I have a Dodge Caravan, I was under it last weekend and I noticed it has a rear steer end take off rack and pinion!
Hmmmm

Does anyone know why I would not want to use an end take off rack vs the Center take off like 'normal'

It seems to me I heard somewhere that an end take off rack is better for bump steer or ackerman angle or something...

 

427V8, I tried an ETO rack from a Ford Taurus (as I recall) 2 years ago or so, I had all sorts of thing I did with that thing, and wound up taking it back as it was just too long, all I did was measure the distance between outer tie rods and assumed it would work, but when going to install it, anything but....
the thing was WAY too long....I must have not measured correctly or something, OR the rack was mounted with the tie rods out of line one way or another much more adding to their length than I figgered on....
I looked and looked in junkyards but didn't spot the minivan setup...so I dunno....

GENE

 

Does this leave us with the Steeroids kit as the only real option? If so, maybe we can stir up another GROUP PURCHASE??? :D

 

I have a rack but it puts the steering wheel on the wrong side!
It can be done, the rack length to the centre of each joint is 615mm
and it is set back a ways from where the drag link sits. I have it set up with correct Acherman angles and it gives 0.00 bump steer over 95% of the suspension travel. the last 5% would require you to be airborne so I am not worried about it!
The only short fall is that it has a much larger turning circle than standard.
Does anone have the measuremnet of the stock Acherman angles?

 

Hi James

Got any more info on that? A guy in our club used a Commodore(?) rack.

:cheers:

Chris O'

 

Chris,
So you want all my secrets before I head off to the Big Birthday bash in the US! Alright, Alright, stop twisting my arm. I also used a Commodore rack, A VL to be precise. It is modified by reducing the overall length by about 10mm each end (from Memory) to give the joint centre to joint centre distance of 615mm.
The tie rods are stock and with the appropriate tie rod ends have length of 315mm centre of joint to centre of joint.Now this total length of Rack and Tie rods exceeds the width of the front end. So what do you do? you set the rack back so the tie rod ends meet the steering arms. Now the VL rack only has a travel of 125mm. The travel required to get full lock is way in excess of this with the standard steering arms. The solution is to shorten the steering arms to 140mm from the centre of the stub axle (spindle to you americanos) to the centre of the tierod end. this gives the full lock with the 125 mm travel. By adjusting the amount of offset the hole in the steering arm you adjust the amount of Acherman angle. As I stated in the previous post I have set mine up with the theoretical perfect acherman angle. This makes it feel like its on rails around a corner but limits the turning circle.
The next hurdle is Bump steer.
The arms at 315mm are not long enough to give the arc the steering arm travels through with the suspension if they were straight. BUT because the rack is set back and the tie rods are on an angle the arc is almost identical. Hence no bump steer.
Demonstration:get a ruler, paper and a pen. with the ruler flat on the paper hold one end and use the pen to draw the arc the end of the ruler travels. Now raise the pivot point of the ruler 100mm above the paper. now draw the arc. the second arc is much shallower and imitates the arc of a much longer ruler/tierod.
You have to make sure the rack end joints have enough angular travel to cope with these angles at full suspension travel and full lock.
I am still waiting for someone to post the factory acherman angles to compare with what I have done.
By the way does anyone want another description of a way to calculate theoretical acherman?
Regards
James :cheers:

 

James,
Very interesting!
Yes please I would very much like to know how to calculate the Acherman angle!

I measured the Caravan rack last weekend and it is about 6" wider from end to end than the vette. I think it could be shortened tho, I'll have to look into it.

 

427V8,
The method requires either access to a drawing program like Autocad 2000 or a good knowledge of Math and a calculator which can work in degrees (COS,SINE TAN).
The first thing is quite simple. You need to know the track and wheelbase of your car. For my '68 I used 1160mm as the track because that was the distance measured between the the Axis of the ball joints at the height of the Axle. It is the point the wheel actually turns about.
The wheelbase is 98inches or 2490mm. Draw yourself a box and put a wheel in each corner.
For simplicity the rear wheels are fixed and we don't count toe in, camber change or suspension travel.
Going straight ahead all the axles are parallel. When we start a turn only the rear axles remain parallel.
Draw a line out of the rear axle to the inside of the corner. Because the rear wheels do not change angle during a turn the car actually turns around the line of the rear axle. This line is the radius of the turn.
For the the front tyres not to scrub out they must both have their axles pointing at the centre of the turn.
This is the point of what Acherman discovered.
OK, lets choose an angle for the outside wheel to turn. try 20 degrees. Draw a line through the axle of the front outside wheel to the extension of the rear axle line. This is the point the rear axle will rotate around. Now draw a line from the point back to the inside wheel axle. By using the drawing program to automatically calculate it you can now find the angle the inside wheel is turning to go around the same point. Or you can do what I did before I got Autocad and use your brain and Trig.

