Undead Sleds - Hot Rods, Rat Rods, Beaters & Bikes... since 2007!

Go Back   Undead Sleds - Hot Rods, Rat Rods, Beaters & Bikes... since 2007! > Frame/Suspension/Brake... Talk & Q&A

Frame/Suspension/Brake... Talk & Q&A All things... Frame/Suspension/Brake

Reply
 
Thread Tools Display Modes
  #1  
Old 11-06-2017, 08:35 PM
Neto Neto is offline
Super Member
 
Join Date: May 2012
Location: Holmes Co, Ohio
Posts: 261
Default Ackerman Principle & King Pin Inclination

I generally prefer to post to existing threads, but the closest to what I wanted to discuss is in a thread that is over 10 years dead. That's a pretty old horse to dig up.

http://ratrodsrule.com/forum/showthr...light=ackerman

So here goes with a clean shot. I do know that the Ackerman Principle is said to be the least of suspension sins, but I want to understand how it is applied.

Here's my question: Isn't it important to consider the KPI, and where it points to on the ground, rather than just go by the top center of the kingpin itself? That is, if the KPI points to the center of the contact patch (which I understand to be in general the ideal objective), shouldn't the line running thorough the center of the idler arm & outer tie rod end back to the center of the rear axle be aligned with the point on the road surface where the KPI points?

(This may sounds like a stupid question, and maybe it is, but if I don't ask, I stay stupid.)
Reply With Quote
  #2  
Old 11-06-2017, 11:16 PM
MercuryMac's Avatar
MercuryMac MercuryMac is offline
Builder Junky!
 
Join Date: Jan 2013
Location: Northern Alberta, Canada.
Posts: 2,160
Default

Neto, you have made me think, whooo-boy, with your angles question. I have made up an answer in my way of thinking. Here it is.
The lines that you draw from the road contact through the tie-rod end and to the rear-end might have to be hitting the rear-end 6" or 8" on the other side of the centre of the rear-end for each front wheel Ackerman angle, whereas the other guys lines are supposed to hit at the centre. It's just easier to explain if you use the top of the kingpin and the centre of the rear-end, but the principle is the same.
The main objective for both sets of lines is to get the front wheels to steer at different rates [the inside one faster than the outside one, and the dang tie-rod stays the same length all of the time]. If your steering arms are bent inwards to sit on the lines previously drawn, then as you steer, they each rotate around their kingpin, the inside steering arm to tie-rod angle gets closer to 180* and the outer tie-rod to steering arm angle gets more to 90*. The angle of the 180* changes quite a bit faster than the 90* one with the movement of the tie-rod.
Reply With Quote
  #3  
Old 11-07-2017, 07:13 AM
Neto Neto is offline
Super Member
 
Join Date: May 2012
Location: Holmes Co, Ohio
Posts: 261
Default

Quote:
Originally Posted by MercuryMac View Post
Neto, you have made me think, whooo-boy, with your angles question. I have made up an answer in my way of thinking. Here it is.
The lines that you draw from the road contact through the tie-rod end and to the rear-end might have to be hitting the rear-end 6" or 8" on the other side of the centre of the rear-end for each front wheel Ackerman angle, whereas the other guys lines are supposed to hit at the centre. It's just easier to explain if you use the top of the kingpin and the centre of the rear-end, but the principle is the same.
The main objective for both sets of lines is to get the front wheels to steer at different rates [the inside one faster than the outside one, and the dang tie-rod stays the same length all of the time]. If your steering arms are bent inwards to sit on the lines previously drawn, then as you steer, they each rotate around their kingpin, the inside steering arm to tie-rod angle gets closer to 180* and the outer tie-rod to steering arm angle gets more to 90*. The angle of the 180* changes quite a bit faster than the 90* one with the movement of the tie-rod.
What got me thinking about this is that in some of the discussions I've read, they get into IFS, with either upper & lower control arms, or even struts. So then they say that KPI is a bit of an archaic term, and refer to Steering Axis Inclination (SAI). With the traditional kingpin design, the question never came to mind, because I just assumed that you would measure from the top center of the king pin - no confusion. But with these other designs, the question is Where on the strut, or Where on the IFS axis do you measure from? So I started thinking that the real point of pivot is the point at which the KPI (or SAI) hits the road surface.

So here's another question: Do oversize tires change the Ackerman on a given vehicle? (Because it certainly would affect the point in the contact patch that corresponds with the KPI, right?) For instance, these (crazy) guys that put 20" wheels on a car that came with 14's. That creates increased positive scrub radius, right? (Again, I'm just trying to wrap my head around all of this.)

