Ackerman caster camber scrub

[ebinmaine 67,717]

Let's talk about steering geometry, shall we?

 

I have enough of an understanding of the terms in the title to get myself in trouble but I'm certainly no mechanical engineer.

 

What prompts the question is wondering how I should construct the front steering for the Colossus project.

I'm going to be using trailer stubs and five lug trailer hubs which originally were going to be welded onto the end of some Wheel Horse spindles.

 

I know several have done that with success.

 

I think it's important to keep in mind that the maximum speed of this tractor is probably going to be five or six miles per hour. Also our rough ground and nearly zero smooth traveling except at the occasional show or gathering.

 

I think the Ackerman angle is going to be fine and the caster is not going to be adjustable and neither one of those really matters much at these very slow speeds.

 

Tow-in I'm going to set around 1/8 or 1/4 and call it close enough.

 

My biggest concern is camber.

I see a lot of tractors that have a very positive camber. Having done some reading on the interwebs I found that positive camber to varying degrees can assist with making steering effort easier.

It also puts you closer to a zero scrub angle which I realize are both tied together. 

 

The one big downside that I can see to this is that obviously the outside edge of the tire is going to wear quite a lot faster than the inside edge and I have tri ribs on there so I want the center of the tire to grab.

 

I'm going to have a set of spindles made and then weld them to the trailer stubs. I'll be able to have the angle of the spindles set. But only once.

 

What type of camber should I go for?

 

 

@OldWorkHorse @Achto @Lee1977 @Stormin @953 nutnut @Greentored @Stepney @Oldskool @Ed KennellKennell @Handy DonDon @WVHillbilly520H @WHX24 

 

[Pullstart 62,923]

You’d be amazed how little camber effects the tread wear of a properly inflated tire...  I don’t understand how, but the tread still meets the surface and the sidewalls squarsh a bit to make up the difference.

 

This guy explains a lot in this video!

 

 

[Stepney 2,325]

Look no further than the front end of a Farmall Cub for .. camber.. My tires are almost definitely the originals from 1951 and still have decent tread.. those are tri rib mind you. It steers very easily and is quite maneuverable. There is atleast a 10-15* list outward from the axle to the top of the tire and its very noticeable. Bites good on the dirt.. almost too good on hardtop. If I were on pavement a lot I would worry. In your case itll almost surely be a benefit. I wouldnt worry about wear for atleast 20 years or so in your use..

 

[Achto 27,624]

This is an interesting question. Most IH row crop tractors have a very noticeable positive camber. On a narrow front tractor, positive camber helps to clean the mud out from between the tires. A necessity with a narrow front. I always figured that IH for some reason carried this on when they started putting wide fronts on a row crop tractor. . The positive camber is not as noticeable on their standard tractors. Most of the other brands of farm tractors also have slight positive camber but not as noticeable as IH.

 

Early Cub Cadets also had a very noticeable positive camber. Most other Gt's  have a slight positive camber but you almost need an angle finder to see it.

 

I guess I would suggest positive camber but I'm not sure that you need to go to IH's extreme.

 

 

Edited January 11, 2021 by Achto

[ebinmaine 67,717]

10 minutes ago, pullstart said:

You’d be amazed how little camber effects the tread wear of a properly inflated tire

I'll add two thoughts to that.

 

 1. These tires are fluid filled and I'm probably never going to check the air pressure again....

 

2. Some of the older farm tractors with wide fronts even had the severely tilted tires and wheels kind of like the narrow front end tractors did.

 

I'm thinking about creating a serious angle like Spenser refers to above.

[ebinmaine 67,717]

Just now, Achto said:

I guess I would suggest positive camber but I'm not sure that you need to go to IH's extreme

 

 

Do you think it would help to have a positive camber enough so that I could try to get the scrub angle down closer to zero? Or should that not be a huge concern?

[Achto 27,624]

2 minutes ago, ebinmaine said:

Or should that not be a huge concern?

 

I don't think that it would be a huge concern.

[Oldskool 6,644]

5 minutes ago, ebinmaine said:

 scrub angle down closer to zero? 

Im just seeing this post. What are you referring to as the scrub angle?

[ebinmaine 67,717]

4 minutes ago, Oldskool said:

Im just seeing this post. What are you referring to as the scrub angle?

Here's a quote from carthrottle.com

 

Essentially it's defined by the location of the point in space where an imaginary line traced down through the centre of the suspension column ...

 

So in my case if you were to draw a line down through the center point of the spindle it would be the point at which that line touches the ground.

It's easier to steer if the center of the tire is as close as possible to that point.

 

 

[Achto 27,624]

8 minutes ago, Oldskool said:

Im just seeing this post. What are you referring to as the scrub angle?

 

Here is a little read for you.

https://www.carthrottle.com/post/what-is-scrub-radius-and-why-is-it-important/

[Oldskool 6,644]

Just now, ebinmaine said:

Here's a quote from carthrottle.com

 

Essentially it's defined by the location of the point in space where an imaginary line traced down through the centre of the suspension column ...

 

So in my case if you were to draw a line down through the center point of the spindle it would be the point at which that line touches the ground.

It's easier to steer if the center of the tire is as close as possible to that point.

 

 

An I wrong in that being caster?

[Oldskool 6,644]

Just now, Achto said:

 

Here is a little read for you.

https://www.carthrottle.com/post/what-is-scrub-radius-and-why-is-it-important/

Ok got it

[ebinmaine 67,717]

Just now, Oldskool said:

An I wrong in that being caster?

Caster is the fore and aft tilt. It's caused by the center line of the kingpin being tilted forward or backwards.

 

Scrub angle can be affected by caster and or camber being changed.

 

[Oldskool 6,644]

Just now, ebinmaine said:

Caster is the fore and aft tilt. It's caused by the center line of the kingpin being tilted forward or backwards.

