OK, first, I want to be sure I have it correct.....it's how when the car is pointed straight ahead, the idler arm and Pitman arm point to the centerline of the rear axles. It determines the difference in right and left wheel angle during a turn so one doesn't scrub? Right? Next.....if this is so......and say I want to narrow a subframe to fit an older car....I can actually adjust the Ackerman to fit the actual car, by carefully measuring and narrowing the center link to make Ackerman correct. I've subbed a lot of cars, and I know that Ackerman couldn't be correct, as the cars had different wheelbase lengths. But it never presented a problem in regular street driving. But if I go and narrow one, quite a bit perhaps, I know it will be far off....so can I fix it? As long as I draw an imaginary line from Pitman and idler to intersect at the rear axle centerline? Thanks in advance!
NOT the idler arm and pitman arm, but the line drawn through the lower ball joint and the outer tie rod end. That works fairly well as a guideline. You have to adjust ackerman by moving/bending steering arms in or out, closer to or farther away from the wheel. On front-steer vehicles, it's sometimes hard to get near correct ackerman because the tie rod end will hit the wheel before you have it out far enough. You can also affect ackerman by moving the center link/rack forward or backward relative to the outer tie rod ends.
I don't know why you would want ackerman built into a street car its primarily used on oval track cars. Mostly dirt.
Old subject but: Angled steering arms permit toe out on turns, allowing the outside tire to turn at a greater radius during turns. This prevents scrubbing of the steer tires while turning. So correct ackerman is more important on a street car than a oval track car. http://www.nationaltbucketalliance.com/tech_info/chassis/ackerman/Ackerman.asp
I agree with TLTony. Ackerman (as you surmised) is the geometry that insures that the inside wheel (in a turn) precribes a smaller radius than the outside wheel. Ackerman's basis is the angular relationship between the spindle center line and the steering arm (tie rod end). Ackerman 101 suggests that an imaginary line drawn from each spindle center line, through the respective steering arm center lines, should intersect at the center of the rear axle. There are lots of variables, more or less application specific. Roundy round cars often have a selection of steering arms to manage Ackerman to control over or understeer, or to force the inside wheel to prescribe more, or less, of a radius. So...in the context of your question: Let's say that the donor car has a 110" wheelbase, and the car your building has the same, but you have to narrow the clip to make it work. As a function of narrowing the clip, the angular relationship between the spindle and steering arm center lines remains the same, but the imaginary intersection point would move forward...follow? Imagine a piece of pizza...6" on a side. Now, trim an inch of the two sides that aren't the crust. Those two sides are still parallel, they still have the same angular relationship to the crust, but the tip, the "imaginary intersection point" is shorter than before, right? As far as the Pitman arm and idler arm go...what you are actually considering is narrowing the center link ("relay rod") . That's fine, because it doesn't change anything that affects Ackerman...keeping in mind that the overall narrowing of the clip DOES affect it.
I have to agree with metalshapes in this link--- http://www.jalopyjournal.com/forum/showthread.php?t=295993&highlight=ackerman+angle A couple more good books are New Directions In Suspension Design by Colin Campbell The Car And It's Wheels by Jan P. Norbye
heres a few more pics to help, shows different set ups rack n pinnion, tie rod in front of axle, tie rod behind axle, and the turning effects. hope it helps, g
"Ackerman" creates Toe Out in corners. There are several ways to do this. As long as it is anywhere near close, good enough. IMHO, most street cars don't have enough Ackerman effect.
While the Ackerman geometery works fine at low speeds when there is zero slip angle, as you go faster the slip angle of the outside tyre on the turn increases at a greater rate than the inside tyre, so the faster you go the less Ackerman you want to keep scrub to zero. So there is no 'ideal' Ackerman angle for any rubber tyred vehicle that goes at more than one speed. For highway speeds zero Ackerman has long been favoured. The problem is that while Ackerman was in one way right, his thinking predated higher vehicle speeds and pnematic tyres which change the dynamics of the whole game.
I think there must be several ideas on what ack. is and does. I thought chopolds was looking to narrow a subframe. To do this ackerman effect is not or need not be altered.
Ackerman is probably the lest understood aspect of front end geometry because it depends on what you are trying to achieve. There is no totally correct answer, it is a compromise. The big car companies spend a fortune messing around with front end geometry, and what they end up with rarely (if ever) ends up as the theoretically perfect steering principle originally proposed by Mr Ackerman. Reading through previous posts in this thread, Thunderace probably comes the closest to my own ideas on this. Race car tuners experiment with Ackerman which mainly effects turn in and transient behaviour. If there was one perfect steering geometry, there would be no need to do any track testing or change anything. On a street car I always try to get close to perfect Ackerman, because at the low speeds used on the street, (and soft grippy tires), it will reduce front tire wear at very large steering angles. If I was road racing at much higher speeds and much greater tire slip angles I would probably try slightly less Ackerman. It mainly depends on cornering speed. Doing a U turn at full steering lock in a supermarket parking lot, requires Ackerman steering angles. Steering into a steeply banked turn at 200+ Mph is just a little bit different.
Those of you responders that are saying that race cars are different, that they require something other than perfect ackerman, please explain why. It seems to me that anything short of "perfect" ackerman is going to result in side slip, or scrub, which to me means lost grip, and a push or a tight situation, unless the design of imperfect ackerman is meant to allow for suspension/steering linkage/tire deflection. But if that's the case, then the attempt is to "deflect back" to perfect ackerman, reinforcing my position that a perfect ackerman angle curve is most desireable. I hope my thoughts are clear Tony
While you guys are arguing I have built ackeman (This discussion subject has been ongoing for a couple of years) into my Dragster and my Jeep altered. I dont like even my race cars scrubbing tires and fighting themselves even when turning in the pits. On the strip i am going straight ahead so even at high speed it hurts nothing. (In fact you would have to make a pretty good case to explain why not to have it. ) It is my sneeky suspicion those who dont have it dont because they dont understand it. I watch my pals with their hidollar REds scrubbing the heck out of their front tires and having difficulty unloading and loading and steering around the pits . Their front ends are about as safe as nothing. They are replacing tires every couple of years after probably 50 miles of actual driving. Does it help at high speed? Probably not that much . Does it hurt at high speed. NO. Does it help at normal drivng speeds and slow ? Certainly. so ???? For two minutes work with a piece of string while building. Why not? Don
At very low speeds and very large steering angles you would be very correct in assuming that perfect Ackerman will always result in minimal tire scrub, best grip, and lowest tire wear. All highly desirable features for a street car driven by your granny to do the shopping. But in a race car driven seriously fast at very high force levels, the front tires are not going to be evenly loaded in a turn. The outside front tire carries far more total vertical load resulting in comparatively much high slip angles than the lightly loaded inside tire. In other words the outside tire does NOT go exactly where it is pointed, but results in a slightly wider radius turn. To compensate for this, and get the maximum available grip from both front tires, the outside tire needs to turn further into the turn than the inside tire. That is exactly the opposite to what Mr Ackerman says. He says the inside tire needs to turn more because it takes the much smaller inside radius (of a tight turn). So who is right ? The race car crew chief that sets his car up for maximum cornering adhesion at full racing speeds, or Mr Ackerman who sets his car up for minimum tire scrub at walking pace ? Both are correct for the application. You will find most modern production cars have something reasonably close to Ackerman steering geometry, and most race cars have set up to have something a bit closer to parallel steering (but not usually full parallel steering). It depends on the tire slip angles you wish to have maximum cornering grip at. Another thing to think about are the type of tire used. Modern steel belted radials have very small slip angles to give that "cornering on rails" feeling. Racing slicks can work at far higher slip angles, and that too effects the steering geometry. A street car on radials can use Ackerman. A real race car driven flat out at racing speeds on sticky wide cross ply slicks can benefit from much less Ackerman, to get the best out of it. It is a case of looking at the whole package, tire, type of car, intended use of the car. There is no one correct solution for every application, it is just one more compromise.
Really? so those funnycars that are 300+ mph are safe as nothing because they lack ackerman? Wow learn something new everyday. Oh and I recommend changing dragster front tires every couple of years due to weather checking and dryrot, not because of mileage.
Drag racing cars are not designed for cornering at speed ! If a "Funny Car" does make a sudden fast turn at over 300 Mph, the results are never good. But for manoeuvring the car around the pits and returning back up the access road, proper Ackerman steering is not going to hurt one bit. Although the wheelbase of a slingshot is so very long that it would almost be parallel steering anyway.
Watch outlaw sprint cars on dirt go thru turns then tell me ackerman steering isn't used in racing, and luagh all you want while I pass you.
Are you sure ? The greater the slip angles the less Ackerman you need. On dirt the slip angles are HUGE. The whole car is going almost sideways. You really need to turn that outside front tire into the turn, but Ackerman does exactly the opposite. See if you can find a video of a sprint car with the front wheels toeing OUT on a turn. (Ackerman toes out on turns). Nobody would set up a dirt car like that. More like anti Ackerman which goes even beyond parallel steering to TOE IN the front wheels on turns.
My comments were to the unsafe without ackerman from Dolmesh, I kinda am aware of how a drag car operates.
I am all in favour of Ackerman on drag cars of any type, and yes if it DOES start going sideways on you, all the front end grip you can find might come in handy. Huge amounts of caster would probably be more useful though to prevent it from going sideways in the first place. I agree Tinman, far to many guys forget about building in some safety and stability, all they care about is launch and sixty foot times when building the REAR suspension. But it is often shortcomings in the front that causes crashes after loss of control for some reason.
Every high performance car I've worked on, including building spindles and steering arms from scratch, were built with MORE toe out on turns (ackerman), not less than what would be considered perfect. The theory being that the inside tire, by turning sharper than dictated, would help pull the outside tire through the turn in high side slip situations. The sprint car example brought up previously is perfect, those guys run extreme ackerman at full lock, where they usually are during a race. On the circle track cars, we set the static toe in at 1/4" OUT, to promote more toe out on turns. My cars have won races, championships and set track records, on dirt and asphalt ovals and in unlimited off road racing classes, in the states and in Baja.