who knows about control arm geometry?

[Donahue]

i've posted a few threads about this subject on some other forums, including dezert dimes, and i am not really getting a good response. i am just researching a design for a sand rail type vehicle, and specifically the front suspension. the questions i have are how good or bad are equal length arms? the design is similar to a-arms, but also similar to a radius arm style. i am not sure if its been done before, but i am interested in knowing people's thoughts on it.

a few pics i've uploaded and did not resize. they are links and not images for this reason. if you have any questions about the design, just ask.

http://www.3leaffab.com/images/front%201.JPG
http://www.3leaffab.com/images/front%202.JPG
http://www.3leaffab.com/images/front%203.JPG
http://www.3leaffab.com/images/front%204.JPG
http://www.3leaffab.com/images/front%205.JPG
http://www.3leaffab.com/images/front%206.JPG
http://www.3leaffab.com/images/front%207.JPG

[DieselS10]

You've got 2 links coming from one direction and one from the other. With only one link connecting to the other two, what will keep the two from twisting? Wouldn't it need two links from both directions?

[s10sixteynine]

im no expert but i belive thats where the bar running from the upper to the lower comes in, and as far as ppl knowing about control arm geometry the only one that comes to mind is Seth (Twisted Minis) he is mostly in the air ride world but builds his own arms and says he said in one of his threads the other day that alot of his influences come from desert racers he might could help out

[Donahue]

correct, there is a fourth "little arm" that keeps the upper and lower control arms from twisting, its what the "radius arm" attaches too. everything would run heims or similar other than this little arm which would run bushings to keep the deflection to a minimum.

[DieselS10]

I did a little research on the equal length situation. From what I've read the disadvantage to equal length arms is that when the suspension is compressed during cornering there is no negative camber gain, like there is with a shorter upper arm, which will cause the tire to lean out at the top and hinder cornering. unequal length arms allow you to build however much negative camber you want for cornering and it also allows the roll center height to be set where ever one chooses.

[greencactus3]

i would love to help, but i have no idea about anything with offroading ish requirements.
i know there's a guy that does the baja SAE in college. he might be able to help you out.

im sure its just the angles of the screenshots, but are you going to be able to run a 2 force member steering arm? looks like the longitudinal const link might be in the way?

while on the subject of 2 force members, the bent arms may look better, but straight line triangle will be better...

and then again, having that 1 link in the middle of the other arms, offset from the other creates a moment... which can easily be avoided.

i dont know about sandrails, but are there front wheels driven? if not, i would recommend (if not wishbones,) the longitudinal const link to be pivoting on the outboard end right at the upright pivot, with the 2 parallel (or not) links just regular 'bars' (no bends, no offsets) which allows every component to not experience any moment

[Donahue]

can you explain moment?

front wheels may or may not be driven, and i plan on sneaking the steering linkage just in front of the short little arm that the "radius arm" attaches to. you and i use different terms, and i am sure mine are incorrect. could you explain what terms describe what parts on my drawings?

[Donahue]

ok, i assume what you call longitudinal i call the radius arm. the upper and lower links would be the lateral ones i suppose. if i make them a straight bar with no bend, how can i keep them from twisting? i figured i needed to capture them together (keeping them parallel) with that little 8" arm.

what i have been trying to do, is convert a rear 5 link commonly used on sand rails to allow for steering by having only 2 points of contact at the knuckle.

[greencactus3]

Quote: Originally Posted by Donahue

can you explain moment?

front wheels may or may not be driven, and i plan on sneaking the steering linkage just in front of the short little arm that the "radius arm" attaches to. you and i use different terms, and i am sure mine are incorrect. could you explain what terms describe what parts on my drawings?

yea sorry about the terminology, i dont know what to call them so i made up names lol

moment is um... bending torque? i guess would be a different name

pretty much anything that is connected in more than 2 points, or anything without any material all the way through the 2 points will have a moment (force that causes material to bend, where a true 2 force member will have forces in ONLY tension and compression, hence 2 force member)

see how that 5 link you posted has only 2 force members? this allows for maximum strength per amount of material, which means 1, you can make the parts lighter/cheaper, or 2, for the same amount of material/weight, you can make the part stronger than a member with more than 2 forces.


i dont know anything about sandrails, but i dont see the benefit of your design over a 'conventional' double wishbone design?

a correctly designed double wishbone utilizes only 2 force members,
depending on the location of the inboardpoints, can even be partially trailing arm, which may help on large bumps, if thats what sand rails hit alot lol

[Twisted Minis]

Okay here is my opinion.

