Progress Pics - new J-Arms

[rokker]

I made some new upper J-arms this weekend - (my first attempt) It cycles very cleanly. Im fairly happy with it. Almost moves exactly how I had it set up on the computer. Not much camber change at all. The tabs for the J-arms are actually only tacked on right now so I can fine tune it a bit more if I please (plug the numbers back into my computer and see what I can get.) I learned alot by drawing everything out on the computer first. This is 20" of travel in the pics. Suggestions are always welcome.

Thanks

fullbump
http://www.myneckisred.com/images/DSCN2306.gif

rideheight
http://www.myneckisred.com/images/DSCN2309.gif

fulldroop
http://www.myneckisred.com/images/DSCN2310.gif

[John_Bitting]

You need some negative camber at the top so you always have full contact patch on the ground while turning or with bodyroll. I have heard the magic number is around 6 degrees at full bump. Looks nice, lets see a pic of that top of the arm.

** I am no expert and this should not be taken as the gospel, just some things I have picked up along the way.**

[jaferri]

What program did you use and what type of truck/class is it??? What type of plans do you have for the rear??? Looks good some more close ups would be nice to see. - Josh

[castlefab]

It looks good, but I agree with John. Try moving the the upper a arm inner pivots closer to the lower inner a arm pivots. The distance between the upper and lower inner pivots should be less than the distance between the upper and lover pivots at the spindle. You should build more camber by doing this.


www.castlefab.net (http://www.castlefab.net)

[025]

Yep, I agree on the need for a little more camber gain and full bump. You probally also have already noticed that the upper arm may try to go down instead of up when at full droop and then landing. Moving the pivots closer vertically at the frame side and a limiting strap might be enough.

[FullsizeFun]

I was thinking that the upper arm might want to go over center also.
How much caster do you have built in? and how much does it gain and lose through the travel? I am no expert, but you will want caster and camber to gain at bump and droop. You will want to move the upper arm pivots down and angle the upper arm pivots front to back. So that the upper arm pivots are not parrallel, they will have more distance at the front than the back. So that your upper arm swings down and back on droop from ride height and up and back on bump from ride height. I am sure there is a lot of people on here that know more than me.
Looks good /ubbthraads/images/graemlins/grin.gif

[rokker]

Jaferri - no race plans for this truck as of now, its just a toy/prerunner ( I just wanna get it done and well see.) Rear will be linked.

Thanks for the input. I plugged the numbers into my computer last night. I moved the upper J-arms frame side pivot 1.25" down and .25" in. I also set the camber at ride height to negative 2 degrees (it was at 0 degrees.)

By doing this I came up with negative 5 degrees (camber) total at full bump and negative 1 degree at full droop while still cycling 20". By moving the upper J-arms frame side pivot in and down also helped with the situation described above by 025. I will make the changes on the truck hopefully this weekend and get some new pics along with a few side shots and close-ups of the J-arm.

What do you guys think of the above camber set-up/change? Acceptable and functional? As far as caster Ill get some numbers but 6 degrees at ride height is good, yes? What about gain? I never really found (been going over as many past threads as possible) a solid number on how much catser gain is acceptable/functional at full bump and droop. Is this number somewhat of a personal preference?

[Brian Mapes]

What you changed it too seams good, that should help with body roll. Your tires will tuck a little better into the fenders too. What kind of truck it that? With the bump cmaber I say no more the 5 degrees otherwise it becomes too much and you are ridding a little to much on the inside edge of the tire. Caster wise i dont quite know as much but i am learning too. I would think that around 5 or 6 would be good for ride height and then gain at full bump and droop to be at like 7 maybe 8. I think what the caster does is makes it easier for the truck to be able to turn, if the caster it negative, so the top pivot point is farther back, i think it also helps it take bumps easier. Correct me if i am wrong on that one. What are you going to be doing shock wise, bypass and coilover? What about bumpsteer, you probably havent even started on that yet.

