I have just been reading the so cal chassis book and the suggested geometry for drag link steering geometry is that the link shouldpass over the pivot of the split wishbone, this seems to give the drag link and the wishbone a simular arc.but working over a different quadrant, and as the draglink is over centre it would have more and more bumpsteer as the suspension moves wouldnt a drag link parrel to the wishbone be better as they would be using the same quadrent of their arcs, if the links were all the same length wouldn't there be no bump steer i have seen both types on different cars which one gives less bump steer UKAde
I'd be interested in some answers on this too. At least it's a TECH question! BTTT. (Damn this board is fast.) Mucho O/T rants lately.
i'll do some drawings over the weekend ,,, if i get time and work some dims out,, as no one seems to want to comment on this UKAde
check out these drawings from the Pete & Jakes catalog...it should answer all your questions on streering geometry.
here's for vega cross streering. i see they are not very clear. maybe if i had your email address i'd send you a better copy....greg
The basic idea of steering geometry to avoid bump steer is quite simple. Suspension movement should not cause wheels to turn sideways! In the real word it's not that easy to accomplish that. If we look at the traditional hot rod set up of solid front axle, transverse spring, mounted by hairpins or wish bone (split or unsplit) the suspension will move in an arc pivoted at the rear connection point. This will cause the axle to move lengthwise. With the steering box solidly mounted the altering distance will cause the front wheels to turn. The Right Way If the steering tie rod is mounted parallel to the wishbone and with the exact same length they will move in the same arc = no bump steer. On Hot Rods this will usually lead to really stupid places to put a steering box. Steering box placement is usually dictated by other components and steering arm lengths are more or less what they are. The Compromise With the car at ride height, extend an imaginary line rearwards through the steering tie rod. This line should coincide with the wish bone pivot point. THIS GEOMETRY WILL CAUSE BUMP STEER. But with the stuff we're using and space available it is the best compromise. Why? With the wheels pointing forward and at ride height, at that very moment, they are both pivoting around the same point. Any suspension movement or steering input will f--k it up and they are pivoting around different points doing different arcs, but it is the good compromise. Or prove me wrong.
Even with that line, the drag link still pivots around the steering arm, which means it pivots in a smaller circle than the wishbone. The best comprimise would be to get them as long and parallel as possible. Dave
that sounds good . my drag link is longer than the wishbone and will be parrell once the new steering arm is made so the arc will be bigger than the wishbone arc sounds like it might work they also run obove each other when looking from obove so the drag link won't get in the way too much for the steering lock UKAde
I don't know what all the hubbbub is about. Most of the raggedy old steering boxes in "Death Rods" are gonna have more slop in them than the arc differences being talked about anyway so when ya hit a dip just don't fight it...Let go of the wheel and let it do it's thing and then "correct it's course" the way old cars have been driven for decades. The first time someone drives an old car they end up steering all over the place because they are used to no-play power rack& pinion steering in the Sonata they just got out of and the old car has at least 2" of steering wheel movement that just don't do a damn thing, so they end up jerking right, too far, then jerking left, then right again.... All you can do is hold a course, like in a frigin Sabot sail boat. Then when you get too far off to the lee, or near a lane line you correct, a little bit, and hold that course till you're coming up high on the windward side, and correct again. So you hit a bump (or a swell) and it does a geographic wig wag. just let it and then head her back up where you was and move on... That's how all old cars were driven before they made the lanes so damn narrow on the freeway....
DrJ, You are so right! The first "old car" I drove was a 68 Fairlane with "voodoo" power steering. I couldn't figure out how to make it goe in a straight line UNTIL I stopped trying to make it go in a straight line. Drove great when I let it do its thing! TDC
At a cruze night in Redondo Beach last mounth I saw this bitchen roadster that was built and raced back in the day.The old dude( I should talk)who owned it clamed the early drag link, (the one that was spring loaded) eliminated bump steer. I think it probably reduced the effect. pogo
[ QUOTE ] DrJ, You are so right! The first "old car" I drove was a 68 Fairlane with "voodoo" power steering. I couldn't figure out how to make it goe in a straight line UNTIL I stopped trying to make it go in a straight line. Drove great when I let it do its thing! TDC [/ QUOTE ] hah, yah i did this for the first hour i drove my burb...i was thinkin.. what the fuck is up with the steering??? now, it just does its own thing, im just along for the ride...
It will be difficult to eliminate bump-steer entirely. You should make an attempt to minimize it. Assuming left-side draglink steering, plot the arc traveled by the left front steering arm (that the draglink attaches to) and see to it that the forward end of the draglink travels that same arc. Any variance results in bump-steer. It's a mathematical thing ! When (if) I design my lakes modified I will use through-the-cowl sprint car type steering. This gives a long draglink which results in a shallower arc travelled at the forward end. The shallower arc will bump-steer less than a short draglink's smaller arc will. If I use split wishbones I will use rears on the front because they are longer. Their overall lengths will be determined by the arc scribed by the forward end of the draglink. If I get those lengths correct and their rear mounting points at the correct height relative to where the rear of the draglink attaces to the pitman arm, I should have very little if any bump-steer.
Dr J's claim about dodgy steering in old cars is reinforced by the experience I had with my old '49 F-1 running the original chassis and steering. I had to really work to keep the ol' truck in the middle of my lane until I got "Smooth" with about 6-8 beers....then, it was one finger steering at any speed I chose....very curious.
I used to run a cross steer on a Model A and had to struggle with unusual road terrain. I put a panard bar on the front and it did help. The next car I built had a Mustang box and I made a front anti-sway bar for the front and that car handled as if it were on rails!! I didn't experience any bump steer and I had full control on uneven surfaces at speed! I used the Pete & Jake geometry set-up for both. Mark
Getting the draglink level with the ground is a big factor in bump steer. A car with no bump steer develops a case of BS simply by adding a dropped axle. The steering box didn't move the axle locating ststem didn't move. The angle of the drag link changed. With it running up hill. the pitman arm now gets pulled and pushed as the steering arm oscilates up and down over a bump. Someone on this board lowered their truck and developed the problem. They couldn't lower the steering arm to get a level draglink. So they raised the steering box to level the drag link. The length of the drag link never changed only the angle. Steering went back to normal.(arm strong) Many people claim the 4 bar front as being the cure but really it was the Mustang steering box with the upward facing pitman arm (leveling the drag link) that was the cure for the bump steer back in the 70's. Both were added at the same time usually. These crude drawing show how much push/pull of the drag link is affected by it's angle. Both are the same length and the up and down travel at the axle is the same.
I thought Old Rusty might bumpsteer, but it's ok. The best way I found for checking for bumpsteer is to (before the front shox are fitted) park the front wheels on a couple of old magazines, stand on the front frame rails and bounce the front of the car up and down. You can get a real good rhythm going and you'll soon see if the wheels are trying to turn with the up and down motion. Normally with the limited suspension travel, a small amount of bumpsteer can be tolerated. Mart.
Tommy cheers mate that is exactly what i was trying to explain... about the drag link being level with the ground and not at an angle pointing at of intercepting the pivot of the wishbone UKAde
Tally ho mate!(always wanted to say that)...just kidding I hope you have that cool British accent. I think people like to over complicate things some times. All the front end geometry that Henry used work ok until the hot rodders started slamming them on the ground.(Oh Yeah) It's really not necessary to reengineer everything. I.M.H.O. Sorry I didn't really understand your original post. It must have been the language barrier.
It's cool Tommy we all speak with that english accent,, and we all have a castle, well actualy my wifes family does ,, but thats another story it's nice to try and get things right ,, and i do like to understand why things are as they are rather than just copying and taking things as read,, my car has got a little out of control,, it was supposed to be something quick to build for my first attempt at anything like this,,, but it's better for it I think, but some of the early stuff i made 18 months ago now seems sub standard,, but if i kept rebuilding stuff it would never be done UKAde
There seem to be at least two different schools on how to set up a side steer draglink to minimize bump steer; parallel to the suspension arm (wishbone), and in the line of action of the suspension arm pivot. Unfortunately, neither of these is ideal and I can’t say for sure that one is always better than the other. Here are a couple sketches comparing the two for the suspension in my Model A. The car is set up with a 35/36 axle, Posies reverse eye spring, stock Model A Gemmer steering box with ½” shortened pitman arm, hairpin steering arm and a stock Model A wishbone (draglink arc indicated in red in pictures below). With the draglink parallel to the suspension arm, the wheels will oscillate left and right of center as the suspension travels up or down. With the draglink in the line of action of the wishbone pivot, the wheels will only oscillate to the left. Generally speaking, it is probably better for the wheels oscillate around center, but if you compare the degree of motion of the two setups, you will see that the oscillations are significantly smaller with the draglink in the line of action of the wishbone pivot. I can’t say conclusively that one way or the other will always produce better results but for this particular geometry, bump steer will be less noticeable with the draglink in the line of action of the wishbone pivot. I believe Ford originally set the side steer cars up with the draglink in the line of action of the wishbone pivot, and Vern Tardel recommends setting up the draglink parallel to the suspension arm. I don’t think there is any way around laying out the geometry for the lengths and pivot locations of the setup you are running to decide what is the best way to go.
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