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Topic Review (Newest First)
04-11-2012 12:18 AM
enjenjo The "intersection" you are talking about is imaginary, and in my explanation it is called the instant center. It is the axis arond which the body rolls on turns. It can be, and usually is different at the front axle, than it is at the rear.

The plane of the ball joint centerline, or the king pin axis, will normally be outside the plane of the outer tie rod end on a rear steer, this is correct according to the Ackerman principal. Below is a drawing showing how Ackerman is figured.

http://www.optimumg.com/OptimumGWebS...ann_legacy.htm

Ackerman is rarely totally correct, nor does it have to be. Without Ackerman one of the front tires will scrub, or slide sideways on a turn, more or lees depending on the radius of the turn. Also tires have a slip angle, an angle somewhere between the degree of turn, and straight ahead, which the tire will slide in a turn, and this angle depends on the traction, and the tire construction. Radials as a group, have a different slip angle than bias tires. This can be taken into account when figuring Ackerman. Also the same spindle may be used on several vehicles with a different wheelbase. The basic spindle used on a GM pickup mat be used on trucks with a wheelbase of from 105" to 167". Obviously the Ackerman won't be the same for all, and in use may not be exactly correct for any of them. So usually 75% Ackerman is generally acceptable.

I hope this answered your question
04-10-2012 01:47 PM
Mutt's37Buick Can someone help me with this questions on bump steer?
In reviewing the near bump steer article they show a diagram that shows one intersection points of upper, lower, and tie rod. (See attached). This looks like a view from the front of the car.
In my suspension the tie rods do not rotate in a plane that is parallel to the control arms. (See attached).
1) it seems that I should cut a plane through the spindle, and control arms to draw this intersection. Then point the tie rod so that it intercepts the crossing point of the upper & lower control arms. Is this right?
2) Right now my tie rods point straight out and are perpendicular to the spindles. Is this best, or is it better for them to be brought closer to being parallel to the cut plane described above.
Thanks
Mutt
04-09-2012 12:35 PM
enjenjo
Quote:
Originally Posted by MARTINSR
Very well thought out and explained, thanks.

Brian

You are welcome. You motivated me to give a detailed explanation, instead of just saying this is what you should do.
04-09-2012 11:14 AM
MARTINSR
Quote:
Originally Posted by enjenjo
Here is more info that may be useful.

The first thing the length of the outer tie rod is determined by the distance between the king pin axis, a line through the upper and lower ball joint centers, and a line drawn through the upper and lower control arm inner pivot shafts, at the height above the lower arm pivot that the steering arm tie rod end center line is above the horizontal center line of the lower ball joint. So if the steering arm height is near the lower control arm height at the ball joint center, then yes, the outer tie rod would be near the same length. But if the steering arm is much higher, as it is on some spindles, the tie rod will be shorter.

You have to then consider the Ackerman angle, because on a rear steer car, the outer tie rod vertical center in as much as an inch inside the king pin axis. This will move the ideal position of the inner tie rod end in by the same distance, which would make the center link shorter. On a front steer, the opposite takes place, and the center link becomes longer with the same tie rod length.

One other thing that is not real critical on a street car, but a big consideration on a race car is the instant center. To be ideal, the outer tie rod should point to the instant center of the car to minimise bump steer caused by body roll. This can cause the tie rods to angle up or down depending on the roll center. In most cases, it will angle down toward the lower control arm pivot a bit, but for most street cars that are not driven to the edge, it is not critical.

All of this is for a perfect world, and in the real world we have to compromise. but if you can take all this into consideration, you should be pretty close to what is needed.

I have not covered everything here, there are other angles that can figure into this too. steering arm length vs center link centerline can cause problems, as can the angles between the steering arm, and the tie rod. But in most cases these are not critical, as long as you are close to correct.
Very well thought out and explained, thanks.

Brian
04-09-2012 08:27 AM
Mutt's37Buick Thanks very much for your detailed description. That along with the link you sent should give me a good start. I'll post added questions if I cannot figure it out.
Thanks
Mutt
04-08-2012 08:46 PM
enjenjo Here is more info that may be useful.

The first thing the length of the outer tie rod is determined by the distance between the king pin axis, a line through the upper and lower ball joint centers, and a line drawn through the upper and lower control arm inner pivot shafts, at the height above the lower arm pivot that the steering arm tie rod end center line is above the horizontal center line of the lower ball joint. So if the steering arm height is near the lower control arm height at the ball joint center, then yes, the outer tie rod would be near the same length. But if the steering arm is much higher, as it is on some spindles, the tie rod will be shorter.

You have to then consider the Ackerman angle, because on a rear steer car, the outer tie rod vertical center in as much as an inch inside the king pin axis. This will move the ideal position of the inner tie rod end in by the same distance, which would make the center link shorter. On a front steer, the opposite takes place, and the center link becomes longer with the same tie rod length.

One other thing that is not real critical on a street car, but a big consideration on a race car is the instant center. To be ideal, the outer tie rod should point to the instant center of the car to minimise bump steer caused by body roll. This can cause the tie rods to angle up or down depending on the roll center. In most cases, it will angle down toward the lower control arm pivot a bit, but for most street cars that are not driven to the edge, it is not critical.

