Mark Ortiz chassis newsletter-“anti” effects - JKowners.com : Jeep Wrangler JK Forum
 1Likes
  • 1 Post By gt1guy
 
LinkBack Thread Tools Display Modes
post #1 of 10 Old 07-24-2015, 12:37 AM Thread Starter
JKO Addict!
 
gt1guy's Avatar
 
Join Date: Sep 2012
Location: New Iberia, La.
Posts: 3,851
Garage
Feedback: 0 reviews
Mark Ortiz chassis newsletter-“anti” effects

I wrote Mark Ortiz a while back with some questions about designing a link front suspension on my Jeep. For those who have never heard of him, he runs a chassis consulting service, has a monthly column in Racecar Engineering and puts out the monthly Chassis Newsletter. This months news letter is on the questions I had, so I thought I'd share. The bolded part in quotes is my questions to Mark.


July 2015

Quote:
I was wondering if you could shed some light on designing a 4-link suspension for the front of a 4 wheel drive vehicle? Vehicle in question is a new style 4 door Jeep JK. I'm designing a custom long arm 4-link front and rear.

The stock geometry is four longitudinal links and a Panhard bar. What I'm doing is a double triangulated with no Panhard bar. Uppers converging at the axle and the lowers converging at the crossmember for the trans. Steering will be full hydro w/double ended ram. No mechanical steering box or drag link (so no bump steer). Coilover shocks mounted outboard on the axles.

I will be running this set up front and rear (rear, minus the steering of course). It's actually a pretty standard design in the 4x4/rock crawling world.

As I said, I'm kind of at a loss as to how the front reacts to the forces.

I've attached the Excel 4link calculator with my design on it.

Main question has to do with anti-squat. On acceleration there is weight transfer off the front end, so would designed in A/S actually end up being pro-lift? I'm thinking it would only act as A/S during braking. I'm kind of at a loss as to how to look at things with regards to the front end.

There is a large offroad community that would be very interested in your thoughts on this.
I am a relative newcomer to offroad chassis. I haven’t had clients in offroad motorsports. However, I have new neighbors here at the shop who do quite a bit of fabrication for offroad vehicles and also run offroad vehicles of their own. I find the whole field very interesting, but I should emphasize that I’m still in the steep part of my own learning curve and my thinking is still rapidly evolving.

I have at least gotten wise to this: there is a huge diversity of “offroad” vehicles and activities. If anything, there is considerably more variety than there is “on road”. And there isn’t any single set of desired properties for an offroad suspension system, any more than there is a single set of desired properties for all vehicles operating on pavement.

Returning to the original question, how do we understand longitudinal “anti” effects in a vehicle where all four wheels are driven, especially at the front, and what properties do we want in this regard? Taking the last part of this first, there is not a single answer for all applications. It depends on what we’re doing with the vehicle.

Usually, we do not speak of anti-squat when referring to the front wheels. Anti-squat means a tendency of the rear suspension to jack up under power, countering the tendency for the rear suspension to compress due to rearward load transfer. The corresponding property at the front is anti-lift: a tendency to jack down under power, countering the tendency for the suspension to extend. Under braking, we can have anti-lift at the rear. The corresponding upward jacking tendency in braking at the front is called anti-dive. All of these can be considered forms of anti-pitch.

Negative anti-lift is pro-lift; negative anti-dive is pro-dive – and so on.

100% anti-squat is the amount of anti-squat that will make the rear suspension neither extend nor compress in forward acceleration. That doesn’t mean the car won’t pitch. It just means it will pitch entirely by rising at the front; the rear won’t go down.

For front wheel drive, 100% anti-lift is the amount that will cause the front suspension to neither extend nor compress in forward acceleration. Again, the car will still pitch, but it will pitch entirely by squatting at the rear; the front won’t come up.

Likewise, in braking 100% anti-dive or anti-lift is the amount that will result in zero displacement at the end in question when braking.

Although linguistic evolution has given us four different terms for these effects, they are all fundamentally the same thing: jacking effects resulting from longitudinal ground plane forces.

