A friend and I were working on his normally suspended 'A this evening and got to talking about suicide front end steering geometry and how it works (or doesn't). It seems the Ackerman principal gets tossed out the window when the tie rod is in front of the axle. Are we right? Is there a way to set up a suicide car to steer well? Thanks.
steer well...? nope. tossed out the window...? yup. Looks cool as **** though! But it seems to me that ford and chevy always had it out for each other, and while the mustang had nearly perfect Ackerman, the early camaro flew in the face of Rudoph Ackerman. I think both cars handled fair, but neither was "great". am I wrong about the camaro?
Followed you there Tingler , right up til you mentioned pumpkins ! Sorry to be the dumb*** Brit , but what the hell's a pumpkin ?
There is no reason you can't have the steering arms in the proper location, at the rear of the spindles, with a suicide perch. With the steering arms in front it should be called a "stupid" front end.
when you have your imaginary lines pointing at the pumpkin thats 100% Ackerman, which isn't quite right. Right now 50-70% is accepted as "right" So that means you lines should intersect at 50-70% the distance from the front axle centerline to the rear.
I thought this "new" theory (I read about it in a book publised by Colin Chapman, engineer for Lotus, in the 60's) said that LESS Ackerman angle could be used, meaning that you could get away with having your extended imaginary lines converge 50%-70% BEHIND the rear axle, not in front of it. Reason being, true Ackerman works on a buggy ch***is wheel going about 3 mph, in otherwords at parallel parking speed, but at higher speeds, tread distortion and the dynamics of the cars center of gravity make the rear wheels/tires accept some of the radius different forces allowing for and calling for LESS Ackerman angle than pure Ackerman Angle on the steering, not more. At speed it acts like it has a 50%-70% longer wheelbase, not shorter, and the Ackerman can be set accordingly. Another thing, if you have bolt on steering arms, if you mount them in front and at the top of the spindle you can use the kingpin inclination to your advantage because the higher they are the further outside the kingpin pivot they will actually be. (behind the axle, the reverse is true.) Just don't paint it red or yellow if it's hangin out in front... It's going to be functional but, but, they don't look good in front anyway so what the diff? Paint it the steering arms the color of a sore thumb...!
Now Johnny is my new hero... that drawing makes all this geometry **** make sense to me now... not that im gonna FOLLOW it or nuttin, but hell least now i know what the hell Tingler was talkin about tanks John
Can you email me the drawing and I'll try and post it for you? I think I get what you're saying, but I've got to think about it some more. I'm thinking that rasing or lowering the tie rod might affect the ackerman number, but not actually affect the way the wheels turn. And since nobody really explained what ackerman is, I'll go for it. The simple answer is that since the inside and outside tires go around different diameter circles as you turn, so the inside tire has to turn tighter. But, theres alot more to it than that. As you turn you get more weight on the outside tire, as weight increases you change your slip angles. Toe-in is to correct the difference in slip-angle. But the problem is that you can only set the toe-in perfect for one turn radius. So ackerman changes toe as you turn so you have close to correct toe for every turn. But most hot-rodders probably don't care too much about the last part
While this conversation centers around a series of alignment principles, it can be easily addressed by jacking up the vehicle, rotating the steering and viewing the results of reversing the spindles. You should observe that the ackerman principle is defeated and your car will handle poorly. As addressed in previous posts the ackerman principle creates a desired characteristic that allows the wheels turning left or right to operate in two concentric arcs of different radius when turning, eliminating tire scuff and lowering roll resistance. This impoves handling and cuts down tire wear. When you have reversed the spindles to put the steering arms in front, you have mistakenly created a reverse ackerman effect in which the outer turning wheel turns sharper than the inner turning wheel, A big no-no. This produces tire scuff, reduces toe, alters the camber gain when turning and increases the rolling resistance. Makes a car ill handing and spooky to drive. While it has been pointed out the steering arms relationship to the vehicle center line, no amount of bending will cure the problem in this application. Front steer is possible on IFS cars due to the fact that the steering arms can be positioned outside the ball joint inclination line. These conversations have called this an ackerman problem, when probably a better way to describe this is a turning radius problem. I hope you can see why it is not good design. The drawing of the relationship of the center line of the king pin, center line of tie rod and the central pivot point in the rear end is a good basis to work from when reengineering. This varies up the centerline of the vehicle based on tire size also but that drawing is a good start. To further complicate this, the axle caster also has an effect on steering and raising or lowering the angle of the steering arm then has an effect of camber gain/loss as well as the toe loss/gain on individual wheels when turning. It is a case of one leads to another and they alll work together. Good luck
I think the geometry of a rock crawling 4x4 is a little different than say a land speed record car the further forward this imaginary intersection is, the more drastic the difference between turning radiuses of the front wheels becomes move this point forward and the slow moving car can crawl around the rock whereas if you move it back the land speed car can gently sweep around the lake without plowing the inside wheel I just made that up, but it makes sense to me.
Who says you can't have suicide and akerman? The angle of the picture makes things look like they might hit, but in person it's a beautifully engineered system with lots of miles and no chipped paint. I took this picture just for posts like these. You don't have to swap spindles.