Equal length control arms should never be used on any suspension setup. And the arms should NOT be parallel with each other. You need to establish a front roll center, or the suspension will feel sloppy. Also your spindle inclination should probably be a bit steeper than it looks there. If you where to move the upper arm mount out on the frame a bit, and make the top of the spindle a bit taller and move the mount closer to the frame you would probably have a good setup.

I don't necessarily agree with building the stabilizer the way you have it. Especially if you want a lot of travel. Its too short really, and will pull your wheel front to back considerably. If you want to run it that way, you would really be better off with one on each control arm, that extend back to roughly the half way point of the vehicle, with the upper being shorter and angle down slightly more. Again, instant center, and also good caster gain. What you have now will pull the wheel evenly, and too much, which will create bump steer during travel.

Personally I think a standard A-arm setup would treat you much better, in a lot of ways. But it can be done this way, and has been done before. It just really isn't the best design out there I feel.

Thats about all I can tell you just by looking. You are aware of ackerman when designing the steerign system I assume?

[greencactus3]

quick post from work. on a sandrail, i doubt ackerman is much of an issue.
just run more of it for better turn in response.

[Twisted Minis]

Ackerman is always an issue. If its designed wrong the truck may not even steer. I know in the sand the tires wouldn't scrub like on the street, but it should still be addressed properly. Theres no excuse for ignoring any portion of the suspension system.

[greencactus3]

Quote: Originally Posted by Twisted Minis

Ackerman is always an issue. If its designed wrong the truck may not even steer. I know in the sand the tires wouldn't scrub like on the street, but it should still be addressed properly. Theres no excuse for ignoring any portion of the suspension system.

im not saying ignore it... i just meant, how do you know what is the appropriate ackerman? are you positive you want the same slip angle on your inside and outside tires at all times? i sure dont

[Twisted Minis]

Slip angle?

Ackerman is progressive. Your inside tire should always turn farther than your outside tire. The center line of the tires should intersect on a line that runs parallel with the center of the rear axle shafts. Slightly in front of or behind that line is okay, but you don't want your front wheels to be parallel to eachother at all times.

[greencactus3]

Quote: Originally Posted by Twisted Minis

Slip angle?

Ackerman is progressive. Your inside tire should always turn farther than your outside tire. The center line of the tires should intersect on a line that runs parallel with the center of the rear axle shafts. Slightly in front of or behind that line is okay, but you don't want your front wheels to be parallel to eachother at all times.

you are behind, look at all the F1 guys. they run reverse ackerman nowadays.
see this pic i picked off google for reference

Quote:

Your inside tire should always turn farther than your outside tire.

without complete tire data, this is simply put, WRONG info. yes, to have theoretical zero scrub, perfect ackerman may seem to be ideal, but since tires need slip to grip, this is not always the case.

i dont mean to be insulting, but (racecar) dynamics is still a very 'non-scientific' portion of physics, you will hear different things at different times from different people, and people will believe what they want, but i believe you do not have a firm grasp of the whats and whys of ackerman etc

[Donahue]

ok, a few things. the longitudinal arm would be a lot longer, probably at least 36". if i made the lateral arms unequal length, then that throws the whole design out the window because it relies on that little 8" arm that connects the upper and lower laterals. ackerman would be addressed, but i will add that my blazer has reverse ackerman and i cant even tell. its not probably all that much, but most OEM's vaguely cover this concept anyways, just look at all the different wheel based vehicles using the same steering knuckles, and all having different values for ackerman.

[greencactus3]

Quote: Originally Posted by Donahue

ok, a few things. the longitudinal arm would be a lot longer, probably at least 36". if i made the lateral arms unequal length, then that throws the whole design out the window because it relies on that little 8" arm that connects the upper and lower laterals. ackerman would be addressed, but i will add that my blazer has reverse ackerman and i cant even tell. its not probably all that much, but most OEM's vaguely cover this concept anyways, just look at all the different wheel based vehicles using the same steering knuckles, and all having different values for ackerman.

i guess the major factor in whether you want negative camber gain through sus stroke or not depends on whether or not having the tire lean into the turns helps traction on sand or not. the benefits of a sticky tire being flat or less on the sticky ground is easy to tell, but depending on if you are 2wd or 4wd, your tire changes too correct?
on 2wds, you get the blade style? and on 4x4s you get paddles up front?
depending on tire compound/tread design, negative camber may help or hurt you

you can use equal length arms and still achieve neg camber with a taller outboard balljoint to balljoint distance than the inboard pivots.

also, you can (and obviously will be designing) play with caster angles and kpis which will affect everything too.


but imo, offroad vehicles, unless you are racing top of the line stuff, the dynamics are fairly loose, and because of not enough consistency between runs, not as important as a road hugging vehicle.

but damper valving is where you wanna look into.... or well, look away from

[Twisted Minis]

Thats an F1 car, I build shit to be street driven, predictable and smooth.

