Is Ackerman really that important?

This is one case, where "Close enough" IS good enough.

 

Cool. Thanks.

Seems like I learn something new every day, on here.

 

Rodster says;

Good question...I think the front end stuff,spindles and arms, are the same within a given weight class....
10 wheel straight trucks have the frames stretched many feet for whatever reasons.I don't believe the Ackermen is changed.
Maybe it doesn't matter on a longer wheelbase.

 

[CoolHand;5876633] ... We've found that on racing cars, anything from zero ackerman to 100% ackerman will work, more or less without a lot of handling change.

CoolHand,
What is zero ackerman? Positive? Negative? If you extend the chassis, is the ackerman changed more to positive or negative?

When we built my 27, it ended up with about a 115" WB chassis. The front end is still your standard Ford 32-48 straight axle deal. We haven't got to adjusting the ackerman, yet. So, will my ride tend to under or over steer?

Not to steal the thread... and IVO the other "stretched" chassis truck comments here (as in TT, AA or BB trucks)... did Ford have a variety of steering parts to compensate for the WB changes, or are the problems just too insignificant on a slow old cargo truck? One day, I'd hope I'll have the chance to flick my truck around on a track day and see what it will do. So the plan is to keep making the mods and upgrades to make it as quick as I can - steering, brakes, posi, etc.

Any thoughts? Gary

 

It would be fixed by moving the tie rod pivot point outward.

 

A 67' Chevelle has independent front suspension,... Ackerman is predominantly a principal used to set up straight, solid beam or tube axle front suspensions.
(If the old gray matter serves me correctly)

 

all front suspentions need to utilize ackerman however some do it different ways, the chevelle uses a relay rod setup (if I remember the term correctly) to acheive the same effect while also eliminating the bump steer that would be caused if you used a straight tie rod with independant suspention

 

I remember when the new camaro came out in 81 or so. The wheels scrubed so bad around corners i couldn't believe it. Yet the car handled like it was on rails in it's day. Ackerman shmakerman

 

Unkl Ian and RottenLeonard have it pegged. The center link/tie rod system on independant suspensions uses the steering arm/tie rod angle to make the inside wheel turn sharper. For wht it's worth we saw significant improvements by playing with ackerman on paved ovals. There's some wiggle room on a street car, but when it's way off you'll know it.

 

This is the way I explain it.

Locate the plane intersecting the upper and lower ball joint. Do this on both sides. From here a line to the center of the rear of the car right above the rear axle. This will make a "V" that starts in the rear at that central location and intersects the Steering Access Inclination line or SAI, between the ball joints of both sides. The tie rod location should be in this "V", either in front of the ball joint plane or behind it. Obviously the rear intersection points will be narrower than the ones in front.

 

Zero ackerman is just that, zero.

If you turn the steering wheel and the right front turns 15 degs, the left front turns 15 degs.

100% ackerman for a solid front axle with the tie rod behind would be the steering arms set up such that their point of projected intersection coincides with the centerline of the rear end housing (IE intersection point = 1X wheelbase).

50% ackerman for the same car would have the steering arm intersection at 2X the wheelbase back from the centerline of the front axle.

Zero ackerman (or 0% if you prefer) has no intersection between the steering arms because they are parallel.

See where this is going?

For an IFS, you can't really rate ackerman by percentage like that.

Generally we shoot for between 2 and 4 degrees of positive ackerman, meaning that the inside tire turns 2-4 degs more than the outside tire does. Anything down to zero ackerman will work on an IFS car. Negative ackerman is the reverse of positive (obviously), and should be avoided.

Also, just an FYI for you guys drawing the lines to the center of the turn radius. That setup is only valid for that turn radius. Meaning that for smaller (tighter) turns, there isn't enough ackerman, and for larger turns there is too much. This is why the traditional depiction is to use the converging lines back to the rear axle CL. It's the same effect though, that setting will only be "perfect" for one turn radius.

That's why arguing over small changes in the amount of ackerman is pretty much pointless. The setting can only ever be "perfect" for one radius of turn. Changing the ackerman angles just changes which radius that is.