I also had thought that plotting the Ackerman angles was just something that you would do with the wheels straight ahead. On another site, a guy said that you are supposed to be able to plot it out for any degree of turn, which would then show the different wheel angles at different stages of the turn, or tightness of turn. (The purpose of this, as I understand, would be to figure out the turning radius of a given steering design.) Then he just flat out said that it doesn't work. But (with the wheels turned) I think he may have been trying to plot through the king pin and steering arm pivot points (like you do with the wheels straight ahead), instead of plotting through the spindle out to the point at which these lines from the two wheels intersect, which, if I understand correctly, (for 100% Ackerman) should be exactly on a line extended out through the rear axle.
Reply With Quote
  #4  
Old 11-07-2017, 12:08 PM
MercuryMac's Avatar
MercuryMac MercuryMac is offline
Builder Junky!
 
Join Date: Jan 2013
Location: Northern Alberta, Canada.
Posts: 2,160
Default

You got me thinking again, sheesh, two days in a row, now. For your first paragraph question, I would say just run your string from the centre of the rear-end, over the top of the tie-rod-end hole, to the imaginary almost-vertical line between the ball joints, [because the steering arm is pivoting around the king pin or steering axis, level with the tie-rod hole].
For the second question, [big tires], I don't think the Ackerman angle would change even though the positive scrub would, but I really don't know. Some of the other guys must have opinions on these weighty subjects.
I agree with your last paragraph, that the lines we draw from the rear centre to the king pin should be done with the steering tires pointing straight ahead. If we draw lines after the wheels are turned, then we better draw them at 90* to each wheel and find where they converge a way out to the side in line with the rear-end housing.
Reply With Quote
  #5  
Old 11-07-2017, 01:14 PM
Neto Neto is offline
Super Member
 
Join Date: May 2012
Location: Holmes Co, Ohio
Posts: 261
Default

Quote:
Originally Posted by MercuryMac View Post
You got me thinking again, sheesh, two days in a row, now. For your first paragraph question, I would say just run your string from the centre of the rear-end, over the top of the tie-rod-end hole, to the imaginary almost-vertical line between the ball joints, [because the steering arm is pivoting around the king pin or steering axis, level with the tie-rod hole].
For the second question, [big tires], I don't think the Ackerman angle would change even though the positive scrub would, but I really don't know. Some of the other guys must have opinions on these weighty subjects.
I agree with your last paragraph, that the lines we draw from the rear centre to the king pin should be done with the steering tires pointing straight ahead. If we draw lines after the wheels are turned, then we better draw them at 90* to each wheel and find where they converge a way out to the side in line with the rear-end housing.
Yes. My understanding is that (with the wheels turned clear to lock) the distance from the center of the outside tire out to that point where the two lines cross, is the turning radius. That's where that part gets practical, at least in my mind. (Because you want to be able to turn around in less than a football field, or the "back forty".)

(And thanks for your responses. I can only think something through so far by myself, then I'm just left with lots of questions.)
Reply With Quote
  #6  
Old 11-07-2017, 01:24 PM
Old Iron's Avatar
Old Iron Old Iron is offline
Rusty Rat Rodder
 
Join Date: May 2011
Location: Willard Mo.
Posts: 8,914
Default

Akerman on a street driven vehicle is a general setting. The circumstances are constantly changing in this scenario.
On a race car it is critical, akerman is set for the corner radius that the vehicle will experience on that particular track then, changed for the next track raced on. The track width of the vehicle dictates what that setting will be. A narrow vehicle will have less akerman than a wide vehicle.
This is a way to see how much farther the outer tire will travel apposed to the inner and how much tighter circle is.
The two lines are the most common track width of a vehicle.
Attached Images
File Type: jpg Akerman.JPG (88.4 KB, 87 views)

Last edited by Old Iron; 11-07-2017 at 01:42 PM.
Reply With Quote
  #7  
Old 12-22-2017, 09:18 PM
Neto Neto is offline
Super Member
 
Join Date: May 2012
Location: Holmes Co, Ohio
Posts: 261
Default

Quote:
Originally Posted by MercuryMac View Post
You got me thinking again, sheesh, two days in a row, now. For your first paragraph question, I would say just run your string from the centre of the rear-end, over the top of the tie-rod-end hole, to the imaginary almost-vertical line between the ball joints, [because the steering arm is pivoting around the king pin or steering axis, level with the tie-rod hole].
For the second question, [big tires], I don't think the Ackerman angle would change even though the positive scrub would, but I really don't know. Some of the other guys must have opinions on these weighty subjects.
I agree with your last paragraph, that the lines we draw from the rear centre to the king pin should be done with the steering tires pointing straight ahead. If we draw lines after the wheels are turned, then we better draw them at 90* to each wheel and find where they converge a way out to the side in line with the rear-end housing.
It has been awhile since you posted this reply, but I continue to think about this, and have another question, this time relating to a front-end design where the tie rod is in front of the axle. OK, here goes. It is said that it is very difficult to get anywhere near correct Ackerman on this design, because the idler arm end needs to be over into the tire to achieve that. So, IF the imaginary line back to the center of the rear axle passes from the center of the tie rod end on the idler arm, through a point in the King Pin Inclination THAT IS LEVEL WITH THE HOLE IN THE IDLER ARM, then it seems to me that you can get closer to 100% Ackerman on a front end with this configuration by raising the idler arm in relation to the spindle as much as possible. This would be because the KPI slants toward the middle of the vehicle as you project it upwards.
Conversely, in a front end design where the tie rod is BEHIND the axle, it would seem that moving the idler arm down lower than the spindle height would increase the Ackerman angle.
So, am I completely nuts??
Reply With Quote
  #8  
Old 12-23-2017, 02:57 PM
MercuryMac's Avatar
MercuryMac MercuryMac is offline
Builder Junky!
 