If you read Carrol Smiths book he addresses the reverse ackerman, and also playing with ackerman to achieve different affects, and even adjustable steering setups to tune ackerman for different courses.

I'll admit I don't know everything about ackerman, and I obviously play with it in a much different fiel than you do. A Mercedes I worked on lately had reverse ackerman. It drives like shit at low speeds.

[dillon_b12]

I am involved with Baja SAE and have been for a few years now. I am not an expert but I think I have a pretty good general grasp on the subject.

1. A-arms should never be equal length. Positive camber is bad...period. Hence you want some camber gain to counteract body roll. On our baja car we have unequal length, unparallel A-Arms.

2. Ackerman remains a hotly debated topic in the off-road world. Remember that in the buggy world often times you have gobs of power available(Busa engines in SMALL buggys or small blocks in the big cars) to spin the tires. Personally, I think that unless the buggy is severely underpowered Ackerman is not that big a deal. Also, you are putting rear-steer on it...it's gonna turn like a madman anyways.

3. Tie-rods should be parallel with lower A-arms.

4. If you are having trouble getting a particular characteristic out of your suspension design(say bumpsteer), it needs to be at full droop not full compression. If you happen to get lots of bumpsteer at full droop it's not that big of a deal because if the tire is at or near full droop it has very little load on it or you are flying. Either way it has little affect on driving dynamics.
However, if the bumpsteer is located at full compression, you got major problems. Imagine jumping the car on a tight woods trail... landing, and despite your steering input, the car darts into that nice oak on the side of the trail.

Lastly, suspension design is a HUGE can of worms. Even when you do find "EXPERTS" their opinions will often vary.

This would be a great read for you...
http://www.rorty-design.com/content/Ed_McCannick.htm

Also, www.minibuggy.net is a great forum with guys that have been designing offroad suspension since before I was born. Between those two sites, you WILL find what you need. I promise.

[greencactus3]

Quote: Originally Posted by Twisted Minis

Thats an F1 car, I build shit to be street driven, predictable and smooth.

If you read Carrol Smiths book he addresses the reverse ackerman, and also playing with ackerman to achieve different affects, and even adjustable steering setups to tune ackerman for different courses.

I'll admit I don't know everything about ackerman, and I obviously play with it in a much different fiel than you do. A Mercedes I worked on lately had reverse ackerman. It drives like shit at low speeds.

as i said, different setups for different uses.
how can you say a sand rail is closer to a street driven car vs an f1 car?

it pretty much comes down to the tires anyways.

i havent read tune to win yet (currently in the middle of engineer to win) but i have heard the milikens (sp?) are all for reverse ackerman

[Twisted Minis]

I'm not saying its closer to street driven then an F1. I do know that the desert built cars run standard ackerman, to an extent.

[greencactus3]

standard ackerman? what is that?

[dillon_b12]

Quote: Originally Posted by Twisted Minis

I'm not saying its closer to street driven then an F1. I do know that the desert built cars run standard ackerman, to an extent.

We run near neutral. Is this what you are calling "standard"?

[Twisted Minis]

I am talking about the standard form, in which most cars come designed from the factory. Meaning the outside tire turns more than the inside. You called the F1 setup reverse, I didn't know what else to call what most vehicles run.

[greencactus3]

Quote: Originally Posted by Twisted Minis

I am talking about the standard form, in which most cars come designed from the factory. Meaning the outside tire turns more than the inside. You called the F1 setup reverse, I didn't know what else to call what most vehicles run.

ohh i see. i thought you were talking like a specific percentage and rate.
i guess the term should be like pro-ackerman or something. i dunno.


oh wait... outside tire turns more than the inside? typo i think

i guess we got side tracked here.... anyways donahue, i think it's best you decide on if its driven or not before you do much more preliminary planning. it just makes too big of a difference.

[Twisted Minis]

Quote: Originally Posted by greencactus3

ohh i see. i thought you were talking like a specific percentage and rate.
i guess the term should be like pro-ackerman or something. i dunno.


oh wait... outside tire turns more than the inside? typo i think

i guess we got side tracked here.... anyways donahue, i think it's best you decide on if its driven or not before you do much more preliminary planning. it just makes too big of a difference.

Yeah, major typo. Lol.

I would agree. I would also ask some people that build sand cars how they set up their ackerman, because the F1 type set up makes sense in a situation where you don't have a lot of traction, like in the sand.

[What?]

jason at gmachine

otherwise matt at bajacustom

those for starter

if still need help, PM me and Ill give you the number to some locals that build these daily, anything goes from them