327/350 chev and rear mount rack?

Driveline (including transmissions, differentials, tailshafts) discussion.

Re: 327/350 chev and rear mount rack?

by benji [old] » Fri Dec 10, 2010 5:52 am

That is the best way Lobster, it gets a bit shady when your control arms are skewed in the top and side views though (like HR's), finding where the correct pivot point is for the arc the arm is travelling gets difficult. Playing around on a suspension program it took it 5000 odd different positions for it to decide where the best position for the rack to go, and it was no where near where I'd put it following the normal procedure.
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Re: 327/350 chev and rear mount rack?

by utility8 » Sat Dec 11, 2010 9:40 am

I have again found & posted this simple article for the Ackerman principle.

Geometry is geometry & can't be ignored.

If this simple principle is not adhered to, the car will not corner well.

It is not positioning of the rack that is the issue, it is the length of the rack, therefore, the angle of the steering arms that will either satisfy this principle or not.

For a HR front crossmember fitted with a front rack, to satisfy Ackermans principle, the steering arms would need to be bent outward.

For a HR front crossmember fitted with a rear rack, to satisfy Ackermans principle, standard steering arms would suffice.

Positioning of the rack, tie rod & steering arm lengths & ratios are what will vary bump steer. Bump steer is the changing angle of the stub axle as it travels up & down with no input change at the steering wheel.

Although there are many people offering front racks for HR front crossmembers, some far better than others, IT IS & ALWAYS WILL BE A COMPROMISE THAT IS FAR FROM PERFECT.

I have had one of the better designs of front rack on a HR crossmember for near on 25 years. I understand it's limitations & cope with it. If I were to build the car in this day & age, I would not travel the same path of a front rack on a HR crossmember.

Comparing Torana/HQ/Clubman/Lotus etc., or any other car for that matter that uses a front rack, when fitting one to a HR crossmember is not relevent. Race car set up is vastly different also.

Apples/apples & oranges/oranges.


For those that are interested in the theory of Front Rack versus Rear Rack on HR Front End conversions.
I have added this information about The Ackerman Principle to allow quick & easy searching.
Read on, it is well worth it.

If you follow how it works, then consider using a front rack on a frontend designed for a rear link assembly, such as a HR unit.
The outer tie rod ends will need to be where the front tyres are. Can that be done?
No, it can't.

Images 3 & 4 are typical of the problems created by a rack mounted to the front of a HR front crossmember.
Image 3 is what you have with a front mount rack.
Image 4 is what you get when trying to steer.
Image 5 is what you are unable to achieve as the front tyres are in the way.

As I said in my previous post, I have a HR cross member in an EH with a front rack. 23 years of it. It is a compromise at best. The geometry is not right & really, with what I now know, should this design of steering modification be able to be legally registered?

This information below was sourced from the following link. http://www.nationaltbucketalliance.com/tech_info/chassis/ackerman/Ackerman.asp


Maybe this will help some of you guys to understand the Ackerman principle and how it affects T-Buckets. The basic theory is that the front wheels of a 2 wheel steer vehicle with the steering on the front should remain tangent to the turning circles of each individual wheel. It is theorized that the center point of these circles falls on a line that is the same as the centerline of the rear axle housing projected out into space. The center of the both arcs is at the same point on the rear axle centerline. That centerpoint slides along that line as the amount of steering input is changed. In other words, on a small amount of turn the centerpoint is way out there, in a hard turn the center is closer to the car. In the straight-ahead position the centerpoint is at infinity. That's way, way, way out there! It's time for a little drawing to keep from confusing you with this attempt at an explanation.

As you can see, the left wheel turns on a shorter radius circle and needs to turn sharper than the right wheel to remain tangent to its turning circle. Why do these wheels need to stay tangent? That places the least amount of side loading on the tires and suspension components. In other words tire and parts wear is going to be minimized. How do we get the geometry such that this desired effect is achieved? Fortunately Mr. Ackerman came along and figured out that if he arranged the mechanical parts of the steering system so that the pivot points of the linkage (tie rod) that connected the two front wheels were closer than the pivot points for the front wheel mounting assembly (spindles with their kingpins), it would affect how the wheels reacted when turning was occurring. He apparently noticed that if he made these points such that if you drew a line from the center of the kingpin to the center of the rear axle housing and placed the tie rod ends center on that line, it would give the desired change in the angles that the two wheels turned. Old Mr. Ackerman found that he had a principle that applied universally. I bet he was pretty proud!

Time for another illustration. O.K., automobiles steered fine and everyone was happy. And along comes Joe Hot Rod and he decides that his heap needs to be nice and low. Hey, no problem. Let's move the spring back behind the axle and down nice and low. Looks great, in da weeds! Oops, small problem, no place for the tie rod to run back there. Imagine that you can see a "great idea" light bulb over Joe's head. Hey that's easy, just swap the spindles side for side and put the tie rod on the front. Look, the wheels still are connected and turn when you give the steering wheel a twist. That will cure all of my problems.