 

What he said with emphasis on slip angle of the tires at the speeds they are most often in a turning stress situation.
(And hotrods shoud be be considered race cars or otherwise, what the hell are they for? Stylin'?)
The basic drawing of how Ackermann works is how it works in a parking lot at 1 MPH. When the car starts moving a LOT of other elements come into the formula.

Simply put, if you swap spindles on a beam axle to put the tie rod in front for clearance of everything else on your lowered frame and don't bend the arms out to as close to the backing plates as you can you are going to be squirrely in turns

 

Wrong.


Ackerman makes the inside tire turn more in a corner.

Doesn't matter what kind of suspension you run, if any.

 

Say WHAT? "Generally we shoot for between 2 and 4 degrees of positive ackerman, meaning that the inside tire turns 2-4 degs more than the outside tire does." and "The setting can only ever be "perfect" for ONE radius of turn". Please correct me if I mis-understand what you're saying. What I understand you to say, in effect, is that once you move the steering wheel off CENTER, to ANY degree of front wheel turning movement (slight turn OR sharp turn), that YOUR front wheels will INSTANTANEOUSLY hit a 2° to 4° difference in direction, AND maintain that SAME degree of difference, throughout the entire range of steering wheel movement? Hopefully, I'm mis-understanding your statement 'cause what I understand you to say ain't gonna happen that way. Thanks...DD

 

You misunderstood, AND I was not terribly clear in my presentation.

We want 2-4 degrees TOTAL, at 18 degs of steering input.

In that case, you've got 18 degs of turn on the right front, and 22 degs of turn on the left front.

At steering angles less than 18 degs, the additional angle induced due to ackerman will be reduced proportionally. It goes from basically zero steering gain at the center progressively upwards until you get to 2-4 degs of lead on the inside tire at 18 degs of steering.

Clearer now?

With an IFS steering system, you have the interaction of several arcs simultaneously, so the gain is progressive, not linear. I haven't calculated it, so I don't know the exponent, per se, but the function will be an exponential.

Because you are dealing with three intersecting arcs, the ackerman will come on slower and then ramp up quickly as it neared the end of its travel. The gain curve will look quite different from an IFS car when compared with a straight tie rod car.

 

PROPORTIONALLY and PROGRESSIVE are very key words here. Thanks for your clarification. DD

 

Those are some words of wisdom that can be applied to many a problem.

 

Gee, I always thought that this Ackerman design was developed because the Queen was mad that the 4 wheel horse carts tore up ruts in the turn of the roads because the wheels turned parallel to themsleves but I kinda think that may be tall tale. I'm a kinda siden with metalshapes and Ryan on this Ackerman thing. One of the issues here is that the approach to interpretation is not mix up IFS suspension with beam axle suspension when trying to grasp the concept. They both have the same resultant but have different functioning parts to achieve that.

Ackerman angle might easiest described as just a number that determines the angle of the front tires take when making a steady state turn so the front end does not washout and understeer or the rear end swing out and over steer. Because of the tire slip angle, the inner tire turns slightly further inward than the outer tire. This allows the vehicle to turn in a circle evenly to the right or the left. It just happens that if you ran a line from the center of the hub inclination angle thru the steering arm pivot center line the intersection of the opposing steering arm lines happens to fall close to the center line of the rear axle and this is accepted. Normal ackerman is where the steering arms point to the rear, reverse ackerman is where the steering arms point out ward to the front. Pointing out ward to the front is more accepted in the IFS design because the placement of the idler links,idler control arm and tie rods can be positioned so the movement of the steering still causes the inner wheel to turn sharper than the outer wheel.

Where there are issues centers around the reverse ackerman on I beam axles when the spindles are flopped and the steering arms point outward. Because the steering design was engineered around normal ackerman where the arms point in and rearward, pointing them forward and outward creates a situation where the outer wheel turns at a sharper radius than the inner and produces a serious scuff issue and excessive tire wear..

The actual ackerman angle will change during the motion of an IFS system but remains constant with an I beam.

 

What most people miss in looking at ackerman on a front steer independent suspension is the 'kingpin' inclination. Yeah I know, they dont have kingpins, but the angle is still there, think about scrub radius.
Veiwed front on, an imaginary line between upper & lower ball joints gives the kingpin inclination. The steering arm/tie rod pivot is usually close to this line. It still looks way short compared with what's needed for decent ackerman on a front steer straight axle, but that's because the upper joints on most domestic front ends (the 67 Chevelle in question is a perfect example) are much further inboard than the lowers.
Those ackerman diagrams looking down on the car show a line from the kingpin, thru the steering joint to the rear end. Same applies if the line goes thru the (imaginary) kingpin inclination, at the height of the rack/steering mechanism.

 

Thanx for the info. Two more questions, please.

Minor point of confusion. Most of the info here and elsewhere talks about the best Ackerman is being achieved on that imaginary line that would run thru the steering parts to the diff center, on the drive shaft centerline. But if "1X wheelbase" you mention above is the vehicle's actual wheel base, wouldn't this intersection with the drive line now be at some place on the driveshaft slightly AHEAD of the diff, as the car's wheel base is shorter than the diagional (long side of the triangle) from the front end steering components to the center of the diff / rear axle?

One of my first questions was overlooked and the answer interests me the most. My 27 truck has a straight axle up front and a 9" out back, normal track dimensions with zero offset wheels. When all was said and done, my 27 ended up with about a 115" WB chassis. The front end is still your standard Ford 32-48 straight axle deal. We haven't got to adjusting the ackerman, yet. We didn't fool with during the build because everyone kept saying it didn't matter much. Ha ha. So, given the stretch, do you think will my ride would tend to under or over steer in it's present condition when compared to, say, a stock 32 Ford passenger car's OEM wheelbase? I'm wondering for two reasons... will it be worth correcting (the gist of this thread, I think I know that answer now) and, perhaps considering other factors like front rear weight bias, could correcting the Ackerman back to ideal actually make things a little worse if by doing so I exaggerated another negative tendancy x 2 instead of bringing things more back to neutral.

Thanx, Gary

Thanx again...

 

Well Gary, think of it like this, if you have stock ford front end parts, the wheel base of a stock 35-41 ford was 112" and the 42-48 ford was 116" depending on the spindles you used you are pretty close to stock ackerman anyway so the change is going to be minimal. Since you have a I beam, the intersection point forward or rear ward on the center line would only change by the width of the axle you are using.. If you are making a little street fighter you are going to be more concerned with the weight balance of your vehicle and camber issues of the I beam.

 

Take a look at a road grader going around the corner next time it's out plowing snow. And how the blade remains level due to ackerman. That could be your car remaining balanced and level to the road while taking a corner.

 

Gary, I'm with Dick on this one.

You're so close, there's no point in obsessing.

The camber gain (or rather, lack thereof) on the beam axle will be the limiting factor in your truck's cornering ability.

If it is currently wearing radial tires and the front axle has not been bent to induce negative camber in the front wheels, your truck will tend toward understeer, because the radials on the front really want some static negative camber to generate their maximum lateral traction, and in stock form your axle is actually giving them positive camber. When you load them laterally, they'll be over on the sidewalls somewhat and thus never make the maximum lateral G that they're capable of.

The only way to fix that is to bend the axle though, so you're probably stuck with it.

The effect is made worse the wider the tires are, so simply changing to wider tires won't fix the problem either.

If you search the HAMB here, you will find a youtube video of a guy running his 34 Ford truck in an autocross. He mounted the camera on the outside of the passenger side door, so you can see the right front tire and how it deflects in the corners.

Take a look at it and you'll see exactly what I'm talking about in action. His truck is wearing bias ply tires, but the principle is the same (the radials just want more negative camber to start with).

Havi - You're going to have to draw me a picture.

 

I'm just a guy listening in. Let me say thanks to all you guys that asked the questions and all the terrific answers I really learned a lot!!

 

Dick and Coolhand,
Thanx for the info and advice. Just FYI, wheel base wise I had it my head that the 1932's WB were around 106 inches or so, hence the reason I was worried about the 9-10 inch stretch in my chassis over "stock." But if the cars thru thru 48 used the same steering components, then no worry, eh? My front end has stock specs - no camber adjustments and 7 deg negative caster.