In all cases, including also jacking resulting from lateral ground plane forces, jacking force equals ground plane force times jacking coefficient. Referring to the graphic in the main page of the attached spreadsheet from the questioner, the jacking coefficient corresponds to the slope of the force line, the green line lowermost in the frame. The slope of this line equates to the ratio between jacking force induced by the suspension linkage and the ground plane force applied to the system.

The slope of this line is also the inverse of the instantaneous slope of the path that the contact patch center follows as the suspension moves, when the wheel is locked in a manner appropriate to the situation being considered (i.e. braking or propulsion). In the case shown in the spreadsheet, the force line has about a 1 in 4 slope. This means that for every pound of longitudinal ground plane force, the suspension induces a jacking force of about a quarter of a pound. In this case, when the force is forward (propulsion), the jacking force is downward (anti-lift).

The spreadsheet is evidently designed with rear wheel drive in mind. The 100% anti-squat line shown is correct, assuming that the other wheel pair doesn’t contribute to propulsion. In that situation, 100% anti (in this case anti-lift) happens when the force line intercepts the opposite axle plane at sprung mass c.g. height (light blue horizontal line). In that situation, it is also impossible to get any jacking effect at the opposite axle at all, because there is no ground plane force there.

That of course is not the case with four wheel drive, except maybe if drive to the rear is disabled. When both front and rear wheels contribute longitudinal force, as in braking with most vehicles and with all wheels driven, we need a steeper force line slope to get 100% anti at a given axle. However, we can get jacking forces at both axles.

The procedure when solving graphically is to lay in what I call a resolution line at a location corresponding to the ground plane force distribution between the two axles, and compare the heights of the intercepts of the front and rear force lines and that resolution line to the height of the sprung mass c.g. If, for example, the front wheels make 60% of the ground plane force, the resolution line is 60% of the wheelbase from the front axle. If the front wheels make all the ground plane force, as in the spreadsheet, the resolution line is 100% of the wheelbase from the front axle, as shown.

If the vehicle has a center differential, we have a known ground plane force distribution, at least until some locking is imposed on the center differential. However, when we have a locked transfer case, we do not have a known torque distribution. We have a 1:1 driveshaft speed distribution, and a highly variable torque distribution and ground plane force distribution.

If the vehicle is running straight and there is similar traction at both ends, we will have close to 50/50 ground plane force distribution. However, if one end has more traction than the other, there will be more torque to that axle and more ground plane force from that wheel pair. When traction is good at both ends and the vehicle is turning, often the torques and ground plane forces are not only

unequal but opposite in direction. The front wheels will follow a longer path and consequently need to turn faster than the rears, but be unable to. The rear wheels will then drive and the front wheels will drag. There will be reverse torque on the front drive shaft and extra torque on the rear drive shaft to counter that. The ground will exert rearward force on the front contact patches and forward force on the rear contact patches. In the questioner’s vehicle, the front will try to lift under these conditions. When it’s propelling the vehicle, its jacking forces will try to hold it down instead.

So there’s considerable uncertainty about what the induced jacking forces are going to be, because of the extreme variability of the ground plane force distribution. Do we at least know what we want the jacking forces to be?

Sort of, but that varies with what we’re doing with the vehicle. For an application such as offroad racing or Global Rallycross, we want the jacking forces to fight pitch, but not too much. If we get too greedy with our antis, we will get wheel hop on pavement or other high-traction surfaces.

For mud, things are different. There, we want both ends to jack up under power, vigorously. Why? Because when we’re stuck, sometimes the momentary tire load increase when we goose the throttle and the suspension pushes up against the frame will get us moving. It doesn’t always work, but in a useful percentage of cases it will.

And for crawling? I’m not sure it matters a whole lot, since the speeds and accelerations are so modest. I think probably the most important thing for a crawling suspension is to have huge travel, and a combination of stiffness in roll and softness in warp.



Kevin
BlackOps82 likes this.
gt1guy is offline  
Sponsored Links
Advertisement
 
post #2 of 10 Old 07-24-2015, 10:20 AM
Rock God
 
Join Date: Aug 2014
Location: Grosse Pointe, MI
Posts: 570
Feedback: 0 reviews

This is really good stuff. I probably spent 100 hours reading and analyzing suspension geometry, and I am still learning every day.