Well not quite Joe. You have just wiped out Mr. Ackerman's principle. Your tie rod attachment points (tie rod end or heim joint) no longer fall on that imaginary line. So what happens now? Well, either the inside wheel does what it is supposed to do, or the outside wheel behaves correctly, but not both at the same time. So now old Joe has that nice set of new high dollar tires grinding themselves up on the local asphalt every time he goes around a corner. The car also has a tendency to get a little quirky because the wheels can't make up their minds which one is going to be in charge of where they are going to point.

Is old Joe just screwed on this deal now, stuck with this problem? Nope! He just needs to get those pesky attachment points back over on Mr. Ackerman's imaginary line. It works just as well on a front mounted tie rod as a rear mounted one. Remember, Mr. Ackerman found out that it was universal. It depends on what he has for front-end hardware as to what he can do to correct this situation. Early Ford spindles with the built on steering arms can be heated and bent to get back out there where they should be. Just be darn careful doing it; if you don't know what you're doing find someone who does and have them do it. The aftermarket offers some parts that can take care of this problem. Sometimes special design parts will need to be made. A lot depends on the individual situation. Brake configurations (calipers and rotors mostly) can create some interesting obstacles. If you can't get out there where Mr. Ackerman says you should be, at least get as close as you can.

Lo and behold, old Joe made the changes and guess what? His lo and in da weeds bucket is cruising along life's highway is fine style. Tire life is improved, steering is less quirky (still needs a little work on the bumpsteer deal) and he is all smiles.

Well, old Joe is not alone on this deal, he's got lots of buddies with the same problem and they haven't done a thing about it. So what happens? Well they just go cruising alongof course they stop by the tire store a little more often than Joe and leave some of their hard earned. And they have to pay a little closer attention to where their missile is headed when all of the guys are out for a cruise and find that great little road with all of the curves that just beg for a guy to open it up just a tad.a tad? Yah right!

By George Barnes.

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Re: 327/350 chev and rear mount rack?

by benji [old] » Tue Dec 21, 2010 10:33 pm

Thought I'd explain my former posts with some pictures as I had some time today playing aroung with some front ends, it may make it a bit easier to understand.

The first is of a front steer rack set up in the way explained above.

I've shown the included angle of the rack rod and the spindle steer arm.

Image Insert:


The next image is with the wheels turned. Shown is the amount each wheel is turned, you can see the inside wheel is turning in. This is the dynamic toe in which is acheived when you use ackerman.

Image Insert:


This is a front end which has the steer arms moved in to clear the rim brake disc assembly. I have moved the rack back so that the included angle is still the same as the technically correct one above.

Image Insert:


With the wheels turned the same amount you can see they are both nearly the same as the second picture.

Image Insert:


Both the vehicles are identical, the rack is the same width it has only been moved back. They also have the same bumpsteer qualities to incedently.

Unfortunatly it probably wont be able to fix all the problems on a HR front end, but it may help a bit in your final positioning.

You can use the same principle on a rear mounted rack, only you move the rack forward to gain ackerman opposed to the front mounted rack set up.

If you have a chance stick your head under some late model cars and they all use this method to achieve the desired ackerman geometry

Cheers Ben.
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Re: 327/350 chev and rear mount rack?

by utility8 » Wed Dec 22, 2010 5:53 pm

And, on a HR front end, how far do you suggest to "move the rack back" to "use Ackerman"?
Positioning of the rack is limited by the design & sheer bulk of the HR front end. To accommodate the fourth image when considering a HR front end, it is not possible.
When you look under some late model cars, you will also find late model componentery & not 44 year old upper & lower control arm design.
Theoretical design is fine to rationalise principles. Applying said principles to a completely different front end is irrational.
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Re: 327/350 chev and rear mount rack?

by benji [old] » Wed Dec 22, 2010 6:15 pm

As I said in my posts, it wont be able to fix the design issues in the HR, you may be able to improve the blankity blank you have to work with to some degree. I have a popular rear mounted rack kit, and from my measurements it has neither ackerman or bumpsteer qualities that are any good, but as I posted above, by moving the rack forward and up I can improve both.
Ackerman design as you have posted is only relevent if the steering arms are correct position and the rack is dead in line in plan view, which it was not in my situation. Personally IMO the HR/mustang II front isn't worth spending the money on.

My appoligies for irrationally trying to continue a tech discusion, I mistakenly took this for a forum where people come to share ideas and help each other learn more about their hobby. It wont happen again, I will leave it to the purple circle.
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Re: 327/350 chev and rear mount rack?

by s4eh » Tue Jan 18, 2011 8:31 pm

Thanks muchly for responses. As been mentioned want to try steer clear of front mount (see what i did there) rack setup to avoid the issues with ackerman etc..Ill be getting in touch with rod in the next few weeks to sus it out more.
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