishbone/axle/etc. ***embly. At a fairly generous extreme case of 3" of suspension travel and 40° of steering lock — i.e. bottoming out at full lock — the additional fore-aft movement at the front end of the drag link would be in the order of .011", or a quarter of a millimetre. That translates to 0.104° of bump steer at the driver's-side spindle. Depending on the steering gear's ratio, that might be about a degree at the steering wheel. In actual driving, even given properly designed suspension with a decent amount of travel, you'd seldom see 20% of that. On the exact same principle, cross steering will exhibit a similar tiny bit of bump steer as soon as there is roll and steering input at the same time. I'd think that properly located drag-link steering on a 4-link or 3-link axle should be closest to the ideal.
Correct. Minimal, but not zero. For the folks who don't want to mess with the maths, the difference is mathematically indistinguishable from zero, as opposed to far away from zero.
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