I think his response highlighted two of the main takeaway's I had. First is that the variation in use case, vehicle, and terrain, is so varied, that there really is no correct answer in terms of setup. What is important is to understand how all the variables interact and to design a system that on paper is most likely to achieve the target you desire. In addition, one should design in adjustability for fine tuning later on.

The second takeaway is that the outputs of a model are only as good as the inputs. You quickly realize that many of the variables are educated guesses. I literally reached a point where I was trying to get a Jeep engineer friend to sneak my JK in to their department to get the COG height measured.

Eventually, I reached a point of knowledge where it was clear to me that I understood what I was building and that the best approach was to build it with the intent to hit a bullseye, but with enough adjustability that I can get to a bullseye if I hit the edge of the board.
Invest2m4 is offline  
post #3 of 10 Old 07-24-2015, 03:04 PM Thread Starter
JKO Addict!
 
gt1guy's Avatar
 
Join Date: Sep 2012
Location: New Iberia, La.
Posts: 3,851
Garage
Feedback: 0 reviews

Your right, actually knowing the exact CG would make all the calcs so much more accurate.


BTW, the 4 link calculator design I sent him was the one we all have from Pirate. I just slowly came to the realization that for the front, it wasn't telling me what I wanted to know.

His idea of the resolution line is fantastic. But again, CG height is needed. Now I just need to find my giant graph paper.

Kevin
gt1guy is offline  
Sponsored Links
Advertisement
 
post #4 of 10 Old 08-19-2016, 08:54 PM
Granite Guru
 
Wannarun's Avatar
 
Join Date: Feb 2012
Location: Tulsa, OK
Posts: 106
Garage
Feedback: 0 reviews

Any chance you figured out the balance of jacking front and rear? What did you end up building? I'm in my infancy of playing with the 4 link calculator.


Sent from my iPad using Tapatalk

2012 JKUR w/ couple modifications
2002 BMW E46 M3
Wannarun is offline  
post #5 of 10 Old 08-20-2016, 03:44 AM
Granite Guru
 
BlackOps82's Avatar
 
Join Date: Dec 2015
Location: Denver Colorado
Age: 37
Posts: 212
Feedback: 0 reviews

Great article.. I enjoy reading Race Car Engineering... i'll have to go check it out.. still in my design phase as well doing my suspension.. but certainly great information.

Float like a Cadillac Sting like a Bimmer.
BlackOps82 is offline  
post #6 of 10 Old 08-20-2016, 01:41 PM Thread Starter
JKO Addict!
 
gt1guy's Avatar
 
Join Date: Sep 2012
Location: New Iberia, La.
Posts: 3,851
Garage
Feedback: 0 reviews

Quote:
Originally Posted by Wannarun View Post
Any chance you figured out the balance of jacking front and rear? What did you end up building? I'm in my infancy of playing with the 4 link calculator.


Sent from my iPad using Tapatalk
The biggest take-away I got from what Ortiz said was the need for adjustability and that without knowing your exact COG...........it's all guesses.

I ended up with what I think will be a good starting point. A somewhat neutral point with adjustability for more or less anti's front and rear. F&R upper frame brackets have 3 mounting holes.

Rear A/S gives me 35%, 50% and 64%. Front A/S gives me 48%, 64% and 80%. Links are as long as will fit, so nothing happens fast. Lowers will mount as far out on the axles as possible to add stability (tri-4link).

Right now I'm in the process of collecting parts for the LS swap, I have everything for the new axles and suspension. I want to do everything at once, which will probably bite me in the ass.

I'll be very happy if I end up with a suspension that's "ok" everywhere, rather than doing some things fantastic and making me think I'm about to die other times. Time will tell.

Kevin
gt1guy is offline  
post #7 of 10 Old 08-20-2016, 03:03 PM
Granite Guru
 
Wannarun's Avatar
 
Join Date: Feb 2012
Location: Tulsa, OK
Posts: 106
Garage
Feedback: 0 reviews
Mark Ortiz chassis newsletter-“anti” effects

It is possible to find the CG of a Jeep. Takes a bit of work but very doable if you have access to a set of 4 corner scales.

