[budetta]

I suppose the easy way to say it is to say arms should be at right angle to chassis centre line.

 

[nobody]

What are youre front uprights off. If off a car with the rack in front and the rack is still in front,youre have no issues

 

[blackpopracing]

Read this, it might help explain some things.
Steering explained

 

[budetta]

Just thought what is the rack position on the donner car ? that should put it right.

 

[AustinHarley]

Hi all.
MX5 uprights and wishbones. Track narrowed 80mm.
Escort rack, inner balljoints sit on a line drawn between upper and lower wishbone pivots.
Track rods parallel with lower wishbone, which at ride height is paralell to the ground.
Wheelbase significantly longer than mx5, so Akerman is way out because of both dimensional changes.

Moving the rack back and forward changes the steering efficiency. For a steering input resulting in 50mm left/right movement of the rack, I can get anywhere from 5 to 10 degrees of steering angle.
The ratio of the inner and outer wheel angles also changes.
I'll keep reading, but my fallback solution is to take the fore/aft measurement from the MNR chassis I have sitting here. Same wheelbase/track so it'll be in the ballpark.

 

[AustinHarley]

Borrowed from "Suspension Geometry and Computation" by John C. Dixon - ISBN 978-0-47051-021-6.

I grabbed this from another forum and it points toward what I'm trying to acheive. If I interpret correctly I think I can draw out the correct position for the rack to get Akerman sorted.

 

[blackpopracing]

To be honest Ackerman is quite flexible. I've read several suspension books and anywhere between about a metre in front or behind the diff works ok. Getting it to sit exactly on the rear axle line is only a preference - not mandatory.
From your description you have the rack in the correct position, you could heat and bend (depending on material) the TRE bars on the uprights to tweak the angle a little but the worst you will get if the ackerman is not perfect is a little more tyre wear on the front and slightly heavier steering (if you can even tell the difference).
The mileage we do in our cars you wont even notice the tyre wear.

 

[AustinHarley]

Ok, thanks for the reassurance.
I've come to a conclusion by waving the tape measure about on the Mazda, and the MNR chassis, and my own little doodle, I'm gonna set the rack back 40 or 50mm from the arm balljoints.
That's if my crossmember doesn't get in the way!

 

[budetta]

Setting it forward should be the same as well?

 

[nobody]

not the clearest picture but this was one i did earlier using mx5

 

[mygasser]

Setting it forward should be the same as well?

no, as the steering arm reaches full lock the track rod end mounting point is further back than when 'straight ahead'. if the measured difference is say 30mm mount the rack back 15mm. then when on full lock the rack is 15mm in front and 15mm behind when straight ahead. if you mounted the rack in front when straight ahead the steering angles would get more out of line as lock increased. hope that makes sense.
neil.

 

[budetta]

yes thanks.

 

[AustinHarley]

I guess I need to go through and add all the photos again?
Anyway, some progress. Been a bit slow. I think the cold just saps motivation!
Got the rack positioned, and made the front horns. The horns were a bit freehand as I'd not done any drawings. Doesn't need to be mm perfect as it only holds the bumper! It's also covered by the wing and a front apron.
Rack position was calculated and drawn so it should be in the right place.

 

[AustinHarley]

shocks and springs all round as well, so it actually rolls and bounces!

 

[budetta]

Coming along nicely

 

[AustinHarley]

This weekend I'm planning to remove the rear floor (via the riveted seams) and drop the body on the chassis for the first time. That will enable me to finish the rear of the chassis rails/rear body mount. I say finish.... I mean tack together the main bits and get the rear bumper in place.
We'll see how well all this measuring, planning, and drawing works out!

 

[AustinHarley]

Managed to stick with the plan, rear floor out, body on for the first time.

 

[AustinHarley]

It's not a great picture, but, in my opinion, that is superb. The stance is just right. Even with the grotty old rover wheels it looks cool! Might have to band the arches 10mm and can hopefully do it so the seam is far enough in to get covered up.

 

[AustinHarley]

Now I can put in the rear body mount, rear horns, bumper, fuel tank

 

[mygasser]

i think the rover wheels actually suit it