has anyone seen any of these SAE documents?

geoff

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Some interesting papers i found while searching the SAE site. Jsut curious if anyone has read any and which ones might be worth purchasing.

Title: Elimination of Roll, Squat, and Dive Through Biased Suspension Response
http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=2000-01-3572

Numerical Design of Racecar Suspension Parameters
http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=921662

Title: Performance Considerations for Run-Off-Road Countermeasure Systems for Cars and Trucks
<A target="_blank" HREF=http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=1999-01-0820>http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=1999-01-0820</A>


thanks, maybe someone else will find one of these interesting too.

"We are the music makers, and we are the dreamers of the dreams." -- Willy Wonka
 

Bob_Sheaves

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I have read all of them and all have points to make that should be considered WITHIN THE INTENDED APPLICATION. I am NOT saying I agree in all instances with the included conclusions, but overall they make compelling cases for specific advancements far over and above what the "homebrewer" can compete with. As a side note- these types of papers are one reason why:

"If you want to run with the big dogs, you better learn to pee on tall trees."

Best regards,

Bob
 

geoff

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I appreciate that bob, but i am having a bit of trouble finding any real data for a long travel 4 link design. I have done as much research as i can, but the more i research and the more i look at trucks on the net, etc i begin to think that most fabricators are clueless as to what they are doing, most of this site excluded.


any input for reading, or a good software package? I have been using working model 2d as a bit of a start but i really cant get too used to that GUI. This is for our senior mechanical engineering design project, not just fun. Thanks.


"We are the music makers, and we are the dreamers of the dreams." -- Willy Wonka
 

FABRICATOR

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Never underestimate the "homebrewer" :)

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

FABRICATOR

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Geoff, do you know exactly what you are looking for.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

geoff

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ive seen one truck that had lower arms mounted in the traditional style, then uppers that went to the *back* i mean actually behind the top of the axle. Didnt make any sense, and when i asked the guy who built it why he did that, he said he doesnt want to share his secrets...

i have seen a number of 4 links where the lowers attach only in front of the axle, not at all below them. however most of these had the shocks mounted on the axle, not the lower link.

lots of stuff very sketchy. i dont know too much, but i am pretty sure these werent such great ideas...

As for our project, it is just going to be a very traditional 4 link. Just trying to kepe driveshaft plunge down so that i dont have to go to a SYE and a CV driveshaft. The truck isnt going to need ridiculous amounts of travel, either, i just need it to have some clean travel.

"We are the music makers, and we are the dreamers of the dreams." -- Willy Wonka
 

Bob_Sheaves

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Sorry if I appeared to denigrate or "put down" anyone on here...my meaning of the "homebrewer" comment was that all of the writers of those mentioned papers are professional suspension and vehicle dynamics engineers with years of experience and statistically, they have a big edge over the homebrewer that only deals with suspensions as a hobby or part time job.

I am emphatically NOT saying anyone is less intelligent, but rather the odds are than the professional will make bigger and more repeatable advances in the sport and business of automobiles simply because they are paid to look for any and all advances in the science of dynamics.

As an aside, I have never met a vehicle dynamics specialist (male or female) that could honestly say they just had a job at work. In every case-they were ALL motorsports enthusiasts of some type. All were willing to help out anyone that asked for their help.

Best as always,

Bob
 

FABRICATOR

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The debate or confusion as to where the attachment points should be is not limited to 4 linking a solid rear axle. There is a similar situation on front ends as well with spindles and A-arms. Some spindles are short, some tall, and some super tall. Some lower arms are attached below the axle centerline and others dead center. Some ball joints are completely within the wheel and some completely outside. They all work.

The best way to see the best 4 links is at the races. These, usually, are a no bull approach because they have to work well. They are part of a much bigger package and are usually not subject to hobbyist experimenting. Exactly where the pivot points are on the axle is not all that relevant, especially with shorter travel. However, these points must be far enough out, radially from the axle, to properly handle brake torque and other loads. They must also be close enough in so that they do not themselves induce torque or otherwise interfere with desirable suspension movement.

The old Chevy Chevelles (1964? and up), which may be where this all began, had attachment points on the axle that are very close to a modern race truck. The top ones are about even or slightly above the uppermost part of the center section, and slightly ahead of the axle centerline. The lowers are basically right below the axle about 3 inches or so. Once the points are chosen on the axle, the front mounting points can be determined. Sometimes offsetting the rear, in any direction, will help out in front by avoiding something that is in the way, help the driveshaft movement, or help with the shock absorber upper mounting location.

Shock absorber and spring loads, if mounted on the LCAs, should not affect things as long as you observe the torque affects mentioned above. It is very important to have the suspension stops on the axle and not allow any shock to bottom out. One other aspect that should be considered, if there is freedom to do so, is component wear. Shorter control arms mean increased angle on all joints. Shorter torque radiuses mean more pressure on joints. As long as you stay within the “torque donut” you can efficiently build as needed.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

geoff

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thanks for the response, that definately helps to get me going.

regarding the strengths of the lowers, we used COSMOS to do a strss analysis on the lower using 600lb springs and it turned out to be pretty good.

You make a good point by noting that he mountin location of the uppers at the axle, is definately not a big deal, but the two main reasons i have yet to build mounts for uppers are 1) i dont know where to put the instant center and 2) i cant figure out what the intersection of the uppers, behind the axle, which should be the upper arm pair lateral restraint point, as milliken calls it "A" really affects? So as you point out the overall distance between the lower and the upper mount is not critical, although (if i am correct) the spacing between the two upper mounts is somewhat more important?

Someone i spoke to is freinds with rick geiser, and asked him about link placement. Apparently he was saying the that the intersection on the upper and lower links should be out in front of the truck and as low as possible. This particular guy located his instant center out near his crank pulley.

If i were to put the instant center all the way up there, and left the lower arms parallel to the ground, it seems we would have very little antisquat, and to keep the links low (keep IC low as a result) we could mount the lowers outside of the frame rail, at the bottom of the rail or even slightly below it, maybe not doing that for ground clearance however.

then the last thign i am having trouble with is driveshaft plunge. I believe this is dictated by a combination of arm lengths, but cant really find too much on the subject. Any way to determine this would be appreciated.


let me know what you think. thanks

"We are the music makers, and we are the dreamers of the dreams." -- Willy Wonka
 

FABRICATOR

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Geoff, for your application, I’m sure Bob could better explain the nuances of all the intersecting points.

Bob, I doubt anyone was offended, that was just thrown in there because I have tried to get the message across many times that things like those SAE papers and a good part of the rules relating to pavement cars have very little relevance to going fast off-road. The basic suspension function for a car designed for traversing smooth surfaces centers around maintaining a predetermined ride height. This is not the case off-road. A state-of-the-art 2WD off-road race vehicle must be thought of as having dual modes. Ride height is determined by the terrain and the cars reaction to it rather than spring or shock absorber settings, and is always radically changing. The only time normal rules apply is when turning or braking. Even then these rules are applied in a very basic manner because the car is really set up for another mode; which is the ability to go fast forward over rough terrain. By fast forward I mean throttle on, balls out, acceleration, across whatever is out there. For one thing, you need weight transfer, lots of it, clear up to 100 percent. This is not provided by the suspension, but it must allow it to take place. The transfer is provided by the combination of lots of power, suspension that will squat, and having enough wheel travel left to keep that mode from being interrupted. At this point you will have the maximum traction available. It may sound silly, but except for the bump capability, the off-road race vehicle in that mode is more comparable to a dragster than a pavement car. Anything that resists body roll will detract from traction. The only body roll control that would be helpful is front to rear. The cool looking but superfluous stance created by the topped out front suspension is not helpful in seeing where you are going and some of the negative camber turns into toe-out. There is a very complex set of things going on and this all must accommodate tremendous variables. The speeds that are achieved now are absolutely amazing and must be witnessed first hand to appreciate. Detroit could never figure this stuff out without going out there and doing it similar to the way the little guy does. Computers can keep up with nearly all aspects of something like F1 suspension, dynamics of the track and vehicle movements. And although computer logging and analysis could be very beneficial, off-road racing is likely way beyond what any computer program can even begin to sort out without significant human input. The science of dynamics of the desert would be hard to tame.


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TxPhPrerunner

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After reading your post I wanted to know if I understood you correctly. A desert truck should be designed to go as fast as possible in the straight rough. This is done by eliminating any hinderance to travel in the designed travel range be it 8" or 40". Am I reading this much correctly? You state that weight transfer should come from power. Should weight placement also be considered? If so would placing any movable weight in the center left to right and as low as possible help? Also is the classic 50-50 weight distribution good for off-road. Finally would basing the design around the drag race concept help. I have a fare idea of how they work. If I'm right then I assume that such a vehicle would be driven like I learned to ride dirt bikes in the late 80s. Ride hard from corner to corner, ride as deep into the corner as possible, brake hard, turn then get on the gas and repeat.

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FABRICATOR

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1) Hindrance to wheel movement must always be there. It is the job of some of the suspension geometry and the sole job of the shock absorber and suspension stop. But you want only that which is necessary. Short travel won’t cut it. This should not be misconstrued into some sort of balancing act.

2) Weight transfer must be brought about by power but allowed or aided by other things. Aside from power, it is also very affected by power to weight ratio, type of rear suspension, weight distribution, wheel travel, wheel base, tire size and design, spring and shock absorption rates, and rear geometry.

3) Weight placement is always very important in any type of racing vehicle.

4) All weight should be placed as low as possible or practical. If you are referring to the movable weight theory for an off road car, it has possibilities but would probably weigh more than it is worth. If things are right, using the throttle would seem much more effective.

5) There is no set or even preferred ratio of front to rear weight distribution. It is dynamic, and dependent on the items mentioned in #2.

6) The only similarity to a drag car is the weight transfer numbers. In function it is much more similar to a dirt bike. The most significant thing is the amount of vehicle control provided by the throttle alone.

Requirements are somewhat different between IRS and solid axle. They are even more different are with all-wheel, or all-wheel/two wheel drive.



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TxPhPrerunner

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By movable weight I was referring only to things such as the fuel tank, battery and other such things. I'm looking at a Chevelle stile 4 link for the rear. Power to weight should be somewhere between 10 and 14 to 1depending on the final weight of the truck. My idea so far is to shoot for 20" front and close to or over 30" in the rear while keeping the static ride height as low as possible. Mount anything I can move as far back, low and close to vehicle center as I can without loosing ground clearance. For the 4 link I would choose front pivot points close to those of a drag racer looking for that perfect body level light in the nose launch (I know this changes even in a drag car). I have noticed that many of the readers rides sit nose up but most of the trucks in skunkz sit level or at a slight downward rake like a dirt bike or a drag car. I intended to replicate this concept (the latter) in my truck. Am I headed in the right direction?

I don't live on the edge. I fell over long ago.
 

TxPhPrerunner

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A few things I left out. My truck is a 77 F150 Super Cab Long bed 2wd. I intend to shorten it and use short bed glass. I'm not trying to build the perfect truck just one that will work well off-road. I had assumed most of what was said in #1 but didn't state it. If I could build a truck that comes even close to a a dirt bike in the way it works I would be in heaven.

I don't live on the edge. I fell over long ago.<P ID="edit"><FONT SIZE=-1>Edited by TxPhPrerunner on 03/15/03 03:03 PM (server time).</FONT></P>
 

FABRICATOR

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Sounds OK so far. Front travel of 20" will probably be a good match for 12 (or more) lb/hp. A ratio having less than 10 lb/hp would not be able to hook up. With a full load of gas some TTs sit nose up at rest, but most are pretty level. The nose up stance of prerunners is for looks.

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TxPhPrerunner

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Thanks for the help.
I know I'm still missing some things, but at this point I'm not even sure what questions to ask. Not having ever driven a modified 2wd off-road vehicle, I have no basis of comparison. I'm starting with a clean sheet.
I think I understand the factors that are most important to making a desert truck fast. If I'm right I can apply the concepts to my truck and build something that will work very well.
In any race the highest average speed wins.
To get the highest average speed in the desert you need:

1) first and foremost wheel travel (anytime your wheels are off the ground you are loosing speed and control)

2) acceleration (a direct result of the power you can get to the ground, travel and weight transfer as important as hp, but hp affects weight transfer)

3) good brakes (less time/distance braking=higher average speed=faster truck)

4)I know that desert races arn't run in strait line. You do have to be able to turn at speeds much greater than 5 mph. High speed cornering ability, however, if I'm correct, should not be a primary focus. (I'm not suggesting that it should be ignored at all.)

I know a book or two could be written on each of these points and each are affected by other factors.


I don't live on the edge. I fell over long ago.
 

geoff

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<blockquote><font size=1>In reply to:</font><hr>

and rear geometry

<hr></blockquote>



more on this please =)

"We are the music makers, and we are the dreamers of the dreams." -- Willy Wonka
 

FABRICATOR

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If I am mind reading you right...you are referring to item #2. Rear geometry, as in squat characteristics, has some affect on weight transfer. The rear could conceivably be set up to allow almost no squat and yet still move for bumps. Fortunately and practically, they are somewhat related, like shock absorption and spring leverage, so if one is right the other is at least close.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 
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