All of this is for a perfect world, and in the real world we have to compromise. but if you can take all this into consideration, you should be pretty close to what is needed.

I have not covered everything here, there are other angles that can figure into this too. steering arm length vs center link centerline can cause problems, as can the angles between the steering arm, and the tie rod. But in most cases these are not critical, as long as you are close to correct.
04-08-2012 08:28 PM
MARTINSR
Quote:
Originally Posted by enjenjo
I am not chopping on you. The statement that all that matters is that the outer tie rod is the same length as the lower control arm is very broad, and not always correct. In the OPs case, with the lower control arm inner pivot shafts angled as they are, there is a very large difference between the correct lengths for front steer vs rear steer, with no other changes. The link I posted at the top, explains how to figure the correct lengths taking all the variables into consideration.

I also am not an engineer, but have engineered a lot of cars, both street and race. I have made a lot of mistakes also, and had to do it over. I am just trying to save some one from making mistakes, and having to do things over.

I am sorry if I offended you, I am sure you are answering from experience, and in a lot of cases, your answer would be close enough. But in this case, since the inner control arm shafts are angled so much, the rule of thumb is not the answer.
I understand what you are saying being I was just talking about the "basic" idea which is why I said, "I think in a nut shell the link where it pivots needs...."

I am with you that it is more complex than I led it to be, I didn't mean to sound that it was the answer that I was posting, it was more of a general answer. With the beautiful layout he posted I am sure Mutt knew that he didn't get the whole answer in my post. Thanks for clarifying though, it's all good.

Brian
04-08-2012 08:11 PM
enjenjo I am not chopping on you. The statement that all that matters is that the outer tie rod is the same length as the lower control arm is very broad, and not always correct. In the OPs case, with the lower control arm inner pivot shafts angled as they are, there is a very large difference between the correct lengths for front steer vs rear steer, with no other changes. The link I posted at the top, explains how to figure the correct lengths taking all the variables into consideration.

I also am not an engineer, but have engineered a lot of cars, both street and race. I have made a lot of mistakes also, and had to do it over. I am just trying to save some one from making mistakes, and having to do things over.

I am sorry if I offended you, I am sure you are answering from experience, and in a lot of cases, your answer would be close enough. But in this case, since the inner control arm shafts are angled so much, the rule of thumb is not the answer.
04-08-2012 06:03 PM
MARTINSR I am saying that without all the specs of distance apart the control arms are (how wide in the frame they are mounted) the "length" of the center link can not be determined.

I never mentioned location of the outer tie rod ends because that wasn't asked. The "length" of the center link has nothing what so ever to do with location of the tie rod ends.

The location of the tie rod ends in relation to the ball joints absolutely have every thing to do with Ackerman angle, but that wasn't the question.


What exactly are you after, are you simply chopping on me, sounds like it, or do you have a simple question. I am by no means an engineer of independent suspension but offered basic information in relation to the center link.

Brian
04-08-2012 05:48 PM
enjenjo
Quote:
Originally Posted by MARTINSR
I think in a nut shell the link where it pivots needs to be as close to the same area as the pivot of the lower control arm.

The "length" of the center link hasn't a bit to do with anything, other than the the pivot needs to be the same as the lower control arm so when the knuckle (spindle) goes up and down the inner/outer tie rod assemblies have the same arch. If they have a different arch they effectively will have different "lengths" than the control arm and thus you will have "bump steer".

Brian

So you are saying the location of the outer tie rod end on the spindle centerline through the ball joints , in for the Ackerman in the rear, out for Ackerman at the front does not affect the center link length? And the height of the steering arm on the spindle above the lower ball joint has no affect on the tie rod length, and the center link length? And the upper control arm inner pivot location, versus the lower control arm inner pivot have no affect on the center link length? And the height of the center link vs the outer tie rod end, and the inner control arm inner pivots has no affect? Just that the outer tie rod is the same length as the lower control arm, correct?
04-08-2012 01:44 PM
Mutt's37Buick Thanks for the explanation.
04-08-2012 12:29 AM
MARTINSR I think in a nut shell the link where it pivots needs to be as close to the same area as the pivot of the lower control arm.

The "length" of the center link hasn't a bit to do with anything, other than the the pivot needs to be the same as the lower control arm so when the knuckle (spindle) goes up and down the inner/outer tie rod assemblies have the same arch. If they have a different arch they effectively will have different "lengths" than the control arm and thus you will have "bump steer".

Brian
04-06-2012 11:13 PM
Mutt's37Buick Thanks for your help.
Definitely more involved than I thought.
Mutt
04-06-2012 06:31 PM
enjenjo Read this, it will help http://www.circletrack.com/chassiste.../photo_03.html
04-06-2012 02:46 PM
Mutt's37Buick
Center Link length on steering?

I'm working on a selecting, or designing, a center Link length for my rear steering on my '37 Buick Roadmaster. (See sketch).
I'm hoping one of you experts can help me with some questions:
1) Is it ideal to adjust the center link so the connection to the tie rods is in line with the axis of the lower control arm (as shown)?
2) If not, then what is the best design to avoid bump steer?
Thanks

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