Join Date: Jan 2013
Location: Northern Alberta, Canada.
Posts: 2,160
Default

Neto, I have to come clean here, I don't KNOW everything, but I like to think everything out. So, thank you for taxing my brain sometimes.
When you asked about front steer designed cars you partly answered your own question. Factory designed cars have their steering arms right out there near the tires or running in a smaller radius than the inner part of the wheel and inside the wheel. It's the guys who put a rear steer axle in their hotrod backwards and call it front steer who are in for a lot of tire squealing and head scratching, on hard turns, because then the Akerman angle is a way off. The reason I said you partly answered your own question is, in this last scenario --- nobody designed the steering geometry, and you asked about a Designed front steer set up.
I think, for practical purposes, the steering arm [or idler arm] rotates around the steering axis or kingpin at the level of the tierod end. So raising the tierod end should help you get the right Akerman angle on front steer arrangements.
Then you asked "Am I completely nuts". Well Sir, ------ I myself have walked beside the path sometimes and so has everyone else on this forum. We are all creative people so we think outside the box a wee bit. Welcome to the club.
Reply With Quote
  #9  
Old 12-23-2017, 04:26 PM
Neto Neto is offline
Super Member
 
Join Date: May 2012
Location: Holmes Co, Ohio
Posts: 261
Default

Quote:
Originally Posted by MercuryMac View Post
Neto, I have to come clean here, I don't KNOW everything, but I like to think everything out. So, thank you for taxing my brain sometimes.
When you asked about front steer designed cars you partly answered your own question. Factory designed cars have their steering arms right out there near the tires or running in a smaller radius than the inner part of the wheel and inside the wheel. It's the guys who put a rear steer axle in their hotrod backwards and call it front steer who are in for a lot of tire squealing and head scratching, on hard turns, because then the Akerman angle is a way off. The reason I said you partly answered your own question is, in this last scenario --- nobody designed the steering geometry, and you asked about a Designed front steer set up.
I think, for practical purposes, the steering arm [or idler arm] rotates around the steering axis or kingpin at the level of the tierod end. So raising the tierod end should help you get the right Akerman angle on front steer arrangements.
Then you asked "Am I completely nuts". Well Sir, ------ I myself have walked beside the path sometimes and so has everyone else on this forum. We are all creative people so we think outside the box a wee bit. Welcome to the club.
Thanks for your response. I hadn't known that people didn't significantly modify the steering geometry for front steer vehicles. But I now think I (finally) understand how this is to be figured. The deal about going by the level of the steering arm makes it all clear to me. I wasn't too sure at all about my previous suggestion that it should go by the point at which the KPI projects to the road surface, but I couldn't understand why it would not figure into the equation in some way or another. I figured there had to be some main reference point, and it makes perfect sense now - that it is the level of the steering arm, as you stated.
Thanks again, and Christmas Blessings to you & your family.
Reply With Quote
  #10  
Old 12-24-2017, 04:17 PM
Dr Crankenstein's Avatar
Dr Crankenstein Dr Crankenstein is offline
Rocket Surgeon
 
Join Date: Mar 2011
Location: SK CAN
Posts: 4,303
Default

I have to disagree with my friend Mac. (My apologies if I misunderstood your post. I'm good at that.)


Raising the steering arm will not affect Ackermann. It will raise the tie rod.

Look at the diagram views below. You'll notice distances and angles are measured from the point marked KPI @ the spindle centerline and center of the kingpin boss. This is ground zero.

You'll also notice the "tie rod angle" reference lines do not pass thru the top center of the kingpin, but slightly offset, again, to coincide with the point marked KPI.

To change Ackermann, is to change the "tie rod angle". The steering arm could be three feet tall and have no effect whatsoever, assuming measurements "G" and "H" remain the same...




.
Attached Images
File Type: png Ackermann KPI.png (109.7 KB, 34 views)
__________________
- The distinction between psychological illness and creative thinking is wafer thin -
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -5. The time now is 08:40 PM.


Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Rat Rods Rule Inc.