Geometry drawing of method to achieve toe in allowable 1

[dsldon]

10 years ago I restored a 1962 Mercedes Benz 190SL. Because I changed the engine I had to remove the original worm and sector steering. I installed a manual steer rack and pinion steering from a 1990 Ford Escort. After some changing of angles and repositioning I was able to remove almost all of the tow in (bump steer) to where it was not visible when bouncing the car and when driving the car fast. I have recently installed the power steering rack from the same model car and have been unable to correct the tow in problem. I don't understand exactly what I am trying to accomplish. If someone could explain the basics and the geometry involved in correcting this problem I would be very happy.

Thanks guys,

Don Harmon

PS: Steering rack is behind the axle.

 

[GregLocock]

Ow. I don't suppose you can find another original car and measure the installed geometry?

Failing that your first priority is to do no evil, that is, the optimum setting will be near having no toe change in bump, so I'll describe that.

You have 3 parameters available.

Tie rod length, vertical postion of the inner ball joint (ie the rack or the pitman arm), and longitudinal position of the inner ball joint.

Hopefully the first and last are correct, so all you have left is the height of the inner ball joint. If this is not so please describe your suspension in as much detail as possible.

With the rack behind the wheel raising the IBJ will tend to push the wheel toe in, in bump, and toe out in rebound. Dropping the IBJ will have the opposite effect.





Cheers

Greg Locock

 

[GregLocock]

Sorry, I didn't answer the question in the title.

Draw the suspension in front view at nominal ride height, and with the wheel raised by 50 mm.

Now adjust the height of the IBJ so that the tie rod (distance OBJ to IBJ) is the same length in both positions.

That will give you zero bump steer.

In practice you probably want some toe-in with bump. I'll look out some likely figures tomorrow.






Cheers

Greg Locock

 

[JakubMech]

Greg,

Can you explain the reason of toe-in with bump - in regards to production cars I assume, ive always been under the impression to achieve the minimal toe during bump as possible.
Would this be used to increase the wheel angle during turn/roll, increasing turn response?

Jakub

 

[GregLocock]

Yes. As the outer wheel loads up in a turn you get progressively less steering effect for a given steering wheel angle, so one way to compensate is to toe in when the car rolls, which is the same as bump steer. Obviously for bumps in themselves zero bump steer would be preferred.

OK Dixon gives the following range for bump steer

40 degrees of toe in to 5 degrees of toe out, per metre of jounce travel. (ie -2 to +.25 at 50 mm)

That range is so wide as to be useless as guidance, I think you'd have to have very good reasons to move away from 0 change in the first instance. This will give slight understeer as mentioned in the first paragraph, but will minimise tyre wear, and maximise stability on rougher roads.




Cheers

Greg Locock

 

[dsldon]

Dear Greg,

Thank you for the prompt reply. I like your idea of finding an original 1990 Ford Escort. I will work on that. At least I will have some original dimensions to start with. As I told you I was able to get a manual rack to work OK. Of course I didn't take any measurements before removing it and installing the power rack. Since the manual rack did work OK I took measurements on it. The distance between the inner ball joints is 22.5". This is 2.5" more than the attachment pints for the Lower A Arms (20"). I was under the impression that the Lower A arms and the IBJ should be as close to the same dimension. How much tolerance would be acceptable?

Your test for zero bump steer is not quite clear to me. You want 2 drawings, one in normal and one with the wheel raised 50mm. Is that 50mm off the floor or just jack up the car frame by 50mm? My guess is 50mm on a jack under the frame. This will take some time as I have no program to draw with on the computer.

I can measure the toe in difference when at normal and at 50mm or I can measure the toe in of each side separately by using a center line reference point on the frame. This should be enough for now.

My plan is to have a friend come over and measure the power rack that is now in the car. My problem is that I can't see very well anymore. I am 78 years old and my eyes are failing, among other things. I hope to get this problem resolved before I can't drive anymore.

Thank you,

Don Harmon

 

[GregLocock]

"I was under the impression that the Lower A arms and the IBJ should be as close to the same dimension. How much tolerance would be acceptable?"

Depends on the rest of the setup. If the rack is above the plane of the lower arms then I'd expect the tie rods to be a bit shorter than the lower arms.

"Your test for zero bump steer is not quite clear to me. You want 2 drawings, one in normal and one with the wheel raised 50mm. Is that 50mm off the floor or just jack up the car frame by 50mm? My guess is 50mm on a jack under the frame. This will take some time as I have no program to draw with on the computer."

No, this is with the wheel jacked up by 50 mm compared with the previous drawing. It only needs to have a few points on it: wheel centre, top of strut, lower arm inner bush and outer ball joint, and each end of the tie rod.






Cheers

Greg Locock