The procedure and calcs are described in the website below. I haven't tried this but the theory should be valid.

http://www.longacreracing.com/techni...spx?item=42586

Edit: this method is only for front to rear CG plane. You would need to perform a left to right measurement and calculation to find the CG point in all planes.

2012 JKUR w/ couple modifications
2002 BMW E46 M3

Last edited by Wannarun; 08-20-2016 at 03:06 PM.
Wannarun is offline  
post #8 of 10 Old 08-21-2016, 10:35 AM
JKO Addict!
 
thedirtman's Avatar
 
Join Date: Aug 2010
Location: southwest reno, nv
Age: 51
Posts: 4,344
Feedback: 0 reviews

Some good info here.
To the OP, I would re-do the rear to get the anti squat numbers up a bit more. I have been messing around with mine lately and currently running around 38% anti-squat on the rear and it squats bad, note I run 150/250 coils which you also have to take into account for handling characteristics and squat.

Before I made the recent changes I was at 107% and the jeep handled pretty well in all my driving situations. I do street, crawling, and a little high speed. In all aspects there the jeep handled very good and was predictable with still a bit of squat in the rear when accelerating from a stop. Right now I get squat thru all acceleration, even slightly, and it does not feel as stable driving on the road. I did some good crawling yesterday and could not tell a difference on the rocks. I did a little desert running at speeds of around 45 mph and it felt like the rear end danced a bit more now then when I was running the Rubicon last month at comparable speeds.

The main reasons I made the changes is because I was building a new sub frame with all my suspension mount attached to it what would give me the ability to pull the transmission without removing any of the suspension. Before the changes I had to remove the entire suspension to change out the clutch.

My original design had poor link separation according to what the "ideal" target is with the 25% tire height. At the axle I had 7" and at the frame I had 3". Not "ideal" but I got good results from it. Now I have 8" at the frame and will be raising the axle uppers so I am at around 10". This should give me as#'s in the 80% range. While I did add separation at the frame it did cost me about 1" of belly clearance.

In the end you can shoot for perfection according the the ideal spec's but doing so will limit you in other places since you have started with eh JK and its limitations in clearance mainly the upper frame side link mounts. Anti-squat is only one part of the whole suspension set up and don't let it trump other factors in your suspension needs.

Don't get too hung up on the numbers as if you are crawling you may need the ground clearance and massive articulation, or if you are racing squat and under/oversteer may be more important.

Some build info here:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


[COLOR="Red"]New to jeeps, check this link
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
thedirtman is offline  
post #9 of 10 Old 08-21-2016, 01:44 PM Thread Starter
JKO Addict!
 
gt1guy's Avatar
 
Join Date: Sep 2012
Location: New Iberia, La.
Posts: 3,851
Garage
Feedback: 0 reviews

Thanks Dirtman, it's good to hear how actual numbers work out in the real world.

At 107% (assume in the rear) did you ever notice any wheel hop under braking on the street? According to Ortiz at anything over 100%, wheel hop under braking can start to manifest itself.

Right now with the AS% numbers I listed I have separations of F= 6"/8.5" and R= 6.5"/8.5". I know I have wiggle room to move things a bit, just from the preliminary measurements I've done. When I actually start cutting I'm sure I'll find more room. For me, there's really no reason to play with the numbers anymore until I'm actually digging in. I need the LS swap sitting in place and I "hope" the Hero 3spd is roughly the same size as an Atlas.

Kevin
gt1guy is offline  
post #10 of 10 Old 08-21-2016, 02:54 PM
JKO Addict!
 
thedirtman's Avatar
 
Join Date: Aug 2010
Location: southwest reno, nv
Age: 51
Posts: 4,344
Feedback: 0 reviews

No wheel hop at 107 on my set up and still had a bit of squat in the rear end when accelerating. Separation is more about strength at the brackets then anything else and how they triangulate dictates the anti-squat

Some build info here:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


[COLOR="Red"]New to jeeps, check this link
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
thedirtman is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the JKowners.com : Jeep Wrangler JK Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may 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
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome