Is Ackerman really that important?

So if I getting this right, the use of original unaltered spindles were designed for a particular wheel base. However no one has commented on what happens when they are put on a different axle, or the axle is dropped, which changes the king pin centers. In my particular case I have late 40's square back spindles on a stock undropped model A axle. I have lenghtened the wheelbase somewhat, although I can't remember the final number off the top of my head. My take away is that I'm probably in the ball park akerman wise, because the later axles were wider, and my wheel base is longer but not extremely longer.

 

No it doesn't.

Measured from the wheel in, the kingpin is in exactly the same position.

Geometry wise, all that changes with a dropped axle ( unless its narrowed as well as dropped ), is the attachment point of the spring.

Which affects the ride height and front roll center.


Edit.

A wider axle will move the point where the lines going from the steering arms cross the centerline back, slightly.
( on a rear steer setup )

So it will very slightly decrease the Ackermann.

 

On the diffrerent straight axle cars I've worked on. I always notice the front mounted tie rod axles always pushed in the turns especially at low speed. I've even experienced speed wobble with these setups.

 

Also to note is scrub radius.

Ackermann angle for figuring the turning radius'

The Scrub radius from the pivot point in inches from the center of the kingpin/balljoint. 3" is max if I remember right.

Together these can both cause ill effects in handling and turning.

Using stock spindles, hubs and wheels will probably make this point mute.

 

Actually, scrub radius is the distance from the CL of the tire contact patch to the projected intersection of the kingpin with the ground line. If the kingpin was vertical, the scrub radius would equal the distance from ball joint to wheel CL, but since the kingpin is always inclined somewhat, you have to project a line through the kingpin to the ground and take that distance.

The larger the scrub radius, the more the wheelbase changes when you turn the wheels (outside wheelbase gets longer, inside wheelbase gets shorter). In extreme cases, this can cause the car to feel/act very twitchy to steering inputs. It will be very stable in a straight line though, as both wheels will want to toe out very badly, keeping the entire steering system loaded at all times (no slop = no wandering).

Large scrub radius also leads to high steering forces, which can be a problem if you have manual steering.

3" is about all you'd ever want on a street car, less will be easier to steer. The lower you go, the less feedback the driver gets (less "feel" for the driver). This is why some folks have a hard time driving a race car with zero scrub radius, 'cause it's got no steering feel, by design.

As you say though, the only control you have over scrub radius is wheel offset. Everything else is set by the OEM when they forge and machine the parts (unless you are the OEM, in which case. . . ).

 

Mrs. Ackerman might think so!

 

You mean positive caster, right?

BTW, 7* positive caster is excellent for high speed handling, which I'm sure that's what you meant. I put 6* caster shims on my straight axle, can't wait to try it out.

As far as adjustments go, Ackerman would only need checked if the frame has been modified or the axle has been moved from the orig position. If toe, caster, & camber were out of adjustment, you would "feel" this a lot more, especially toe & caster. You could write an entire book about the science of front end geometry & alignment, but in a nutshell, this is what most people want:

Returnability - this is the ability for the steering wheel to return to a straight ahead position after turning. Caster & toe are a big factor in this. It can be hard to drive a car without this feature. See Coolhands "scrub" note above - 3* pos.

Straight line stability - the ability to let go of the steering wheel & have the car go straight without any darting or correction needed. Toe & caster are the factors for this is that order, along with tire type (bias ply vs radial)

Cornering - Since it's difficult to get a car to handle perfectly in a corner, IMO, it's more important to have an understeer condition (when driven to the limit) rather than oversteer. It's much easier to correct an understeer condition while it's occurring than oversteer, unless you drive a road race car for a living. Caster, camber, & Ackerman are the influences here.

BTW, oversteer is what causes more movie stars to crash Vipers, Porsche's, Corvette's & Ferrari's

 

I think Coolhand pretty much nailed the way Chevy sets up the front end on a lot of their cars/trucks I have an 88 van and it scrubs like crazy on gravel @ low speeds but drives fine down the road. I think that why people like 80's Camaro front clips on their dirt cars because the big scrub radius makes the dart into the corner which frees up the rear end. This is good on a track with tight corners but on a track with big sweeping turns less scrub would probably be better. Great topic in any event

 

I like your explaination better. I was keeping it simple for good reason, although I could see where it might have caused more confusion, thanks for tuning it up.

 

I know what you mean. I have about 2.75* pos caster in my 71 vette & it felt good until I removed about 100 lbs from the motor ( installed alum heads, intake, etc). Caster works better if there's more weight pushing down on the turning tires.

 

Sorry if this is beating a dead horse but I am confused. Unless I don't understand what the post below is saying (Post 8). This looks like, when I draw it out anyway, that the lines from the king-pin centers through the tie rod end will intersect at the center line of the axle about the same distance forward of the axle as the rear yoke is behind. Which means a "short" tie rod. VERY crude drawing below

The one below indicates a "long" tie rod so that the tie rod ends are closer to the wheel. So that the line through the tie rod ends passes through the king-pin centers a intersect at the center of the yoke. The same extended line as for a rear tie rod location.

Crude picture. Top is Post 8, short tie rod. Bottom is the other post, long tie rod and conventional rear tie rod.



My T. Tie rod ends out toward the wheel as far as I could. The lines from the tie rod ends through the king-pin centers intersect pretty close to the center of the rear yoke.




It looks to me that the inner wheel is turning sharper than the outside wheel. This would be reasonable Ackerman. So I think I have it right. Just trying to help with my confusion.

