Ackerman Angle Question

[Is1BadFord]

I'm setting up a totally custom rack system in my 66 Mustang. This system's based on a Sweet manual rack, and I'm using tubular tie rods in order to connect to the spindles. This is where my question comes in. I've found some great information regarding Ackerman, and how to eliminate it in the cars this kind of rack was designed for....stock cars. Thing is, it's all regarding a front steer spindle. I'm using the stock 66 spindle, which is a rear steer. This question may seem silly, but does reversing the direction the steering is coming from reverse how positive and negative Ackerman is determined? Meaning, on a front steer car, positive ackerman (the good kind from what I understand) is measured to the front of the car (the steering arms move forward, rather than rearward?). Switching to a rear steer, would this reverse it or would positive Ackerman still be forward. I need this information to determine the best mounting position for the rack, either centered on the steering arm of the spindle, or a bit to the front or rear.

Thanks!

[gen3bu]

you may want to reconsider that rack. if it is designed for a front steer suspension, then it will turn the opposite direction in a rear steer suspension.

[RaceMan]

my experience with ackerman is all in race cars but there is no good or bad ackerman its only better or worse for what I've done, basicly ackerman is the difference (speed or distance) in the turning of the front wheels , if your racing oval tracks theres (and its guestionable) a need for this but I don't think you want any on a street car . All you need to do is get the steering arms and balljoints the same distance apart and inline with your rack , ok theres alittle more to it than that but thats about it good luck

[GBodyGMachine]

I think you would have the opposite reaction to ackerman in the rear of the spindle than if it were mounted up front. I was just reading in my book though, the closer to axle centerline your rack is, the less effect the rack has on ackerman. So, if you are going to use this rack, I would put it as close to the axle centerline as possible.

JEff

[Is1BadFord]

you may want to reconsider that rack. if it is designed for a front steer suspension, then it will turn the opposite direction in a rear steer suspension.

This is the rack I'm intending to use, my understanding is that you specify front or rear steer when ordering =).
http://www.pitstopusa.com/detail.aspx?ID=3289

It's just that the setup page I was reading only talked about front steer cars, which is why I wasn't sure if the setup reversed on changing the steering position.

Anyhow, thanks for all your replies! Much appreciated.

[paul67]

If you do not use ackerman the car will not steer all that happens is you turn the wheel and the car goes straight on ,I know as it did it on my street rod and that was just trying to get out the drive .

[Is1BadFord]

I thought 'some' ackerman was necessary, but I'm far from a suspension expert. I've always 'modified' stock setups with better parts, this will be my first ground up build. Guess I'll go ahead and do a search online for 'Ackerman / rear steer and see what comes up =).

[GBodyGMachine]

I thought 'some' ackerman was necessary, but I'm far from a suspension expert. I've always 'modified' stock setups with better parts, this will be my first ground up build. Guess I'll go ahead and do a search online for 'Ackerman / rear steer and see what comes up =).

You dont NEED Ackerman. Some cars have Parrallel Steer. This is basically, no to out/in on turns if I remember correctly.

[astroracer]

Check out the articles here:
http://www.longacreracing.com/articl...at.asp?CATID=2
Ackerman is one thing to think about but BUMPSTEER is the real issue. Swapping a rack into a current suspension design is not the thing to do. The inner tie rod points HAVE to be set up correctly with the upper and lower control arm pivots. If they are not you will have terrible bumpsteer and the car will not be fun to drive....
Mark

[DLinson]

This is a three dimensional problem. Before you determine your rack size you need to determine the tie rod assembly length. Then you will have to determine how much ackerman you want to use. The ackerman is determined by drawing a line from both lower ball joint to the center of the rear axle. The outer tierod should land on this line (if your rack is at the same level as the lower control arm). For front steer systems it is outboard of the lower ball joint and on rear steer it will be inboard of the lower ball joint. If your rack is at the same level as the lower control arm, your tie rod length will be the same length as the lower control arm. If it is higher, you will have to shorten the tie rod assembly length by the ratio of the top control arm compared to the bottom contol arm with respect to how far it is between the two. I believe there may be a picture this in Herb Adams book Chassis Engineering. You may want to order it, it's only about $15 at Amazon. Check out the bumpsteer section of that longacer link above, it shows a diagram that should help explain it.
Also check out this link: Ackerman link