 

Scrub angle can be affected by caster and or camber being changed.

 

I knew what it was just trying to see if we were on the same page. Thanks

[8ntruck 7,025]

Positive camber might be there to center the tire contact patch over the spot on the ground where the steering axis of the spindle intersects the ground to minimize the scrub angle.

 

In off road use on a solid axle and at low speed, it probably won't make much difference.

 

Most antique cars that ran solid front axles had positive camber.

[Oldskool 6,644]

@ebinmaine your front axle is already built? Correct? If so wouldnt the scrub angle be somewhat built in along with your caster (pivot pin,) and camber ( kingpin bosses)? I realize the scrub angle may play part of it but if the trailer hubs are going to make your track width wider then wouldnt that make that part of the geometry moot? If I was doing it I would copy the original spindles with a dial type angle finder. Why reinvent the wheel. Once together install long enough to run your straight edge back to the center of the rear for your ackerman then tack your steering arms on.

[Handy Don 12,258]

Good questions.

I agree with @pullstartthat in your use case (low speeds, unpaved and rough ground, numerous small steering corrections, properly inflated tires) tire wear isn't a big factor

• The Ackerman calculations are based on the wheelbase and the track width of the front tires on their tire patch centerlines plus the lengths of the steering arms and the kingpin to centerline distance (it starts complicated and gets even more more complicated if the front and rear don't follow the same track!) You can get the formulas online.
  Castor (positive or negative) lengthens or shortens, respectively, the wheelbase; Camber (positive or negative) narrows or widens (respectively) the track. For tires 16" in diameter, though, the effect for small changes in castor and camber will be likely not affect the Ackerman calculations.
• My observation on the 310 and 520 (which both steer well, though the 520 reduction is easier!) is that they are mostly straight "toe-d", and have a "less" Ackerman --i.e. when going straight ahead, both wheels scrub very little and on sharp turns, the inner wheel scrubs noticeably.
• On both the castor is somewhat "slack" or positive but only a very few degrees -- this makes the steering feedback light on starting a turn but it increases with a sharper turn and it also uses gravity to help keep the wheels pointed forward-- tomorrow I can measure the 520 which I have nearby if you need it but someone may have the spec somewhere on this
• On both the camber is somewhat positive but again, only a very few degrees -- this tends to keep the tractor running in a straight line with little or no steering effort and it tends to try to "center" the steering over rough ground -- again, I can to measure this tomorrow on the 520 if you need it

IMHO, emulating a C- or 500 series steering geometry will serve you just fine with your fairly heavy tractor. You may want to consider more reduction in the steering linkage to make it easier to turn and justify a spinner knob (or, anecdotally, a "suicide" knob)!

[Oldskool 6,644]

After reading the other responses I feel like I went the caveman (ugh hit with rock) method. 

[Handy Don 12,258]

27 minutes ago, Oldskool said:

After reading the other responses I feel like I went the caveman (ugh hit with rock) method. 

I feel like I overshot by quite a bit.  We can form the over/under club.

[Lee1977 6,673]

At 4 or 5 Mph you don't need much of either Caster can be neutral with a very small positive Camber.

[ebinmaine 67,717]

1 hour ago, Oldskool said:

if the trailer hubs are going to make your track width wider then wouldnt that make that part of the geometry moot?

That's a thought I've had ....

 

Part of the reason for this thread is to get us all thinking on it. 

Part of it is also to help me figure out if I should go to a positive camber that is more in line with a narrow front tractor. ... BUT....I don't think it'll help. 

 

1 hour ago, Oldskool said:

your front axle is already built? Correct? If so wouldnt the scrub angle be somewhat built in

Yes but..... Because I haven't attached the spindles yet the camber could be changed. 

 

 

1 hour ago, Handy Don said:

• On both the camber is somewhat positive but again, only a very few degrees -- this tends to keep the tractor running in a straight line with little or no steering effort and it tends to try to "center" the steering over rough ground -- again, I can to measure this tomorrow on the 520 if you need it

IMHO, emulating a C- or 500 series steering geometry will serve you just fine with your fairly heavy tractor. You may want to consider more reduction in the steering linkage to make it easier to turn and justify a spinner knob

I'm thinking what I'll do is give it a little positive camber. 

 

The steering wheel is well oversized. I think about 16 or 17"?? Compared to a stock 13" of that vintage. 

And will very likely have a knob of spinnability. 

 

[Oldskool 6,644]

@ebinmaine but wouldnt you want negitive camber so the outside tire digs in at the bottom on corners? Isnt this going to be in the woods alot? 

[WVHillbilly520H 10,373]

I grew up on an Allis Chalmers "C" narrow front and Cab Cadets , as mentioned above lots of positive camber, but not mentioned is with the AC there was no adjustable "toe" as all this was set as cast, this tractor wasn't super heavy but not all that light either, it had 2 different brands of front tri-rib tires so they wore diffidently but both on the outside edges more so than the wide front "B" but again how many years and inflation rates before any of this happened, so they short version is yes add a bit of chamber but don't go overboard and 1/8" toe and drive happy.

Edited January 12, 2021 by WVHillbilly520H

[ebinmaine 67,717]

41 minutes ago, Oldskool said:

@ebinmaine but wouldnt you want negitive camber so the outside tire digs in at the bottom on corners? Isnt this going to be in the woods alot? 

That was my logic as well but apparently it's positive camber that helps with traction and steering. 

As the wheel is turned to the outside of a turn the tilt changes. Causing grab. 

 

[953 nut 55,372]

Camber,           did someone say camber?

In NASCAR they use negative camber on the right front wheel so that the contact patch will become flat on the track while making high speed left turns. However I don't think that will be your situation.