Bill

 

LongT...You've got it! It's not EASY to see with your wheels turned away from "straight-ahead", but it does look like you're at least in the ballpark. You understood well enough that the steering arms on the spindle (tie rod end attach points) needed to be bent farther outward (necessitating a longer tie rod) to keep that imaginary line straight from the tie rod end, through the king pin center line, and on to the center of the rear end. You obviously have the theory well in hand. Nice T-bucket! DD

 

The bottom drawing is right on. Your T looks like its in the ball park.

Lenghty wheelbases and a short tract width will make any rod a bear to turn. This is true as ackerman narrows. Turning radius is larger.

 

Thanks. I took the picture to show that the inside wheel appears to be at a sharper angle than the outside one.

Bill

 

Ding Ding Ding! Thats a winner! perfect example of what to do. The top drawing you made is what happens when someone flips a rear steer to front steer without taking ackerman into account.

The second drawing is after the arms have been bent out toward the wheel so the ackerman line meets toward the rear axle center. You can only work with what you've got and only bend them so far before clearance is an issue.

Nice Car!

rick

 

You nailed it man....
Cool little T too!

 

Excellent.

I've stolen these pictures to redeploy any time a nay-sayer declares that tie rod in front can't be done.

Very nice T, sir.

 

As long as this thread is going on lets keep the information rolling.

This is a good example of a front steer application with Ackerman principle. The convergent line principle is similar to the diagram of "long T" however this only works in a limited number of applications and only with the correct designed parts. If you examine most newer front steer vehicles you will see that the tie rod end/steering arm end is positioned to the outside of the longitudinal center line of the spindle hub inclination. That is indicated by the top view of the T bucket, even though this is a straight axle the steering junction falls outside the king pin axis center and this approach is correct. I believe these parts are Total Performance chevy spindles with the bolt on steering arms. This is one of the few applications that work.

The most common application leading to problems is to use the stock or hot rod Ford style spindles which are designed for a rear steer vehicle. When reversing this application things get skewed out of wack because it was not the intended design parameter. Using Ford steering arms in front steer the positioning of the steering arm/tie rod end is inside the spindle inclination longitudinal center line and this creates an over rotation of the outer tire and outer tire scuff. There are no steering arms made to conform to front steer dynamics for these spindles and bending arms do not lead to sucess.

To further add information the only way most of the front steer straight axle kits work is to use disc brakes. The offset of the disc rotor provides the addition clearence necessary to bring the steering arm/tierod end into the correct alignment position for adequate Ackerman principle.

Attempted front steer application with drum brakes of any kind do not provide the additional clearence necessary to get the correct steering arm/tie rod end relationship. The steering arm angle conflicts with the backing plate and the alignment falls inward of the spindle angle resulting in the over rotated Ackerman angle.

 

There is a lot of great info here. To sum up for the original poster, you can drive a car without exact ackerman and not die. However, just like everywhere else on the car, the more attention to detail you spend in the steering geometry, the better the overall product will be.

 

Great info, thanks to all who posted. Now, I have a long driveway with a big circular end for easy truck/trailer turn-arounds. It is ABC material means compacted sand/rock mix. Anytime someone passes thru with a late model vehicle (mail deliv., meter reader, whatever) it re-arranges my roadbed due to tire scrub. Easy to see, it tears a groove in it almost. My old rides don't do that to it. Up in the City I hear the tires squeal as modern cars turn tightly thru convenience store & gas station smooth cement parking areas at walking speeds. My '55 just silently cruises thru. Is this due to a neglect of ackerman pricpals by modern engineers?

 

Modern suspensions use a twist on old suspension geometry. The SAI or steering axis inclination is varied with design. Moving the pivot points outward for the upper ball joint causes the outside of the tire to recieve higher pressure durring turning than older style suspensions. Making the contact patch of the tire to the road "better".

Go crawl underneath a new car and check out the suspension design sometime.

There is much more to say about this but I think it would be mute and maybe cause confusion.

 

Many good posts here. Kudos to contributors. Now, my F100 with Twin I-Beam and a race car trailer on the back seems to...oh, never mind. Ha! Ha!

 

I think that it is likely that this is more due to the differences in what a radial tire wants from a suspension, and what a bias ply tire wants.

Radials need more slip angle to generate their maximum grip (which is higher than a bias ply tire BTW). They also need to be leaned slightly away from the direction of load (IE the outside tire when turning a corner wants a little negative camber) to generate their peak grip. Radials also complain more while they're giving their maximum grip. A radial will squeal and whine for a long time while it's getting up to its maximum grip. If it isn't squealing at all, you're not getting everything out of it. It's very easy to make a radial holler at low loads and/or low coefficients of friction (say like low speed turns in a parking garage with a slick sealed floor).

Because the radial is the tire of choice for basically all new cars, the suspension is designed around them, and since pavement is pretty much the universal road medium now, designers don't worry a lot about what the contact patch and traction circle look like on gravel or dirt surfaces.

The tires need some thrust to make their max traction (that's what camber and slip angle provide), so the suspension design takes this into account and provides it. If the surface you're driving on has a very low coefficient of friction or is easy to push around, the thrust provided will overcome the road material and either plow a furrow in it or make the tires squeal at super low speeds while driving poorly.

Old cars had suspensions designed to drive on anything reasonably well, wearing bias ply tires (which want to be upright and given as little thrust as possible), and not bottom out over big chuck holes or cow path ruts.

New cars have suspensions that are optimized to make the cars handle beautifully on standard pavement with radial tires.

As always, when you optimize one aspect of a system, you generally compromise another. Nothing is free. The designers just decided that increased pavement performance was worth the tradeoff.

 

Ditto