When the Ackerman lines intersect at the center of the rear axle this is 100% ackerman. This will allow the front tires to turn at different rates so that they are both traveling in concentric arcs with their centers landing on the rear axle centerline. This is good for the grocery getter but not always the best for performance cars. A friend here at work raced an S2000 sports car and also a GT car in SCCA and he set his with about 66% ackermant. So he drew a line from the ball joint to about 1/3 of the way out from the center of the rear axle. I believe this helped out with the turn in and also working with the slip angles of the tires when loaded differently side to side. Another friend with racing experience also set his up about the same.

Carol Smith who wrote several racing engineering books also mensioned that he believed that there was a lot to gain out of a race car by tuning the ackerman.

Dennis

[Marcus SC&C]

Ackerman is present and desirable on most street cars. If the inside tire doesn`t turn farther than the outside tire so that it can follow it`s smaller turning radius then the tires have to scrub and work against each other. Most solid axle 4WD trucks have reverse ackerman (LandRovers are a notable exception) and it`s easy to feel the scrubbing and binding when turning tightly. This basically gives you a lot more toe in when turning which tends to make the car want to go straight (understeer). We`ve found that some cars have too little ackerman from the factory. G bodys for example respond very nicely to more ackerman with quicker,more responsive turn in. Some race cars run zero ackerman and do well that way but you don`t have to park them at the mall and they operate on the ragged edge with very high loads and often cornering on the curbing etc.
The proper engineering and installation for a R&P in a chassis not intended for a R&P is a whole other kettle of fish. Ackerman is the least of your worries. Mark SC&C

[Is1BadFord]

Wanted to say thanks to you guys. Responses like these are the reason I joined this website. You guys have pointed out a whole bucketload of things that I can now use to search more information, which will of course help me with my project. The longacre racing site was particularly helpful and informative.

We've been basing the suspension and now the steering of this car mostly on a mix of factory 1966 Mustang, and a Dirt Late Model circle track car (taking into consideration of course offsets and preset mounting locations for turning left etc). For example, we're using the stock mustang length upper and lower control arms which utilize the stock mounting locations (or modified stock, in the case of the upper), but changing the spring system to adjustable coilovers, and using tubular control arms. The spindle is the stock 1966, but the brakes are all Wilwood, utilizing brake hats and oversized two piece rotors front and rear. Rear suspension is a 4 link design, centered (turning left is cool, but this thing WILL likely be parked at the mall on occasion, not to mention road coarses are notorious for the odd right turn) and utilizing bird cages (as a stock car would) in order to eliminate suspension bind. And as for the steering, thanks to you all I've learned quite a bit (which was the purpose of this thread) about steering, and will be learning a lot more before I actually begin the fabrication and installation of the system. I seriously do appreciate all the help!!

[gen3bu]

mark -- how did you increase the ackerman on the g-body. just curious.

kevin

[sinned]

If you do not use ackerman the car will not steer all that happens is you turn the wheel and the car goes straight on ,I know as it did it on my street rod and that was just trying to get out the drive .

Huh? Whether you have positive, negative or neutral Ackerman the car will still steer. I little positive is recommended for a street car, oval track cars often use negative Ackerman or even different length steer arms to increase turn-in bite on left turns, this of course hurts turning to the right.

[Is1BadFord]

Actually, our circle track car uses ackerman to increase steering to the *right*! But we don't race on asphault =). We use the brakes in order to set the car, the left front is the only one that grabs, right front is set to little or no pressure, rear brakes are pressured evenly, and then the steering is used to correct the forced oversteer. Wouldn't be very fast on asphault, but on clay, it's the only way to go =p.

I think what Paul67 meant was that his car liked to push, probably pretty bad. A lot of street rods haven't got much rubber up front, and a mix of no ackerman + lots of power + light weight and big rear tires...I'm sure his fronts were scrubbing pretty badly, in particular if there was any weight transfer going on.

[sinned]

Correction: Asphalt driven circle track cars! I can see how you would need to reverse that if you spend your entire race counter steering

.

[Travis Rankin]

Originally designed so rich peoples driveways wouldn't get ripped up :D