[mattrick8888]

is that an a arm ranger?

[rokker]

mattrick8888 - yes an A-arm Ranger would be correct.

WorkInProgress - As far as the degree(s) in negative camber at full bump, my J-arm has delrin bushings on the frame side pivot and a heim on the spindle side pivot so I can adjust the amount of camber slightly after I have the tabs re-aligned.

As far as caster - maybe Im getting confused but I thought that when looking from the side of the vehilce when the top pivot point is further back, that is known as "positive" caster? Either way what I am shooting for (when looking from the side of the truck) is the upper pivot to be tilted "back" around 6 degrees at ride height and gain slightly a few degrees at full bump and droop so that the top pivot point would than be 8 degrees titlted back. Someone please correct me if Im wrong.

Bumpsteer - this is what I was doing to figure out tie-rod placement. When I was setting the new upper J-arm frame side pivot mounts into the computer, I also drew a circle large enough so that the circles outer path would cross through the spindle at full bump, ride height, and full droop. Once I found the exact size of the circle that would intersect in the same place of the spindle in all three positions, I found the exact center of the circle. The exact center of this circle would be my inner tie rod placement. Where the circles path crosses through the spindle would be the outer tie-rod placement.

Thanks in advance for the help.

[FullsizeFun]

what cad program are you using? autocad, mastercam, surfcam, solid works?

[ntsqd]

[ QUOTE ]
Bumpsteer - this is what I was doing to figure out tie-rod placement. When I was setting the new upper J-arm frame side pivot mounts into the computer, I also drew a circle large enough so that the circles outer path would cross through the spindle at full bump, ride height, and full droop. Once I found the exact size of the circle that would intersect in the same place of the spindle in all three positions, I found the exact center of the circle. The exact center of this circle would be my inner tie rod placement. Where the circles path crosses through the spindle would be the outer tie-rod placement.

[/ QUOTE ]

Ackerman Angle has an effect on inner tie rod pivot placement. If those are ready-made uprights then your Ackerman is probably fixed. So your inner tie rod pivot point will be offset inboard by the amount that the outer pivot point is offset inboard. Your circle needs to intersect the location of the outer TR pivot point at all three positions. If you have yet to build the steering arm then you need to figure out how much Ackerman you want/need, build the steering arm (in steel or computer model), and then start on the inner pivot location.

Fore/aft location of the inner TR pivot not only plays a role in effective Ackerman (effects the rate of angularity change), but also has a role in bumpsteer.

[025]

When it comes to bumpsteer try this: Figure your ackerman and the change in distance outward viewing from the front. Pin point a spot where you want your arms to go (height) take in to account your change because of ackerman. Then snap a 3-point arc starting from the full bump spindle to the full droop spindle and then pull it till you hit the spindle at ride height. The center of that arc will give you as close to zero bumpsteer as possible. I would though take this with a grain of salt, Unless you had bulkheads waterjet cut that incorporate top and bottom pivots your computer to actual is probally a little different.
Your camber change looks a lot better. Dont forget that when you steer your castor will also give you more camber or less depending on inside or outside wheel. With 9 deg. of castor it could be as much as 3 deg. of camber change. This works for you and not against you. Most of the race trucks are running close to 9deg. of castor for street driving try closer to 5-6 deg. Dont forget to take into account leading angle which is the amount that the suspension is rocked back. Not too much or the shocks will have to work too hard to keep from packing up when braking into whoops. I would personally stay away from castor change (rear pivots larger spread than front pivots on the upper arm only). Its starts a z-axis that can get real confusing when trying to figure bumpsteer. And does little for performance considering not everyones suspension will have the upper arms flat at ride height. If they are not then it will actually reduce castor before increasing. Otherwise things look good and you should be real happy that you took the time and effort to make you geometry correct before spending days of welding. Matt

[rokker]

Thanks for all the input and suggestions. Ill keep you posted on the changes I make and the numbers I come up with for everything as well as some more pics when I get them.