The king pins and the tie rod ends are the 4 corners of a t****zoid - ( see Johnnyfast's drawings). The tie rod has to be the shorter side when compared to the axle to work right. It works with the arc that the steering arms swing through to turn the inside wheel more in relation to the outside wheel the tighter you turn. With the tie rod in front and wider than the axle it changes the sweep of the steering arms - it won't work "right" no matter what you do with the arms. You can make it less wrong - but not "right". Front steer cars have a pitman and idler arm connected by a tie rod plus outer tie rods to the wheels to make the geometry work. If you have a single tie rod between the spindle steering arms, I can't see how the ackerman can work right unless the tie rod is behind the axle. I'm not saying you can't get away with it, I just don't think you can make it "right".
Saw this one last weekend at a show in Moravia NY. A roadster made from a coupe body powered by a long block MOPAR L 6 with t5 ******. Says it steers nice no bump steer and it S#!+s and gits.
Answer to Tommy's post. While picture looks to help explain situation this set up appears to have a steering arm bent upward to clear the radius rods. Form the angle of the picture there is not enough inclination of the steering arm and therfore does not induce enough toe out during turns. This produces a tire scuff and some roll resistance. Since most operation is in a straight line much of this goes unobserved and may be also masked in this case by the large offset of the spoke wheels on Buicks. This setup creates excessive camber gain when turning and a real positive road feel at the sacrifice of tires. This probably drives ok but by the book has a few issues. It would be interesting to know what the turn toe out on this ride is, as it is neat workmanship.
As was mentioned breifly before, ackerman has to do with the placement of the steering arms and not (not always at least) the type of suspension. Front steer is best suited for zero ackerman (parallel) steering or reverse ackerman steering. For high speed racing cars (formula/sports racers) usually parallel or reverse ackerman steering is needed. This (also briefly mentioned) is because of the slip angle. A race car is a "high lateral acceleration" car, that means basically that the car corners fast hence developing large lateral accelerations. Load shifts when under high lateral acceleration. Basically you lift the load off of the inside tire in the corner. This means that the tire requires less slip angle (slip angle is a function of the normal load on the tire). Less slip angle means less steering input (for the inside tire) to negotiate the corner. This means less ackerman. In some instances you even need reverse ackerman to avoid dragging the inside tire (and hence understeer). You have lifted so much load off of the inside tire that the steering angle needs to be LESS than the outside tire. The problem when dealing with a race car is that there is much more involved with ackerman than just the steering geometry. The torsional stiffness of the ch***is and suspension, the roll axis, and spring rates all play a large part in determining the load on each individual tire in a corner. The load is part of determining the slip angle and therefore the correct steering geometry needed. Also, keep in mind that the "correct" steering geometry (ackerman, parallel, or reverse ackerman) will only be right for one radius corner at one speed (I say speed because that will determine load). Now, on to the part that really matters.....why do you all care? Your tire choice will have a big effect on the entire handling of the car but no one takes that into account. The only tire data I've ever used is race tire data. If you are looking for some simple ways to determine ackerman for a street car I have some old notes I could post I guess. However, you have to realize that even with ackerman geometry it will only be "perfect" for one turn circle radius. For all others there will be some "off-ackerman" which is the deviation from the perfect ackerman for all other turn circle radii.
Johny fasts drawing is exactly right for general practice... It's just a matter of where you place the steering arm. Rule of thumb: Front Steer: Pivot point for draglink must be on the outside of the kingpin Rear Steer: Pivot point for draglink must be ono the inside of the kingpin.. The only thing that is tricky for a car with the tie rod in the front is that it is difficult to get the steering arm to be in the correct place due to tire interferrence since it has to go outside the plane of the king pin. But as Blair says above.. it's not that critical for a street car.. cheat it a little so it fits and you'll be fine.. Think about this - everybody is using the same steering arms on different width axles and different wheel bases... it works for most folks without any problem... More of an indication that it's not that critical.
You won't find Ackerman being used on race cars (circle track and road race) 'cuz it scrubs the tires and screws up the ch***is set-up...Years ago the Winston Cup guys got away from the "rear steer" ch***is and that was one reason...
I have to stop reading my head hurts and I think my brain is about to explode........... Good info though
If you really want your brain to explode, check out this automotive ch***is text book: Fundamentals of Vehicle Dynamics by Thomas D. Gillespie (a good guy, teaches/taught at the University of Michigan) http://www.amazon.com/exec/obidos/tg/detail/-/1560911999/qid=1126328078/sr=8-1/ref=pd_bbs_1/103-6581386-8492600?v=glance&s=books&n=507846 Damn text books are expensive, I have it, and I would share a few graphics but my scanner is down. There are some good Ackerman and "parallelogram" images and some crazy formulas...
I have it and it is a good book. I took a hybrid vehicle design/systems engineering course from a MIT PHD who took ground vehicle dynamics from Gillespie, small world huh? Race Car Vehicle dynamics is a good one too it's by Milliken and Milliken.
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