that's your homework. Now go to it!
( this is also why it feels like the nose of our Vettes moves around in front of us compared to a sedan when we go round corners. We are almost sitting on the rear axle)

:cheers:

 

Re the rack from the Caravan, It would need to be 615mm or shorter from the centre of each tierod joint. This is the length of the "rack" Usually the joints are replaceable so you can change the length of the tierods. Again the travel of the rack end to end will need to be 125mm or more to work with the 140mm steering arms. If the travel is much shorter the Steering arms need to be shorter as well to get the steering lock required. This creates greater stresses. The shorter the rack and the longer the travel makes the installation calculations easier

:cheers:

 

I am getting confussed with this post. It is talking like Acherman is designed in by the factory?
The ACKERMAN that I know is a problem associated with rack and pinion steering and it strictly involves toe out or toe in when the wheel is turned. Due to the racks position this is inevitable.
To combat this the sprindle steering arms are curved on a certain angle to create reverse Ackerman to cancel this effect.
I don't believe the factory designes ackerman into a system to go around corners and make the wheels point to any center. They just want the toe to remain constant.
Bump steer is a whole seperate problem related to tie rod length and angles between the upper and lower A arms.


[Modified by norvalwilhelm, 12:51 PM 5/14/2003]

 

All cars have ackerman in ther steering. the inside wheel must have a tighter turning radius or the inside tire would scrub.

 

Just to clarify, our Australian friends are talking about using a rack from a Holden Commodore (a car manufactured by GM in Australia). They could be a bit tricky to source here in the US of A, unless GM uses this rack on another vehicle available in this market...

 

I have got to get me a new suspension book. My last one tried to get rid of ackerman. Bad book. :) :)

 

Actually the acherman calculations don't sound all that bad.

I guess this is another thread to save...

 

Thanks James! :crazy: I wish I had known this at the Convention - I could of had a good look at what you have done.

Have a great trip and share the pics when you get back.

:cheers:

 

Let me see if I got this straight.. I need to fly to australia and find a Holden to get a used rack then get a 4 year physics degree to figure out how to ionstall it just to 'save' me some money on a rack and pinion steering rack? Glad I only spent a grande on Steeriods. :p:

J/K I find it interesting but sometimes you got know your limits, I figure it would take me 100 hrs to figure out how to graft a junk yard rack on to me vette. That's like getting paid $10/hr for my time. :eek:

 

I don't know about you but this is a hobby to me....like camping?

 

why use all kinds of programs? just set the steering to straight ahead and draw lines from the steering knuckle through the ball joints. If the 2 lines meet at the center of the rear axle (differential) you have 0 toe, if they meet behind the diff you have toe in, in front of it toe out.

Look at this:



(spindle is on backwards.. I know)

Either I'm blind or there's no ackerman here. Now if you had manual steering the hole in the knuckle would be farther forward and you'd have some ackermans

Now for bump steer, if one could get the thing perfectly aligned with the lower control arm it would have nearly 0 bump steer throughout the suspension travel,. The steering rod would be the yellow line, the place where the yellow intersects the green on the inside would be the pivot point for the steering rod. This will only work with an ETO.



and, Holden Commodore, isn;t that the same as an Opel Omega and a Cadillac Catera???

 

Guys after reading all the replys I am getting the impression that most of you don't know what ackerman is. You certainly can't measure Ackerman with the wheels pointing straight ahead. Car's do not turn with one wheel tighter then the other , both are to turn an equal amount.
Ackerman is what happens when the wheel is turned and the toe suddenly takes a jump.
Sure you can look at a rack and see if it is going to have ackerman or reverse ackerman but it is strickly to do with where the centerline of the rack is in relationship to where the centerline of the 2 ball joints. If the rack's centerline is behind the centerline drawn between the 2 ball joints you have an ackerman effect. If the rack centerline is in front of the ball joint centerline you have reverse ackerman.

 

This is a very intresting topic. I am ABSOLUTLY with out a doubt going to have to go to rack and pinion, Considering my headers just won't do with the box in the way. Which really pisses me off considering I really wanted to just finish the car and enjoy it. Now it looks like I am going to have to spend another grand before I can get to that point. I am going to have to research both sides of the story, "steeroids/complete custom chop job" if I "CAN" key work is Can, do it my self and it would come out much cheaper then I am going that route.