Still, the stretch still seems like a lot to me. It justs blows my mind that it can't matter that much - given all that has been said about trying to get the Ackerman correct here on the HAMB. But why worry, eh? I'm not that hot of a solo driver and I doubt I'd ever put up ALL the cash really needed to truly make the truck super fast, anyway. Still, I'd like to be able to flick it around now and then just for fun when no one is looking without finding out the hard way something was really not right.

I'm a dot.. Gary

 

Its useless to drive with incorrect ackerman angle.

 

OK, I'm kinda following this. My car has a suicide setup, steering arms forward. I do have Ackerman issues. It is really noticable at low speeds, parking lots ect. How can I reduce or eliminate it? I really don't see any way to run my steering arms towards the rear though. Thanks, Todd

 

Ackermann steering geometry

<!-- /firstHeading --><!-- bodyContent --><!-- tagline -->From Wikipedia, the free encyclopedia
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Ackermann geometry


Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii. It was invented by the German Carriage Builder "Lankensperger" in 1817, then patented by his agent in England Rudolph Ackermann (1764–1834) in 1818 for horse drawn carriages. Erasmus Darwin may have a prior claim as the inventor dating from 1758.<SUP id=cite_ref-0 class=reference>[1]</SUP>
The intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path around a curve.<SUP id=cite_ref-Norris.2C_Modern_Steam_Road_Wagons.2C_Ackermann_steering_1-0 class=reference>[2]</SUP> The geometrical solution to this is for all wheels to have their axles arranged as radii of a circle with a common centre point. As the rear wheels are fixed, this centre point must be on a line extended from the rear axle. Intersecting the axes of the front wheels on this line as well requires that the inside front wheel is turned, when steering, through a greater angle than the outside wheel.<SUP id=cite_ref-Norris.2C_Modern_Steam_Road_Wagons.2C_Ackermann_steering_1-1 class=reference>[2]</SUP>
Rather than the preceding "turntable" steering, where both front wheels turned around a common pivot, each wheel gained its own pivot, close to its own hub. A linkage between these hubs moved the two wheels together, and by careful arrangement of the linkage dimensions the Ackermann geometry could be approximated. This was achieved by making the linkage not a simple parallelogram, but by making the length of the track rod (the moving link between the hubs) shorter than that of the axle, so that the steering arms of the hubs appeared to "toe out". As the steering moved, the wheels turned according to Ackermann, with the inner wheel turning further.<SUP id=cite_ref-Norris.2C_Modern_Steam_Road_Wagons.2C_Ackermann_steering_1-2 class=reference>[2]</SUP> If the track rod is placed ahead of the axle, it should instead be longer in comparison, thus preserving this same "toe out".

Simple approximation for designing Ackermann geometry


A simple approximation to perfect Ackermann steering geometry may be generated by moving the steering pivot points inward so as to lie on a line drawn between the steering kingpins and the centre of the rear axle.<SUP id=cite_ref-Norris.2C_Modern_Steam_Road_Wagons.2C_Ackermann_steering_1-3 class=reference>[2]</SUP> The steering pivot points are joined by a rigid bar called the tie rod which can also be part of the steering mechanism, in the form of a rack and pinion for instance. With perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point. Note that this may be difficult to arrange in practice with simple linkages, and designers are advised to draw or analyze their steering systems over the full range of steering angles.
Modern cars do not use pure Ackermann steering, partly because it ignores important dynamic and compliant effects, but the principle is sound for low speed manoeuvres. Some race cars use reverse Ackermann geometry to compensate for the large difference in slip angle between the inner and outer front tyres while cornering at high speed. The use of such geometry helps reduce tyre temperatures during high-speed cornering but compromises performance in low speed maneuvers.<SUP id=cite_ref-2 class=reference>[3]</SUP>

 

See post #8

 

You need to bend the steering arm outward (toward the drum/disk/tire) until the tie rod pivot is at least even with the kingpin. Farther outboard is better (to a point, which you won't be able to reach), so bend them until you run into clearance issues.

That will help your parking lot blues. Won't be perfect, but if you can't run the tie rod behind, this fix will at least let the car drive better at low speed and make the front tires live longer.